The application of pH and ORP process control parameters within the aerobic denitrification process
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Aerobic denitrification is a biological nitrogen removal process in which nitrification and denitrification occur simultaneously (in the same reactor) under identical environmental conditions. This contrasts to traditional separate stage nitrification denitrification in which the nitrification and denitrification processes occur sequentially in different reactors under opposing environmental conditions. While aerobic denitrification has long been identified in other ecosystems (such as the nitrogen cycle within soil) it was not thought possible within wastewater treatment processes. The existence of aerobic denitrification within wastewater treatment systems was first identified in the early 1980's following nitrogen mass balances that suggested unexplained nitrogen losses were occurring in the aeration tanks of many full-scale biological nutrient removal facilities (total nitrogen losses of up to 30% were frequently occurring U.S.EPA (1987)). Since then researchers and engineers have attempted to elucidate the mechanics behind the aerobic denitrification phenomenon and the conditions required for its optimization. It is thought that aerobic denitrification may offer advantages and possible savings when compared to alternative traditional separate stage nitrification denitrification processes. The use of real time parameters such as ORP, pH, DO and airflow rate (oxygen demand) can provide immediate insight into a biological treatment process. This knowledge can be used to ensure optimum performance in terms of real time pollutant concentrations and hydraulic loads. This research aimed to elucidate some operational aspects of the aerobic denitrification phenomenon, to investigate opportunities for several types of real time control (ORP, pH, DO, and airflow), and to develop a process control system using the online parameters. An activated sludge process was established within two lab-scale sequencing batch reactors. The reactors were operated under a range of conditions using raw domestic wastewater as the feed. ORP, pH, DO, and airflow were measured online in real time while other biochemical parameters (such as the various forms of nitrogen) were measured periodically using HACH photometric procedures. Dissolved oxygen concentration was the operational variable (dissolved oxygen set points (DOSP) 4.0-0.5 mg/L), other parameters such as MLSS concentration and feed strength were maintained (where possible) at a consistent value (~3000 mg/L and ~600 mg/L COD respectively). The system was operated under both nitrification and aerobic denitrification conditions with the dissolved oxygen concentration determining the degree to which aerobic denitrification existed (~40% TN removal at DOSP 0.5 mg/L). The biochemical event of interest was the depletion of ammonia nitrogen. The key online profiles of interest were the ORP-time profile and the pH-time profile. The research sought to demonstrate the credibility of ORP and pH as real time control parameters for the depletion of ammonia nitrogen in the aerobic denitrification process. To achieve this a microprocessor-software based process control system was developed by using the relationship between online measurements and biochemical events. The results indicated the ORP-time profile does not provide any feature for the depletion of ammonia nitrogen when the dissolved oxygen is maintained at a fixed concentration. That is the previously identified "ammonia elbow" is probably the result of dissolved oxygen concentration breakthrough rather than nutrient depletion. The lack of an ammonia depletion elbow meant that ORP could not be used for process control within the aerobic denitrification process. The pH-time profile showed an "ammonia valley" feature at the point of ammonia depletion. This feature was consistently present in both the nitrification and aerobic denitrification processes. The research incorporated the feature into the process control system and successfully used it to control the length of the aerobic denitrification treatment sequence. With respect to elucidating some operational aspects of the aerobic denitrification phenomenon the main variable of interest was the dissolved oxygen concentration. The results indicated the aerobic denitrification process has an optimum dissolved oxygen concentration around but probably below 0.5 mg/L. The process probably does not have an optimum concentration but an optimum range. It is likely this range is influenced by variables such as the biomass concentration and the release of reducing power in terms of the ability to hydrolyze stored carbon polymers. A secondary objective of this research was to elucidate advantages of aerobic denitrification relative to alternative traditional separate stage nitrification denitrification processes. For example it has been proposed that aerobic denitrification may require smaller treatment reactors, require less air for nitrification, produce less sludge per unit of wastewater treated (relative to a traditional nitrification-denitrification process), and have less dependence on organic carbon for denitrification. The results suggested the low dissolved oxygen concentrations required for the aerobic denitrification process significantly inhibit the nitrification process. This causes a considerable extension in the required aeration times for the oxidation of ammonia nitrogen (~300% increase in aeration time relative to a traditional nitrification process). The longer aeration times suggest the process may not offer savings in terms of aeration requirements (aerobic denitrification required ~200% more air per unit of wastewater treated relative to a traditional nitrification process) or treatment tank sizes (relative to traditional separate stage processes). A reduction in the quantity of sludge produced (per unit of wastewater treated) of over 30% was demonstrated for the aerobic denitrification process. While aerobic nitrogen removal has been achieved under certain conditions autotrophically by other researchers this work found the process is probably undertaken predominantly by heterotrophic micro-organisms. The low dissolved oxygen concentrations required for the process also appear to favor heterotrophic denitrification using stored intracellular carbon (biosorption). This research demonstrated the aerobic denitrification process was able to remove nitrogen with less dependence on organic carbon (the organic carbon requirements for aerobic denitrification were not quantified but experimental data suggests a possible 40% saving) either by the use of a shortened nitrification-denitrification pathway and/or the ability to use stored carbon.