- Email: [email protected]
The role of advanced treatment in wastewater reclamation and reuse
l.'I)
Pergamon
War SCI Tech Vol. 40, No. 4-5, pp. 1-9 , 1999 C 1999
Published by Elsevier Science LIdon behalfof the IAWQ Pnnted In Great Bntarn, All rights reserved 0273-1223199 $20.00 + 0.00
PU: S0273-1223(99)00479-S
THE ROLE OF ADVANCED TREATMENT IN WASTEWATER RECLAMATION AND REUSE Rafael Mujeriego* and Takashi Asano** • Escuela Tecnica Super ior de lngenieros de Caminos , Canales y Puertos de Barcelona, Universidad Politecnica de Cataluiia, Gran Capitan, sin . 08034 Barcelona. Spain •• Department ofCivil and Environmental Engineering. University ofCalifornia at Davis. Davis. CA 95616-231 I, USA
ABSTRACT The role of advanced wastewater treatment in wastewater reclamation and reuse is reviewed. Most of the current wastewater reclamation and reuse technologies are essent ially denved from those used in water and wastewater treatment. However, opportunities for adopting technological innovations are much greater for water reuse applications. because reclaimed water will have an economic value as an alternative water supply. Significant progress has been made in developing sound technical approaches to producing high quality and reliable water sources from reclaimed wastewater. This overview paper provides an assessment of technological advances in water reclamation and reuse, which has been dubbed as "the greatest challenge of the 21st century." The paper was presented at the Opening Plenary Sess ion of the lAWQ's Advanced Wastewater Treatment, Recychng and Reuse Conference in Milan, Italy on September 14, 1998 . iO 1999 Published by Elsevier SCIence Ltd on behalf of the lAWQ . All nghts reserved
KEYWORDS Advanced treatment; public health; recycling; reliability; reuse; water purification; water quality; wastewater treatment; water reclamation. INTRODUCTION Advanced treatment plays a critical role in the effective treatment of municipal and industrial wastewater to meet water quality objectives for water reuse and to protect public health. Conventional and advanced wastewater treatment consist of a combination of physical, chemical, and biological processes and operations to remove settleable, suspended, and dissolved solids, organic matter, metals, nutrients, and pathogens from wastewater. Most of the current wastewater reclamation and reuse technologies are essential1y derived from those used in water and wastewater treatment. However, opportunities for adopting technological innovations are much greater for water reuse applications, because reclaimed water will have an economic value as an alternative water supply. Furthermore, contrary to the disposal of treated effiuent, where federal or national regulations are enforced by "command and control" methods of water pollution control, water reclamation and reuse allow more flexibility in water quality management, and consequently more possibilities for adopting new technologies. In evaluating wastewater reclamation technologies, however, the overriding considerations are the operational reliability of each unit process or operation, and the overall capability of the complete treatment
R. MUJERIEGOand T. ASANO
2
system to provide a reclaimed water that meets established wastewater reclamation criteria. As a result, additional treatment processes and operations may be required in certain cases for removal of chemical contaminants and removal or inactivation of microbiological pathogens. Similarly to conventional wastewater treatment, the general terms used to describe different degrees of treatment, in order of increasing treatment level, are preliminary, primary, secondary, and tertiary/advanced treatment. A disinfection step for removal or inactivation of pathogenic organisms is often the final treatment prior to storage and distribution. Because of cost considerations, preliminary, primary, and secondary treatments are generally considered water pollution control requirements; the additional treatment required for water reuse is normally designated as tertiary or advanced treatment. The goal in designing a wastewater reclamation and reuse system is to develop integrated cost-effective process combinations that are capable of reliably meeting the water quality objectives required for water reuse. Figure 1 shows a generalized view of wastewater treatment processes and operations as well as effluent reuse schemes. Based on water quality requirements, any effluent stream can be used as reclaimed water for various beneficial uses. The range of applicable technologies may include: 1) lagoons, wetlands, and natural treatment systems, 2) advanced physical-chemical treatment, 3) advanced biological treatment including biological nutrient removal (BNR), 4) advanced oxidation processes, 5) membrane separation and membrane bioreactors, 5) disinfection technologies, and 6) innovative reactor designs such as sequencing batch reactors and advanced mixing devices.
Preliminary
Secondary
Primary
Advanced E",uon'
E",uon'
Low Rate Proc_... otabohutoon pondI __ted "goonl
D_flCtion
High RI'" Proc_ activated ,Iudge trickling "her. rotating biocontactors
Nhrogon RI""",01 nltrlflcallon - denltrif,catlon -'ective ion exchange break po,nl chlorinet,on gllllrlpping overiand flow Phoophorul R_II chemica' precipitation
Sulpended BoIIdI Remo.oI
Sludge ProcHllng
I
I
cfte"\lcal coagulltlon filtration
8laloglc0l
Non blolOll1cl1
1II1cklnlng dlg..tlon
tIIlckenfng
Orglnlce Ind Mllale Remo.1II
condtttoning
dewatering
dewa'e,ing
carbon adsorption
IlIIlr centrifuge
drying bedl
filter centrifuge ,nCIneration
DI......ed SoUdl lIemo.oI
,ev. . . OBrnoetl etectrocUalYSl1 dlstlll.hon
Figure 1. Generalizedwastewatertreatmentprocesses and operations, and effluent reuse schemes (adapted from Asano, Smith and Tchobanoglous, 1985).
Figure 2 shows a conceptual view of the water quality changes that take place during municipal use of water in a time sequence. Through the process of water treatment, unpolluted sources are used to produce water of such a high quality that it meets applicable standards for drinking water. Municipal and industrial uses degrade water quality, and the processes necessary to recover wastewater quality then become a matter of concern for wastewater treatment. In the actual case, treatment is carried out to the point required by regulatory agencies for protection of aquatic environments and other beneficial uses. The dashed line represents an increase in treated water quality as required by water reuse. Ultimately as the quality of treated
Role of advanced treatment in wastewater reclamation and reuse
3
wastewater approaches that of unpolluted natural water, the concept of wastewater reclamation and reuse is generated. Further advanced wastewater reclamation technologies, such as activated carbon adsorption, advanced oxidation, and reverse osmosis, can generate a water of much higher quality than conventional drinking water, and the product obtained is thus designated repurified water. Today, technically proven wastewater reclamation or purification processes exist to provide water of almost any quality desired.
