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Safe Water for Everyone: 
membrane bioreactor technology

Experts suggest that membrane bioreactors may be a key to global water sustainability

In the evolution of life on earth, the membrane was essential in that it allowed the formation of cells, and later the compartmentalisation of processes in cells. As humans have learned how to produce more complicated and efficient synthetic membranes, so too have we developed the ability to compartmentalise processes. In this way, membranes can be used to filter cells from for example waste water. If the filtered cells play a role in breaking down additional waste flowing through the membranes, a membrane bioreactor has been created. A membrane bioreactor consists of some biological item or items in association with a membrane. A membrane is a surface that has the ability to let some things through it and will block others.

This article summarises developments in water treatment membrane bioreactors. Within the African context, the article has particular relevance to those involved in the provision of clean water and safer environments. The technologies described allow decentralised water treatment and hence given the size of the continent and the population spread, these technologies may provide answers to many planners.

Article by Francis A. DiGiano et al.

Reuse and decentralization will be essential for meeting human needs for water and sanitation in both developing and developed countries. Membrane bioreactors (MBRs) will be an essential part of advancing such water sustainability, because they encourage water reuse and open up opportunities for decentralized treatment.

These were the conclusions of a Rockefeller Foundation-sponsored Team Residency held at the Bellagio (Italy) Study and Conference Center on April 23-26, 2003. The foundation invited 14 experts on membrane technology, water treatment technologies, and water sustainability from the United States, United Kingdom, Germany, Italy, Australia, Israel, South Africa, and Malaysia to explore the role of MBRs and other membrane processes in achieving sustainable water and sanitation. The foundation periodically brings together up to 14 participants from developed and developing countries to discuss topics of global importance. The format permits structured and unstructured time to explore common ground and forge shared solutions to tough challenges.

Membrane Bioreactors Come of Age

MBRs discussed in this instance combine the activated sludge found in high throughput sewerage treatment plants with membrane filtration (see image below). So, in addition to removing biodegradable organics, suspended solids, and inorganic nutrients (such as nitrogen and phosphorus), MBRs retain particulate and slow-growing organisms (thereby treating more slowly biodegraded organics) and remove a very high percentage of pathogens (thereby reducing chemical disinfection requirements). They also require less space than traditional activated sludge systems because less hydraulic residence time (HRT) is needed to achieve a given solids retention time (SRT). In addition, MBRs are more automated, making them ideal for decentralized treatment because they are simpler to operate.

 Description of MBR technology
An MBR is a combination of the activated sludge process, a wastewater treatment process characterized by a suspended growth of biomass, with a micro- or ultra-filtration membrane system that rejects particles. The membrane system replaces the traditional gravity sedimentation unit (clarifier) in the activated sludge process. The turbidity and suspended solids concentration of the effluent is far lower than in conventional treatment. All biomass is retained and becomes returned activated sludge. Biological growth leaves the system as waste activated sludge. The figure shows an immersed MBR that is market by several vendors with various proprietary features.

We base the readiness of MBR technology on the following reasons:
- The engineering principles underlying MBRs are familiar enough to ensure reliability. Because MBRs combine two familiar technologies - activated sludge and membrane filtration - significant engineering expertise can be applied to MBR design and operation. Several studies already have applied activated-sludge-related biology to MBRs, although current investigations suggest potentially important differences in growth, population diversity, cell activity, and competition. One obvious difference is that MBR membranes have to be cleaned periodically to minimize biological and chemical fouling, and MBR manufacturers are developing cleaning methods.
- MBRs have been used in enough applications to verify successful performance and identify critical design and operating factors. MBRs have been used to treat a wide range of municipal and industrial wastewaters, and currently are installed at more than 1000 sites in Asia, Europe, and North America, according to a database assembled by the Water Environment Research Foundation. Most currently treat a few hundred m³/d (the largest treats less than 40,000 m³/d). But plans are underway to build MBRs that will treat 30,000 to 150,000 m³/d, and the technology could be used to treat 300,000 to 800,000 m³/d, according to an assessment by a major consulting engineering firm.
· Enough reliable equipment and technological support are commercially available to meet existing and developing demand. Membrane-manufacturing capacity is expanding, so unit costs are declining. The long-term trend is a "virtuous cycle" in which declining costs spur more demand, which spurs further cost reductions.

