Pannon Egyetem

Etvs Jzsef Fiskola

Kaposvri Egyetem

Edutus Nonprofit Zrt.

IBS Development

Nonprofit Kft.

TMOP-4.1.1.C-

12/1/KONV-2012-0015

Felsoktatsi

egyttmkds a

vzgyi gazatrt

Contactor technologies in

wastewater treatment

2

Rotating biological contactors (RBCs)

The rotating biological contactors are actually a special application of the combination of

biofilm and activated sludge technologies. The secondary wastewater treatment spread

over the past decadesand it is still used especially for smaller wastewater plants. Such

treatment was used to remove only organic matter or only nitrification or remove both

organic matter and nitrification by combined. As secondary biological wastewater

treatment, it could be used in cases, where 30 mg BOD5/liter and the same amount of

suspended solids were the host limit value.

It could be used also for intensified secondary treatment, but in that case the filtration of

the treated water must be ensured to achieve BOD5 concentration below 10 mg/liter

(limits set for wastewater treatment in the United States). In these cases, the treatment

could achieve ammonium concentration below 1 mg/litre in the treated water. At the same

time, denitrification was also possible on the RBC lines as a result of their easy

conversion. It was been widely used for treating industrial wastewaters.

The rotaring biological contactors can be considered as combined solutions for biofilm

and activated sludge systems. In this case, the biofilm can be build up on some sort of a

panel, moving cyclically between the water and fluid phase. This is achived by fixing the

thin panels or discs on a suitable shaft and the shaft is lowered in the pool until the water

level. This way, nearly 40% of the discs will be underwater and its 60% will be in the air

as a result of their rotation. Apart from the disc solution, later variants were also

developed where a cylindrical, perforated supporting structure or wire mesh support unit

was fixed around the horizontal rotating shaft and some type of plastic charge was placed

in, usually slotted plastic pipes. The shaft rotates slowly, meaning that the rotational speed

of the entire disc or block immersed in the water is around 1-1.6 revolutions per minute.

Overall, the fluid flow is perpendicular to the axis of the shaft. Therefore the water is

flowing through the rotating discs and supplies the adhered biofilm with nutrients. The

latter thus is suitable for wetting, therefore the biofilm may only dry out if the the rotation

stops.

The rotating discs ensure the organic nutrient, ammonium and oxygen supply of the

cyclically flooded film. When lifted above the water surface, the discs release part of the

fluid, thereby continuously wetting and aerating of the biofilm occurs as a result of the

rotate. The continuously generated biomass adhering to the surfaces gets washed off as a

result of this alternating movement of wetting, trickling and aeration, once its grown too

thick. As a result of the relatively slow rotation, the biofilm may also grow sufficiently

thick, but the concentration of the fluid phase underneath the discs remains constant over

time. When linked in series, they can therefore nitrify well.

Figure 6.1 shows the schematic and actual construction of the rotating biological

contactor as well as its installation in the surrounding environment. They have been built

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in large numbers in Switzerland (approximately 1500 plants) to implement regional or

local wastewater treatment.

Figure 6.1: Typical construction and installation of an RBC in its environment.

4

A later variant of rotating biological contactors is the so called submerged contactors or

SBC solutions. In this configuration, about 70-90% of the rotating discs are submerged

in water. An advantage of this solution is that the weight of the discs on the shaft is far

less, exerting smaller load on it and its bearings. Such solutions have been applied in

many cases to activated sludge tanks already to modernise and expand them. Of course,

these hybrid solutions required an increase of the biofilm's and thereby the entire system's

oxygen supply, which was provided by inlet air from below. The aerating element or

diffuser was earlier a simple, slit tube, later it was developed into an advanced air

dispersion system with a perforated membrane. It was also built in a variant, where little

"bags" were fitted onto the rotating discs with their opening downwards and the air

accumulated therein, therefore rotated the disc on its own. In spite of these advantages,

the submerged biological contactors have only made it to relatively limited practical use.

It was necessary to create a uniform design for the rotaring biological contactors;

standardised disc diameters and rotating shaft lengths were eventually defined as 3.5

metres and 7.5 metres respectively. In case of such standard equipments has become

general for the biofilm surfaceformed on a single shaft to be 9300 m2. During the

development naturally brought units with larger surfaces. The discs were packed closed

together and the surface per shaft increased to 13900 m2. Units of lower charge density

or disc density, which were having smaller surfaces per unit of volume, were applied in

treatment plants where the organic matter load was relatively higher and pursuant to

which more biofilm thickness was formed in the system. In this case, the appropriate disc

gap - the distance between neighbouring discs - had to be ensured to allow the passage of

air and thereby the biofilm's oxygen supply.

Rotaring biological contactors of higher density or smaller disc gap were primarily

applied in nitrifying (post-nitrifying) contactors, where the organic matter load was

considerably lower and the contactor's role in the wastewater treatment was rather a

nitrifying post treatment. It has also become general in practice, that such biological

contactors should be operated in series, because at the end of the series was more cleaner

water could be available In these cases, often the disc units having smaller specific

surfaces were placed at the front of the series, while those with smaller disc gaps at the

end. Naturally, in newer contactors the rotaring discs have been tried to replace with

plastic charges in appropriate cylindrical supporting structures, i.e. drums. The charge in

this case was made of some type of perforated tubes, pressed tube pieces (fine cut tube-

like substrate) on which the biofilm could grow well, while the water was also able to

penetrate it sufficiently. For such biofilm substrates, the specific surface could naturally

fluctuate highly and the oxygen intake could also be a problem. This is why these had to

be occasionally aerated from below.

Rotaring biological contactors were usually deployed in covered from. There were several

reasons for this. It prevented the overgrowth of algae on the surfaces of the discs,

respectively the biofilm forming on it. It considerably reduced the system's heat loss in

areas of colder climate. Besides, it offered protection for polyethylene charges against the

5

harmful effects of UV radiation. Fiberglass polyester proved very useful for building such

protective roofs. Despite that traditional construction techniques were perfectly suitable

for building wastewater treatment plants, therefore it was suitable buldings with

traditional roof also.

In practice, RBC units were built in multiple steps and they were sometimes even

configured side by side turning them with the same driving mechanism, with this reducing

the specific investment costs. The latter construction was possible even within the same

tank where the RBC units were separated only by submersible partition walls from each

other. The current number of RBC units depended on the level of contamination of the

raw wastewater and the water quality that had to be reached. One or tow stages may only

ensure a rough, secondary biological treatment, while six or more RBCs linked in series

can remove organic matter and are capable of full nitrification. In this configuration the

drive shafts follow one another and placed perpendicularly to the flow and the

aforementioned partitioning boards ensure the appropriate storage or reactor space. In

cases where relatively small fluid flows had to be treated, the RBC treatment line shafts

can be placed with parallel to the direction of flow.

The shafts are generally mechanically powered. However, there are air powered systems,

where the intake air at the bottom ensure the rotation using appropriate collection cups.

The rpm of mechanically powered shafts is generally constant, although the variable

rotational speed disc contactorshave also been built. The speed of rotation is relatively

low to allow the fluid trickle out of the biofilm properly. The separation of the biofilm

from the surface of the disc is ensured by this very trickling of the fluid.

The advantage of the rotaring biological contactors is their extreme simplicity, minimal

operational and controlling requirement and the resulting relatively low costs of operation

and investment. A specific advantage is that the biofilm is able to regenerate very quickly

from impulsive loads. However, a number of RBC malfunctions have emerged as a result

of the following:

- failure of supporting shaft or discs,

- malfunction of the supporting structure of the charge,

- lower treatment capacity than what the treatment plant has been designed for,

- occasional overgrowth of higher organisms in the biofilm,

- possible insufficient biomass growth on the surface of the discs,

- inadequate rotation speed for the air-driven discs.

The widespread of the rotaring biological contactors may also been inhibited by the highly

negative attitude of some state institutions and regulatory authorities in the United States

to the construction of such systems, mainly due to hazards caused by the malfunctions

listed above. The stumbling block was the spread of RBCs, that the development of the

filters suitable to eliminate the residual sludge from the treated water followed the

development of RBCs with some delay. Usually, some fine filter was used for this, but

these broke down very fast.

6

Key aspects for planning RBC

Suitable substrate surface for the biofilm

Tha appropriate substarte surface of biofilm is indispensable for proper treatment. Proper

oxygen supply of the biofilm corresponding to the speed of nutrient removal is also

necessary, otherwise neither oxidation, nor assimilation will function properly in the

biofilm. The appropriate size and operation of the biofilm surface primarily the control

of the biofilm's thickness and may ensure the absence of parasitic microorganisms in the

biofilm. These are flies, their larvae and the snails. The formation of excessively thick

biofilm on the discs may also lead to the damage of the supporting shaft.

