Evaluation of tertiary filtration and disinfection systems for upgrading high-purity oxygen-activated sludge plant effluent

The Joint Water Pollution Control Plant (JWPCP) of the Sanitation Districts of Los Angeles County provides advanced primary and partial secondary treatment for 15.3 m(3)/s (350 MGD) of wastewater. A 7-mo pilot plant study evaluated several tertiary treatment alternatives for in-plant water reuse. The influent to the tertiary treatment system was the high-purity oxygen-activated sludge plant effluent. Three filtration systems were evaluated: a deep-bed anthracite filter, a shallow-depth pulsed-bed sand filter, and a continuous backwash deep-bed sand filter. Chlorination and ultraviolet (UV) disinfection were also compared. All three filters were capable of consistently meeting the State of California's effluent turbidity limit. The differences in turbidity removal efficiencies were insignificant. Changes in hydraulic loading rate, up to 0.2 m(3)/m(2) . min (5 gpm/ft(2)), had insignificant effects on the quality of all filter effluents when filtering normal secondary effluent. The effect of chemical filter-aids on turbidity removal depended strongly on the characteristics of the filter influent and the filter design configuration. Breakthrough of alum might occur with an excessive dosage of alum. The estimated costs for filtering the secondary effluent by the three filters were similar. A high-UV dose of 300 mW-s/cm(2) would be required to reduce the total coliform concentrations less than or equal to 2.2 MPN/100 mL, mainly because of the low UV transmittance (40-63%). The 2.2 MPN/100 mL coliform standard could be met either by a chlorine residual of 5 mg/L with a contact time of 120 min or 10 mg/L for 60 min.