The Dulux Gracefield site in Wellington, NZ, has a treatment plant which treats waste from the water-based paint plant. The last step in the treatment process is dewatering the sludge that builds up in the clarifier, by using modified waste skips which have drain holes fitted and a layer of sawdust in the bottom. This has been quite successful over the years, but the sludge only dries to an average of 10 to 15 per cent solids over a 2 – 4 week period in the summer, less in the winter.
This, combined with the desire to reduce the quantity of waste to landfill, resulted in us thinking how this final, dewatering step in treatment could be done better. Several ideas were suggested and trialled, with little success due to the expense, cumbersome process and lack of improvement.
Then, we decided to trial a belt press. Belt press processing involves a belt that drags the wet solids into a gate where the solids are progressively ‘squeezed’ until what comes out the end is mostly solids. The waste water from the belt press can be collected and recycled. Belt presses are used extensively in the processing end of the mining industry, to separate valuable solids from sludge.
We got in touch with our waste contractor and developed a partnership. After discussing the concept and coming up with an agreement, our site sent a trial load of 10,000 litres of treatment plant clarifier sludge to the contractor’s site.
This trial was extremely successful.
The contractor then installed a belt press in its local branch, and after a few more trials, we went ‘online’ sending all the clarifier waste directly to the contractor for belt press treatment, rather than the slow drying process in the modified skips on site. The process provides a 60% improvement in reducing the quantity (by weight) of waste that now ends up at the local landfill.
The working relationship between Dulux and the contractor has also improved as we all strive to reach the goal of “No harm”.
Sludge cake during pressing on the belt press