A new sludge treatment plant at Harrogate South Sewage Treatment Works is designed to handle up to 4 tonnes (dry solids) daily. Sludge is thickened continuously up to 8% (ds) and is then treated in parallel anaerobic mesophilic (AD) and thermophilic aerobic digestion (TAD) plants each with a maximum working volume of 530m3. Microbiological studies were carried out to compare the destruction of pathogens and faecal indicator bacteria. The AD plant operated with a mean retention of 26 days at 34 °C and achieved 49% reduction of volatile solids. The TAD plant operated with a mean retention of 28 days at 55 °C and reduced volatile solids by 35%. Operation was on a pump in-pump out cycle, guaranteeing 4h retention for all sludge. The disinfecting ability of TAD exceeded that of AD since it reduced counts of Enterobacteriaceae, thermotolerant coliforms and faecal streptococci to below 103/100ml, rendered cytopathic enteroviruses undetectable and destroyed viability of Ascaris suum ova within 4h. The AD process reduced bacterial counts by 90% and enteroviruses by 99%, but has no effect upon viability of Ascaris ova.

A procedure for counting viable heterotrophic bacteria in activated sludge was evolved from a study of the effects of modifications to procedures at the different stages of enumeration. Optimal counts were obtained with Casitone-glycerol-yeast extract agar (CGY) with incubation for 6 days at 22". Homogenization of mixed liquor was conveniently performed, with minimal lethal effect on the bacteria, by treating samples, diluted 1/10 in sodium tripolyphosphate solution (5 mg/l), in a boiling tube immersed in the Kerry ultrltsonic cleaning bath for 1 min. Counts were significantly affected by the pH value of diluent and CGY, but not by the homogenization method or by treating homogenized samples with enzymes or N-acetyi cysteinc, or by adding colloidal peptizing agents to the diluent. Replicate colony counts showed variances greater than the mean, although precision increased with increasing number of colonies/dish ; there was a direct relationship between colony counts and volume plated for up to c. 1000 colonies/dish. Counts on spread plates tended to be higher and more precise than on dilution frequency plates, although the 2 methods showed satisfactory correlation. Counts were not significantly affected by the method of sampling and preparing the initial dilution, and it was considered prudent to examine samples immediately after collection.

Methods were evaluated for the recovery of Cryptosporidium oocysts from water. Various types of cartridge filter were tested but proved to be inferior to the standard Cuno Micro-Wynd filter. Laboratory scale sand columns were also evaluated. The retention within the column material was satisfactory at low flow rates. However, the system was judged inadequate for monitoring because of the poor retention of oocysts within the column matrix at realistic flow rates. The continuous flow centrifuges tested were shown not to be capable of yielding satisfactory recoveries, although it is considered more refined machines currently available may warrant investigation. The vortex-flow filtration technique using the Membrex Benchmark device gave fairly consistent recoveries of 30-40%. However, the comparatively long process times would militate against the use of this device for monitoring purposes. A cross-flow filtration module was evaluated and gave relatively good recoveries (approximately 40-80%) at moderately high flow rates (0.9-1.01/min) from clean water samples spiked with Cryptosporidium oocysts.

The inactivation of Salmonella duesseldorf in sewage sludge during anaerobic digestion was investigated at 35 and 48°C with mean retention periods of between 10 and 20 days. Digesters were fed daily with raw sludge containing added Salm. duesseldorf after removal of digested sludge. During steady operation, the levels of Salm. duesseldorf in the digested and the feed sludge were determined and their specific rates of decay were estimated. The latter were: (i) greater at 48°C than at 35°C for the same retention time; (ii) similar for retention periods greater than 15 d, but lower for 10 d; (iii) greater when the level of salmonellas in the feed was lower. Gas production, a measure of steady state, was gradually lost when the mean retention period was reduced to 6.7 d. In experiments in which a single dose of Salm. duesseldorf was added to digesting sludge, the inactivation appeared to follow first‐order kinetics at 35°C and the decimal decay rate, 1.6/d, was similar to that in the daily feeding experiments (1.4/d) with larger and similar inocula of Salm. duesseldorf. At 48°C, however, the rate of inactivation declined with decreasing time from inoculation suggesting that the culture contained cells differing in thermal resistance. The degrees and rates of inactivation of salmonellas in those experiments were greater than in full‐scale digesters, because the latter seldom operated under conditions ideal for inactivation or because indigenous salmonellas are more resistant.

With funding from the European Commission, a consortium of members of the European Water Research Institutes is carrying out a programme of work with the objective of optimising and standardising a method for determining the presence in water of (oo)cysts of Cryptosporidium and Giardia. Each of the stages of the conventional analysis procedure (initial concentration, recoveryand identification and enumeration) are being investigated and the relative merits of existing and new methods are being assessed. Newly developed filters (Envirochek and Filta-Max) have been shown to be more efficient for initial recovery of (oo)cysts from water than the previously used Cuno cartridge filters. In addition, for the analysis of raw waters, flocculationwith ferric sulphate has been shown to give recoveries similar to the Envirochek and Filta Max. Modern purification systems such as immunomagnetic separation have also been assessed and found to offer some advantages over flotation although optimisation of the latter has brought improved efficiency. Preliminary assessment of solid phase cytometry has indicated that this technique could offer significant time savings compared to conventional microscopic counting. The results of the study will be used to propose a revised standard method to CEN.