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Efficiency of Autothermal Thermophilic Aerobic Digestion Under Two Different Oxygen Flow Rates

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This paper evaluates two dual digestion processes in comparison with conventional digestion. The dual digestion process consists of an autothermal thermophilic aerobic digestion (ATAD) process ahead of an anaerobic digestion process. All the three processes evaluated had the same overall 10 day SRT. The objective of the work is to observe the differences between two different oxygen flow rates (0.105 v/v/h and 0.210 v/v/h) for an ATAD process using blended municipal solids with the help of profile experiments that were conducted for both conditions by sampling every 2 hours for duration of 24 hours. The two processes were compared with the conventional process in terms of VS removal, biogas yield and pathogen destruction. Oxygen utilization per volatile solids removed were found to be 1.20 lb/lb VS removed for the oxygen flow rate of 0.210 v/v/h and 0.83 lb/lb VS removed for 0.105 v/v/h. For the oxygen flow rate 0.210 v/v/h, higher (50% greater) ATAD effluent ammonia concentration was observed compared to the lower airflow rate. Furthermore, for the 0.201 v/v/h flow rate, there was no further increase in ammonia in the subsequent anaerobic step, suggesting that the hydrolytic reactions were complete for this ATAD reactor within the 2.25 day SRT. For the ATAD reactor with oxygen supply of 0.210 v/v/h, higher VS destruction of 23.8% was achieved when compared to 17.8% VS removal for oxygen flow rate of 0.105 v/v/h. However, the two different oxygen flow rates applied to the ATAD reactor did not affect the overall (ATAD + anaerobic digestion) VS removal efficiency, suggesting that the lower oxygen application rates were sufficient to produce a stable digestion process with an excess of 50% overall VS destruction in a relatively short 10 day overall SRT. Thus, there does not appear to be an obvious advantage for completion of the hydrolytic reactions within the ATAD process. The use of the higher oxygen flow rate should be solely considered for producing more heat to maintain thermophilic conditions and not for overall VS removal. Final effluent of both conditions, met the 40CFR Part 503 regulations with undetectable FC levels. The biological heat of oxidations were calculated to be 14,300 J/g VS removed and 15,900 J/g VS removed for the oxygen flow rates of 0.105 v/v/h and 0.210 v/v/h, respectively.
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Keywords: ATAD; Class A biosolids; Dual Digestion; ORP; heat of biological oxidation; oxygen utilization

Document Type: Research Article

Publication date: 2009-01-01

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