| Summary: | 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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