decentralised waste water treatment contents

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Decentralised Waste Water Treatment

Tasks, function and selection of small-scale purification plants

Sub-tasks

Overview of components

Components of purification process

Components of post-treatment system

Selecting a small-scale purification plant

Components of preliminary treatment system

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Sub-tasks of waste water purificationOverview of processes

Tasks

Components

Selection

Wastewater pondPre-composting tank

(Rottebehälter)Single-chamber settlement

tankMulti-chamber settlement

tankSingle-chamber slurry pitMulti-chamber slurry pitFilter sack

Wastewater pondInfiltration trench, filter bedFilter chamberSediment filterPlant bedsPercolating filterBiological contactor systemSmall-scale activation systemSBR process

Lamella separatorComposting bedPolishing pondSand filterUnderground trickle systemDrip trough (trickle area)Seepage trench, soak-awayCisternUnderground irrigationLiquid manure trench,

agricultural recycling

Preliminary treatment Purification process Post-treatment,recycling

Toilet wasteRainwater Household waste water

Black waterGrey water

Return to water cycle

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Refuse

Sub-tasks of waste water purification

Primary sludge

Organic carbon compounds are convertedinto biomass

secondary sludge

Waste water purification: sub-tasks

The entire purification process for domestic waste water can be divided into individual sub-stages or sub-tasks, regardless of the size of a water treatment plant:

Separating out of coarsematerials

Separating out of solidmaterials

Separating out of sand

Elimination of nitrogen

Elimination of phosphorous

Biological purification

Sludge storage

Sludge treatment

Separating out of biomass Sludge disposal

Post-treatment of outflow

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1. GeneralSince it would be impossible to perform all the required sub-tasks in one single stage or in one single systemcomponent, each and every water treatment plant consists of the appropriate combination of individualcomponents. A percolating filter, for example, is not a water purification system in itself: it is only acomponent which, when combined with other components such as a multi-chamber settlement tank, asedimentation tank, sludge storage tank etc., constitutes a water purification system. Frequently, several sub-stages are found in one single component.

Multi-chamber slurry pit Infiltration trench

Separating out of coarsematerials

Separating out of solidmaterials

Sludge treatment

Sludge disposal

Sludge treatment

Sludge storage


Separating out of biomass

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2. Separating out of coarse materialsCoarse materials found in untreated waste water include paper, disposable nappies and other hygieneproducts, food remains and kitchen waste, razor blades, curlers, plastic bags, cotton buds, plasters and soon: in other words, mainly materials which could in fact be disposed of with the rest of the householdrubbish and need not end up in the waste water.

With small-scale purification plants it has been shown that - unlike with large-scale plants - local residentscan be encouraged by means of appropriate education of the public to dispose of these materials along withnormal refuse and not to flush them down the toilet. Experience has also shown, however, that all coarsematerials cannot be prevented from ending up in the waste water. These have to be separated out to preventblockages and other malfunctions in the water treatment plant. However, the dividing line between coarsematerials and the primary solid materials described in section 6.3.4 (Separation of solid materials) is flexibleand cannot be clearly defined.

The separating out of coarse materials is usually achieved by the settling of sedimentation and floating inmulti-chamber settlement tanks and multi-chamber slurry pits. One special component is the filter sack.When activation tanks are used without preliminary treatment, secured refuse baskets and specially designedscum baffles are used.

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3. Separating out of sandSand and small stones get into domestic waste water through the washing of vegetables, dusting,vacuuming, boot-cleaning and similar activities. Sand also gets into the sewage network through ventilationholes in manhole covers. Poorly maintained rainwater inlets, which are the main cause of sand in urbanmixed-sewage systems, are not an issue in small-scale purification plants because with this kind of system thecollection of waste water always follows the separation process. As a result, the amount of sand in small-scale treatment plants is small and there is usually no need for a dedicated structure for separating out sand.Sand is simply filtered out together with other solid materials.

Solid materials are all those particulate materials which are not categorised as coarse materials or sand,which can be separated out within a few hours by settling or floating in still water. This includes ground-uppaper, solid faecal elements and other particles which get into the waste water as a result of householderswashing dishes, cleaning or having baths or showers. It is vital that solids be removed in a separate process ifthe treatment plant contains components which must be protected against silting up, e.g. percolating filters,rotating biological contactors (RBCs), biological contactors and in particular all 'natural' treatment stages,such as infiltration trenches and plant beds. When using activation tanks, there is no need to separate outsolid materials first.

The components used for separation are multi-chamber settlement tanks and multi-chamber slurry pits.Another possibility, mentioned previously, is the filter sack. This allows some materials which are categorisedas solids to be separated out in addition to coarse materials.

4. Separating out of solid materials

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5. Biological purificationApproximately two thirds of the organic carbon compounds found in untreated waste water which can bedetected using the 5-day BOD parameter are present either in solution or in highly distributed form. Theycannot therefore be removed using physical methods, but can only be removed using a 'biological cleaning'process. The basic principle underpinning all biological processes is that dissolved organic matter is mostlyconverted into bacterial biomass by the metabolic activity of aerobic microorganisms in the presence of freedissolved oxygen, biomass which is then separated from purified waste water using physical methods. Aproportion of the organic matter is used by the microorganisms to cover their requirements for CO2 and H2O.A key precondition for the success of this process is the presence of free dissolved oxygen.

Infiltration trenches, filter beds, filter chambers, sediment filters and plant beds are known as 'semi-natural'or 'natural' processes because they allow oxygenation without the use of pumps, fans or similar mechanicaldevices and therefore without the use of external energy.

Percolating filters, rotating biological contactors, biological contactors and activation tanks are known as'mechanical' or 'artificial' processes, for which machines are needed for oxygenation.

Another differentiating criterion is whether the microorganisms are present as a fixed growth on growthsurfaces (biofilm) or are suspended in the water, i.e. can move freely. In all natural processes and when usingpercolating filters the microorganisms settle on the growth areas, while in the activation tank they aresuspended and free to move about. Rotation biological contactors and biological contactors feature bothfixed and suspended organisms.

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...Biological purificationIn the case of all biological purification components, the biomass created by bacterial growth (excess sludgeor secondary sludge) must be removed on a regular basis, unless it is continually broken down within thecomponent itself - as is the case with infiltration trenches and plant beds.


Semi-natural processes Mechanical processes

Sediment filter

Filter chamber

Infiltration trench

Plant bed

Percolating filter Rotating biological contactor

Biological contactor Activation tank

Microorganisms

fixed position

suspended

Various stages of the biologicalpurification process

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6. Separating out the biomassThe bacterial biomass which performs the biological purification is usually separated out from the purifiedwaste water in a downstream sedimentation tank by means of sedimentation. Clarifying by means ofsedimentation can also be performed using a lamella separator. Activation tanks, which are operated using adamming process with discontinuous aeration, contribute to both the biological purification of the waterand the separating out of the biomass through sedimentation, alternating periodically.

In the case of 'natural' biological purification processes (filter chamber / plant bed), biomass is separated outin the structure itself by the filtering effect of the construction material used. The continuous eating by thehigher organisms (protozoa, grubs, insect larvae etc.) offsets the growth of biomass, which means that underthe right conditions a balance is achieved between the amount of existing biomass and the amount brokendown, preventing the filter material from becoming overgrown and blocked. If this balance is not achieved,blocked material may have to be replaced.

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7. Oxidation of nitrogen (nitrification)The nitrogen found in untreated household waste water comes mainly from human excrement. In the formof ammonium-nitrogen (NH4-N) it produces a nitrate (NO-

3), causing unwanted oxygen consumptionin thedischarge system. In Germany this fact is tolerated in the case of small-scale purification plants, while inAustria the oxidation of nitrogen (nitrification) must take place in the purification system, even in the case ofpurification systems for individual homes, and not in the receiving water after it has been discharged.

Although the oxidation of ammonium-nitrogen is not a desired feature of small waste water treatmentsystems, it often arises 'by itself' as a result of the design regulations imposed on small-scale plants.Favourable conditions exist for nitrifiers in activation tanks, for example. This can be of significance in termsof the process.

The oxidation of nitrogen is performed by special microorganisms (nitrifiers) according to the summaryreaction equation: NH4

+ +2O2 → NO3- +2H+ +H2O + energy. As the equation shows, H+ions are formed

during the nitrification process. In soft drinking water with a low acid buffer capacity (carbonate hardness <10 °dH), this can lead to a lowering of the pH with serious disruption to the biological purification processes.There are small-scale activation systems for which normal operation as a purification system is only possiblethrough the regular addition of alkaline agents, e.g. 'lime hydrate' (calcium hydroxide Ca(OH)2 ).

