GERMAN ATV RULES AND STANDARDS

W A S T E W A T E R - W A S T E

ADVISORY LEAFLET ATV-M 268E

Control and regulation of N Elimination Using the Activated Sludge Process

February 1997 ISBN 3-934984-57-6

Marketing: Gesellschaft zur Förderung der Abwassertechnik e.V. (GFA) Theodor-Heuß-Allee 17 D-53773 Hennef Postfach 11 65 . D-53758 Hennef

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Advisory Leaflet ATV-M 268 has been elaborated by the ATV Specialist Committee 1.13 "Automation of Sewage Treatment Plants". The following have collaborated:

Dr. Rer. Nat. J. Arnold, Bergisch Gladbach Dr.-Ing. P. Baumann, Stuttgart Dr.-Ing. P. Hartwig, Hannover Prof. Dr.-Ing. W. Hegemann, Berlin Dr.-Ing. J. Lohmann Dipl.-Ing. J. Maschlanka, karlsruhe Dipl.-Ing. E. Michael. Dr.-Ing. S Schlegel, Essen (Chairmann Dr.--ing.H.-H. Schneider, Berlin

All rights, in particular those of translation into other languages, are reserved. No part of this Standard may be reproduced in any form by photocopy, microfilm or any other process or transferred or translated into a language usable in machines, in particular data processing machines, without the written approval of the publisher. GFA -Publishing Company of ATV - Wastewater, Water and Water Environment, Hennef 1997 Original German Edition produced by: JF. CARTHAUS, Bonn

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Contents

Notes for Users 4

1. Objective 4

2. Fundamentals and process description 2.1 Nitrification 2.2 Denitrification 2.3 Processes

3 5

3. Health hazards through wastewater discharges into lakes and rivers 7

4. Hygienic loading and self-cleaning of lakes and rivers 8 4.1 Loading due to wastewater and the possibilities for its reduction 9 4.2 Diffuse loading and the possibility of its reduction 9 4.3 Self-cleaning of lakes and rivers 10

5. Methods of wastewater disinfection 11 5.1 UV irradiation 11 5.1.1 Fundamentals 12 5.1.2 Design and operation 14 5.1.3 Costs 19 5.1.4 Accident protection, side effects on lakes and rivers 19 5.2 Membrane filtration 20 5.2.1 Fundamentals 20 5.2.2 Design and operation 21 5.2.3 Costs 22 5.2.4 Accident protection, side effects on lakes and rivers 23 5.3 Ozonisation 23 5.3.1 Fundamentals 24 5.3.2 Design and operation 24 5.3.3 Costs 25 5.3.4 Accident protection, side effects on lakes and rivers 25 5.4 Chlorination 26 5.4.1 Fundamentals 26 5.4.2 Design and operation 27 5.4.3 Costs 27 5.4.4 Accident protection, side effects on lakes and rivers 27 5.5 Other procedures 28

6. Conclusions and recommendations 29

7. Bibliography 31

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Notes for Users This ATV Advisory Leaflet is the result of honorary, technical-scientific/economic collaboration which has been achieved in accordance with the principles applicable for this (statutes, Rules of Procedure of the ATV and ATV Standard ATV-A 400). For this, according to precedents, there exists an actual presumption that it is textually and technically correct and also generally recognised.

The application of this Advisory Leaflet is open to everyone. However, an obligation for application can arise from legal or administrative regulations, a contract or other legal reason.

This Advisory Leaflet is an important, however, not the sole source of information for correct solutions. With its application no one avoids responsibility for his own action or for the correct application in specific cases; this applies in particular for the correct handling of the margins described in the Advisory Leaflet.

Foreword The ATV Guide ATV-H 205 "Disinfection of Wastewater" was produced in 1993. The hygienic properties of surface waters have, for some years, been discussed in public and within the specialist world, inter alia together with the EU Directive on the quality of bathing waters [9]. Whether a disinfection comes into question is to be considered carefully. The success achievable is determined by the respective formulation of the problem and is dependent on the local constraints such as the status, usage and course of the body of water. The influence of other sources of loading on the body of water in addition to the discharge of wastewater must be included in the decision. Therefore, the ATV Guide has been transformed into a revised and supplemented edition in the form of this ATV Advisory Leaflet ATV-M 205E "Disinfection of Biologically Treated Wastewater".

1. Introduction Communal wastewater, even after biological and advanced treatment, still contain a large number of micro-organisms including pathogenic agents (bacteria, viruses, parasites). The majority of the pathogenic agents originate from the digestive tract of humans and animals. Surface waters are loaded both in point form through wastewater discharges from sewage treatment plants as well as diffusely, in particular through discharges from scattered settlement areas, and flooding of agriculturally used areas, as well as through animals in and around the body of water.

Static inland waters can, for example, be kept relatively free of wastewater through ring sewer systems. Diffuse loadings can be avoided by using deliberate measures, for example, protective strips in river bank areas. With flowing waters and in coastal areas these preventative measures can be used to a limited extent only.

Disinfection of wastewater has the task to so reduce the pathogenic agent through removal, destruction or deactivation, that a health hazard through the discharge of wastewater into a lake or river is no longer to be feared. The hazard is dependent on the type of usage of the processed wastewater or the body of water into which the water is discharged. Disinfection processes may not negatively change the characteristics of the treated water.

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In Germany, for the discharge of wastewater into a lake or river or into the sewer system, there are no limits or guidance values laid down for bacteria, viruses or protozoa. A disinfection or sterilisation is laid down or recommended for raw wastewater from the following source areas:

− genetic engineering plants in accordance with the Genetic Engineering Safety Ordinance [1],

− animal cadaver disposal in accordance with the Animal Cadaver Disposal Site Ordinance [2],

− tanneries in accordance with the German Federal Health Office (BGA) lists of approved and recognised disinfectants and disinfection procedures [3],

− (highly) infectious hospitals and sanatorium areas such as tuberculosis sanatoria, special isolation wards, infectious disease hospitals, special experimental animal farms and similar in accordance with the BGA advisory leaflet on the discharge of hospital wastewater [4], with DIN 19520 on the treatment of wastewater from hospitals [5] and from the BGA directive for hospital hygiene and prevention of infection [6].

The treatment of wastewater will not be dealt with further but only the disinfection of communal wastewater following biological treatment. This is, above all, very common in North America.

A disinfection of biologically treated communal wastewater can be sensible if an increased risk of the transfer of the pathogenic agent exists through the discharge of wastewater. Above all, an increased risk is to be feared with bathing waters. As the diffuse loading in lakes and rivers cannot be dealt with through the disinfection of wastewater alone the effectiveness for the body of water is to be decided in each individual case.

The employment of wastewater disinfection also comes into consideration with the processing of a partial flow of biologically treated wastewater as process water.

This Advisory Leaflet is to provide the necessary bases and specialist information for the possibilities and limitations for decisions on the employment of wastewater disinfection and the associated disinfection technique for biologically treated wastewater.

2. Terms and Fundamentals of Wastewater Disinfection With wastewater disinfection the term disinfection is not employed in the strict medical sense as "destruction or deactivation of all pathogenic micro-organisms" but rather as "treatment of wastewater or sludge to reduce the activity of pathogenic agents below a certain laiddown value" [8]. The reduction can also be effected by retention. Sterilisation means the "destruction or removal of all micro-organisms including their permanent forms" (thermal or filtration method). The terms decontamination and hygienisation are not specific and should no longer be used.

The selection of the disinfection process is based on the properties of the wastewater (in particular characterised by suspended, dissolved organic and oxidisable substances, pH value, temperature, coloration, transmission), according to the type of pathogenic agent

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which is to be expected in the wastewater and according to the utilisation of the treated wastewater. The destruction or deactivation rates are influenced essentially by the following factors:

− organic loading, suspended solids and turbidity of the wastewater, − initial concentration, type and characteristics of the micro-organisms, − type and characteristics of the disinfectant or disinfection process, − concentration of the disinfectant or intensity of radiation and reaction time.

