Zahller, Jeffrey

Zahller, Jeffrey

Jeffrey Zahller is a speaker at WEFTEC 2026.

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About Zahller, Jeffrey

Jeffrey Zahller is a speaker at WEFTEC 2026.

Zahller, Jeffrey

Jeffrey Zahller is a speaker at WEFTEC 2026.

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

Keeping It Simple: Principles for Application of Traditional Mesophilic Digestion

Abstract

Many municipalities have considered using high-rate anaerobic digestion, including thermophilic reactors, thermal hydrolysis processes (THP), thermal-chemical hydrolysis processes (TCHP), and temperature phased anaerobic digestion (TPAD). These technologies come with advantages (higher biogas rates, greater solids destruction) and challenges (instability, corrosion, operational complexity). However, traditional mesophilic digestion (95-100°F) remains the workhorse across the industry due to simplicity and stability. This paper proposes a series of fundamental principles to evaluate and optimize mesophilic digestion operation and design. The principles can be applied to any form of digestion but are critical to effectively evaluate mesophilic systems prior to considering high-rate modifications.

• [b]True Mass Balancing[/b] – accurate data and methods to calculate volatile solids reduction (VSR).
• [b]Feed Design and Control[/b] –maximize kinetic advantages and stability.
• [b]High Value Operational Features[/b] – efficient mixing and foam/gas management.
• [b]Keeping Options Available[/b] – flexibility in design for future changes.

These four principles will be reviewed using the case study of the Central Kitsap Treatment Plant (CKTP), Poulsbo, WA, as they designed a complete digester replacement project. [b]Case Study[/b] CKTP is a 6 million gallon per day (MGD) biological nutrient removal plant with two mesophilic digesters fed from four separate sources: co-thickened septage and primary sludge (PS), thickened waste activated sludge (WAS), scum, and FOG. This feed mixture has a large portion of low volatile solid (VS) waste (septage, WAS), combined with higher VS feed (PS and FOG). This has made VSR historically difficult to measure and confirm. Figure 1 emphasizes the large (27 percent) fraction of septage and WAS. Consistently achieving the necessary 38 percent VSR to meet Class B requirements was historically challenging. CKTP desired to maintain simple mesophilic operation as part of the new facilities, but only if VSR challenges could be addressed by using each of the principles noted above. [i]Principle 1 – True Mass Balancing[/i] In 2021 a study of the digester feed estimated volatile solids fractions from each feed source (Table 1, Table 2) and compared VSR calculated by three methods: Van Kleeck (VK, Equation 1), a true mass balance (U, Equation 2), and an empirical method (V[sub]d[/sub], Equation 3). As shown by Switzenbaum et al. (2003), VK calculations (used by CKTP), which estimate VSR using only volatile solids fractions, may underestimate VSR due to assumptions regarding inert solids. VS[sub]F[/sub] = Volatile Solids Fraction (Feed) VS[sub]W[/sub] = Volatile Solids Fraction (Digester Effluent) V[sub]d[/sub] = Volatile Solids Destroyed SRT = Solids Retention Time (1) Van Kleeck (Switzenbaum et al., 2003): VSR = (VS[sub]F[/sub]-VS[sub]W[/sub])/(VS[sub]F[/sub]-(VS[sub]F[/sub]*VS[sub]W[/sub])) (2) True Mass Balance (Switzenbaum et al., 2003): U(%) = (VMass[sub]in[/sub]-VMass[sub]out[/sub])/(VMass[sub]in[/sub]) (3) Empirical Method (Metcalf & Eddy/AECOM, 2014): V[sub]d[/sub] =13.7 ln⁡(SRT)+18.9 Figures 3 and 4 present the results of the comparison, with the empirical method using both the calculated SRT and the reported mean cell residence time (MCRT). The VK method underestimated VSR. This was corroborated when comparing the VSR to estimated gas production rates using a range of 12 to 18 cubic feet per pound of VS destroyed (Metcalf & Eddy/AECOM, 2014). The mass balance method predicted the actual biogas production as well or better than VK. The mesophilic system was achieving the necessary VSR. [i]Principle 2 – Feed Design and Control[/i] Operating digesters in parallel versus series will alter the kinetics. Series operation allows for increased VS removal of 3-5 percent over parallel operation but increases the loading rate to the first stage with less protection against instability (Zahller et al., 2007). Following this approach, CKTP designed their new digesters to allow for series operation. [i]Principle 3 – High Value Operational Features[/i] Poorly mixed, poorly measured and irregular pulses in VS feeding create inconsistent gas production, poor temperature distribution, and reduce reactor stability. Mesophilic efficiency is partly a product of how a digester is fed, not only what it is fed. To address this, CKTP chose to integrate the following: - Separate feed lines for control and measuring individual feed stock (Figure 6). - Flow meters and inline solids meters to improve data quality. - Biogas flow meters for each digester to measure performance. - Mixing and foam suppression designs for system stability. [i]Principle 4 – Keeping Options Available[/i] While ultimately deciding to keep mesophilic digestion, CKTP maintained long-term options by designing for future flexibility: - Wall thickness and insulation to allow for higher thermal loads (thermophilic operation). - Maintaining site space for pre-treatment systems such as THP or THCP. [b]Summary[/b] While high-rate digester technology may be necessary, the advantages of mesophilic digestion are often the best choice for a facility. The design principles in this paper provide a framework to give traditional technology thoughtful consideration, ultimately providing more efficient operation of mesophilic systems or better justification for a high-rate approach.

