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a Public Health Section, United Utilities Water, Lingley Mere Business Park, Great Sankey, Warrington, WA5 3LP, UK
b Environmental Consultant, 1117 Stormy Way, Cincinnati, OH 45230
* Corresponding author (alan.godfree{at}uuplc.co.uk)
Received for publication February 27, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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Abbreviations: CCP, critical control point • HACCP, hazard analysis critical control point • MAD, mesophilic anaerobic digestion • STEC, shiga toxin–producing Escherichia coli
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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Historically, the development of sewage treatment was driven by the need to reduce environmental contamination brought about by the uncontrolled discharge of human wastes to rivers and streams. The principal goal of the various forms of sewage treatment was (and remains) removal of gross solids and to mitigate the polluting effect by reducing the readily assimilable organic fraction of the settled sewage. Commonly used treatment processes such as anaerobic digestion were neither designed, nor operated specifically, to remove or inactivate pathogenic microorganisms, and the mechanisms of pathogen inactivation are poorly understood in some instances. For example, process objectives such as volatile solids reduction and pathogen reduction may respond to changes in process controls in different ways. This can result in practical difficulties for operators of treatment facilities trying to achieve specific microbiological standards or pathogen reduction goals where these are a relatively new requirement, as in the UK, for example.
This paper reviews the effect of the wastewater and sludge treatment on pathogens. Information about the mechanisms of pathogen inactivation obtained from laboratory and field studies can be used to prepare operating regimes designed to assure that pathogen reduction goals are achieved consistently. Impending changes to the European and United Kingdom regulations herald a move away from end product testing of microbiological content to a more proactive means of assuring product quality based on a system of quality management, predicated on hazard analysis critical control point (HACCP) principles.
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EFFECTS OF TREATMENT ON MICROORGANISMS |
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TOPABSTRACTINTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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The range of pathogen inactivation efficiency reported is large, and depends on the extent of the treatment process and variation between operating conditions even for the same generic treatment process (Table 3).
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The inactivation of indigenous E. coli in full-scale sludge treatment processes was investigated during a 3-mo study of nine different sludge treatment processes at 35 sites in the UK, all of which were operating in accordance with national Codes of Practice (Department of the Environment, 1989). All processes surveyed reduced the numbers of E. coli. So-called "enhanced" treatment processes (analogous to Class A), for example, composting, lime addition, and thermal drying, reduced numbers of E. coli to the detection limit of the analytical method (United Kingdom Water Industry Research, 1999). For all of these methods, 
90% of results showed bacterial reductions of 6 log. Lagooning of sludge also significantly reduced numbers of E. coli and, depending on the method of operation, reductions in the order of 5 log were observed. Mesophilic anaerobic digestion (MAD), the process performed at the majority of sites surveyed, reduced numbers of E. coli by, on average, between 1.4 and 2.3 log depending on the solids content of the product. For sites producing a liquid product (2–4% dry solids), 78% of all reductions were in the range 1 to 2 log. Where digested sludge was subsequently dewatered to produce a cake, 89% of analyses showed reductions in the range 2 to 4 log (Table 4). This study demonstrated clearly the variability in the degree of microbial reduction achieved at sludge treatment facilities operating the same generic treatment process. The reasons for the differences are unclear.
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Studies of anaerobic mesophilic digestion under laboratory conditions showed that oocysts added to the contents of a digester operating at 35°C rapidly lost viability (as measured by excystation), decreasing to 17% after 3d from an initial 81% viability (Whitmore and Robertson, 1995). Losses of viability in distilled water and anaerobic sludge at 35°C were similar, amounting to 90% after 18 d, indicating that the principal effect on viability was temperature. Oocysts exposed to mesophilic anaerobic digestion for 3 d and then stored for a further 14 d were completely inactivated. Aerobic digestion or pasteurization, both at 55°C, caused 92% loss of viability in 5 min. Thermophilic anaerobic digestion at 50°C resulted in complete inactivation within the first 24 h (Whitmore and Robertson, 1995).
