STATE-OF-THE-PRACTICE IN BIOSOLIDS/POLYMER BLENDING FOR BIOSOLIDS DEWATERING

摘要

Gwinnett County owns and operates three water reclamation facilities (WRFs): the F. Wayne Hill Water Resources Center (FWHWRC), the Crooked Creek WRF (CCWRF) and the Yellow River WRF (YRWRF). The FWHWRC is the largest of the three WRFs with a maximum month average daily flow (MMADF) rated capacity of 60 million gallons per day (MGD) or 227 ML/d and a current average annual daily flow (AADF) of approximately 32 MGD (121 ML/d). The YRWRF has a MMADF rated capacity of 22 MGD (83 ML/d) with an AADF of approximately 13 MGD (49 ML/d). The CCWRF has a MMADF rate capacity of 16 MGD (60 ML/d) with an AADF of approximately 7.5 MGD (28 ML/d), was not part of this study and preliminary design project. Gwinnett County Department of Water Resources (GCDWR) transfers primary sludge (PSL), primary scum and waste activated sludge (WAS) produced at the YRWRF to the FWHWRC via the County sewer collection and conveyance system for treatment. The solids treatment process at the FWHWRC incorporates storage of PSL and WAS in a common tank for phosphorus release and nutrient recovery, mechanical thickening of this combined PSL and WAS utilizing rotary drum thickeners; anaerobic co-digestion of thickened sludge with delivered fats, oil and grease (FOG) and high strength waste (HSW); stabilized liquid biosolids storage; and centrifuge dewatering of the stabilized sludge and the chemical sludge from the tertiary treatment process. Liquid streams from thickening and dewatering are used to feed a nutrient recovery system utilizing struvite precipitation (Ostara, Inc.). Trucks haul the resulting biosolids to landfill for disposal. FWHWRC spends approximately $1.2 million of cationic polymer per year for thickening ($402,000) and dewatering ($802,000) operations. The dewatering polymer system used an average of 31.8 pounds of active polymer per dry ton of solids produced (1b/Dt or 15.9 kg/dMT) at a concentration of 0.74 percent, and the thickening polymer system used an average of 8.3 1b/dT (4.2 kg/dMT) of active polymer solution. Solution concentration for the thickening process is not clearly known and is a reported concern of facility staff. Since most of the polymer usage and cost is related to the dewatering process, field investigations were scheduled to ascertain if opportunities exist to help reduce polymer usage at FWHWRC. GCDWR requested that the existing polymer systems in the solids handling facilities at FWHWRC be evaluated by Jacobs Engineering and offer recommendations to improve operability and safety within the building. The project goals and success factors were as follows: 1. Improve safety with grating and non-slip surfaces 2. Provide improved polymer dose control for dewatering polymer system equipment that was 15 years old 3. Improve operational flexibility and optimize equipment space 4. Maintain solids capture [(less than 200 parts per million (ppm) to nutrient recovery] 5. Add polymer system for Chemical Solids Thickeners 6. Improve instrumentation 7. Reduce overall polymer consumption at current locations AND save some money Polymer activation is a fundamental performance metric for optimizing polymer usage. Sub-optimal activation is a likely cause of the higher than expected polymer dosage required in the FWHWRC dewatering process. Recent advancements in polymer blending and feed equipment have demonstrated proper polymer activation at other utilities. Two such devices with mechanical mixing of the emulsion polymer with the dilution water were selected to perform pilot tests on the dewatering centrifuge polymer blending equipment at FWHWRC in November and December 2017 (CH2M, 2018). A UGSI Chemical Feed Solutions PolyBlend Magnum M Series (MMM1800-MM2400) and a ProMinent Fluid Controls ProMix L-Series (KS-14) were pilot tested separately. Performance of each pilot skid was compared to the existing single-stage low speed non-mechanical (relies on water pressure for mixing) Polydyne SNF FloQuip EA-Series (EA70P) polymer blending unit. Results showed the two-stage pilot equipment used 10 to 25 percent less polymer than the existing one-stage equipment, while producing much cleaner centrate with no decrease in cake solids dryness. Reduction in polymer usage was attributed to improved activation of the polymer solution. Replacing the existing one-stage polymer feed system with a state-of-the-practice two-stage polymer feed system that was successfully pilot tested is anticipated to provide and annual savings of $100,000 to $200,000 (10 to 25 percent polymer savings). GCDWR has hired Jacobs to design improvements to the rotary drum thickener and centrifuge polymer systems that incorporate the two-stage biosolids/polymer blending equipment and new polymer feed pumps along with other auxiliary equipment to enhance the effectiveness of both polymer systems. The implementation of this state-of-the-practice biosolids/sludge blending systems and other polymer system enhancements should result in a safer operator work environment, improve polymer dose control and instrumentation, more operational flexibility and significant 10 to 25 percent centrifuge polymer savings. This manuscript also includes preliminary design criteria and cost estimates for the recommended enhancements for the rotary drum thickening, centrifuge dewatering and chemical thickener polymer systems.