Treatment of municipal wastewater UASB reactor effluent by unconventional flotation and UV disinfection

Post-treatment of an UASB reactor effluent, fed with domestic sewage, was conducted using two-stage flotation and UV disinfection. Results were compared to those obtained in a parallel stabilisation pond. The first flotation stage employed 5–7.5 mg L⁻¹ cationic flocculant to separate off more than 99% of the suspended solids. Then, phosphate ions were completely recovered using carrier flotation with 5–25 mg L⁻¹ of Fe (FeCl₃) at pH 6.3–7.0. This staged flotation led to high recoveries of water and allowed us to separate organic matter and phosphate bearing sludge. The water still contained about 1 × 10² NMP/100 mL total coliforms, which were removed using UV radiation to below detection levels. Final water turbidity was <1.0 NTU, COD <20 mg L⁻¹ O₂ and 71 mNm⁻¹, the liquid/air interfacial tension. This flotation-UV flowsheet was found to be more efficient than the treatment in the stabilisation pond and appears to have some potential for water reuse. Results were discussed in terms of the biological, chemical and physicochemical mechanisms involved.