Side effects of flux enhancing chemicals in membrane bioreactors (MBRs): study on their biological toxicity and their residual fouling propensity.

Soluble and colloidal materials like soluble microbial products (SMP) or extracellular polymeric substances (EPS) are considered to be major foulants in membrane bioreactors (MBRs). Removing these fouling causing substances is thus thought to reduce the fouling of the membrane in general. In addition to traditional strategies for fouling prevention which mostly try to remedy the effects of fouling by air scour, etc., the new and promising method of adding chemicals is being investigated here. Previous tests with 30 different substances have shown that several of these reduce SMP concentration in the supernatant and enhance filtration. Nevertheless, additive dosing might have unknown side effects in filtration systems. Results presented in this study indicate that these additives may themselves cause severe fouling on different membranes if they remain unbound in the liquid phase. Therefore, the thorough control of the dosing rate of these chemicals will be of paramount importance in full scale applications. Biological toxicity of additives was measured in terms of respiration. OUR tests did not show inhibiting effects for most additives. Chitosan even showed an enhanced OUR due to biodegradability. Oxygen transfer could be enhanced for 25% with the addition of a polymer.