Excessive fluid and organic matter loads will naturally leave a higher contaminant

concentration in the fluid phase as well. In this case a single treatment phase must be

multiplied by linking the phases in series. If only one RBC functions as a treatment unit,

then effective preliminary treatment is indispensable in order to achieve the desired water

quality. If the wastewater enters the RBC at a lower organic matter concentration, it is

necessary that the speed of organic matter removal, namely the speed of nutrient

utilizatiion be reduced as well. Biological transformation will then take place according

to the first-order reaction dynamics, at a lower velocity. Therefore an RBC line developed

with several units is far more economical. In that configuration, the first stages function

as high load units, removing organic matter at high speeds, but only partial removed,

while in the subsequent stages, as the organic matter load is gradually reduced, even

nitrification is possible. However, care must be taken to prevent the first stage from

receiving such excessive organic matter load, which would cause lack of oxygen and

consequently the anaerobic fermentation of the biofilm layers and odour formation.

Significant overload increasing the weight of the biofilm may even lead to the fracture of

the shaft.

In the early stage of the development, the capacity calculations for rotaring biological

contactors assumed better performance as proven later As a consequence of this the

authorities were skeptical concerning the construction of such treatment plants in the

United States. In the early stages, they did not consider that rotaring biological contactors

necessary treatment of the sludge at the end of the series of the biological tanks must be

provided. If filters were applied, arrangements had to be made for their appropriate

backwashing. From here the side flows, usually recirculated to the start of the treatment

plant resulted in significant increase of load. Therefore, upon designing RBC units, the

engineers must perform careful material balance calculations for the entire system in

order not to underdimension the first stages, which would lead to the aforementioned

problem.

pH and nutrient balance

7

As in any biological treatment facility, setting the appropriate pH and controlling it is also

important for RBCs (Metcalf and Eddy, 1979). As a general recommendation, pH should

be kept between 6.5-7. Wherein nitrification is necessary in the treatment plant, keeping

a higher pH is necessary as the nitrification will slow down at a pH of 7 (Brenner et al.,

1984). pH between 7.5-8.5 were found more suitable, especially in case of RBCs

containing a higher volume of suspended sludge (Savier et al., 1973). In well stabilised

RBC units, pH remains within a relatively narrow range, especially if no nitrification

takes place in there. However, it may occur in case of softer waters that pH must be

readjusted by adding chemicals as the nitrification starts.

As mentioned earlier, nitrification generats two moles of acid for each mole of ammonium

oxidised to nitrite. In lack of sufficient buffer capacity of the water, this causes significant

acidification and pH reduction. This must be reset by adding some basic chemical.

Sodium hydroxide is generally used for this. As a general experience, alkalinity in the

wet phase is advised to be kept between 50-100 mg/l (0.8-1.6 mmol/l) to ensure

continuous nitrification. This is capable of ensuring a pH of about 7 in the given system.

Otherwise, in these treatment plants, as in the case of any biological treatment, setting the

appropriate organic carbon: nitrogen: phosphorus ratio is indispensable. The generally

recommended BOD: N: P ratio value for this is 100:5:1.

Oxygen intake or aeration

In rotaring biological contactors, oxygen intake should be such that create a clearly oxic

environment in their biolfilms. The organic matter load is in proportion to the oxygen

demand. Its absence leads to the aforementioned operational problems. Oxygen demand

has been observed to be generally around 6.8-7.3 gram oxygen/m2d. It has also been

observed when RBCs aerated from below, the oxygen utilisation from the air taken in

turned out to be only around 2-2.5% (Chou 1978). If we assume that the oxygen utilisation

of such an air driven rotaring biological contactor is only 2.5%, then for the biofilm of

9300 m2 surface area where the air supply can be characterized by 7 m3/min, the resulting

oxygen intake will be approximately 8.3 g O2/m2d. The aforementioned oxygen intake

speeds, i.e. those around 6.8-8.3 g O2/m2d correspond well to the oxygen surplus demand

of 1.5 g/m2 required for the maximum speed of ammonia-N removal. The latter value has

been naturally calculated without denitrification, namely with a specific value of 4.6 kg

O2/kg ammonium-N.

The initial phase of the development of RBCs, oxygen intake was necessarily fine tuned

using small pilot equipment. In case of these, due to the smaller diameter, the smaller

peripheral velocity was a disturbing factor also. Perhaps it is a result of this that the

rotaring biological contactors built in the first period were operated with insufficient

aeration and thereby with insufficient wastewater treatment capability, which may have

led to the aforementioned skeptical attitude.

8

Concerning the biological load of rotaring contactors, daily loads of about 20 g BOD5/m2

of substrate surface area were recommended at the beginning. As this value had been

precisely clarified for dissolved BOD5, this also means that the double of this was

allowable for total BOI5 values for municipal wastewaters. The ratio of total and

dissolved BOD is usually around 1:1. It was soon discovered, however that this load is

excessive and as a consequence such RBCs became oxigenlimited for biological

oxidation. Today, the former load value of 20 g/m2d dissolved BOD5 is cautiously taken

as 12-15 g of dissolved BOD5/m2d only for RBCs, where no additional aeration takes

place in the fluid phase. Nowadays, a number of companies consider this the safe range

for designing rotaring biological contactors, considering the periodical peak loads as well

(McCann and Sullivan, 1980; Weston, Inc. 1985; Envirex, 1989).

At present, the further improve of aeration the permissible loads for RBCs are rather taken

as 30 g total BOD5/m2d. This does not lead to lack of oxygen, and neither will be

interfering macro fauna develop inside the biofilm.

Fluid and organic matter load

It is generally accepted that biofilm systems, especially RBCs remain far more stable to

load variations than activated sludge systems. This does not result from fluid flow or

nutrient convection, but perhaps the biofilm is able to better fix and store the nutrients

and such systems only have provide far lower levels of treatment in terms of their effluent

water than the activated sludge systems. Regardless of this, it is possible to reach a load

value for RBCs where the organic matter "breaks through" it, producing a far poorer water

quality. In cases like this, it is advisable to apply some sort of balancing before the

rotaring biological contactor. Another option, if somehow it is possible to shift the daily

peak load over to the night, balancing the RBC's biological load.

Such solutions may be useful for compensating the peak loads of the wastewaters

containing suspended solids coming from backwashing of the filters. Because of this, like

in the case of activated sludge systems, tank volumes or necessary surfaces for rotaring

biological contactors must also be dimensioned with a certain security factor.

The effect of temperature on rotaring biological contactors

Rotaring biological contactors are as sensitive to the temperature variations as any other

biological system. This means that during the cold winter periods, it is worth to protect

them from significant cooling. This why covering such systems is a fortunate solution.

The designers believe that the speed of organic matter removal in case of RBCs is more

or less constant above 13 oC and significant deceleration only occurring below this

temperature (Envirex, Inc., 1989; Lynco, Inc., 1992; Walker Process, Inc., 1992). The

same can be said for nitrification and it decreases dramatically downwards from this

temperature.

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Sludge yield

Clarifying the precise sludge yield is important for RBCs as something has to be done

with the sludge following wastewater treatment. Therefore the scope of the design must

extend to designing the necessary equipment for this. It is generally true that rotaring

contactors have sludge yields similar to other, fixed film systems. Considering this, the

specific sludge yield is generally expected around a specific value of 0.4-0.6 kg organic

matter/kg total BOD5. Smaller values may occur in case of moderately loaded systems,

where higher endogenous oxygen uptake also takes place (intensified mineralisation of

the material of the cells). Higher specific sludge yields have been measured in units

operating at high specific loads. The total from the above is then usually 0.5-0.8 kg

projected on the removed BOD5 quantity (Envirex, Inc. 1989). The difference between

the two is the inorganic matter content of the sludge (heating residue). As usually for

other wastewater sludges, about 80-95% of the sludge is organic matter (heating loss).

The sludge of the RBCs thickens well; it can be condensed to dry matter concentrations

of 2.5-3% by gravitationally. This is why it is an accepted solution to use secondary

sedimentator for RBCs to remove the suspended solids. In these cases, using a sludge

excavator at the bottom of the tank is also useful as in mixed biofilm and activated sludge

systems. The thickening of the sludge however may be problematic. The sludge may

easily ferment at the bottom of the tank and float to the surface, making subsequent sludge

treatment difficult.

