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.

Increased anaerobic selector hydraulic retention times (HRTs) in a high-purity oxygen activated sludge process resulted in an increase in soluble orthophosphate release and biodegradable COD removal at aeration solids retention times (SRTs) ranging from 1.1 and 1.7 days. Under operating conditions that included biological foam recycling, a well established phosphorus accumulating organism community was observed at HRTs higher than 55 minutes that resulted in a decrease in filament counts and foam stability, providing effective operating conditions for foam control. While enhanced biological phosphorus removal was observed at HRTs between 20 and 60 minutes, its effectiveness with respect to foam control was compromised due to excessive foam trapping and recycling. Thus, increased anaerobic selector HRTs and low aeration SRTs can provide effective measures for biological foam control under conditions that minimize the impacts of foam trapping or that eliminate biological foam recycling.

Egg-shaped digesters have been promoted as free from maintenance expenses that plague conventional digesters. In 2001, however, OSP suffered digestion process upset that was exacerbated by clogging and mixing problems that developed because of deferred maintenance and the accumulation of grit. A Nocardioform bloom in the secondary system was unable to be brought under control because pumping and mixing problems reduced the digester active volume and thus substantially limited the amount of feed that could be introduced. Biological foam from the WAS appeared to correlate with substantial digester foaming problems that led to the uncontrolled transfer of digester contents to the belt press day tank and poor dewaterability. In addition, episodic foaming caused interruption in gas collection and affected the cogeneration process. Only by putting a stand-by digester in service and by clearing the accumulated grit and debris in two of the remaining digesters could the mixing lines be reopened and full-digester volume utilized. The versatility of the egg-shaped digester design allowed the maintenance staff to clear grit through the bottom withdrawal piping and the staff was able to complete the cleaning job in less than three weeks (480 man-hours).Historically, the digestion system has been under loaded, achieving over 61% volatile solids reduction and averaging 83 mg/L volatile acids and a VA/ALK ratio of 0.02. During the peak of the upset conditions, the volatile acids reached over 1900 mg/l and the VA/ALK ratio increased to 0.28. The volatile solids reduction dropped to around 14%. The sour conditions that developed during the upset period quickly subsided once the pumping and mixing problems were addressed. Because of the reduced wasting during the digester upset, Nocardioform filaments predominated in the mixed liquor and substantial foaming problems persisted in both the secondary system and in the digesters until wasting rates could bring the filament counts to historic levels (<1.0 x 10 6 intersections/g VSS).The paper describes in detail the causes of the OSP digesters upset, the impact to upstream and downstream processes, and the corrective measures taken.

Solids retention time (SRT), biological scum trapping and recycle and the dynamic equilibrium between Nocardioform populations in the foam and the ML are the controlling factors in activated sludge foaming events caused by Nocardioform bacteria. The combination of selective wasting and SRT control can ensure long-term foam control to the operation of a pure-oxygen activated sludge system. Polymer addition to the ML, followed by selective wasting of foam can cure a severely foaming ML in a matter of weeks provided the SRT remains below 1.5 days. SRT control and selective wasting will also alleviate a severely foaming ML, but effects will only be observed after three or four months after implementation. A SRT of 0.3 days will result in the complete wash out of Nocardioform bacteria from the activated sludge system, which can then operate at a SRT of 3 days free of Nocardioform.