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

Introduction and Objectives
The LOTT (Lacey, Olympia, Tumwater, Thurston County) Clean Water Alliance is located on Puget Sound in Olympia, WA. LOTT operates the Budd Inlet Treatment Plant (BITP), an advanced nutrient removal facility with an average flow of 13 million gallons per day (MGD). BITP relies on four large dissolved air flotation (DAFT) tanks for co-thickening waste activated sludge (WAS) and primary sludge (PS) prior to digestion. The DAFTs, constructed in 1982, perform well, but much of the 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 revitalize the DAFT with modern technology.
Key objectives this paper will address:
- Demonstrate strategies to refurbish and reuse existing DAFTs with more modern technology, specifically self-aspirating recirculation (SAR) pumps.
- Develop a method and present results for a full-scale pilot to test SAR pumps and determine design criteria and performance.
- Summarize results and lessons learned for the final design, construction, startup, and testing of the full-scale system based on the pilot work.

Methodology - 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. The tanks are rectangular and include a traditional internal flight system for collecting thickened solids. A recirculation pump with a large pressurization tank is used to dissolve air into the recirculated liquid. Figure 1 provides a photo of the original system along with an internal view of a DAFT prior to the upgrades.

In 2015, LOTT began to review options to replace/upgrade the aging DAFTs. Table 1 provides a summary of the design criteria used for the initial evaluation. The DAFTs needed to produce a 6-8 percent THS and accommodate a firm capacity of 80,000 lb/d to serve for the next 20-25 years. The existing DAFT tanks, while unwieldy, perform well and the large tank volume provides a buffering capacity to maintain resiliency across varying 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, etc.).
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 new internal components.

The second condition was addressed by a proposal to replace the pressurization system with SAR pumps. The proposed SAR pumps (Nikuni America, Inc.), shown in Figure 2, are utilized by World Water Works (WWW) in their package DAFT units. They are a compact means to transfer air into solution, acting as both the recirculation pump and the pressurization system. A rotometer allows air into the volute of the pump, where it is compressed into the discharge. However, LOTT's DAFTs are large units and there was no simple means to determine the size or number of pumps necessary to meet the required solids loading.

Methodology and Findings - SAR Full-Scale Pilot Testing
To determine the design of the SAR pumps, DAFT 4 was utilized as a full-scale pilot. In 2017, a single SAR pump, Nikuni model M80FP, was installed in place of the existing pressurization tank and recirculation pump. Figure 3 provides a photo of the layout. The SAR pump operates at a 280 gpm flow rate with a 70-80 psig discharge pressure. Flow from the pump is directed to a pipe manifold in which the following occurs:
- Entrained air passes across a throttled knife gate valve, releasing the entrained air in a fine bubble foam.
- WAS and PS, with polymer, are injected into the recirculation line and mix with the fine bubbles prior to entry into the DAFT.

LOTT stress tested the DAFT in 2019 until the primary design criteria (6 percent THS for effluent solids) was no longer achievable. The system reached approximately 12,000 lb/d, which fell short of the required target. 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 designed to better control the air from the SAR pump. A second SAR pump was installed and stress tested in late 2020 and early 2021. Figure 4 illustrates the second pilot system. Dual SAR pump pilot testing achieved loadings above 30,000 lb/d per DAFT, exceeding the target goal and providing critical design information. Table 2 provides a summary of the key pilot results.

Full-Scale Operation Status and Significance of Work
Given the pilot success, a full upgrade for the remaining DAFTs was completed in 2023-2024. Figure 5 shows the final SAR system. Operational in October 2024, the system is producing consistent effluent solids at 6-8 percent THS.

The key significance of this work is twofold:
1) Development of a pilot methodology, with proven results, that allowed the customized application of SAR pumps to an existing DAFT.
2) Validation of the pilot method with a full-scale installation and start-up. The design methods used in this paper can be applied by operators and engineers at any facility to match SAR technology with their specific DAFT application.