Tim. Sequence (No Sc.le)
Figure 2. Water quality changes during municipal uses of water
10
a time sequence (after Asano, 1998).
WASTEWATER RECLAMAnON TECHNOLOGIES Table 1 shows a summary of the major unit operations and processes used for wastewater reclamation. At present, the dominant wastewater reuse applications, worldwide, are irrigation of agricultural lands, parks and golf courses. However, there has been considerable progress in reclaimed water applications in the urban setting such as toilet flushing, cooling, fire fighting, and stream flow augmentation. Furthermore, future use of reclaimed water may involve a completely controlled "pipe-to-pipe" system with an intermittent storage step, or it may include blending of reclaimed water with non-reclaimed water, either directly in an engineered system or indirectly through a surface water supply reservoir or a groundwater recharge scheme. When reclaimed wastewater applications have a potential route for human exposure, the major acute health risks are those associated with exposure to biological pathogens including bacterial pathogens, helminths, protozoa, and enteric viruses. From a public health and process control perspective, the most critical group of pathogenic organisms is enteric viruses, because of the possibility of infection from exposure to low doses and the lack of routine, cost-effective methods for virus detection and quantification. In addition, treatment systems that can effectively remove viruses will most likely be effective for controlling other pathogenic organisms. Thus, it is essential to produce virtually pathogen-free effluent for water reuse applications that have the potential for significant human exposure or contact, such as spray irrigation of food crops eaten uncooked, irrigation of parks and playgrounds, and supply of unrestricted recreational impoundments where swimming may take place. Tertiarv and advanced treatment After conventional biological treatment processes (e.g., activated sludge process, trickling filters, and oxidation ponds), tertiary or advanced treatment can be applied to remove additional dissolved and suspended contaminants, nutrients, specific metals, and other harmful constituents (see Fig. 1). Filtration. Filtration is a solid/liquid separation process that effectively removes suspended particles larger than about 3 um. When wastewater passes through a column of granular media, particles are removed by impaction, interception, and physical straining. As particles accumulate in the filter media, headloss through the filter increases, making it necessary for the filter to be cleaned by backwashing using a combination of air and water scour. To avoid rapid build-up of headloss and reduction of run time, filtration is most effective if the particle concentration (as TSS) is lower than 20 mg/L, Filtration can be used downstream of
4
R. MUIERIEGO and T. ASANO
primary sedimentation (primary effluent filtration), or downstream of secondary sedimentation (tertiary filtration). Table I. Unit operations and processes used in wastewater reclamation (Adapted from Asano and Levine, 1998) Process
Description
Application
Solldlliquld separation Coagulation Flocculation Filtration Sedimentation
Additionof chemicalsto destabilize colloids and suspendedmatter Particleaggregation Particleremovalby granularmedium Gravitysettlingof particulatematter, chemicalfloc, and precipitates
Promoteparticle destabilization to improve flocculation and solids removal Particleagglomeration upstreamof liquid/solidseparationoperations Removalof particleslarger than about 3 11m Liquid/solids separation
Blolo&lcal treatment Aerobicbiological treatment Oxidationpond Disinfection
Biologicalmetabolismof wastewaterand solids by microorganisms in an aeration basin Ponds with 2 to 3 feet of waterdepth for mixingand sunlightpenetrationand oxidationand synthesisby algae The inactivationor removalof pathogenic organismsusing oxidizingchemicals, ultravioletlight, caustic chemicals,heat, or physical separationprocesses
Incorporationand removalof organicmatter from wastewaterby synthesisinto microbial cells and CO2 and H20 Reductionof suspendedsolids,BOD, fecal bacteria and ammonia
Process by whichcontaminants are physicallyadsorbed onto the carbon surface Wastewateris distributedover a packing through which forced air is drawn to extractammoniaand volatileorganicsfrom the water droplets Exchangeof ions between an exchange resin and water using a flow through reactor
Removalof hydrophobic organiccompounds
Protectionof public health
Advanced treatment Activatedcarbon Air stripping
Ion exchange
Lime treatment Membrane processesand reverseosmosis
Removalof ammonianitrogenand some volatileorganics
Softeningand removalof selectedionic contaminants; Effectivefor removalof cations such as calcium,magnesium, iron and anions such as nitrate The use oflime to precipitatein high pH Used to stabilizelime-treated water, to reduce variouscations and metals from water and its scale formingpotential wastewater Removalof impurities,bacteriaand viruses, Pressuredriven membrane processesto dissolvedsalts from water and wastewater separateimpurities,colloids,ions from water,based on size exclusionor molecular diffusion
As pathogenic organisms are associated with particles, filtration is an effective process for reducing pathogen concentration in wastewater streams, and provides an excellent pre-treatment for disinfection. Filtration is stipulated as a required treatment process in many applications, because of its favorable effect on aesthetics, particle removal, and disinfection effectiveness. If water is to be treated by activated carbon, ion exchange, or reverse osmosis, filtration is used to reduce solids loading on these processes and improve their overall effectiveness. Adsorption. Activated carbon adsorption is effective in removing hydrophobic organic compounds from surface and groundwater sources. Compounds with low water solubilities, such as organic solvents and chlorinated organic solvents, are adsorbable because of their low water solubility. Water soluble compounds
Role of advanced treatment in wastewater reclamation and reuse
s
and larger compounds are better removed by oxidation or ultrafiltration. In most cases testing is necessary (isotherm evaluation, dynamic adsorption testing) to determine the applicability of activated carbon treatment to a water of specific quality. Membrane Processes. Among advanced treatment processes, membrane applications have clearly emerged as one of the promising alternatives to conventional advanced physical-chemical treatment, which usually includes chemical coagulation, flocculation, and granular-medium filtration. Membrane processes, ranging from microfiltration to reverse osmosis, are finding their way to cost-comparable applications for removal of microorganisms in membrane bioreactors, and for removal of trace organic substances, specific ions, and dissolved solids. Membrane processes include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis. Microfiltration is effective for removal of particles, and can be competitive with conventional granular medium filtration. Ultrafiltration is effective for removal of particles and macromolecules. Nanofiltration and reverse osmosis are effective for removal of dissolved ions from liquids. While membranes have multiple applications, the useful life of a membrane closely depends on avoiding conditions that will cause fouling, scaling, or chemical interactions. Membrane process success is highly dependent on appropriate pretreatment. Pretreatment options include filtration for coarse particle removal, scale control, and chemical addition. Post treatment includes water pH stabilization to prevent corrosion, and air stripping. Disinfection. Disinfection is an essential component of many wastewater reclamation and reuse treatment systems. The objective of disinfection processes