A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources and Development

Thesis (M.Eng.) - Asian Institute of Technology, 1991

Two reactors, with similar feed substrate except different phosphorus addition and similar operating conditions, were operated with the view of developing poly-P and non-poly-P sludge. The sludge withdrawn from the last aeration tank of the reactors was tested to study the possible metabolic reactions taking place and to identify the rate limiting reaction(s) during anaerobic metabolism of these substrates in bio-P organisms. The substrates tested are lactic acid, acetic acid and glucose. Acetic acid during anaerobic metabolism, proposed to be consuming intracellular carbohydrate to get required !educing power for the formation of PHB (Poly-B-hydroxybutyrate) from acetoacetyl-CoA (in addition to other changes). Because of the inability of the experimental results, presented as proof, to identify the role of intracellular carbohydrate study was undertaken to prove it. From the study it was found that the concept holds true for non-poly-P organisms also though this was originally proposed for poly-P sludge. Though a number of models were proposed by different researchers to understand anaerobic metabolism of acetic acid that of other substrates were not proposed or the proposed models were not complete. In this report a biochemical model was proposed to understand the metabolism of lactic acid and glucose and the model was compared and found to be valid within the scope of this study. Further possible rate limiting reactions were identified during anaerobic metabolism of acetic acid, lactic acid and glucose. For lactic acid this was found to be uptake or conversion to pyruvic acid. For acetic acid this was the reaction supplying reducing power and finally for glucose this was metabolizing through glycolytic pathway. Lactic acid uptake in two hrs when phosphorus content (on MLSS basis) was more than 5% was almost same and was about 23 mg-COD/g-MLSS irrespective of the phosphorus content of the sludge. For glucose uptake was found to be same throughout the range (0.83<Px<8) tested. On contrast acetic acid uptake greatly depended on Px as long as Px was greater than about 3% - and was proportional to Px. For non-poly-P sludge (Px =0.83%) acetic acid uptake was almost equal to that with poly-P sludge with Px 4.9%. Due to the inability of the non-poly-P sludge to supply energy by hydrolysing poly-P reserves, the role of carbohydrate as an energy source was tested and found not to be the case. Further possible energy sources were pointed out.