The influence of substrate and temperature on biological nitrogen removal in wastewater treatment systems

Previous evaluations have indicated that savings in operating costs of greater than 25% can be realized if denitrification is added to a system already achieving nitrification. Energy requirements are reduced because organics are consumed under both aerobic and anoxic conditions. Reduced expenditures for energy, and lower sludge handling and disposal costs due to decreased biosolids production make nitrogen removal an economically attractive proposition. A number of recent articles in the literature suggest nitrogen removal is not as well understood as it once was believed. Conflicting reports of sludge production rates for nitrogen removal systems and systems operating under low temperatures are particularly interesting because they call into question the current economic evaluations of nitrogen removal.; This research examined the influence of substrate and temperature on biological nitrogen removal. Proteins were chosen as model substrates because they account for approximately 28% of the COD in raw sewage. In this study, sequencing batch reactors (SBRs) achieving aerobic carbon removal and anoxic carbon removal were run in parallel to assess and compare the overall efficacy of the two processes. Areas of process performance considered were effluent quality and biosolids production.; It was found that aerobic and anoxic systems produced an effluent of acceptable quality. Biosolids production under anoxic conditions was found to be 24% and 32% higher than the aerobic reactors when operating at temperatures of 20 and 14{dollar}spcirc{dollar}C, respectively. Batch rate studies using cultures generated from the SBR examined the effect of using complex substrates. Proteins because of their high molecular weight require enzymatic degradation before the cell can use the substrate for growth. Batch testing proceeded with: one half of a stock culture sample being fed a protein hydrolysate (PH) while the remaining portion was fed a whole protein. Ovalbumin, {dollar}alpha{dollar}-casein and lactalbumin were chosen for further study on the basis of their molecular weight and physical properties.; Results show ovalbumin and {dollar}alpha{dollar}-casein were used at a rate comparable to that of the PH under many of the test conditions. Lactalbumin was metabolized at a much slower rate than the PH under both aerobic and anoxic conditions and at temperatures of 14 and 20{dollar}spcirc{dollar}C. The molecular weight of lactalbumin (18 kd) is comparable to {dollar}alpha{dollar}-casein (22-25 kd) and is less than that of ovalbumin (45 kd).; Based on the results of this study it is proposed that the overall structure of the protein may be important in defining the rate of uptake and hydrolysis. Proteins containing a large number of cross-links may undergo uptake and hydrolysis very slowly while proteins with minimal cross-linking appear to be utilized more rapidly. The overall structure of the protein will play a role in determining the protein's solubility. Lactalbumin, an example of a cross-linked protein, forms settleable particles in solution. Ovalbumin and {dollar}alpha{dollar}-casein are soluble proteins with minimal cross-linking.