This paper was presented at WEFTEC 2026 in New Orleans, Louisiana.

Presentation time

15:30:00

15:45:00

Session time

15:30:00

17:00:00

SessionOptimizing Anaerobic Digestion Operations

Session locationErnest N. Morial Convention Center

TopicBiosolids and Residuals, Facility Operations and Maintenance

Author(s)

Zahller, Jeffrey, Nolan, Laura, Parmenter, Adam, martin, nick

Author(s)J. Zahller1, L. Nolan1, A. Parmenter1, n. martin2

Author affiliation(s)HDR Inc, 1HDR Inc, 1HDR Engineering Inc., 1Kitsap County Wasterwater Division, 2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date Sep 2026

DOI10.2175/193864718825160311

Volume / Issue

Content sourceWEFTEC

Word count11

Keeping it Simple: Principles for Application of Traditional Mesophilic Digestion

Abstract

As demands and performance requirements have continued to become more aggressive, many municipalities have considered or implemented high-rate forms of anaerobic digestion, including thermophilic reactors, thermal hydrolysis processes (THP), thermal-chemical hydrolysis process (TCHP), and temperature phased anaerobic digestion (TPAD) applications. These technologies come with advantages (higher biogas rates, greater solids destruction) as well as challenges for operation and maintenance (higher levels of instability, inhibition, odors, corrosion, and operational complexity). Despite the many available options, traditional mesophilic digestion (operational range of approximately 95-100Â°F) remains the workhorse across the majority of water resource and recovery facilities (WRRFs) over the last 50 years. While it lacks some of the kinetic advantages of high-rate technologies, it comes with a variety of benefits including simplicity, stability, safety and low cost of operation. This paper proposes a series of fundamental principles to evaluate and optimize mesophilic digestion operation and design. The principles can be applied to any form of digestion, but are critical to effectively evaluate mesophilic systems prior to considering high-rate modifications. -True Mass Balancing - accurate data and appropriate methods to calculate volatile solids reduction (VSR) performance. The calculation methods can vary significantly and can drastically change the perceived performance. -Feed Design and Control - the ability to feed consistently and slowly across a variety of sources as well as maximizing the kinetic advantages of mesophilic digester option (parallel and series control). -High Value Operational Features - incorporating simple, but effective means to address foam, mixing, overflow and chemical addition. -Keeping Options Available - flexibility in a design and approach that allows for changes (including to high-rate systems) in the future without losing the value of established infrastructure. These four principles will be reviewed using the case study of the Central Kitsap Treatment Plant (CKTP), operated by Kitsap County (Poulsbo, WA), as they developed the design of a complete digester replacement project. The County was looking at next steps to upgrade an aging mesophilic digestion system while having unique challenges with low volatile solids (VS) waste streams. A thoughtful application of these principles allowed them to move forward with a trusted technology while keeping a wide variety of options available for the future. Case Study CKTP presents a set of unique challenges that could be solved through either traditional or high-rate digestion technologies. The facility is a 6 million gallon per day (MGD) biological nutrient removal plant with two mesophilic digesters fed from four separate sources: co-thickened septage and primary sludge (PS), thickened waste activated sludge (WAS), scum, and FOG. This feed mixture has a large portion of low volatile solid (VS) waste (septage, WAS), combined with higher VS feed (PS and FOG). This has made VSR historically difficult to measure and confirm. Mechanical failures of the existing digesters (originally constructed in the 1970s) led to a project for replacement with two new 1.4 MG tanks. CKTP desired to maintain the simplicity of the mesophilic operation, but only if a clear pathway to address historic challenges could be achieved. Each of the principles below are discussed in context of their application to CKTP in determining if they should keep their mesophilic system. Principle 1 - True Mass Balancing Plants often lack instrumentation (flow meters, solids meters) or staffing (grab samples, monitoring) to obtain accurate data regarding the flow and volatile solids content of each portion of the feed stock. This makes it difficult to accurately correlate VSR to the hydraulic residence time (HRT). This is critical for CKTP, as the variety of feed stocks and proportions could drastically change the assumed/calculated VSR. Figure 1 provides a summary of the solids proportions feeding the CKTP digestion system, which emphasizes the large (27 percent) fraction of septage solids and high portions of WAS and biological