The capacity of rotaring biological contactors

As was mentioned in the introduction, rotaring biological contactors may be designed

purely for organic matter removal, for combined organic matter and ammonium removal

and for post-nitrification following the activated sludge treatment. The latter is used less

often than at the other biofilm systems. The various designers estimate nitrification

capacity quite differently. Envirex Inc., for example, if the ammonium concentration of

the treated water provided above 5 mg/l, the nitrification capacity of the RBC as 1.5

g/m2/d. Probably, this is only possible for post-nitrification filters. At this range, as a

function of the concentration of ammonia, its removal takes place according to the first

order dynamics.

Unfortunately, the presence of higher organisms in the sludge such as consumers and

predators will greatly deteriorate the nitrification capacity. As a consequence of this, the

oxidation capacity of 1.5 g/m2/d may decrease significantly, meaning it should only be

designed with adequate security.

An American company reported highly varying oxidation speeds as a function of the

ammonium concentration of the treated water. Between 4.5-5.5 mg/l, this oxidation speed

is 1.5 g/m2d and between 4.5-3 it is only 1.2 g/m2d, while between ammonium

concentrations of 3-2 mg/l it is only 0.75 g/m2d. This clearly indicates that the speed of

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oxidation is highly dependent on the supply of ammonium. This could be observed for

trickling filter unit systems as well, and it is even more accentuated for RBCs.

References

Chou, C. C. (1978) Oxygen Transfer Capacity of Clean Media Pilot Reactors at South

Shore. Autrotrol Corporation: Milwaukee, Wisconsin.

Envirex, Inc. (1989) Specific RBC Process Design Criteria. Waukesha. Wisconsin.

Lyco, Inc. (1992) Rotating Biological Surface (RBS) Wastewater Equipment: RBS

Design Manual. Lyco Inc., Marlboro, New Jersey.

McCann, K. J., Sullivan, R. A. (1980) Aerated Rotating Biological Contactors: What are

the Benefits? Proceedings of the 1st National Symposium on Rotating Biological

Contactor Technology, Vol. I, EPA-600/9-80-046a; Champion, Pennsylvania.

Metcalf and Eddy, Inc. (1979) Wastewater Engineering: Treatment, Disposal, and Reuse;

McGraw-Hill. New-York

Walker Process, Inc. Aurora, Illinois (1992) Personal communication

Weston, Inc. (1985) Reuse of Currant RBC (Rotating Biological Contactor) Performance

and Design Procedure, EPA 600/2-85-033, Cincinnati, Ohio.

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6. Moving Bed Biofilm Reactors (MBBR)

Over the past 20 years the use of so called moving bed biological reactors (MBBR) widely

spread. Thanks to their simplicity and appropriate flexibility, their various biofilm

substrate variants are widely used. MBBR systems with appropriate reactor

configurations may be successfully applied for the removal of both the organic matter,

therefore the biological oxygen demand and then for the oxidation of ammonium even

for the removal of nitrate and the total nitrogen content. Unaerated biofilm, but at the

same time adequately supplied with nitrates is capable of very effective denitrification.

This is partly because of the organic matter supply in the anoxic environment and partly

as a result of the hydrolysis of the deeper sludge layers. Together, these may meet the

strict limits corresponding to the nutrient removal in case of lower COD/TKN ratios of

the raw wastewater. At the same time, this solution also offers a cheap way of nitrate

reduction in the unaerated sludge space (preliminary denitrification).

Naturally, a special biofilm substrate had to be developed for the MBBR reactors, which,

considering its size, is capable of allowing free movement in the fluid or activated sludge

phase, however, it is ensuring adequate turbulence in the charge or on its surface and

provides the adequate nutrient and oxygen supply of the biofilm by aerating the fluid

phase. Of course it is important, that the presence of the charge or biofilm substrate do

not inhibit substantially the movement of the air bubbles as this may result in higher

energy demand for treatment. It is common that about one third one half of the reactors

are filled with biofilm substrates while the rest is free fluid volume. Some kind of

filtration must be built on the appropriate surfaces of the reactors to prevent the substrate

being carried or washed out. Besides, the appropriate removal of the biofilm from the

surfaces of the substrate particles must also be ensured.

Single tank moving bed biological reactors can be considered perfectly mixed tank

reactors. In case of longitudinal tank configuration and fluid flow, a certain concentration

gradient will form between the end points, especially in the reactors containing activated

sludge, therefore they can be considered some kind of reactor cascade, in which nutrient

concentration gradient evolves. This may be further increased by the solution of linking

number of tanks in series, whose one or two tanks containing moving biofilm substrates

In case of an anaerobic/anoxic/oxic series of tanks, the biofilm substrate is perfectly

unnecessary in the anaerobic tank, but it can even be omitted from the oxic tank. A

fundamental advantage of the MBBR reactors in comparison to fixed film systems is that

can ensure the advantages of both biofilm and activated sludge, while the aforementioned

inhibiting effects remain less prevail as a result of the continuous movement of the

biofilm.

In flooded biofilm as well as mobile bed systems, the specific advantage of the biofilm

prevails, namely that its various layers provide various environments for the accumulation

and activity of various microorganism groups. However, the moving biological charge

12

allows the formation of larger surfaces in the fluid layer, increasing the surface of the

biofilm and the volumetric capacity of the biofilm system.

The fluid flow is much larger around the layers of the moving biofilm substrate than the

simple flooded bed, static (fixed) biofilm systems. This causes a lesser thickening of the

biofilm in moving biological substrates and thereby a less frequent needs to bakcwash the

reactor. It is also necessary, that the hydraulic resistance of moving biological substrates

is perhaps even less than in the case of fixed bed systems, therefore the pressure loss of

the fluid passing through.

Moving biological charge or moving bed reactors, thanks to their simple spatial

arrangement similar to activated sludge systems practically allow the implementation of

the same combined (A2/O or A/O) reactors or stages as in case of an activated sludge

system. This makes such hybrid combinations particularly advantageous in terms of

achievable organic matter removal and nitrification as well for preliminary or post-

denitrification. However, this does not mean that such combined arrangements should

necessarily increase pumping energy demand and aeration may also be similar.

The main advantage of such biofilm reactors in comparison to activated sludge systems

is that the biomass forming or formed in the biofilm remains fixed in the reactor. No

separate clarification volume needs to be designed for its separation and sedimentation.

This results in the average sludge age increasing considerably in comparison to activated

sludge flown through directly (although recirculated). As a result of its longer sludge age,

biofilm is far more applicable both for nitrification and hardly biodegradable organic

matter. As a combined result, moving bed bioreactors may not only be combined with

conventional secondary clarification techniques, but the separation of their activated

sludge may be implemented using other, more modern solutions, e.g. ultrafilters.

The diversity of moving bed biological contactors stems from the fact that the tank may

be configured in a large number of ways. But perhaps even more importantly, that

biological tanks built earlier can be easily converted without any difficulty to such

charged moving bed biological reactors. In such a case, their sludge mass or total

activated sludge and biofilm mass in comparison to the activated sludge mass can be

increased considerably in the aerated tank. The secondary sedimentator will only be

loaded by the activated sludge part and the minimal sludge mass breaking off from the

biofilm. In this sense, the hydraulic load of a secondary sedimentator may be increased

without any increase in volume or reconstruction works.

Moving bed biofilm filters

Due to its favourable properties in the field of producing and developing moving

biological charges an intense competition has developed. Such biofilm substrates have

been made from many materials and in many different forms. It was important that their

densities and sizes not be too large, allowing them easy movement in the fluid phase.

13

Their specific surfaces must be also big. To achieve this, rougher surfaces were used

(Lazarova and Manem, 1994), to help the biofilm's adhesion. Easy manufacturing of the

biofilm substrate is likewise definitive and will probably remain so in the future, although

its technology is constantly developing.

In systems simultaneously nitrifying and removing organic matter from municipal

wastewaters, limitation of organic matter must always be applied to help nitrification. In

activated sludge systems, this can be achieved by maintaining a small

nutrient/microorganism (F/M) ratio or a longer sludge age to sufficiently starve the

heterotrophs. In activates sludge systems, full nitrification is only achievable at a

maximum load of 0.16 kg BOD5/kg sludge dry matter x d (Halling-Sorensen et al., 1993;

Henze et al., 2002). In biofilm systems, the maximum organic matter load has been given

as 4-6 g BOD5/m2 d (Rittmann et al., 2001; Henze et al., 2002). Only in case of smaller

surface organic matter loads can be achieved the nitrification capacity which is required

for the oxidation of the surplus ammonium of municipal wastewatersin biofilm systems.

The first significantly successful biofilm substrate was developed by Norwegian

researchers in Trondheim (Odegaard, 2006). The development was started in the middle

of the 1980s, perhaps in order to reduce the load of the North Sea and the Baltic Sea using

biofilm nitrification and denitrification. In this region, physico-chemical treatment was

widely used, which had a very poor nitrogen removing efficiency. The seas of the region

had become eutrophic at that time for long periods as a result.