8. Elimination of oxidised nitrogen (denitrification)Nitrogen which has been oxidised into a nitrate (NO-

3 ) is used amongst other things as a plant fertiliser andcan lead to excessive growth of algae. This is tolerated in small purification plants.

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9. Elimination of phosphorousIn domestic waste water, phosphorous comes mainly from human excrement. The phosphorisation caused bydetergents containing phosphate, which at one time was a key issue, has now lost much of its relevance. Inwater, phosphorous is a more effective fertiliser than nitrate and therefore promotes the growth of algae.This is tolerated in small purification plants.

10. Sludge treatmentThe sludge which accumulates during the treatment of waste water, i.e.

§ the precipitate solid material from the untreated waste water (primary sludge) and

§ the continuous growth of biomass produced during biological purification (secondary sludge)must be treated in such a way that its ability to decompose is largely eliminated before it is disposed of.

We talk about sludge being 'stabilised'. Stabilisation can be achieved aerobically or anaerobically.

In anaerobic sludge stabilisation, also known as sludge decomposition, the sludge is allowed to decay in theabsence of oxygen with the help of various groups of bacteria, to such a degree that when utilised onagricultural land, the influx of air soon creates aerobic conditions and most of the odour which is still presentat this stage disappears within a few days. At normal temperatures, anaerobic sludge stabilisation takes avery long time and can only be guaranteed in multi-chamber slurry pits which are evacuated approximatelyevery 2 years. In multi-chamber settlement tanks with an evacuation interval of ≤1 year as recommended byDIN 4261, the sludge is not stabilised, merely stored.

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... Sludge treatmentThe principle of the aerobic sludge stabilisation process is that the microorganisms in the effluent sludge arekept in a constant state of 'hunger' in the presence of oxygen, forcing them to use virtually all the reserve,storage and other reusable materials available to them as food. Using this aerobic process, the sludge isstabilised such that when it is disposed of no further decomposition takes place and no odours develop.Because of design regulations, activation tanks for small-scale purification plants are consistently subjected tosuch low demands that the desired state of hunger of the microorganisms is created, allowing biologicalpurification and stabilisation of the sludge to take place simultaneously in the activation tank.

'Natural' purification processes which use various types of soil filters as biological stages of purification are apeculiarity insofar as the only sludge which accumulates in these systems is primary sludge, with nosecondary sludge. The primary sludge is stabilised anaerobically in the upstream multi-chamber slurry pit.

11. Sludge storageBecause there are long intervals between the removal of sludge from small-scale purification plants, sufficientstorage capacity is essential. Particularly large storage volumes allowing an accumulation time of at least sixmonths are required if the sludge is to be used directly for agricultural purposes. This is because effluentsludge cannot be used on agricultural land in winter or during the growing season.

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... Sludge storageIn mechanical small-scale purification systems, the secondary sludge which accumulates during biologicalstages of purification is transported periodically to the upstream multi-chamber pit. If this is a multi-chamberslurry system, it serves simultaneously as a sludge storage facility. Assuming that the sludge is to be stored inthe multi-chamber settlement tank, the tank volume should be enlarged to 'sludge storage supplement' level.If there is no preliminary treatment stage with sufficient sludge storage capacity, a separate sludge storagetank will need to be built. There are also 'enlarged' activation tanks, a special case in which the tank alsoperforms the function of the sludge storage tank.

12. Post-treatmentBefore being reintroduced into sensitive waters, additional outflow enhancement measures can be applied.Polishing ponds and downstream sand filters are possible options. These structures also serve to moderateoutflow volumes. A polishing pond can also function as a storage facility if the treated waste water is to beused for irrigation purposes.

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INNOCHEMComponents of a small-scalepurification plant

Single-chamber settlement tank

Multi-chamber settlement tank

Filter sack

Tasks

Components

Selection

Infiltration trench

Filter chamber

Sediment filter

Plant bed

Percolating filter

Rotating biologicalcontactor

Biological contactor

Activation tank

SBR system

Underground tricklesystem

Filter bed

Underground tricklesystem

Sedimentation tank

Lamella separator

Sludge storage tank

Composting bed

Polishing pond

Sand filter

Seepage basin

Soak-away

Preliminary treatment Post-treatmentPurification process

Multi-chamber slurry pit

Single-chamber slurry pit

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In multi-chamber settlement tanks,undissolved material is removed from thewaste water by settling to the ground orfloating to the surface. Floating sludge isprevented from passing into the outflowof the tank by a scum baffle or similarfixture. At normal usage levels withdomestic waste water, the volumeproduced by the depth of the scum baffleis sufficient to store the floating sludge.

Multi-chamber settlement tanks can bebuilt as two- or three-chamber tanks. Thebaffles have filter slots in them, and thedistance between the base of the tank andthe bottom line of these slots must be atleast half the depth of the water.

1. Function and design

Because the waste water spends less time here than in a multi-chamber slurry pit, it is relatively fresh andthere is only minimal decomposition. Fluctuating sewage levels in terms of volume and concentration are onlyequalised and/or buffered to a slight degree.

Multi-chamber settlement tankaccording to DIN 4261 Part 1

Sc hnit t E-F

Schnitt M- LSchnit t I -K

S chni t t G- H

L W 6 00

3 00

2 00

3 00

0,5 t

50-1 00

150 10015 0

FE

I M

G

H

L K

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Cross-section

Cross-section

Cross-section

Cross-section

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Rating and construction in accordance with DIN4261, Part 1 (Small-scale purification systems);Systems without aeration of waste water;Application, rating and design:

Usable volume ≥0.3 m³/ per inhabitantpopulation equivalent), min. 3 m³,provided that there is an additional sludge storagetank for accumulating secondary sludge,

If there is not, the volume must be increased by0.05 m³ per inhabitant for percolating tanks,rotating biological contactors and biologicalcontactors and by 0.125 m³ per inhabitant foractivation tanks (extra volume for sludge storage),

For total volumes of up to 4 m³, two-chambersystems are permissible,

Usable water depth at least 1.2 m, maximumwater depth 1.9 - 3 m depending on size offacility.

2. RatingMulti-chamber settlement tanks may be used as apreliminary to 'mechanical' purification processes(percolating tanks, rotating biological contactors,biological contactors and activation tanks).

3. Application

If emptied regularly, multi-chamber settlementtanks have high levels of mechanical cleaningeffectiveness. The volume of solid material in theoutflow is usually below 0.5 ml/l; coarse materialsare consistently removed. As with primarysettlement tanks, it is often assumed of largepurification plants that reductions of 30% - 35%can be achieved when using using settlement tanks,five-day BOD

4. Effectiveness

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Experience has shown, however, that with smalltwo-chamber systems in particular the complexinterplay of

sedimentation and biological processes,

the decomposition of sludge, formation of gasand therefore disruption of the settling andfloating of precipitate sludge and

the passage back into solution of the products ofreaction of anaerobic decomposition

the 5-day BOD reduction can last varying periods oftime and any value between 0% and 50% is to beexpected. The same applies to the COD.

In all cases the discharged water will bedecomposed, can cause nuisance odours and givesoff harmful gases. This should be taken intoconsideration when choosing construction materialsand protecting against corrosion.

... EffectivenessAccording to DIN 4261, Part 3 (Small-scalepurification systems; Systems without aeration ofwaste water; Operation and maintenance [10],multi-chamber settlement tanks must be evacuatedat least once per year . The sludge whichaccumulates consists of coarse materials, primarysludge and the secondary sludge which results fromthe downstream aerobic biological purificationstage.

It is not fully decomposed (stabilised) and containsnon-decayed, visible coarse materials from hygieneproducts. When all chambers have been completelyemptied, the volume of the sludge/water mixturewill of course correspond to the total volume of thesettlement tank.

5. Residual material

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Multi-chamber slurry pits can be designed as three- or four-chambersystems. Undissolved materials and those which can float are separatedout in the same way as in the multi-chamber settlement tank. Due to thelarge precipitation area, the process of decomposition achieves a morethorough separation of undissolved materials plus the partly anaerobicbiological breakdown of undissolved pollutants in the water. The sludgearea, which is considerably larger than in a multi-chamber settlementtank, can store much larger volumes of sludge. Most of the sludge willdecompose. Due to the larger overall volume, fluctuations in the amountof waste water to be handled can be better balanced out. Theseadditional characteristics of the multi-chamber slurry pit enhance theoperational reliability of subsequent biological treatment stages.


