Process Intensification with Low Apparent Sludge Age at Monkmoor STW Using Membrane Aerated Biofilm Reactor

SUMMARY OF KEY FINDINGS
The installation of 48 OxyMem™ Membrane Aerated Biofilm Reactor (MABR) modules at Monkmoor Sewage Treatment Works (STW) in Shrewsbury demonstrated the ability of the MABR system to seamlessly retrofit an existing treatment plant to provide process intensification and in this case to manage increased flows and loads associated with population increase. The MABR system achieved ammonia removal rates of 243 kgN/d which is a nitrification rate of 3.4 gN/m2.d and increased the nitrifier aerobic sludge age to 9.2 days enabling effluent ammonia-N concentrations below 1 mg/L even in cold weather.

INTRODUCTION & OBJECTIVE
Monkmoor STW in Shrewsbury serves a population PE of 116,000, which is projected to increase to 146,000 by 2033. Prior to the MABR upgrade, the facility operated as a conventional activated sludge plant (ASP), treating a mix of domestic and trade waste, including landfill leachate, meat processing wastewater, sludge imports and wastewater from other traders. The site comprises an inlet works, four storm tanks, four primary settlement tanks, four ASP lanes, four final settlement tanks, and a sludge treatment facility for digesting both indigenous and imported sludge from nearby locations.

A target removal of 162 kg of ammonia-nitrogen (N-NH3) per day was set for the MABR system, when installed in the anoxic zones. To achieve this, 48 OxyMem™ MABR units were installed in April 2024. At the time of install, the Monkmoor MABR IFAS system was the largest MABR system in the UK and can be considered the first large-scale commercial installation within the UK.

Performance testing was carried out in December 2024, to coincide with the start of the cold weather operation.

RESULTS AND DISCUSSION
Installation challenge
A key advantage of the MABR system is the potential to drop MABR modules into existing tanks without draining the tanks or stopping the flow, meaning the MABR upgrade can be non-disruptive. The main challenge was the presence of walkways on top of some of the walls required to support the MABR modules. It was not practical to remove these walkways for the installation of the MABR support beams and instead a cantilevered steel structure was fabricated that slotted in underneath the existing walkways (see Figure 2 and Figure 3).

Nitrification rates
The average target ammonia removal load was 161.6 kgN/d. The mass of ammonia removed was calculated by measuring the concentration change before and after the modules and multiplying it by the total flow through the ASP. The measured load and removal is shown in Figure 4. During the sampling campaign, the ammonia load was 1239 kgN/d, 1.28 times the design load and removal was 243 kgN/d, which was 1.5 times the design target.

#The nitrification rate during this period varied between 1.2 and 6.6 gN/m2.d (Figure 5). The MABR performance is a diffusion-driven process; thus, when the ammonia load to the system is higher, there is a higher driving force for the diffusion of ammonia and thus the nitrification rate is higher Figure 6.

The required SRT to achieve the measured Monkmoor STW residual ammonia concentration (Nad) was calculated following Haandel and Lubbe 2012. In theory a sludge age of at least 10.2 days is required to achieve the final effluent ammonia-N concentration of 0.41 mg/L but Figure 7 shows that the MABR IFAS system achieved it with a sludge age of 8.8 days. The better than expected ammonia removal can be attributed to the contribution of nitrifiers attached to the MABR fibers.

The impact of the MABR biomass was quantified through plant simulations using SIMBA# 6.0 software. The simulations suggest that 18.5% of the aerobic ammonia-oxidizing bacteria (AOB), which facilitate the conversion of ammonia to nitrite, were located within the biofilm. Monkmoor STW performance test results show that the MABR contributed to the oxidation of 243 kgN/d, representing 19.6% of the inlet ammonia-N load, which aligns closely with the proportion of AOB mass in the MABR system estimated in the simulation. Consequently, the MABR biofilm increased the effective aerobic sludge age to 10.7 days, supporting the observed low final effluent ammonia-N concentrations at Monkmoor STW. Achieving a minimum aerobic sludge age of 10.7 days at Monkmoor STW without MABR, would have required an additional 2,639 m3 of reactor aerobic volume, equivalent almost to 1 additional ASP lane.

CONCLUSIONS AND LESSONS LEARNED
The successful installation commissioning and start-up of the OxyMem™ MABR IFAS system at the Monkmoor STW reveal several critical insights:

Rapid and non-disruptive retrofit solution
The MABR modules were dropped into the existing tank without ever stopping the flow — demonstrating that IFAS MABR systems can be used to intensify existing plants quickly and non-disruptively.

Process intensification
243 kgN/d of ammonia was removed by the MABR system, equating to a nitrification rate of 3.4 gN/m2.d. As a result, the effluent ammonia concentration was less than 0.5 mg/L despite an apparent sludge age of just 8.8 days.

This installation demonstrates that municipalities can quickly increase their nitrification capacity in a matter of weeks with modular drop-in MABR units and can serve as an example for other facilities with capacity challenges.