United States
There has been a growing concern in the United States, generally originating from residents near biosolids application sites, that land application endangers their health and welfare. A recent report by the National Research Council (2002), addressing alleged inadequacies in the USEPA's regulatory and research program, received much attention from the news media and intensified the discussion of risks from biosolids application to land.
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.
The UK Department for Environment, Food and Rural Affairs (DEFRA) announced that they intend to revise the Regulations and Code of Practice to take account of the safe sludge matrix (Department for Environment, Food and Rural Affairs, 2002). It is proposed to establish process monitoring based on the principles of hazard analysis and critical control point (HACCP) allied to an end product standard (Table 6). The rationale behind this approach is to establish the critical control points within the sludge treatment process that assure pathogen reduction. Wherever possible, monitoring should be performed to demonstrate that the process is operating within the control limits set. Microbiological analysis of the final treated sludge serves to verify that the controls are effective. The sampling frequency necessary for verification is significantly less than required if product quality was to be assessed solely on the basis of microbiological analysis.
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