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Domestic waste water re-use—aspects of the treatment system
Water Research Vol. 9, pp. 655 to 657. Pergamon Press I975. Printed in Great Britain.
DOMESTIC WASTE WATER RE-USE--ASPECTS OF THE TREATMENT SYSTEM D. KUIPER and R.
WECHSLER*
Testing and Research Institute of the Netherlands Waterworks KIWA N.V., Rijswijk, Netherlands (Received 13 October 1973) Abstract--The domestic water re-use is discussed with special emphasis on the requirements for
the drinking water quality and the process reliability. From these, special requirements for the treatment system are derived. A possible treatment system is presented which is capable of producing water of the desired quality. This system is built up from a sequence of available processes. The effectiveness of each of the processes as well as of the whole sequence is discussed. INTRODUCTION
The re-use of water for domestic purposes has been seriously considered in recent years, mainly as a means to produce supplementary water in arid zone areas. Re-use may be of importance also for industrialised temperate areas with no principal water shortage. In general these areas are situated along rivers and estuaries and a substantial part of the drinking water is produced from the riverwater. The increased pollution of the rivers in recent years has depreciated the drinking water quality. As a result the waterworks are forced to intensify the treatment system by advanced processes. Meanwhile the environmental legislation requires extension to the treatment of sewage prior to discharge. In view of this situation it is attractive to close the cycle and to re-use the water. This implies a recycle system in which drinking water is produced from sewage. Such a recycle system offers the advantage of being relatively independent of the conventional sources (rivers) of which the quality is decreasing, uncontrollable and may be exposed to catastrophic events. Complete independence of supplementary water is not possible as waterlosses occur in the recycle system because of leakages, evaporation and treatment. In arid zone areas these losses amount to about 50% of the supplied drinking water and in temperate areas only 20-30%. The cost of a recycle system presents no serious objections to introducing the system as it appears to be competitive, concerning present prices, with the combined cost of an advanced drinking water treatment and sewage treatment. In addition cost of water transport and long term storage may be saved. The essential aspect of water re-use is the quality of the reclaimed water. Many people are concerned about the quality (see requirements section) of drinking water which may be produced from municipal waste
water. The existing drinking water standards are based on the assumption that the drinking water is produced from uncontaminated ground or surface water. The quality criteria are specified by consideration of hygiene and by maximum allowable concentrations of a limited number of relevant substances. In recent years maximum allowable concentrations for a great number of toxic compounds have been established, but this number is only a small fraction of all potential hazardous compounds which may be present in municipal waste waters. It can be argued that in several areas in the world the drinking water has no obvious deleterious health effect, though it is produced from surface water which is substantially contaminated with waste water. However such an argument does not take into account the fact that in a recycle system compounds may be present in higher concentrations because of accumulation. The extent of accumulation will depend on local conditions. In arid zone areas the concentration of a compound can reach two times the concentration in "once through" sewage and in temperate areas about five times if that particular compound is not removed in the treatment system. This implies that the concentration of that particular compound in the drinking water in the recycle system may reach about 10 times its value in drinking water produced from a polluted river. [1] In general drinking water produced from a new "source" will be less readily accepted than from conventional, well-known sources, with which long term experience has been acquired even though these sources are increasingly polluted. REQUIREMENTS
In view of the above mentioned aspect the quality of the drinking water of a recycle system must not be considered by existing limited standards, but has * Present address: Department of Health, Water Polluto be based on a positive interpretation of the more tion Control Branch, Sydney NSW 2000, Australia. general but basic statement that drinking water must This paper was received for the Paris Conference but together with several more was accepted for Water be wholesome and safe. This implies that a recycle Research as there was no place for it on the Conference system which will be acceptable and applied as an Programme. additional source of water supply, in competition with 655
656
D. KUIPER and R. WECHSLI!R water from waste water sources. The storage of the product water is required to enable such analyses to be carried out. (5) There must be sufficient physical distance between the inttuent and effluent of the reclamation process to prevent cross infection. (61 It is advantageous to control the quality of sewage with respect to industrial dumping of water borne wastes which contain specific components, that may adversely affect the plant performance and or the product quality. Legislation may be useful in this respect.
