Zahller, Jeffrey

Jeff is a registered chemical engineer and has worked in wastewater process design for 23 years. He is a Senior Professional Associate at HDR, where...

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About Zahller, Jeffrey

Jeff is a registered chemical engineer and has worked in wastewater process design for 23 years. He is a Senior Professional Associate at HDR, where he has lead process mechanical design and field work across the treatment plant, from the headworks to disinfection, from clarifiers to solids dewatering and digestion. He also serves as a technical lead for HDRs odor control technology group.

Zahller, Jeffrey

Titles from this speaker

Description: Advanced Thickening Upgrades: Maximizing Existing Assets by Integration of New...

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

Abstract

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.

This paper was presented at WEFTEC 2025, held September 27-October 1, 2025 in Chicago, Illinois.

Presentation time

13:30:00

14:00:00

Session time

13:30:00

15:00:00

SessionThickening to Drying: Optimizing Solids Handling Through Innovation

Session locationMcCormick Place, Chicago, Illinois, USA

TopicBiosolids & Residuals

Author(s)

Zahller, Jeffrey, AGUSTSSON, OSKAR, Roe, Patrick, Summers, Kip, Zuchowski, Tyle

Author(s)J. Zahller1, O. AGUSTSSON1, P. Roe1, K. Summers2, T. Zuchowski2

Author affiliation(s)HDR Inc1, LOTT Clean Water Alliance2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2025

DOI10.2175/193864718825159939

Volume / Issue

Content sourceWEFTEC

Word count12

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

Abstract

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.

This paper was presented at the WEF Residuals & Biosolids and Innovations in Treatment Technology Joint Conference, May 6-9, 2025.

SpeakerZahller, Jeffrey

Presentation time

09:10:00

09:30:00

Session time

08:30:00

10:00:00

SessionAdvancements in Thickening Technologies: Operational Optimization and Cost Savings

Session number23

Session locationBaltimore Convention Center, Baltimore, Maryland, USA

TopicAsset Management, Efficiency, Solids Thickening

Author(s)

Zahller, Jeffrey, Agustsson, Oskar, Roe, Patrick, Summers, Kip, Zuchowski, Tyle

Author(s)J. Zahller1, O. Agustsson1, P. Roe1, K. Summers2, T. Zuchowski2

Author affiliation(s)HDR, 1LOTT Clean Water Alliance, 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2025

DOI10.2175/193864718825159799

Volume / Issue

Content sourceResiduals and Biosolids Conference

Word count12

Description: Small System with Large Challenges: Operating a Wastewater Treatment Plant that Does...

Small System with Large Challenges: Operating a Wastewater Treatment Plant that Does Not Follow the Rules

Abstract

Small community wastewater treatment plants must overcome unique challenges that larger facilities do not face. Small plants often have the same mechanical complexity and effluent standards as larger facilities, but without the benefits of stable flow/loadings, operational flexibility, easy access, and buffered protections from toxicity or industrial loadings. The Stevens Pass Sewer District (SPSD) provides a successful case study of a plant which must weather such challenges and merge a technically advanced treatment system (a 379 m3 /d [0.1 MGD] membrane system with nitrogen removal capabilities) with the unique challenges of small system dynamics. The facility reduces ammonia levels to near non-detect despite being carbon deficient, while also responding to nearly instantaneous shoulder season shifts in influent flows and loading. Staff work through extreme logistics challenges with full winter isolation, while working with residents to address the significant impacts of acute events to the plant process.

Small system wastewater treatment plants (379 m3/d [0.1 MGD]), such as Stevens Pass Sewer District (SPSD), require unique design and operational standards. A modest inconvenience for a large plant can be a critical failure for a small facility. Process improvements (such as carbon addition to improve denitrification) and construction projects have extremely challenging logistics, cost, and staffing issues. The SPSD case study provides an example of merging technically advanced treatment systems.

SpeakerZahller, Jeffrey

Presentation time

15:30:00

Session time

15:30:00

17:00:00

SessionSmall Community Applications of Decentralization and Associated Management Approaches

Session locationRoom S403a - Level 4

TopicIntermediate Level, Research and Innovation, Small Communities and Decentralized Systems, Sustainability and Climate Change

Author(s)

Zahller, Jeffrey

Author(s)J. Zahller 1; A. Staples 1 ; S. O'Brien 2; T. Maher 2; J. Zahller 1; T. Schlect 3;