With increasing pressure on water resources, efficient and reliable wastewater treatment systems are crucial. Plants are searching for solutions that offer a smaller footprint and higher quality effluent. Over the last 15 years, membrane bioreactors (MBRs) have become increasingly common in both municipal and industrial wastewater treatment plants and help mitigate against effluent suspended solid issues. These systems combine a membrane filter with an activated sludge process where microorganisms are able to thrive and break down contaminants.
One of the biggest challenges with MBRs is that solids and biomass can build up on the membranes and reduce the separation efficiency. Therefore, preventative control strategies and effective monitoring tools are essential to maintaining optimal operation. Petros Gkotsis and fellow researchers reviewed membrane fouling mechanisms and highlight several common applied control strategies including:
- Feed pretreatment (screening, pre-sedimentation)
- Cleaning protocols (physical and chemical)
- Membrane surface modification (increased hydrophilicity, smoothness, charge)
- Modified mixed liquor (chemical treatments: alum, ferric chloride, PAC, etc.)
- Operational modifications (reduced flux, increased aeration)
With any control or preventative action strategy however, impacts on the process must be carefully monitored.
Researchers from Monash University and the School of Applied and Biomedical Sciences in Australia published a study in early 2016 that evaluated practical tools for rapid MBR monitoring. Sixteen tools were reviewed and ranked on a scale of 1-5 under six categories: reagent cost, equipment cost, ability to test on-site, time required to perform the analysis, simplicity and ease of use, and confidence and utility of the data.
Capillary suction time (CST), originally used to predict sludge dewaterability, correlates with specific resistance to filtration (SRF) and ranked first overall. This method measures how quickly MLSS supernatant takes to pass through a filter paper and offers an indication of filterability. Because suspended solids have a significant effect on CST results, it’s recommended that the results be normalized against TSS data.
Suspended solids (TSS and VSS) remain important parameters to monitor as high TSS/VSS can result in significant cake layer formation and reduced efficiency. TSS/VSS measurements however, do not offer any insight into the size or health of the biological population. LuminUltra’s 2nd Generation ATP testing – using the QG21W test method – ranked 3rd overall of the 16 tools evaluated and was cited as the preferred method for monitoring the biological population. The ability of the method to differentiate between cellular ATP from active microbes and dissolved ATP from dead cells, allows for a measure of both the size and relative health of the population. This can help operators establish cause-and-effect relationships between operational parameters and the biological population.
Finally, it is also important to monitor extracellular polymer (ECP) concentrations through turbidity based bulk characterization, or turbidity after centrifugation. While ECP’s are crucial for flocculation and settling, it’s been found that many of these molecules are products of cell death and break down and can lead to increased bio-fouling and filterability issues. It’s important to note that the cause of ECP formation and its effect on MBR efficiency are still debated therefore, it’s recommended that ECP levels be monitored and correlated with other parameters so that triggers for both ECP formation and membrane fouling can be identified for each specific system.
These tools allow operators to identify and manage effective preventative action and control strategies to maintain high quality effluent and help us meet our increasing water demands.
Data Source: Scholes E., Verheyen, V., Brook-Carter, P. 2016. A review of practical tools for rapid monitoring of membrane bioreactors.