Multi-chamber slurry pitaccording to DIN 4261 Part 1

In accordance with DIN 4261, Part 1 (Small-scale purification systems; Systemswithout aeration of waste water; Application; Rating and design:Usable volume ≥1.5 m³ per inhabitant, min. 6 m³,

Design always with at least three chambers, volume of first chamber1/2 of total volume,

Usable water depth min. 1.2 m, max. water depth 1.9 - 3 m depending on size

2. Rating

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Inflow Discharge

50 to 100

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Multi-chamber slurry pits are approved withoutrestrictions for use as preliminary purification andsludge storage tanks upstream of all types ofaerobic biological purification. They are particularlyappropriate for use upstream of 'natural' biologicalpurification components which present an increasedrisk of blockages (infiltration trenches, plant bedsetc).

When dealing with volumes of domestic foul waterof up to max. 8 m³/d, which corresponds to acommunity of around 50 inhabitants, in accordancewith DIN 4261 they are considered only inexceptional cases and only as transitional solutionsor as stand-alone purification stages .

It should be noted that multi-chamber slurry pitsalso involve decomposition of the waste waster andgive off harmful gases.

3. ApplicationThe disruption to precipitation caused bydecomposition in the base sludge occurs in the firstchamber of multi-chamber slurry pits in the sameway as in settlement tanks. Durch das größerespezifische Volumen der folgenden Kammernwirken sie sich aber kaum auf die Ablaufbeschaffen-heit aus. Because of this, and the additionalbiological partial purification, the purifying effect ofmulti-chamber slurry pits is considerably higher thanthat of settlement tanks.

If the system is well-maintained, the mechanicalpurifying effect is extremely good. The volume ofsolid material in the outflow is usually significantlybelow 0.3 ml/l; coarse materials are consistentlyremoved. A 5-day BOD reduction of 30% - 50% isconsidered feasible. The 5-day BOD concentrationsin the outflow are generally between 200 mg/l and300 mg/l.

4. Performance

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According to DIN 4261, Part 3 (Small-scale purification systems; Systems without aeration of waste water;Operation and maintenance [10]), multi-chamber slurry pits must be evacuated at least once every two years,i.e. floating sludge must be completely removed and a proportion of the base sludge should be left in placeas seeding sludge. The sludge consists of coarse materials and solids (primary sludge) and, in some cases, thesecondary sludge from a downstream aerobic biological purification stage. Although it contains non-decayed, visible coarse materials from hygiene products, unlike the sludge from multi-chamber settlementtanks (provided that they are evacuated every two years), most of it will decompose (stabilise).


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Filter sack

The inevitable decomposition of waste water in multi-chamber settlement tanks and the associateddisadvantages led to the development of the filter sacksystem in Austria in 1986. In this process, which is largelyunknown in Germany, the untreated waste water flowsthrough non-clogging fabric sacks suspended in the air.Coarse materials are then retained in the sacks. Thefiltering effect of the fabric is enhanced by the fact that,during operation, a filter of separated materials rapidlyforms which produces a high degree of separation ofundissolved materials.

The sacks are arranged in underground chambers in twolines such that the sacks in one line are used first and inorder. When all the sacks in this line have reached fullcapacity, the second line is put into operation by thecontrolling mechanism. After the second line has gone intooperation, the filled sacks can be left to drip off underaerobic conditions for an extended period of time. A rateof drainage to 15 - 20% TS as well as a certain compostingeffect can be observed.


Chamber with filter sacks formechanical sewage treatment

3,00

40 40 40 40 40 40 60

>1,0Res.

1,00

3,00

Stützen D N 100 FlachschieberNotüber lauf (Putzöffnung)

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Emergency overflow(opening for cleaning)

Gate valveDN 100 supports

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Filter sack

There are no mandatory standards which apply to the filter sacksystem.Rating may be designed following:

Ingerle, K and Stegner, U: ten filter sacks with a usable volume of20 kg for 50 inhabitants; following the decision of the department

of the Styrian provincial government relating tosewage disposal in rural areas: one sack per inhabitant.

2. Rating

The filter sack system is an alternative toconventional mechanical multi-chamber tanks,provided that a height of approx. 1 m is available.It is suitable for use in small waste water treatmentplants with serving a community of < 10inhabitants .

The advantage of the filter sack system lies in thefact that waste water remains fresh and that onlysolid materials and no sludge water need bedisposed of; the disadvantages include the fact thatsacks have to be handled and the height loss withinthe facility.

3. Application

Only sporadic reports have so far been producedregarding the effectiveness of this system. Thereductions in BOD and COD assays appear to becomparable to those in multi-chamber slurry pits.

Once filled, the sacks are left to drip off for severalweeks and are then removed from the filterchamber, at which point new sacks are put in place.

4. Effectiveness, residual material

Chamber with filter sacks; cross-section

80 60

>1,0Sackhalter

Stützen DN100

Schachtbei tie fer

Kanallage

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Shaft fordeep ducts

Sack holder

DN 100 supports

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Infiltration trench and filter bed

Infiltration trenches are systems for 'naturally' purifying sewagewhich has gone through preliminary sedimentation using aerobicbiological purification. In addition, the secondary sludge producedby this process is separated from the purified waste water, stored,treated and finally 'disposed of'.

Infiltration trenches feature two drainpipes laid on top of oneanother, the top one for incoming waste water and the bottomone for discharged water. The space in between consists of a filterlayer. The purification processes are basically identical to that ofan underground trickling system with a sand/gravel base.However, the two drainpipes arranged one on top of the otherprovide better aeration of the filter layer.


Biological purification is carried out bymicroorganisms which are present in the fine gravel.Oxygen is supplied from air which is drawn into thegravel after each feeding process through the deep-seeping waste water. A high oxygen supply volumecan thus be produced through intermittent feedingof the infiltration trench.

Infiltration trench according toDIN 4261 Part 1

100

0 , 6 0

0,20

0,50

gegebenenfallsZwischenschicht

Grobsand oderFeinkies

Verfüllung

abgeglicheneSohle

1:500

150

30m

Lüftung SammelschachtVerteilerkammer

bei TonrohrenAbdeckung derStöße

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Selection

Filler

Intermediate layer(if required)

Coarse sand or finegravel

Aligned base

Distribution chamberCovering for joints ifusing clay pipes Ventilation Collection shaft

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2. RatingIf space limitations mean that parallel filter trencheshave to be laid out so close together that there areno earth mounds separating them, this gives rise toa infiltration bed. Infiltration trenches and filterbeds are basically identical in terms of theirfunction.

3. Application

In accordance with DIN 4261, Part 1 (Small-scalepurification systems; Systems without aeration ofwaste water; Application, rating and design):

specific length of trench≥6 m per inhabitant;base width ≥0.5 m; min. depth 1.25 m; length ofone single line max. 30 m,Filter gravel 2/8 mm, min. 60 cm high; distribution

and collection drainage: nominal value 100 withgradient of approx. 1:500; aeration required,

Base layer must be slightly impermeable to water;use artificial seal if necessary.

The grain size of the filter material suggested in DIN4261 is a compromise between the desire to achievethe most effective purification - which wouldrequire a very fine filter material - and the desire tokeep the system functioning for as long as possiblewithout blockages developing.

Infiltration trenches are suitable for use in very smallsystems because the costs of the pipe material,replacing the soil and supplying the gravel make thisan uneconomical option for larger plants incomparison with other solutions. Infiltrationtrenches do not require energy but do require asufficient difference in height between the incomingwater and the outflow. It is not possible tointervene in the purification process to control it.

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3. PerformanceFew details are available about the actualperformance of infiltration trenches and beds builtin Germany; most research has been performed inother countries and cannot be easily compared withthe results of German filter trenches because thefilter material used abroad is often sand and not thefine gravel mentioned by DIN 4261. Because ofdiffering construction methods, impacts andconcentrations of waste water in the infiltrationtrenches studied, the published dischargeconcentrations differ widely and in a 5-day BODassay lie between > 100 mg/l and < 10 mg/l.

A careful evaluation of the results brings us to theconclusion that an infiltration trench designed andbuilt in accordance with DIN 4261 can achieve a 5-day BOD of <40 mg/l and a COD of <150 mg/l .Extensive oxidation of nitrogen with a NH4-Ndischarge value of <10 mg/l, as is required inAustria even for the smallest systems, is also notpresent.

Given the specific reactor volume of around 5 m³per inhabitant, more effective purification might beexpected. However, bear in mind that due to thenature of the process, the waste waster only everflows through a small proportion of the availablevolume and only this volume is used for purification.

