Sewage
The primary purpose of treatment is to reduce the amount of carbonaceous material and ammonia to a level consistent with the assimilative capacity of the receiving water. Coincidentally, treatment reduces the numbers of indicator organisms and pathogens to an extent that depends on the nature of the treatment process. Reduction in numbers of indicator organisms and pathogens is brought about by the combined effects of separation of solid material (most microorganisms are associated with solids), predation, competition from naturally occurring organisms, and inactivation due to changes in pH and temperature. The effect of various treatment processes on the numbers of indicator bacteria and pathogens is summarized in Tables 1 and 2, and represents removal of solids-associated organisms or inactivation due to biotic factors such as predation or exposure to sunlight.
Sewage SludgeThe treatment of solids is designed to stabilize sewage sludge and reduce its odor. Depending on the process, some degree of pathogen inactivation will occur, but optimizing sludge stabilization specifically for pathogen reduction has not been normal practice.
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).
Until recently, research into the effects of treatment concentrated on those pathogens traditionally associated with excreta and that were widespread in the community (e.g., salmonellas) or those more likely to exhibit resistance to sludge treatment processes (e.g., Ascaris). In comparison, little is known about the pathogens that have only recently emerged as public health issues, most notably Escherichia coli O157:H7 and Cryptosporidium. There is little information available on the fate of E. coli O157:H7 (and other shiga toxin–producing E. coli [STEC]) during the treatment of wastewater and sewage sludge. Despite the highly infectious nature of STEC and the presence of multiple virulence factors, there is evidence that it is no more resistant to inactivation during sludge treatment than the non-STEC strains that constitute the majority of E. coli normally present in sewage and sludge (United Kingdom Water Industry Research, 2002).
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.
Cryptosporidium parvum is another pathogen of increasing importance. Wastewater discharges and runoff from agricultural land are an important source of Cryptosporidium oocysts found in watersheds. The transmission of cryptosporidiosis is often zoonotic and the possibility exists of foodborne infection arising from the use of sewage sludge in agriculture. Stadterman et al. (1995) found that a laboratory activated sludge plant removed 98.6% of seeded C. parvum oocysts. In a comparison of different treatment regimes, activated sludge and anaerobic digestion were the most effective means of removing oocysts, the latter destroying 99.9% in 24 h.
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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