Design of an efficient tannin extract effluent treatment system

Abstract

A study was undertaken on the combined wastewater effluent from a Tannin extract plant (90%) and a Mushroom Production Unit (10%). The effluent had BOD5 of 2350 mg/l, COD of 4600 mg/l, and high levels of total suspended solid (0.43-4.20 mg/l), phosphorus (2078 mg/l), free ammonia (2982 mg/l) and colour index of 50 to 150°H. These values are effluent Standards of Eldoret Town in Kenya. To address these problems, three potential treatment processes were considered: VITOX system using pure oxygen, lagoon evaporation system, and Step-alum coagulation/flocculation process using wood ash leachate for pH adjustment. The latter was chosen because of its efficiency for not only BOD5, COD, phosphorus and free ammonia reduction but also for the complete colour removal when alum was added in two stages as laboratory Jar-tests indicated. A plant was subsequently designed that incorporated a two-step alum coagulation/flocculation process in two upflow tanks with wood-ash leachate for pH adjustment during and after flocculation.