Technologies like CEB, CIP, Backwashing, submerged rotating MBR and air sparging have been promising and have given a direction on countering the fouling of membranes
Wastewater collection and drainage has been prevalent in various parts of the world since ancient period. Cloaca Maxima, a vaulted channel sewer of the ancient Roman drainage system is the oldest existing Roman engineering monument that was used to transport drainage water to the Tiber River. However, unplanned drainage of wastewater in water bodies raised concerns about environment and public health. Urban development, improved quality of life and expanding cities lead to the adoption of more advanced and efficient treatment techniques of wastewater. The treatment involved construction of large number of treatment units in a process treatment train which led to the sprouting of another challenge scarcity of land for the establishment of these treatment facilities. This made it essential to develop engineered treatment methods to meet our needs. Wastewater treatment process comprise unique units grouped together to provide primary, secondary, tertiary and advanced treatment. Wastewater treatment schematic:
Among these various stages, this article focuses on the membrane bioreactors (MBR) technology, which is utilized in the secondary treatment stage of wastewater. Urban wastewater secondary treatment usually consist of conventional activated sludge treatment process (ASP) which utilizes heterotrophic bacteria in aerobic environment. Sludge produced from the process is removed by gravity settling. However, building up of sludge in this process imposes construction of large size of aerated bioreactor and also sludge treatment needs to be provided. MBR is an improved version of the conventional ASP where secondary clarifier has been replaced by the membrane unit to isolate treated water from the influent.
2. Membranes Bio Reactor
MBR consist of an activated sludge bioreactor built with a membrane separation module to retain the biomass and sludge coming with the influent wastewater. Since, the effective pore size of the membrane is close to 0.04 μm or below, the effluent produced from the MBR is considered highly treated and clarified. There are basically two main process configurations of MBR with pumped and airlift hydraulic operations:
1. Submerged or immersed MBR (iMBR)
2. Side stream MBR (sMBR)
Out of these two MBRs, iMBR is considered more energy efficient as compared to sMBR due to the fact that in side stream MBR, the membrane module requires pumped crossflow to counter high pressure and volumetric flows which consumes significant amount of energy. In sMBRs, there always has to be a balance between the pumping energy demand and the flux. To achieve higher flux, a high transmembrane pressure is required with a high retentate velocity since energy demand is directly proportional to the flowrate times pressure. On the contrary, if flowrate is decreased by reducing cross sectional area of the membrane, it would result in the pressure drop along the length of the module on the retentate side. This is due to the inverse relationship between resistance to flow and area. Among the two MBRs, sMBR is more susceptible to fouling than iMBR, since it operates under higher flux and fouling increases with increasing flux and lowers permeability especially above the critical flux.
Membranes are solid semi permeable material medium which selectively allows transport of physical or chemical components through it, depending upon the membrane pore sizes and their structural variations. The membrane materials used for MBR are majorly categorized into two types-polymeric and ceramic. However, a variety of polymeric or ceramic materials are used to form membranes such as: Polymeric - Polyacrylonitrile (PAN), high density polyethylene (HDPE), polyethylsulphone (PES), polysulphone (PS), polytetrafluoro ethylene (PTFE) and polyvinylidine difluoride (PVDF).
Ceramic- Aluminum oxide/ Alumina (Al2O3), silicon carbide (SiC), Titanium dioxide/ Titania (T iO2) and Zirconium dioxide/Zirconia (ZrO2) Membranes are manufactured such that they have high porosity, narrow pore size distribution to provide higher selectivity and throughput, be mechanically, thermally and chemically strong to resist temperature and pH concentrations. Ceramic membranes perform better than polymeric to resist fouling and chemical attack. However, their application is limited due to its high cost. Majority of MBR membrane modules available in the market are polyvinylidene difluoride (PVDF) based, after PVDF most selling is PES and PE.
There are 4 key membrane separation processes utilized, which includes reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). Any of these membrane separation processes can be identified based on the following factors:
1. Membrane Configuration
2. Material of Membrane
3. Type of Driving Force
4. Separation Mechanism
5. Nominal Size of the separation
Typical operating ranges (in terms of particle size separation) of these membrane separation processes are as follows:
1. Microfiltration - 0.07 −2.0 μm
2. Ultrafiltration - 0.008 −0.2 μm
3. Nanofiltration - 0.0009 −0.01 μm
4. Reverse osmosis - 0.0001 −0.002 μm
For MBR, the separation processes mainly utilized is ultrafiltration along with the biological treatment in ASP.