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Prompted by the potential benefits of multi-phase anaerobic digestion, such as improved digester performance, increased process reliability, sludge dewaterability, and the potential to produce Class A biosolids, the City of Phoenix Water Services Department (WSD) initiated a study in January 2003 to evaluate the feasibility of converting the existing single-stage high-rate anaerobic digestion process at the 23rd Avenue Wastewater Treatment Plant (WWTP) to a multiphase digestion process. Accompanying a higher volatile suspended solids reduction, multiphase digestion can result in elevated ammonia levels in the digested sludge. Dewatered sludge centrate with elevated ammonia would be returned to the plant's liquid stream via a side stream recycle and potentially impact nitrification performance. To assess the impact of multi-phase digestion on liquid stream treatment and provide a basis for plant expansion, process modeling using Biowin32 was performed at steady-state and with diurnally varying influent loads.

Plant historical records were evaluated to obtain influent wastewater characteristics, unit process performance, and plant operational configuration. 2-weeks of intensive sampling were conducted to supplement plant data for characterizing influent, establishing influent diurnal patterns, and the process response to dynamic loading, all of which was used to validate the Biowin model calibration. Due to the configuration of the plant's sampling systems, intensive sampling data were obtained almost exclusively on the primary effluent. A mass balance was performed to back-calculate the raw influent characteristics based on the plant's historical primary treatment efficiency. Both the steady state and the extended period dynamic modeling for the intensive sampling period resulted in predictions that closely match key plant measurements obtained during the same period, such as aeration basin mixed liquor suspended solids (MLSS) and volatile suspended solids (MLVSS), effluent NO3 - N, TKN, and TSS.

The calibrated model was then used to evaluate the maximum treatable flow and ammonia load, in order to determine the requirement for side stream treatment after the conversion to multiphase digestion. A set of limits was established to gauge acceptable plant operation and performance. At the winter average day maximum month (ADMM) condition, both steady state and dynamic modeling results indicate that the plant can produce satisfactory effluent while operating the existing process within the recommended limits at flow up to 63 MGD. At the rated flow, the hydraulic loadings of primary and secondary clarifiers, the BOD5 loading on aeration basins, and the total oxygen transfer rate (OTR) at the corresponding peak day condition would exceed the recommended limits. At summer ADMM loadings with one aeration basin out of service for maintenance, the plant could treat up to 61 MGD with the hydraulic loadings of primary and secondary clarifiers, the BOD5 loading on aeration basins, and the oxygen uptake rate (OUR) exceeding recommended limits. At both conditions, the total nitrogen loading to the aeration basins resulting from the modeling is regarded as the maximum allowable nitrogen loading. This is because any additional ammonia loading caused by multi-phase digestion would further increase the OTR or OUR requirement. Treatment for the ammonia in the side stream would be needed after the conversion to multi-phase digestion.
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Document Type: Research Article

Publication date: 01 January 2005

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