Active anti-fouling and defouling of membranes using electrochemical methods

Abstract

Industrial wastewater treatment from cattle slaughterhouses traditionally use dissolved air floatation (DAF) to remove the high organic content of fat, oil and grease (FOG) from wastewaters and recover them for producing other products, including tallow, gelatine and food additives. Dissolved air floatation presents several operating difficulties including the large size, high energy use and odour issues resulting in increased operation costs and decreased efficiency. Replacing a DAF unit with microfiltration or ultrafiltration membranes to recover FOG and protein could reduce operating costs and space, increase product quality through fractionation of various valuable nutrients and reduce or eliminate the smell. However, membranes have rarely been considered for such applications due to the high fouling propensity of the feed and the inability of conventional methods and membranes to effectively deal with this. One relatively unexplored fouling mitigation / cleaning technique involves electrochemistry to either prevent fouling through charge repulsion or remove fouling via electrochemical reactions. The key target of this work therefore is to investigate the suitability of conductive stainless steel membranes for filtration of a FOG / protein stream utilising electrochemical cleaning methods. In theory, in-situ membrane antifouling (the inhibition of fouling) and defouling (the removal of fouling) can be achieved by varying the applied potential on an electrochemically active and conductive membrane. Oils and organic matter tend to carry a negative charge in aqueous solutions at neutral pH and increase in negative charge density with alkali pH. For antifouling, maintaining a negative charge on a membrane surface repulses negatively charged molecules in solution, which in turn, both inhibits adsorption and/or induces them to migrate away from the electrode / membrane surface. The major finding here was that reductive potentials, at or below the hydrogen evolution reaction were successful in minimising fouling on an electrode and also succeeded in mitigating fouling by 67% in a full membrane setup. There is an ideal operating window for negative potential to achieve antifouling, enough to induce repulsion but not negative enough to initiate redox reactions at the membrane surface between -0.4 V to -1.2 V. However, to achieve the best performance antifouling probably requires constant operation at potentials negative enough to initiate the hydrogen evolution reaction or reductive desorption. The commercial implications of this are likely to be power requirements, on the order of 10W/m2. Or to put it another way, this translates to an energy requirement of 1 kWh/kL of permeate flux through the membrane, which for treating a stick water stream in typical abattoir. Defouling by contrast uses electrochemical reactions to either oxidise the organic foulants or produce gas bubbles which can lift off foulants through physical shear forces. The major finding here is that, when operated as anodic electrodes, membranes which have oxidative potentials applied, demonstrate some defouling. However, there is also polymerisation of organics into a gel layer which frequently remains attached to the membrane as well as some bulk polymerization. Further there is corrosion of the membrane surface. For reductive potential, there is an ideal operating window to achieve defouling, enough to produce hydrogen bubbles to remove fouling but not enough to initiate extensive base catalysed polymerization reactions in the bulk between -2 V to -3 V. For the full membrane setup the mitigation of fouling was not as successful as antifouling and resulted in mitigating fouling by only 3%. The membrane fouled very quickly and was unable to recover its performance effectively using electrochemical means. This suggests that long-term antifouling is a more suitable choice for effective fouling mitigation in abattoir waste water streams. This is the first known application of reactive electrochemical membranes to abattoir wastewater treatment. The ultimate aim here would be to clean the membrane without damaging the recovered products, without requiring cost inhibitive chemicals, whilst simultaneously reducing, and possibly eliminating, back washing time. Future areas of research include continuous operation under both anti-fouling and defouling protocols to see the long term impacts on both the membrane and fats, oils and proteins. It is anticipated that this study can be a worthwhile entry point for more sophisticated electrochemical engineering membrane fouling mitigation studies.