In one of the largest studies of its kind, Horan and colleagues examined the survival of a range of enteric pathogens under laboratory conditions (United Kingdom Water Industry Research, 2002). The pathogens evaluated were salmonellae (Salmonella senftenberg, S. dublin, S. enteritidis, S. typhimurium), Campylobacter jejuni, Listeria monocytogenes, E. coli O157:H7, Cryptosporidium, and poliovirus. The sludge treatment processes were MAD, pasteurization followed by MAD, lime treatment, and composting.
Mesophilic Anaerobic Digestion
A series of bench-top chemostats (10-L capacity) were used to model the process operating at 35 ± 3°C with a mean hydraulic retention of 12 d (Horan et al., 2004). The feed sludge was spiked with the organisms of interest to assess inactivation across a wide range with the digesters operated on a semicontinuous basis, being fed manually once a day. The numbers of all bacterial pathogens were reduced during MAD. Log inactivation ranged from 0.34 log for C. jejuni, 2.23 log for L. monocytogenes, 3.8 log for E. coli, and 4.24 log for S. senftenberg. The primary sludge digestion stage of MAD was very effective at removing both poliovirus and Cryptosporidium. Poliovirus was inactivated very rapidly and a 6.2 log removal was demonstrated. Primary sludge digestion completely reduced the viability of Cryptosporidium oocysts, from 76 to 96% to 0%, equivalent to a 3.2 log removal. Secondary sludge digestion at 15°C for 14 d (operated as a batch process), provided additional inactivation of pathogens surviving the primary stage of digestion and resulted in log removals of 1 of E. coli, 1.95 log removal of S. senftenberg, and 0.34 log removal of C. jejuni. A survey of three full-scale treatment works revealed that the viability of Giardia cysts was between 5.45 and 21.38% in raw sludge. After MAD, viability was below the detection limit in all cases and represented a 1 log inactivation.
Pasteurization followed by Primary Digestion
Raw sludge was subjected to two heating regimes: 70°C for 30 min or 55°C for 240 min (Department of the Environment, 1989). All added bacteria were eliminated by pasteurization at 70°C for 30 min and this gave values for log removal in the range 5.31 to 9.0. The observed log removal achieved by pasteurization depended on the numbers of added organisms that could be achieved in the feed sludge. Pasteurization at 55°C for 240 min also eliminated all the bacterial spikes in one trial, with log removals again ranging from 5.31 to 9.0. In a second trial, low numbers of L. monocytogenes, C. jejuni, and S. senftenberg survived, which the investigators ascribed to poor mixing of the sample. Poliovirus was eliminated both at 70°C for 30 min (an 8.41 log removal) and at 55°C for 240 min (an 8.43 log removal). The viability of Cryptosporidium oocysts was reduced from 53 to 1.3% in pasteurization at both 70°C for 30 min and at 55°C for 240 min. Those oocysts that survived were completely killed after just 2 d of primary digestion.
Lime Stabilization
Trials were performed using a combined primary and secondary sludge (waste activated sludge) dewatered to a cake containing about 25% dry solids content. Finely divided crushed quicklime (calcium oxide) was added at a rate of 12% (w/w), sufficient to raise the pH to at least 12 for 2 h. The process was highly effective at eliminating enteric pathogens and a complete kill was demonstrated for all bacterial species (L. monocytogenes, C. jejuni, S. senftenberg, S. typhimurium, and S. dublin), with a log removal in the range 6.1 to 9.7. On one occasion, small numbers of S. senftenberg survived treatment. Non-STEC strains of E. coli were also completely eliminated. In the two trials with poliovirus, >6 log (6.50 and 6.82 log) inactivation of poliovirus was observed following lime addition. The performance of the lime process against Cryptosporidium was variable, ranging from a 2 log loss in viability to no loss.
