In an effort to minimize their short and long-term environmental impact, industries across the globe are facing increasing challenges and costs in the treatment of their process wastewater. As a result, many of these industries have built their own treatment systems in order to treat and reuse process streams, reducing discharge costs to municipal treatment plants and recovering valuable products from effluent.
Traditional wastewater treatment plants consist of pre-treatment systems to capture large floating particles and coarse suspensions (e.g. wood, textile, plastic and stones), followed by biological aerobic digestion that converts organic waste into carbon dioxide, nitrogen gas and water. After the biological process, the cleaned water is typically separated by gravitational force in settlers or clarifiers in which part of the biomass is then fed again to the biological process. Excess biomass then flows to the thickener for further dehydration. The last step consists of granular (sand) filtration and chemical treatment which filters and decontaminates the solution from pathogenic microorganisms.
A membrane bioreactor (MBR), widely used in industrial and municipal applications, is an effective wastewater treatment system that produces high quality effluent suitable for reuse applications. The membranes retain the biomass and mixed liquor suspended solids (MLSS) within the bioreactor, thus producing effluent that is free of particles, bacteria and viruses. MBRs remove between 96-99% of COD, compared to 95% in conventional activated sludge processes. Additionally, MBRs can be operated at higher MLSS concentrations of up to 30 g/L.
The external principle
In both aerobic and anaerobic MBR wastewater treatment systems, ultrafiltration (UF) membranes are commonly used either in submerged configurations or in external (side-stream) crossflow systems which are located outside the bioreactor tank.
Submerged MBR systems place the membranes directly inside the bioreactor tank. In this system, permeate is obtained using vacuum suction at 5-30kPa which results in permeability of around 15-35LMH at MLSS of up to 12 g/L. In aerobic MBR, submerged membranes rely on air scouring as a cleaning method to remove fouling and keep the membranes clean. In anaerobic MBR, this type of membrane cleaning can only be done by recompressing the biogas and using that gas as the scouring agent.
External membrane filtration is positioned outside of the bioreactor and uses crossflow to keep the membranes working at optimal conditions. If more rigorous cleaning is necessary, backwash protocols (temporarily reversed permeate flow) or chemical backwash are implemented. Tubular membranes are frequently used in external MBR systems because they are robust, easy to clean, and emit zero odors. The membrane’s performance is dependent on the system’s crossflow velocity, transmembrane pressure and sludge condition. Healthy sludge and clean biomass results in optimized performance of the MBR system. Normally, permeate is produced using pressures of 300-600 kPa resulting in permeability between 50 and 150LMH at MLSS up to 30 g/L. Typical COD removal is above 98%, reaching COD levels of 100ppm and lower.
Membrane products for MBR’s
Berghof Membranes offers a complete line of membrane solutions for MBR applications. See below for the related membrane products.
A large truck manufacturer produces two wastewater streams: (1) industrial wastewater containing oil and metals with a high COD concentration, and (2) common wastewater stream containing high levels of organic matter from the factory‘s 3,000 employees. The volume and strength of the combined wastewater vary considerably from day to day. They requested a wastewater system to effectively treat the variating wastewater stream.
“To meet strict discharge regulations, think outside the box.”