For the disinfection results, respectively the product of the reaction time and concentration or the intensity of the disinfectant as well as the evenness of treatment are decisive. With thermal disinfection, temperature and reaction time, for the chemical process the concentration and the reaction time and with UV irradiation the intensity of the radiation and the retention time are important for the results. What is important is that every wastewater particle is treated with the same intensity and same reaction time. With membrane filtration the pore width of the membrane and the pressure difference for the retention of the micro-organisms are crucial.

A statement on the effectiveness of wastewater disinfection is difficult and only possible through microbiological verification. As a rule, with sewage treatment plant effluents, general hygiene parameters such as the so-called faecal indicator bacteria are used for the determination of effectiveness. These serve as proof of faecal pollution in the water as they are always present in large numbers in the intestines of warm blooded mammals and it can therefore be assumed that pathogenic agents can also be present. The following count as faecal indicator bacteria:

− (total) coliform bacteria which give only an indication of a faecal pollution as they do not originate exclusively from the intestines of warm blooded mammals,

− faecal coliform bacteria or escerichia coli (E. Coli), which can be seen as verification of a faecal pollution as they only occur in the intestines of warm blooded mammals frequently also

− faecal eneterocochen, which equally represent a verification of faecal pollution.

Under certain circumstances the verification of pathogenic bacteria (salmonella) and enteroviruses is required. Coliphagen can also be employed for the verification of the deactivation of intestinal viruses.

The effectiveness of wastewater disinfection is to be verified through the operation of the process technology employed in accordance with the intended application, and is to be examined at the sewage treatment plant effluent before admixture with the body of water. The concentration of certain indicator organisms in the inflow is determined immediately before and after the disinfection facility for the assessment of the reduction effect. The reduction rate is given, as a rule, logarithmically or in powers of ten and is important for the evaluation of the efficiency of a disinfection process. Fundamentally, however, it should not be only the reduction rate achieved but also always the achieved number following treatment which should be given, as only these are decisive for the assessment of the treated water.

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3. Health Hazards through Wastewater Discharges into Lakes and Rivers

Infectious diseases can be spread by wastewater. Some pathogens, which can occur in wastewater, as well as the infection hazards resulting from these and infectious diseases are listed in Table 3.1.

Table 3.1: Some human and animal pathogens which can cause illnesses, directly or indirectly, via wastewater

Type of pathogenic agent Illnesses Viruses

Polio viruses Cocsackievirus A,B ECHO viruses Hepatitis A

Meningits, Infantile paralysis Meningitis eczema Meningitis, diarrhoea Epidemic hepatitis

Bacteria Salmonella typhi Enteric salmonella i.a. Shigella sp Enteropathogenic Escorichia coli Yersinia species Pseudomonas aeruginosa Vibrio cholerae Campylobacter jejuni Leptospira species Listeria monocytogenes Francisella tularensis Bacillus anthracis Clostridium botulinum Mycobacterium species Chlamydia trachomatis

Typhus Enteric salmonella Bacterial dysentery Enteritis, enterotoxins Enteritis Dermatitis, otitis Cholera Enteritis Leptospirosis Listeriosis Tularaemia Anthrax Botulism, Gas gangrene Skin ulceration tuberculosis Conjunctivitis, trachoma

Protozoa Entamoeba histolytica Giardia lambia Cryptosporidium species

Amoebic dysentery Lambliasis Cryptosporidosis

Worms Ascaris lumbricoides Taenia species

Ascarids Tape worms

The presence of pathogens need not lead inevitably to a disease. This first occurs if the pathogenic agents (bacteria, fungi, parasites) or viruses penetrate a micro-organism (e.g. humans) and multiply there. Infectious illnesses can be nonapparent, that is clinically unsymptomatic or proceed together with a manifest illness characterised by clinical symptoms.

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The occurrence of an infectious disease is dependent on:

− the number of germs (different depending on type and method of contraction), − the virulence of the pathogen, − the resistance or the health of the recipient.

The contraction of illnesses through the above given germs as a result of bathing in surface waters is seldom reported. The verification between illness and source of infection is here, however, very difficult. Insofar as microbiological values of the EU Directive on the quality of bathing waters are maintained [9] it is assumed that the epidemic-hygienic hazard is negligibly small. The quality requirements are given in Table 3.2.

Table 3.2: Microbiological quality requirements on bathing waters in accordance with the EC Directive on the Quality of Bathing Waters

Microbiological parameters Guidance/G value Limiting/L value

Total coliform bacteria in 100 ml

500 (80)

10,000 (95)

Faecal coliform bacteria in 100 ml

100 (80)

2,000 (95)

Streptococci faecalis in 100 ml 100 (90) - Salmonella in 1000 ml - 0 (95) Intestinal viruses in 10 ml - 0 (95)

The figures in brackets give the percentages in which the values may not be exceeded.

For the processing of drinking water from surface waters there are Guidance/G Values for total coliform bacteria, faecal coliform bacteria streptococcus faecalis and salmonella laid down in the EU Directive on quality requirements on surface waters for the abstraction of drinking water.

4. Hygienic Loading and Self-cleaning of Lakes and Rivers

In unloaded bodies of water there are bacteria, which belong to the intestinal flora of vertebrates, which are as a rule undetectable. The hygienic loading is caused by point form discharges, for example from wastewater and diffuse inputs from surfaces, e.g. agriculture. In the course of flow they are reduced by the self-cleaning effect of the body of water. Measures for disinfection can only be applied at the point source, for example through the treatment of the sewage treatment plant effluent before discharge into the lake or river. Before taking such measures, the drawing up of a balance as to whether the desired target, e.g. bathing waters quality, can also be achieved in this way, should be used as an aid. In many cases the loading from the diffuse area is so high that technical measures at point sources alone cannot lead to the achievement of the target.

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4.1 Loading Due to Wastewater and the Possibilities for its Reduction

The hygienic loading of lakes and rivers due to wastewater comes mainly from the discharges from communities [13]. These are predominantly point form such as, for example, with collected wastewater from stormwater or combined sewer systems and with communal sewage treatment plants, which respectively are discharged into bodies of water via defined discharge points. In part they are also of a diffuse type such as, for example, with unofficial disposal via small type sewage treatment plants or with widespread scouring of roads and from residential areas which are loaded with animal excretions.

To avoid hygienic loading from diffuses wastewater inputs the connection of all inhabitants to communal wastewater systems is to be sought as the first measure. Widespread scouring of precipitation water should not be percolated directly into a lake or river but rather via active topsoil. As a result of passage through the ground one no longer has to reckon with a loading of lakes and rivers.

With the discharge of collected precipitation water, water loaded with animal excretions is introduced into a lake or river, usually without treatment, intermittently and in surges. It is similar for the relief of combined wastewater systems, via stormwater overflows. Here the hygienic loading through human pathogenic agents is higher. In both cases technical measures for wastewater disinfection are barely feasible. The best result is potentially a modification of the drainage area with the aim of separating all the slightly loaded stormwater and percolate it over a wide area. Otherwise, and supplementary to this, only partial success can be achieved using settling and retention facilities.

During the course of the treatment process in the sewage treatment plan there is a reduction of the bacteria, depending on the effectiveness of the individual process stages (Fig. 4.1). However, taking into account the processes of N and P elimination as well as filtration or polishing ponds, this is often insufficient for an effective relief [14]. Viruses are reduced by a factor of some two powers of ten [15] through adsorption in the sludge. Worm parasites and/or their permanent form are increased by 70 to 90 % in the sludge of the mechanical stage; the rest, to a great extent arrives in the surplus sludge. The disinfection of wastewater in sewage treatment plants is only sensible following extensive biological wastewater treatment and can, in certain cases contribute to a significant relief of lakes and rivers.

