Effect of coagulant addition on membrane fouling and nutrient removal in a submerged membrane bioreactor

This study was conducted to investigate the characteristics of membrane fouling and removal of phosphorous in a membrane bioreactor process by addition of inorganic coagulants directly into aerobic tank of a pre-anoxic nutrient removal system. In order to determine the specific cake resistance of activated sludge floc for different coagulants used, jar test and dead-end filtration experiments were performed. The permeate flow rate increased as well as phosphate removed effectively, as the dosage of alum increased, both in the jar test and dead-end filtration experiments. Considering the phosphorous removal capacity, flux variations, pH range and specific resistance, it seems that the use of alum was effective in concentration range from 200 to 500 mg/L. Ferric chloride was efficient in causing decline in specific resistance, on the other hand, it caused decrease in pH than that of alum. The bench scale MBR performance test was conducted to evaluate the effect of coagulant addition into a MBR process. It was found that addition of alum in the aeration tank had a positive effect on phosphorus removal along with minimization of membrane fouling. The MBR performance elucidated that addition of 30 mg/L of alum was effective to remove 3 mg/L phosphorous in feed for MBR process with reduction in membrane fouling factor without deterioration in nitrogen removal efficiency. The particle size analysis further confirmed that coagulation with alum could minimize membrane fouling because the relative small particles in the MBR reactor, which are considered as a cause of membrane fouling, could became bigger particles due to the addition of coagulant.

The permeate flow rate increased as well as phosphate removed effectively as the dosage of coagulant increased in the batch coagulation tests using alum and ferric chloride as coagulants. The flux increased more than twice at coagulant dosage over 200 mg/L than that of without coagulant addition. Considering the flux, pH and specific resistance, it seems to be effective that the use of alum in concentration range from 200 to 500 mg/L. The use of ferric chloride in bench scale MBR was ruled out, although it caused good reduction in specific resistance, because ferric chloride decrease the pH of the system comparatively more than alum. In the MBR process, addition of alum could increase phosphorus removal but its impacts on nitrogen removal were not thoroughly investigated in present study. Furthermore, alum injection could be helpful in minimization of membrane fouling because the relative small particles, which are considered as a cause of membrane fouling, could became bigger particles having less fouling tendency due to the coagulant.