Background Many industries, both within and outside the chemical sector, produce wastewater containing biodegradable organic compounds. Within the chemical industry the traditional aerobic activated sludge process is still the most common way to treat such wastewater. However, aerobic microbiological breakdown of organic compounds leads to the build up of biomass (bacteria) of which the excess has to be discharged. Due to the presence of potentially hazardous components the discharge of this waste stream can be problematic. Figures show that on average the excess sludge production amounts to roughly 40 to 50% of the chemical oxygen demand (COD) (organic) reduced in the aerobic process. With the introduction of mesophile operated membrane bioreactor (MBR) the amount of excess sludge produced can be reduced to roughly 25% of the COD (organic) due to further mineralisation of the sludge. The sludge production, however, is still very significant and amounts to millions of tonnes for the EU. These sludge streams require a significant amount of chemicals and energy for stabilisation and dewatering. Further processing of these sludge streams also involves an enormous logistical effort with its own environmental impact and risk. After these processing and handling activities, a significant part of the organic sludge streams can be usefully applied (digestion or incineration with energy production), but the primary focus should be on prevention of the production of these massive amounts of sludge from an environmental as well as from an economic point of view. Full in process mineralisation of the organic sludge can contribute to this prevention target and will lead to a concentration of minerals, possibly creating a future reuse and recycling potential for these minerals, e.g. the phosphates. Objectives The PROMOTHE MBR project aimed to demonstrate and disseminate the effectiveness and efficiency of a thermophile operated membrane bio reaction for aerobic digestions of sludge that: originates from the chemical industry (chemical carbon activation); strongly varies over time in flow, load and constitution (due to plant operation); is thermally polluted (approximately 50-55°C) and contains hard biodegradable (recalcitrant) organic compounds. The operation aims to result in almost zero sludge production; high COD and TSS removal efficiency, making effluent suitable for reuse; reuse of effluent as process water; reduced energy consumption, compared to mesophile “side stream” MBR systems, and reduction of chemical usage for sludge processing or pH correction in the case of direct discharge to sewer. This will be done by operating the first Thermophile operated MBR within the chemical industry for treatment of wastewater containing recalcitrant organic compounds, which should show a significant performance improvement on sludge production. The system will treat approximately 500 000m3 of wastewater every year with an average COD-level of 5 400mg/l. Results The PROMOTHE MBR project established the MBR plant at a very early stage in the project cycle and moved forward to testing the bioreactor using influent from the associated activated carbon plant. It ran into some technical difficulties as the plant was not able to deliver sufficient quantities of influent to design specifications due to the plant shutting down. Also the pumps were frequently blocked by activated carbon grit and other materials from the plant floor leading to additional filtering systems that were not in the original design specification. Further problems were encountered with seals which failed due to the wrong choice of material. These operational problems delayed the project but were eventually resolved satisfactorily. After this first stage the beneficiary was able to move on to the mesophilic operation of the MBR. At this stage of the operation, the project had to demonstrate that the bacteria operated in stable conditions and this was successfully achieved. The results of the mesophilic operation were well above expectations and met all the project objectives. However, as the innovative part of the project was to test thermophilic bacteria, the beneciary switched to the use of high temperature dependent bacteria. Again the plant experienced difficulties in maintaining the temperature of the influent high enough for the bacteria to operate optimally. This was partly due to the fact that the regulator (SEPA) had changed the maximum permissible temperatures for the stack emissions, and this constraint meant that the influent could not be increased above 45°C where temperatures of 55°C were required. This led to a decline in the performance of the bacteria and resulted in negative sludge growth, indicating that the bacteria were in fact dying. In December 2007, the MBR plant was switched back to the fall-back position of mesophilic operations. The project made good progress in effecting the switch back and the biology responded well demonstrating the flexibility of the system. Unfortunately, in March 2008 the plant suffered damage to the aeration system and consequently oxygen to the biomass deteriorated to such an extent that the project team stopped the operation and emptied the reactor. The MBR plant was repaired and operations restarted in February 2009 so that the beneficiary could begin transfer of the technology. They completed the paperwork to nominate the technology as BREF (as a best available technique) with the intention of submitting the forms to the European Integrated Pollution Prevention and Control Bureau (EIPPCB) in May 2009. Finally, since the transfer to thermophilic operation has not been successful, the ultimate level of innovation has not been realised. However, the hybrid design of the MBR plant enabled mesophile operations far beyond expectations, i.e. 97% of COD removal, 100% solids removal and only 7% sludge yield on COD removed. Economically the project has demonstrated an overall saving. Assuming that the plant could reach optimal (design) capacity rather than 51%, the overall annual savings would be in the region of £300 000 giving a pay back period on the overall investment cost [€2 231 000] of between 5 and 6 years. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).