Advanced Thickening Upgrades: Maximizing Existing Assets by Integration of New Technology

At wastewater treatment and water reclamation facilities, thickening of solids is critical to maximize process efficiency. Facilities that co-thicken, mixing waste activated sludge (WAS) and primary sludge (PS), rely on a single, crucial solids treatment process. Many facilities have aging historic assets, such as dissolved air flotation thickeners (DAFTs), and are looking for creative ways to extend the life of existing systems, improve performance, and increase ease of operation. The LOTT (Lacey, Olympia, Tumwater, Thurston County) Clean Water Alliance is located at the south end of Puget Sound in Olympia, WA. LOTT operates the Budd Inlet Treatment Plant (BITP), an advanced biological nutrient removal facility with and average flow of 13 million gallons per day (MGD) and peak flows of over 70 MGD. BITP relies on four large concrete DAFT tanks for co-thickening WAS and PS prior to anaerobic digestion. These DAFTs, constructed in 1982, perform reasonably well, but much of the associated mechanical equipment has reached the end of its useful life. While a simple solution to replace all equipment in-kind was possible, LOTT was interested in creative ways to take older thickening technology and revitalize it by blending it with more efficient, modern technology. This paper provides an overview of planning and pilot testing to upgrade a DAFT solids thickening system by creatively blending new technology with existing infrastructure to maximize value and improve operation. The information presented provides a methodology and key pilot results demonstrating how to apply alternative technology to upgrade a more traditional thickener. Key topics include: - Evaluating the existing DAFTs in comparison with newer thickening technologies such as rotary drum thickeners (RDTs) and gravity belt thickeners (GBTs). - Determining how to refurbish and reuse existing DAFTs with more modern technology such as self-aspirating recirculation (SAR) pumps. - Developing a full-scale pilot to test SAR pumps and determine treatment capacity and thickening performance when applied to the existing DAFT hydraulics. The full-scale pilot was conducted over multiple years to verify performance and long-term capacity. - Final design, construction, startup, and testing of the complete system that validated the successful integration of modern technology with classic, well understood thickening equipment. The goal of this overview is to provide design engineers, planners, and operators, with a means to carefully assess critical existing thickening assets and creatively apply technology to bring an 'older' system into peak condition. Technology Selection The LOTT DAFTs were originally rated for approximately 15,000 lb/d solids removal (WAS only), with a target thickened solids (THS) concentration of 3.5 percent. Each of the four tanks is approximately 46 ft long by 16 ft wide. The tanks include a traditional internal flight system, with upper flights that carry solids to a beach for collection in a hopper and lower flights/cross collectors that remove residual settled solids. Each system included a centrifugal recirculation pump with a large pressurization tank that was used to dissolve air into the recirculated liquid and mix it with influent solids. Figure 1 provides a photo of the original pressurization tank system along with an internal view of a DAFT prior to the recent upgrades. In 2015, LOTT began the process to review options to either replace the aging DAFTs or upgrade them. The DAFTs were suffering from mechanical failures and excessive maintenance time to operate the pressurization tanks. In addition, LOTT was looking at long-term loading and performance needs. Table 1 provides a summary of the design conditions during the initial evaluation. LOTT determined that the DAFTs needed to produce a 6-8 percent THS and be able to accommodate a firm capacity of 80,000 lb/d to serve for the next 20-25 years. The review process included two broad options: (1) replace the existing system with a mechanical thickening option such as an RDT, or (2) refurbish the existing thickening system to provide long-term reliability. The RDT technology appeared promising, as it would use an efficient technology that is commonly installed in new wastewater facilities and allow LOTT to reclaim process space in the plant. The existing DAFT tanks, while unwieldy and aging, perform well and the large tank volume provides a buffering capacity to maintain consistency and resiliency across various solids loads. Refurbishment of the DAFTs was the preferred approach, assuming two key conditions could be met: 1)Complete replacement of all internal components (including flights, drives, shafts, chains, sprockets, cross-collector, etc.) and repair of any damaged baffles or concrete. 2)Replacement of the complicated pressure vessel system with technology that would be reliable and easier to operate. To address the first condition, an assessment of the existing DAFT tanks was performed and a vendor was selected (Brentwood Industries, LLC) to provide all new internal components that would be procured by LOTT for installation in each DAFT tank. The second condition was addressed by a proposal to replace the pressurization system with simpler, easy to operate, SAR pumps. The proposed SAR pumps (Nikuni America, Inc.), as shown in Figure 2, are utilized by World Water Works (WWW) in their package and custom DAFT units. The technology provides a more compact and efficient means to transfer air into solution, as the SAR pumps act as both the recirculation pump and the pressurization system. A small rotometer allows air to flow into the volute of the pump, driven by the venturi effect of the impeller, where it is compressed into the discharge. The challenge with this approach is that LOTT's DAFTs are particularly large units and there was no simple way to ensure the size or number of pumps necessary to meet the required solids loading. SAR Full-Scale Pilot Testing To determine the design of the SAR pumps, DAFT 4 was utilized as a full-scale pilot system. In 2017, a single SAR pump, Nikuni's largest model M80FP, was procured and installed by plant staff as a substitute for the existing pressurization tank and recirculation pump. Figure 3 provides a photo of the full-scale layout. The SAR pump operates at a constant speed, 280 gpm flow rate with a 70-80 psig discharge pressure. Flow from the pump is directed to an injection pipe manifold in which the following occurs: - Large residual air bubbles are removed from the line via a vent valve. - The remaining entrained air passes across a knife gate valve throttled to burn the residual head from the pump. The large pressure drop across the valve releases the entrained and dissolved air in a fine bubble foam. - Influent solids (WAS and PS), mixed with polymer, are injected into the liquid recirculation line, where they mix with the fine bubbles prior to entry into the DAFT. The pilot system was operated with the help of WWW for over a year to verify operation and to test the capacity. LOTT staff stress tested the DAFT in 2019 with solids at higher and higher loading rates until the primary criteria (6 percent THS for effluent solids) was no longer achievable. The system achieved approximately 12,000 lb/d, which fell short of the required design criteria. At this point, the pilot study was modified to determine if a second SAR pump in parallel could improve the results. Working with WWW, a customized header with a series of ball valves was developed to better control the air from the SAR pump. A second SAR pump was installed in parallel with the previous model and stress tested in late 2020 and early 2021. Figure 4 provides a photo of the second pilot system. Dual SAR pump pilot testing achieved loadings above 25,000 lb/d per DAFT, exceeding the necessary design criteria and providing critical information to integrate SAR pumps into the existing DAFTs. Table 2 provides a summary of the key pilot results. Construction and Full-Scale Operation Given the success of the SAR pump pilot, design and installation of the full upgrade for the remaining DAFTs was completed over the course of 2023-2024. The pilot work had validated the necessary SAR pump capacity, ease of operation, and piping design that allowed for consistent, steady results. Figure 5 provides a picture of the final SAR system, integrated into the existing DAFT tanks based on lessons learned from the pilot work. The system was fully operational in October 2024 and is producing consistent effluent solids in the 6-8 percent THS range. The work in this paper demonstrates a methodology, operational approach, and potential design criteria to creatively apply the alternative SAR pump technology to existing DAFT systems to allow them to operate effectively and efficiently for decades to come.