4.2 Diffuse Loading and the Possibility of Its Reduction

The diffuse loading of lakes and rivers originates mainly from the agricultural side. The main input takes place through scouring of liquid manures and sewage sludge from arable areas as well as from animal grazing and manure heaps in the area of the banks of lakes and rivers. Flooding contributes to this if agriculturally utilised bank regions are swamped or loaded sediment is stirred up in the lake or river. For an effective reduction the edge areas of lakes and rivers and flooding areas should be kept free from agricultural utilisation, that is from organic inputs.

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Fig. 4.1: Content of faecal coliform bacteria in raw wastewater and inthe effleunts of individual treatment stages of a communal sewage treatment plant

Salmonella and other pathogens can be introduced directly into the water via the excrement of water birds. The congregation of birds at certain locations can be avoided by the banning of feeding.

The adsorption of micro-organisms in the sediment leads first to a reduction of the hygienic loading of areas of free water but, however, not to the dying off of the former. In the sediment in which the organism can survive for a long time, they increase. Through flooding or heavy turbulence they can again get into the free water zone.

4.3 Self-cleaning of Lakes and Rivers

In the lake or river the majority of pathogenic agents, as locally foreign micro-organisms, are confronted with extreme conditions. Growth takes place only in exceptional cases [18]. The half-life periods of the mortality rates have a large variation from several hours up to weeks [19]. Among the most important influencing factors are temperature, pH value, turbidity, range of nutrients, osmotic environmental conditions, irradiation taking into account water depth and shadow effect, grazing and competition from local types of micro-organism. Hydraulic engineering measures such as channelling, banking and water diversion can reduce the self-cleaning force.

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Overall the number of faecal indicator bacteria - and thus also pathogenic organisms - barely decrease in the majority of German waters in the course of their flow as, due to the dense population and agricultural utilisation up to the edges of the banks, the mortality rate is compensated by a permanent resupply through renewed wastewater discharges and surface flooding [20].

5. Methods of Wastewater Disinfection For the disinfection of wastewater there are various procedures available on a physical or chemical basis. The most important are:

Physical procedures Thermal treatment UV irradiation Membrane filtration

Chemical procedures Ozoning Chlorination using chlorine and chlorine separating compounds or chlorine dioxide Employment of peracetic acid or hydrogen peroxide

According to practical experience (in part only with semi-industrial plants) as well as for economic and environmental reasons, according to Table 5.1 only a few procedures can be considered for application with biologically treated wastewater before the outlet of the sewage treatment plant [21].

Table 5.1: Common procedures of wastewater disinfection

Procedure Disinfection effect

Operational experience

Environmental compatibility

Costs DM/m3

UV irradiation + ++ + 0.05 - 0.10

Membrane filtration

++ - ++ 0.40 - 1.601)

Ozoning + + - 0.10 - 0.35

Chlorination2) ++ ++ - 0.08 - 0.12

1) In addition other economic advantages can be achieved in the operation of the sewage treatment plant 2) Only to be used in emergency

5.1 UV Irradiation

The disinfection effect of UV-C rays (wavelength range 200 to 280 nm) in water have already been used since the beginning of the 20th century for the disinfection of drinking and process water. Basic fundamentals and information on this are contained in the DVGW [German Association for Gas and Water] and FIGAWA [Technical Association of Firms in the Field of Gas and Water] Standards [22],[23],[24]. Since the middle of the seventies UV irradiation has been used more and more for the disinfection of wastewater. In North America there are several hundred plants in operation [25] which, in part, have already been disinfecting biologically treated wastewater with a throughflow of

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10 m3/h to 16000 m3/h and also increasingly replace existing chlorination plants. The plants up until 1996 in Germany at Cuxhaven, Cismar, Norden and Wyk on the island of Föhr for the disinfection of sewage treatment plant discharges, are UV irradiation plants. On several sewage treatment plants a partial flow of the effluent is disinfected using UV irradiation plants for the production of safe process water.

5.1.1 Fundamentals The effectiveness of UV irradiation depends on the sensitivity of the micro-organisms to be deactivated. In the wastewater it is further reduced through UV absorbent substances and the type and quantity of suspended solids as well as other chemical-physical properties of the wastewater, in particular the content of humic acids, iron and manganese compounds. As measure for the UV-C transparency of the water is the transmission measured with a layer thickness of 1 cm and wavelength of 254 nm. A reduction of the UV radiation is also effected by films which form during operation on the water side surface of the silica glass jacket tubes of UV radiators. They must be removed regularly. Type and intensity of the mineral and organic (biofilm) film formation depends on the respective local conditions, whereby the content and form of iron and manganese compounds play a role.

The effect of the UV irradiation comes about in that the energy rich rays effect changes in the structure of the nucleic acids on absorption into the core of the cells (dimer formation). The changes lead to a loss in the reproductive capability of the cells, inasmuch as a level is achieved such that the capability of repair of the cells is overloaded. Most effective is the radiation using a wave length of 260 nm, the absorption maximum of nucleic acids. Medium pressure radiators cover a wide UV spectrum in particular in the range 200 to 280 nm. The radiation produced from low pressure mercury radiators with a wave length of 254 nm very closely approaches the absorption maximum. Outside the optimum wave length range the biocide effect of the UV radiation reduces rapidly.

The ability to repair damage caused by UV radiation is characterised differently with the various strains of micro-organism. Numbered among the UV sensitive types are the gram-negative bacteria (e.g. coliform bacteria, salmonella), among the lesser sensitive the gram-positive bacteria (e.g. staphylococci, enterococci). Consequently, following UV irradiation, there is a change of the remaining biocoenosis (floral displacement). Fungi and spores have the highest UV resistance. Viruses are to be graded between gram-negative and gram-positive bacteria. With the cell's own repair mechanisms there is a differentiation between photo and dark reactivation, depending on whether the repair processes are induced by light or not. According to investigations a maximum of an increase of the tracer microbial balances by ca. a power of ten is to be expected with favourable boundary conditions.

The more hits a cell has through UV quanten the greater is the probability of an irreversible deactivation of the cell. The number of hits per cell is dependent on the irradiation in J/m2 (DIN 5031, Part 1,previously the exposure dose), which every cell receives with passage through the irradiation space [27]. As the flow in the irradiation space and the irradiation strengths (Fig 5.1) is unevenly distributed, the actual irradiation which a cell receives can only be estimated by calculation.

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A sufficient inactivation only occurs if both the free as well as all micro-organisms which are bonded with an agglomerate or solid particle are irreversibly damaged. For this a significantly higher volumetric irradiation is required. Due to the exponential relationship between the inactivation and irradiation an as ideal as possible plug flow and ideal cross mixture are important. Short circuit flows of up to 1.0 % can place the results in doubt. The determination of the actual effectiveness of an UV irradiation plant, in practice, is only possible experimentally [29]. This is best and most accurate through determination using biodosimetry [23]. With this the relationship of radiation, throughput and reduction are determined for a test organism with known and suitable UV sensitivity.

Fig. 5.1: Distribution of the irradiation strengths in the area of four UV radiators arranged in a square [28]

According to investigations on trials and practice plants with low pressure radiators [28],[30],[31],[32] the minimum irradiation was some 300 to 450 J/m2 for the certain maintenance of the limiting and guidance values of the EC Bathing Watesr Directive for biologically treated wastewater with a content of filterable solids of 5 to 20 mg/l. With a transmission between 55 % and 60 % in the biologically treated water the electrical energy consumption is then some 40 Wh/m3 of wastewater and the electrical radiator performance at some 0.15 kW/(l/s) throughput [33],[34].