Throughflow in an infiltration trench

1

2

3

4

S i c ke r r o hr R i e s el gr ab e n

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Seepage duct Seepage trough

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4. Residual materialThe biomass produced in the filter unit (secondary sludge) is disposed of continuously as it is consumed byhigher organisms and is also continually rinsed out to some degree by the relatively coarse filter material ofDIN 4261 standard normally used in Germany, in a similar way to a percolating filter operating with lowdemand levels. An infiltration trench can therefore be expected to have a useful life of many years. If after anextended period of operation a blockage does occur, it is best to leave the filter material on the bottom andlay a new trench alongside. Residual material therefore normally accumulates only in the preliminary stagebut not in the infiltration trench or filter bed itself.

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Filter chamber

The filter chamber (not commonly used in Germany) has been introduced into the Austria's national standardÖNORM B 2502 Part 1 (Small-scale purification systems (domestic purification systems) for systems serving upto 50 inhabitants; Application, construction and operation) as an alternative to the infiltration trench. Thereason behind this was the required length of infiltration trenches in Austria of 20 m per inhabitant and theresultant high costs for these systems.


Biological purification is carriedout by microorganisms which arepresent in the fine gravel. Oxygenis supplied from air which isdrawn into the gravel after eachfeeding process through thedeep-seeping waste water. A highoxygen supply volume can thus beproduced through intermittentfeeding of the infiltration trench.

Filter chamber according toÖNORM 2502 Part 1

20-30

7 0

20-30

1 0

20-30

D N > = 15 0

L üf tu n g s ö f f n u n ge n o d e rEn tl ü f t u n g ü b e r D a c h

T r en n g ew e b e

Pr a l l pl a t t ep e rf o r i e rt

g e wa s c h e n erSa n d 2 / 6 ( F e in k ie s )

p e rf o r i e rt e s D r ä n r oh r Ki e s 1 6 / 32

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Ventilation apertures or ventilationvia roof

Perforated deflector

Separating web

Washed sand 2/6 (fine gravel)

Perforated drainpipe Gravel

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Filter chamber

2. RatingDIN 4261 does not cover filter chambers but doesprovide for soak-aways.In accordance with ÖNORM B 2502, Part 1 (Small-scale purification systems (domestic purificationsystems) for systems serving up to 50 inhabitants;Application, rating, construction and operation):horizontal filter surface AF ≥2 m² per inhabitant,min. 6 m²,

Height of biological fine gravel layer(2/4 mm) min. 0.7 m.

3. ApplicationFor cost reasons, filter chambers are only a viableoption for very small waste water treatment plants.

No details are available about the performance ofÖNORM-standard filter chambers because so few ofthis type of system have been built as primarystages of biological purification. We can thereforeonly surmise that a 5-day BOD of < 40 mg/l andCOD of < 150 mg/l could be achieved. Nitrificationwith an NH4-N value of < 10 mg/l is not to beexpected.

4. Performance

Depending on the demand placed on the filterchamber, the top 10 - 15 cm of the filter sand willbuild up sooner or later and will need to bereplaced. However, there are no field reports yetavailable as to when this becomes necessary. Thesand which is removed is not 'sludge'; itscharacteristics are more like the sand collected incommunity purification plants.


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Sediment filter

Sediment filters are anotherfurther development of the filterchamber produced in Austriawhich have also been introducedto the German market. Sedimentfilters are characterised by thefact that the the total requiredfilter surface is provided byindividual 'cups' which arepositioned on top of one anotherand through which materialflows in sequence from top tobottom.


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Einkornsickerbet on

The total floor space required is therefore less than a filter chamber but withthe same filtering area. The concrete cups are filled with a special filtermaterial. The facility is usually fed intermittently with pre-treated wastewater by tipping tanks.To enhance the effectiveness of purification, sediment filter systems can becombined with a downstream recirculation shaft - discharged water beingpumped back into the pre-treatment system - or an upstream percolatingfilter.

Sediment filter, design and cross-section

Tasks

Components

Selection

Distribution rocker

Support

Ventilation necks

Gravel

Perforated discharge pipe

Distribution mat

Fine gravel

Separating web

Plastic granulate

Fine-grain permeable concrete

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Sediment filter

2. RatingNo standards apply to the rating. With reference tothe rating for filter chambers in ÖNORM B 2502,Part 1,

a specific total filter surface of AF ≥2 m² perinhabitant is usually observed.

If an upstream percolating filter is present,sediment filters are rated at AF = 0.3 or 0.5 m² perinhabitant

.

3. ApplicationThe preferred area of application is a communitysize of < 50 inhabitants, although facilities havebeen built which serve a community size of > 100inhabitants.

The remarks made about the filter chamber alsoapply to the filter material of the filter cups in asediment filter. It is important that the filter materialof the topmost cup be changed before signs of ablockage appear in the second cup. This is becauseit is extremely difficult to change the filter materialin the second and subsequent cups.


In the case of simple sediment filter systems thelimit value for the 5-day BOD of < 40 mg/l and thatof the COD, < 150 mg/l, can be achieved butnitrification should not be expected. If nitrification isrequired, the sediment filter system must becombined with an upstream percolating filter.

4. Performance

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Plant beds

Active principle: plant beds are a component of plant purification systems. They serve to biologically purifywaste water which has already gone through preliminary sedimentation. In principle, all plant beds consist ofa base planted with marsh plants and covered with sand and gravel. Purification is performed mainly by theactivity of microorganisms which live in this base layer, similar to the processes in an infiltration trench. Thegrowth of the plant roots is intended to maintain the hydraulic permeability of the base layer. It is surmisedthat the plants may have other effects too but there are very few experimental results relating to the effect ofthe plants. The most important factor for the effectiveness of plant beds is a sufficient supply of oxygen forthe microorganism population. The plants themselves do not however make a significant contribution to this.

Process variants: there is a wide range of possible construction and operation methods. A distinction can bemade between the following main types:with regard to the hydraulic conditions:primarily horizontal andprimarily vertical flow types and

with regard to the water table situation:clogging and non-clogging

plant beds.

The diagram shows an example ofthe structure of anon-clogging plant bed.


Non-clogging plant bed

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Plant beds

2. RatingAccording to Process Sheet A 262 (Draft)(Principles of rating, construction andoperation of plant beds for district wastewater for community sizes of up to 1000inhabitants) published by the GermanAssociation for Waste Water Treatment(ATV):

Surface required for horizontal flowAPKA ≥5 m² per inhabitant, min. 20 m²,sediment filter layer thickness 0.6 m,freeboard 0.3 m,

Surface required for vertical flowAPKA ≥2.5 m² per inhabitant, min. 15 m²,sediment filter layer thickness 0.8 m,freeboard 0.3 m,

permeability of sediment filterkf ≥10-4 to 10-3 m/s,

Base layer must be sealed (kf ≥10-8 m/s);if necessary, use artificial seal,

Plant with reeds, cattails,irises.

For the system to be effective, a sufficient supply of oxygenfor the microorganism population is essential. Vertical-flow,non-clogging and intermittent-feed plant beds all featurefavourable conditions for the supply of oxygen, such that a5-day BODof <20 mg/l, a COD of <100 mg/l and extensiveoxidation of nitrogen with NH4-N <10 mg/l can be achieved.Horizontal-flow plant beds are less effective because withthis system, clogging beds do not provide sufficient oxygenand non-clogging beds only allow material to flow through apart of the sediment filter. With these systems nitrificationcannot be guaranteed, but a 5-day BOD of < 40 mg/l andCOD of < 150 mg/l can be achieved. As long as thevegetation is not cut or mowed, no residual material shouldaccumulate in the plant bed itself.

4. Performance, residual material

Plant beds exist in all sizes up to around 1000 inhabitants(population equivalent). They are particularly well suited tosmall communities (individual structures); when consideringlarger plant purification systems, the issue of cost-effectivenessshould be investigated in comparison with 'mechanical'systems.

3. Area of application

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Percolating filter

Percolating filters are 'mechanical' components used in the biological treatment of pre-treated waste water.Waste water which has had the sludge removed is spread over a porous padding material with a largesurface upon which microorganisms settle. Stone chippings or plastic chips are used as padding material.These microorganisms remove the utilisable, dissolved or finely dispersed matter from the waste water as ittrickles over the padding and convert it into biomass which is continuously rinsed out of the percolating filterand separated out in a sedimentation tank further down the process chain.


Small percolating tanks arenormally operated using 'returnpumps', for example such that thesludge/water mixture pumpedfrom the sedimentation tank to thepreliminary treatment stage is amultiple of the of the inflowvolume and the sewage is routedvia the percolating filter severaltimes.

