On site, and very much in mind

Hannah Scott of Eco Technology looks at the advantages and options for small to medium sized organisations (SMEs) considering on-site effluent treatment.

The on-site treatment of industrial effluent and wastewater has become an increasingly attractive choice for commercial organisations. Historically treatment plants have been restricted to those organisations with sufficiently large effluent volumes to warrant the capital investment.

Current advances in technology and competition in the marketplace have opened up a number of cost-effective options for smaller organisations. The main benefits of on-site treatment are: Reduction in the volume and strength of effluent discharged to sewer or watercourse, significant reductions in effluent bills or tankered waste disposal costs, and taking control of environmental impact and performance. (e.g. to help achieve Best Available Techniques (BAT) under Integrated Pollution Prevention and Control (IPPC)).

True cost

The 'true' cost of industrial effluent is often unrealised, and may be more than three times the total amount billed for water supply and effluent disposal. For a typical manufacturing organisation, the following costs need to be considered: Supply of clean mains water, energy to heat and transfer water through the process system, expensive chemicals and additives used in the process, additional chemicals used to treat effluent prior to discharge, system maintenance costs, losses through leaks or batch dumps, and finally effluent disposal costs. The hidden costs are surprising.

It makes both financial and environmental sense to address water consumption and effluent discharges from site. In particular to identify methods to reduce volumes and strength of effluent, and re-use valuable heat, water and process chemicals. Typically a company will have a mains water supply, from the local water company. (Certain sites have an abstraction licence for boreholes or watercourses, permitted and regulated by the Environment Agency (EA) in England and Wales, or Scottish Environmental Protection Agency (SEPA)). A metered water bill typically has a standing charge for water supply (which is often dependent on the size of the water meter) together with a charge per cubic metre for the water used.

Any company discharging effluent into a river or watercourse has a legal obligation to obtain a 'discharge consent' from the EA or SEPA. The effluent is regularly monitored by the Agency to check that it meets the consent parameters. Charges are calculated according to the composition of the effluent, the quantity discharged and the quality of the receiving water. Illegal discharges and water pollution are prosecutable offences.

Rainwater from roofs, carparks and other outside areas typically discharge into the surface water drainage system, which in turn will run to the local watercourse. There is not normally a charge for the discharge of clean rainwater, but companies do have a duty to ensure that it is not contaminated.

Sewer discharge

Typically a company has two types of discharge to the sewer, namely domestic sewage and trade effluent. The two are usually kept separate and individually monitored or metered (although they may mix when they later join the main sewage system). Domestic sewage comprises wastewater from staff toilets, hand basins, showers, and kitchen areas.

Trade effluent comprises effluent from all processes on site, and is 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".

For any discharge into the sewer, a company must legally obtain a 'consent' or permission from the sewerage undertaker (which is often the same company as the water provider). 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. Certain substances may be prohibited.

In terms of costs, the utility bill often includes a standing charge for domestic sewage (often based on the size of the water meter, number of staff on site, canteen facilities) and an additional charge per cubic metre for the amount of effluent discharged to sewer.

The charges levied for trade effluent are calculated using the 'Mogden formula' which takes account of the volume, chemical oxygen demand (COD) and suspended solids (SS) in the effluent (see table above).

Treatment technologies

There are a range of treatment technologies available, typically categorised as physical, chemical and biological processes. They can be used alone or in combination with other processes. Systems may be used to remove expensive or prohibited contaminants, remove suspended solids, reduce COD or volume, or adjust pH, ultimately resulting in reduced effluent disposal charges.

Systems may be used to treat effluent that has traditionally been discharged straight to sewer, or even wastes previously tankered away from site for disposal by a waste contractor at a licensed treatment facility.

The choice of system depends on the volume and nature of your effluent, together with your treatment requirements and financial constraints. Aerobic and anaerobic processes use microbes to degrade effluents. For example, aerobic digestion has traditionally has been used for treating domestic sewage. Biological systems are suited to biodegradable effluents, including those from dairy, paper, beverage, food, textile, chemical and pharmaceutical industries. These systems can reduce effluent COD by up to 90 per cent but typically have high capital and running costs. Both are vulnerable to harmful biocides, cleaning agents and other chemicals, sensitive to pH balance, feedstock and temperature, all of which can reduce or destroy microbial activity.

Chemical processes

Chemical processes require the addition of chemicals to bring about precipitation, oxidation or chemical reactions to remove or degrade contaminants in the effluent. Typically chemicals are used to maximise performance of subsequent processes, to 'crack' emulsions, precipitate metal ions and other solids. Chemical systems may be suited to most effluents, and can be fully automated using pH monitoring and control. However, the chemicals required may mean that the process is expensive, and has inherent health and safety risks.

Micro filtration, ultra filtration, nano filtration and reverse osmosis use membranes to selectively remove contaminants from wastewater, including oils, greases, metals, dyes, proteins, pigments, and colloids. One limitation with membranes is their sensitivity to substances such as chlorine, trichloroethylene, and other halogenated solvents, but this can often be overcome with pre-treatment. These systems typically have low operating costs, with a small footprint (compared for example to chemical or flotation systems) and may be fully or semi automated. Membrane systems typically require less capital expenditure and lower running costs than floatation systems.

Settlement tanks or basins allow suspended solids to settle out by gravity, sometimes enhanced by the addition of coagulants. Performance is effected by effluent flow and sedimentation time, the nature of the SS and sludge build up problems.

Dissolved air floatation

Floatation systems are generally used to remove less dense pollutants such as suspended solids, oil and grease. Dissolved air floatation (DAF) is particularly common, and uses fine bubbles of air passed up through the liquid to induce floatation. The resulting layer of sludge and contaminants produced on the surface of the tank is skimmed off for alternative disposal. Treated effluent is removed from the bottom. Performance is effected by coagulant or flocculent dosing, air saturation, and flow. DAF systems are best suited to consistent effluent streams. Adsorption and ion exchange systems use specialist materials that attract and absorb pollutants on to their surface. Examples include activated carbons, which can absorb a wide variety of contaminants from effluent including oil, diesel, chlorines, dyes, PCBs, herbicides and trike. The carbon must be exchanged periodically as it becomes 'saturated' and can result in expensive running costs, but for low volumes of effluent this can still be the cheapest option (depending on application).

Looking at re-use of treated industrial effluent, a simple example is 'greywater recycling' where wastewater from washroom basins and showers is re-used for flushing toilets or watering garden areas. The result is a saving in mains water and sewage disposal costs.

Membrane filtration

Component wash and floor wash water can be recycled and re-used through membrane filtration. The membranes will selectively remove contaminants (such as oil, grease, solids) and allow the cleaned wash water (still containing the valuable cleaning chemicals) to pass through. Similarly spent acid wastes can be treated to remove solids and contaminants, and produce an acidic solution that simply needs 'topping' up before re-use.

The selective nature of membranes can be exploited to selectively recover or discard specific components of an effluent stream, and are currently used in many pharmaceutical and fine chemical reaction loops. Other industry examples include the treatment of water-based paint washings to produce concentrated paint and wash water separately, both of which can be re-used. Likewise propylene glycol (antifreeze) can be recovered and concentrated from washings.

As these examples illustrate there are substantial savings in mains water, fresh chemicals or materials, and effluent disposal costs. Charges for effluent discharge doubled between 1990 and 2000 and the increases are expected to continue above the rate of inflation. Now is the time to investigate the 'true' cost of your effluent disposal and consider the options and potential cost savings available to your company, whatever your size, turnover or industry.



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