Harnessing Hydraulic and Chemical Flocculation to Improve SVI – Experience at LACSD

Presentation Description: The concept of using hydraulic flocculation to improve solids setting and reduce sludge volume index (SVI) has been evaluated at the LACSD Saugus Water Reclamation Plant (WRP). The study included evaluating the effect of hydraulic flocculation alone and with chemical flocculation. Hydraulic flocculation can be achieved in the secondary clarifiers by adding a flocculation chamber at the upstream end of rectangular clarifiers and in the center well of circular tanks. Chemical flocculation is usually accomplished though the addition of flocculant (polymers). The study also established the relationship between polymer dosage and reduction in SVI.
Bench-scale tests were conducted using the emulsion polymer currently applied at Saugus WRP, along with the representative mixed liquor (ML) and return activated sludge (RAS) samples from LACSD’s Pomona WRP, which currently does not use polymer. Flocculation in these tests was simulated by mixing at 50 rpm for 10 minutes, reflecting the flocculation state that can be achieved with a flocculation baffle. SVI was used as the primary performance metric. The experimental setup included a control that experiences rapid mixing only, a sample subjected to both rapid mixing and flocculation, and four samples receiving gradually increasing polymer doses. Each testing day included two sessions: one where polymer-dosed samples underwent only rapid mixing, and another where they also experienced hydraulic flocculation.
The relationship between SVI and polymer dose has been established, and the optimal polymer dose for reducing SVI has been identified as 2 mg/L under the current operating conditions. Results from these tests indicate that hydraulic flocculation alone can reduce SVI by 3 – 10%. In most cases, hydraulic flocculation alone achieved SVI reductions comparable to adding 1 mg/L of polymer. With no polymer or at a low polymer dose of 1 mg/L, hydraulic flocculation further lowered the SVI and enhanced polymer performance; however, as polymer doses increased, the additional benefit of hydraulic flocculation diminished, indicating that chemical flocculation becomes dominant at higher doses. These findings suggest that improving hydraulic flocculation may allow for reduced contingency factor in polymer dosing in the field, leading to more cost-effective operations.