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Concurrent Evaluation of Large High-Solids Dewatering Centrifuges: Initial Results

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Equipment startup for a long-term evaluation of three large high-solids centrifuges from two different manufacturers began in May 2003 at the Los Angeles County Sanitation Districts' Joint Water Pollution Control Plant in Carson, California. These three centrifuges, two of which are identical except for the backdrive configuration, are in addition to a fourth high-solids centrifuge from a third manufacturer that has been in operation at the plant for more than four years. The results of the multi-year evaluation of centrifuge performance, operational reliability and dewatering economics will be used to select centrifuge equipment for a future 600 dry ton per day biosolids dewatering facility. The future facility will replace an existing low speed scroll centrifuge facility constructed in the early 1980's that is currently operating near its design capacity.

The three centrifuge models selected for the evaluation project are advertised as producing greater than 30% cake solids and are expected to process 75-100 dry tons per day of solids each. That level of performance suggests that significantly fewer high-solids centrifuges will be required for solids processing. In addition, the high-solids centrifuges will produce significantly drier cake compared to the existing low speed centrifuges. However, to be more economical than the existing centrifuges, the larger throughput and increased cake solids must offset the higher capital cost, higher polymer dosage and increased maintenance costs.

A mass balance and an economic model were developed in order to evaluate the economic impacts of variations in centrifuge performance. Feed flow, cake solids, solids recovery, power consumption and polymer dose are considered, along with the cost of power, polymer, cake reuse/disposal, maintenance and the annualized capital costs, to calculate an overall cost of dewatering and reuse/disposal on a dollars per dry ton basis.

The various manufacturers bid their equipment based on an overall cost per dry ton of solids processed utilizing the economic model constrained by minimum required cake solids, minimum recovery and minimum and maximum hydraulic throughput. The existing high-solids centrifuge provides a performance benchmark to quantify changes in biosolids dewatering characteristics. Maintenance costs for each centrifuge were established by requiring the manufacturer to offer a five year extended warranty and service contract at a fixed price. Failure to meet the guaranteed minimum performance or equipment availability may, at the Districts' discretion, result in equipment rejection with forfeiture of a letter of credit in the full amount of the purchase price.

An overview of the unique procurement process will be presented. The initial start-up and performance test results for the centrifuges, as well as observations regarding the effects of operating speed, backdrive design, and biosolids conditioning on centrifuge performance, will be discussed. Comparison will also be made to economics of operation of the existing low speed centrifuge equipment.
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Document Type: Research Article

Publication date: 2004-01-01

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