is to inactivate/destroy pathogenic organisms. Disinfection is typically used as one of the final treatment processes in a treatment system. Chemical disinfection practices are based on addition of a strong oxidizing chemical such as chlorine, ozone, hydrogen peroxide, or bromine. Ultraviolet radiation is an alternative to oxidation process that can achieve disinfection. Other methods for decreasing the microbial content of reclaimed wastewater include exposing pathogenic organisms to high pH environments as in lime treatment. Alternatively, organisms can be effectively removed by physical methods, such as membrane filtration systems. The most common type of disinfection system is chlorine at dosages ranging from 5 to 15 mg/L, with a recommended contact time of 30 minutes to 2 hours. For wastewater reuse, it is important to remove residual chlorine to prevent complications in downstream beneficial uses. Dechlorination is often used as a final step, and is accomplished using sulfur dioxide or other reducing agents. Activated carbon adsorption is also effective for removal of residual chlorine. Ultraviolet disinfection has been demonstrated to provide a viable alternative to chemical disinfection processes, at doses ranging from 100 to 120 mWs/cm2, and higher. The performance ofUV disinfection is influenced by water turbidity and UV lamp intensity, which in tum can be reduced by lamp age and the fouling characteristics of the wastewater. Upstream filtration is usually essential to prevent particle shielding of pathogenic microorganisms thus reducing the effectiveness ofUV disinfection. WASTEWATER REUSE APPLICAnONS The purpose of this section is to provide an overview on water quality requirements for various water reuse applications, with particular emphasis on public health aspects of wastewater reuse. Table 2 shows a summary of the seven major categories of wastewater reuse, with their treatment goals and some illustrative applications. Considerable experience is available in many parts ofthe world on agricultural and landscape irrigation with reclaimed water, with an increasing number of projects devoted to the beneficial reuse for recreational and environmental purposes. Although industrial reuse has been traditionally concentrated in cooling water systems, increasing interest has developed in industrial areas for reclaimed water as an alternative and reliable supply for process water. Water management in industry involves: 1) the use of higher quality raw materials, 2) the use of cleaner technologies, 3) the adoption of more water efficient technologies, and 4) the application of advanced treatment technologies to promote materials recovery and water reuse. Membrane,
R. MUJERIEGO and T. ASANO
6
sorption-desorption, and photo-oxidation processes are common alternatives in the textile industry, either in current application or in the demonstration phase. In addition to technical performance characteristics, investment and operation and maintenance costs appear as determining factors in the final selection process. This multiple objective approach clearly illustrates that advanced treatment for wastewater reclamation and reuse isjust one, but an important element of water resources management. Table 2. Categories of municipal wastewater reuse (Adapted from U.S. EPA, 1992, and Asano and Levine, 1998)
Category of wastewater reuse Urban use Unrestricted
Restricted access irrigation
Food crops
Non-food crops and food crops consumed after processing
Recreational use Unrestricted
Treatment goals
Secondary, filtration, disinfection BODs: <10 mg/L Fecal coliform: NO/I 00 mL Turbidity: < 2NTIJ Clz residual: I mg/L pH 6t09 Secondary and disinfection BODs: < 30 mg/L TSS: < 30 mg/L Fecal coliform: < 200/100 mL Clz residual: I mgIL pH 6t09 Secondary, filtration, disinfection BODs: < 10 mg/L Turbidity: < 2NTU Fecal coliforms: NO/I 00 mL ci, residual: I mg/L pH 6t09 Secondary, disinfection BODs: < 30 mg/L TSS: < 30 mglL Fecal coliforms: < 200/100 mL Clz residual: I mg/L pH 6 t09
Secondary, filtration, disinfection BODs: < 10 mg/L Turbidity: < 2NTIJ Fecal Coliforrns: NO/I 00 mL Clz residual: I rng/L pH 6 to 9 Restricted Secondary, disinfection BODs: < 30 mg/L TSS: < 30 mg/l, Fecal Coliforms: < 200/100 mL Clz residual: I mg/l, pH 6t09 Environmental reuse Site specific treatment levels pH Dissolved oxygen Coliforms, Nutrients Site specific Groundwater recharge Industrial reuse Secondary and disinfection BODs: < 30 mg/L TSS: < 30 mg/L Fecal coliform: < 200/100 mL Safe Drinking Water Requirements Potable reuse
Example apphcations
Landscape irrigation: parks, playgrounds, school yards, fire protection, construction, ornamental fountains, aesthetic impoundments, In-building uses: toilet flushing, air conditioning
Imgation of areas where public access is infrequent and controlled: freeway medians; golf courses; cemeteries; residential areas; greenbelts
Crops grown for human consumption and consumed uncooked
Fodder, fiber, seed crops, pastures, commercial nurseries, sod farms. Commercial aquaculture
No limitations on body-contact: lakes and ponds used for swimming, snowmaking
Fishing, boating, and other non-contact t recreational activities
Use of reclaimed wastewater to create artificial wetlands, enhance natural wetlands and sustain stream flows Groundwater replenishment; salt water intrusion control; subsidence control Coohng-system make-up water, process waters, boiler feed water, construction activities and washdown waters Blending in municipal water supply reservoir; pipe to pipe supply
Role ofadvanced treatment ID wastewater reclamation and reuse
7
To accomplish the high degree of treatment and reliability required for potable reuse, a treatment train of advanced unit operations and processes has to be implemented, including lime clarification, nutrient removal, recarbonation, filtration, activated carbon adsorption, demineralization by reverse osmosis, and disinfection with chlorine, ozone, ultraviolet radiation, alone or in different combinations. The conventional advanced treatment train alternatives used for potable reuse include nutrient removal by high lime and granular activated carbon, with or without reverse osmosis. The treatment alternative of high lime treatment and carbon adsorption, followed by disinfection has typically been applied for water reclamation before indirect potable reuse. The reverse osmosis process is normally applied when build up of dissolved solids in the system is to be prevented. Granular activated carbon followed by reverse osmosis is very effective in removing a large number of pollutants. However, the electrical energy requirements of reverse osmosis, together with the cost of membrane replacement and antifouling controls, and brine disposal make this alternative treatment expensive, and only applicable in areas where water availability is low and its cost is high. Technological advances in membrane development and manufacturing have been brought down the cost of membrane replacement and maintenance over the last decade, and it can be expected to become an increasingly competitive treatment option for water and wastewater treatment in the near future. Expected reliability and average treatment train performance are summarized in Table 3. The concentration levels indicated in Table 3 are comparable or better than those commonly applicable to conventional sources for potable water production. However, treatment reliability and the presence of trace organic compounds make direct potable reuse an alternative applicable only in extreme situations. Table 3. Reliability data and average treatment train performance for carbon adsorption and lime treated activated sludge effiuent (Adapted from Metcalf & Eddy, 1991)
Constituent
BOD COD TSS NHJ-N
Phosphorus Oil & grease Arsenic Cadmium Chromium Copper Lead Mercury Selenium Zinc TOC Turbidity, ntu Color Foaming agents TOS 1.0.=Insufficient data.
