Fouling and Cleaning During Treatment of Thermomechanical Pulp Mill Process Water

Abstract: One of many possible applications for membranes in industries is to separate valuable products from wastewater. However, usage of membranes inevitably leads to fouling. Fouling means that the treated solution affects the membrane and causes a permanent decrease of the permeate flux. That means that the membrane requires cleaning so that the flux can be restored and the full potential of the membrane can be utilized. Membrane cleaning can be performed by different methods, one of them being chemical cleaning. This master thesis aims to investigate which parameters that affect how the fouling takes place, as well as which parameters that affects the chemical cleaning of the fouled membranes. The experimental work has been performed by treating an ultrafiltration membrane with process water from a thermomechanical pulp mill. The experimental work consisted of two main parts, the fouling experiments and the cleaning experiments. The fouling experiments investigated the fouling influence of temperature, duration and concentration of the treated solution. The results from the experiments showed that the fouling is influenced by the temperature and that more severe fouling is achieved at higher temperatures. The cleaning experiments showed that the efficiency of a cleaning method is dependent on both the cleaning agent concentration and the cleaning time. Two cleaning agents, Ultrasil 10 and sodium hypochlorite, were used in the cleaning experiments. A conclusion that can be drawn from this work is that it would be interesting to investigate if the fouling in membrane plants could be reduced by using lower temperatures industrially. Less fouling would lead to a reduced need for chemical cleaning, which probably would increase the membrane lifetime as chemical cleaning often is the most damaging procedure that a membrane is subjected to. A lower temperature could lead to financial savings both by an increase of the membrane lifetime, but also as the operating time between the required cleaning can increase. However, a lower filtration temperature is negative from an economical perspective as the flux is lower at lower temperatures and the production will thus be more time consuming. A way of overcoming the reduced production pace could be to increase the membrane area.