Municipal Wastewater Treatment Plants

Dewatering Treatment Process:

SEDIMENTATION & CENTRIFUGATION NEWSLETTER - October 1996, No. 202 McIlvaine Visits Denver WWTP In conjunction with the 10th annual Residuals & Biosolids Management conference held in Denver this past August, we had the opportunity to visit the Metro Wastewater Reclamation District plant just north of Denver. The plant serves 1.3 million people at an average flowrate of 150 mgd. It is the largest wastewater treatment facility between the Mississippi River and the West Coast. Treatment units include 9 mechanical bar screens; 4 vortex, 2 aerated, and 2 velocity grit chambers; 14 primary clarifiers; 12 nitrification/denitrification aeration basins, with anoxic and oxic zones; 8 pure oxygen activated sludge aeration basins; 22 secondary clarifiers; and chlorination/dechlorination disinfection equipment. Primary solids and flotation-thickened, waste-activated solids are processed in 10 anaerobic digesters. After digestion, the 75 dry tons per day of biosolids are then dewatered using centrifuges to 16 percent - 17 percent solids and trucked to private and District-owned agricultural sites where they are applied at agronomic rates and incorporated into the soil. Approximately 3 percent of the dewatered biosolids are composted and sold to homeowners, commercial landscapers and nurserymen as a high-quality fertilizer and lawn top-dressing. Methane gas generated during biosolids digestion is used as fuel for four engines that generate electricity that is sold to Public Service Company of Colorado, producing approximately $1.6 million annually in revenues.

SEDIMENTATION & CENTRIFUGATION NEWSLETTER December 2001 No. 264 Denver Finds that Preheating Sludge Increases Solids Content and Decreases Trucking Costs The Metro Wastewater Reclamation District’s (MWRD) Central Plant currently treats approximately 150 mgd of wastewater generated in the Denver, CO metropolitan area. Residual solids (primary sludge and thickened waste activated sludge) are anaerobically digested, and currently dewatered with older centrifuges before truck-transport to land application or composting. Methane gas produced by the anaerobic digesters is used to produce electricity with onsite turbine generators, and waste heat from these turbines is used for building and digester heating. As part of a major project to replace the existing centrifuges with high-solids technology units, a pilot test was conducted to determine the impact of pre-heating the digested biosolids feed to the new high-solids centrifuges. Based on testing performed at other municipalities, it was expected that heating the digested biosolids to temperatures up to 140° F (60° C) prior to dewatering would produce a significantly drier dewatered cake. This would, in turn, cause a significant reduction in the trucking costs borne by MWRD for transport to agricultural land application sites up to 120 miles from the plant. The pilot test experimental setup involved feeding digested biosolids through a re-piped existing raw sludge spiral heat exchanger and then dewatering the material in a trailer-mounted, high-solids centrifuge after polymer conditioning. Projected optimum cake dryness for each temperature increment indicates a cake total solids increase of approximately 0.65 percentage points per 10° increase in feed temperature. There was no significant difference in polymer dosage and solids recovery at the higher feed temperatures. However, centrate BOD5 increased from approximately 200 mg/l to 400 mg/l. An economic analysis recommended incorporation of the pre-heating process in the new facilities

SEDIMENTATION/CENTRIFUGATION NEWSLETTER - December 2001, No. 264 Denver Finds that Preheating Sludge Increases Solids Content and Decreases Trucking Costs The Metro Wastewater Reclamation District’s (MWRD) Central Plant currently treats approximately 150 mgd of wastewater generated in the Denver, CO metropolitan area. Residual solids (primary sludge and thickened waste activated sludge) are anaerobically digested, and currently dewatered with older centrifuges before truck-transport to land application or composting. Methane gas produced by the anaerobic digesters is used to produce electricity with onsite turbine generators, and waste heat from these turbines is used for building and digester heating. As part of a major project to replace the existing centrifuges with high-solids technology units, a pilot test was conducted to determine the impact of pre-heating the digested biosolids feed to the new high-solids centrifuges. Based on testing performed at other municipalities, it was expected that heating the digested biosolids to temperatures up to 140° F (60° C) prior to dewatering would produce a significantly drier dewatered cake. This would, in turn, cause a significant reduction in the trucking costs borne by MWRD for transport to agricultural land application sites up to 120 miles from the plant. The pilot test experimental setup involved feeding digested biosolids through a re-piped existing raw sludge spiral heat exchanger and then dewatering the material in a trailer-mounted, high-solids centrifuge after polymer conditioning. Projected optimum cake dryness for each temperature increment indicates a cake total solids increase of approximately 0.65 percentage points per 10° increase in feed temperature. There was no significant difference in polymer dosage and solids recovery at the higher feed temperatures. However, centrate BOD5 increased from approximately 200 mg/l to 400 mg/l. An economic analysis recommended incorporation of the pre-heating process in the new facilities.