Average reliability
Average removal, % 100 100 100 100 100 97 61 98 100 98 99 23 7 98 100 100 93 92 95
10%
50%
90%
100 100 100 97 100 100 93 100 100 100 100 31 26 100 100 100 100
100 100 99 81 100 98 63 98 98 99 98 18 12 95 98 100 94 84
89 97 87 48 99 73 0 87 84 98 78 0 0 58 83 95 56
1.0.
!.D.
!.D.
!.D. !.D.
Average effluent concentration,mgfL 0 0 0 0 0 2 0.003 0.0002 0 0.002 0.001 0.028 0.006 0.008 0 0 5 0.17 129
WASTEWATER REUSE BENEFITS AND CHALLENGES Through integrated water resources planning, the use of reclaimed wastewater may provide sufficient flexibility to allow water agencies, as well as individual industrial users, to respond to short-terms needs and
8
R. MUJERIEGO and T. ASANO
to increase sustainable, long-term water supply reliability, without constructing additional storage or conveyance facilities at substantial economic and environmental expenditures. Significant progress has been made in developing sound technical approaches to producing reliable sources of quality water by wastewater reclamation processes. Continued research and demonstration efforts will result in further progress in the development of water reuse applications. Some key topics involved are: 1) 2) 3) 4) 5) 6)
Assessment of health risks associated with trace organic substances. Improvements in monitoring approaches to evaluate microbiological quality. Application of membrane processes to produce high quality reclaimed water. Evaluation of the effect of reclaimed water storage on water quality. Evaluation of the fate of microbiological, chemical, and organic contaminants in reclaimed water. Evaluation of the long-term sustainability of soil-aquifer treatment systems.
To date the major emphasis on wastewater reclamation and reuse has been for nonpotable applications such as agricultural and landscape irrigation, industrial cooling, and in-building applications such as toilet flushing. Direct potable reuse of reclaimed municipal wastewater is, at present, limited to extreme situations. While indirect potable reuse by groundwater recharge or surface water augmentation has gained support, some concerns still remain regarding trace organic substances, treatment and reuse reliability, and particularly, public acceptance. A key-determining factor in promoting wastewater reclamation, recycling and reuse is the continued development of cost-effective treatment systems. While water supply reliability may justify the adoption of advanced treatment systems for industrial reuse, which brings the cost of reclaimed water to equal or higher values than those of conventional water supplies, other potential beneficial uses may require the least cost reclamation alternatives before they may be considered for implementation. The impressive cost reductions and performance improvement experienced by the advanced treatment process during the last two decades anticipates a most promising future for expanding its application. With development progress driven by the cost margins of the industrial sector, numerous advanced treatment processes are expected to be cost-effective for application in other potential beneficial uses of reclaimed water, such as urban use, and groundwater recharge. SUMMARY AND CONCLUSIONS Advanced wastewater treatment plays an increasingly critical role in the treatment of municipal and industrial wastewater to meet water quality objectives for water reuse and protect public health. Opportunities of adopting technological innovations are much greater in water reuse applications, because reclaimed water will have an economic value as an alternative water supply source. In evaluating water reclamation technologies, overriding considerations are the operational reliability of each unit process or operation, and the overall capability of the complete treatment system to provide reclaimed water that meets established wastewater reclamation criteria. After conventional biological processes, tertiary or advanced treatment can be used to remove additional dissolved and suspended contaminants, nutrients, specific metals, and other harmful constituents. The range of advanced treatment technologies currently in use includes granular medium filtration, adsorption, membrane processes, and disinfection. Using these existing as well as emerging treatment technologies, future water management options may include 1) use of high quality raw materials, 2) the use of cleaner technologies, 3) the adoption of more water efficient technologies, and 4) the application of advanced treatment technologies to promote materials recovery, energy, and water recycling and reuse. Significant progress has been made in developing sound technical approaches to producing reliable sources of quality water by wastewater reclamation technologies. Continued research and demonstration efforts will result in further progress in the development of more cost-effective treatment processes and operations, which will be adopted in many water reuse applications in the future.
Role of advanced treatment in wastewater reclamation and reuse
9
REFERENCES Asano, T., Smith, R. G. and Tchobanoglous, G. (1985). Municipal Wastewater: Treatment and Reclaimed Water Characteristics. In: Irrigation with Reclaimed Municipal Wastewater -A Guidance Manual, Pettygrove, G. S. and Asano, T. (eds), Lewis Publishers, Inc. Chelsea, Michigan. Asano, T. (1998). Reclaimed Wastewater as a Water Resource. Asian Water, 14(1), 16-21. Asano, T. and Levine, A. D. (1998). Wastewater Reclamation, Recycling, and Reuse: An Introduction. In: Wastewater Reclamation and Reuse, Asano, T. (ed). Water Quality Management Library - Volume 10, Technomic Publishing Co., Inc. Lancaster, Pennsylvania. Metcalf and Eddy, Inc. (1991). Wastewater Engineering, Treatment, Disposal, and Reuse. Third Edition. McGraw-Hill, Inc., New York. U.S. Environmental Protection Agency and U.S. Agency for International Development (1992). Guidelines for Water Reuse, EPAl625/R·92/004, September, Washington, D.C.