Small percolating filter with sedimentation pump,sludge pump and circulation pump

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Tasks

Components

Selection

Sludge returned to anaerobic digester Distributor unitVentilation duct

Maintenance aperture

Alarm

Overflow

Submersible motor-drivenpump with floating switch

Sludge pump

Sloping concretebottom (made on site)

Concrete base withflow-through apertures

Percolating filterpadding material

Distribution rocker

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Percolating filter

... Function and designIn exposed locations, percolating filters may need tobe covered during the winter months to ensure thatthey do not malfunction. Nowadays, smallpercolating filters are no longer planned as 'one-offs'because a large number of suppliers in partnershipwith concrete works now offer complete kits whichsimply need to be correctly put together on site.

2. RatingUp to 50 inhabitants: in accordance with DIN 4261Part 2 (Small-scale purification systems; Systemswith aeration of waste water; Application, rating,design and inspection:

permissible volume load BV ≤0.15 kg/m³.d;for waste water which has gone throughpreliminary sedimentation, required percolatingfilter volume VPF ≥0.27 m³ per inhabitant,

Height of percolating filter hPF ≥1.5 m;return ratio RR = 3,

when HPF =2.5m, RR = 1.

Provided that the rating and design recommendationsof DIN 4261 Part 2 are observed, a 5-day BOD of <40mg/l and COD of <150 mg/l (discharge values) andpartial oxidation of nitrogen can be expected. Thecontinuously rinsed secondary sludge and thatseparated from the waste water in the sedimentationtank accumulates in the percolating filter. It isnormally transported to the component where pre-treatment is taking place and disposed of togetherwith the primary sludge.


There are no upper or lower limits on the areas of usefor percolating filters. It should be noted, however,that the applicable standard for the rating of largerpercolating filters (>50 inhabitants) is not DIN 4261but the ATV Process Sheet A 122 (Principles for rating,construction and operation of small purificationsystems with aerobic biological purification processesfor community sizes of between 50 and 500inhabitants). Percolating filters require less energythan the activation process but the associatedconstruction costs are much higher.


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Rotating biological contactor

Rotating biological contactors are used to purify district waste water which has gone through preliminarysedimentation using a process of aerobic biological purification. Rotating biological contactors consist ofroller-shaped growth-encouraging units for microorganisms which pivot around a horizontal axis. Variousdifferent versions exist. The rollers are immersed about halfway in vats of pre-treated waste water and are setto slowly rotate so that the microorganisms alternately come into contact with waste water and air. There istherefore a periodic alternation between the absorption of food and the absorption of oxygen. Both themicroorganisms which are present on the special units and those which move freely about in the vats areinvolved in the process of biological purification. Nowadays, small rotating biological contactors areprefabricated in the factory and then simply erected or put together on site.



Cross-section of vertical axis ofrotation

View Cross-section of axis of rotation

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2. RatingUp to 50 inhabitants: in accordance with DIN 4261Part 2 (Small-scale purification systems; Systemswith aeration of waste water; Application, rating,design and inspection):

permissible surface load of BA ≥0.004 kg/(m² · d)

for waste water which has gone throughpreliminary sedimentation, requiredgrowth surface area ARBC ≥10 m² per inhabitant

Min. surface area 45 m².

3. Area of applicationThere are no upper or lower limits on the areas ofuse for rotating biological contactors. It shouldhowever be noted that, like the percolating filter,the applicable standard for the rating of larger units(> 50 inhabitants) is the ATV Process Sheet A 122.RBCs are also lower energy consumers than theactivation process but the associated constructioncosts are usually higher, assuming that the samepurification output is required.

Provided that the rating and design recommendationsof DIN 4261 Part 2 are observed, a 5-day BODof <40mg/l and a COD of <150 mg/l plus partial oxidation ofnitrogen can be expected.

The continuously rinsed-out secondary sludge andthat which is separated out further down the process(in a sedimentation tank / lamella separator)accumulates in the rotating biological contactor. It isnormally transported to the component where pre-treatment is taking place (multi-chamber settlementtank or slurry pit) and disposed of along with theprimary sludge.


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Biological contactor system

Biological contactors, which are still not very frequently used in Germany in small purification plants, areused to purify district waste water which has gone through preliminary sedimentation using a process ofaerobic biological purification. Biological contractors are 'packed-bed reactors' where fixed, permanentlyimmersed plastic elements serve as growth-encouraging surfaces for microorganisms. Free-movingsuspended microorganisms are also found in the water. Oxygen is supplied from below by diffused air.


Activation process using growth-encouraging units

The required reaction volumeshould be divided betweenseveral cascades through whichmaterial flows one after theother. The packed bed can besubsequently integrated intoexisting tanks, e.g. into thethird chamber of a multi-chamber slurry pit, and thisthen converted into an aerobicbiological purification system.

Biological contactor system withair diffusion

DN 150

Pump

DN 150

Biological contactor

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Biological contactor system

4. EffectivenessOnly isolated reports have been produced so farregarding the effectiveness of small biologicalcontactors. According to these, a 5-day BODof <40 mg/l , a COD of < 150 mg/l (in discharge) andextensive oxidation of nitrogen can be expected.

The secondary sludge which is continuouslydischarged with the waste water and separated fromthe waste water in subsequent separation stages(sedimentation tank / lamella separator) accumulatesin the biological contactor. It is normally transportedto the component where pre-treatment is taking place(multi-chamber settlement tank / slurry pit) anddisposed of together with the primary sludge.


Activation process using growth-encouraging units

2. RatingNo standards yet apply to the rating; for the timebeing, it would seem reasonable to observe therecommendations given for the rating of RBCs inDIN 4261 Part 2 (Small-scale purification systems;Systems with aeration of waste water; Application,rating, design and inspection):permissible surface load of BA ≤0.004 kg/(m² · d)

for waste water which has gone throughpreliminary sedimentation, requiredgrowth surface area ABC ≥10 m² per inhabitantMin. surface area 45 m².

From a technical point of view there are no upper orlower limits on the areas of use for biologicalcontactors. It is not yet possible, however, to saydefinitely whether biological contactors are a viablesolution in small-scale purification plants for up to 50inhabitants in comparison with other options.


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Activation tank

Process: continuously-fed activation tanks combined with a pre-treatment stage and downstreamsedimentation tank are the most frequently-used 'mechanical' components in both large and small sewagetreatment plants for traditional aerobic biological purification. In the traditional, generally familiar version,the waste water which has been through preliminary sedimentation is artificially aerated and powerfullycirculated in the activation tank. Free-moving, suspended microorganisms remove the biologically utilisablematter from the water and convert it into biomass. This is separated from the treated waste water in asedimentation tank further down the process chain. Most of the biomass is returned to the activation tankeither continuously or discontinuously (return sludge). Only the continuously forming growth ofmicroorganisms (excess sludge) is removed from circulation and transported to the upstream multi-chamberpit.


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For systems serving up to 50 inhabitants,the activation tank is usually aerated bythe diffusion of air. Numerousmanufacturers offer type-specific and (forsmaller systems) pre-fabricated activationsystems. These are activation tankshandling low demand levels 'withsimultaneous aerobic sludge stabilisation'.

Diagram showingtraditional activation process

continuously-fed

Tasks

Components

Selection

Return sludge

Waste water Pump

Discharge

Oxygen

ACTIVATION TANKSEDIMENTATION TANK

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Activation tankcontinuously-fed

2. Operation with preliminary sedimentationIn most of these systems a multi-chamber settlement tank isfitted upstream of the activation tank to pre-treat the wastewater. This is advantageous, particularly in terms ofseparating out problematic coarse materials, but it also hasdisadvantages:

The inevitable sewage decomposition in the settlementtank can be undesirable for the activation tank (activatedsludge, lack of growth particles for the nitrifiers).

The accumulating sludge is not stabilised and requiresseparate treatment.Preliminary sedimentation involves costswhich are in no

way commensurate with the actual contribution to thepurification process.

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It can therefore be beneficial to dispense with preliminary sedimentation and only to separate out coarsematerials contained in the untreated waste water in the interests of operational reliability. The following maybe considered:Screen baskets or sieve baskets which are suspended in the inflow zone of the activation tank,

Suspended scum baffles as described by Renner, H. to separate out floating coarse materials.

When comparing costs, bear in mind that if there is no preliminary sedimentation then a separate sludgestorage tank will need to be built.

Small activation system with preliminarysedimentation

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3. RatingUp to 50 inhabitants: in accordance with DIN4261 Part 2 (Small-scale purification systems;Systems with aeration of waste water;Application, rating, design and inspection):

permissible volume load LV≤0.2 kg/m³ · d;for waste water which has gone throughpreliminary sedimentation,

required activation tank volume VAT≥0.2 m³per inhabitant; min. volume 1 m³,

aeration unit should ensure O2 content of≥2 mg/l,

freeboard (distance between maximumwater level and upper edge of tank)≥0.3 m.

