| Endogenous nitrate respiration occurs in different processes of the biological wastewater treatment plants; post anoxic stage of the biological nutrient removal (BNR) systems, final clarifiers and aerobic/anoxic (A/A) sludge digestion. This research is undertaken to address some issues related to ENR. The effects of several parameters, such as temperature (15-30{dollar}spcirc{dollar}C), initial NO{dollar}sb3sp-{dollar} concentration (5- 30 mg N/L), initial solids concentration (1000-4000 mg/L) and solids retention time (7, 10 and 15 days), on batch ENR were investigated. Additionally, continuous ENR experiments were performed at different NO{dollar}sb3sp-{dollar} loadings (5-60 mg NO{dollar}sb3sp-{dollar}-N/hr) to observe the effect of nitrate loading on ENR. Both the solids concentration and the initial NO{dollar}sb3sp-{dollar} concentration did not affect k{dollar}sb{lcub}rm ENR{rcub}{dollar} values. The effects of temperature and SRT were quantified and compared to the literature. In the continuous experiments, changing flow rates or NO{dollar}sb3sp-{dollar} concentrations at constant loading did not appear to exhibit any noticeable changes in k{dollar}sb{lcub}rm ENR{rcub}{dollar}.; An extension of the solids flux theory in conjunction with nitrate mass balance was used to model the fate of nitrate in a sludge settling column. A new term, sludge stability index, was introduced to reflect the sludge rising time and the corresponding critical nitrate concentration necessary for causing the sludge rising. The model predicted well the nitrate concentrations and observed sludge rising times occurring under batch settling column experiments with different initial boimass (2000 to 5000 mg/L) and nitrate (10 to 30 mg N/L) concentrations.; A computerized data acquisition and control system was used to continuously monitor pH and ORP of laboratory scale digesters to investigate the feasibility of using pH as a real-time control parameter for the A/A process. The monitoring results revealed two well-defined control points which correspond to complete nitrification (ammonia valley) and denitrification (nitrate apex). Based on these findings, two real time control strategies were developed and tested. First, pH was allowed to increase (anoxic) and decrease (aerobic) within pre-set limits to control A/A cycling. A second approach detected the points of complete nitrification and denitrification using the differential of the pH signal (dpH/dt). Comparison of the digestion results with aerobic digesters indicated similar MLVSS and fecal coliform destruction but with superior TN removal and better dewatering characteristics. |
