Startup of an Extended Granular Sludge Bed (EGSB) Reactor for the Treatment of Citric Acid Production Plant Wastewater.

Wastewater streams generated in the production of citric acid are typically contaminated with high levels of organics as well as high concentrations of sulfates and calcium. This wastewater is suitable for anaerobic biological treatment in high rate reactors for efficient and cost effective removal of COD. This kind of treatment is however, sensitive to the high levels of sulfates and calcium. The first are known for their inhibitory effect on methanogenesis due to conversion to sulfides, partially in the form of hydrogen sulfide. The latter may precipitate in the process as CaCO3, a phenomenon which is accelerated in the relatively highly alkaline (bicarbonate) environment, and in the temperature conditions (35deg. Centigrade) prevailing in the high rate anaerobic reactor.This paper describes the startup of an anaerobic treatment plant, utilizing the high rate reactor known as the Extended Granular Sludge Bed (EGSB), treating a heavily contaminated effluent stream from a citric acid production plant. The wastewater stream of 800 m3/day contains up to 17,000 mg/l COD, 1000 mg/l SO4 −2 and 600 mg/l Ca+2 with a typical temperature of 60°C (which is pre-cooled to 40°C by a dedicated heat exchanger).Maintaining a proper pH range in the reactor was a main concern in the operation of the treatment plant. To prevent hydrogen sulfide inhibition, pH in the reactor should be in the 7.2 – 7.8 range or higher. To reduce CaCO3 precipitation however, pH in the bioreactor should be lower than 6.7. Thus, an optimum pH level must be maintained.The anaerobic effluent is further treated in a stripping/oxidation/aeration stage to remove residual sulfides in the form of H2S and HS−, before discharging to a municipal WWTP.Biogas (5,000 m3/d, 75% methane) is collected in a balloon shaped storage tank mounted on top of the EGSB reactor. The biogas is utilized for steam production (1.8 ton/hour) by a dedicated steam boiler, or otherwise burned in an on site flare.The EGSB high rate anaerobic reactor has shown, during 12 months period, the ability to remove 88% of the total BOD present in the wastewater stream. No operating problems related to the high sulfate levels were observed. However, calcium salts precipitation took place in the bioreactor and was followed by blockage of the influent distribution system. Antiscalant polymer based on organophosphate compounds was dosed to the reactor in order to reduce calcium precipitation.