Water Sustainability and the Role of MBRs

Water sustainability requires a holistic approach to water management, one that emphasizes decentralized systems to encourage water reuse, while providing safe water to those currently unserved or underserved in developing countries. Overall, MBRs meet the water sustainability criteria, but several important improvements still are needed (see table below).

For example, although the cost of membrane processes has dropped by up to 30-fold since 1990, economic sustainability is rated as "improvement needed." Future cost reductions should come from continued technical improvements and the benefits of a growing demand for membrane production. MBRs have not been in operation long enough to have data on membrane life, so this cost is unknown; reducing water flux may increase membrane life, but it will increase the capital cost. Affordability also depends on institutional and government policies, which could include rebates or subsidies as incentives to reuse water in order to reduce freshwater demands.

Table 1. Sustainability Criteria for MBR Technology
(Balkema et al, 2002 and indicates the Team=s ratings for MBRs)

Environmental sustainability. 
Although MBRs received a "good now" rating for most environmental sustainability indicators - effluent water quality and optimal water, nutrients, and land use - improvements are needed in the system's chemical and energy use. Since MBRs primarily use chemicals and energy to control fouling found that two-thirds of the energy used in municipal MBRs is needed to generate crossflow from air sparging to control fouling], a better understanding of the fouling process might reduce their use. For example, Guibert and team found that intermittent and cyclic aeration with submerged hollow fibers reduced the air-sparging demand (and related energy use) by about 50%. Also, an anaerobic MBR could be a net energy producer due to biogas generation. MBRs also may be more sustainable than conventional activated sludge systems when considering biosolids volumes and effluent levels of heavy metals and persistent organic pollutants, but more research is needed to confirm these effects.

Technical sustainability. 
MBRs also received a "good now" rating for most technical sustainability indicators, except ease of use. Experience suggests that membrane capacity and life can be optimized by appropriate preliminary treatment, especially removing fibrous material (such as hair) using screens with openings of 2 mm or less. However, the quantity and noxious nature of such screenings are problematic for most operations, and a proper balance has not yet been established between screening's advantages and disadvantages in MBR-based treatment facilities.

Another important unresolved technical issue is the optimum mixed-liquor suspended solids (MLSS) concentration that allows for acceptably high water flux and small reactor footprint, without reducing oxygen transfer so much that it limits reactor size. MLSS concentration is controlled by biomass retention time, which in turn determines biomass withdrawal volumes and the energy and costs related to treating and disposing of waste activated sludge.

Also, while rated "good now," reliability could be improved by reducing the failure rate of individual components and the need for redundancy. On-line testing (such as pressure decay tests and particle counting) is the preferred option for monitoring performance to ensure reliability. To make on-line monitoring feasible for small, decentralized facilities, test systems must be inexpensive and reliable, and their outputs must be relayed telemetrically to a centralized facility that can deploy trained technicians.
Socio-Cultural Sustainability. MBRs received "improvement needed" ratings for all three socio-cultural sustainability indicators, which are difficult to quantify and thus, overlooked. "Institutional requirements" has to do with local standards and regulations for wastewater treatment, discharge, and reuse. The acceptance of water reuse and novel sanitation methods depends on culture and facility management. Other indicators have to do with implementation issues, like the availability of technical expertise and ability to accept responsibility for operations at a more centralized level.

MBRs in Decentralized Wastewater Reuse

Lately, researchers have been noting the advantages of decentralized treatment systems over centralized ones in achieving water sustainability. The perceived benefits include less need for major infrastructure development and/or maintenance; potentially lower costs; less discharge to receiving waters; and more opportunities for water reuse because the reclaimed water is locally available and the pathogen risk is lower.

In theory, decentralized systems can be used for a single dwelling, housing cluster, subdivision, or a satellite development, but the smallest practical scale may be housing clusters. MBRs can provide significant opportunities for reuse in a decentralized wastewater management system (see image below). In decentralized water management, valuable resources in wastewater - water, nutrients, and the organic material's energy content - are "mined" and reused close to their point of generation. The water can be reused safely to flush toilets, to irrigate landscapes, in various industrial processes, and to extinguish fires. Nutrients can be reused via irrigation, and the extracted energy can be used to generate heat and electricity.