Pergamon
War SCI Tech Vol. 40, No. 4-5, pp. 1-9 , 1999 C 1999
Published by Elsevier Science LIdon behalfof the IAWQ Pnnted In Great Bntarn, All rights reserved 0273-1223199 $20.00 + 0.00
PU: S0273-1223(99)00479-S
THE ROLE OF ADVANCED TREATMENT IN WASTEWATER RECLAMATION AND REUSE Rafael Mujeriego* and Takashi Asano** • Escuela Tecnica Super ior de lngenieros de Caminos , Canales y Puertos de Barcelona, Universidad Politecnica de Cataluiia, Gran Capitan, sin . 08034 Barcelona. Spain •• Department ofCivil and Environmental Engineering. University ofCalifornia at Davis. Davis. CA 95616-231 I, USA
ABSTRACT The role of advanced wastewater treatment in wastewater reclamation and reuse is reviewed. Most of the current wastewater reclamation and reuse technologies are essent ially denved from those used in water and wastewater treatment. However, opportunities for adopting technological innovations are much greater for water reuse applications. because reclaimed water will have an economic value as an alternative water supply. Significant progress has been made in developing sound technical approaches to producing high quality and reliable water sources from reclaimed wastewater. This overview paper provides an assessment of technological advances in water reclamation and reuse, which has been dubbed as "the greatest challenge of the 21st century." The paper was presented at the Opening Plenary Sess ion of the lAWQ's Advanced Wastewater Treatment, Recychng and Reuse Conference in Milan, Italy on September 14, 1998 . iO 1999 Published by Elsevier SCIence Ltd on behalf of the lAWQ . All nghts reserved
KEYWORDS Advanced treatment; public health; recycling; reliability; reuse; water purification; water quality; wastewater treatment; water reclamation. INTRODUCTION Advanced treatment plays a critical role in the effective treatment of municipal and industrial wastewater to meet water quality objectives for water reuse and to protect public health. Conventional and advanced wastewater treatment consist of a combination of physical, chemical, and biological processes and operations to remove settleable, suspended, and dissolved solids, organic matter, metals, nutrients, and pathogens from wastewater. Most of the current wastewater reclamation and reuse technologies are essential1y derived from those used in water and wastewater treatment. However, opportunities for adopting technological innovations are much greater for water reuse applications, because reclaimed water will have an economic value as an alternative water supply. Furthermore, contrary to the disposal of treated effiuent, where federal or national regulations are enforced by "command and control" methods of water pollution control, water reclamation and reuse allow more flexibility in water quality management, and consequently more possibilities for adopting new technologies. In evaluating wastewater reclamation technologies, however, the overriding considerations are the operational reliability of each unit process or operation, and the overall capability of the complete treatment
R. MUJERIEGOand T. ASANO
2
system to provide a reclaimed water that meets established wastewater reclamation criteria. As a result, additional treatment processes and operations may be required in certain cases for removal of chemical contaminants and removal or inactivation of microbiological pathogens. Similarly to conventional wastewater treatment, the general terms used to describe different degrees of treatment, in order of increasing treatment level, are preliminary, primary, secondary, and tertiary/advanced treatment. A disinfection step for removal or inactivation of pathogenic organisms is often the final treatment prior to storage and distribution. Because of cost considerations, preliminary, primary, and secondary treatments are generally considered water pollution control requirements; the additional treatment required for water reuse is normally designated as tertiary or advanced treatment. The goal in designing a wastewater reclamation and reuse system is to develop integrated cost-effective process combinations that are capable of reliably meeting the water quality objectives required for water reuse. Figure 1 shows a generalized view of wastewater treatment processes and operations as well as effluent reuse schemes. Based on water quality requirements, any effluent stream can be used as reclaimed water for various beneficial uses. The range of applicable technologies may include: 1) lagoons, wetlands, and natural treatment systems, 2) advanced physical-chemical treatment, 3) advanced biological treatment including biological nutrient removal (BNR), 4) advanced oxidation processes, 5) membrane separation and membrane bioreactors, 5) disinfection technologies, and 6) innovative reactor designs such as sequencing batch reactors and advanced mixing devices.
Preliminary
Secondary
Primary
Advanced E",uon'
E",uon'
Low Rate Proc_... otabohutoon pondI __ted "goonl
D_flCtion
High RI'" Proc_ activated ,Iudge trickling "her. rotating biocontactors
Nhrogon RI""",01 nltrlflcallon - denltrif,catlon -'ective ion exchange break po,nl chlorinet,on gllllrlpping overiand flow Phoophorul R_II chemica' precipitation
Sulpended BoIIdI Remo.oI
Sludge ProcHllng
I
I
cfte"\lcal coagulltlon filtration
8laloglc0l
Non blolOll1cl1
1II1cklnlng dlg..tlon
tIIlckenfng
Orglnlce Ind Mllale Remo.1II
condtttoning
dewatering
dewa'e,ing
carbon adsorption
IlIIlr centrifuge
drying bedl
filter centrifuge ,nCIneration
DI......ed SoUdl lIemo.oI
,ev. . . OBrnoetl etectrocUalYSl1 dlstlll.hon
Figure 1. Generalizedwastewatertreatmentprocesses and operations, and effluent reuse schemes (adapted from Asano, Smith and Tchobanoglous, 1985).
Figure 2 shows a conceptual view of the water quality changes that take place during municipal use of water in a time sequence. Through the process of water treatment, unpolluted sources are used to produce water of such a high quality that it meets applicable standards for drinking water. Municipal and industrial uses degrade water quality, and the processes necessary to recover wastewater quality then become a matter of concern for wastewater treatment. In the actual case, treatment is carried out to the point required by regulatory agencies for protection of aquatic environments and other beneficial uses. The dashed line represents an increase in treated water quality as required by water reuse. Ultimately as the quality of treated
Role of advanced treatment in wastewater reclamation and reuse
3
wastewater approaches that of unpolluted natural water, the concept of wastewater reclamation and reuse is generated. Further advanced wastewater reclamation technologies, such as activated carbon adsorption, advanced oxidation, and reverse osmosis, can generate a water of much higher quality than conventional drinking water, and the product obtained is thus designated repurified water. Today, technically proven wastewater reclamation or purification processes exist to provide water of almost any quality desired.
