Application of membrane filtration in system closure of white water systems in newsprint mills.
The pulp and paper industry is considered to be one of the most water intensive industries in S.A. With increasing environmental regulations and awareness the industry is leaning towards system closure. By recycling water and using it back in the process, the industry can considerably reduce its consumption of fresh water as well as its production of waste water. One method that is gaining momentum for the purification of water is membrane filtration. Membrane filtration does not require any sophisticated heat-generating equipment as compared to conventional separation methods like evaporation making it a viable choice. It is, however, prone to fouling and requires long membrane cleaning cycles. The first part of the study involved the simulation of system closure in the laboratory using a Rapid-Kothen sheet forming machine. This was conducted in order to determine the accumulation of the different contaminants as the white water is recycled. Results obtained indicate that the accumulation of contaminants with increasing number of cycles tends to exhibit a linear relationship. The burst index and brightness of the paper decreased as the level of closure increased. The main aim of the project was to evaluate ultrafiltration as the core process for purification of white water in terms of productivity, retention, flux decline, fouling and cleanability of the membrane as well as to determine the optimum operating conditions that reduce fouling. Polymer membranes having different molecular weight cut offs (10, 50, 100 and 150 kDa) were tested. Overall it was established that the 100 kDa membrane exhibited the lowest degree of irreversible fouling, the best cleanability, the highest productivity and average permeate flux and the permeate obtained from this membrane met most of the water quality requirements for the reuse of water in the paper manufacturing process. The 100 kDa membrane was used in further studies. The optimum operating conditions that reduce fouling was determined using the Taguchi method. Three parameters were investigated at three levels i.e. temperature (20, 40 and 60°C), pressure (1,2 and 3 bar) and volume reduction factor (VRF) (0.63, 0.71 and 0.86). Results obtained indicated that low temperatures, pressures and VRF values reduced the fouling hence the optimum operating conditions were a temperature of 20°C, a pressure of 1 bar and a VRF of 0.63. The permeate quality obtained at the optimum conditions is in accordance with the water quality standards for the reuse of water in the process. Alum and FeCl3 coagulation were used as pre-treatments to ultrafiltration to reduce the membrane fouling thereby increasing the membrane life. Results obtained indicate that twice as much FeCl3 than alum is required to obtain a similar reduction in suspended solids and turbidity and a low sludge volume index (SVI). FeCl3 is more expensive than Alum; requiring twice as much would considerably increase the cost of treatment. Hence alum was chosen as the coagulant to be used in further tests. It was found that the optimum dosage and pH were 288.8 mg/L and 7.68 respectively.