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Activated sludge is a well-established wastewater treatment technology. Over the years, this technology has been improved to treat water to meet ever-stringent effluent discharge standards.
Aerobic, suspended-growth activated sludge—a widely used process—is relatively simple in reactor design and associated mechanical equipment but highly complex in regard to physical, chemical and biological transformations occurring within the process. An accurate description of these reactions and microbial population associated with the process are areas of active research and likely to remain so in the foreseeable future. Today, applications of molecular biology techniques are helping the industry better understand the process.
The success of the activated sludge process depends on effective separation of the microbial aggregates (sludge) produced in the treatment process from the treated water before discharge or further treatment for reuse. Traditionally, solid-liquid separation is achieved by gravity settling through a secondary clarifier. Gravity settling is sensitive to a number of variables, including the intrinsic settling quality of produced sludge, which is dependent on the right combination of floc-forming and filamentous organisms.
Thus, solid-liquid separation can be a rate-limiting step to produce the desired quality of effluent. This constraint can be eliminated by employing membranes with sub-micron size pores. The membrane acts as a positive barrier for passage of any particulate matter, including bacteria and pathogens. Depending on the membrane design, a pressure or vacuum is applied to separate the liquid from solids.
MBR Pros & Cons
The application of membrane bioreactors (MBRs) has been growing rapidly due to the advantages this technology offers in generating a high quality of water and increasing protection to the receiving environment. The treated water (permeate) from the MBR process can be reused with minimal further treatment, augmenting water resources where necessary.
Some other advantages of the MBR process include: process operation at higher solids concentration, smaller reactor volume and footprint, longer sludge retention time (SRT) and potential reduction in excess solids production, increased process control, decoupling of SRT and hydraulic retention time and retention of slow-growing and non-settling microorganisms. An additional advantage of the MBR process is a reduction in disinfection requirement to meet bacteriological discharge quality; permeate contains negligible solids concentration.
A major challenge in using MBRs is membrane fouling, which has been controlled by agitation created by supplying air or cleaning chemicals, or a combination of both these techniques. These steps add to the capital and operating costs of the system. A better understanding of the fouling phenomenon and use of improved membrane materials can eliminate fouling and lead to longer membrane life.
Advances in MBR
A recent survey by the National Water Research Institute indicates an exponential growth of low-pressure membrane application to water and wastewater treatment, with an estimated total installed capacity of 3,500 million gal per day (mgd) as of 2006.1 Increasingly, industrial wastewater is treated by an MBR process prior to reverse osmosis (RO) to produce water for reuse in industrial operations. Such an approach is very attractive for treatment of municipal wastewater where sewer-mining and an MBR-RO combination can result in alternate water resources for industrial or other uses.
Where drivers are favorable, MBR technology competes successfully with conventional technology, producing equivalent water quality. It should be noted that an additional filtration step is required to produce water quality equivalent to that stemming from a traditional approach. The MBR total capital and operating costs are a function of treatment capacity, influent wastewater quality and expected effluent quality. One of the analyses has shown that the total life cycle cost of MBR process ranged from $1.68 to $2.58 per 1,000 gal treated for plants with a capacity of 1 to 5 mgd.2 Larger MBR systems tend to have lower life cycle costs.
Given the water scarcity across the globe and increasing global population, application of MBR processes likely will grow more rapidly than in the past. This will lead to enhanced protection of water quality and conservation of water resources. Ongoing research in material science is expected to lead to better membranes that have longer life. The advances in wastewater research and improved understanding of microbial population dynamics will lead to better designs and promote MBR technology.