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5.1.2 Design and Operation Technically, UV radiation is produced through low pressure or medium pressure mercury vapour discharge lamps. Formation of ozone is prevented by a suitable doping of the quartz glass mantle of the radiator. In Fig. 5.2 the spectral emission bands of both radiator types are compared with the spectral effect curve for the inactivation of micro-organisms. The most important characteristics and properties of low and medium pressure radiators are given in Table 5.2. For some time radiator types have also been on offer which are to be classified between low and medium pressure radiators.

Fig. 5.2: Spectral emissions from mercury low pressure and medium pressure radiators and the spectral effect curve for the inactivation of micro-organisms [2]

Table 5.2: Characteristics and properties of low and medium power UV radiators

Characteristics and properties Low pressure radiators

Medium pressure radiators

Mercury vapour pressure [bar] 0.001 1 - 20

Surface temperature [°C] 40 - 100 600 - 900

Radiation in the UV range

Wave lengths [nm]

monochromatic

254

broadband

200 - 400

Yield in the UV-C range (200 - 280 nm):referred to the electrical power [%]],referred to the length of radiator [W/cm]

30 - 40

0.2 - 0.7

12 - 15 4 - 15

Power drop over the life [%] 30 - 40 25 - 40

Life (h) 8000 - 15000 3000 - 8000

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In practice predominantly low pressure radiators are employed. Referred to the electrical energy consumption they have a high UV-C yield. The low surface temperatures are favourable with regard to the danger of the formation of coatings. As a rule, as a result of the small power density, many radiators are required. With the employment of medium pressure radiators the number of radiators can be reduced by a factor of 10 to 20 with more compact construction, due to the higher power density. The lesser UV-C yield, however, leads to a significantly higher energy consumption. The higher surface temperature increases the danger of the formation of coating and, as a rule, requires automatic cleaning systems, which are also already on offer in a combination of mechanical and chemical cleaning. Recently in North America, where electricity costs are low, medium pressure radiators are being increasingly installed with automatic cleaning systems which have been developed from designs for the disinfection of combined wastewater overflows.

Wastewater, before irradiation, should be extensively biologically treated, be fed to the irradiation plant free of air bubbles and have a low concentration of filterable solids (< 20 mg/l, better, for example following filtration, < 5 mg/l).

Fig. 5.3: Sketch of the principle of UV irradiation systems [60]

Primarily open channel UV irradiation systems with low pressure radiators are used for wastewater disinfection (Fig. 5.3). Apart from these, closed systems with completely filled cross-sections - mainly with medium pressure radiators - are also employed. With open channel systems one is concerned with channels with gravity flow in which the UV radiators are arranged in a tight grid and which are operated with a constant water level using suitable discharge facilities. Closed UV irradiation systems with built-in submerged radiators or a central quartz glass tube radiated from outside are more often employed for small flow volumes, e.g. for the processing of partial flows of sewage treatment plant effluent to obtain process water.

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Fig. 5.4: Open channel UV irradiation system with horizontal radiator arrangement (schematic)

Fig. 5.4 shows, as an example, the layout of an open channel system in the most frequently installed method, with horizontally arranged UV low pressure radiators, parallel to the direction to the flow. Essentially the following designs and information refer to this. 2 to 30 radiators are arranged parallel to each other in removable modules. With plants with radiators which are transversely or vertically flowed past, attention is in particular to be paid that no hydraulically unfavourable back-up or eddy zones are formed through which the irradiation becomes too uneven. The inlet structure must ensure an as even, calm and bubble free flow as is possible. The outlet structure, here a shutter tail weir, must provide for a constant water level in the irradiation channel.

For a throughput of some 1000 m3/h an area of ca. 6 m x 20 m is required. Ca. 400 x 100 watt radiators with each 20 to 25 W UV-C output at 254 nm can be installed in the channel. With this the transmission of biologically treated wastewater lies between 50 and 60 % referred to a 1 cm layer thickness. The dimensions of the channel are ca. 1.2 m depth, 1.3 m width and 20 m length. The hydraulic loss is some 0.5 m to 1.0 m. Control system and facilities for cleaning and maintenance of the UV module are, as a rule, accommodated on both sides of the channel.

Before installing an UV irradiation system investigations on a semi-industrial scale using the subject wastewater over a long period for the determination of the basic data for the operational plant and the operational constraints (transmission, filterable solids, hydraulics etc.) and the operational constraints, are recommended. With transmission values below 55 - 50 % first the cause of the low values should be ascertained and corrected otherwise no economic operation of the UV irradiation can be expected. An improvement of transmission of 25 % effects a halving of the necessary irradiation power. Already with forecast filterable solids of the order of 10 - 20 mg/l, it is to be examined whether a filtration of the wastewater beforehand is not necessary for a certain reduction. The design of the UV irradiation systems is carried out by the manufacturers

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taking into account previous investigations, the properties of the wastewater and the required reduction rate. Method of operation and ease of handling of systems offered should always be checked against a reference system, in particular with regard to cleaning and exchange of radiators.

Notes on the layout, on the offer and on the acceptance of the system:

− The determination of the process and design-dependent minimum volumetric irradiation takes place with the consideration of the maximum volume flow, of the minimum UV transmission, of the maximum anticipated content of suspended solids, of the required reduction and of the maximum content of indicator organisms to be maintained, with confidence, in the effluent.

− It is to verified that, for the required reduction, a satisfactory irradiation by the UV irradiation system in the wastewater which is to be treated and an even throughflow can be ensured with all operating conditions. For this a diagram of the distribution of volume irradiation strength and a verifiable calculation of the mean volumetric irradiation [35],[28] as well as a retention time study are to be presented with the offer. In addition, a verification of the disinfection potential of the system using biodosimetry based on the DVGW or EPA process should be presented.

− The service lives of the UV radiators including the switching procedures, the UV performance of the radiator at the end of the service life and the associated energy consumption of the radiators per m3 of wastewater throughput through the UV system are to be given and guaranteed In the suppliers offer. Reference sources, prices and verification on the long-term availability for UV radiators are to be given.

− The required equipment and facilities for cleaning together with a description of the necessary cleaning expenditure and the cleaning procedure as well as the anticipated costs for cleaning should be contained in the supplier's offer

− Before acceptance of the system the required reduction in the wastewater is to be verified using microbiological examination and, at least with larger systems, it is recommended that an examination for even flow is undertaken in accordance with EPA using tracer measurement [8].

Notes on design and equipping:

− With heavy variations in throughflow, but always with flows greater than 1000 m3/h, the distribution of the total flow into parallel channels is recommended.

− Systems are to be so equipped that that they remain fully functional with the expected weather and climatic conditions.

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− The UV modules should be stable and capable of being installed and removed by operational personnel without special aids, be capable of being submerged in water and diluted acids and ensure a sufficient protection of installed radiators against damage. The quartz glass envelope tubes and the UV radiators should be easily accessible for assembly and dismantling as well as for manual cleaning.

− All electrical installations and connections must be easily accessible and meet the operational conditions in wet and damp environments. They are to be of IP 67.

− Switching equipment is preferably to be made of IP 67 or be protected against sprayed water.

Notes on measurement and control technology:

− The flow to be treated in the irradiation system is to be continuously recorded and documented.

− With larger systems a matching to the flow conditions is to take place by switching on and off the irradiation channels or of the radiators or, with medium pressure radiators, by regulation.

− The water level in the irradiation channels must, in particular with horizontally arranged radiators, be regulated accurately (+ 1 cm), rapidly and without backing up and an even discharge has to be achieved over the complete cross-section of the irradiation channel. Rapid reaction, electromechanical discharge devices and shutter tail weirs have proved themselves in practice. On falling below a minimum water level the system is to be shut off for safety reasons.

− The UV radiators are to be equipped with running time meters, on/off counters and electronic chokes in accordance with the status of technology. As required, automatic emergency cut-offs as well as fault and alarm indicators (e.g. for radiator or modulator failure) have to be provided, which document date, time and nature of the fault.

