INTERNAL MEMORANDUM

DATE: 2026-07-09

TO: Kurt Fairbanks, Kale Anderson, Rebecca Hurley, George Sprouse

FROM: Mariah Dorner, Adam Sealock, Josh Koopman

SUBJECT: Seneca Afterburner Temperature Increase Study

Introduction

Cyanide is a by-product of incomplete combustion in the multiple hearth furnace at Seneca Water Resource Recovery Facility (WRRF). The cyanide is removed from incinerator exhaust, along with other air pollutants, in the venturi and after-cooler wet scrubbers. Spent venturi and after-cooler water containing cyanide is sent to the head of the plant, mixing with plant influent before entering the primary clarifiers. Cyanide is a known inhibitor of nitrification and is thought to have contributed to the 2025 Seneca plant upset where incomplete nitrification during secondary treatment lead to high nitrite concentrations in plant effluent and subsequent failure of chlorine disinfection (See the Report titled Seneca Nitrifier Washout and Disinfection Upset for reference).

Starting in August of 2025, weekly monitoring of cyanide concentrations in venturi and after-cooler waste streams was implemented. A thorough attempt was made, using multiple regression and principle component analyses, to relate cyanide production in Seneca’s incineration system with various operating parameters including incinerator, afterburner, venturi scrubber, and after-cooler setpoints and process values. However, no variables were found to strongly correlate with cyanide concentration in scrubber water streams. Multiple sources, including a publication by Daigger et al. (1998) and a 2000 WEFTEC Report by the Central Conta Costa Sanitary District (Martinez, CA) have reported that increasing the afterburner temperature to > 1300°F in a multiple hearth furnace system can serve to destroy cyanide before it is collected into aqueous streams by the wet scrubbers. Seneca WRRF currently operates their afterburners at 1250°F with process values controlling tightly to this setpoint. The following study was performed on April 30, 2026 at Seneca WRRF to determine the efficacy of increased afterburner temperatures for destroying cyanide at the plant.

Methods

Seneca WRRF has two identical multiple hearth incinerators, each with their own afterburner and wet scrubber systems. They operate one incinerator at a time, transitioning between Incinerator 1 (INC1) and Incinerator 2 (INC2) once per year. Annual cleaning and maintenance is performed When an incinerator is taken offline for the year. Incinerator 1 was put online in September of 2025 and was the operational incinerator during the time of this study.

During the study, the afterburner temperature setpoint was increased by increments of 50°F from 1250°F. The goal was to reach a maximum temperature setpoint of 1500°F, however the operator controlling INC1 was inexperienced with incinerator control and was unable to achieve afterburner temperatures > 1435°F.

Each temperature setpoint was held for 1-2 hours, after which samples of venturi and after-cooler water were collected and analyzed for cyanide concentration. Free cyanide (HCN) was measured using the Hach kit, and additional sample was pH-adjusted to pH > 10 and submitted the MCES Analytical Lab for total cyanide (CNtotal) analysis. The time and afterburner process temperature were recorded for each sample.

Results

The concentrations of HCN and CNtotal in both venturi and after-cooler waste streams decreased as a function of increasing afterburner temperature.

Compared to 1256°F, increasing the afterburner operating temperature to 1432°F decreased the concentration of CNtotal in venturi water by 87% and in after-cooler water by 63%.

The rate of cyanide loading to the plant (g/min) as a function of afterburner operating temperature was calculated using scrubber water flow rates (gpm). Tag SENDB0091FT was used for venturi water flow, and SENDB0107FT was used for after-cooler flow at each sampling time. Total cyanide loading at Seneca is the sum of the loading from the venturi waste stream plus the loading from the after-cooler waste stream. Linear regression modeling shows CNtotal loading to Seneca as a function of afterburner temperature (below), with a decrease in CNtotal of 0.9 g/min for every 10°F increase in afterburner temperature.

Additional plant flow data was used to predict the cyanide concentration at the start of secondary treatment under different afterburner operating temperatures. Tag SENAB0200FT was used for Seneca influent flow and tags SENBCNRASFLOW and SENBCSRASFLOW were used for Seneca RAS flow. Flows of additional un-metered streams into secondary treatment, including centrifuge centrate, gravity belt thickener water, and drain flow, were estimated to be approximately 4.6 gpm. The figure below shows how diurnal influent patterns (April 29 - May 2, 2026) would likely affect cyanide concentration at the start of secondary treatment, as predicted under different afterburner operating temperatures. The dashed line indicates the concentration of cyanide generally understood to be inhibitory towards nitrifying bacteria (0.1 mg/L).

