Severn Trent - MABR WWTP Expansion Trial

The consumption of energy is of major concern for Severn Trent Water (STW), PLC, UK. One example is that as part of their customer commitment STW1, plan to make their processes more efficient so that they use less energy and at the same time reduce carbon footprint by 7%. To do this the company wanted a solution that would benefit their 4.2 million customers, their business, and the wider environment, a suitable technology that decreases energy requirements and subsequently reduces emissions of greenhouse gasses.

Solution

One of the ways STW achieve this is through their R&D team, evaluating newer, more energy efficient technologies for inclusion into their treatment plants. In 2013 STW and OxyMem began a collaboration to demonstrate the benefits of OxyMem’s Membrane Aerated Biofilm Reactor (MABR) in meeting a very stringent water quality requirement while also reducing energy consumption. OxyMem’s MABR is an innovative solution to secondary biological wastewater treatment, which can be deployed as a modular unit and be easily fitted into an existing aeration tank. The OxyMem MABR combines bubble-less aeration and fixed films to provide a treatment process for carbon and nitrogen removal bringing the effluent to required discharge standards, in a compact structure.

During the course of the 12-month trial the MABR we demonstrated:

• Good effluent quality,
• High rates of COD removal,
• Impressive SOTE 4.5kgO2/kWh
• Low sludge yield 0.13kgTSS/kg COD applied
• Simultaneous nitrification/denitrification

In particular, it should be noted the excellent effluent quality that can be achieved in a pure biofilm, single pass system, without any supplementary solids separation, and that this excellent effluent quality was achieved at a volumetric loading rate twice that normally applied in an activated sludge system.

Along with energy savings, the low sludge production compared to 0.3 – 04 kg TSS/kg COD applied for activated sludge offers significant savings. To put this in context the savings for a 200,000 PE activated sludge plant would be approximately $220,000/y, assuming a sludge yield of 0.3 kg TSS/ kg COD applied for the activated sludge plant and a disposal cost of $30/ton.

The low energy use of the MABR which was verified during this trial, (4.5 kg O2/kWh in comparison with operational fine bubble diffusers at 2 kg O2/kWh). A simple comparison for a 200,000 PE plant shows that the energy savings achieved by an MABR system, compared to an activated sludge system, would be 895,000 kWh/year. If the plant was located in the USA then this would result in the avoidance of over 470 ton CO2eq/y and a saving of $62,600/y at an electricity price of $0.07/kWh.