Treatment optimization to address water quality challenges in a ZLD scheme – 4 years of operation of the Twin Oaks Valley WTP

摘要

The Twin Oaks Valley WTP (TOVWTP) has supplied drinking water to the member agencies ofthe San Diego County Water Authority since April of 2008. The facility was designed toproduce 100-mgd of drinking water using mechanical strainers, membrane ultrafiltration, ozonewith advanced oxidation, biologically active carbon contactors and chloramines. Plant sourcewater is a variable blend of Colorado River and State Water Project water, with provisions to usean emergency supply (Lake Hodges). The plant is a zero-liquid discharge (ZLD) facility. Allliquid waste streams, including membrane cleaning chemicals, are equalized, coagulated andclarified. Clarifier overflow is returned to the head of the plant while underflow is thickened andcentrifuged. Thickener overflow and centrate are returned to the equalization basins. Centrifugecake is hauled off site. More than 99% of the untreated water is recovered as drinking water.This paper presents a firsthand account of how water quality and operational challenges havebeen addressed to improve plant reliability. The overall methodology to address each challenge,including treatability and monitored data, will be presented. ZLD facilities present uniquechallenges since any constituents that enter the facility in the untreated water or throughchemical addition must be efficiently removed in the residual solids or leave in the treated waterat permissible levels. Accumulation of these constituents can quickly impact plant capacity, andthus must be monitored and controlled proactively. Due to the closed loop nature of the ZLDsystem, mitigation strategies need to balance competing requirements. For example, pHsuppression was evaluated to improve solids stream clarifier performance, but high source wateralkalinity and subsequent needs for pH addition for treated water corrosion control drove coststoo high. Polymer addition and relocation of the centrate return stream were ultimately effective,but even these required evaluating the potential for membrane fouling due to polymer carryoverand speciation of DOC fractions in the recycle stream.