Composting
The time and temperature conditions specified in the UK Code of Practice for composting (Department of the Environment, 1989) were simulated by heating 100-g amounts of a 1:1 (v/v) mix of sludge and chopped barley straw, with the sludge having previously been spiked with the target organisms. Two heating regimes were investigated: 5 d at 40°C followed by 55°C for 4 h or 4 h at 55°C followed by 5 d at 40°C. Complete inactivation of non-STEC E. coli, equivalent to 6.18 log, was observed and C. jejuni, S. enteritidis, and S. dublin were completely removed. In a similar way, composting proved very effective for poliovirus and a complete removal of 9.55 log was demonstrated. For all of these organisms, the observed removal was the same regardless of whether the initial temperature was 40°C held for 4 d or whether it was 55°C held for 5 h. In contrast, the removal was less effective for L. monocytogenes and S. senftenberg, but was improved if the higher temperature regime was applied first; this provided additional removal in the order 0.3 to 0.8 log. Salmonella senftenberg exhibits increased resistance to heat compared with other strains of salmonellae and has been used in the validation of food production processes that rely on heat to inactivate pathogens (Murphy et al., 1999).
Thermophilic Processes
United Utilities Water has added an Alpha Biotherm thermophilic aerobic digestion (TAD) system to an existing MAD sludge treatment plant at Ellesmere Port (Cheshire, UK). Details of the installation and its operation have been described (Davies and Messerli, 2000). Briefly, following thickening (to between 4.5 and 8.6% dry solids), sludge is heated to approximately 35°C. Sludge is transferred to the TAD reactor where it undergoes mixing and aeration until the target temperature of between 65 and 70°C is attained. The reactor is locked out for an hour to allow pasteurization to take place. After 1 h, the treated sludge is cooled via a heat exchanger (to recover heat) and passed forward to the conventional MAD stage. Pasteurization temperatures are achieved through a combination of externally applied heat, by means of hot water heat exchanger, and biological heat from the exothermic growth of thermophiles in the TAD reactor. Microbiological data from a continuous 7-d commissioning period showed that indicator organisms and salmonellae were not detected, equivalent to a minimum 5 log reduction in Enterobacteriacae, 4.1 log for E. coli, and 3.5 log for salmonellae. Similar results were obtained from an operational autothermal thermophilic aerobic digestion (ATAD) plant in the Czech Republic with reported log reductions of 5.6 and 4.8 for fecal coliforms and Enterococci, respectively (Zabranska et al., 2003). Salmonellae (presence in 4 g) were detected in virtually all samples of raw sludge, whereas only 7 to 27% of samples taken at the end of the process were positive.
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RECENT DEVELOPMENTS |
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TOPABSTRACTINTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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Even before this increased awareness, some communities and counties (e.g., Kern County in California; Hay et al., 2000) responded to the concerns of citizens by adopting restrictive rules that essentially prohibited the application to land of Class B biosolids. In response, municipalities can either convert to Class A treatment of the biosolids or abandon the use of land application.
The movement to Class A, although generally increasing the cost of production, is not without benefit. Some costs of controlling and monitoring the processing and application of the biosolids are reduced, because record-keeping and application limits are less complex for a Class A product, particularly if the metals content is low enough to meet the "exceptional quality" requirements (USEPA, 1994). Transport costs are reduced if application sites closer to the treatment works can be found where Class A (but not Class B) biosolids can be utilized. The higher dry solids content of Class A products further reduces transport costs.
The process options usually selected for upgrading to Class A pathogen reduction have been heat-drying, composting, lime pasteurization, the N-Viro process (an alternative type of lime pasteurization), and thermophilic aerobic digestion. Composting is the method selected by most communities, although several large cities have chosen heat-drying (Milwaukee, Boston, New York). Lime pasteurization and thermophilic aerobic digestion have often been selected by smaller treatment facilities.
