KlarAid* lowers SDIs for power plant’s RO system, reduces overall operating expenses
A large southeast power station treats river water for boiler water makeup by removing influent turbidity in a lamella clarifier/sand filter pretreatment system prior to removing dissolved ions by reverse osmosis (RO). Historically, the plant has used a cationic coagulant blend fed at approximately 30 ppm (mg/L). This has resulted in filter effluent turbidity of 1 to 2 NTU. However, silt density index (SDI) readings, an indication of the quantity of colloidal and particulate matter remaining after pretreatment, have historically ranged from 2.6 to 3.2. In addition, there was a growing concern over the quantity of sludge generated with this program and rising disposal costs. With the installation of a new RO system a few years ago, this plant began looking at the issue of sludge disposal. In addition, plant workers were concerned with the degree to which the thin film composite membranes were becoming fouled, as a result of continuing to operate at these SDI levels. The cumulative colloidal loading threatened to significantly reduce the membranes’ serviceable life and to limit the plant’s overall throughput. Attempts to replace the existing coagulant program, first with an alum/polymer blend and then with a ferric chloride solution, failed to provide any reduction in SDI levels, although ferric treatment did produce reliable turbidity removal. In addition, ferric fed at its optimal dosage of 50 to 60 ppm (mg/L) resulted in a massive buildup of sludge and an increasing frequency of filter backwash as a result of carryover and post precipitation.
A specialized KlarAid coagulant blend of inorganic and organic polymers was substituted for the existing coagulant blend at 20% of the feedrate. Flocculant feed decreased from close to 1 ppm (mg/L) to approximately 0.4 to 0.5 ppm (mg/L). KlarAid produced exceptionally low filter effluent turbidities (1 to 1.2 NTU) over a wide range of operating flows and raw water turbidities. More impressively, the SDI levels dropped to approximately 1.65, or roughly half of the best achievable result from any program prior to KlarAid
This power station benefited in many ways by switching to a KlarAid program. The impressive results of KlarAid, compared with ferric chloride eliminate the need for pH adjustment. While keeping chemical treatment costs virtually the same, KlarAid lowers concern over increasing sludge disposal costs and the handling of hazardous chemicals.
- Reduced solids loading on the filters has resulted in significantly fewer backwashes and lower overall water use.
- Significantly lower colloidal solids loading on RO membranes is expected to save on maintenance costs, extend their critical service life and elevate performance between periodic cleanings.
- Reduced sludge volume and better settling in the lamella clarifier have increased efficiency of solids removal from the pretreatment system.