CFD Modelling for DEMON Process Optimization at the Blue Plains Advanced Wastewater Treatment Plant (AWTP) of District of Columbia Water (US)

Abstract: Computational Fluid Dynamics (CFD) was used to understand and optimise the design of a deammonification (DEMON®) process to solve the problem for sludge wash-out. This project runs from problem investigation to design solutions and implementation at the plant. CFD was used to understand the hydraulic risks of the hyperbolic mixers and to optimise the design to achieve optimal settling conditions.

INTRODUCTION
The DEMON® reactor is an intermittent aeration system to provide enough oxygen to convert about 50% of the incoming ammonia to nitrite. The produced nitrite is then used by anammox bacteria to oxidize the remaining ammonia to nitrogen gas. Since the anammox bacteria are slowly growing, the anammox granules requires a sludge retention over 50 days (vs. ca. 5 d for Ammonia Oxidizing Bacteria (AOB)). The treated effluent is decanted from the reactor via a baffle wall after sedimentation of the biomass. The largest deammonification plant is installed at the Blue Plains Advanced Wastewater Treatment Plant (AWTP) of District of Columbia Water (US). At the start of the project, DC Water had to run the process as an SBR system due to a sludge wash-out and the breakage of the decanter inside the DEMON tank due to too high turbulence coming from the hyperbolic mixers. In this study, CFD was used to investigate the problem and to optimise the design to achieve optimal settling conditions.

MATERIALS AND METHODS
The modelling domain of the CFD simulations (with Ansys Fluent) can be seen in Figure 1. A Eularian-Eularian model was used to model the water and the air. The moving reference frame (MRF) methodology was used to model the hyperbolic mixers (type Invent Hyperclassic®).

RESULTS
An analysis of the current situation was conducted to reproduce and gain insights on the issues experienced in practice. Based on the simulation results (see Figure 1Figure 2), high velocities and turbulence could be observed at the location were the original decanter was placed. Even when the aeration was off, the velocities were to high according to the simulation to find a good location for the decanter. Hence, this was leading to the breakage of the supporting structure of the decanter and DC Water had to operate the plant as SBR instead of a continuous system. This reduced significantly the potential of the plant, and a solution was investigated in this project. To solve this problem, a baffle was designed to induce the outflow at the side of the tank and to allow settling between the baffles. The conceptual design is shown in the left figure in Figure 3. The sludge would settle in between the two baffles and flow back into the tank (indicated by the green arrows in Figure 3, left). A potential risk identified from the first CFD results was the high flow over the bottom and the sides of the reactor induced by the hyperbolic mixer. To solve this, it was decided to place a deflector baffle below the opening of the sludge to avoid upwards flow into the settling region. The deflector is shown as a red triangle in Figure 3 (left).
A problem related to the deflector was identified from the initial simulation. A rotational flow was created just above the deflector (see Figure 3, middle) which can lead to some air entrapment and air intrusion to the settling region. The air would disturb the settling region, and more sludge can potentially be washed out. Multiple what-if scenarios were tested in few weeks to optimise the deflector design and to ensure no air intrusion in the baffles to have optimal settling conditions. Finally, the air intrusion to the baffles minimized by re-sizing and optimizing the location of the deflector (Figure 4). DC Water implemented the suggested structures (Figure 5) and could operate the installation as expected.

CONCLUSIONS
In this study the mixing behaviour in a DEMON® reactor was investigated with the help of CFD and the problems with the decanter were identified. Afterwards, CFD was used to find the optimal design of a deflector in the DEMON® reactor to ensure optimal settling conditions and no air intrusion. In a short time frame (a few weeks), multiple designs were virtually tested and analysed. This helped DC water to confidently invest in the new baffle structure and gave extra insights in the operation of the reactor and the mixers. The suggested baffle was put in place after the project and is currently operating as expected.