Activation tanks without preliminarysedimentation are not covered by DIN 4261;for such systems, ÖNORMen B 2502 Parts 1and 2 prescribe a minimum volume of 0.3 m³per inhabitant.

There are no upper or lower limits on the areas of use forthe activation process. The activation process can produceparticularly good purifying effects, but the energy requiredis generally greater than that for percolating filters andRBCs.


Activation tankcontinuously-fed

Using the specific tank volume stated above, a 5-day BODof<40 mg/l, COD of < 150 mg/l and NH4-N of <10 mg/l(discharge) can be expected.In an activation tank with preliminary sedimentation, theexcess sludge (secondary sludge) produced by microbialactivity and separated out in the sedimentation tankaccumulates and is usually transported to the componentwhere pre-treatment is taking place. In the case ofactivation tanks without preliminary sedimentation, theexcess sludge which accumulates in the activation tank is anaerobically stabilised mixture of primary and secondarysludge. The sludge requires no further treatment; it isstored in a separate tank until it can be disposed of.


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Activation tank

Function and design

Sequencing batch method

An activation tank using the sequencing batchmethod is a remake of an old idea. With thismethod, the phases

aeration phase (aeration switched on, interruptedby pauses if necessary; no outflow; water tablerises according to inflow to the system),settlement phase (aeration switched off; no

outflow; water table continues to rise; activatedsludge sediments) and

removal phase (aeration switched off; clarifiedwater is discharged or pumped off)

alternate periodically once or several times a day.The timing of the three phases can be controlled interms of volume or time or a combination of thetwo.

The advantage of the sequencing batch method isthat there is no need for a sedimentation tank. Onthe other hand, the release of treated water can beintermittent in sensitive discharge systems. It wouldnot be correct to describe any and every kind ofsequencing batch process as a 'single-tank system'

because although this process dispenses with thesedimentation tank, a sludge storage tank must stillbe present.

DIN 4261, Part 2 does not cover the activation processusing the sequencing batch method. In principle, theremarks made about continuously-fed activation tanksapply with regard to rating, area of application andresidual materials.

Aeration

Fill

Sedimentation

Empty

Phase 1

Phase 2

Phase 3

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Sequencing Batch Reactor (SBR)

1. Function and designSequencing batch reactors (SBRs) are purification systems which use the activated sludge process whichoperates using batches. Here, processes which in the biological stage of a conventional community plantwould take place in separate locations all function in the same tank in sequential steps. In certain size rangesthe price of a container does not increase in a direct relation to the tank volume, and it is often cheaper toset up two identical components than to build an activation tank and sedimentation tank. SBRs are thereforeparticularly suitable for smaller community sizes as of about 8 inhabitants because in these scenarios theycan be built more cheaply than continuous systems. However, a control system is always required.

The advantages compared withconventional small activation plantsinclude:can be controlled

regardless of load

functions with more energy savings

usually produce betterdischarge values.

Reactors

Discharge

Pre-thickener

Inflow

Screen Buffer tank

Fan Agitator Aeration

SBR system with tworeactors by MALLBETON

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Sequencing Batch Reactor (SBR)

... Function and designA small SBR system generally consists of a two-chamber intake tank. This is where mechanical pre-treatment take place, i.e. the settling of large, heavy solids, and where the volume and concentration ofwaste water is buffered. From here, the waste water passes into the actual reactor tank at predefinedcontrol intervals.

There are various control processes, which may take thefollowing form: During filling and for a defined periodthereafter, the sludge is artificially aerated. After aerationthe sludge settles to the bottom of the reactor tank. Fromhere, part of the activated sludge (excess sludge) ispumped into the sludge tank before the clarified waterflows off. The reactor, which will still be around 30 - 70%full, is then aerated at defined intervals until fresh wastewater is again pumped in under aeration. If there is noaeration, an agitator is activated. By selecting theappropriate agitation interval, denitrification can beperformed in systems with nitrification. To do this,agitation should be performed during and after the fillingprocess so that the nitrate formed during the previouscycle can be processed. Phases of a cycle in

sequential biological purification

2. Treatment phases

Aeration

Fill

Agitation

Aeration

Fill

AgitationFill

Agitation

Aeration

Sedimentation

Empty

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Enlarged activation tank

1. Function and designOne special form of the sequencing batch process is the 'authentic' single-tank small activation system, firstbuilt in the Austrian province of Styria, which does in fact consist of only one tank (the activation tank) anddoes not feature preliminary sedimentation, a sedimentation tank or a sludge storage tank. One special formof the sequencing batch process is the 'authentic' single-tank small activation system, first built in theAustrian province of Styria, which does in fact consist of only one tank (the activation tank) and does notfeature preliminary sedimentation, a sedimentation tank or a sludgestorage tank.

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This is made possible by incorporating anenlarged activation tank with a specificvolume of 1.5 - 2 m 3 per inhabitant. With acontainer of this size, the amount of sludge inthe activation tank increases so gradually thatit is only necessary to remove excess sludgeevery one to one and a half years, such thatthe activation tank also takes over the functionof the sludge storage tank. In this system,treated water is pumped off once a day,overnight, by an air-lift pump (Renner, H.).This method makes the facility completelyinsensitive to fits and starts in the inflow;however, the disadvantage is the intermittentdischarge of the treated waste water. Single-tank small activation system

Tasks

Components

Selection

Compressed air

Scum baffle

Floating materials

Activation tank

Pneumatic elevator

Aeration

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Enlarged activation tank

2. RatingNo standards apply to the rating. The specificvolume of 1.5 - 2 m³ per inhabitant stems from anassumed volume of accumulated sludge of 35g/(inhabitant · d) and the requirement that thesludge accumulated in one year can be stored in theactivation tank between two removals of excesssludge.

3. Area of applicationFor small communities these systems arestraightforward and cheap to build, but becomemore expensive for larger community sizes of > 20inhabitants.

Nitrification possible: in the discharge, values below a5-day BODof <10 mg/l, COD of <70 mg/l and NH4 -Nof <5 mg/l can be achieved on a regular basis. Fearsthat an activation tank with a specific volume of 1.5 -2 m³ per inhabitant would be constantly underloadedand therefore provide poor treatment quality have, inpractice, been proved unfounded; in fact, theopposite seems to be the case.

When required (when the system is working at fullcapacity, once a year) the tank is completely emptiedso that only a small amount of seeding sludge is left.The sludge is aerobically stabilised but may containnon-decayed, visible coarse material from hygieneproducts if the residents who depend on the facilitydo not exercise sufficient discipline.

4. Effectiveness

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Components

Selection

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Sedimentation tank

S ch la mm

Abwasser

Sedimentation tank (hopper tank of a small-scale purificationplant)

1. Function and designThe sedimentation tank is the component most frequently used for separating out biomass from treatedwaste water. In small-scale purification plants, it normally takes the form of a hopper tank. In thesesystems, the sedimentation tank is frequently combined with the biological reactor to form one singlestructure. It is unnecessary to go into detail about function and design principles.

2. RatingUp to 50 inhabitants: in accordance with DIN 4261Part 2 (Small-scale purification systems; Systemswith aeration of waste water; Application, rating,design and inspection:

Throughflow time tST ≥3.5 h,

Surface area AST ≥0.7 m²,Water depth hST ≥1 m,

permissible surface loading depends onthe type of upstream biological reactor,

for activation tanks, qA ≤0.3 m/h;for packed-bed reactors (percolating filters,RBCs, biological contactors) qA ≤0.4 m/h.

Tasks

Components

Selection

Sludge

Waste water

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Sedimentation tank

3. Area of applicationThere are no upper or lower limits on the areas ofuse for sedimentation tanks.

4. EffectivenessThe Institut für Bautechnik (Institute forConstruction Technology) in Berlin demands a limitvalue of 0.3 ml/l of solid materials in the dischargefrom the sedimentation tank before a quality markcan be awarded. This figure must be met inqualified random sampling and according to the '4out of 5' rule. An indirect regulation also applies tothe performance (separating effect of the biomass)of the sedimentation tank, because a 5-day BODof<40 mg/l and a COD of <150 mg/l may not beexceeded in the discharge. The limit values givenhere can be met under normal operating conditions;major hydraulic overload may however cause thesevalues to be exceeded as a result of the discharge ofsludge.

