Effect of COD to sulphate ratio and temperature in expanded-granular-sludge-blanket reactors for sulphate reduction

In an ethanol-fed expanded-granular-sludge-blanket (EGSB) reactor at 33 deg rees C, 80-90% of the sulphate load was removed at a rate of 4 g S/1 d, pro vided that at least 6 g chemical oxygen demand (COD) per g sulphate-sulphur was supplied. The reactor started up in a matter of days. Gradually decrea sing the ethanol to sulphate ratio (R) to about stoichiometry, resulted in 60-70% sulphate removal at rates of 7 g S/1 d. Similar tendencies were obse rved with ethylene glycol as sole carbon and energy source. Total COD remov al never reached more than 70-75%. This was related to a rather high biomas s washout. The sulphate removal efficiency decrease when R was set at level s below 6, apparently because sulphate reducing bacteria (SRB) could not co mpete with methane producing bacteria (MPB) for acetate produced from the s ubstrate dosed. Thermophilic operation at 55 degrees C, after a stepwise in crease in the reactor temperature over a period of 23 days, did not favour acetotrophic sulphate reduction. Yet, operation at 48 degrees C and subsequ ently returning the temperature to 33 degrees C clearly enhanced acetate co nversion by SRB. In the case of an electron donor price of 0.035-0.075 USD/ kg COD, the cost for operation at R = 6 was found to be competitive to that at stoichiometry, i.e. R = 2, provided the biogas produced was effectively used. (C) 1999 Elsevier Science Ltd. All rights reserved.