District of Sechelt Preliminary Report

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District of Sechelt Bio-Solids Processing Options Comparison Pre-Design Report 10.0 SHORT-LISTED TECHNOLOGIES 10.1 Stabilization The comparison of stabilization processes indicates that aerobic digestion and lime stabilization are appropriate for this application. Aerobic digestion has the advantage of volatile solids reduction, simplicity of operation, and inherent thickening. However, the product can be difficult to dewater mechanically as flocs do not easily release water. Pre-conditioning will be required before dewatering. Lime stabilization has the advantages of relatively simple operation, low capital costs and has the potential to produce a Class A product (pH of 12 or higher and temperature greater than 70˚C for 30 minutes). The lime reaction is exothermic and will raise the temperature of the mixture. This method also results in better dewatering characteristics; however, the total solids mass is increased because of the lime addition. The process has a relatively high annual cost because of chemical consumption. Nevertheless it merits more detailed investigation and, at minimum, bench scale testing for effectiveness and development of design parameters. Anaerobic digestion is typically more costly for smaller plants and single stage reactors are usually insufficient for pathogen kill. Multi-stage anaerobic digesters are recommended if a Class A product is desired. Liquid lime stabilization is more commonly used over dry lime addition. Dry lime is more difficult to handle, requires dry chemical feeders and a pug mill to mix the dewatered biosolids cake. Calcium Hydroxide and Calcium Oxide (Quicklime) can both be used for stabilization. Calcium Hydroxide has a higher delivered cost, but since it has already been hydrated, a slaker is not required. A slaker is required for Quicklime. The selection of stabilizing chemical and the appropriate dosages for Sechelt biosolids should be undertaken with bench scale testing and pilot testing. Dosages of Calcium Hydroxide can range from 0.1kg / kg dry solids to 0.3kg / kg dry solids. 10.2 Thickening The most economical thickening process is gravity thickening. Thickening is not required if aerobic digestion is employed. Gravity thickening is useful prior to lime stabilization to enable concentration of solids and reduce the dosage of Calcium Hydroxide. Gravity thickening is also useful if the biosolids are to proceed directly to the dewatering process. Page 41 1592.0026.01 / November, 2010 U:\Projects_KEL\1592\0026\01\R-Reports-Studies-Documents\Draft\2010-11-REP-BiosolidsProcessing Options Comparison Pre-Design Study (rev3).doc
District of Sechelt Bio-Solids Processing Options Comparison Pre-Design Report 10.3 Dewatering The comparison shows that centrifuge dewatering is the most practical and least offensive process in terms of odour control. The screw press process may also be considered, but pilot testing should be undertaken to confirm its effectiveness. The supply only costs of a centrifuge system complete with a polymer pre-conditioning skid range from $250,000 to $350,000. The supply cost of a screw press complete with polymer injection system is approximately $250,000. Both have approximately the same floor space requirement. Both options can be considered in further detail before a final selection is made. 10.4 Thermal Drying The most commonly used thermal drying process are rotary drum mechanisms with either direct heat (utilizing hot gases to cause evaporation), or indirect heat (thermal oil or steam jackets). Indirect dryers are more commonly used in smaller plants as they are easier to operate and require less energy. If an anaerobic digestion is used in conjunction with a dryer, the methane produced in the digester can be used as fuel to produce the heat required for the dryer. Page 42 1592.0026.01 / November, 2010 U:\Projects_KEL\1592\0026\01\R-Reports-Studies-Documents\Draft\2010-11-REP-BiosolidsProcessing Options Comparison Pre-Design Study (rev3).doc
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1.0 COST ESTIMATES 1.1 Capital Cost
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1.2 Operating Cost Estimates Table
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District of Sechelt Preliminary Report

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