An audit of the Power House at Ketchikan Pulp Company (KPC) by JANSEN revealed a difficult and growing problem in dealing with secondary wastewater treatment sludge. The traditional means of sludge disposal--mixing secondary and primary sludge together and burning it in the mill's two hog fuel boilers--was causing the following problems:
- Low solids content of the secondary and primary sludge mixture from the screw press.
- High expense due to increased polymer usage in the screw presses.
- The need to burn more fuel oil in the boilers to sustain combustion of the wet wood waste mixture.
- The inability to burn all of the mill-generated hog fuel.
- High mixed liquor solids levels in the activated sludge aeration basin due to the inability to dispose of as much sludge as necessary.
The proposed solution to the problem was to thicken the secondary sludge, mix it with the red liquor in the evaporation plant, and burn the material in the recovery boilers.
The first step was a short-term combustion trial, performed in August 1995 in one of the mill's four recovery boilers. The short-term test gave promising results. The sludge's high heating value of 23,200 kJ/kg on a dry basis (10,000 Btu/lb) actually improved combustion conditions in the furnace. The next step was to determine long-term operating effects.
In the summer of 1996, KPC began a long-term trial, using temporary sludge thickening and handling equipment to determine the viability of this approach. After six months of operation, the burning of secondary sludge in the recovery boilers has proven to be successful. Reductions in polymer usage in the screw presses and reductions in fuel oil firing in the hog fuel boilers resulted in significant cost savings. There was no indication of accelerated scaling in the evaporators, and the recovery boilers remained on their normal cleaning cycle. Hog fuel firing rates increased and the wastewater treatment plant aeration basin had increased sludge disposal capacity.
This paper describes this secondary sludge disposal process and the benefits gained by KPC by burning the sludge in the recovery boilers.