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The effect of varying air supply upon supernatant quality in autoheated thermophilic aerobic digesters treating waste sludge from a biological phosphorus removal process Boulanger, Mary Louise


Return flows from sludge stabilization processes can have a significant impact on overall plant design. This is especially true for biological phosphorus removal (Bio-P) processes, because high phosphorus levels in return supernatant can defeat the purpose of the process. Previous research determined that excess stored phosphorus in Bio-P waste sludge is released to solution under both mesophilic aerobic and anaerobic digestion conditions. This research investigated thermophilic aerobic digestion (commonly referred to as AT AD) of Bio-P waste sludge, to determine the extent of phosphorus release. Because dissolved oxygen conditions affect the release and uptake of phosphorus in Bio-P treatment, the effect of different aeration levels on phosphorus release and general supernatant quality was also studied. Phosphorus (P) release in ATADs was of special interest because field results indicated that these types of digesters were capable of generating high concentrations of the volatile fatty acids (VFA's) required to drive the phosphorus storage mechanism in Bio-P plants. Two 72 liter pilot scale ATADs were used, operating in series with a 6 day total retention time. The sludge feed was an average combination, in terms of VS, of 44 percent primary sludge and 56 percent Bio-P waste activated secondary sludge. The digesters were operated in batch mode on a 24 hour cycle. The temperature in ATAD 1 varied between 35 and 56°C, and the temperature in ATAD 2 varied between 55 and 64°C. Average influent volatile solids concentrations varied between 16600 and 18400 mg/L. Three aeration conditions were defined by on-line monitoring of oxidation reduction potential (ORP) and dissolved oxygen concentration (DO). The condition with the lowest airflow rate was labelled "oxygen deprived", and was characterized by ORP generally less than -300 mV and DO concentrations generally less than 1 mg/L in both digesters . The condition with a medium airflow rate was labelled "oxygen satisfied", because ORP was above +100 mV and DO was greater than 1 mg/L by the end of the 24 hour cycle in ATAD 1, and conditions were always aerobic in ATAD 2. The condition with the highest airflow rate was labelled "oxygen excess", because ORP was generally higher than +100 mV and DO was generally greater than 1 mg/L in both digesters. Phosphorus and nitrogen balances were done for each aeration condition, and solids balances were done for the oxygen deprived and the oxygen excess conditions. Other parameters measured were total and soluble COD, volatile fatty acids, pH, and alkalinity. Results indicated that total VS reduction was the same for both the oxygen deprived and the oxygen excess conditions. A comparison of influent and effluent total COD concentration confirmed that overall sludge stabilization in ATADs was not affected by airflow rate within the range studied. Total VS reduction in the first digester in the series was similar to that predicted by the EPA Design Curve for aerobic digesters. Although total VS and COD reduction was not affected by airflow rate, the proportion of soluble COD increased with decreasing airflow and the concentration of acetic acid was greater in the oxygen deprived experiment than with the higher airflow rates. Dissolved nitrogen also increased with decreasing airflow. Supernatant quality thus generally declined with decreasing airflow. The least amount of phosphorus released occurred under the oxygen satisfied condition, which was characterized by alternate low ORP and high ORP conditions in ATAD 1. The greatest amount of phosphorus release occurred under the oxygen deprived condition. In all cases, the concentration of phosphorus in the supernatant would be of concern if that supernatant was returned to the influent of a Bio-P treatment process.

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