− For each irradiation channel at least one UV sensor for the measurement of the irradiation level with lower alarm limit is to be installed for continuous monitoring of operation. It also serves for the observation of the formation of deposits and indicates the necessity for cleaning the radiators. Following on-site investigations an area with low irradiation level and unfavourable flow conditions is to be established as installation point. If successive irradiation spaces are to be activated and deactivated dependent on the flow, at least two UV sensors are necessary, one in the permanently active area and one in the most frequently inactive area.

− Sampling points are to be installed before and after each irradiation line for microbiological examination, which allow a representative sampling in a thoroughly mixed area.

Notes on the formation of deposits and the cleaning of radiators:

− In addition to failure and ageing of the radiators the formation of deposits on the quartz envelope tubes are responsible for the lowering of the UV irradiation level during operation. They can, dependent on the properties of the wastewater, be very different

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and are influenced by the type of flocculant used for phosphorous elimination [36]. The formation of deposits is therefore to be checked regularly. Cleaning is to be carried out when there is a falling below the determined sensor value, depending on the composition of the wastewater, approximately every 14 days to 3 months.

− According to previous experience an effective cleaning is only possible with diluted acid [37]. Ultrasonic cleaning systems have not proved themselves in practice. The formation of deposits can be delayed using mechanical wiper systems. There is no long-term experience available for mechanical/chemical cleaning systems.

− With small and medium sized systems it has proved to be practical for periodic chemical cleaning, to remove individual UV modules from the irradiation channels, to place these for some 5 to 10 minutes in a tank with diluted acid, finally to rinse them clean and replace them in the channel. With larger systems with over 100 UV modules it can be practical to lift out and clean a complete irradiation field all at one time from the channel with the aid of a crane. For each 1000 m3/h throughput performance three hours work can be estimated for the cleaning of the UV radiators without using a crane, one hour, assuming an operator friendly design.

5.1.3 Costs The costs for UV irradiation, without water filtration with favourable conditions (high transmission of some 60 %, low deposit formation) are some 0.05 DM/m3 of wastewater. With unfavourable conditions or with expensive equipment it can rise to over 0.10 DM/m3 of wastewater. With low pressure radiators it is made up of 50 % each capital costs and operating costs. Half the operating costs arise from power consumption, the other half from maintenance expenses including radiator replacement costs. With medium pressure radiators higher electrical costs arise with lower investment and radiator replacement costs.

5.1.4 Accident Protection, Side Effects on Lakes and Rivers With UV irradiation, secondary effects in the form of depot effects are excluded. With the irradiation levels which are employed up until now no formation of undesirable compounds or bacterial mutants have been observed [31]. It has not been possible to verify any toxic degradation products with the various investigations in the field of drinking water [38].

UV rays can lead to damage of eyes and skin. With regards to safety precautions [39], [40] the UV systems must be so constructed that, in operation, no radiation can escape from the systems. The UV irradiation zones must therefore be made lightproof. Appropriate warning signs are to be fixed clearly on the system. Cleaning and maintenance tasks are to be undertaken with the UV radiators switched off.

For the cleaning of radiators as far as possible organic acids are to be used which can be disposed of in an environmentally friendly manner. Used UV radiators can be disposed of together with fluorescent tubes. As they contain mercury they do not belong in domestic rubbish.

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5.2 Membrane Filtration

Micro/ultrafiltration (Table 5.3) offers a very extensive retention of bacteria and solids. Due to the absorptive bonding of viruses to solids a reduction of the number of viruses can be achieved.

Table 5.3: Membrane procedures, separation limits and retained components

Reverse osmosis Nanofiltration Ultrafiltration Microfiltration

Separation limit 0.1 - 10 nm 1 - 10 nm 0.005 - 0.2 µm 0.1 - 5 µm

Pressure range 7 - 120 bar 5 - 40 bar 1 - 10 bar overpressure procedure: 0.5 - 5 bar

vacuum procedure:0.1 - 0.9 bar

Retained components

molecules ions,

viruses, molecules

colloids, macromolecules,viruses

particles, bacteria, colloids

The process technology of membrane filtration is already part of the status of technology for the treatment of certain wastewater (e.g. landfill leachate). For the disinfection of communal wastewater micro/ultrafiltration has until now only been applied occasionally abroad for small capacities (ca. 1000 m3/d) [41],[42],[43] and for this it is still in development [44],[45]. This applies for both membrane technology as well as for the associated mechanical, measurement and control technologies. Thus it still does not come into consideration for a wider application in practice. Should it be considered pretrials should always be carried out.

5.2.1 Fundamentals

With micro/ultrafiltration the permeate of the wastewater to be treated flows through a membrane as a result of overpressure or vacuum. For the aim of wastewater disinfection a medium pore separation of 0.2 µm is sufficient. Particles which are greater than the pore diameter are retained and accumulate in front of the membrane. During operation a layer forms on the membrane, made up from the retained particles, which reinforces the filtration effect through adherence to the permeate flow but, after a certain time, it is so limiting that at certain time intervals or having achieved a set pressure loss it has to be removed by flushing. In continuous operation deposits form on the membrane which lead to blockages due to colloids, micro-organisms and metal oxides. Furthermore, the formation of biolfilms on the pure water side, for example in the area of faults, cannot be excluded. These deposits must be removed regularly using chemical cleaners (peroxide, acids or alkalines) using an automatic programme following the switching off of individual modules or lines.

Dead-end filtration (static filtration and cross-flow filtration (dynamic filtration) are differentiated (Fig. 5.5). With dead-end filtration the wastewater is fed vertically to the membrane. The covering layer grows rapidly. The membrane, after relatively short operating intervals, has to be flushed. With cross-flow filtration there are two main streams, the permeate flow through the membrane and the overflow parallel to the

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membrane. The overflow delays the formation of the covering layer. The intervals between flushing are longer.

Fig. 5.5: Schematic principle of dead-end and cross-flow filtration

The permeate flow through the membrane is dependent on the pressure difference and the overflow velocity which, dependent on the procedure, can be applied as overpressure or vacuum. The vacuum procedure takes place in cross-flow operation, the overpressure procedure both in dead-end as well as cross-flow operation.

For all methods of micro/ultrafiltration a good mechanical precleaning already before the biological treatment of the wastewater is important, for example using a fine sieve of ca. 1 mm gap width in order reliably to prevent damage to the membrane surface. The performance of the micro/ultrafiltration must be continuously harmonised with the wastewater inflow to the sewage treatment plant. This takes place via a measurement and regulation system, first by the matching of the throughflow and subsequently by switching in and out individual modules or also via an upstream buffer reservoir.

5.2.2 Design and Operation Both variants of vacuum and overpressure filtration can be applied both combined with the biological part of the activated sludge process as well as with the secondary settling stage with all biological treatment processes (Fig. 5.6).

Fig. 5.6: Possible arrangement of the micro/ultrafiltration system in aeration plants

Micro/ultrafiltration in combination with the biological part with the activated sludge process

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With the application of micro/ultrafiltration immediately together with the biological reactor, sludge concentrations of activated sludge can be achieved up to over 20 g/l. A secondary settlement using sludge return is dispensed with. There are savings in tank volume compared with normal biological reactors which, however, are in part compensated by possibly necessary upstream buffer storage. To the following questions with this procedure there is still no satisfactory operational experience available: enrichment effect in the biological reactor and its effect on the activated sludge, characteristics and the processing capability of the surplus sludge, influences on the biological P and N elimination through reduced contact times, applicability of precipitation and flocculation agents, for example for phosphorus elimination, operation with stormwater flow conditions.