Pre-pasteurization before anaerobic digestion, popular in Europe, has not seen significant application in the United States, probably due to concern about problems with heating raw sludge in continuous flow heat exchangers. Numerous installations use thermophilic aerobic digestion before anaerobic digestion. The detention time between feeding of raw sludge and withdrawal of product is adequate to meet the requirements of the Part 503 time–temperature equation. Hay et al. (2000) report the use of post-pasteurization, that is, pasteurization following instead of preceding a vector attraction process such as mesophilic anaerobic digestion. The USEPA regulations (USEPA, 1993) address the elevated risk of growth of residual or contaminating bacterial pathogens under this condition. If the process that reduces pathogens is the terminal process in a Class A process train and it does not simultaneously reduce vector attraction, the biosolids must be applied to the soil surface within 8 h after treatment. The biosolids must then either be injected immediately into the soil, or be plowed in within 6 h.
In recent years there has been great interest in adapting thermophilic anaerobic digestion to meet the thermal treatment requirements of the Part 503 regulation. Conventional thermophilic digestion is not listed in the Part 503 regulation as a process for further reduction of pathogens (PFRP). As Shimp et al. (2003) noted, conventional means of achieving anaerobic digestion must be modified. For digestion conducted at 55°C in a continuously fed well-mixed digester, some feed "short-circuits," that is, it leaves the digester after a relatively short residence time, with the result that pathogens are not reduced to the level required by the regulation for a Class A biosolids. The time–temperature equation in the regulation requires that all particles be treated for a specified time at the temperature of the operation. For example, at 55°C, the time required is 24 h. As Shimp et al. (2003) observe, this requirement has been met by carrying out the digestion in two or more stages, with one of the stages operating on a fill–hold–draw sequence. Variations of this approach have been used by Los Angeles at its Hyperion plant (Wert et al., 2003) and at its Terminal Island plant (Shao et al., 2002) and by the Orange Water and Sewer Authority (OWASA) in Chapel Hill, NC (Willis et al., 2003).
The Orange Water and Sewer Authority's original plan (Farrell et al., 1996) demonstrates one of the problems faced by innovators attempting to develop new Class A processes. Three thermophilic digesters were intended to operate at 52°C in series, followed by a mesophilic digester, with each of the thermophilic digesters fed on a draw–hold–fill basis. The sum of the calculated reductions in pathogens in each additional stage were expected to produce the overall desired pathogen reduction. The proposed option would have required a full-scale demonstration that the process met the requirements of the USEPA's Pathogen Equivalency Committee (PEC). Not only is proving adequate pathogen reduction difficult on a full scale, but also the uncertainty of obtaining approval discourages the faint-hearted. Fortunately, OWASA subsequently discovered that the thermophilic digesters could operate successfully at temperatures above 55°C. Thus, the USEPA time–temperature requirement was satisfied by operating one of the digesters at >55.3°C for a hold time of 22 h (Willis and Gottschalk, 2001) and approval by the PEC was not required. The full-scale facility has been operating successfully for about 2 yr.
An alternative approach to draw–hold–fill operation has been developed by the city of Columbus, Georgia (Willis et al., 2003). This plant will utilize a continuous-flow, well-mixed thermophilic digester followed by a large-diameter pipe designed to accomplish a reasonable approximation of plug flow. The plug flow unit will operate at the same temperature as the digester. Because the digestion will continue in the plug flow unit, it satisfies the Part 503 regulation requirement that the vector attraction reduction process occur after or at the same time as pathogen inactivation. Because this configuration does not fit the specific requirements for use of the time–temperature requirement, PEC approval is required before the process qualifies as producing a Class A product. Data obtained on a pilot-scale demonstrating the validity of this approach are being examined by the PEC. Meanwhile, construction of a full-scale demonstration facility is underway at Columbus.
Adequate pathogen reduction can be obtained at time–temperature combinations that are considerably less severe than given by the USEPA's time–temperature equation. Tests at Perris, CA, showed that pasteurization at a temperature of 60°C and a detention time of 35 min would reduce pathogens to levels comparable with the levels achieved with pasteurization at 70°C with a detention time of 30 min (Hay et al., 2000). At 60°C, the Part 503 equation requires 4.78 h of contact. Ferran et al. (2002) demonstrated that at 55°C, approximately 4 h was needed in an acid-phase thermophilic digester, using a draw–hold–feed procedure, to reduce enteroviruses and viable helminth eggs by the required 3 and 2 logs, respectively. This is much less than the 24-h requirement of the Part 503 equation. Aitken et al. (2003) obtained similar results. Additional research will likely establish the excessive degree of conservatism in the USEPA equation with an expected reduction in the holding times required for a given temperature. This will provide shorter hold times, which will simplify operation of process schemes for producing Class A biosolids.
