Effect of digestion temperature and pH on treatment efficiency and evolution of volatile fatty acids during thermophilic aerobic digestion of model high strength agricultural waste

Thermophilic aerobic digestion (TAD) of a model agricultural waste, potato peel slurry, a( soluble chemical oxygen demand (COD) load equivalent to approximately 8.0 gl(-1), was carried out under batch conditions at 0.5 vvm aeration rate. Digestions were carried out at temperatures of 45, 50, 55, 60 and 65 degreesC (or left unregulated) without pH control to study the effect of digestion iemperatures on TAD. The effects of digestion pH on the process were studied at pH 6.0. 7.0. 8.0. 9.0 and 9-5 (and in unregulated control) all at 55 degreesC. Except for digestion at 65 degreesC, which was inoculated extraneously using culture of Bacillus strearothermophilus all reactions were carried out using the populations indigenous to the waste. During digestion at different temperatures. the removal of soluble COD increased with temperature to reach a peak at 60 degreesC before declining slightly. removal of soluble solid (SS) followed similar pattern and reached peak at 65 degreesC being the highest temperature studied. while the degradation of TSS and TS (TSS divided by TS) decreased with an increase in temperature. Digestion at pH 7.0 was more efficient than at other pH values. Acetate was the predominant volatile fatty acid (VFA) in all the reactions and accounted for up to 90% of the total. Digestion at 60 degreesC led to the greatest accumulation of acetate, and this coincided with the period of highest oxygen uptake, and rapid consumption of soluble carbohy Iso-valerate was also produced at all pH values. Digestion at 55 degreesC and also at pH 7.0 led to rapid and efficient processesuith least accumulation of VFA and should be of interest in full-scale processes whenever it is practicable to regulate the digestion pH and temperature. The result of digestion at unregulated pH indicates that gradual adaptation may be used to achieve efficient treatment at elevated pH values. This would be of interest in full-scale processes where it is not practicable to tightly regulate digestion pH, and where the waste is produced at a pH value much higher than neutral. (C) 2004 Elsevier Ltd. All rights reserved.