With the vacuum procedure, submerged hollow fibre or plate modules are applied directly in the biological reactor. The following operational parameters are laid down [42],[43]. With a vacuum of 0.1 to 0.9 bar a permeate flow of 20 to 50 l/(m2 . h) is possible. The cross-flow effect is created by pressure aeration in the area of the module, which can be used simultaneously for oxygen supply. Electrical energy of 0.1 to 0.7 kWh/m3 of permeate is needed depending on the procedure (creation of vacuum, flushing, pressure aeration for cross-flow operation). The vacuum procedure directly behind the biological reactor using pipe or plate modules and cross-flow filtration [46] delivers a permeate flow of 20 to 300 l/(m2 . h) with an applied pressure of 1 to 10 bar. The energy requirement, based on the procedure, is 2 to 7 kWh/m3 of permeate. Sieving with a mesh of 50 to 500 µm is recommended.

Micro/ultrafiltration combined with secondary settlement

With micro/ultrafiltration combined with secondary settlement is only sensible with external operation. With large flow volumes with small solids loading (filterable solids < 20 mg) dead-end filtration according to the overpressure procedure is particularly useful [44],[41]. According to manufacturer's details, the procedure delivers permeate flows of 40 to 100 l/(m2 . h) with a pressure of 0.5 to 3 bar and a procedure dependent energy requirement of 0.2 to 0.4 kWh/m3 of permeate. Long-term operational results have resulted in permeate flows of 70 l/(m2 . h) with a pressure of 0.5 to 1.5 bar and an energy requirement of 0.25 kWh/m3 of permeate [44].

Micro/ultrafiltration, including disinfection of the wastewater, is proposed as a possible alternative in order to rehabilitate sewage treatment plants with insufficient biological reactor volume.

5.2.3 Costs Overall, according to estimates, considerable investment and operational costs have to be applied for micro/ultrafiltration. All cost estimates and cost comparisons must always include all elements of wastewater treatment in the sewage treatment plant due to the mutual dependencies. A higher overall economic efficiency is possible if, in addition to the disinfection effect even further treatment aims are realised; however, up until now, there has been hardly any industrial scale experience:

− with micro/ultrafiltration combined with the biological part following the biological reactor economical advantages can result as one can dispense with a secondary settling stage and savings through smaller biological reactor volume are possible.

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− If, through micro/ultrafiltration not only a reduction but also a deliberate P removal to below 0.2 mg/litre Ptot [44] is sought, the employment of micro/ultrafiltration combined with secondary settlement as alternative to the process combination of flocculation filtration and wastewater disinfection using UV irradiation can be cost effective.

As procedure dependent costs, taking into account investment and operational costs, 0.7 to over 1.6 DM/m3 of permeate are quoted by suppliers for micro/ultrafiltration combined with the biological filter. For micro/ultrafiltration combined with secondary settlement 0.4 to over 1.0 DM/m3 of permeate are to be applied. With internal operation in cross-flow and external operation in dead-end operation, the capital cost are the greater (over 50 %). With external operation in cross-flow energy costs make up the main share of ca. 50 %.

Due to the increasing employment of micro/ultrafiltration and the competition which has appeared in the meantime, it is to be expected that the investment costs will sink. A reliable consideration of all costs, such as chemical treatment, use of membranes, pre-treatment and treatment of concentrates in the sewage treatment plant is first possible following the conclusion of currently running research projects and the first industrial scale practical experience.

5.2.4 Accident Protection, Side Effects on Lakes and Rivers Due to the purely physical separation of the micro-organisms using micro/ultrafiltration the appearance of toxic and undesirable side products during filtration can be excluded. However, account is to be taken of the certain requirements (e.g. chlorine free, EDTA and NTA free) placed on the chemicals used for chemical treatment. Furthermore, account is to be taken that the micro-organisms retained in the activated sludge of the sewage treatment plant become enriched.

5.3 Ozonisation

Some 100 years ago ozonisation was used for the first time in the Netherlands for the processing of drinking water and since then has had a wide propagation in this field. In 1986 there were well over 1000 ozone drinking water processing plants [28]. Following the turn of the century ozone was used mainly and primarily for the disinfection of drinking water. Today's plants are mainly employed for the removal of organic content substances, particularly of odour and taste intensive substances.

Ozonisation was first employed for the treatment of wastewater some 15 years ago, mainly in the USA and Canada. There, in 1986, there were over 40 plants. Within the Federal Republic of Germany there are currently no industrial scale plants for the ozonisation of biologically treated wastewater. However, the procedure has, since the mid-seventies, already been investigated in various trials and pilot plants with regard to the disinfection performance [48],[49],]50],[51], partly also in combination with UV irradiation [31]. Occasionally ozone is employed to remove colour from wastewater, for example from the textile industry.

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5.3.1 Fundamentals The characteristic property of ozone is its extreme oxidation capacity; the latter lies considerably higher than that of chlorine. The effectiveness of ozonisation depends on the dosing, the reaction time, the organic preloading of the water to be treated and the respective pH value. With high pH values the ozone disintegrates more quickly than at low values which has the result of reduction of the disinfection effect [52].

5.3.2 Design and Operation Ozone is produced endothermically. The reaction product formed disintegrates rapidly into molecular and atomic oxygen. Due to its instability ozone cannot be filled and transported and is therefore to be produced on site. For the economical production of larger quantities of ozone silent electrical discharge is used. An ozone plant (Fig. 5.7) consists of an ozone production plant, a mixing facility, a reaction container and a residual ozone removal plant [53].

Fig. 5.7: Ozone plant for wastewater disinfection

Ozone is produced either from dust-free, dried atmospheric air or from pure oxygen under the effect of high voltage electrical energy in the ozone reactor (Siemens ozonisator). The necessary cooling is by using air or water. Prerequisite for a yield of 40 g/m3 with the employment of air and 80 - 100 g/m3 using industrial oxygen is a thorough drying of the gas employed. According to details from the firms the ozone yield when using pure oxygen under favourable conditions is ca. 15 % of the oxygen input. For 1 g of ozone some 6 Wh to 15 Wh of electrical energy is required with oxygen and 10 Wh to 30 Wh with atmospheric air.

The ozone transfer into water is dependent on the transfer system, on the injection pressure and on the characteristics and temperature of the water. The effectiveness drops with increasing water temperature and reducing injection pressure. The ozone can be added either in the main stream or fed, via an injector, into a subsidiary stream. The mixture of the ozonic water with untreated water then takes place subsequently according to the contraflow principle. As a result of the chemical instability of the ozone a surplus breaks down rapidly so that the ozone can be described as having only a very limited "depot effect" with regard to disinfection.

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With regard to the dosage, the reactor time and the desired residual concentration of the disinfection agent the following orientation data can be given for biologically treated wastewater:

Dosage: 5 - 35 g ozone/m3 wastewater: Reaction time: 5 - 30 min Residual ozone content: 0.1 - 1 g ozone/m3 wastewater In trials and pilot plants micro-organisms and virus reduction rates of two to three powers of ten have been achieved [49],[50],[54],[55]. Better efficiency can be achieved through higher ozone dosing or the combination of ozone with UV irradiation, whereby this is associated with an increasing energy and cost expenditure. For the operation of ozone plants the following requirements are to be placed on the erection sites: − the ambient temperature should not exceed 30 °C, the relative humidity 60 %. − the erection site is to be kept free of dust. − aggressive gases and oxidisable contamination should not be present in the vicinity. − ozone resistant materials are to be employed. Residual ozone must be removed from the processed water before introduction into the wastewater. The effectiveness of removal is guaranteed if the ozone concentration in the off-gas is less than 0.02 mg/m3. The most usual residual ozone removal systems are thermal, catalytic or operate via active carbon filters.

5.3.3 Costs As up until now ozonisation for the disinfection of wastewater has not been carried out on an industrial scale, the cost calculations are based on results which have been gathered from the processing of drinking water. In addition to capital services (65.%), the costs for energy and cooling water consumption (25 %) have, in particular, to be taken into account; some 10 % of the total expenditure must be used for maintenance, repair and personnel costs. In accordance with the dosing rates given in Sect. 5.3.2., costs for wastewater disinfection using ozone, are of the order of 0.10 to 0.35 DM/m3 of wastewater.

