The effect of aeration rates on odour and solids of pig slurry

Abstract

The effects of various aeration rates on the levels of Total Solids (TS) and odour in pig slurry were studied over a period of 4 months in 1·5-litre laboratory scale digesters stirred at 80 rpm at approximately 20°C. The levels (g litre −1) of TS and VS in the slurry were 63·7 (±2·7) and 54·1 (±2·7), respectively. The maximum oxygen transfer rates to the digesters were estimated to be 118·4, 23·8, 15·9 and 3·1 mg O 2 litre −1 h −1. The results of this study indicated that all treated effluents had significantly (P < 0·05) lower levels of solids than untreated wastes. The reduction in the levels of TS associated with these aeration rates were 35%, 20%, 25% and 26%, respectively. Redox values (E cal(mV)) became more negative with decreasing aeration, providing an indication of the degree of anaerobiosis in each digester. The values were approximately −20(±4), −253(±6), −251(±8) and −321(±5) for the four aeration rates examined. Odours associated with each of the treatments were determined by a panel (odour threshold dilution (OTD) and odour offensiveness) and by chemical means (volatile sulphides, ammonia). Aeration rates of 15·9 to 23·8 mg O 2 litre −1 h −1 ( redox ⋍ −250 mV) reduced the OTD by more than 80% and the levels of volatile sulphides and ammonia by 95% compared with unaerated wastes. Increasing aeration approximately sixfold resulted in a positive level of dissolved oxygen (> 1 mg litre −1) in the treated effluent and reduced the OTD and the levels of volatile sulphides and ammonia by a further 15% and 4%, respectively. Odour offensiveness, the most subjective of the measures of odour, also decreased, with increasing aeration.