The Difference Between Conventional Activated Sludge Process and Membrane Bioreactor Process

MBR VS CAS/ASP for Sewage and Wastewater Treatment 


What is Conventional Activated Sludge Process (CAS/ASP)?

The Conventional Activated Sludge Process is a well-established technology for sewage and wastewater treatment projects, involving the use of bacteria to degrade biodegradable organics and air for respiration. 



With ASP, wastewater passes an aeration tank where air is continuously applied using a blower or a surface aerator. The bacterial biomass is processed in this aeration tank that acts as a bioreactor, undergoing an aerobic treatment to break down amino acids and organic carbon. Wastewater then overflows into a settling tank also popularly called a clarifier or a sedimentary tank. This is where cell separation happens with the biomass or activated sludge settling and separating from the wastewater. Resulting clearwater flows out of the clarifier and the settled sludge is recycled back into the aeration tank.


How Does a Membrane Bioreactor (MBR) Work?


Membrane Bioreactors are also widely used in municipal and industrial wastewater treatment projects. Compared to CAS/ASP, these MBRs provide an advanced level of organic, suspended solids, and nutrients removal as it combines the conventional activated sludge process with membrane filtration. 



With MBRs, the biomass is processed in an aeration lagoon, also known as an aeration basin, by supplying air and nutrients. The liquor mixture is then filtered through submerged (internal) or lateral (external) microfiltration (MF) or ultrafiltration (UF) membranes. Powerful pumps assist in filtering mixed liquor and with the release of clear water from the system. Settled solids or the biomass is then transported back to the aeration lagoon. 


Advantages of MBR Systems


An MBR is a solid-liquid separation system that replaces the role of a sedimentation tank in a conventional activated sludge process. Even without a sedimentation tank, it can effectively remove sediments. It also adapts to high concentrations of MLSS with almost 100% Suspended Solids (SS) removal. A typical activated sludge treatment system and MBR system are shown below. 


Compared to the conventional activated sludge process, the MBR offers the following advantages: 


1. High Removal Efficiency of Pollutants


An MBR exhibits excellent efficiency when it comes to SS and turbidity removal. Since the membrane pore size of the membrane module is usually very small, at around 0.1μm, all suspended solids and sludge in the bioreactor can be retained. Additionally, MBRs have an SS removal rate of almost 100%. MLSS concentration in reaction tanks can also be increased to 5000-12000 mg/L.  


2. Greater Flexibility and Practicality 


The traditional activated sludge method requires multiple reaction tanks, resulting in larger land area requirements and higher operating costs. This conventional method also involves a more complicated operation and therefore needs highly skilled operators.


On the other hand, using an MBR offers a shorter turnaround time, a smaller footprint, and a more flexible water treatment process. The water output can also be easily adjusted by increasing or decreasing the number of membrane modules, making it a lot more convenient compared to CAS/ASP. 


3. Solution to Excess Sludge Disposal


The disposal of excess sludge also affects the operation of a sewage treatment plant. In the MBR process, the sludge yield is very low, so the production of excess sludge is kept at minimum. If the sludge volume is high, dehydration can be carried out directly without sludge concentration, greatly saving sludge treatment costs. 


Four Reasons to Choose Membrane Solutions’ MBR System


Membrane Solutions’ Membrane Bioreactor (MBR) system is designed to effectively remove suspended solids and organic pollutants in wastewater, making it the best technology available for treating industrial wastewater, chemical wastewater, and commercial wastewater. The MS Hollow Fiber MBR System adopts a reinforced PVDF hollow fiber membrane that showcases strong chemical resistance, high strength, high porosity, and large influx. The effluent quality of the system strictly adheres to reclaimed water standards.