Although increasing the afterburner temperature may not completely remove CNtotal below the inhibitory level during periods of low flow, the concentration would be greatly reduced. Importantly, concentrations of CNtotal in MLSS predicted in the figure above represent estimated concentrations at the start of Seneca’s anaerobic Zone A, and some degradation of cyanide may occur during anaerobic treatment before it reaches aerobic zones. Bacteria in MLSS are know to consume cyanide under aerobic conditions, but anaerobic bacterial degradation of cyanide is less understood. Ongoing work within MCES R&D group seeks to understand how much degradation of cyanide may occur under anaerobic conditions in MLSS. Remaining CNtotal that enters aerobic zones will inhibit nitrifiers if the concentration is ≥ 0.1 mg/L, but will degrade under aerobic treatment until the concentration reaches < 0.1 mg/L and nitrification may resume.

When inhibitory levels of cyanide are present at the start of aerobic treatment, peak nitrification activity moves further down the aerobic tanks where diffuser density is lower and where it may be more challenging to meet the oxygen demand associated with nitrification. Lowering the cyanide loading to Seneca secondary treatment may support improved aeration performance and control. MCES process engineers and R&D staff are currently working to understand the performance of Seneca’s aeration system and any relationship between aeration challenges and cyanide at the plant.

Gas usage by Seneca afterburner

Increasing the afterburner temperature at Seneca will remove cyanide and support nitrification performance and plant stability. However, these benefits should be considered along with the added cost of increased natural gas used by operating at the higher afterburner temperature.

Natural gas flow (SCFH) is monitored at Seneca plant for INC1 incinerator burners (SENDB0001FT), INC1 afterburners (SENDB0002FT), and the plant’s boiler (SENDB0005FT). When the afterburner temperature was increased for this study, the afterburner natural gas usage increased from 2091 ± 448 SCFH (average ± sd for Nov 2025 – May 2026) to 5181 ± 9.29 SCFH when the afterburner setpoint was at 1400°F or greater.

Gas flow to the afterburner appeared to reach a maximum value at around 5000 SCFH, which may be a mechanical or process control limitation. Gas flow to the afterburner did not increase substantially beyond ~ 5000 SCFH, despite the afterburner being unable to reach setpoint temperatures ≥ 1450°F.

The total natural gas usage at Seneca can be calculated as the sum of gas flow (SCFH) to incinerator burners, incinerator afterburners, and the plant’s boiler (SENDB0001FT + SENDB0002FT + SENDB0005FT). The total gas usage at Seneca increased from an average of 5913.9 ± 1430.3 SCFH on 4/30/26 (4/29/26 0:00 to 4/30/26 6:00) before the afterburner temperature was increased, to a maximum of 11087.8 SCFH when the afterburner was operating at ≤ 1400 °F.

While it is clear that the natural gas usage increased as a result of increasing the afterburner temperature, it is unclear how operating at greater temperatures will impact the consumption of gas by the incinerator system as a whole. Gas used by the afterburner fluctuates and is likely affected by incinerator operation and the temperature of incinerator exhaust upon entering the afterburner.

For example, despite operation of the afterburner at the standard operating temperature of 1250 °F on the evening of 5/1/26, afterburner gas usage spiked up to 4609.3 SCFH. Total gas usage on 5/1/26 likewise spiked to 11648.6 SCFH, higher than the maximum gas usage observed during the afterburner study.

Gas usage at Seneca also follows a diurnal annual trend, with increased gas use during cold winter months being associated with boiler operation. At the time of the afterburner study performed on 4/30/26, total gas usage at Seneca was trending downward, and average gas usage on 4/30/26 (6936.3 ± 1751.7 SCFH) fit the seasonal trend. Since the afterburner study only took 5.67 hours total, the increase in gas consumption that day is not representative of daily average gas usage during continuous operation at an increased afterburner temperature. An investigation involving longer-term operation of the afterburner at an increased temperature is required to fully asses the impacts on gas consumption.

Conclusions

Increasing the afterburner temperature at Seneca Plant can substantially lower the cyanide loading to Seneca’s secondary treatment train, which would reduce stress on nitrifying bacteria and would likely support improved process stability and aeration system performance. Ongoing work should consider these benefits of removing cyanide along with the increased gas costs associated with operating at an increased afterburner temperature. Mechanical or process control restrictions on gas flow to Seneca’s afterburners should be investigated, as they may hinder operation at desired high afterburner temperatures. Importantly, removing cyanide stress from Seneca’s secondary treatment may allow the plant to operate under more energy efficient conditions to offset increased gas costs (lower SRT, low DO), and this should also be considered.