Low-technology processes for producing a Class A product, long-term lagooning and/or drying, have been used, most notably by the city of Chicago (Tata et al., 2000). The Chicago process has received approval by the USEPA Region 5, with the provision that the product, produced in large batches, be demonstrated to be free from viable helminth eggs. The need for this additional expensive testing is doubtlessly limiting more extensive use of such low-technology processing to produce Class A products.
The trend toward Class A processing is expected to continue. Many facilities are expected to use modifications of thermophilic digestion, because so many plants already have mesophilic digesters that can be adapted to run at thermophilic temperatures. The trend will probably accelerate if it is demonstrated that the Part 503 time–temperature equation is overly conservative and that lower temperatures will produce satisfactory pathogen destruction.
United Kingdom
In contrast to the United States, where direct health effects (of biosolids application) are a major concern, the driver to increase safeguards in the UK was the issue of food safety. Against this background of concern over food production methods, the water industry in the UK agreed to a set of guidelines matching the level of sludge treatment with the crop under cultivation. This agreement, made under the auspices of Water UK and representatives of the food suppliers, was concluded in 1998.
The safe sludge matrix (ADAS International, 2004) forms the basis of the agreement. It consists of a table of crop types, together with clear guidance on the minimum acceptable level of treatment for any sewage sludge–based product, which may be applied to that crop or rotation.
The main effect of the safe sludge matrix was the cessation of raw or untreated sewage sludge being used on agricultural land. From the end of 1999, all untreated sludges have been banned from application to agricultural land used to grow food crops. The matrix introduces the concept of two classes of treatment, analogous to the U.S. 503 Regulations (Table 5). When enacted in legislation, the regulations will introduce two categories of sludge: treated and enhanced treated. Treated sludge can only be applied to grazed grassland and must be deep-injected into the soil. The regulations require that there will be no grazing or harvesting within 3 wk of application. Where grassland is reseeded, sludge must be plowed down or deep-injected into the soil. More stringent requirements apply where sludge is applied to land growing vegetable crops and, in particular, those crops that may be eaten raw (e.g., salad crops). Treated sludge can be applied to agricultural land used to grow vegetables provided that at least 12 mo have elapsed between application and harvest of the following vegetable crop. Where the crop is a salad, which might be eaten raw, the harvest interval must be at least 30 mo. Where enhanced treated sludges are used, a 10-mo harvest interval applies.
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QUALITY MANAGEMENT SYSTEMS |
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TOPABSTRACTINTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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Obtaining the necessary pathogen reduction and/or end product standard requires assurance that treatment processes operate consistently within the specified parameters. In the UK, operators are employing quality management systems (QMS) as a means of demonstrating compliance. These are based on the HACCP principle. The HACCP system, as applied to food safety, originated in the 1960s. It was developed jointly by the Pillsbury Company, the U.S. Army, and the National Aeronautics and Space Administration (NASA) with the objective of ensuring the safety of foods being developed for the manned space program. The starting point was Failure Mode and Effect Analysis, an engineering system that looks at a process in its entirety (components and manufacturing stages) and seeks to identify what can go wrong. The HACCP system has become the universally accepted strategy for ensuring food safety (National Advisory Committee on Microbiological Criteria for Foods, 1997). Briefly, HACCP is a systematic approach to the identification, evaluation, and control of food safety hazards based on the following seven principles:
The first-stage hazard analysis is common for treatment works producing biosolids for land application and the scope can be reduced to the production of a process flow diagram. This should include all inputs into the sludge treatment, including imports such as primary sludges from smaller outlying facilities or other organic materials.