Norwegian researchers realised at this time that the simplest and most economical method

for oxidising ammonium and removing nitrate is the construction of a compact biofilm

substrate relatively small in size. but having a large specific surface (Odegaard et al.,

1991). At the end of the 1980s, the production of specially structured and small sized

biofilm substrates began in Norway. This Norwegian substrate became globally known

under the brand name of Kaldnes, but similar products are manufactured in many

places. The Japanese were perhaps the first to produce similar products at the end of the

1980s - biofilm substrates marketed under the name "biotube".

At the end of the 1980s, the success of the Kaldnes charge soon led to the patenting of

the moving bed biofilm technology and its successful practical implementation in 1989.

It was also realised that such a moving biofilm system, having been successful in treating

municipal wastewaters, can be used successfully for treating industrial wastewaters as

well. Those waters need even longer retention time in the wastewater for the

microorganisms to adapt and decompose the organic matter. Such were the wastewaters

of the paper industry, in which cellulose and shorter carbon chain fragments represented

decomposition problems for the microorganisms. The Norwegian company and its

competitors worldwide later developed many different biofilm substrates. Such charges

are shown in table 7.1.

Table 7.1: Plastic biofilm substrates and their geometric design

14

Manufucturer Name Specific surface* Nominal substrate

dimensions (thickness and diameter)

Veolia, Inc. AnoxKaldnesTM K1 500 m2/m3 7 mm, 9mm

AnoxKaldnesTM K3 500 m2/m3 12 mm, 25 mm

AnoxKaldnesTM

biofilm chip (M) 1200 m2/m3 2 mm, 48 mm

AnoxKaldnesTM

biofilm chip (P) 900 m2/m3 2 mm, 48 mm

Infilco Degremont, Inc. ActiveCellTM 450 450 m2/m3 15 mm, 22 mm

ActiveCellTM 515 515 m2/m3 15 mm, 22 mm

Siemens Water

Technologies Corp. ABC4TM 600 m2/m3 14 mm, 14 mm

ABC5TM 660 m2/m3 12 mm, 12 mm

Entex Technologies, Inc. BioportzTM 589 m2/m3 14 mm x 18 mm

*Based on information by producers

Interestingly, the most frequently applied biofilm substrate in practice is the K-type of the

Kaldnes substrate. Retention of the Kaldnes charge in the aerated tank is possible by

building special filter-like walls or by building in appropriate filter tubes. The position of

the filter surfaces vertical or horizontal construction is very important for the self

purification of the filter; therefore separation from the filtered biofilm substrate. The filter

panels placed vertically were and are primarily applied in tanks without any network of

15

aeration pipes at the bottom of the tank. Such are the anoxic reactors, in which the biofilm

may help denitrification next to the activated sludge. Naturally, in case of vertical filters,

an appropriate aeration pipe must be placed under them, which ensures, through cyclical

introduction of air, the removal of the filtered biofilm substrates from the surface of the

filter. The air cannot be blown underneath continuously because the reactors in question

are anoxic. The air flow must pulsate. Figure 7.1 shows a typical, horizontal and vertical

filter configuration.

Figure 7.1: Vertically placed filter panel with aeration from below (a), horizontal filters

with appropriate stiffening pipe network above the aeration elements (b).

The aeration is not necessary in the anoxic area, but it may also be ensured in there. In

the oxic areas, surface aeration is out of the question, but deep aeration may be

implemented in a variety of ways. Aeration using rougher bubbles with more intensive

fluid mixing is advantageous. Such intensification may even be carried out using

hyperboloid mixers. The developers of the original MBBR used relatively small diameter

perforations distribution tubes (4 mm boreholes) at the bottom of the tank then the

Kaldnes charges. This aeration proved favourable as the larger air bubbles in the space

filled with the biological film substrate could ensure far better turbulence and a better

supply of oxygen for the biofilm. Such configuration of the aerator also helped not to

cause excessive activated sludge sedimentation in the aerated tank. Laser perforated

rubber membranes are not necessarily favourable in these biofilm systems.

It was also experienced that if aeration was stopped, the charge therefore the biofilm

substrates could settle without further ado on the bottom of the tank, releasing the fine air

bubbles bound therein. By the start of aeration, the settled biofilm substrates can be easily

reintroduced in the aerated area. Their majority is placed above the aeration elements and

their circulation and flow can move the charge underneath the aeration tubes as well. This

configuration also has the advantage of being very stable, air may be introduced through

acid proof steel tubes, which results in the extreme durability and usability of the aeration

system. Figure 7.2 shows a typical configuration for such an aeration and pipe distribution

system.

16

Figure 7.2: The configuration for the aeration system on the bottom of the tank (a) and

the acid proof distribution tubes on the bottom of the tank with 4 mm holes on the sides

(b).

By today, a large number of plastic biofilm substrates have been developed. Significant

development took place in this field in the United States, Canada and Israel. The last one

and a half decades evidently saw the success of the moving bed biological reactors and

moving biological charge operated systems.

The development of the structured biofilm substrates led to the integrated, fixed film

activated sludge systems, in which the most important task was perhaps improving the

nitrification at cold temperatures. The usage of such biofilm substrates was then patented

in technological variants such as airlift or the centrally aerated reactors operated with

surrounding anoxic areas. These proved successful primarily in Israel, in small

wastewater treatment plants. An important aspect in developing biofilm substrate

particles was to increase the quantity of biofilm in their inner spaces, better protected

against washdown, while ensuring the adequate oxygen supply of this latter as well.

Soon the reactors were not formed as a single tank, but by using appropriate partitioning

panels instead, transforming them into series of reactors, shifting the system configuration

towards multi stage biofilm or activated sludge systems using multiple environments as

discussed earlier. The partitioning walls or panels may then offer further opportunities of

diversity. If the substrates are filtered out over the entire cross section, thy system will

function quite differently than if filters are only built in a relatively shallow lower or an

upper part of it. The flow relations between the two systems differ greatly.

Design considerations of the moving bed biofilm filters

Designing moving bed biofilm or hybrid reactors fundamentally assumes that treatment

does not take place in a single tank, but in a series of tanks instead, namely in a series of

treatment units linked together. This is definitely beneficalas a special biofilm may form

in each reactor on the surface of the substrate that may operate close to its optimum

conditions in each area. Such configuration is very simple for moving bed biological

17

reactors. This is a significant advantage for moving bed biofilm reactors against activated

sludge systems. For those the simultaneous competition of all microorganisms takes place

in each sludge floc. In the latter case an average suspended solid or sludge retention time

or sludge age must be maintained for the mixed culture. This must ensure that a

composition of the microorganisms is formed, which are capable of removing all nutrients

in contact with the wastewater to the desired extent. As the time for removal is actually

the duration ensured by the hydraulic retention time, the biofilm systems have greater

flexibility, as in those a concentrated microorganism culture may perform the treatment

under the same duration of time.

Biofilm substrates

The biofilm systems are determined by the formation of the appropriate sludge

concentration, active biomass and biofilm in a particular reactor. In case of moving bed

reactors, the biofilm substrate is capable of fixing relatively large mass of biofilm. Based

on investigations, the sludge mass fixed on the biofilm, given the adequate charge and

filling rate, may even reach the value of 1-5 gram/litre of dry sludge matter. However, if

the capacity of such biofilm systems is investigated relative to the volume, results will

show that the efficiency or organic matter removal will be far superior, than in the

corresponding activated sludge systems. (Rusten et al., 1995). The biofilm system or more

precisely such increase of its capacity can be explained by the following factors:

- relatively active biomass formation on the biofilm substrate,

- its mass may be increased considerably with the thickness of the biofilm,

- its appropriate nutrient supply may be increased by the movement of the

bubbles,

- the air movement and the turbulence likewise intensifies this.

Such solutions in the particular reactors or treatment stages may help the formation of

sludge mass better specialised for the various nutrient types on the biofilm substrate

(Figure 7.3).

18

Figure 7.3: Sludge forming on a particular substrate in case of a 4 stage biological

filter.

The thickness and the colour of the biofilm forming on the substrate show well that the

sludge mass forming in the various tank areas are different in terms of nature and function.

In the first stage, species specialised on organic matter removal dominate while those

specialised on nitrification can be found in the last.

Moving bed biological reactors or units, similarly to the continuous flow activated sludge

systems, may be arranged in various combinations to achieve organic matter removal,

nitrification and denitrification. In table 7.2. the general arrangement principles of such

moving bed reactors can be found.

Table 7.2: General configuration variants of moving bed biofilm filters

19

After treatment, the efficiency of all component units depends on the following:

- the composition of the wastewater to be treated, which is limiting the formation

of the technological steps,

- the available system of biological tanks upon proper reconstruction or

intensification,

- the efficiency expected from the treatment or other quality requirements.

