CASE STUDY: Management of Wastewater Recycle Streams at the West Basin Water Recycling Plant

The West Basin Water Recycling Plant (WBWRP) began producing recycled water in mid-1995 and has the capacity to deliver 37.5 million gallons per day (mgd) of recycled water. WBWRP produces tertiary-treated recycled water for landscape irrigation and industrial applications and by advanced lime clarification/reverse osmosis (LC/RO) or microfiltration/reverse osmosis (MF/RO) treatment for groundwater injection and high-purity industrial applications. Both processes, tertiary and advanced, currently operate at the facility.Secondary-treated wastewater effluent from the City of Los Angeles Hyperion Treatment Plant is pumped to the WBWRP. The secondary-treated wastewater entering the facility is treated by two different treatment processes known as “Title 22” and “Barrier” systems. The recycled water produced by the Title 22 treatment process (30 mgd capacity) consists of coagulation/flocculation, direct mono-media filtration, and chlorine disinfection, and is delivered to customers through a distribution system owned and operated by WBMWD. Current recycled water uses include golf courses, schools, parks, street medians and greenbelts, and industrial uses (i.e. cooling towers & boilers). The Barrier treatment process consists of two treatment processes; one of process is lime clarification/tri-media filtration and reverse osmosis using cellulose acetate membranes (5 mgd capacity), the other process is microfiltration, reverse osmosis using polyamide membranes, and chlorine disinfection (2.5 mgd capacity). The high purity recycled water produced by the Barrier treatment process is injected into the West Coast Basin Barrier Project (WCBBP) to protect coastal groundwater against saltwater intrusion. The high purity recycled water is blended with potable water prior to injection in the WCBBP.The liquid waste streams that are generated within WBWRP are: microfiltration backwash, mono and multi-media filtration backwash, decant from solids thickener, filtrate from solids filter presses, and brine from the reverse osmosis processes. All liquid waste streams generated at WBWRP except of the reverse osmosis brine are diverted to the Title 22 backwash waste equalization basin and processed through the high rate clarifier. The reverse osmosis brine is directly discharged to the ocean via Hyperion's 5-mile ocean outfall. Decant from the high rate clarifier fed to the head the Title 22 treatment system. The clarifier is operated to maintain an effluent turbidity of 4 NTU or less with a ferric chloride dose of about 60 mg/L, and a cationic polymer dosage of about 1 mg/L. The clarifier has a capacity of 3.0 mgd. Effluent quality from the clarifier does not negatively impact Title 22 treatment train efficiencies since the ratio between waste streams and plant influent is about 4–6%.The solids generated within WBWRP consist mostly of lime from the lime clarification process with a lesser amount of biosolids from the filter and microfilter backwashes. On a dry weight basis approximately 85% of the solids is composed of calcium oxide and calcium carbonate. On average, about one cubic yard per day of lime grit is generated and added to the lime clarification sludge. Lime grit is a by-product of the lime slaking process. Solids are blended with lime slurry, thickened in gravity thickeners and dewatered using two (7.5 ton) plate and frame presses to approximately 50% solids. The solids generated during the treatment processes are dewatered and beneficially reused offsite (i.e. landfill cover). Sludge dewatering operations occur at night, six days per week, to advantage of off-peak power rates. There are normally two to three truck loads per day. Each load weighs about 20 tons. Solids production in 1999 totaled 15,700 tons (average monthly quantity was 1,300).Treatment operations at WBWRP are continuously optimized in order to produce recycled water that meets regulatory standards and at the same time minimize the amount of waste generated by the facility. None of the liquid or solid waste streams generated within WBWRP are discharged to the local sewer system.