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digesters operated in series to one mesophilic anaerobic digester operated at a solids retention time (SRT) equivalent to the series digesters. In recent years, many studies on enhanced digestion processes have indicated improved performance from temperature-phased anaerobic digestion (TPAD), “acid-gas” (AG), or other phased digestion processes. A shared configuration in all these approaches is the use of staged reactors, digester tanks operated in series. The purpose of this investigation was to very simply understand the enhancements, i.e. changes in digester performance and sludge dewaterability, resulting solely from series operation of mesophilic anaerobic digesters and to assess the operational complexities of routine series digestion operation at the South Treatment Plant (South Plant) in Renton, Washington.

The South Plant is a conventional activated sludge wastewater treatment plant with a rated wet weather capacity of 115 million gallons per day (mgd). Solids handling facilities consist of six dissolved air flotation thickening (DAFT) tanks for co-thickening primary and waste activated sludge, four mesophilic anaerobic digesters, and eight belt filter presses for dewatering. Each anaerobic digester is 100-feet in diameter and has an effective volume of 2.75 million gallons. In normal parallel operation, the digesters are continuously fed raw thickened sludge and are continuously mixed using digester-gas recirculation and two hydraulic sludge recirculation systems. One hydraulic mixing system recirculates sludge from the bottom of the digester to the top; the other recirculates sludge from the digester lower sidewall to the upper sidewall. Digested sludge is withdrawn from the bottom-to-top recirculation line. A blending/storage tank (BL/ST), downstream from the digesters, aids in attaining a homogenous product for digestion and dewatering.

The series digestion test was conducted for approximately 60 days. During this time the withdrawal from one digester was modified to discharge to the side circulation loop of the second digester in series. The SRT in each series digester was maintained between 17 and 21 days. Loading to the primary digester varied from 0.15 to 0.25 lb. volatile solids (VS)/ft3/day. Loading to the secondary digester was approximately 0.07 lb VS/ft3/day. The control digester was operated at a SRT between 29 and 45 days and was loaded between 0.08 and 0.13 lb VS/ft3/day.

Once the series digestion system stabilized, it achieved a combined volatile solids reduction (VSR) between 67 and 70 percent, consistently 3 percentage points or more higher than achieved in the control digester. As expected, gas production in the first series digester was higher compared to the secondary digester. Volatile acid sampling of all digesters (series and control) showed that acetic acid was the predominant volatile acid; furthermore total volatile acids did not exceed 150 mg/L in any digester. Microbiological analyses for fecal coliforms and Salmonella spp. showed that the secondary digester had lower concentrations of these bacteria than the control digester.

At the end of the test period, sludge from the second series digester and the control digester were dewatered on the plant's belt filter presses. Using South Plant's current dewatering polymer, the staged digester sludge required a higher polymer dose to yield the same dewatering performance as the control digester. However, qualitative observation and analyses by the polymer manufacturer show that the polymer type was not optimized during the test. Tests performed by the manufacturer indicated, with the addition of a different polymer, the dewatered product and drainage characteristics of the series digester may be better in comparison to the control digester.

In summary, analyses indicate improvements in VSR and pathogen reduction for staged mesophilic digestion at South Plant without other enhancements (additional heat, or acid/gas phasing); in addition, digester gas analyses show higher methane concentrations in the secondary staged digester than in the control digester. Qualitative changes in dewatering performance, including a change in the polymer dose, were also observed.
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Document Type: Research Article

Publication date: 01 January 2004

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