In such moving bed biofilm and hybrid wastewater treatment plants, the size of the

available biofilm surface is a key parameter (Odegaard et al., 2000). During the design

process, the removal speeds of the particular nutrients and the surface of the substrate

need to be taken into consideration. The removal of the organic matter on the heterotroph-

dominated surface of the biofilm may take place at particularly high speed, while the

surface of the biofilm is definitive for the nitrification capacity of the autotrophic

microorganism layer beneath it. Therefore in biofilm systems the surface removal

capacity (Surface Area Removal Rate- SARR) and the surface load (Surface Area

Loading Rate - SALR) need to be considert regarding to organic matter removal and

nitrification. As in any nutrient removal, the removal of organic matter in such biological

substrates is rapid in case of good nutrient supply. The speed of removal in this case is

zeroth order (independent from the organic nutrient concentration), while it can shift to

first order (dependent on concentration) at smaller concentrations (Monod dynamics).

The most general correlation for nutrient removal is true in this case, too:

r = rmax (L/K+L)

where:

r = the speed of removal (g/m2d)

rmax = the maximum removal speed (g/m2d)

L = applied load (g/ m2d) and

K = half velocity constant

Organic matter removal

The speed of organic matter removal (SALR Surface Area Loading Rate) in case of a

moving bed biofilm reactor is obviously the treatment objective, namely it must be

designed as a function of the quality of the treated water to achieve. Table 7.3 contains

typical design values for BOD5 load, considering the objective of treatment. If such a

treatment plant needs to ensure nitrification as well, far less organic matter load should

be considered for its design. Reversly, if organic matter removal is the sole objective, far

greater organic matter loads are permissible. Removal of organic-C however requires

appropriate oxygen supply in the fluid phase, a value somewhere between 2-3 mg/l. This

much is enough for the removal of organic matter to take place at the appropriate speed.

Keeping oxygen concentration at higher levels will not improve the surface removal

speed of organic matter.

20

6.3.table: Expected BOD removal efficiency and BOD5 of treated water

Organic matter removal

(%)

Organic matter load

(g BOD5 /m2d)

High load (75-80 % BOD5 removal) >20

Normal load (80-90 % BOD5 removal) 5-10

Low load systems (stable nitrification)

21

Figure 7.5: BOD removal as a function of specific BOD5 load upon high load moving

bed biofilm filtration.

Normal load systems

As the organic matter load decreases, nitrification begins in the biofilm after a certain

load value. Practical experience has shown that in a moderate load range (7-10 g

BOD5/m2d) adequate treatment can be ensured even at temperature around 10 oC, butthe

phosphorus needs to be removed from the system through chemical precipitation. Linking

two reactor tanks in series may be recommended for such treatment loads, which

significantly improves the system's security of nitrification and the quality of the treated

water.

Low load systems

Such solutions may be recommended for the nitrifying reactor, if it is preceded by an

organic matter removing reactor. This can then ensure the appropriate nitrification in the

second unit respectively this double combination also results in an economical solution.

If the organic matter removal is inappropriate in the first moving biofilm filter unit, then

the nitrification may significantly deteriorate in the next unit and the effect of the reactor

may even be practilcally ineffective. As shown in figure 7.6 clearly, the nitrification

capacity of the biofilm forming on the substrate decreases dramatically as the BOD5 load

increases. Accordingly, the two biological stages must be designed very carefully for the

removal of various nutrients.

22

Figure 7.6: The effect of the BOD5 load and oxygen concentration on the nitrification

speed at a temperature of 15 oC (a) the nitrification speed observed in the various stages

in case of a moving bed reactor series (b) (Heim et al., 1994).

It can be seen well in figure 7.6 that in case of a load of 2 g BOD5/m2d, a nitrification

capacity of 0.8 g/m2d can be achieved, if the dissolved oxygen concentration in a given

system is 6 mg/l. However, the nitrification capacity may be reduced by 50%, if the

organic matter load is increased by one and a half times, therefore to 3 g BOI5 /m2d. To

achieve this objective, the designer or the operator must either increase the oxygen

concentration in the fluid phase or the surface ratio in the system in order to increase the

available biofilm surface. But it is clear, that neither solution can improve the economy

of the wastewater treatment beyond a certain optimum. It is better to design the first stage

of such MBBR systems for smaller specific organic matter load to avoid nitrification

related problems in the second reactor stage.

Figure 7.6 b shows the characteristics of a three stage moving bed biological filter. Based

on the studies that were used to clarify the nitrification capacity of the sludges in the

various stages, it can be very well seen, that the nitrification capacity in the first biological

unit is far lower than in the following two units. It can also be well seen, that there is no

significant difference between the 2nd and 3rd stages. On the other hand the parameters

seen in the left corner show the dissolved COD load that could be observed in the reactors

of the system. In case of a small load, only a moderate surface load may be considered

according to the above and in this case it is practical to design the surface load with

appropriate temperature correction (Sen et al., 2000).

LT = L10 1.06 (T-10)

where

T= ToC is the temperature,

L10 = 4.5 g/m2d at a temperature of 10 oC

Nitrification

In case of the nitrifying moving bed biofilm filters, a number of aspects have to be

considered during their design. Most important among these are the specific organic

matter load, the dissolved oxygen concentration in the fluid phase, the ammonium

concentration, the temperature of the wastewater and the pH of the wastewater, namely

the alkalinity. As shown well in figure 7.6, that in the respect of determining the

nitrification, the extent to which dissolved organic nutrients (BOD) have been removed.

If this is insufficient, a terminal competition may form between the heterotrophs and the

nitrifiers to consume the oxygen. If the organic matter supply decreases (carbon

limitation), the speed of nitrification will clearly increase in the biofilm until the oxygen

concentration in it will become rate determining. The ammonium concentration of the

wastewater will only be a rate determining of nitrification if it decreases below the value

of 2 mg/l in the fluid phase (Hem et al., 1994; Odegaard et al., 1994; Odegaard, 2006).

23

This may occur in situations, where nitrification is very important accurately the limit set

for the ammonium concentration of the treated water is very low.

In case of post-nitrification moving bed biofilm filters composed into two reactors, where

each of only performs nitrification, it is possible that the first would operate under low

oxygen conditions while the second would run under low ammonium conditions. The

temperature always has a significant effect on nitrification, but such biofilm combinations

may be used to significantly compensate the effect of cooling through oxygen

concentration. However, it is important to achieve a proper alkalinity of the water,

otherwise the decrease of the pH may also contribute to considerably decreasing the

nitrification in the biofilm.

It was observed, that at a well-operating moving bed biofilm filter the oxygen will

become definitive for the speed of nitrification, when the oxygen/ammonium-N ratio in

the wet phase decreases below 2 (Hem et al., 1994; Odegaard et al., 1994). In this sense,

contrary to the activated sludge systems, the oxidation speed of ammonium may be

different such in biofilm filters as a function of the dissolved oxygen concentration. This

is highly probably the result of the diffusion inhibition of the oxygen, which should enter

the biofilm at the appropriate speed to achieve nitrification. (Hem et al., 1994).

Higher oxygen concentrations significantly increase the diffusion speed of oxygen from

the wet phase. It is obvious, that the entry of oxygen into the biofilm may not only be

improved by the concentration gradient, but by the intensified mixing of the fluid phase.

Figure 7.7 shows that in case of an adequately low ammonium concentration in the

wastewater, the speed of nitrification will grow proportionately to the dissolved oxygen

concentration, but after reaching a certain level of ammonium-N concentration, it will

stabilise regardless of any change of the ammonium concentration. In case of an

appropriately developed or adapted nitrifying biofilm the ammonium concentration will

not influence the speed of nitrification until the aforementioned oxygen-ammonium-

nitrogen ration decreases below the range of 5-2. The ratio to assume for design purposes

is advised as 3.2, obviously calculated backwards from the ammonium-nitrogen limit

value of the treated water. For such nitrification, its speed must be dimensioned by

considering the limitation and the water temperature (Rusten et al., 1995; Salvetti et al.

2006).

24

Figure 7.7: The effect of oxygen concentration on the capacity of the biofilm for low

ammonium concentrations.

Denitrification

The moving bed biofilm reactors may be applied for denitrification without any difficulty.

Naturally, they do not need to be aerated in this case and even the nitrified fluid needs to

be recirculated. Only then denitrification can occur in them. It is advisable to place the

denitrifying reactors before these aerated reactors. A good organic matter and nitrate

supply can be ensured in the preliminary denitrification unit. Well built preliminary

denitrifiers may be capable of nitrogen removal up to 50-70%. This does not even require

a particularly high internal recirculation ratio. Recirculating the nitrated water is sufficient

up to ratios of about 1:1-3:1. Typical denitrification speeds can be seen in table 7.4.

