The Decolourisation or Carbon Plant at Chelsea is used to remove colour from the filtered sugar liquor by filtering it through carbon granules. A number of waste streams are generated by the Decolourisation Plant. The wastewater tank receives these waste process flows which include discharge from the carbon columns, the dewatering screw, the cyclone, excess transport water, overflow from the spent carbon tank and various floor drains. The flows of water contain black carbon dust and varying amounts of ‘good’ sized carbon granules.
If this ‘slurry’ material is not stirred then the wastewater tank begins filling with sediment that can contain up to 50 percent good carbon granules and 50 percent fine carbon sludge. In the past, this sludge was removed using suction trucks at great expense. A new recovery system pumps the outflow from the wastewater tank up to the spent carbon tank. In this tank, any ‘good’ sized carbon granules settle out and are added to the “spent carbon stock”, ready to be kilned or dried for reuse. The fine carbon dust that is not required overflows from the spent tank and is directed to tradewaste.
Approximately 45 tonnes of granular carbon is used each year, however with this process improvement, around 15 tonnes of this material is recovered. This equates to a cost saving of around $60,000 annually. As a consequence, the trade waste suspended solids loading has been reduced giving us an additional saving of around $2,000 per annum in tradewaste charges.
Fig 1. Wastewater tank drains via cess pit to tradewaste.
Fig 2. Wastewater tank showing stirrer.