Tim. Sequence (No Sc.le)
Figure 2. Water quality changes during municipal uses of water
10
a time sequence (after Asano, 1998).
WASTEWATER RECLAMAnON TECHNOLOGIES Table 1 shows a summary of the major unit operations and processes used for wastewater reclamation. At present, the dominant wastewater reuse applications, worldwide, are irrigation of agricultural lands, parks and golf courses. However, there has been considerable progress in reclaimed water applications in the urban setting such as toilet flushing, cooling, fire fighting, and stream flow augmentation. Furthermore, future use of reclaimed water may involve a completely controlled "pipe-to-pipe" system with an intermittent storage step, or it may include blending of reclaimed water with non-reclaimed water, either directly in an engineered system or indirectly through a surface water supply reservoir or a groundwater recharge scheme. When reclaimed wastewater applications have a potential route for human exposure, the major acute health risks are those associated with exposure to biological pathogens including bacterial pathogens, helminths, protozoa, and enteric viruses. From a public health and process control perspective, the most critical group of pathogenic organisms is enteric viruses, because of the possibility of infection from exposure to low doses and the lack of routine, cost-effective methods for virus detection and quantification. In addition, treatment systems that can effectively remove viruses will most likely be effective for controlling other pathogenic organisms. Thus, it is essential to produce virtually pathogen-free effluent for water reuse applications that have the potential for significant human exposure or contact, such as spray irrigation of food crops eaten uncooked, irrigation of parks and playgrounds, and supply of unrestricted recreational impoundments where swimming may take place. Tertiarv and advanced treatment After conventional biological treatment processes (e.g., activated sludge process, trickling filters, and oxidation ponds), tertiary or advanced treatment can be applied to remove additional dissolved and suspended contaminants, nutrients, specific metals, and other harmful constituents (see Fig. 1). Filtration. Filtration is a solid/liquid separation process that effectively removes suspended particles larger than about 3 um. When wastewater passes through a column of granular media, particles are removed by impaction, interception, and physical straining. As particles accumulate in the filter media, headloss through the filter increases, making it necessary for the filter to be cleaned by backwashing using a combination of air and water scour. To avoid rapid build-up of headloss and reduction of run time, filtration is most effective if the particle concentration (as TSS) is lower than 20 mg/L, Filtration can be used downstream of
4
R. MUIERIEGO and T. ASANO
primary sedimentation (primary effluent filtration), or downstream of secondary sedimentation (tertiary filtration). Table I. Unit operations and processes used in wastewater reclamation (Adapted from Asano and Levine, 1998) Process
Description
Application
Solldlliquld separation Coagulation Flocculation Filtration Sedimentation
Additionof chemicalsto destabilize colloids and suspendedmatter Particleaggregation Particleremovalby granularmedium Gravitysettlingof particulatematter, chemicalfloc, and precipitates
Promoteparticle destabilization to improve flocculation and solids removal Particleagglomeration upstreamof liquid/solidseparationoperations Removalof particleslarger than about 3 11m Liquid/solids separation
Blolo&lcal treatment Aerobicbiological treatment Oxidationpond Disinfection
Biologicalmetabolismof wastewaterand solids by microorganisms in an aeration basin Ponds with 2 to 3 feet of waterdepth for mixingand sunlightpenetrationand oxidationand synthesisby algae The inactivationor removalof pathogenic organismsusing oxidizingchemicals, ultravioletlight, caustic chemicals,heat, or physical separationprocesses
Incorporationand removalof organicmatter from wastewaterby synthesisinto microbial cells and CO2 and H20 Reductionof suspendedsolids,BOD, fecal bacteria and ammonia
Process by whichcontaminants are physicallyadsorbed onto the carbon surface Wastewateris distributedover a packing through which forced air is drawn to extractammoniaand volatileorganicsfrom the water droplets Exchangeof ions between an exchange resin and water using a flow through reactor
Removalof hydrophobic organiccompounds
Protectionof public health
Advanced treatment Activatedcarbon Air stripping
Ion exchange
Lime treatment Membrane processesand reverseosmosis
Removalof ammonianitrogenand some volatileorganics
Softeningand removalof selectedionic contaminants; Effectivefor removalof cations such as calcium,magnesium, iron and anions such as nitrate The use oflime to precipitatein high pH Used to stabilizelime-treated water, to reduce variouscations and metals from water and its scale formingpotential wastewater Removalof impurities,bacteriaand viruses, Pressuredriven membrane processesto dissolvedsalts from water and wastewater separateimpurities,colloids,ions from water,based on size exclusionor molecular diffusion
As pathogenic organisms are associated with particles, filtration is an effective process for reducing pathogen concentration in wastewater streams, and provides an excellent pre-treatment for disinfection. Filtration is stipulated as a required treatment process in many applications, because of its favorable effect on aesthetics, particle removal, and disinfection effectiveness. If water is to be treated by activated carbon, ion exchange, or reverse osmosis, filtration is used to reduce solids loading on these processes and improve their overall effectiveness. Adsorption. Activated carbon adsorption is effective in removing hydrophobic organic compounds from surface and groundwater sources. Compounds with low water solubilities, such as organic solvents and chlorinated organic solvents, are adsorbable because of their low water solubility. Water soluble compounds
Role of advanced treatment in wastewater reclamation and reuse
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and larger compounds are better removed by oxidation or ultrafiltration. In most cases testing is necessary (isotherm evaluation, dynamic adsorption testing) to determine the applicability of activated carbon treatment to a water of specific quality. Membrane Processes. Among advanced treatment processes, membrane applications have clearly emerged as one of the promising alternatives to conventional advanced physical-chemical treatment, which usually includes chemical coagulation, flocculation, and granular-medium filtration. Membrane processes, ranging from microfiltration to reverse osmosis, are finding their way to cost-comparable applications for removal of microorganisms in membrane bioreactors, and for removal of trace organic substances, specific ions, and dissolved solids. Membrane processes include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis. Microfiltration is effective for removal of particles, and can be competitive with conventional granular medium filtration. Ultrafiltration is effective for removal of particles and macromolecules. Nanofiltration and reverse osmosis are effective for removal of dissolved ions from liquids. While membranes have multiple applications, the useful life of a membrane closely depends on avoiding conditions that will cause fouling, scaling, or chemical interactions. Membrane process success is highly dependent on appropriate pretreatment. Pretreatment options include filtration for coarse particle removal, scale control, and chemical addition. Post treatment includes water pH stabilization to prevent corrosion, and air stripping. Disinfection. Disinfection is an essential component of many wastewater reclamation and reuse treatment systems. The objective of disinfection processes is to inactivate/destroy