Table 7.4: Design values for denitrification speeds for preliminary denitrification of

municipal wastewaters.

Denitrification speed (NO3-N equivalent)

Reference

0.4 1.0 g/m2d (Gardemore wastewater treatment plant,

Norway)

Rusten and Odegaard,

1995

0.15-0.5 g/m2d (Freevart wastewater treatment plant,

experimental semi-industrial unit, Norway)

Rusten et al., 2000

0.25-0.8 g/m2d (Crow Creek wastewater treatment plant,

experimental semi-industrial unit, USA)

McQuarrie and Maxwell,

2003

Post-denitrification in case of MBBR reactors

Post-denitrification may be appropriate for moving biofilm reactors and even for

activated sludge systems. But the appropriate organic nutrient supply is required to

consume the oxygen of the nitrate. This requires the addition of some sort of auxiliary

nutrient. The maximum nitrate removal in these cases (ensuring an adequate external

nutrient supply) may exceed the value of 2 g nitrate-N /m2d. Figure 7.8 shows the

denitrification speed measured for the various organic nutrients.

25

Figure 7.8: Denitrification speed as a function of temperature and various auxiliary

nutrients. (Rusten et al., 1996).

The post-denitrification may even ensure 100% nitrogen removal. Naturally,

denitrification may be imagined in combinations, where both preliminary and post-

denitrification are built for a given system. Preliminary denitrification during the winter

period may even be paused as necessary. More precisely, by aerating the unit, the system's

nitrification capacity may be increased to the desired extent. When such post-

denitrification will require more auxiliary nutrientswill make the solution quite a degree

more expensive. In case of denitrification, it is evident that if aeration is inactive, the

movement of the moving bed biofilm charge or the fluid and thereby supplying the

microorganisms with nutrients must be ensured by contributing some external kinetic

energy to the system. The required mixing energy is usually around 25-36 W/m3.

References

Halling-Sorensen, B. and Jorgensen, S. E. (1993) The Removal of Nitrogen Compounds

from Wastewater. 1st Edition, Elsevier Science Publishers, Amsterdam

Hem, L., Rusten, B., Odegaard, H. (1994) Nitrification in a Moving Bed Reactor. Water

Res. , 28, 1425-1433.m

Henze, M., Harremoes, P., Jansen, J. L. C. and Arvin, E. (2002) Wastewater Treatment:

Biological and Chemical Processes. 3rd Edition, Springer, Germany

Lazarova, V., Manem, J. (1994) Advances in Biofilm Aerobic Reactors Ensuring

Effective Biofilm Activity Control. Water Sci. Technol., 29, 319-327.

Mc Quarrie, J., Maxwell, M. (2003) Pilot-Scale Performance of the MBBR Process at the

Crow Creek WWTP. Proceedings of the 76th Annual WEF Exposition and

Conference. Los Angeles, USA

Melin, E., Odegaard, H., Helness, H., Kenakkala, T. (2004) High-Rate Wastewater

Treatment Based on Moving Bed Biofilm Reactors. Chemical Water and

Wastewater Treatment VIII, IWA Publishing: London, United Kingdom, 39-48.

Odegaard, H., Gisvold, B., Strickland, J. (2000) The Influence of Carrier size and Shape

in the Moving Bed Biofilm Process. Water Sci. Technol., 41, 383-391.

Odegaard, H., Paulsrud, B., Bilstad, T., Pettersen, J. (1991) Norwegian Strategies in the

Treatment of Municipal Wastewater Towards Reduction of Nutrient Discharges to

the North Sea. Water Sci. Technol., 24, 179-186.

Odegaard, H., Rusten, B., Westrum, T. (1994) A New Moving Bed Biofilm Reactor-

Application and Results. Proceedings of the 2nd International Specialised

Conference on Biofilm Reactors, Paris, France, Sept 29-Oct 1., International

Association on Water Quality: London, United Kingdom, 221-229.

Odegaard H. (2006) Innovations in wastewater treatment: the moving bed biofilm

process, Water Sci. Technol., 53 (9) 1733.

Rittmann, B. E. and McCarty, P. L. (2001) Environmental Biotechnology: Principle and

Applications. International Edition 2001, McGraw-Hill Book Co., Singapore

26

Rusten, B., Hem, L., Odegaard, H. (1995), Nitrification of Municipal Wastewater in

Novel Moving Bed Biofilm Reactors. Water Environ. Res., 67., 75-86.

Rusten, B., Wien, A., Skjefstad, J. (1996) Spent Aircraft Deicing Fluid as External

Carbon Source for Denitrification of Wastewater: From Waste Problem to

Beneficial Use. Proceedings of the 51st Purdue Industrial Waste Conference, West

Lafayette, Indiana, May 6-8, Purdue University: West Lafayette, Indiana.

Salvetti, R., Azzekino, R., Cabziani, R. Bonomo, L. (2006) Effect of Temperature on

Tertiary Nitrification in Moving-Bed Biofilm Reactors. Water Sci., 40, 2981-2993.

Sen, D., Copithorn, R., Randall, C., Jones, R., Phago, D., Rusten, B. (2000) Investigation

of Hybrid Systems for Enhanced Nutrient control, Project 96-CTS-4, Water

Environment Research Foundation: Alexandria, Virginia.

27

Hybrid, biological wastewater treatments

An overview of the integrated biofilm and activated sludge systems

The idea of linking two biological growths and utilising them in a common reactor first

emerged in the 1970s. Such solutions are today known as hybrid reactors or systems.

Despite the fact, that this solution is generally used to improve the efficiency and

capacities of treatment plants built earlier, knowledge rather incomplete on their precise

design and optimal operational possibilities (Daigger et al., 1998; Boltz and Daigger,

2010). The precise clarification of the combined operation of the biofilm and the activated

sludge may therefore hold further development opportunities in the future for the

construction, intensification and nitrogen removal of both aerobic and anaerobic

combined systems. The latter may partly reduce the required specific treatment volume

and partly the nitrogen content of the treated waters further. The following description is

aimed at introducing the aerobic hybrid systems and further development opportunities.

Theoretically, the name hybrid applies to any treatment that contains two different

treatment methods or solutions. The first such system in practice was the combination of

fixed film and activated sludge, the so called IFAS process has become widely

recognized, with widespread.. The name IFAS stands for integrated fixed film activated

sludge. It is a combination of the conventional biofilm filtration and the activated sludge,

where the biofilm load is significantly influenced by the sludge growing in suspended

form next to it.

The development of IFAS targeted to increase the total sludge mass in the activated

sludge tank and the volumetric treatment capacity of the treatment plant with biofilm.

This may be done by slightly overloading the activated sludge, which can be compensated

by the growth of the nitrifying microorganisms in the biofilm. In this sense, the IFAS

solution means an increase of nitrification capacity as well. This effect may be increased

by linking some tank areas in series, which may ensure even better nitrification while the

overloading of the first ones may take place simultaneously.

The first hybrid systems were built to increase the treatment capacities of the activated

sludge units built earlier. Ganczarczyk (1983) reported that at the late 1960s, Japanese

experts attempted to decrease the sludge load of the secondary clarifier by implanting

some substrate in the aerator. The first industrial scale hybrid system was built in 1975 in

Philadelphia (USA), where a rotaring biological contactor was built on an activated

sludge system (Guarino et al., 1980). In the 1980s then a number of hybrid solutions were

implemented in Europe (CAPTOR and LINFUR) and in Japan (Ringlace) (Nicol et al.,

1988). The development of such systems was accelerated by the favourable effects of

biofilm treatment. In the latter period, artificial bacteria immobilisation in the fixing or

substrate material has also been proven successful (Emori et al., 1994; Tanaka et al.,

1991). In addition to this, a number of suspended substrate hybrid systems have also been

built (Tijhuis et al., 1994a, b; Chudoba et al., 1994b; Rusten et al., 1994a,b; Mnch et al.,

28

2000; Lee et al., 2002; Nigueire et al., 2002). Industrial and semi-industrial units

confirmed their simplicity and cost efficiency for intensifying existing plants, especially

if no land area was available to further expand it or if such land acquisition was very

costly (Morper et al., 1990; Sen et al., 1994; Randall et al., 1996; Rusten et al., 1994a;

Mnch et al., 2000).

The fundamental operational differences of the conventional activated sludge and the

IFAS systems, thus the fixed biofilm - activated sludge system, are shown in figure 8.1.

Figure 8.1: The IFAS treatment and the conventional activated sludge treatment.

