Many industries from oil refining to pharmaceutical manufacturing produce high strength wastewaters with COD concentrations above 1000mg/l. Where these wastewaters are treated to reduce the COD before discharge to sewer or surface water, it’s usually by conventional activated sludge systems. They do a pretty good job, but the resulting effluent usually requires considerably more treatment if it is to be recycled for use in cooling systems, boiler make-up, cleaning in place or other applications where potable water is not strictly necessary. But membrane technologies are changing that.
One of the important design parameters in activated sludge plants is the organic loading or the ratio of food (COD) to micro-organisms (effectively the Mixed Liquor Suspended Solids or MLSS), usually abbreviated to F:M ratio. In a conventional activated sludge plant the operating MLSS concentration is limited to around 4000mg/l by the efficiency, or lack of efficiency, of the final settling tank in retaining sludge, and this sets the overall design and performance. In membrane bioreactors (MBRs) the final settling tank is replaced by a membrane – microfiltration or ultrafiltration – which gives two big advantages. Firstly it allows the plant to operate at MLSS concentrations three times higher than a conventional plant and, secondly, it produces a much higher quality effluent, typically meeting turbidities below 1NTU and silt density index (SDI) less than 3. If the membrane is an ultrafiltration type, it can also remove large organic molecules that are not biodegradable.
Operating at a high MLSS concentration means a smaller aeration tank and more efficient COD removal from high strength wastewaters like those in the food and dairy industries. But the big advantage is the clarity of the effluent. It can be pumped directly to a reverse osmosis (RO) plant without any further pre-treatment, and the quality of the RO permeate will usually be at least equal to, and often a lot better than that of mains water: good enough to be recycled for a wide range of uses.
Those UK companies in the food processing and dairy industries that have already installed MBR-RO systems have found that the promises of reduced water and carbon footprints have been met. But they’ve been surprised at just how economically beneficial the schemes have been. Firstly there’s the obvious savings from reduced sewer discharge and mains water consumption. Secondly, the low SDI produced by the MBR means that RO membrane life, the main element of the operating costs, is typically more than five years and plant operating costs are so low that, even after allowing for the capital amortisation, the RO permeate is cheaper than mains water. Payback in less than 3 years is the norm.
Anaerobic treatment of high COD wastewaters, using technologies like the Upflow Anaerobic Sludge Blanket (UASB), generates biogas as a renewable energy source. They consume less energy and produce much lower sludge volumes than aerobic activated sludge systems but, almost invariably, have to be followed by an aerobic polishing stage, typically using activated sludge. The activated sludge system can, of course, be an MBR but the recent development of anaerobic MBRs does it all in one unit operation, giving reduced capital and operating costs and lower carbon footprints.
Fundamental to MBRs is the membrane that is used as the final barrier. Aside from the choice of pore size (microfiltration or ultrafiltration) there is the question of module configuration. Some manufacturers have opted for submerged membranes – either flat sheet of hollow fibre – with an extraction pump which draws the effluent through the membrane under vacuum. Others prefer the external crossflow configuration with membrane modules – usually tubular – contained in pressure tubes like RO modules through which the mixed liquor is pumped.
To keep the membrane surface clean, the external module configuration uses a high crossflow velocity which increases pumping costs by comparison with submerged membranes. On the other hand, submerged membrane modules are cleaned by “sweeping” the surface with air bubbles and that increases the air requirement, and hence the cost, above that needed for COD oxidation. The problem for the customer is that there is not a great deal of interchangeability between membranes, so when you buy an MBR, you commit to a particular membrane supplier. However, as happens with all technologies, inevitably an industry standard membrane configuration will emerge, and the resulting competition among membrane suppliers will bring membrane prices down further, reducing both capital and operating costs.