A study of the biodegradability of residual COD

Biological treatment for removal of soluble waste organic substrates has proven highly successful. Still, treatment efficiency is seldom, if ever, 100% based on chemical oxygen demand (COD), total organic carbon (TOC), or biochemical oxygen demand (BOD). Residual organic material remains even when simple monosaccharides or organic acids (which can be con sidered to be totally available metabolically) are fed to an acclimated microbial population under ideal aerobic growth conditions. The biodegradability of such residuals requires practical kinetic interpretation and definition. The question is important, not only from a scientific and engineering stand point, but from a regulatory standpoint as well, because it deals with the assessment of plant performance and with punitive measures for noncompliance. The critical question is: What is the biodegradable portion of a wastewater; what is the residual amount of organic matter that a well-operated and designed plant should deliver? When a readily metabolizable compound such as glucose is fed at a concentration of 1000 mg/L (1060 mg/L COD) to an acclimated microbial population under aerobic conditions in either a batch reactor or a continuous-flow reactor, the lowest residual soluble COD usually attainable is between 30 and 50 mg/L. This residual is properly designated as non-biodegradable in practical engineering terms because it is residual with respect to the acclimated population. Over the years, the authors have been interested in deter mining the nature and fate of this residual COD. Thabaraj and Gaudy1 reported the transient presence of small concen trations of ribose and ketoacids in residual COD. In addition, several unpublished studies were aimed at gaining insight into the long-term kinetic mode and magnitude of 02 uptake resulting from residual COD in effluents from well-treated synthetic wastewaters. This work provided some preliminary evidence that a significant portion of residual COD is biode gradable. The rate of BOD exertion was much slower than for comparable concentrations of original substrate and the kinetic mode was more often than not first-order decreasing; it was more like the idealized BOD kinetics determined by earlier workers2 than the more rapid S curve with plateau obtained by later workers from BOD exertion by feed stock rather than treated effluent.34 The major purpose of this study was to examine the long term fate of the COD remaining after aerobic treatment of a synthetic waste. Previously, much research was done on total retention of biomass5,6 and it seemed that the simplest and most direct approach was to totally retain both the biomass and the residual COD and to monitor changes in each over time with daily feeding of a given concentration of carbon source. MATERIALS AND METHODS