In comparison to an activated sludge system, the IFAS system may double the sludge

concentration, more precisely the biomass mass in the tank. As mostly the heterotrophs

growing on the surface are broken down from the biofilm fixed on the substrate, the

nitrifying layer forming beneath or the average retention time of the biomass will be far

greater than the suspended part (sludge age). The fluid flow will only carry the suspended

solid to the secondary clarifier from the designed hybrid tank. The sludge load of the

secondary clarifier hardly changes in comparison to the activated sludge system. If the

mass of microorganisms can be increased using the biofilm, its capacity will also change

likewise. The question is how the capacity of the biofilm grows with the increase of the

load with respect to organic matter and ammonium oxidation. It properly nitrifies, the

lagging nitrification film parts will enable the activated sludge to perform some degree of

nitrification by inoculating it. It has been indicated earlier, the settling parts of the biofilm

breaking away is clearly improved by such a combination (TF/SC). This can be observed

by the decrease of the sludge index and the growth of filamentous microorganisms. This

combination may result in a somewhat reduced specific sludge production as a

consequence of the better sludge oxidation of the biofilm.

In line with earlier, in moderately loaded hybrid systems adequate simultaneous

nitrification and denitrification may be solved next to organic matter removal. It has

likewise been observed, that such combined or hybrid systems regenerate far quicker after

an operational disorder than conventional activated sludge systems.

29

Current technical developments are trying to make use of further advantages of the hybrid

systems. A treatment plant in which the growth of the heterotrophs and nitrifiers may be

regulated such that the decomposition of the organic matter should take place definitively

in the activated sludge with nitrification dominating in the biofilm, the organic matter

load limitation of the latter may be reduced to bearable levels and the nitrification of the

biofilm may ensure far greater average volumetric capacity in the entire system (Tijhuis

et al., 1994b; van Benthum et al., 1996, 1997). Such systems give the biofilm a very

specific role. In the future, this solution may come to widespread application, if the

regulation of operation may actually succeed in separating the various microorganism

species and their activities into two selectively operating reactor systems working in the

same area.

Accordingly, the IFAS system may practically be evaluated as the intensification and the

optimisation of two conventional systems. The static or moving biofilm substrate placed

in the pool may allow intensification of the volumetric capacity. This may free up space

in the tanks, which may be separated for creating an anoxic reactor space. The same could

be by creating further (anaerobic) tank spaces, which may enable the intensified

biological phosphorus removal of the activated sludge. It is practical to separate the

hybrid MBBR reactor into a number of separate stages linked in series to remove organic

matter. This is advantageous for nitrification. This solution, with appropriate

reconstruction, proved useful for increasing the capacity, treatment efficiency and the

nutrient removal of the activated sludge tanks built previously.

In this sense, the biofilm installation into the activated sludge may therefore be

implemented in a wide variety of reactor configurations. Not only the sludge volume can

be increased in the aerobic tank using biofilm, but even in the anoxic tank, considerably

improving nitrogen removal. Necessarily, the anoxic tank must then be mixed for the

adequate nutrient supply of the biofilm and the activated sludge and appropriate nitrate

recirculation (sludge water recirculation) must also be arranged. Although it is

theoretically possible to install charges in the anaerobic tank, this solution has not come

into general use, there is no particular advantage. Such increase of sludge mass in the

aerated tank is getting increasingly common and a wide variety of hybrid

(nitrifiying/denitrifying) systems are being built today. Preliminary and post-

denitrification variants have also been implemented.

Installation of biofilms into the activated sludge tanks may be implemented in case of

perfectly mixed tank reactors or pipe reactors as well. In case of fixed static panels, no

concerns arise in relation to the substrate retention. This applies for substrates made of

curtain-like threads, installed in a similar manner. In case of moving biofilms and biofilm

substrates, it is indispensable to somehow repatriate the biofilm substrate retained by

filtration at the exit point of the tank to the front of the aeration tank.

The integrated biofilm, activated sludge system may be easily confused in terms of

principle with the moving bed biofilm filters. The earlier IFAS configurations typically

30

did not operate by sludge recirculation, but the accumulated activated sludge had to be

cyclically removed by backwashing operations. Fixed, but loosely positioned biofilm

substrates offer greater possibilities for limiting the biofilm's overgrowth by aerating and

mixing the fluid phase more intensively. The biofilm substrate hybrid solutions (MBBR)

applied at present day for municipal wastewater treatment, which uses activated sludge

recirculation in each case to maximise the volume of sludge operating in the reactor.

The recirculated suspended sludge significantly increases the treatment capacity of the

system (Hamoda et al., 2000). Due to the larger particle size of substrates with low

specific surface, the concentration of the biofilm in the reactor will only be a few hundred

mg/l. Thereforethe removal of the contaminants must be mostly carried out by the sludge

in suspended form. In these cases, biofilm may significantly improve nitrification,

especially at lower temperatures (Randall et al., 1996; Sen et al., 1994; Jones et al., 1998)

respectively in case of peak loads, it may also stabilise the capacity of the treatment plant

(You et al., 2003; Jones et al 1998).

Resulting from the above, the IFAS systems emphasising the work of the biofilm better

are highly suitable forhigh load activated sludge treatment following post-denitrification

also. In this case, the ammonium load in the biofilm may be significantly increased due

to the dominance of the nitrifiers. These are generally utilized for post-nitrification after

activated sludge systems. Due to the low sludge growth allows its implementation using

biofilm substrates, which are heavier than water, at which practically the biofilm filtration

will be definitive. Denitrification is naturally a problem in this case, which regularly

ensured by subsequent addition nutrient in new or similar unit like IFAS. The cyclical

sludge washdown of this latter is more frequent and the denitrification itself is likewise

very costly. The auxiliary nutrient demand for the reduction of the nitrate generated from

the ammonium and the price allow the precise calculation of this. In practice, this system

has been implemented in the South Pest wastewater treatment plant without expansion of

the former activated sludge capacity. The advanced MBBRs capable of municipal

wastewater treatment apply simultaneous organic matter and nitrogen removal,

necessarily with sludge recirculation.

In hybrid systems, the improvement of the sedimentation of the sludge and a reduction of

the growth of filamentous organisms have been observed (Wanner et al., 1988; Morper

et al.,1990; Chudoba et al., 1994b; Lessel et al., 1994; Dalentoft et al., 1997; Jones et al.,

1998; Wang et al., 2000, Kim et al., 2010). This was well proven by industrial

investigations. Wanner et al. (1988) reported having reduced of the sludge index from

1500 to 150 in 20 days through the rapid washout of the filamentous organisms. In our

case, 90% of the total biomass was present as biofilm in the system converted into hybrid.

By increasing the sludge age, the sludge index set to a value around 100 ml/g, favourable

from the aspect of operation. The biofilm leads to a somewhat better sludgy consistency

in the activated sludge of the hybrid systems, restricting the growth of filamentous

organisms in it (Wanner et al., 1988; Chudoba et al., 1994b).

31

In addition to thesludge swelling, a similarly inconvenient phenomenon of the activated

sludge systems is excessive foaming, namely the formation of a stable foam layer on the

surface of the aeration tank. It was proven earlier, that the microorganisms that cause this

belong in the group of actinomycetes. One responsible species among them is Nocardia

amarae (Ganczarczyk, 1983; Mori et al., 1992). These become dominant in oily, fatty

wastewaters at longer sludge ages and higher temperatures (Ganczarczyk, 1983; Pitt et

al., 1990; Sezgin et al., 1985). Among other suggestions, reduction of the sludge age to

1.5-2 days has been recommended to reduce foaming (Sezgin et al., 1985; Pitt et al., 1990;

Cha et al., 1992; Mori et al., 1992). However, such a sludge age may not ensure

nitrification in the conventional activated sludge systems. Reduction of the foaming may

however also be ensured without reduction of nitrification in the hybrid systems, in which

the sludge part of biofilm may ensure adequate nitrification even at such a short sludge

age. Sen et al. (1994) measured full nitrification at a load of 0.9 kg COD m3d and 0.1 kg

N/m3d at a sludge age of 1.7 days with a hybrid system, while an activated sludge system

with the same parameters only ensured nitrification at a sludge age of 3-4 days. Their

examination was carried out on an A2/O system. In this, the anaerobic and anoxic zones

(with 3 and 2 units linked in series respectively) greatly reduced the average load of the

system. This effect was not investigated for greater organic matter loads.

An unfavourable property of these combined systems is that significant odour formation,

which takes place upon stoppage of the tanks for some cleaning, by the anaerobic

fermentation of the biomass. This means that other stages and storage capacities must

also be incorporated into the treatment process. These can be an appropriate tank for the

periodical removal of the biofilm substrate. It is perhaps also mention worthy, that the

biofilm substrate in comparison to the activated sludge system will increase its hydraulic

resistance, requiring large bubble aeration.