Wastewater Reuse in Decentralized MBR Systems

MBRs provide a reliable, high-quality, reusable effluent. For example, its particle-free effluent allows more effective post-disinfection, as required before reuse. Moreover, MBRs provide excellent pretreatment when reverse osmosis (RO) is needed to generate very high-quality reclaimed water. MBRs may also remove fouling fractions of organic matter more effectively than microfiltration prior to RO.

However, effective decentralized wastewater management systems will depend on the technical resources of a centralized authority, including monitoring, maintenance, and technical service. Ideally, each decentralized system's performance would be monitored by a centralized service provider whose technical staff can respond rapidly to local needs.

Membrane Technology in Developing Countries

The United Nations' Millenium Goals and the Johannesburg Earth Summit's findings (see table below) define the challenge for sustainable sanitation services in developing countries. Improvements in wastewater management are inextricably linked with the desperate need to provide safe drinking water to those currently unserved or underserved.

The magnitude of the problem cannot be understated: In 2000, an estimated 1.1 billion people lacked access to safe drinking water and 2.4 billion to adequate sanitation. Put another way, 40% of Africa's people, 19% of Asia's people, and 15% of Latin America's and the Caribbean's people lack access to safe water, and 40% of Africa's people, 52% of Asia's people, and 22% of Latin America's and the Caribbean's people lack adequate sanitation. Meanwhile, the urban populations of Africa and Asia are expected to nearly double in 25 years, while those of Latin America and the Caribbean are expected to increase by 50%.

At present, the use of membranes to meet this demand is limited to a few research and development projects. In order to achieve the Millennium Goals, membrane technologies will have to effectively address the following issues:

• the per capita water demand will be small (on the order of 25 L/person/d);
• most poor people will be in dense, periurban settlements;
• local water sources will be contaminated with faecal matter and turbidity;
• urban water will receive uncontrolled industrial effluent discharges;
• membrane system concentrates will be discharged locally;
• electrical supply will be scarce and intermittent;
• local technical support will be a challenge;
• low pressure, low energy systems will be preferred;
• local sources of indigenous flocculants, chelating agents, and enzyme cleaning chemicals need to be developed; and
• modular systems will best suit the dispersed need.

A "first cut" analysis of membrane technology's potential use in a developing country can be generated using two important statistics: the human development index (HDI) and the water resources per capita. Countries with a high HDI (greater affordability) and low water resources per capita (greater need) may be ideal candidates for MBRs in order to promote water conservation and reuse. Those with both high HDI and water resources per capita may find MBRs better protect their abundant water resources. Low HDI countries obviously will need financial assistance but still are entitled to clean water and public health protection. In these countries, decentralized MBRs in dense urban settlements would reduce sewer requirements, encourage local agricultural reuse, and eliminate the need for chlorine disinfection.

Water sustainability is a critical issue in developing countries. In the Triple Bottom Line, J. Elkington urges that projects in these areas be socially responsible, environmentally sound, and economically viable. Membrane technology may be effective here, but its utility or service needs to be assessed holistically to avoid repeating the mistakes many nongovernmental organizations have made in developed countries.

The Bellagio Framework

As we noted in our Bellagio Framework on MBR Technology (see table below), attaining water sustainability will require commitment from policy makers, planners, funding agencies, educators, implementing agencies, and technology providers. The need is urgent. MBRs can help achieve water sustainability and prevent unnecessary human misery.

This article is dedicated to the memory of Prof. Alberto Rozzi, who planted the seed for this Bellagio Team Residency.

More information:

This article written by: Francis A. DiGiano, Gianni Andreottola, Samer Adham, Chris Buckley, Peter Cornel, Glen T. Daigger, A G (Tony) Fane, Noah Galil, Joe Jacangelo, Alfieri Pollice, Bruce E. Rittmann, Alberto Rozzi, Tom Stephenson, and Zaini Ujang

Related Articles:

Membrane Development In South Africa

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