Preparing for the Future: WAS Only Indirect Thermal Drying at Manchester Creek WWTP

Summary This paper and presentation will discuss the planning that went into the design of a new solids processing facility at the City of Rock Hill's wastewater treatment plant, the Manchester Creek WWTP (MCWWTP), to accommodate a future secondary treatment process change and plant expansion. Specifically, the paper will discuss the accommodations that were made in design for existing and future feedstocks to the solids train. The pre-selection process for the belt filter presses (BFPs) and the indirect thermal dryer will be discussed including details on the manufacturer coordination and selection parameters. The benefits of thermal drying and flexibility inherent to the design of the MCWWTP's biosolids facilities will also be explored. Background The Manchester Creek WWTP (MCWWTP) treats wastewater for the City of Rock Hill, South Carolina. The MCWWTP has a currently permitted maximum month capacity of 20 million gallons per day (MGD). Final treated effluent is discharged directly to the Catawba River upstream of its confluence with Manchester Creek. A liquid train expansion and improvement project is currently under design to expand the plant to a maximum month capacity of 30 MGD, with an ultimate buildout capacity of 50 MGD. In the existing solids handling process, solids from primary and secondary clarifiers are currently blended and dewatered with two 2.0-meter width belt filer presses (BFPs) to approximately 22% total solids (%TS). Dewatered cake is transported to either the Water and Sewer Authority of Cabarrus County's (WSACC) sewage sludge incinerator to be incinerated; to the Anson County landfill for disposal; or to the McGill Environmental New Hill (NC) Facility for composting. The MCWWTP contracts dewatering operations, dewatered cake transportation, and dewatered cake disposal services to a contract service provider. In accordance with the latest MCWWTP Biosolids Master Plan, the City of Rock Hill is pursuing improvements at MCWWTP that will improve the dependability of the existing solids management options and expand the plant's biosolids facilities to handle the load associated with the plant expansion. Following the completion of the 30 MGD expansion, the MCWWTP will eliminate the primary clarifiers and utilize an aerobic granular sludge process leading to a change in solids feedstock to the solids train. The solids project will be constructed prior to the 30 MGD expansion and therefore must accommodate both the existing and future solids feedstocks as a part of the design. The updated biosolids facilities will improve plant operation, provide multiple outlet options (potential for Class A Biosolids), and provide improvements to outdated facilities. The project includes the following facilities: -WAS Storage - BFP Dewatering - Indirect Thermal Drying - All Ancillary Equipment The new Thermal Dryer Facility will be designed for WAS only thermal drying. By eliminating primary sludge (PS) in the dryer feedstock, the heat transfer efficiency (lb-water/hour per square foot heat transfer surface area) of the thermal dryers is significantly increased in comparison to PS + WAS solids, due to the recalcitrant nature of the blend. Furthermore, undigested PS does not form a granular product but tends to be more fibrous in nature, whereas undigested WAS typically forms a granular product with higher bulk density more suitable for land application, storage, and handling. Because the solids project will be completed in advance of the liquid train expansion there will be an approximately five-year period when primary solids will be produced at the MCWWTP and will still need to be processed, but not dried. Therefore, the new solids facilities include the provision to process PS and WAS separately during the five-year period prior to the expansion and process change completion. See Figure 1 for the initial process flow diagram with two separate treatment trains for PS and WAS and Figure 2 for the ultimate planned solids process flow diagram. Flexibility in Solids Facility Design Because all new infrastructure associated with the primary solids management train will be for a short period of time the overall project goal was to minimize capital expenditures on PS only equipment that could not be used following the process change. For solids storage, a small, bolted steel tank (25,000 gal) with a vertical turbine mixer was proposed for PS storage. Following the liquid train conversion, the PS tank would be taken out of service and could be repurposed elsewhere onsite or demolished. The capital expenditure associated with the PS tank was considered necessary for operator flexibility, but tank size was reduced to keep costs minimal. A new, larger WAS storage tanks was proposed to support the long-term plan for a conversion to a WAS only process and a new dryer operational schedule of 24 hours per day, 4 days per week at annual average conditions. The larger WAS storage allows the facility to practice continuous wasting and decouple the liquid train operations from the solids operations by providing a wide spot in the line as well as provide a homogenous feed to the BFPs. The proposed Dewatering Facility is designed to house up to four (4) belt filter presses to meet long-term buildout. Initially three BFPs will be installed in the building and PS and WAS will be dewatered on separate BFPs. Piping and valving interconnections will allow the two streams to remain completely separate but provide needed equipment flexibility and redundancy to have a shared standby machine. The WAS dewatered cake will be conveyed to the thermal dryer facility while the PS dewatered cake will be hauled offsite for processing. The dewatering building should see a seamless transition to all BFPs dewatering the same feedstock from the WAS Storage Tanks following the process conversion. Because the Thermal Dryer Facility will only be designed for a WAS only stream the facility should transition easily with the liquid train process upgrades. Future planning discussions with the paddle/screw dryer manufacturers revolved around the impact of the liquid train process change on solids dewaterability and solids loading to the dryer. Overall, the thermal dryer facility will significantly decrease the water content of the biosolids, which will reduce both the volume and wet mass to be transported and final product to be managed by Rock Hill and increase their product end use alternatives. Equipment Selection Two requests for proposals (RFP) were developed for pre-selection of the belt filter press manufacturer and thermal dryer system supplier to help expedite the design process. Pre-selection of the thermal dryer system supplier was an important step as each manufacturer has their own intricacies and design details. Working with only one dryer manufacturer following preliminary design allowed manufacturer specific details and requirements to be coordinated in advance of construction. Manufacturer coordination prior to issuing the RFPs included: bench scale testing across a range of primary and secondary sludge blends to understand the impact of the PS only, WAS only and PS/WAS blend on dewatering equipment throughput, polymer usage and dewatered cake concentrations and thermal dryer performance (fouling potential, heat transfer, final product quality). In addition, potential technology suppliers were invited to meet with Rock Hill to discuss their technology offerings and differentiators from their competitors. Manufacturers were given the opportunity to review the equipment specifications that were a part of the RFP package. RFPs were evaluated using the methodology shown in Tables 1 and 2. The paper and presentation will detail the selection process.