The effects of mixing and volatile fatty acid concentrations on anaerobic digester performance

Abstract

The literature on the effects of mixing and Volatile Fatty Acid concentrations on anaerobic digestion is reviewed, and results of further experiments, carried out on laboratory scale digesters, reported. It is shown that mixing regimes for anaerobic digesters control the rate of biogas production. For impeller speeds of between 140 and 1000 rpm, in a digester system treating sewage sludge, no improvement in gas yields was obtained. At the higher speeds, a slight reduction in gas production occurred, due possibly to shear forces separating the hydrolytic bacteria from their polymer substrates. Propionic acid concentrations, within digesters fed with sewage sludge, increased shortly after dosing and particularly when lactic acid was a feed additive. Acetate concentrations up to 4000 mg litre −1 did not inhibit gas production within digesters and could be stimulatory. Digesters showing (propionic + butyric acids): (acetic acid) ratios of less than 80:1 produced good gas yields.