the conventional water supplies, has to meet the tollowing general requirements. (a) The quality of the drinking water must meet the highest existing drinking water standards or goals. (b) The treatment process must remove any conceivable pollutant. (c) The operation of the process must be highly reliable. In order to meet these general requirements the treatment system of a recycle system must be based on the following points. (1) The treatment system must consist of several processes in series with overlapping functions and rely on different and unrelated mechanisms. The consequences of this are. (a) There are several barriers of protection between the consumer and the polluter. (b) The use of processes relying on different and unrelated mechanisms permits the removal of a wide range of pollutants. (c) The overlap of process function gives an increased removal of any substance by the whole chain of processes. (2) The treatment system must have overload capability with respect to flow and pollutant concentration. This is necessary because of daily and seasonal flow variations and because of variations in composition, both the overall composition and shock loads of certain specific pollutants. (3) The performance of each of the processes must be monitored to enable fast response of the operators to adverse conditions. (4) The product water must meet the required quality and hence must be analysed prior to its release as drinking water. It is necessary to consider more detailed analytical procedures than currently used in sanitary engineering. Currently used analytical procedures were not evolved for examination of drinking
A POSSIBLE TREATMENT SYSTEM
Based on the existing technology it is conceivable that the forementioned requirements can be met by the following process sequence: 1. Conventional primary and secondary biological treatment. 2. Lime clarification. 3. Hyperfiltration (reverse osmosis). 4. Activated carbon treatment. 5. Disinfection. Each of these operations allows a considerable flexibility in performance. In addition, because of the overlap in functions the overall performance may be varied and so a wide variety of product quality goals can be achieved. The actual design will be the optimum of desired product quality, reliability of the process and cost. The general performance of each of the processes is demonstrated in Table 1. It will be seen that each of the sewage compounds is removed by more than
Table 1. The removal of substances by the treatment process Effect Substance Bacteria and virusses Suspended matter Total organic matter Toxic organics Inorganic salts Toxic inorganics Phosphate Nitrate Ammonia Cyanide Urea Phenols Taste and odour Oil Detergents Hydrocarbons Chlorinated hydrocarbons Volatile organic acids Carbohydrates } Amino acids Fatty acids Proteins
Lime clarification
Hyperfiltration
x
x
x
x
x
x
x
x
x
x
x
x
×
X
X
x
x
x
x
x
X
X
x
x
x
x
x
X
X
x
x x
of:
Biological treatment
x
x x
x
x
x
....
Activated carbon
Chlorine X
X
X
X
x x x
x
x
x
x
x x
x
x
x*
-
x
x
x*
-
x x
x x
x
x
x
x
x
x
×
×
x
x >(X
x
x x x x
x
x
x
x x x ~<
x x
x
x
x
x
x
x
x
x
×
)4
X
×
x
x
x x x
x
x
x
x
x
>,
x
x
×
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
The effect of the removal is expressed as: × × x 90-100%. × × 50-.90%. × 10-50%. * The removal can be realised but the process has to be adjusted.
< 10%;.
X*
X
X
X ~
X
X
X
Domestic waste water re-use one process with the exception of inorganic salts which are only removed by hyperfiltration. In general the presented data in Table 1 are rather conservative and better results can certainly be obtained in practice. It is clear that duplication of function within the process sequence greatly increases the extent of pollutant removal compared with the removal by each individual process. Concerning the conventional sewage treatment
Any of the well established processes might be used, depending on local conditions, available installations and influent quality. Because of the duplication of functions within the sequence the quality of the reclaimed water is not critically influenced by the performance of this operation. It is used to remove the bulk of the suspended matter and dissolved organics. In addition because of the nature of biological reactions it damps out peakloads of BOD concentration. The effluent of this process can be allowed to vary to a certain extent, in quality and quantity. The subsequent processes, being costly must be operated at maximum capacity all the time. Hence a certain reservoir is necessary to provide a buffer capacity. At this stage a low quality supplementary water can be added to the recycle system. The remaining processes are operated at constant flow conditions: (a) Lime clarification provides disinfection by detention at high pH. Removal of colloidal matter and calcium salts is also provided and is essential as these substances cause membrane fouling in
657
the reverse osmosis process. (b) Hyperfiltration is principally a desalting process but it is clear that it effects a wide range of pollutant removal. In general substances with molecular weight greater than about 200 are almost completely removed. The product of the reverse osmosis contains low molecular weight dissolved organics including some taste and odour determining compounds. These substances are further removed by activated carbon treatment. Finally a disinfection by chlorine is necessary before the water is released. Chlorine has the added advantage of removal of any residual ammonia. At the end of the process sequence a storage reservoir for the product water is necessary for quality control. Type and capacity may depend on local conditions, the residence time should be several days. CONCLUSION It can be concluded that domestic water re-use can be realised in competition with conventional water supplies, provided that no serious technical objection can be raised against the drinking water quality and the process reliability. In this light, process requirements are discussed and a treatment process is suggested, consisting of a sequence of processes, which provides a great flexibility to achieve the desired goals under various local conditions. REFERENCE
1. Kuiper D. and Wechsler R. (1974) Conceptual aspects of water re-use, Water Res. 8, 529.
DOMESTIC WASTE WATER RE-USE--ASPECTS OF THE TREATMENT SYSTEM D. KUIPER and R.
