MBBRs: Putting wastewater to bed
Moving Bed Bioreactors (MBBRs) are the filtration method of choice for many operators looking to remove BOD, ammonia and nitrogen from wastewater. Here we take a close look at the technology and the plastic media at the heart of the process
Q&A with Derek Allen, Business Consultant, Warden Biomedia
Q: What is a Moving Bed Bioreactor and how was this technology developed?
A: MBBR technology was developed in the 1980s and was primarily the natural evolution of the established, well proven and robust fixed film process. Initially the conventional mineral media trickling filter, it then progressed to submerged aerated filter [SAF] technology and finally to a moving bed [MBBR] configuration. Each process was driven by the need to achieve a higher discharge standard and a smaller footprint.
Significant improvements were made to the fixed film process with the introduction of light weight, high efficiency polypropylene random media in the 1960s. Today, MBBR technology is rapidly becoming the treatment plant operators’ choice due to its ease of operation, consistent process performance and reduced operator input.
Q: How does an MBBR work?
The principle of MBBR technology is based on an active biofilm growing on small specially designed polypropylene elements that are suspended in the reactor vessel. The process utilises the advantages of both activated sludge and fixed film technologies without being restricted by their disadvantages. The MBBR process can be used for BOD removal, nitrification, or denitrification.
The media only partially fills the retaining tank, typically between 35% and 67%. This depends on the required removal efficiency or required effluent quality and is immersed in the wastewater undergoing treatment, enabling it to circulate freely.
For BOD removal or nitrifying plants, both process and motive air is blown in to the tank through a floor mounted diffuser grid, normally coarse bubble, providing intense mixing of the reactor contents. Specially selected propeller type mechanical mixers are used in denitrifying plants. The mixing action promotes the diffusion of substrates into the biofilm and the removal of the reaction products.
The carrier elements provide a large protected surface area on which a biofilm develops. The bacteria and other micro-organisms which form the biofilm break down the pollutants in the wastewater. Consequently, their population increases causing the biofilm to become thicker. The motion of the media dislodges surplus biomass, so maintaining a thin and efficient biofilm. The biofilm fragments removed from the media are held in suspension in the process liquor and are discharged in the effluent. The solids are subsequently removed from the MBBR effluent in a solids separation process, typically gravity settlement. In a conventional MBBR a return activated sludge loop is not required as the separated sludge is not returned to the MBBR reactor.
Q: What filter media are used?
The core component within the MBBR system is the plastic media. Wastewater treatment is complicated and can be expensive if the incorrect media is used, therefore selecting the right media is crucial to the success of any fixed film process. Through innovation, R&D and collaboration with specialist research institutions Warden have created the optimum range of media for use in biological wastewater treatment applications.
The unique shape of each element enables higher surface area media to be used within the reactor vessel allowing for maximum usage of the available surface area. This optimises oxygen transfer efficiency thereby improving process performance. These individual plastic elements naturally promote the development of a biofilm comprising of biologically active microorganisms (heterotrophic and autotrophic bacteria).
Warden’s MBBR media is a cylindrically shaped random filter media with internal and external fins and a variety of surface areas. Its innovative design creates a high percentage of protected surface area for microorganisms to adhere. In turn, this increases the overall biomass concentration and can reduce the tank volume required for wastewater treatment. Large openings allow the wastewater to freely pass through the media which in turn controls the solids inventory by scouring the biofilm thereby maintaining a healthy and thin biofilm.
The combination of the higher surface area and open void ratio, ensures that a high active population of biomass can be maintained at low biomass concentrations, thus maintaining process stability and reduced operational costs. The high porosity and minimal resistance to flow, facilitates completely mixed operating conditions within the reactor, reducing the effects of toxic shocks and ensuring even biofilm development throughout the reactor volume.
Q: How are the media manufactured?
The media is manufactured in Warden’s ‘state of the art’ manufacturing facility in Luton, reducing delivery costs and ensuring the carbon footprint of the complete process is controlled and minimised.
The material of manufacture is a strong, lightweight, thermoplastic resin (polypropylene) suitable for wastewater treatment applications. It is non-toxic to micro-organisms, immune to rot, fungus, or bacterial attack. It has a high resistance to ultra violet degradation, ageing, erosion, or disintegration. Polypropylene also ensures longevity, has structural integrity, and can be recycled. Client feedback indicates that biological start up times are also reduced when using polypropylene media.
Additional process capacity or increased removal efficiency can be achieved by simply adding more media, reducing the need to build more reactors.
Q: What are the key applications of the technology?
The flexibility of MBBR technology makes it ideal for use in both industrial and municipal applications. The MBBR system can be used as a stand-alone process or configured within an existing process stream to increase capacity or provide an improved removal efficiency. MBBR is suitable for use in roughing applications, secondary or combined secondary and tertiary systems, making it ideal for BOD removal, nitrification and Denitrification.
Over the past three decades MBBR technology has been used to treat municipal wastewater, pharmaceutical, pulp and paper, distillery, brewery, abattoir, dairy, petrochemical and fine chemical effluent streams.
Q: What are the benefits of MBBRs over the alternatives?
As a third- generation fixed film technology, MBBR is both robust and well established, providing a system that is resilient to shock and toxic shock loading, fits into a smaller footprint, requires minimal man-power input, provides improved process security, and ensures consistent and high-quality discharge standards. In addition to treating conventional municipal wastewaters to a high standard, MBBR is ideal for the treatment of complex industrial effluent streams. The technology can be installed into existing tanks of varying sizes and shapes lending itself to sites that have limited land availability or utilizing existing assets, thereby offering savings on excavation and construction costs.
-This article appeared in the May 2018 issue of WWT magazine.
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