Coagulation Flocculation

A process of coagulation followed by sedimentation has been applied extensively for the removal of suspended and colloidal materials from water. Lime, alum, and ferric salts are the most commonly used coagulating agents. Floc formation is sensitive to parameters such as pH, alkalinity, turbidity, and temperature. Most of these variables have been studied, and their effects on the removal of turbidity of water supplies have been evaluated. In the case of the chemical treatment of wastewater stabilization pond effluents, however, the data are not comprehensive.

Shindala and Stewart (1971) investigated the chemical treatment of stabilization pond effluents as a post-treatment process to remove the algae and improve the quality of the effluent. They found that the optimum dosage for best removal o£ the parameters studied was 75 to 100 mg/L of alum. When this dosage was used, the removal of phosphate was 90% and the chemical oxygen demand (COD) was 70%.

Tenney (1968) has shown that, at a pH range of 2 to 4, algal flocculation was effective when a constant concentration of a cationic polyelectrolyte (10 mg/L of C-31) was used. Golueke and Oswald (1965) conducted a series of experiments to investigate the relation of hydrogen ion concentrations to algal flocculation. In this study, only H2SO4 was used, and only to lower the pH. Golueke and Oswald (1965) found that flocculation was most extensive at a pH value of 3, with which Tenney's results agree. They obtained algal removals of about 80 to 90%. Algal removal efficiencies were not affected in the pH range of 6 to 10 by cationic polyelectrolytes.

The California Department of Water Resources (1971) reported that of 60 polyelectrolytes tested, 17 compounds were effective in the coagulation of algae and were economically competitive when they were compared with mineral coagulation used alone. Generally, less than 10 mg/L of the polyelectrolytes was required for effective coagulation. A daily addition of 1 mg/L of ferric chloride to the algal growth pond resulted in significant reductions in the required dosage of both organic and inorganic coagulants.

McGarry (1970) studied the coagulation of algae in stabilization pond effluents and reported the results of a complete factorial-designed experiment using the common jar test. Tests were performed to determine the economic feasibility of using polyelectrolytes as primary coagulants alone or in combination with alum. He also investigated some of the independent variables that affected the flocculation process, such as concentration of alum, flocculation turbulence, concentration of polyelectrolyte, pH after the addition of coagulants, chemical dispersal conditions, and high-rate oxidation pond suspension characteristics. Alum was found to be effective for coagulation of algae from high-rate oxidation pond effluents, and the polyelectrolytes used did not reduce the overall costs of algal removal. The minimum cost per unit algal removal was obtained with alum alone (75 to 100 mg/L). The most significant effects occurred with alum and polyelectrolyte concentrations. The time of polyelectrolyte addition alone had no significant effect. The more important interactions occurred between alum and polyelectrolyte, alum and polyelectrolyte concentrations, time of polyelec-trolyte addition and alum concentration, and time of polyelectrolyte addition and polyelectrolyte concentration.

Al-Layla and Middlebrooks (1975) evaluated the effects of temperature on algae removal using coagulation-flocculation-sedimentation. Algae removal at a given alum dosage decreased as the temperature increased. Maximum algae removal generally occurred at an alum dosage of approximately 300 mg/L at 10°C. At higher temperatures, alum dosages as high as 600 mg/L did not produce removals equivalent to the results obtained at 10°C with 300 mg/L of alum. The settling time required to achieve good removals, flocculation time, organic carbon removal, total phosphorus removal, and turbidity removal were found to vary adversely as the temperature of the wastewater increased.

Dryden and Stern (1968) and Parker (1976) reported on the performance and operating costs of a coagulation-flocculation system followed by sedimentation, filtration, and chlorination with discharge to recreational lakes. This system probably has the longest operating record of any coagulation-flocculation system treating wastewater stabilization pond effluent. The TSS concentrations applied to the plant have ranged from about 120 to 175 mg/L, and the plant has produced an effluent with a turbidity of less than 1 Jackson turbidity unit (JTU) most of the time. Aluminum sulfate dosages have ranged from 200 to 360 mg/L. The design capacity is 0.5 mgd (1893 m3/d), and the plant was constructed in 1970 at a cost of $243,000. Operating and maintenance costs for 1973-1974 were $304/mg ($0.08/m3). Because of seasonal flow variations, operations and maintenance costs ranged from $200 to $800 per mg ($0.053 to $0.21 per m3).

Coagulation-flocculation is not easily controlled and requires expert operating personnel at all times. A large volume of sludge is produced, which introduces an additional operating problem that would very likely be ignored in a small community that is accustomed to a minimum of operation and maintenance of a wastewater lagoon. Therefore, coagulation-flocculation does not seem feasible for application in small communities.

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