Filter Press Optimization Reduces Incineration Costs
The city of Edmonds, Wash., about 20 miles north of Seattle on Puget Sound, optimized the belt filter press at its wastewater treatment plant to reduce the amount of moisture in its biosolids, thereby improving the performance of its incineration system.
The Edmonds Wastewater Treatment Plant is an 11 mgd activated sludge plant providing secondary treatment. It sits on 4½ acres in the middle of a tourist, residential, and mixed-use community referred to locally as Edmonds Bowl (in reference to the notable geographic feature of the area).
?Because of the sensitive location of our plant, we?ve always studied our solids-handling approaches and options carefully,? said Chris Sheridan, Plant Supervisor. ?When we put the pencil to it, it was determined that on-site incineration was clearly our best option ? especially compared to the pollution that would have resulted from trucking the solids to a less sensitive area.?
In 1996 the plant participated in two national benchmarking studies, Sheridan said. ?These indicated that there was room for improvement in our solids-handling activities. For the next year or so, we collected data and documented unit costs on our solids-handling operation to better quantify our situation. Our data showed we were burning 40 gallons of diesel fuel to incinerate each dry ton of solid wastes. We definitely wanted to reduce this operating expense.?
After reviewing the plant?s solids-handling data, operators decided to take on several pilot projects to see if they could increase solids dryness on the existing belt filter press, a two-meter Ashbrook™ Klampress®, which at the time had been in operation for nearly 10 years.
After eliminating all assignable variables - especially those associated with the belt press operating parameters and operating procedures ? operators were able to reduce the per ton auxiliary fuel use down to around 31 gallons per dry ton. They also were able to shave about half an hour off the incinerator burn time, which had been running up to 18 hours a day.
?From there, we continued to benchmark with similar plants back east, in New Jersey and Connecticut,? Sheridan said. ?As it turned out, they were beating us in several categories. Most importantly, they were burning autogenously (without the addition of auxiliary fuel), which meant they were burning quite a bit less fuel than us. It didn?t take us long to decide that we should make it our goal to burn autogenously, or close to it.
?Theoretically, if you can get above 24 percent solids in the sludge prior to incineration, you just don?t need to add any fuel to the process ? this can represent a significant saving in both fuel and staffing time. We were running at 20 to 22 percent solids. Ultimately, we set our goal as 80 percent autogenous operation.?
Staff at the plant contacted several plants that used belt presses and toured the facilities ? examining closely the arrangement and functioning of their systems. At the time, they were thinking about putting additional equipment in front of the belt filter press to help increase solids.
"We looked at the rotary drum thickeners and gravity belt thickeners in use ahead of the belt presses at other plants. We also contacted Ashbrook™ about their press extensions," Sheridan said.
Plant staff eventually settled on an optimization plan offered by Ashbrook that boosted the dry solids content produced by the existing belt filter press.
?The optimization for higher solids on the existing Klampress® included the extension of the filtration area of the pressure section by approximately 50 percent,? said Glenn Allen, Business Unit Manager of Opti-ProSM Retrofit and Aftermarket Optimization Products, Ashbrook Corporation. ?When we do an extension of this sort we add rollers, bearings, and frame components which are engineered to significantly increase the performance of the existing equipment.?
Plant personnel conducted a 14 day trial in which the optimized press had to meet 24 percent or better cake at specific sludge ratios, Sheridan said. During the trial, the filter press averaged 24.5 percent solids to the incinerator line feed.
?Based on the sludge characteristics going in, we were well pleased with the result,? he said. ?Since the completion of the Ashbrook™ optimization in September of 1999, we have enjoyed a tremendous drop in the auxiliary diesel fuel required to operate our incinerator.
"We dropped from an average of 31 gallons per ton to only 11 gallons per ton during wet weather flows -- thats an annual savings of $40,000 to $50,000. Plus, instead of having to burn 17 hours a day, we are able to burn for just 16 hours a day at increased solids loading through the plant. And, we?re also saving about $3,000 a year in reduced polymer demand. In fact, we have more throughput capacity, if we should need it.?
About the Author: Bob Bierhorst is Marketing Manager for Ashbrook Corporation.