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Evaluating and Selecting Class a Biosolids Heat Drying Options in the Pacific Northwest

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The Alderwood Water and Wastewater District (District) is currently in the process of upgrading the Picnic Point Wastewater Treatment Facility (WWTF). This project involves replacement of all liquid and solid stream processes and expansion of the existing 3 mgd plant to 6 mgd while maintaining existing treatment plant operations. The plant will be upgraded from a conventional activated sludge plant with sodium hypochlorite disinfection and belt filter press dewatering producing a Class B product, to a state-of-art plant with membrane bioreactors, UV disinfection, centrifuge dewatering, and heat drying. The facility will not include digestion, and production of Class A biosolids is achieved by heat drying alone.

With the assistance of the Design Consultant, the District selected centrifuges and heat drying as their biosolids management option. The District elected to go through an evaluated bid selection format (including economic and non-economic factors) to select the appropriate drying technology and manufacturer, and then assign the contract to the general contractor. The selection process was divided into four stages to narrow down and select the technology and manufacturer. The first phase involved creation of a Dryer Team to develop and implement a dryer evaluation and selection process. The second phase requested a statement of qualifications from several manufacturers. For the second phase, the manufacturers who submitted were asked to give a presentation for the selection committee. Based on the statement of qualifications and presentations, the Dryer Team (consisting of District and Design Consultant representatives) evaluated five manufacturers, representing four different dryer types:

Indirect heating of thermal oil with conductive heat transfer

Indirect heating of circulation air with convective heat transfer; single-pass of solids

Direct heating of circulation air with convective heat transfer; recycling of solids

Fluidized bed dryer

The District selected three manufacturers as pre-qualified bidders for the third phase and the Dryer Team developed the comprehensive procurement documents. The selection process included product quality, operations and maintenance considerations, manufacturer's experience, operating parameters, corporate responsibility, risk, reliability, project implementation, and lifecycle costs. The process allowed each manufacturer to propose their drying system according to the performance specifications and include a separate bid item for a dried biosolids conveyance and storage system. Additionally, the manufacturer included a lump sum bid item for design and consulting services since they would be required to take part in the design process and a lumpsum bid item for performance optimization services for a one year period following final acceptance. These contractual terms included requirements for the selected manufacturer to participate in the design process as a member of the design team, and, unlike traditional design approaches, the manufacturer would therefore have a higher stake in the successful outcome of the design and project.

The request for proposals included several forms and a questionnaire to facilitate and streamline the evaluation process, as well as provide an equitable selection process. For the fourth phase, the District evaluated the proposals. Based on the District's scoring of the selection criteria for each proposer, the District ultimately selected Kruger's belt drying system. This belt dryer is currently the first under contract for a municipality in North America.

During the design process, the Design Consultant worked with the local utility company, Puget Sound Energy (PSE), under their conservation grant program to receive funds for upgrading the thermal system with a high efficiency heat exchanger. The high efficiency heat exchanger has an efficiency of 92.7% as compared to only 82% for the conventional heat exchanger. PSE has agreed to provide approximately 70% of the additional cost of upgrading from the conventional to the high efficiency heat exchanger, resulting in a very short payback period.

The Design Consultant investigated different types of dried product conveyance methods, considering site constraints, product characteristics, and dust creation. A pneumatic conveyance system was selected as the best option for this application. The specifications for the dried product system included safety measures for the conveyance, storage, and dust collection system to ensure dried product handling safety.

Keywords: Class A biosolids; belt dryer; biosolids heat drying; conductive drying; convective drying; direct heating; dried product safety; energy incentive programs; evaluated bid; high efficiency heat exchanger; indirect heating; life-cycle cost analysis

Document Type: Research Article


Publication date: January 1, 2009

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