WASTE DISPOSAL systems

Appraising wastewater treatment options is not straightforward, considering that composition and volume of waste is highly variable and subject to influences beyond the control of the treatment provider.

One needs to establish whether a given technology will suit industries with different types of waste and whether a single process design offers a treatment for several wastes. For example, does the process allow the same plant to handle a number of different wastes? Furthermore, reliability and costings are key considerations.

Some people contend that landfilling does not constitute treatment, but many associated processes occurring in more highly engineered treatment methods (absorption, filtration and biodegradation) also take place when a liquid waste is co-disposed with dry waste. The cost of liquid landfill is low, the process can cope with changes in the nature of the waste due to its large attenuation capacity and design data are relatively cheap and easy to obtain.

To avert pollution problems and possible punitive action, careful screening, analysis and management of incoming waste are necessary. Public perception of liquid landfill is negative so licences can be restrictive and difficult to obtain. Furthermore, potential EU legislation restricting landfill practices could threaten the long-term viability of such schemes. Generally, only the larger landfills receiving substantial volumes of dry waste will take liquid input.

Anaerobic exercises

Anaerobic systems treat biodegradable wastes, such as food and drink industry wastes; sewage sludge and liquor; and pulp and paper mill effluents. Such wastes would be expensive to treat in an aerobic biological system, due to their high oxygen demand. Conversely, high COD is an advantage in an anaerobic system, since the organic compounds may be converted to biogas which can be burnt to generate power or to heat the process.

On the downside, anaerobic digestion is sensitive to concentrations of heavy metals, phenols, sulphides and volatile fatty acids; the pH of the feed must be closely controlled and there is a high bicarbonate alkalinity demand.

Membrane separation

Recent advances in membranes allow them to remove virtually any contaminants with a diameter >0.1nm from aqueous solution, either through preferential diffusion of the contaminant through the membrane, reverse osmosis, or electrodialysis. Waste streams rich in organics, inorganics and heavy metals can all be treated by this means. To avoid fouling, the waste stream must be free of grease or solids, so upstream filtration is essential.

A contaminant which contains a component with a significantly different volatility, solubility or freezing point to water may be separated by a technique based on phase equilibria, such as distillation, evaporation, freeze crystallisation (see photographs), stripping or solvent extraction. Capital cost of such plant can be comparatively high, depending on the process and the waste.

As both the water companies and the Environment Agency are starting to implement toxicity-based trade effluent and discharge consents, it is becoming increasingly important to remove inhibitory or toxic components from liquid wastes. Refractory, toxic or inhibitory organics which are not amenable to conventional biological oxidation in processes such as activated sludge may be treatable by advanced oxidation processes, such as wet air oxidation, ozonation or catalysed hydrogen peroxide treatment. Capital and operating costs are relatively high due to the high temperature and/or use of ozone. Aggressive operating conditions increase the required specification of plant and trade effluent charges for the treated waste may still be significant.

Some wastes suitable for treatment by advanced oxidation can be more economically treated by incineration, or even be suitable as secondary liquid fuel.

Global Environmental's bulk liquid effluent treatment plant in Leeds can deal with wastes ranging from complex mixtures of hazardous components to simple oil and water mixtures. Implementation of any of the above wastewater treatment techniques would give the benefit of expanding the range of waste types which can be treated.

However, I believe that there is no obvious candidate for commercial development. Every option has some disadvantages, which include high costs, limitations in process flexibility and lack of reliable process design data. Market risks must be considered, as the liquid waste treatment sector is competitive and over-supplied.

In the short term, the wisest course of action would be to maximise the performance of your existing assets, whilst keeping a close eye on technological developments.

Jayne Myatt is recycling development manager at Global Environmental

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