This technology uses a membrane or a ‘perm-selective’ membrane such as a microfiltration or ultrafiltration. The membrane is porous and allows the passage of water and dissolved solids while restricting the passage of solid materials on the other side of the membrane. The retained solids are decomposed and broken down by the presence of microorganisms into finer solids that can pass through the membrane as a mixed liquor.
The remaining solids are removed as sludge in to the sludge facilities. This method employs the technique of the activated sludge system only that it doesn’t require settlement tanks for the removal of sludge. This reduces the number of suspended solids in the effluent generated and with lesser number of pathogen population improving its quality.
The membrane selectivity depends on the pore sizes which ranges from microfiltration range of 0.1-0.4 μm to the coarse ultrafiltration range of 0.02- 0.1 μm. This system has a good solid retention time hence providing more for microbial activity on the solids ‘bio treatment’. The membranes are made up of ceramic or polymeric materials.
Properties of the Membranes used;
• They should be strong enough for mechanical and structural efficiency
• Should be highly porous to increase the surface area of solids permeability.
• The pores should be narrow and well distributed to provide high solid selectivity as
possible.
• The material used should be resistant to chemical corrosion, temperature and pH.
The Design and mechanism
i. Flow equalization
This chamber acts as a waste water holding facility before its pumped in to the plant for treatment. It is equipped with a pumping mechanism that pump water into the anoxic chamber. The design depends on the volume of waste generated.
ii. Anoxic chamber
This chamber is air tight to create room for anoxic processes such nitrification on the nitrogenous waste present in the waste water before it enters the aeration chambers. This reduces the nutrient load of the final effluent improving its quality.
Depending on the waste water capacity, the anoxic chambers can be several with a minimum of two designed to operate in a sequence from one chamber to another. Municipal and domestic waste contain most the nitrogen load.
iii. Aeration chambers
Water from the anoxic chambers is fed in to this chamber where it is continuously aerated by the help of air generated by air blowers.
This provides ambient environment for breakdown and consumption of the carbonaceous organic suspended solids. They have media strands that are thread like that provide a large surface area for microorganisms’ attachment for faster breakdown and decomposition of suspended solids.
The mixed liquor moves by the help of the gravity. The chamber as well can be as many according to the capacity volume of waste water generated.
iv. Membrane chambers
This chamber contains the membranes that filter out the suspended solids from the aeration chambers through a selective technique allowing only the dissolved solids and water to pass through. It is equipped with mechanized suction system that help in the suction process of the water and dissolved solids through the membranes.
The retained solids are again worked on by the microorganisms and broken down to finer suspended solids that can pass through the membrane as mixed liquor suspended solids. The remaining solids are collected as sludge by a sludge suction pump to flow equalizer chamber reducing the number of suspended solids in the effluent.
The membranes suffer fouling and hence need frequent maintenance like washing off the solid particles that clog the membranes. They are also not suitable dealing with fats, grease and oils since they may form scum that will clog the membrane pores.
v. Treatment and storage
The effluent produced is treated through chlorination that is constantly dosed by the help of a chlorine dosing pump. Since the quality of the effluent contain less
suspended solids, the chlorine dosed is enough for free chlorination that is used for disinfection before the water is pumped for storage.
Advantages of MBR
a. Improved effluent quality with low suspended solids hence low BOD5 and COD load.
b. It has a small footprint in the treatment of waste water due to high solid retention time with a high effluent production that is independent of hydraulic retention time.
c. It has a better bio treatment of waste due to a longer retention time hence improved effluent quality.
d. It can utilize a smaller space area while at the same time treating large volume capacity of the generated waste water. Making it a viable option for commercial wastewater treatment plants where there is little space available.
e. It is very effective in the removal of waste water nutrients i.e. nitrogen and phosphorus through the anoxic chamber.