Introduction

Water costs have risen and may continue to do so. Metal finishing chemicals are expensive and trade effluent charges are increasing. The use of excess water to dilute the effluent is bad for the environment and often bad for profits.

Minimising the consumption of water and treatment chemicals is essential to a successful metal finishing business. This Reference Note discusses how this can be done.

Information

Background

Substantial cost savings can often be made in this area with virtually no investment, while further savings can be made with comparatively simple techniques. Attention to water use cuts water and trade effluent costs. Reducing water flow can sometimes stop an old effluent treatment plant contravening the quality conditions of a trade effluent licence. Studies by the ETBPP and industrial experience, especially in Germany, suggests that it is rarely necessary to use sophisticated technology to cut water costs.

Turn the Water Off

Metal finishers leave water running for many reasons, some of them unjustifiable when considered against the current cost of water and effluent disposal. Water left running over a one hour lunch break adds 12.5% to the water bill for the day. Businesses have located losses and made good savings by simple methods such as:

    • checking the water meter to make sure none was used over Christmas and the weekends
    • fitting self-closing valves on the ends of hoses
    • having one main isolating valve so that everything can be shut off at night without disturbing the careful adjustments of the valves on each tank.
These ideas may not have been worth the bother in the past but they certainly are now.

Watch the Little Things

A small trickle of water can result in a large bill if it runs for many hours.

Use Multi-Stage Counter Current Rinsing

Rinsing is a dilution process. The film of treatment solution carried on the work piece (drag out) has to be diluted with clean water. If a dilution ratio of 1000 is required, a single rinse tank will consume 1000 litres of water for every 1 litre of drag out. Three tanks arranged for counter current rinsing only consume 10 litres as indicated in the following sketch.

The multi stage counter current approach can also be applied to pickle systems.


If you can achieve a high concentration of solution in the rinse water by three or more rinsing stages (and if necessary concentrate it more using an evaporative device), you may be able to use the rinse water as make-up to the treatment tank and eliminate all rinse effluent.

Halve Solution Consumption

A static tank, initially filled with water, can be placed after a rinse and before the next treatment. If the work is passed through it in sequence but is returned to it and dipped once again after the treatment, it will cut losses of treatment solution by 50% and halve the requirement for rinse water.

A New Effluent Treatment Plant

Installing a bigger and better effluent treatment plant should be viewed as a last resort. It will be much more profitable to satisfy trade effluent licence conditions by reducing the load on the present plant.

Benefits

More attention to water and chemical economy can improve profits, and put off the need to upgrade the effluent plant without detracting from product quality.

Legislation

"Installations for surface treatment of metals and plastic materials using an electrolyte or chemical process where the volume of the treatment vats exceeds 30m3" are to be subject to Integrated Pollution Prevention and Control legislation (IPPC). The definition of "treatment vats" is under discussion and likely to be resolved shortly.

Metal finishers must comply with COSHH regulations and The Health and Safety Executive provides a number of relevant guidance notes.

The discharge of effluents to sewer requires a trade effluent licence and effluent discharged anywhere else requires an effluent discharge consent from the Environment Agency, the Scottish Environment Protection Agency, the Environment Agency Wales, or the Northern Ireland Environment and Heritage Service according to its location.

The Future

IPPC is likely to affect large metal finishers and possibly most metal finishers depending on the final interpretation of "30m3 treatment vats". Greater use of existing, relatively simple water and chemical conservation technologies - rather than technological breakthroughs - are likely to be of most importance in the near future.