Determination of Critical Control Points
A critical control point (CCP) is defined as a step (in the treatment process) at which control can be used to prevent or eliminate a hazard, or reduce it to an acceptable level. The CCPs depend on the type of sludge treatment being used and the configuration of the works. Stressors used to inactivate pathogens include heat, pH, time, and moisture content.
Setting Critical Limits
A critical limit is one that separates acceptability from unacceptability. The critical limit may be specified within the definition of processes to significantly reduce pathogens (PSRPs) or processes to further reduce pathogens (PFRPs) (USEPA, 1993) or the UK Code of Practice (Department of the Environment, 1989). Where these are not specified, it will necessary to conduct a statistically robust program of sampling and analysis. In preparation for the proposed UK regulations, operators of sludge treatment facilities have performed sampling of the E. coli content of raw and treated sludges over typically a 28-d period. Data on potential CCPs is collected over the duration of the trial using spot measurements or the output from continuous monitors (e.g., temperature or pH).
Critical Control Point Monitoring
Each CCP requires defining a monitoring program to assess whether the CCP is under control. Ideally, this should make use of continuous, real-time measurements. This minimizes the requirement for additional sampling and analysis, but more importantly it allows for early warning that process conditions are moving out of specification.
Corrective Action
Under the proposed UK regulations, failure to achieve the critical limits for a CCP must trigger an action specified in the site HACCP plan. This may be to analyze the batch(es) of sludge for E. coli content to determine whether it complies with the microbiological standards for treated or enhanced sludge (Table 6). Biosolids not achieving the minimum end product standard are not permitted to be land-applied. In the absence of microbiological results, biosolids produced at times that the critical limits have not been achieved cannot be land-applied until such time as the process comes back into control. The options available for disposal of noncompliant biosolids include return to the sludge treatment process, landfilling, incineration, or application to nonagricultural land.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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To address concerns raised about the robustness of risk reduction measures in programs where historically reliance had been placed on post-application controls, utilities are considering additional treatment to produce Class A or enhanced biosolids. This puts operators of sludge treatment facilities in control of the greater part of the overall pathogen reduction goal. It is important, therefore, that operators understand the ability of the process to inactivate pathogens, the associated control points, and the critical limits. Carrington et al. (1982) observed that destruction of salmonellas in full-scale operational anaerobic digesters was consistently lower than that achieved during laboratory experiments using sludge from the same works. Factors affecting the efficiency of anaerobic digestion include poor mixing, short circuiting, uneven heating, and reduced hydraulic retention time due to accumulated debris in the base of the digesters. The efficiency of full-scale operational sludge treatment plants also varies over time as shown by Soares et al. (1994) when they investigated the removal of enteroviruses as measured in 12 monthly sampling events. Operational efficiency (in terms of pathogen inactivation) can significantly affect the net pathogen reduction, particularly for a Class A or enhanced treatment process. The effect of operational efficiency on human health risks associated with growing crops of land receiving biosolids has been modeled (United Kingdom Water Industry Research, 2003; Gale, 2004). Modeling showed that a process achieving >6 log destruction (of pathogens) for 99% of the time, but with a 0 log reduction for the remaining 1% of the time (e.g., due to short circuiting), achieves a net reduction of just 2 log, giving rise to a 10000-fold increase in the relative risk. The relative effect on less efficient processes (Class B, Treated) of operational underperformance was markedly smaller. Consider a process capable of achieving 2 log reduction under ideal conditions. Inefficiency resulting in 1% of the sludge receiving no pathogen inactivation is equivalent to net destruction of 1.7 log. The modeling demonstrates the crucial importance of characterizing the sludge treatment process, identifying the critical control points, and establishing critical limits linked to monitoring. Thereafter, routine measurement of the CCPs may be used to demonstrate that the process is operating efficiently and achieving the necessary level of pathogen destruction.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONEFFECTS OF TREATMENT ON...RECENT DEVELOPMENTSQUALITY MANAGEMENT SYSTEMSCONCLUSIONSREFERENCES |
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