Predicting and Controlling RO Membrane Fouling treating Wastewater Effluent

Abstract

The combination of MBR membrane technology with reverse osmosis (RO) for wastewater treatment has been proven to provide a very high quality effluent. The MBR efficiently removes nutrients (C, N & P) from the influent wastewater together with suspended solids and colloids making it suitable to feed it to a RO membrane which removes most of the remainder of the dissolved compounds, to produce very high quality effluent which is of drinking water quality. The main advantages of the wastewater treatment systems based on MBR technology are the quality of output water and the need of little installation space, in comparison with conventional activated sludge (CAS) systems. However, fouling particularly of the RO membrane, remains a problem when using the RO membrane as a final stage in a wastewater reclamation plant. Membranes require frequent chemical cleaning processes to control the build-up of organic, inorganic and biological fouling on the membrane surface. As the membrane gets fouled, increasing amounts of energy are required to produce the same amount of product water. Therefore it is essential to understand what is causing the fouling and what control measures must be applied to achieve long-term sustainable operation. The objective of this project is to understand how the composition of the RO influent water (MBR effluent) in terms of inorganic contaminants, temperature, pH and the system recovery affects membrane fouling. To achieve this objective, the feedwater of an MBR-RO pilot plant located at Quart was studied to obtain a characterisation of RO feedwater quality. The results of a membrane autopsy were used to see what types of membrane fouling are observed during operation. An RO membrane simulation model was then used to understand through multiple simulations how the feed water quality, temperature, pH and system recovery affect membrane fouling, the product water quality and the energy consumption. The results are then used to propose strategies to control membrane fouling and optimize process operation in terms of energy use and permeate quality. The results from the autopsy and those from the modelling studies confirm that the main component of inorganic fouling is Calcium Carbonate (CaCO3) particularly on the concentrate side of the membrane (the end of the membrane). On the feed side of the membrane, organic fouling was predominant. The modelling studies showed that the addition of acid to control the pH could control the CaCO3 precipitation. Furthermore, we have shown that the optimal process recovery to run the RO membrane at the lowest energy consumption for different feedwater temperatures between 24 and 27ºC is 70-75%. The final part of the project utilized online process data to monitor membrane fouling during pilot plant operation and normalize process data in terms of permeate flux and salt rejection