WECHSLER*
Testing and Research Institute of the Netherlands Waterworks KIWA N.V., Rijswijk, Netherlands (Received 13 October 1973) Abstract--The domestic water re-use is discussed with special emphasis on the requirements for
the drinking water quality and the process reliability. From these, special requirements for the treatment system are derived. A possible treatment system is presented which is capable of producing water of the desired quality. This system is built up from a sequence of available processes. The effectiveness of each of the processes as well as of the whole sequence is discussed. INTRODUCTION
The re-use of water for domestic purposes has been seriously considered in recent years, mainly as a means to produce supplementary water in arid zone areas. Re-use may be of importance also for industrialised temperate areas with no principal water shortage. In general these areas are situated along rivers and estuaries and a substantial part of the drinking water is produced from the riverwater. The increased pollution of the rivers in recent years has depreciated the drinking water quality. As a result the waterworks are forced to intensify the treatment system by advanced processes. Meanwhile the environmental legislation requires extension to the treatment of sewage prior to discharge. In view of this situation it is attractive to close the cycle and to re-use the water. This implies a recycle system in which drinking water is produced from sewage. Such a recycle system offers the advantage of being relatively independent of the conventional sources (rivers) of which the quality is decreasing, uncontrollable and may be exposed to catastrophic events. Complete independence of supplementary water is not possible as waterlosses occur in the recycle system because of leakages, evaporation and treatment. In arid zone areas these losses amount to about 50% of the supplied drinking water and in temperate areas only 20-30%. The cost of a recycle system presents no serious objections to introducing the system as it appears to be competitive, concerning present prices, with the combined cost of an advanced drinking water treatment and sewage treatment. In addition cost of water transport and long term storage may be saved. The essential aspect of water re-use is the quality of the reclaimed water. Many people are concerned about the quality (see requirements section) of drinking water which may be produced from municipal waste
water. The existing drinking water standards are based on the assumption that the drinking water is produced from uncontaminated ground or surface water. The quality criteria are specified by consideration of hygiene and by maximum allowable concentrations of a limited number of relevant substances. In recent years maximum allowable concentrations for a great number of toxic compounds have been established, but this number is only a small fraction of all potential hazardous compounds which may be present in municipal waste waters. It can be argued that in several areas in the world the drinking water has no obvious deleterious health effect, though it is produced from surface water which is substantially contaminated with waste water. However such an argument does not take into account the fact that in a recycle system compounds may be present in higher concentrations because of accumulation. The extent of accumulation will depend on local conditions. In arid zone areas the concentration of a compound can reach two times the concentration in "once through" sewage and in temperate areas about five times if that particular compound is not removed in the treatment system. This implies that the concentration of that particular compound in the drinking water in the recycle system may reach about 10 times its value in drinking water produced from a polluted river. [1] In general drinking water produced from a new "source" will be less readily accepted than from conventional, well-known sources, with which long term experience has been acquired even though these sources are increasingly polluted. REQUIREMENTS
In view of the above mentioned aspect the quality of the drinking water of a recycle system must not be considered by existing limited standards, but has * Present address: Department of Health, Water Polluto be based on a positive interpretation of the more tion Control Branch, Sydney NSW 2000, Australia. general but basic statement that drinking water must This paper was received for the Paris Conference but together with several more was accepted for Water be wholesome and safe. This implies that a recycle Research as there was no place for it on the Conference system which will be acceptable and applied as an Programme. additional source of water supply, in competition with 655
656
D. KUIPER and R. WECHSLI!R water from waste water sources. The storage of the product water is required to enable such analyses to be carried out. (5) There must be sufficient physical distance between the inttuent and effluent of the reclamation process to prevent cross infection. (61 It is advantageous to control the quality of sewage with respect to industrial dumping of water borne wastes which contain specific components, that may adversely affect the plant performance and or the product quality. Legislation may be useful in this respect.
