IMPROVING ANAEROBIC DIGESTER EFFICIENCY BY HOMOGENIZATION OF WASTE ACTIVATED SLUDGE

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Abstract:

MicroSludge™ is a patented chemical and pressure pre-treatment process that increases both the rate and extent that waste activated sludge (WAS) is degraded in a conventional mesophilic anaerobic digester. The process uses alkaline pre-treatment to weaken cell membranes and an industrial-scale homogenizer to provide an enormous and sudden pressure change to burst the cells. The resulting liquefied WAS is readily converted to biogas in a conventional mesophilic anaerobic digester.

The Chilliwack WWTP is located approximately 100 km east of Vancouver, British Columbia, and serves a population of 70,000. The first full-scale prototype MicroSludge process was commissioned at the plant in January 2004. A single 4,000 L/h homogenizer was operated for 8 to 10 hours per day to process all of the WAS generated at the facility during the demonstration program. Thickened WAS from the plant was chemically pre-treated, homogenized, and anaerobically digested together with the primary sludge at a 13 day HRT. Volatile solids, COD, volatile fatty acids, BOD, TKN, ammonia, chemical consumption, dewaterability of digested solids, and process reliability were monitored throughout the demonstration program.

The full-scale demonstration clearly showed that alkaline pre-treatment followed by homogenization significantly increases both the rate and extent that thickened WAS is degraded in a conventional mesophilic anaerobic digester. In a 65:35 primary:secondary sludge mix at an HRT of 13 days, volatile solids reductions of up to 90% were achieved, with the average VSr being 78%. This implies a volatile solids reduction of greater than 90% for secondary sludge alone. When conventionally treating the same mix of primary sludge and untreated WAS under similar operating conditions, the anaerobic digesters at the Chilliwack WWTP achieved an average VSr of 60%.

The cost of alkaline pre-treatment was found to be approximately US$11 per dry tonne of WAS processed at a solids concentration of 4 percent. Homogenization required approximately 760 kW of electrical power per dry tonne of WAS processed at a solids concentration of 4 percent. Assuming a unit power cost of US$0.05/kWh, the power cost of homogenization was approximately US$38 per dry tonne of TWAS processed. Caustic addition was approximately US$11 per dry tonne WAS. The homogenizer maintenance and parts replacement costs, including labour, was estimated to be approximately US$18 per dry tonne of WAS processed, for a total O&M cost of approximately US$70 per dry tonne of WAS processed. The total cost of homogenization is projected to decrease linearly from approximately US$70 per dry tonne of WAS processed at 4 percent solids to approximately US$40 per dry tonne processed at 7 percent solids.

The total energy that can potentially be recovered from alkaline pre-treatment and homogenization of WAS is the sum of the additional electrical energy and heat energy generated from biogas conversion in the anaerobic digester, and the heat energy due to high-pressure homogenization.

Overall, it is projected that approximately 1,100 kWh/dry tonne WAS of additional electrical energy and 1,900 kWh/dry tonne WAS of additional heat energy can potentially be recovered by anaerobic digestion of WAS after homogenization compared to conventional mesophilic anaerobic digestion. The total value of the additional recovered energy is estimated to be approximately US$100 per dry tonne of WAS processed.

Other benefits of homogenization are increased digested capacity, a reduction in the digester mixing and heating requirements, and the reduced quantity of digested biosolids that require disposal. In a plant having a primary: secondary sludge ratio of 2:1, the total quantity of dry solids requiring disposal is reduced by approximately 850 kg for every 1,000 kg of WAS homogenized. This implies that both the volatile solids and the fixed solids originally present in the WAS are liquefied by homogenization. For a sludge cake solids content of 20 percent and a residuals disposal cost of US$25 per wet tonne, the cost savings are approximately US$100 per dry ton of WAS processed.

The economic benefits of homogenization are greater for plants with higher electrical utility costs and higher biosolids disposal costs.

Document Type: Research Article

DOI: http://dx.doi.org/10.2175/193864705783865578

Publication date: January 1, 2005

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