Utilities Change Pumping Systems to Handle Drier Sludge

Two northern California municipalities chose hydraulically actuated positive displacement piston pumps to help them handle the higher solids content sludges required by their local landfills. The East Bay Municipal Utility District and the Daly City Waste Treatment Plant both encountered problems with their old pumping systems after upgrading their centrifuges to provide drier sludge cake.

The East Bay Municipal Utility District (EBMUD), in Oakland, Calif., serves 1.1 million residents in the San Francisco area. EBMUDs Special District No. 1 is responsible for interception, treatment, and disposal of wastewater collected from sanitary sewer systems in seven communities located in the area. The wastewater treatment plant is a combined primary and secondary facility that treats an average of 75 mgd of residential and industrial flows.

The plant currently produces about 65 dry tons/day of anaerobically digested sludge, of which 20 percent is composted and 80 percent is hauled to a landfill. In the late 1980s, The landfill used by EBMUD started to impose a 20 percent surcharge on any sludge material below 20 percent solids content, even though California law at the time mandated only a minimum 15 percent solids sludge cake for landfill disposal.

EBMUD elected to upgrade its three existing dewatering centrifuges to avoid the penalties. During the centrifuge upgrade phase in the summer of 1989, a fourth new centrifuge was added to the EBMUD dewatering facility. At the time, the district installed a new twin cylinder, all-hydraulic Schwing KSP 10 H(K) pump to handle sludge from the new centrifuge.

After completing the upgrade, the district encountered problems pumping the higher solids sludge cake from the older centrifuges with its existing progressive cavity pumps. As the solids content of the sludge cake increased, it lost the lubricating characteristics of wetter sludge. Bridging and higher pumping pressure caused premature rotor and stator failures and extensive downtime. The progressive cavity pumping systems were not able to withstand the threshold of 20 percent solids for sludge conveyance.

The district began using the Schwing pump as the lead unit, handling approximately 50 percent of the total sludge cake produced. The pump was supplied with a 30 hp power unit and fed by an SD 250 screw feeder. It conveyed the dewatered sludge approximately 100 feet to a truck loading hopper. Schwings high pressure "SE" sludge coupling system was installed to replace older grooved type couplings to provide a leak-proof system over the entire operating pressure range.

After evaluating the performance of the new pump, EBMUD began a program to replace its progressive cavity pumps with the piston pumps. The final retrofit was completed in early 1995. The new system allows EBMUD to handle sludge cake between 20 and 25 percent solids. When compared with the progressive cavity pumps, the KSP 10 pumps reduced maintenance costs by 75 percent. Maintenance associated with the progressive pumps had topped $10,000 per month each after the District upgraded its centrifuges to produce higher solids sludge.

At the Daly City Waste Treatment Plant, a similar Schwing unit replaced a progressive cavity pump after the municipal plant upgraded its dewatering system with a new centrifuge. The resulting 20-24 percent solids digested sludge cake is lifted 35 feet for discharge into a truck loading hopper.

The North San Mateo County Sanitary District serves the Daly City area. In early 1988, the facility undertook a modernization to produce drier sludge. As with EBMUD, the main objective was to comply with higher solids requirements for material dumped into landfills. Landfill costs had increased from $200 to $460 per truck load from the Daly City site. Plant personnel calculated that for every 2 percent increase in solids, they could eliminate one truck load per week.

The pumping system includes a hopper adjacent to the centrifuge with a screw feeder charging the pump. The sludge is pumped through glass-lined, 6-inch pipe. Five 90-degree elbows are incorporated in the pipeline to route the sludge to the new truck loading station. While the pump is outdoors, the electrical power pack and control panel are inside the centrifuge room.

"The Schwing pump handles 650 gallons of material per hour to the truck loading station. Other than the routine replacement of piston heads, weve experienced no downtime and a recent inspection revealed virtually no wear after 1 1/2 years of operation," said Falko.