solids. Historically, despite a relatively high solids retention time (SRT) within the digesters, CKTP was challenged in consistently achieving the necessary 38 percent VSR to meet the biosolids goal of Class B (Code of Federal Regulations [CFR] Title 40 Part 503). In 2021 a study of the digester feed estimated volatile solids fractions from each feed source (Table 1, Table 2) and compared VSR calculated by three methods: Van Kleeck (VK, Equation 1), the true mass balance (U, Equation 2), and the empirical method (Vd, Equation 3). As shown by Switzenbaum et al. (2003), VK calculations (traditionally used by CKTP), which estimate VSR using only volatile solids fractions, may underestimate VSR due to assumptions regarding inert solids. VSf = Volatile Solids Fraction (Feed) VSw = Volatile Solids Fraction (Digester Effluent) Vd = Volatile Solids Destroyed SRT = Solids Retention Time Van Kleeck (Switzenbaum et al., 2003): VSR=(VSf-VSw)/(VSf-(VSf*VSw))=(1-((VSw)/(VSf)))/(1-VSw) (1)True Mass Balance (Switzenbaum et al., 2003): U(%)=(VMass_in-VMass_out)/(VMass_in ) (2) Empirical Method (Metcalf & Eddy/AECOM, 2014): VSR (Vd)=13.7 lnâ¡(SRT)+18.9 (3) Figures 2 through 5 present the results of the comparison, with the empirical method using both the calculated SRT and the reported mean cell residence time (MCRT) from the original plant data. The analysis demonstrated, decisively, that the VK method underestimated the VSR and the mesophilic system was performing better than previously thought. This conclusion was corroborated when comparing the VSR to estimated gas production rates using a bracketed range of 12 to 18 cubic feet per pound of VS destroyed (Metcalf & Eddy/AECOM, 2014). The mass balance method predicted the actual biogas production as well as or better than the VK. Consequently, the mesophilic system was achieving the necessary VSR and a primary driver for moving to high-rate systems was less significant. Principle 2 - Feed Design and Control Operating digesters in parallel versus series will alter the kinetics. Series operation moves toward plug flow, allowing for increased VS removal of 3-5 percent over parallel operation but increases the loading rate to the first stage with less protection against instability (Zahller et al., 2007). Following this approach, CKTP designed their new digesters to allow for the effluent pumps of each digester to serve as feed pumps to the other tank. Simple series operation is efficient and flexible and can gain an extra edge on VSR, maximizing mesophilic performance. Principle 3 - High Value Operational Features Poorly mixed, poorly measured and irregular pulses in VS feeding create inconsistent gas production, poor temperature distribution, and reduce reactor stability. Mesophilic efficiency is partly a product of how a digester is fed, not only what it is fed. To address this, CKTP chose to integrate the following: Separate feed lines for control, metering and measuring individual feed stock (Figure 6). Flow meters and inline solids meters to improve data quality. Biogas flow meters for each digester to measure performance. Aggressive mixing and foam suppression designs for system stability. Principle 4 - Keeping Options Available While ultimately deciding to keep mesophilic digestion, CKTP maintained long-term options by designing for future process flexibility: Wall thickness and insulation to allow for higher thermal loads (thermophilic operation). Space for additional heat exchangers for conversion to high-rate systems. Centralized location of new digesters and control buildings, with available site space allowing for pre-treatment systems such as THP or THCP to easily be added. Summary While high-rate digester operation is often useful and necessary, particularly in situations where Class A biosolids products are needed, the advantages of mesophilic digestion should not be quickly overlooked, as the stability and simplicity of use is often the best choice for a given WRRF. The analysis and design principles presented in this paper provide a useful framework to allow municipalities to give traditional technology thoughtful consideration, ultimately providing more stable and efficient operation of a mesophilic systems, or providing better justification for a high-rate approach.

This paper was presented at the WEF Residuals, Biosolids, and Treatment Technology Conference in Kansas City, MO, May 11-14, 2026.

SpeakerZahller, Jeffrey

Presentation time

15:30:00

16:00:00

Session time

15:30:00

17:00:00

SessionConventional Anaerobic Digestion Is Still Relevant!

Session locationKansas City Convention Center

TopicDigestion

Author(s)

Zahller, Jeffrey, Nolan, Laura, Parmenter, Adam, Martin, Nick

Author(s)J. Zahller1, L. Nolan1, A. Parmenter1, N. Martin2

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2026

DOI10.2175/193864718825160231

Volume / Issue

Content sourceResiduals, Biosolids and Treatment Technology Conference

Word count11

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;