5.3.4 Accident Protection, Side Effects on Lakes and Rivers In higher concentrations ozone is toxic and has the effect of being an irritant particularly on the mucous membranes of the eyes, nose and lungs. With a longer stay in an atmosphere with ozone contents ≥ 0.2 mg ozone/m3 a cough provocation occurs. From 4 mg ozone/m3 upwards one has to reckon with typical irritant gas poisoning effects. Concentrations of 20 mg/m3 and more can quickly lead to death through a lung oedema. Due to the health hazarding properties of ozone sufficient safety measures (residual ozone destruction, forced ventilation, ozone monitors) are to be provided for work places in the vicinity of ozone plants. The accident prevention regulations of the Professional Association of Gas and Water Works as well as the local accident prevention association are to be observed.

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The following safety precautions are particularly important:

- ozone plants must be sited in securable rooms. According to [57], ozone plants can also be installed in equipment rooms. This applies in particular for vacuum systems. With the presence of working places it must be ensured that the MAK value of 0.2 mg/m3 is not exceeded.

- aeration and ventilation must be ensured and controllable from outside the hazardous area.

- rooms in which, in cases of failure, ozone can escape must be fitted with gas warning equipment.

- ozone plants may only be operated and maintained by personnel who have been instructed on the dangers, safety regulations and measures to be taken with faults.

Although no foreign substances are added through the ozonisation of water and, through the relatively rapid breakdown of the ozone into molecular oxygen, the positive effect of oxygen enrichment occurs, the possibility of the formation of unwanted oxidation products (e.g. epoxides, peroxides) have to be taken into account for the wastewater.

5.4 Chlorination Under assessment of the following statements chlorination of biologically treated wastewater should not be employed due to many disadvantages for lakes and rivers. The chlorine gas process has, nevertheless the advantage that it can be installed rapidly without great resources and thus, in any case is available for emergencies.

5.4.1 Fundamentals For the disinfecting effect of chlorine and its compounds and/or hydrolisation products, it is assumed that essential enzymes of the micro-organisms are inhibited or damaged by the chlorination.

With the application of chlorine today only indirect chlorination is employed, whereby the chlorine solution is produced in situ from chlorine gas and water. The effect is based primarily on the undissociated hypochlorous acid (HOCl). The reaction balance, with high temperatures and high pH values, is displaced in the direction of the less effective OCl ion. In addition, chlorine is also used to a limited extent as a disinfectant in the form of hypochlorite, e.g. sodium hypochlorite (NaOCl, also designated as bleaching lye) or calcium hypochlorite (Ca(OCl2)) or in the form of chlorinated lime (CaCl(OCl)).

Chlorine dioxide must also be produced on site and used immediately as it is explosive in concentrations above 30 % by volume and the solution disintegrates rapidly. "Finished products" are not suitable as chlorine dioxide. The effect of chlorine dioxide is based directly on its heavy oxidative properties, which are considerably stronger than those of hypochlorous acid. Through this the reaction time is reduced to a few minutes. In addition the effect is not so pH dependent and, in the range of pH 6 - 9, is almost constant. For the production of chlorine dioxide two processes are used, the chlorite-chlorine process and the chlorite-hydrochloric acid process. With the chlorite-chlorine process an aqueous solution of chlorine gas and sodium chlorite are simultaneously oxided by chlorine. The advantage of the chlorite-hydrochloric acid process is that the handling of hydrochloric acid is, from a technical process aspect, simpler than the handling of chlorine gas. With this process sodium chlorite is converted into chlorine dioxide using hydrochloric acid.

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5.4.2 Design and Operation Today's normal dosing equipment (Fig. 5.8) functions on the vacuum injection principle [58]. The regulation of the input must take place not only proportionally to quantity but also according to the chlorine consumption (residual chlorine content). Dependent on the content of organic substances in the wastewater, 1 - 20 mg of free chlorine per litre, a thorough admixture and a contact time of 15 - 30 minutes are necessary. A controlled disinfection is, however, only guaranteed if a pH range between pH 6 and 8 is maintained. In the wastewater which flows out of the tank, a surplus of free chlorine of the order of 0.2 mg/l has to be removed, in order to ensure a disinfection with the aid of a dechlorination stage before discharge into a lake or river.

Fig. 5.8: Chlorine gas plant for wastewater disinfection [21]

As chlorine dioxide is not stable as a gas, an aqueous solution with 1 - 3 g chlorine dioxide must be prepared immediately before use. Addition can take place in the throughflow process using the suction-injection system or via membrane pumps. The dosing is quantity proportional dependent on the chlorine dioxide surplus in the water and is dependent on the organic loading of the wastewater and the required reduction. When using treated wastewater as process water in sewage treatment plants, some 5 - 10 g chlorine dioxide per m3 wastewater are necessary for the reduction of faecal indicator bacteria by three powers of ten; with sand filtered wastewater with small residual pollution only 1 - 5 g/m3.

5.4.3 Costs Costs for chlorination plants are not given as, because of the given disadvantages, this procedure is only to be employed in emergencies.

5.4.4 Accident Protection, Side Effects on Lakes and Rivers Strict safety precautions are too be observed with the handling of chlorine. Specially trained personnel are to be employed. In areas in which chlorine is stored or employed these must be signed with warnings and entry is to be controlled. The accident

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prevention regulations of the local accident prevention associations as well as the Gas and Waterworks Social Assurance Agencies are to be observed [59].

The MAK value for the extremely poisonous chlorine gas is 1.5 mg/m3. Chlorine is a greenish pungent smelling gas which, already in relatively low concentrations, can irritate the mucous membranes. With higher concentrations it has a paralysing effect on the central nervous system whereby, with prolonged inhalation, death can occur due to a lung oedema. With the introduction of the vacuum technique, safety risks are today extensively excluded. The MAK value for the explosive and extremely poisonous chlorine dioxide is 1.45 mg/m3. Chlorine dioxide is an orange-yellow coloured gas which has a pungent smell similar to chlorine. Even small concentrations in the air can lead to breathing difficulties and irritation of the mucous membranes. As the initial product is rated as water hazardous (WGK 2) authorised tank plants must be employed.

In natural waters and wastewaters chlorine, hypochlorite and chlorine dioxide react with inorganic and organic water content substances /e.g. protein, alchohols, humic acids, phenols). The result of these reactions is the formation of in part unwanted side products (e.g. chlorophenols, trihalide methanes, AOX). In comparison with chlorine gas the danger of the occurrence of environmentally damaging compounds with the employment of chlorine dioxide is less, as no trihalomethane, chlorophenols and reaction products with ammonium and amino compounds result. The AOX formation is reduced by more than 90 %, however, polar organic reaction products such as chlorite, chlorate and chloride result.

With the employment of chlorine without dechlorination stage the biocoenosis in lakes and rivers can be impaired, whereby the self-cleaning and the natural antagonists of pathogens are damaged. Chlorine concentrations of 0.3 - 0.6 mg/l lead to irreparable losses in plant and animal biomasses in lakes and rivers; 0.05 - 0.1 mg/l already have negative effects on the growth and survival of fishes. Furthermore there is the danger that toxic substances accumulate in the food chain up to fish and beyond.

5.5 Other Procedures

With today's status of technology only a few other processes are employed for wastewater disinfection. Although, for technical application and economic reasons and due to the in part unsatisfactory development status, these are employed in a few cases only, they are to be listed for the sake of completeness.