the conventional water supplies, has to meet the tollowing general requirements. (a) The quality of the drinking water must meet the highest existing drinking water standards or goals. (b) The treatment process must remove any conceivable pollutant. (c) The operation of the process must be highly reliable. In order to meet these general requirements the treatment system of a recycle system must be based on the following points. (1) The treatment system must consist of several processes in series with overlapping functions and rely on different and unrelated mechanisms. The consequences of this are. (a) There are several barriers of protection between the consumer and the polluter. (b) The use of processes relying on different and unrelated mechanisms permits the removal of a wide range of pollutants. (c) The overlap of process function gives an increased removal of any substance by the whole chain of processes. (2) The treatment system must have overload capability with respect to flow and pollutant concentration. This is necessary because of daily and seasonal flow variations and because of variations in composition, both the overall composition and shock loads of certain specific pollutants. (3) The performance of each of the processes must be monitored to enable fast response of the operators to adverse conditions. (4) The product water must meet the required quality and hence must be analysed prior to its release as drinking water. It is necessary to consider more detailed analytical procedures than currently used in sanitary engineering. Currently used analytical procedures were not evolved for examination of drinking
A POSSIBLE TREATMENT SYSTEM
Based on the existing technology it is conceivable that the forementioned requirements can be met by the following process sequence: 1. Conventional primary and secondary biological treatment. 2. Lime clarification. 3. Hyperfiltration (reverse osmosis). 4. Activated carbon treatment. 5. Disinfection. Each of these operations allows a considerable flexibility in performance. In addition, because of the overlap in functions the overall performance may be varied and so a wide variety of product quality goals can be achieved. The actual design will be the optimum of desired product quality, reliability of the process and cost. The general performance of each of the processes is demonstrated in Table 1. It will be seen that each of the sewage compounds is removed by more than
Table 1. The removal of substances by the treatment process Effect Substance Bacteria and virusses Suspended matter Total organic matter Toxic organics Inorganic salts Toxic inorganics Phosphate Nitrate Ammonia Cyanide Urea Phenols Taste and odour Oil Detergents Hydrocarbons Chlorinated hydrocarbons Volatile organic acids Carbohydrates } Amino acids Fatty acids Proteins
Lime clarification
Hyperfiltration
x
x
x
x
x
x
x
x
x
x
x
x
×
X
X
x
x
x
x
x
X
X
x
x
x
x
x
X
X
x
x x
of:
Biological treatment
x
x x
x
x
x
....
Activated carbon
Chlorine X
X
X
X
x x x
x
x
x
x
x x
x
x
x*
-
x
x
x*
-
x x
x x
x
x
x
x
x
x
×
×
x
x >(X
x
x x x x
x
x
x
x x x ~<
x x
x
x
x
x
x
x
x
x
×
)4
X
×
x
x
x x x
x
x
x
x
x
>,
x
x
×
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
The effect of the removal is expressed as: × × x 90-100%. × × 50-.90%. × 10-50%. * The removal can be realised but the process has to be adjusted.
< 10%;.
X*
X
X
X ~
X
X
X
Domestic waste water re-use one process with the exception of inorganic salts which are only removed by hyperfiltration. In general the presented data in Table 1 are rather conservative and better results can certainly be obtained in practice. It is clear that duplication of function within the process sequence greatly increases the extent of pollutant removal compared with the removal by each individual process. Concerning the conventional sewage treatment
Any of the well established processes might be used, depending on local conditions, available installations and influent quality. Because of the duplication of functions within the sequence the quality of the reclaimed water is not critically influenced by the performance of this operation. It is used to remove the bulk of the suspended matter and dissolved organics. In addition because of the nature of biological reactions it damps out peakloads of BOD concentration. The effluent of this process can be allowed to vary to a certain extent, in quality and quantity. The subsequent processes, being costly must be operated at maximum capacity all the time. Hence a certain reservoir is necessary to provide a buffer capacity. At this stage a low quality supplementary water can be added to the recycle system. The remaining processes are operated at constant flow conditions: (a) Lime clarification provides disinfection by detention at high pH. Removal of colloidal matter and calcium salts is also provided and is essential as these substances cause membrane fouling in
657
the reverse osmosis process. (b) Hyperfiltration is principally a desalting process but it is clear that it effects a wide range of pollutant removal. In general substances with molecular weight greater than about 200 are almost completely removed. The product of the reverse osmosis contains low molecular weight dissolved organics including some taste and odour determining compounds. These substances are further removed by activated carbon treatment. Finally a disinfection by chlorine is necessary before the water is released. Chlorine has the added advantage of removal of any residual ammonia. At the end of the process sequence a storage reservoir for the product water is necessary for quality control. Type and capacity may depend on local conditions, the residence time should be several days. CONCLUSION It can be concluded that domestic water re-use can be realised in competition with conventional water supplies, provided that no serious technical objection can be raised against the drinking water quality and the process reliability. In this light, process requirements are discussed and a treatment process is suggested, consisting of a sequence of processes, which provides a great flexibility to achieve the desired goals under various local conditions. REFERENCE
1. Kuiper D. and Wechsler R. (1974) Conceptual aspects of water re-use, Water Res. 8, 529.