In the sedimentation tank itself no residual material isproduced; only the biomass from the upstreambiological process is separated from the purifiedwater. If an upstream packed-bed reactor is present,the deposited biomass is usually transported as awhole to the component providing preliminarytreatment. If an activation tank is being used, most ofthe sludge is routed back to the activation tank(return sludge) and only the continuous build-up ofsludge (excess sludge) is removed from circulationusing various different methods and then transportedto the preliminary treatment component or a sludgestorage tank as the case may be.


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Lamella separator

1. Function, design and applicationLike the sedimentation tank, the lamella separator (see Fig. 6.14) serves to separate out the biomass fromtreated waste water. It consists of a bundle of plates or pipes with a parallel incline through which thesludge-containing waste water flows either upwards (counterflow) or downwards (co-current flow). Solidparticles sediment and slide downwards to the plates or the bottom of the pipes. Thus, a lamella separatorcan be likened to several very flat settlement tanks arranged on top of one another with inclined bases. Theadvantage of the lamella separator is that, compared with conventional settlement tanks, it takes up lessroom.Lamella separators are normally used with systems serving > 50 inhabitants, particularly when it isimportant that the facility take up as little space as possible (compact and container treatment plants). Interms of effectiveness and accumulation of residual material, please refer to the remarks given for thesedimentation tank.

Inflow

Water

Sludge

Lamella zone

Extraction of purified water

Sludgecollection funnels

Lamella separator

2. RatingNo standards apply to systems serving up to 50inhabitants; indicators can be found in ProcessSheet A 122 (Principles for rating, construction andoperation of small purification systems with aerobicbiological purification processes for community sizesof between 50 and 500 inhabitants) from theAssociation for Waste Water Treatment (ATV):

permissible surface load qA≤0.4 - 0.6 m/hThe surface is considered to be the total of the

horizontally projecting lamella surfaces.

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Sludge storage tank

1. Function and designSince organisation of the removal of sludge cannotalways be guaranteed at short notice at any time ofthe year, small-scale purification plants need asludge storage capacity of several months. If amulti-chamber slurry pit is being used forpreliminary treatment, there should be sufficientstorage space; if using a multi-chamber settlementtank, extra volume will need to be provided.Activation systems without preliminary purificationrequire their own sludge storage tank, normally asimple tank with a base which inclines towards asunken point.

In accordance with DIN 4261, Part 1 (Small-scalepurification systems; Systems without aeration ofwaste water; Application, rating and design):

If a plant has a multi-chamber slurry pit forpreliminary purification, this can be used to performthe function of the sludge storage tank; noadditional storage space will be required.

If a plant is using a multi-chamber settlement tankfor preliminary purification, this can also be used tostore the sludge if its capacity is enlarged to 0.05 m³per inhabitant for packed-bed reactors (percolatingfilters, RBCs, biological contactors) or by 0.125 m³per inhabitant for activation systems (sludge storagesupplement).Activation systems without preliminary treatment

require a separate sludge storage tank. The volumeshould amount to 0.25 m³ per inhabitant. The issuesof application, effectiveness and residual materialdo not apply to the sludge storage tank.

2. Rating

Tasks

Components

Selection

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Effluent sludge composting bed

1. Function and designConventional sludge drying beds in their traditionalform, with a concrete surround and base drainage,have become less important in small-scale treatmentplants since the introduction of vacuum liquidmanure tanks and gully emptiers for removing anddisposing of liquid sludge.A more recent development is the effluent sludgecomposing bed, which dehydrate and 'refine'anaerobically or aerobically stabilised sludge(Reinhofer, M). These are simple basins of soil whichcan be sealed with a film whose base is laid with agravel drainage layer covered with humus. Similar toplant beds, reeds and other aquatic plants areplanted in the humus and the basin is then fedcontinuously with liquid sludge. Water flowing offthrough the drainage system is routed into theinflow to the purification plant.

The plants help the sludge water to evaporate andcontribute to converting the sludge, over the courseof a few years, into an earthlike product comparableto compost with a water content of approximately60%. The original volume of sludge is considerablyreduced.

Rating: no standards apply to the rating ofcomposting beds. A surface area ofAKVB≥0.25 – 0.5 m³ per inhabitant is recommended.

Composting beds have already been built forcommunities of several hundred inhabitants.

The remaining material, a volume of around15 - 20 l/(inhabitant · a)is similar to humus.

2. Rating and application

Tasks

Components

Selection

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Polishing pond

1. Function and designPolishing ponds provide advanced purification ofbiologically treated sewage before it is allowed toflow into sensitive discharge systems. Theyconstitute a simple process for enhancing outputwater in terms of suspended solids, organic load (5-day BOD, COD), nitrogen, phosphorus and bacteria.The growth of algae during warm months canhowever result in the water discharged from thepolishing pond demonstrating poorer results in the5-day BOD and COD than when it flowed in.

As far as small-scale treatment systems are concerned,polishing ponds have frequently been built forindividual houses downstream of small plantpurification systems. The main focus is often thepond's function as a storage pond to allow purifiedwaste water to be used for watering the garden.

It is not possible to make any general statementsbecause the effectiveness of a polishing ponddepends on the properties of the inflow, thethroughflow time and the season. However, so longas there is no secondary contamination caused byalgae and assuming that all significant parameters arepresent, a significant fall to less than half of theoriginal concentration can normally be expected.

The level of accumulation of sludge is minimal; theheight of sludge deposits grows by only a fewcentimetres a year. The sludge which needs to becleared out at intervals of several years is well-stabilised.


2. RatingNo standards apply to systems serving up to 50inhabitants; indicators can be found in ProcessSheet A 201 (Principles for rating, construction andoperation of waste water ponds etc.) from theAssociation for Waste Water Treatment (ATV):

Throughflow time 1 - 5 days; water depth 1 - 2 m,

Impact surface in inflow area, deflective surfaces ifnecessary; the sludge removal method to be usedmust be considered at the early planning stage.

3. Application

Tasks

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Selection

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Sand filter for post-treatment

1. Function, design and applicationSand filters as a post-treatment stage after 'mechanical' small-scale purification systems are a very effectiveway of enhancing output water before it flows into sensitive discharge systems. Their effect is to hold backall suspended solids, to reduce the 5-day BOD, COD and ammonium-nitrogen levels and, if a ferrous basematerial is used, to eliminate phosphorus to a certain degree. They also serve to moderate the outflow, whichis particularly desirable in sequencing batch activation systems, and provide assurance that brief malfunctionsin the technical facilities will not have an immediate impact on the receiving water. Process: post-treatmentsand filters are similar in structure to filter chambers and plant beds and, provided they are built with anopen design, are usually planted with reeds etc. They may also create themselves a natural cover over time asseeds of various types are spread.

2. RatingRating: no standards apply to downstream sandfilters. Data Sheet No. 3 (Waste water disposal inexternal applications (small-scale purificationsystems)) of the North Rhine-Westphaliaenvironmental agency recommends

a surface area ASF ≥0.2 m² per inhabitantto ensure a long service life, it is better to opt for a

surface area ASF ≥0.5 m² per inhabitant

or, for domestic purification systems, even up to1 m² per inhabitant

.

Various sized post-treatment sand filters havealready been built for communities of severalhundred inhabitants.

With the exception of phosphorus, the dischargevalues of a sand filter are close to those of naturaldischarge systems.With a normally functioning biological primarystage, no residual material requiring disposalaccumulates in a large-sized post-treatment stage.

3. Application, performance

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Selection

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Underground trickle system

1. Function and designThe underground trickle system is not a component of the sewage treatment plant in itself; rather, it servesto route pre-treated waste water into the subsoil following a line or plane. Microorganisms in the naturalbase do however cause biological purification to take place beneath the trickling system. A permeable base isrequired for application of the system.

1:50030m

0,15

LüftungVerte ilerk ammerbei TonrohrenAbdec kun g derStöße

Über deckung

Ausgle ichsschicht

2. Rating

Length of trench l, depending onbase layer: if gravel and sand, ≥10 mper inhabitant,if loamy sand, ≥15 m per inhabitant,if sandy loam, ≥20 m per inhabitant,

Distribution drainage pipes NW 100,gradient 1:500; aeration requiredMinimum distance from ground

water 0.6 m.

In accordance with DIN 4261, Part 1 (Small-scale purification systems; Systems without aeration of wastewater; Application, rating and design):

Underground trickle systemin accordance with DIN 4261 Part 1

Tasks

Components

Selection

Distribution chamberCovering for joints if

using clay pipes Ventilation

Cover

Compensation layer

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Untergrundverrieselung

0 ,50m

100

0,10m

0,60m

0,60m

gegeb ene nfal lsZwisch ens chicht

Gr obsand o derFeink ies

Ve rfül lun g

4. PerformanceThe trickle system itself does not make asignificant contribution to the purificationof waste water. No general statements canbe made as to the performance of the baselayer because the degree of decompositionof the water contents depends on the soilcharacteristics and the vertical distanceunderground through which the watercan seep.