The type of the biofilm substrate

In practice, a large number of substrates have been put to application in activated sludge

systems. The two fundamental types are either fixed or free floating substrates.

The fixed biofilm substrate may usually be made of small tubes, fixed plates or

fixed nets or curtain like threads. The substrate must necessarily be fixed on some

sort of frame structure to keep it fixed in the activated sludge, which is in motion

by aeration.

At the same time, the moving substrate parts are either polyurethane cube cut to

some form or more solid, smaller plastic substrate elements, which may be formed

in the most diverse forms. A relatively well accepted classical solution is the tube

with "spokes" reminiscent of train wheels or platelets or bodies, as detailed in the

chapter describing moving bed biofilm wastewater treatment.

32

Plastic nets (Chudoba et al., 1994a) ceramic plates (Hamoda et al., 2000) plastic discs

(Wanner et al., 1988; You et al., 2003), rings made of wire-like materials (Sen et al., 1994;

Randall et al., 1996; Watchow, 1990) have all been used before as substrates. Although

the intensive aeration of the fixed bed systems does necessitate the cyclical backwashing

of the biofilm partly containing mechanically filtered contaminants. Catastrophic sludge

formation, overgrowth of flies and clogging problems has all been observed in these

systems, destroying their operation, especially their nitrification (Lessel et al., 1994;

Jones et al., 1998; Suzuki et al., 1999). The biofilm forming on the fixed substrates were

found less efficient in comparison to the moving substrates due to its worse oxygen supply

(Wartchow et al., 1990; Wanner et al., 1998; Sen et al., 1994).

Any of the above substrates may be applied in integrated, fixed film activated sludge

systems. At the begining of development, first the plastic platelets, then the trickling filter

unit type constructions were introduced by flooding them with activated sludge water.

The development of the moving biofilm charges started with the aim to increase the

volume of the specific biomass quantity that can be formed in a tank. Sponge cubes with

large specific surface came to general use first, then, as polyethylene processing

technologies developed, the production of rigid bodies took over. A great advantage of

the latter moving charges is that by intensifying the turbulence of the fluid phase

(aeration), the oxygen supply of their inner parts is increased considerably, thereby less

likely to be clogged and to block the filters applied to retain them. Their lifespan is also

considerably longer and the operation of such systems in combination with them is also

simplified to a great extent.

The nutrient transport in moving substrates is more favourable, therefore suspended

substrate biofilm or hybrid systems have become popular. Moving substrates with

densities close to water are the Kaldnes elements (Odegaard et al., 2000; Rusten et al.,

1994), the ANOX rings (Mnch et al. 2000), plastic beds (Emori et al., 1994; Tanaka et

al., 1991; Nogueira et al., 2002) and polyurethane foam particles (Sen et al., 1994; Wang

et al., 2000; Reimann et al., 1990; Morper et al., 1990).

In the initial stages of development, a lot of rotary biological contactor units were

intensified in this way by reintroducing the activated sludge or the biomass generated,

and by oxygen supply through some means of air distribution at the bottom. This way,

the simple or flooded RBC plants were practically converted to hybrid biofilm activated

sludge systems. In case of those functioning in flooded mode, their free fluid space was

filled up to a rate of 40%. While classical RBCs were also operated as continuously

aerated systems by activated sludge reintroduction, it was more practical in flooded rotary

biological contactors to create anoxic units or cycles. This could be implemented by

linking an activated sludge anoxic unit linked into the treatment line to intensify

denitrification.

Fixed substrate systems

33

As mentioned in the previous chapters, the basic principle of the hybrid system began in

the flooded film systems applied in the United States in the 1930s. Another question, that

the solution applied for aeration was in the elementary state, the biofilm-activated sludge

combination still produced good results. It was a particularly positive feature for these

solutions, if two units were linked in series or even the lagging biofilm in the first stage

was removed by a secondary clarifier between the two flooded stages.

Such solutions at that time, however, sludge recirculation was not yet applied. The charge

mostly consisted of asbestos panels, placed at a distance of 3-4 cm in the aerated tank.

The aeration elements used to be some kind of perforated or slit air distribution pipes.

The hydraulic retention time of the wastewater varied between 1.7-3 hours in the aeration

tank. In the United States, the wastewaters were of course always far more diluted than

in Europe. Unfortunately, the suspended biofilm substrate panels significantly obstructed

the horizontal fluid flow and thereby the oxygen diffusion, which led to reduced the

performance in comparison to today's biofilm substrates.

The immobilisation of the biofilm on the surface of the substrate sets appropriate

requirements for this latter. Thermodynamically, the adhesion of the bacteria is

favourable, if it reduces the free energy as well (Teixiera et al., 1998a). Examining the

adhesion of the nitrifying biofilm on various materials, cellulose was found to be the most

suitable. Excellent adhesion was observed within a single day (Kim et al., 1997). It was

further observed, that the cellulose substrate nitrifying film ensured the highest specific

transformation speeds after its appropriate formation (800 g NH3/m3d). This was roughly

the quadruple of that measured value for other polymers, such as polyurethane foam or

rubber shavings. Since bacteria always have a negative surface charge surplus, their

adhesion and colonization is favourable to the substrate, which having a positive charge

surplus.

A quality change of the biofilm substrate was the curtain-like plastic net suspended from

above and appropriately fixed in the aeration tank, which was developed during the 60s

in Japan. Such biofilm substrate at the same time provided better horizontal

interoperability of the biofilm therefore the convection could play a far more significant

role in the biofilm's nutrient supply than in the earlier American developments. In the

next two decades, the use of plastic nets came to widespread in the United States and in

Germany. This biofilm substrate was especially successfully for improving the removal

of biological nutrients. By this time, the fixed and yet moving nets, curtains were used in

the activated sludge tanks, which could be easilgy lifted up and the overgrowth could be

controled with cleaning them. There is no particular difficulty to lift the frame of the

plastic substrate and the sludge mass adhered to it (the blocks) from the activated sludge

tank. Meanwhile, most of the overgrown biofilm will also break off as the water trickles

down.

In recent years, the curtain-like biofilm substrates have become widely popular. It is

indispensable upon their construction to have enough free space available above and

34

below them (about 0.3 above and about the double of this underneath) to allow even

introduction of the air bubbles and their conglomeration at the surface. Therefore the

supporting elements and block must be appropriately positioned in the aeration tanks. In

the fluid phase, a nutrient concentration gradient is formed as the fluid traverses

longitudinally, but this may be formed in the oxygen supply as well (density of aeration

elements and intermittent air supply). Such systems have also been built as circular tanks,

where the movement of the fluid is intensified with various elements besides free flow.

These had to be placed in the sections between the blocks (Figure 8.2). The possibility of

the insertion is shown in figure 8.3, emphasising the importance of the hoisting apparatus.

A fundamental task is considered in designing that the operation of the system is the

average hydraulic retention time of the suspended sludge, namely the activated sludge

should be greater in the hybrid tank than the minimally required sludge age in the

corresponding activated sludge system. If this retention time is shorter than, the fixed film

may easily be washed down and lose its nitrifying capacity. However, this is not

inevitable, as there are several treatment plants, where the average sludge age of the

activated sludge part does not reach the value required for nitrification, but the plant

functions and nitrifies perfectly. This can be the result of a longer sludge age forming in

the fixed film, nitrifying organisms developing in higher concentrations in it. It seems

generally accepted, that in case of such systems, half of the nitrification takes place in the

activated sludge and the other half in the biofilm. However, neither can this be taken as a

strict rule in design. This requires precise knowledge of the system's construction, oxygen

supply, sludge removal, etc.

Figure 8.2: Fixed substrate flooded hybrid wastewater treatment plant.

35

Figure 6.4: Incorporation of a curtain-like charge in the activated sludge tank.

The biofilm substrates, especially in case of easily treated waters, it is more appropriate

that the nutrient supply of the highest possible speed of the biofilm on it should be

implemented. This can be achieved, if the convection of the fluid is ensured

simultaneously to increasing the surface. This, at the same time, allows better regulation

of the biofilm's thickness.

Primarily the net-like biofilm substrates proved most favourable for this, with the best

results. The biofilm substrate threads are made of polyvinyl chloride (PVC), polyester or

polyethylene. The production of such substrates started in Japan and soon followed in the

United States and subsequently, the German producers also started to manufacture their

own instead of using Japanese products.Of course, it was very important, the formed fine

structure of a substrate. This allowed the formation of a sludge layer thick enough in a

single tank that could resulted in significant simultaneous denitrification in it.

Installation of modern, static biofilm substrates and the design of the operating