pathogenic organisms. Disinfection is typically used as one of the final treatment processes in a treatment system. Chemical disinfection practices are based on addition of a strong oxidizing chemical such as chlorine, ozone, hydrogen peroxide, or bromine. Ultraviolet radiation is an alternative to oxidation process that can achieve disinfection. Other methods for decreasing the microbial content of reclaimed wastewater include exposing pathogenic organisms to high pH environments as in lime treatment. Alternatively, organisms can be effectively removed by physical methods, such as membrane filtration systems. The most common type of disinfection system is chlorine at dosages ranging from 5 to 15 mg/L, with a recommended contact time of 30 minutes to 2 hours. For wastewater reuse, it is important to remove residual chlorine to prevent complications in downstream beneficial uses. Dechlorination is often used as a final step, and is accomplished using sulfur dioxide or other reducing agents. Activated carbon adsorption is also effective for removal of residual chlorine. Ultraviolet disinfection has been demonstrated to provide a viable alternative to chemical disinfection processes, at doses ranging from 100 to 120 mWs/cm2, and higher. The performance ofUV disinfection is influenced by water turbidity and UV lamp intensity, which in tum can be reduced by lamp age and the fouling characteristics of the wastewater. Upstream filtration is usually essential to prevent particle shielding of pathogenic microorganisms thus reducing the effectiveness ofUV disinfection. WASTEWATER REUSE APPLICAnONS The purpose of this section is to provide an overview on water quality requirements for various water reuse applications, with particular emphasis on public health aspects of wastewater reuse. Table 2 shows a summary of the seven major categories of wastewater reuse, with their treatment goals and some illustrative applications. Considerable experience is available in many parts ofthe world on agricultural and landscape irrigation with reclaimed water, with an increasing number of projects devoted to the beneficial reuse for recreational and environmental purposes. Although industrial reuse has been traditionally concentrated in cooling water systems, increasing interest has developed in industrial areas for reclaimed water as an alternative and reliable supply for process water. Water management in industry involves: 1) the use of higher quality raw materials, 2) the use of cleaner technologies, 3) the adoption of more water efficient technologies, and 4) the application of advanced treatment technologies to promote materials recovery and water reuse. Membrane,
R. MUJERIEGO and T. ASANO
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sorption-desorption, and photo-oxidation processes are common alternatives in the textile industry, either in current application or in the demonstration phase. In addition to technical performance characteristics, investment and operation and maintenance costs appear as determining factors in the final selection process. This multiple objective approach clearly illustrates that advanced treatment for wastewater reclamation and reuse isjust one, but an important element of water resources management. Table 2. Categories of municipal wastewater reuse (Adapted from U.S. EPA, 1992, and Asano and Levine, 1998)
Category of wastewater reuse Urban use Unrestricted
Restricted access irrigation
Food crops
Non-food crops and food crops consumed after processing
Recreational use Unrestricted
Treatment goals
Secondary, filtration, disinfection BODs: <10 mg/L Fecal coliform: NO/I 00 mL Turbidity: < 2NTIJ Clz residual: I mg/L pH 6t09 Secondary and disinfection BODs: < 30 mg/L TSS: < 30 mg/L Fecal coliform: < 200/100 mL Clz residual: I mgIL pH 6t09 Secondary, filtration, disinfection BODs: < 10 mg/L Turbidity: < 2NTU Fecal coliforms: NO/I 00 mL ci, residual: I mg/L pH 6t09 Secondary, disinfection BODs: < 30 mg/L TSS: < 30 mglL Fecal coliforms: < 200/100 mL Clz residual: I mg/L pH 6 t09
Secondary, filtration, disinfection BODs: < 10 mg/L Turbidity: < 2NTIJ Fecal Coliforrns: NO/I 00 mL Clz residual: I rng/L pH 6 to 9 Restricted Secondary, disinfection BODs: < 30 mg/L TSS: < 30 mg/l, Fecal Coliforms: < 200/100 mL Clz residual: I mg/l, pH 6t09 Environmental reuse Site specific treatment levels pH Dissolved oxygen Coliforms, Nutrients Site specific Groundwater recharge Industrial reuse Secondary and disinfection BODs: < 30 mg/L TSS: < 30 mg/L Fecal coliform: < 200/100 mL Safe Drinking Water Requirements Potable reuse
Example apphcations
Landscape irrigation: parks, playgrounds, school yards, fire protection, construction, ornamental fountains, aesthetic impoundments, In-building uses: toilet flushing, air conditioning
Imgation of areas where public access is infrequent and controlled: freeway medians; golf courses; cemeteries; residential areas; greenbelts
Crops grown for human consumption and consumed uncooked
Fodder, fiber, seed crops, pastures, commercial nurseries, sod farms. Commercial aquaculture
No limitations on body-contact: lakes and ponds used for swimming, snowmaking
Fishing, boating, and other non-contact t recreational activities
Use of reclaimed wastewater to create artificial wetlands, enhance natural wetlands and sustain stream flows Groundwater replenishment; salt water intrusion control; subsidence control Coohng-system make-up water, process waters, boiler feed water, construction activities and washdown waters Blending in municipal water supply reservoir; pipe to pipe supply
Role ofadvanced treatment ID wastewater reclamation and reuse
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To accomplish the high degree of treatment and reliability required for potable reuse, a treatment train of advanced unit operations and processes has to be implemented, including lime clarification, nutrient removal, recarbonation, filtration, activated carbon adsorption, demineralization by reverse osmosis, and disinfection with chlorine, ozone, ultraviolet radiation, alone or in different combinations. The conventional advanced treatment train alternatives used for potable reuse include nutrient removal by high lime and granular activated carbon, with or without reverse osmosis. The treatment alternative of high lime treatment and carbon adsorption, followed by disinfection has typically been applied for water reclamation before indirect potable reuse. The reverse osmosis process is normally applied when build up of dissolved solids in the system is to be prevented. Granular activated carbon followed by reverse osmosis is very effective in removing a large number of pollutants. However, the electrical energy requirements of reverse osmosis, together with the cost of membrane replacement and antifouling controls, and brine disposal make this alternative treatment expensive, and only applicable in areas where water availability is low and its cost is high. Technological advances in membrane development and manufacturing have been brought down the cost of membrane replacement and maintenance over the last decade, and it can be expected to become an increasingly competitive treatment option for water and wastewater treatment in the near future. Expected reliability and average treatment train performance are summarized in Table 3. The concentration levels indicated in Table 3 are comparable or better than those commonly applicable to conventional sources for potable water production. However, treatment reliability and the presence of trace organic compounds make direct potable reuse an alternative applicable only in extreme situations. Table 3. Reliability data and average treatment train performance for carbon adsorption and lime treated activated sludge effiuent (Adapted from Metcalf & Eddy, 1991)
Constituent
BOD COD TSS NHJ-N
Phosphorus Oil & grease Arsenic Cadmium Chromium Copper Lead Mercury Selenium Zinc TOC Turbidity, ntu Color Foaming agents TOS 1.0.=Insufficient data.
