CHEMICAL ENHANCED PRIMARY TREATMENT – PILOT INVESTIGATION AND CASE STUDIES IN BRAZIL

CEPT is a wastewater treatment technology based on suspended solids removal by physical-chemical processes of coagulation, flocculation and sedimentation. Physical-chemical treatment is indeed an old and a well-known technology. It was applied in the U.S. until the 1930s, when it fell into disfavor, with the advent of the activated sludge process, at the time more efficient and less costly. The recent offer of less costly chemicals, and of new polymers, brought back the physical-chemical treatment, now as a primary step ahead of conventional secondary biological treatment, known as Chemical Enhanced Primary Treatment. In the CEPT coagulation step, low dose iron salts are applied, typically ferric chloride, ranging from 25 to 50 mg/L. Flocculation is achieved with supplementary addition of an anionic polymer.The CEPT process increases organic matter (BOD) and suspended solids (TSS) removal up to 50% and 80%, great figures when compared to what can be achieved in conventional primary treatment, 30% and 50% efficiencies. Taking into account these figures, 3 new plants have been built in Brazil under this process (final design flow 3 m3/s), with excellent performance.In the current year, an investigation on the process efficiency under different chemicals and mixing conditions has been pursued, at the Treatment Research Centre (CETE) of the Universidade Federal do Rio de Janeiro, in Rio de Janeiro, Brazil. The project goal is to run the process under diverse conditions, so that its feasibility can be enhanced, technically and economically, for the Brazilian scenario.The experimental pilot plant works with 3 different coagulants, including an organic tannin compound, adding these chemicals to the sewage flow at an aerated grit chamber, where air mixing can be controlled. Two identical clarifiers, side by side, one receiving raw gritted sewage, and the other receiving the chemical enhanced gritted one, allow comparisons at different flows and overflows. Control of air and sewage flows, coagulant dosages, and proper indicators are achieved.Sludge generation is increased in the CEPT concept, and naturally this will increase costs in sludge conditioning, stabilization and drying.