A Novel Membrane Bioreactor for By-Product Recovery

Many inorganic waste streams are contaminated with organic chemicals and, as such, are classified as hazardous waste. The cost implications of disposing of such wastes are high and likely to rise in the future. Some companies, however, are addressing this problem by treating the waste before final disposal. Commonly used techniques, such as steam stripping and carbon adsorption, still leave a residue of concentrated contaminated residue, which requires disposal. At the same time, toxic organic chemicals such as benzene can be degraded using micro-organisms, but the high acidity, alkalinity or salt concentration associated with inorganic waste streams will often preclude such treatment.

Figure 1: Principles of Operation

Membrane Extraction Technology (MET) Ltd has developed an alternative technology to extract and treat such toxic organic molecules from inorganic waste streams. With the support of the Environmental Technology Best Practice Programme (ETBPP), MET has built a pilot plant at Elf Atochem UK Ltd, at Widnes in Cheshire, which uses an Extractive Membrane Bioreactor (EMB) to remove and completely break down the benzene and other organic contaminants from a solution of 30% w/v aluminium trichloride. The ETBPP, a Government Programme managed by AEA Technology plc, has published two documents to inform companies interested in using this technology, entitled: FR200 Recovery of Aluminium Trichloride Solutions Using Membranes and FP200 A Novel Membrane Bioreactor for the Recovery of Valuable By-Products.

Pilot Plant at Elf Atochem

The EMB technology presents a cost-effective alternative to traditional recovery methods. The cost of waste disposal is cut; the cost of operating the bioreactor is much less than the energy costs of steam stripping, or of carbon adsorption units; and the resultant decontaminated waste stream may have other saleable uses. This is indeed the case at Atochem. The company uses Friedel-Crafts reactions to produce ring-substituted aromatic molecules such as benzophenones and isobutylacetophenone. These reactions proceed in aluminium trichloride (AlCl3) and yield an aqueous solution contaminated with traces of benzene and the reaction products. Disposal costs for the waste AlCl₃ solution are £50 ($73 US) per tonne, and for each tonne of product, 2-3 tonnes of waste solution is produced. However, if the contaminants are removed, the decontaminated waste stream can be sold for use as a flocculant in wastewater treatment.

The EMB technology uses a selectively permeable membrane to extract the organic molecules into a biomedium containing specialised micro-organisms. These micro-organisms biodegrade the toxic organics, producing more micro-organisms, carbon dioxide and water. No secondary toxic waste stream is produced.

Pilot Plant

The overall aim of the project was to construct a pilot plant capable of treating 2-3 tonnes of AlCl3 solution per day over a six-month period to generate long-term operating data and provide sufficient material to explore commercial opportunities.

The membrane modules consist of silicone rubber tubes housed in a shell, like a shell and tube heat exchanger. The viscous nature of the AlCl₃ solution results in large pressure drops for a given flowrate and consequently the design calculations indicated that the waste solution should be pumped through the module shells so that the membrane module pressure limits are not exceeded. At a flow rate of 100 litres/hour of AlCl₃ solution and with 120m² of membrane area, greater than 99.9% removal of organic compounds can be achieved.

Elf Atochem provided a feed system for the pilot study, which was designed to separate any carried-over organic phase from AlCl₃ solution prior to the pilot unit and allow easy return of the separated organic phase to the production process. After treatment the pollutant-free solution is collected in a storage tank.

Following initial commissioning, the plant tests were suspended. The original silicone rubber membranes were examined and some were found to have been damaged. Laboratory tests indicated that an unidentified compound in fresh AlCl₃ had weakened the membrane. An alternative material - ethylene-propylene-diene-monomer (EPDM) – was used as a replacement and its effectiveness compared. After a further six months of tests, from May to October 1998, the new membranes showed no sign of damage and the improved design modules allowed organic free AlCl₃ solution to be generated by a single pass through the membranes.

Regular monitoring during the pilot study period showed that the EMB system was effective even during periods of large variations in effluent quality and concentrations of organic molecules. It was also found that very low levels of contamination were treated effectively.

During the study period, the Elf Atochem plant produced several different products and the effectiveness of the bioreactor remained constant. The results suggested that these production changes had little or no effect on the micro-organisms. The main advantage of the technology over its competitors is the bioreactor’s ability to eliminate the organic compounds at source. When comparing the environmental burden of the alternatives, EMB technology has a lower energy demand than steam stripping devices and produces no toxic waste. The energy demand is less than half of that needed for carbon adsorption, when using a regenerative package, and has a reduced demand compared to using a carbon adsorption container system. The technology also benefits from stability in the face of solids contamination. Whereas a carbon adsorption unit would cease to operate if contaminated, the EMB system continued to function when fine inorganic or organic solids were pumped through the membrane modules.

Both the capital and operating costs of the EMB technology are lower than its competitors, and according to Dr. W Sim, technical manager at Elf Atochem,

"The EMB has proved to be a simple, cost effective and reliable technology".  

Possible uses could include point source treatment for diverse and difficult wastes such as dibromoethane in NaBr; dichloroethane in hypochlorite; dichloroaniline in NaOH; chloronitrobenzene in HCl; chlorobenzene in NaOH; aniline in KI; and phenol in saline wastewater. The technology is best applied at the source of emission, typically in scrubber overflow liquors or aqueous phases from aqueous-organic phase separations. Pilot trials have been undertaken ICI Hillhouse in Blackpool, Hickson and Welch in Castleford, Bush Boake Allen in Widnes, and Solvay Deutschland in Hannover, Germany.

For further information and free copies of FR200 Recovery of Aluminium Trichloride Solutions Using Membranes and FP200 A Novel Membrane Bioreactor for the Recovery of Valuable By-Products, contact the Environment and Energy Helpline Tel: 0800 585794. The Environment and Energy Helpline is a free service provided by the Envirowise Programme. Alternatively look on the website: http://www.envirowise.gov.uk for more information about how the Envirowise Programme can help.

By: Dr. Stuart Ballinger, Envirowise, Guest Author
stuart.ballinger@aeat.co.uk