Thermal procedures Thermal procedures in the form of disinfection and sterilisation are among the safest methods of destroying pathogenic agents. A general thermal treatment of wastewater is not feasible for reasons of cost. Thermal disinfection or sterilisation is laid down for raw wastewater from certain of the above named sources. By autoclaving (sterilisation) at at least 120° C over at least 20 minutes, which is required for wastewater from genetic engineering plants [1] and animal cadaver disposal stations [2], a mortality or deactivation of all pathogenic bacteria (including their spores), worm eggs and viruses is guaranteed. A thermal treatment (disinfection) of raw wastewater, for example from certain hospital departments, is carried out at a temperature of at least 100° C and a reaction time of 15 minutes.

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Oxidation agents not containing chlorine

In addition to ozone and chlorine hydrogen peroxide (H2O2), peracetic acid can be named as additional oxidation agents.

Hydrogen peroxide is employed occasionally with wastewater treatment to combat odour problems, hydrogen sulphide and bulking sludge. To a certain extent a disinfection effect is also achieved through this.

Peracetic acid is an effective oxidation agent and is stable over long periods. It possesses a strong microbicidal effect against bacteria, viruses and, under certain preconditions, also against spores. As here the peracetic acid disintegrates into harmless components and barely influences the oxygen consumption it can, according to today's knowledge, be designated as relatively environmentally friendly. In several large scale trials in Great Britain mechanically treated wastewater was disinfected with peracetic acid ("Oximaster") before discharge into the sea. In this way a reduction of the coliform bacteria of 2 - 3 powers of ten was achieved in a plant with 100,000 m3 of wastewater per day through the addition of 15 mg of peracetic acid per litre of wastewater.

Lyes, and industrially produced disinfection and sanitary reagents With lyes (caustic soda, milk of lime) the hydroxyl groups (OH-) are particularly effective against the gram-positive bacteria (however, not against tubercular bacteria). They are sometimes employed in hospital areas, for example for the disinfection of faecal and highly infected (raw) wastewater from isolation stations. However, there they are being increasingly displaced by disinfectants offered by industry. Also to be mentioned are biocide effective sanitary reagents which are employed against odour nuisances in chemical toilets. The discharge of wastewater containing such agents into sewage treatment plants can, with insufficient dilution, lead there to impairment of the biological wastewater treatment.

6. Conclusions and Recommendations The necessity for wastewater disinfection depends crucially on the utilisation of the lake or river into which the wastewater is discharged. If an increased risk of the transfer of diseases and/or their instigators via water exists and only if a subordinate loading via another path occurs is wastewater disinfection to be considered.

In the Federal Republic of Germany only very little drinking water is taken from surface waters into which wastewater is discharged without prior passage underground. This water is disinfected during water processing. The risk of infection via drinking water is therefore relatively slight. Of the remaining forms of utilisation of lakes and rivers, primarily swimming and watersport, commercial fishing, agricultural and horticultural irrigation as well as drinking water for cattle, place increased requirements on the hygienic properties of the lakes and rivers. Disinfection can also be justified with the utilisation of treated wastewater as process water. The following are to be observed with regard to the necessity and practicality of a wastewater disinfection:

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- wastewater which, from an epidemic hygienic aspect, is particularly questionable (e.g. from isolation stations or sanitoria for tuberculosis) must be disinfected directly at source (preferably through the application of a thermal procedure).

- a deactivation or retention of all disease germs is not possible in sewage treatment plant practice, solely a reduction of the concentration of pathogenic micro-organisms to a concentration harmless for humans and animals.

- wastewater disinfection at the runoff from wastewater treatment plants is only practical after exhausting the possibilities of improved biological and advanced wastewater treatment.

- the loading of lakes and rivers from diffuse sources, in particular those from agriculture, represent a considerable share of the problem.

- the loading of lakes and rivers can occasionally be considerably higher than the loading at the runoff of the wastewater treatment plant due to the discharge of stormwater and combined water overflows, which have an effect a long time after the end of the overflow. The biological properties of lakes and rivers can equally be impaired due to the stirring up of suspended mater and sediments.

- with some wastewater disinfection procedures - in particular with chlorination - health and ecologically suspect wastewater content substances can be formed.

With regard to a general improvement in the hygienic conditions of flowing waters in order, for example, to ensure the quality of waters for bathing, the disinfection of wastewater discharged from sewage treatment plants is often insufficient. As a rule the precipitation water loads as well as the diffuse loading sources, in particular those from agriculture and rural settlements, must be decisively reduced. This is dependent on the local conditions and is successful in individual cases only [12],[20].

Should, after consideration of the given aspects and after exhausting possibilities for advanced treatment through which, as a rule, a reduction of the concentrations of pathogenic micro-organisms is also achieved, the decision be made in favour of wastewater disinfection then, for reasons of hygiene and aquatic biology according to the latest status, UV irradiation should be given priority over ozonisation, so long as micro/ultrafiltration is not developed on an industrial scale and is economical.

A chemical disinfection of wastewater from the outfall of a sewage treatment plant, for example using chorine, is to be rejected as a long-term solution and can only be employed in special cases or as an emergency measure, as the input of additional chemicals into the wastewater can lead to uncontrolled reactions and thus to an impairment of the aquatic ecology. An exception could, under environmental aspects, be hydrogen peroxide and peracetic acid.

For aquatic biological and economical reasons a wastewater disinfection should be limited to a period determined by the usage of the lake or river (e.g. summer half year).

In order to be able to inform users downstream of the discharge point and to take emergency measures in time with a failure of disinfection, no bathing and no drawing of water for drinking water processing, watering of field cultivation or drinking water for

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livestock should be undertaken within a safety area, taking into account local conditions. With lateral discharge this area corresponds with the wastewater trace which, depending on the depth of the body of water, linearity and turbulence can still be verified after a reach of up to 1000 times the width of the body of water. With complete admixture of the treated wastewater with the total body of water at the discharge point this area is to be matched to the necessary warning time (e.g. half an hour flow time in the body of water).

7. Bibliography (Translator note: Known versions in English are given. Otherwise a short courtesy translation of the title is given in square brackets).

[1] Verordnung über die Sicherheitsstufen und Sicherheitsmaßnahmen bei gentechnischen Anlagen (GentechniK-Sicherheitsverordnung - GENTSV) [Ordinance on the Safety Levels and Safety Measures with Genetic Engineering Plants]. Dated 24.10.90 in the new edition of 14.03.95. BGBl. I, 2340

[2] Verordnung über Tierkörperbeseitigungsanstalten und Sammelstellen (Tierkörperbeseitigungsanstalten-Verordnung - TierKBVV) [Ordinance on Animal Cadaver Disposal Facilities and Collection Points]. Dated 01.09.76. BGBl. I. 257.

[3] Bundesgesundheitsamt (1994): Anhang zur Liste der vom Bundesgesundheitsamt (BGA) geprüften und anerkannten Desinfektionsmittel und -verfahren, Bundesgesundhbl. 3, 128. [Federal Ministry of Health (1994). Appendix to the List of Disinfectants Certified and Recognised by the Federal Ministry of Health], As at 01. 01.94. 12th Edition..

[4] Bundesgesundheitsamt (1978): BGA-Merkblatt Einleitung von Krankenhausabwasser in Kanalisation oder Gewässer, Bundesgesundhbl. 21, 34 [Federal Ministry of Health (1978): BGA Advisory Leaflet on the Discharge of Hospital Wastewater into Sewerage Systems or Bodies of Water].

[5] DIN (1964): DIN 19520 Behandlung von Abwasser aus Krankenanstalten, Beuth Verlag, Berlin [Treatment of Wastewater from Hospitals]

[6] Bundesgesundheitsamt (1980): Richtlinie für Krankenhaus Hygiene und Infektionsprävention. Anlage zu Ziffer 7.2, Bundesgesundhbl. 23, 56 Guidelines for Hospital Hygiene and Infection Prevention]

[7] Isaak, R.A. (1996): Disinfection Dialogue. Water Environment & Technology, 67 - 72. [8] DIN (1997): DIN EN 1085 Abwasserbehandlung Wörterbuch, Beuth Verlag, Berlin.

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