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Anaerobic Digestion

Thickened sludge has a solids content of approximately 4-5% solids depending on season. While thickening increases the solids concentration of the sludge, the material still contains pathogens and putrescible organic matter. Anaerobic digestion is a means of reducing both the pathogen population and the volatile organic content of the sludge, thereby making the material more stable and easy to manage during ultimate disposal.

Thickened sludge is pumped from both the gravity thickeners and thickening centrifuge into anaerobic digesters. The BCUA operates five 80' diameter anaerobic digesters. During the digestion process, the sludge becomes the food source for anaerobic bacteria, which in the absence of oxygen consume the organic material in the sludge and produce methane gas as a byproduct of respiration. The methane gas rises to the surface of the tanks where it is collected from under the digester covers and used as an energy source for the four boilers which provide the heat for the entire wastewater treatment facility. The microorganisms, by releasing methane gas, reduce the volatile organic content of the sludge by approximately 50-60%.

The anaerobic digestion process must occur under carefully controlled conditions. The temperature must be maintained between 95 to 100 degrees Fahrenheit. This temperature range is the optimum temperature for the anaerobic bacteria that feed on the sludge, but tends to inactivate the pathogens in the sludge, thereby resulting in significant pathogen reduction. Some methane gas is recirculated through the tanks to keep the sludge completely mixed and in contact with the anaerobic bacteria. Other parameters such as the pH and ammonia concentration in the tanks must also be carefully monitored. Since the process depends on the absence of oxygen, the digester covers are designed to float on the surface of the sludge, creating an airtight seal around the edge of the cover. The anaerobic sludge digestion process requires a holding time of 12-16 days. The anaerobic digestion process produces a sludge that once again has a solids content of approximately 3%. So much methane is produced during anaerobic digestion that the mass of the material is reduced considerably, thereby reducing the solids concentration. At this stage in the sludge treatment process, the material is relatively stable and free of pathogens. A second sludge centrifuge is utilized to thicken a portion of the digested sludge approximately 7% solids. The blend of digested sludge and centrifuge thickened digested sludge results is a liquid sludge product between 4-6% solids, depending on the season. As a result of temperature effluents, a thicker liquid sludge can be obtained during colder weather months than during warmer months.

After the digestion and thickening process, the liquid sludge is pumped into two 100' diameter sludge holding tanks. The holding tanks hold about eight days average sludge flow before barging by the Spectraserv Company to the PVSC treatment works in Newark, NJ.

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