Optimizing Water Treatment Plant Operations for Savings of Water, Energy, and Chemicals

摘要

This study investigates the feasibility of reducing the dosage requirement of ferric chloride for the removal of colloidal particles from waters containing low alkalinity by manipulating the pH control of the coagulation process. When metallic salts such as ferric chloride are used as coagulants, the solution pH should be maintained at a certain optimum level to allow the formation of hydroxopolymers or hydroxide precipitates from the added coagulants. Since the chemical reactions result in reducing the solution pH, bases are simultaneously added to to maintain a constant pH at the optimum level. This constant pH method has been practiced for laboratory coagulation studies and in water treatment. The results of laboratory studies indicate that a two-step pH control method can achieve the same removal efficiency as the constant pH method but at lower dosage requirements. The study also investigates the feasibility of reducing the consumption of treated water for filter backwash. The current backwash practice employs a constant backwash rate of 18-25 gpm/sq ft (50% bed expansion) for 5-15 minutes to clean a clogged filter. A new method employs an initial low backwash rate which causes only 10-15% bed expansion. At this bed expansion the sand grains are more efficiently scoured and cleaned than at the higher backwash rate. The backwash rate is then gradually increased to the 18-25 gpm/sq ft level for an effective rinsing of the filter bed. Laboratory studies indicate that the new backwash method is as effective as the constant rate method. Because of the low initial rates employed, the new method results in a saving of 30% water consumption.