Pilot-and bench-scale coliform inactivation tests with UV irradiation were used to show how suspended solids remaining in filtered secondary effluent affect the efficiency of the UV disinfection process. Observed kinetic inactivation rates decreased with increasing suspended particle sizes of 7 m or larger present in tertiary effluent. First-order inactivation rates estimated from collimated beam dose-response curves for discrete ranges of UV doses were substantially different, which should caution researchers not to compare inactivation data obtained with largely dissimilar UV doses or suspended particle distributions. A dose of approximately 800 J/m 2 was identified as the minimum dose that will consistently meet the California wastewater reclamation coliform criterion when applied to in-line filtration effluent. Water Environ. Res., 73, 233 (2001).

High‐solids anaerobic digestion can consistently achieve 55 to 60% volatile solids destruction after thermal hydrolysis pretreatment, which reduces its viscosity and increases the fraction of soluble organic matter. For feed sludge with total solids concentrations between 6.8 and 8.2%, the process is stable at hydraulic retention times of 9 to 12 days, significantly increasing the treatment capacity of existing digesters or, in treatment plants without spare capacity, helping to postpone, reduce, or even avoid costly infrastructure investments. Process stability is related to the high concentration of soluble organic matter in the digesters. High‐solids temperature‐phased digestion appears to be superior to high‐solids mesophilic digestion, with respect to process flexibility and stability, biosolids stabilization, and biogas generation, although ammonia inhibition may have occurred. Implementation of high‐solids digestion could significantly reduce operation and maintenance costs of solids‐handling operations.

Microfiltration and UV disinfection are two alternative technologies for water reclamation. The results of a pilot study combining these two processes are presented. In addition to producing filtrate turbidities averaging 0.06 nephelometric turbidity units, microfiltration was an effective barrier to pathogens, demonstrating average log reductions of 4.5 for total coliforms and 2.9 for MS2 bacteriophage. Ultraviolet disinfection following microfiltration reliably met the California Wastewater Reclamation Criteria (Title 22) total coliform standard of 2.2 colony‐forming units/l00 mL at a UV dose of 450 J/m2. The MS2 bacteriophage standard, which requires a 5‐log reduction, was achieved by microfiltration and a UV dose of 880 J/m2. A model of the kinetics of inactivation of MS2 bacteriophage was used in further analysis of disinfection data. The model indicated that considerable backmixing occurred in the pilot UV disinfection unit, and observed UV doses could be reduced with improved hydraulics.

A study at the Southeast Water Pollution Control Plant, San Francisco, California, was undertaken to evaluate the feasibility of replacing its current secondary effluent chlorination system with a 6.57 m 3 /s ultraviolet (UV) disinfection system. Two vertical lamp units and one horizontal lamp unit were pilot tested in parallel. The effective UV dose for coliform removal and the effects of feedwater characteristics and reactor hydraulics were examined. The horizontal pilot unit gave a more consistent performance than the vertical pilot units. Based on the test results, a UV dose of 65 mW' s/cm 2 would be required for the plant to achieve the target effluent total coliform level (240 CFU/IOO mL) 95% of the time. The large fluctuation of UV disinfection results could be attributed to the wide range of feedwater quality inherent to a combined sewer system. High suspended solids, characteristic of the plant's secondary effluent during storm events, significantly increased the UV dose required to achieve the target coliform level. Existing UV inactivation models were evaluated. The hydraulic behavior of the pilot units was found to significantly affect their test results. Higher virus removal efficiency was observed with the UV systems than the full-scale chlorination system.

Eight types of Class A biosolids were tested for fecal coliform (FC) reactivation and/or regrowth at 20, 35, and 50°C for 21 days. Growth of FC did not occur at 20 or 50°C, but it was observed in two samples incubated at 35°C after a lag period of 48 hours. In undigested biosolids, final FC concentration exceeded 10 4 MPN/g, whereas in thermophilically digested biosolids, the final FC concentration remained below 10 3 MPN/g, as FC regrowth may have been affected by the presence of the anaerobic bacterial consortium responsible for the digestion process. Fecal‐coliform reactivation and regrowth within treatment plant operations seem unlikely but can occur in land application of biosolids.