On-site options

Effluent treatment plants have historically been viewed as high-cost capital equipment, limited to those sites with sufficiently large effluent volumes to justify the investment.

But the rising costs of discharging effluent to drain, coupled with current advances in treatment technology, are making on-site treatment a more attractive, cost-effective option for smaller organisations.

The main benefits of on-site treatment are threefold: a reduction in the volume and strength of effluent discharged to sewers or watercourses; significant reductions in effluent bills or tankered waste disposal costs; and taking control of a plant's environmental impact and performance.

Moreover, the 'true' cost of industrial effluent is often not appreciated. In some circumstances, it can be more than three times the total amount actually billed for water supply and effluent disposal.

Hidden costs include those for heating and transferring clean mains water through the process, chemicals and additives used in the process and to treat the effluent prior to discharge, maintenance costs, leaks, and not forgetting final effluent disposal costs.

It therefore makes both economic and environmental sense to address water consumption and effluent discharges from site. In particular, to identify ways to reduce efluent volumes and strength, and re-use valuable heat, water and process chemicals.

Companies typically discharge contaminated effluent to the foul drainage system, or sewer. Domestic sewage comprises wastewater from toilets, hand basins, showers and kitchen areas and typically incurs set charges - for example, based on the number of staff on site.

Trade effluent comprises effluent from all processes on site, defined as 'any liquid, either with or without suspended particles, which is wholly or partly produced in the course of any trade or industry carried on at trade premises'. Trade effluent is generally monitored by regular sampling and metered.

For any discharge into the sewer, a company must legally obtain a 'consent' or permission from the sewerage undertaker (often the same company that supplies the mains water). Illegal discharges may result in severe fines. The consent typically has restrictions on the composition and strength of the effluent, daily maximum volume allowed, and maximum rate of flow, temperature and pH. The charges levied take account of the volume, chemical oxygen demand (COD) and suspended solids (SS) in the effluent.

So, what are the on-site treatment options available to companies looking to reduce their exposure to these effluent charges? They can be broadly categorised as biological, chemical, and physical processes. Each can be used alone, or in combination. They can be used to remove expensive or prohibited contaminants, remove suspended solids, reduce COD or volume, or adjust pH.

Biological processes - aerobic or anaerobic - use microbes to degrade effluents. For example, aerobic digestion has traditionally been used for treating domestic sewage. Biological systems are suited to biodegradable effluents, such as those from the dairy, paper, beverage, food, textile, chemical and pharmaceutical industries. These systems can reduce effluent COD by up to 90% but typically have high capital and running costs. Both are vulnerable to harmful biocides, cleaning agents and other chemicals, and sensitive to pH balance, feedstock and temperature.

Chemical processes basically add chemicals to bring about precipitation, oxidation or reduction of contaminants in the effluent.

Typically, chemicals are used to maximise performance of subsequent processes - for example, to crack emulsions, precipitate metal ions and other solids. Chemical systems can be fully automated using pH monitoring and control. But the chemicals required may make the process expensive with inherent health and safety risks.

Physical processes include filtration, sedimentation, flotation, adsorption and ion exchange. Membrane filtration systems - microfiltration, ultrafiltration, nanofiltration and reverse osmosis - can selectively remove from wastewater contaminants such as oils, greases, metals, dyes, proteins, pigments and colloids. They tend to have low operating costs, and typically require less capital expenditure than, for instance, flotation systems.

At the process level, the selective nature of membranes can be exploited to selectively recover or discard specific components of an effluent stream, and they are currently used in many pharmaceutical and fine chemical reaction loops.

Flotation systems are generally used to remove less dense pollutants such as suspended solids, oil and grease. Dissolved air flotation (DAF) is particularly common, and uses fine bubbles of air passed up through the liquid to induce sludge and contaminants to float to the surface from where they can be skimmed off.

Also common are sedimentation tanks or basins that allow suspended solids to settle out by gravity, sometimes enhanced by the addition of coagulants. Performance is, however, affected by effluent flow and sedimentation time, the nature of the SS and sludge build-up problems.

For low volumes of effluent, adsorption and ion exchange systems can still be the cheapest option, depending on the application. The most common use activated carbon as the adsorbent, which can remove a variety of contaminants including oil, chlorides, dyes, PCBs, herbicides and trichloroethylene. Running costs can be high with large effluent volumes, as the carbon has to be exchanged periodically as it becomes saturated.

Once treated, much industrial effluent can actually be re-used on site, instead of going straight for discharge. A simple example is the 'greywater recycling' of water from washbasins and showers to be re-used for flushing toilets or watering garden areas.

As these examples illustrate there are substantial savings to be made in mains water, fresh chemicals or materials, and effluent disposal costs. Spending the time to investigate the technology and options available now could result in long term benefits for companies of all sizes.PE

Hannah Scott is Regional Manager for membrane systems specialist Eco Technology.