Taking a Late Phase Detour: Dewatering System Redesign for the Plum Island WPCP

Summary This paper and presentation will discuss the in-plant operating conditions leading to a late-in-the-design-process dewatering technology selection reevaluation for the Charleston Water System's (CWS) main wastewater treatment facility, the Plum Island Water Pollution Control Plant (PIWPCP). Specifically, the paper will discuss the steps taken to reevaluate the solids dewatering design after 90% design completion by transitioning the selected dewatering technology and making this change while minimizing impacts to the construction schedule already underway. The efficient and timely redesign around a new dewatering technology was achieved by leveraging onsite pilots, solids sampling and off-site assessments, as well as manufacturer presentations to assist CWS in reviewing the preferred technology vendor options. Ultimately, CWS selected centrifuges to replace the originally selected rotary fan press technology to ensure improved dewaterability across a wide range of influent characteristics. Furthermore, the paper will discuss the design challenges associated with accommodating a dewatering technology change while concurrently minimizing impacts to the building footprint and structural design in order to curtail construction sequencing design delays and allowing the Construction Manager At Risk (CMAR) contractor to begin pile driving and sub-surface foundation work as originally scheduled. Background The Plum Island Water Pollution Control Plant (PIWPCP) is the main wastewater treatment facility for Charleston Water System (CWS), providing wastewater treatment for the City of Charleston, Daniel Island, James Island, and several surrounding municipalities. The facility is positioned on a 22-acre island surrounded by salt marsh and discharges to the tidally influenced Charleston Harbor at a rated capacity of 36-MGD. This location causes significant constructability issues due to high ground water from tidal influences and local major rain events coupled with challenging subsurface issues and seismic codes requiring major structures to be built on pile supported foundations. Prior to design, a long-term facility master plan had been completed for the PIWPCP identifying facilities required to accommodate currently permitted flows of 36-MGD with expansion up to an ultimate facility capacity of 54-MGD on both the liquid and solids treatment trains. In accordance with the master plan recommended improvements, new solids handling facilities were included as part of the Phase 4 Capital Improvements Project. These capital improvements included the following new solids handling processes: - New Solids Handling and Dewatering Building with Rotary Fan Press dewatering technology - Three (3) Sludge Storage Tanks with Blending Pumps - Dewatered Cake Storage Sludge Hopper and Loadout Facility The existing dewatering unit process equipment at the facility includes three (3) rotary fan press dewatering units which dewater a blend of undigested primary and secondary solids resulting from the liquid treatment process. Secondary solids are gravity thickened prior to blending with the primary sludge solids immediately upstream of dewatering in a blending tank. CWS had many years of acceptable operating experience (See Figure 1, January 2017 through October 2021) with the existing rotary fan press technology entering the detailed design phase of the upgrade and had thus selected the same technology to utilize in the new dewatering facilities to be constructed during the Phase 4 Capital Improvements Project. The originally proposed Phase 4 Solids Handling Building was designed to hold up to seven (7) 8-channel rotary fan presses with ancillary feed, polymer makeup, and conveyance equipment to meet long-term buildout solids handling needs and accommodate facility expansion up to a 54-MGD ultimate treatment capacity. However, during the later stages of the facility design, CWS identified insurmountable issues with the existing rotary fan press technology used for dewatering (See Figure 1, After October 2021). Steep declines in dewatered cake solids content resulted in increased wet cake mass and volumes for transportation and disposal at a local landfill facility, significantly increasing downstream disposal costs. Concurrently, the local landfill was significantly increasing tipping fees for 'wet waste', further exacerbating solids disposal costs for CWS. In addition to reduced dewatered cake solids content, the rotary fan press solids and hydraulic loading rate performance declined which would require increased numbers of dewatering units to process the anticipated solids loadings under current and future operating conditions. This combination of impacts, decreased dewatered cake solids, increased downstream management costs, and reduced rotary fan press performance, caused CWS to investigate and reconsider the dewatering technology selection and basis of design for the new facility after the rotary fan press design had advanced to the 90% completion level. Upon further investigation, it was determined that influent characteristics at the plant had changed since the initial assessments, causing a detrimental impact on the proposed dewatering equipment performance. Some of the impacts were due to a new influent conveyance tunnel system and collection system behavior between wet and dry periods, new headworks facilities with increased grit removal capability, and new primary and secondary treatment facilities which were included with recently completed capital improvements projects (Phase 2 and Phase 3 projects). Following this initial investigation, CWS approved reconsideration of the dewatering technology selection to include other dewatering technologies (e.g., belt presses, centrifuges). Alternative Technology Selection Immediately following the decision to reconsider the dewatering technology selection, work began to conduct on-site pilot and demonstration testing of belt filter presses and centrifuges. Concurrent demonstration testing was conducted on the same feed sludge for the existing rotary fan presses, a trailer-mounted belt filter press, and a trailer-mounted centrifuge to assess dewatered cake performance and other design parameters for each technology. Table 1 and Figure 2 show the results of the concurrent dewatering technology testing. On-site testing was coupled with off-site and bench scale testing across a range of primary and secondary sludge blends to understand the impact of the primary to secondary sludge (PS/WAS) ratio on anticipated dewatering unit performance to inform operational decision making. Lastly, potential technology suppliers were invited to make presentations to CWS regarding their technology offerings. Ultimately, CWS selected high solids, horizontal decanting centrifuges to replace the rotary fan presses to ensure improved dewaterability across a wide range of influent characteristics and PS/WAS ratios. Following the updated technology selection, the planned dewatering facility had to be redesigned on an accelerated schedule for the new technology while staying within the same building envelope to retain as much of the prior design structural framing and layout as possible to expedite the design and permitting processes. This paper will share some of the design challenges associated with and overcome with the conversion from rotary fan press to centrifuge dewatering technology across the various engineering disciplines (e.g., process mechanical, structural / architectural, and electrical and control systems). Updated design of the new dewatering facility has since been completed with centrifuges (See Figure 3 for a rendering comparison) and construction is currently progressing on the facility.