Average reliability
Average removal, % 100 100 100 100 100 97 61 98 100 98 99 23 7 98 100 100 93 92 95
10%
50%
90%
100 100 100 97 100 100 93 100 100 100 100 31 26 100 100 100 100
100 100 99 81 100 98 63 98 98 99 98 18 12 95 98 100 94 84
89 97 87 48 99 73 0 87 84 98 78 0 0 58 83 95 56
1.0.
!.D.
!.D.
!.D. !.D.
Average effluent concentration,mgfL 0 0 0 0 0 2 0.003 0.0002 0 0.002 0.001 0.028 0.006 0.008 0 0 5 0.17 129
WASTEWATER REUSE BENEFITS AND CHALLENGES Through integrated water resources planning, the use of reclaimed wastewater may provide sufficient flexibility to allow water agencies, as well as individual industrial users, to respond to short-terms needs and
8
R. MUJERIEGO and T. ASANO
to increase sustainable, long-term water supply reliability, without constructing additional storage or conveyance facilities at substantial economic and environmental expenditures. Significant progress has been made in developing sound technical approaches to producing reliable sources of quality water by wastewater reclamation processes. Continued research and demonstration efforts will result in further progress in the development of water reuse applications. Some key topics involved are: 1) 2) 3) 4) 5) 6)
Assessment of health risks associated with trace organic substances. Improvements in monitoring approaches to evaluate microbiological quality. Application of membrane processes to produce high quality reclaimed water. Evaluation of the effect of reclaimed water storage on water quality. Evaluation of the fate of microbiological, chemical, and organic contaminants in reclaimed water. Evaluation of the long-term sustainability of soil-aquifer treatment systems.
To date the major emphasis on wastewater reclamation and reuse has been for nonpotable applications such as agricultural and landscape irrigation, industrial cooling, and in-building applications such as toilet flushing. Direct potable reuse of reclaimed municipal wastewater is, at present, limited to extreme situations. While indirect potable reuse by groundwater recharge or surface water augmentation has gained support, some concerns still remain regarding trace organic substances, treatment and reuse reliability, and particularly, public acceptance. A key-determining factor in promoting wastewater reclamation, recycling and reuse is the continued development of cost-effective treatment systems. While water supply reliability may justify the adoption of advanced treatment systems for industrial reuse, which brings the cost of reclaimed water to equal or higher values than those of conventional water supplies, other potential beneficial uses may require the least cost reclamation alternatives before they may be considered for implementation. The impressive cost reductions and performance improvement experienced by the advanced treatment process during the last two decades anticipates a most promising future for expanding its application. With development progress driven by the cost margins of the industrial sector, numerous advanced treatment processes are expected to be cost-effective for application in other potential beneficial uses of reclaimed water, such as urban use, and groundwater recharge. SUMMARY AND CONCLUSIONS Advanced wastewater treatment plays an increasingly critical role in the treatment of municipal and industrial wastewater to meet water quality objectives for water reuse and protect public health. Opportunities of adopting technological innovations are much greater in water reuse applications, because reclaimed water will have an economic value as an alternative water supply source. In evaluating water reclamation technologies, overriding considerations are the operational reliability of each unit process or operation, and the overall capability of the complete treatment system to provide reclaimed water that meets established wastewater reclamation criteria. After conventional biological processes, tertiary or advanced treatment can be used to remove additional dissolved and suspended contaminants, nutrients, specific metals, and other harmful constituents. The range of advanced treatment technologies currently in use includes granular medium filtration, adsorption, membrane processes, and disinfection. Using these existing as well as emerging treatment technologies, future water management options may include 1) use of high quality raw materials, 2) the use of cleaner technologies, 3) the adoption of more water efficient technologies, and 4) the application of advanced treatment technologies to promote materials recovery, energy, and water recycling and reuse. Significant progress has been made in developing sound technical approaches to producing reliable sources of quality water by wastewater reclamation technologies. Continued research and demonstration efforts will result in further progress in the development of more cost-effective treatment processes and operations, which will be adopted in many water reuse applications in the future.
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REFERENCES Asano, T., Smith, R. G. and Tchobanoglous, G. (1985). Municipal Wastewater: Treatment and Reclaimed Water Characteristics. In: Irrigation with Reclaimed Municipal Wastewater -A Guidance Manual, Pettygrove, G. S. and Asano, T. (eds), Lewis Publishers, Inc. Chelsea, Michigan. Asano, T. (1998). Reclaimed Wastewater as a Water Resource. Asian Water, 14(1), 16-21. Asano, T. and Levine, A. D. (1998). Wastewater Reclamation, Recycling, and Reuse: An Introduction. In: Wastewater Reclamation and Reuse, Asano, T. (ed). Water Quality Management Library - Volume 10, Technomic Publishing Co., Inc. Lancaster, Pennsylvania. Metcalf and Eddy, Inc. (1991). Wastewater Engineering, Treatment, Disposal, and Reuse. Third Edition. McGraw-Hill, Inc., New York. U.S. Environmental Protection Agency and U.S. Agency for International Development (1992). Guidelines for Water Reuse, EPAl625/R·92/004, September, Washington, D.C.