Anaerobic wastewater treatment has many benefits over the more widely used aerobic technology, most notably the substantial reduction in sludge production and energy consumption. However, the majority of problems arise when the final step of the digestion process, methanogenesis, becomes unbalanced. Methanogenesis is undertaken by obligate anaerobic archae which, due to a combination of their relatively slow growth rates and more stringent growth requirements, are more sensitive to fluctuations/deficiencies in their environment than the other bacteria involved in anaerobic fermentation. It is a frequent misconception that the majority of wastewater feedstocks contain adequate supplies of the necessary nutrients to promote microbial growth. Therefore, the aim of this work was to gain a better understanding of the nutrient requirements in the methanogenic phase of anaerobic fermentations when treating high strength, soluble wastewaters. Investigations feeding a 30gCOD/l, predominantly sucrose, substrate to a set of CSTRs, required the methanogenic biomass to achieve acetate utilisation rates of 0.25 and 0.5 g/gVSS.day, and it was found that the growth of the methanogenic population was limited by a lack of available nutrients. Results from ICP-AES (inductively coupled plasma - atomic emission spectrometry) analysis carried out under growth limited and unlimited conditions, found that the requirements of group two metal ions, Mg2+ and Ca2+ were dramatically higher than the trace concentrations provided by the protein media supplement.
Metal requirements per gram volatile solids per day for magnesium increased by almost four times and for calcium by a factor of two when the organic loading rate was doubled. The increase in the requirement for magnesium appears to involve two factors: a proportional increase due the increase in methanogenic population in the reactor, and a proportional increase due to the increase in loading rate. Therefore the magnesium requirement in the system was affected by both the loading rate to the reactor and the proportion of methanogens in the biomass. It was determined that there was no increase in calcium requirement when the loading rate was increased; the increase in calcium requirement was suggested to be solely due to the increase in concentration of methanogens in the reactor. Therefore calcium requirements appear to be fixed per gram methanogens, and therefore a more generic requirement than magnesium.
The role of acetate as a precursor to methanogenesis was also investigated. Results from batch experiments involving degrees of methanogenic inhibition to investigate the proportion of substrate converted via acetate were comparable to those of previous authors, finding that approximately 60-70% of methane production from sucrose occurs via acetate. It was concluded that methanogenesis is vital to success of an anaerobic treatment process, however, its sensitivity to nutrient inhibition can be a major problem. Achievement of high acetate utilisation rates requires supplementation of significant concentrations of magnesium and calcium salts. When using high strength soluble wastewaters it could be useful to calculate the % methanogenic biomass present and the acetate loading rate on the methanogenic population. This could give an indication of the possibility of nutrient limitation occurring without significant supplementation of group two metal ions. ICP-AES analysis was concluded to be a useful way of measuring nutrient concentrations in reactors, and it was suggested that monitoring nutrient concentrations in reactors could avoid cases of growth limitation.