3. ApplicationUnderground trickle systems are preferred for smallplants. In terms of observation, one disadvantage isthat the processes of decomposition andpurification taking place underground cannot bemonitored.

3. ApplicationNo residual material accumulates in an undergroundtrickle system. In a natural base layer, if the systemhas been designed properly a balance will emerge inthe long term between the creation of biomass andthe simultaneous consumption of biomass by higherorganisms.

Underground trickle systemin accordance with DIN 4261 Part 1

Tasks

Components

Selection

Filler

Intermediate layer(if required)

Coarse sand or fine gravel

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Seepage basin

1. Function and designA seepage basin is used to convey treated wastewater into the subsoil over a large surface areaabove ground. It is a flat soil basin with a permeablebase, planted with vegetation, which is fedintermittently with treated waste water.

It helps to purify the water by holding back allsuspended solids, reducing the 5-day BOD, COD andammonium-nitrogen levels and, if a ferrous basematerial is used, to eliminate some phosphorus.However, these effects cannot usually bedemonstrated since it is impossible to takerepresentative water samples from the subsoil.

If designed properly no residual material requiringdisposal should accumulate, in the natural base layera balance will emerge between the existing/filteredbiomass and the simultaneous consumption ofbiomass by higher organisms.


2. RatingNo standards apply to seepage basins. Data SheetNo. 3 (Waste water disposal in external applications(small-scale purification systems)) of the NorthRhine-Westphalia environmental agencyrecommends a surface area ASB = 5 -10 m² perinhabitant depending on the permeability of thebase layer.

Cross-section of seepage basin

Tasks

Components

Selection

Distribution shaft

Inflow25 cm Covering layer of topsoil Distance to

ground water ≥1,50 m

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Soak-away

LW 0,6 0

0 , 15

0 , 50

1 ,00

An ga b e n i n M e te r

0, 0 5- 0, 1 0Ve r fü l l u n g

Sa nd

F ei nk ie s

ge w a c h s e ne rBo de n

0 ,2 0

Si c k e rf l ä c h eh öc hs t erG ru n d w as s e rs t an d

Pr a l l p l a t te

1 ,5 0

1. Function and designIf purified waste water cannot be conveyed into a discharge system or underground using laminar trickling /seepage or a system located near the surface, a soak-away can be an essential part of a sewage treatmentsystem. It is used to route treated waste water underground and not to purify it.The sand layer underneath the deflector shouldnot therefore be considered as a biologicalreactor but simply as a mechanism to preventthe ingress of suspended solids into the baselayer and to ensure a long service life for thesoak-away.

Soak-away according toDIN 4261 Part 1

2. RatingIn accordance with DIN 4261, Part 1 (Small-scale purification systems; Systems withoutaeration of waste water; Application, ratingand design):

The required usable seepage surface areadepends on the absorption capacity of theground and the height of the water columnin the soak-away; guide value ASA ≥1 m² perinhabitant.

Diameter of shaft ≥1 m;minimum distance from ground water 1.5m.

Tasks

Components

Selection

Filler

Deflector

Sand

Fine gravel

Planted ground

Seepage areaMax. level of ground water

Figures in metres

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Type BType A Type B Type D

Selecting a small-scale purificationplant

Tasks

Components

Selection

As mentioned previously, one single component does not represent a water purification system in itself; itonly becomes one when combined with other appropriate components. The combination possibilities aremanifold, as is the number of possible types of sewage treatment systems.

1. Possible combinations

Infiltration trench Sedimentfilter

Multi-chambersettlement tank

Plant bed



Plant bed

Filter sack

Semi-natural processesIn the following summary of the differenttypes, purification processes are divided simplyinto the two largest groups, 'semi-natural' and'artificial' processes. This summary does notcover all the conceivable combinations, onlythose which have already been used on manyoccasions.All system types can be equipped with extracomponents such as sludge composting beds,polishing ponds or post-treatment sand filters.Underground trickle systems, seepage basins orsoak-aways can be necessary when there is noreceiving water (discharge system) available toreceive the purified water.

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Type F Type G

Selecting a small-scale purificationplant

Type E


Percolating filter

Sedimentation tank


Sedimentation tank

Mechanical processes

Rotatingbiological contactor

... Possible combinations

Type H Type I Type J Type K


Rotatingbiological contactor

Lamella separator

Sludge storage tank

Activation tank

Sedimentation tank


Sludge storage tank

Activation tank

Sedimentation tank

Sludge storage tank

Activation tank,sequencing

batch method

Sludge storage tank Sludge storage tank

Enlargedactivation tank,

sequencingbatch method

Tasks

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Selection

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Selection criteria

1. SizeIn principle, all the waste water treatment planttypes listed here can be used for all community sizesof up to 50 inhabitants. Types A, D and K areparticularly suitable for domestic systems serving upto around 10 residents.

2. Period of usageFor obvious reasons, if a small purification plant isonly to be used temporarily the 'lost' investmentcosts should be kept as low as possible The idealsolution for short-term usage is transportable unitsin containers. This kind of solution is obviously onlyavailable for 'mechanical' purification methods. Theavailable range of transportable treatment systemsfor communities of up to 50 residents is howeversomewhat meagre. For small domestic purificationsystems, the costs of constructing a multi-chamberslurry pit + plant bed combination (Type C) arerelatively low. If these components are also used asrainwater storage tanks / wetland areas etc. afterbeing connected to the sewage system, there willlogically be no 'lost' investment costs.

The local situation regarding building costs and thetreatment method to be used will influence the costsof investment. The trend is that 'semi-natural'processes are more cost-effective for small communitysizes (domestic systems), while 'mechanical' solutionsare a better cost option for larger community sizes.However, the main reason for the difference in costs isthe scope of services and quality of construction. Theservices supplied range from 'ex-works concrete parts'to turnkey facilities including planning, approval,connection to the national grid etc. When designing asystem, the possible options range from qualityconcrete and stainless steel to coated iron andrecycled plastic. This is not to say that only anexpensive sewage treatment system will work; thepoint is simply that differing prices can represent verydifferent services.

3. Construction and operating costs

Tasks

Components

Selection

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Selection criteria

... Construction and operating costsThe details given about operating costs are notbased on uniform principles. It has been shown thatcost comparisons for operating costs sometimesonly take electricity costs into account,

and companies' documentation often gives noinformation about the time required to manage asystem correctly

sewage treatment operators' hourly rates forongoing system management vary enormouslyand

the costs of regular maintenance by themanufacturing company and the officiallyrequired discharge inspections vary greatly.

For the reasons listed, it is impossible to make anyreasonable absolute statements about the costs ofconstructing and operating small-scale waterpurification systems. We can say with certainty,however, that the smaller the system, the higherthe specific construction and operating costs.

The minimum requirements which apply in Germanycan be observed when using all of the system typeslisted above. For purification effectivenessrequirements which exceed these, large-sizedactivation systems are recommended, supplementedby downstream sand filters. If managed properly, thissystem combination achieves a level of purificationwhich is is no way inferior to a large-scale sewagetreatment plant.

4. Requirements

Tasks

Components

Selection

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Selection criteria

5. Space requirements'Semi-natural' purification methods, particularly theopen beds of plant purification systems, require alot of space which is not always available. Noteveryone is happy to have a wetland area filled withwaste water next to their house if it is not totallyodour-free. No such problems are associated withan underground 'mechanical' system.

6. Disposal of effluent sludgeRegardless of the type of treatment system used,sludge will accumulate which has to be disposed of.If it is possible to dispose of effluent sludge foragricultural purposes in the vicinity of the plant, it isbest to choose combined processes which yieldstabilised sludge, e.g. a combination of a multi-chamber slurry pit and a 'natural' biologicalpurification process or an activation tank withoutpreliminary treatment.

Tasks

Components

Selection

Contents ContinueEnd

Decentralised Waste Water Treatment (End)Tasks, function and selection of small-scale purification plants

Sub-tasks

Overview of components

Components of purification process

Components of post-treatment system

Selecting a small-scale purification plant

Components of preliminary treatment systemINNOCHEM Wasser GmbHWesterburger Weg 18D - 26203 WardenburgTel: +49 (0)4407 716 32-0Fax: +49 (0)4407 716 321E-Mail: [email protected]: www.innochem-online.de

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