The Problem

Under current operating conditions the single clarifier receives high levels of solids, largely glass fibre based material, dumped from the manufacturing process. These results from the effect of cleaning down between grades or when operational problems arise which necessitate discharging to the effluent plant over short periods of time. The clarifier then receives large quantities of solids which is increasingly having a detrimental effect on its performance and was consistently exceeding its discharge limits for COD/SS and zinc.This meant that they were unable to discharge the effluent to the River until it was back in specification. This was done by a process of continuous recirculation through the plant. This of course impacted negatively on production. In the near future with a second machine due on line, this position will become more acute and will undoubtedly lead to the effluent exceeding the EA limits on a more regular basis.

The Solution

Vale Water undertook an initial survey of the whole process and then carried out a detailed sampling programme across the whole plant and it is the results from this survey that has lead to the main issues being resolved. Initially discussions were held with Production to try and control the “dumping” to effluent and this has had some degree of success.The chemistry was amended to ensure that the pH entering the clarifier was held at 10.0+ for optimum zinc removal. A decision was taken to also introduce, on an intermittent basis, a chelating agent to augment zinc removal treatment when certain grades (containing high zincs) were being produced.

The Result

Under most conditions the effluent falls within the limits set by the EA for discharge into the receiving water

course. The new production line is under commission and the net effect is that the effluent quality is being

maintained. Sludge removal and treatment has also benefited by the changes to the chemistry.

The Future

The site needs to find ways of recycling water due to the limits set on withdrawal from the adjacent river and with the improved quality, this is under review. Vale Water are looking at reducing the solids going forward by trialling a wedge wire system and also further downstream processing to achieve the quality of recycled water required.