Howard F. Curren AWTP Biosolids Dewatering Technology Evaluation

Authors:

  • Jacob Porter, Dan Schmidt, Michael Bullard - Hazen and Sawyer

The City of Tampa (City) owns and operates the Howard F. Curren AWTP (HFCAWTP). The HFCAWTP is currently permitted for 96 million gallons per day (MGD) average annual daily flow (AADF) and operates with AADF of approximately 56 MGD. The current anaerobically digested sludge production rate to the dewatering and downstream solids handling process is approximately 1,125 dry pounds per day per million gallons AADF at a concentration of approximately 2% total solids. The biosolids processing portion of the plant downstream of anaerobic stabilization currently consists of belt filter press (BFP) dewatering, and rotary drum drying facilities.

The belt filter press dewatering facility consists of eight (8) 2.0-meter width belt filter presses with a design hydraulic capacity of up to 140 gallons per minute per machine at a feed solids concentration of 2.0% total solids (solids loading rate up to 700 dry pounds per hour per meter). The dewatering facility also includes all associated ancillary equipment for sludge feed pumping, polymer conditioning, and dewatered cake conveyance. The belt filter presses typically produce a dewatered cake with a total solids concentration in the 15% to 17% range. Current unit cost for dewatering at the facility is estimated at approximately $135 per dry ton.

The heat drying facility consists of a two train rotary drum thermal drying system with each rotary drum drying train designed with 12,000 pound per hour evaporation rate. Only one (of the two) thermal drying trains is currently operational; however, the rotary drum drying facility has not been operated since November 2010 due to a higher operational costs associated with production of a Class AA pelletized biosolids production than either Class B dewatered cake land application or dewatered cake landfill disposal.

Ultimate biosolids disposal costs are directly related to the water mass in the dewatered cake. Therefore, reductions in sludge water content will beneficially impact ultimate management operating costs. There are two general approaches to increasing dewatered cake solids content – (1) optimizing the existing belt press operation or (2) consideration of alternate dewatering technologies. In order to objectively assess the various options for improving dewatered cake solids content, the City is pursuing a multi-pathway process involving on-site pilot testing and process optimization to quantify operational advantages of individual or aggregated opportunities.

Pilot testing was conducted concurrently with three dewatering technologies (high solids centrifuge (HSC), screw press (SP), and rotary fan press (RFP)) in March 2013. A “current generation” belt filter press was also pilot tested in June 2013. The goal of the pilot testing was to identify the following comparison factors for each technology.

  • Quantification of operational costs (e.g., polymer usage, electrical power consumption, etc.);
  • Determination of operator and maintenance staff hours and costs;
  • A comparison of performance (e.g., dewatered cake solids content, capture, etc.);
  • The potential for pathogen re-growth and impacts on land application disposal; and
  • Identification of the operational costs and performance of liquid polymer activation systems.

The objective of this evaluation is to provide recommendations for process improvements to the HFCAWTP sludge dewatering process to increase system efficiency, reduce operating costs, and restore system reliability. The results of the assessment and subsequent recommendations are expected to be used as the basis for planning future capital improvement projects (CIP) for the City of Tampa.

For more information, please contact the author at jporter@hazenandsawyer.com.

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