Design of a Chemically Enhanced Wastewater Treatment Lagoon in Brazil

This study describes he design of an upgrade for an overloaded wastewater treatment facility in the state of S˜o Paulo, Brazil. The current facility serves 50,000 people and consists of an anaerobic lagoon followed by a facultative lagoon that only removes 42% of the chemical oxygen demand. The design proposed by Sabesp, the state environmental agency, will replace the existing lagoons with mechanically aerated basins followed by settling basins.The authors were given permission to visit the current lagoons and to offer alternate design(s). We recommend chemically enhanced primary treatment (CEPT) as the central feature of the alternate design. CEPT is the process whereby metal salts are added to the wastewater to enhance the removal of solids, organics, and phosphorus via coagulation and flocculation. Two design alternatives are presented and compared, on the basis of performance and cost, with the proposed Sabesp design. The first involves the construction of a small CEPT tank ahead of the lagoons. The second is the addition of the chemical coagulants directly to the inlet of the first lagoon. Jar-tests conducted at the site show that both design alternatives have comparable removal efficiencies with respect to the proposed Sabesp design, while significantly reducing both capital and operations and maintenance costs.