Sane in the membrane
Dr Paul Zuber of Brightwater Engineering looks at the sometimes difficult but ultimately successful development of MBR technology
In the last 100 years the world's population has tripled, with water use increasing sixfold. Agricultural use, at approximately 80% of the total global water demand, is responsible for a large part of the increase. To control this overuse a range of programmes, such as the Envirowise initiative, which aims at reducing industrial use by 30% (August 2005), have been established. Water continues, however, to become a resource where, in certain localities, demand exceeds supply. This is driving increasing reuse research and development, which is crucial to the local supply in certain parts of the globe. Membranes play a critical role in these developments, and are currently underpinning the pilot and impact investigations for wastewater reuse taking place in areas such as California, Singapore and Sydney.
Since their commercialisation in the 1960s, membranes have found application in many industrial and water sectors. The use of membranes in wastewater treatment, however, has generally lagged behind for a variety of reasons, including economic viability, technical capability and, perhaps most critically, a lack of environmental and legislative drivers.
To understand the emergence of membranes into wastewater treatment it is interesting to draw an analogy with clean water membrane treatment using reverse osmosis (RO). After RO plants started to gain commercial acceptance, there followed a period of operational difficulties during the mid/late 1970s and early 1980s.
RO was a new technology in the UK, with limited understanding of the membrane and associated process. Plants were installed without consideration of pre-treatment requirements - for example, one company drew water directly from a canal onto the plant, resulting in rapid blockage. Reliability was also affected by various design issues.
During the early 1980s RO had a poor operational and reliability reputation. Development work undertaken during this period overcame the design and manufacturing issues, and greatly improved RO reliability. There was also a trend from hollow fibre-based modules to spirally-wound membrane sheet modules. Driven by a combination of environmental requirements, the desire to reduce the use of expensive chemicals, and the need for higher-quality water, the industry returned to RO membranes systems in the late 1980s/early 1990s. Resulting from the advances in system and product design, RO membrane systems went from strength to strength and are now considered in the market to be a stable method of treatment.
Similar issues could be said to exist in the use of membranes in the wastewater treatment sector - with the added complications of an inherently variable feed stream and the requirement for a symbiotic biological treatment process. In the UK a number of water companies investigated the operation of membrane plants using membrane bio-reactors (MBRs). Some operational difficulties have been experienced with these, including issues of membrane robustness, influence of influent grease and flow patterns, biological stability, and operator confidence/experience. Despite such early development issues, the process continues routinely to produce an effluent of excellent quality, in terms of its physical, chemical and biological content. In addition, where there is a driver for these high-quality effluents, MBR technology can prove to be an extremely viable option.
There are now believed to be more than 1,300 MBR installations worldwide, treating both domestic and industrial waste streams and split predominantly between immersed hollow fibre and immersed flat sheets systems. Early demonstration projects concentrated on the smaller-capacity schemes, but as confidence and capability with the technology grows there are now schemes treating population equivalents (PE) of >80,000.
The principal issue associated with the operation of membranes in wastewater has been maintaining the membranes in a clean and operational condition. Over the last 15 years around 40% of the research publications involving MBR technology have reported on the causes and prevention of membrane fouling. Fouling is a general term and includes both surface and in-membrane contamination which can reduce system performance. Much work has been done to characterise the occurrence, to find remedies and to prevent membrane fouling, focusing on contaminant exclusion, membrane development and system enhancements.
The exclusion of substances known to impair performance is now a routine element of improved overall system design. This focuses upon inlet works design. All membrane processes require screening at 3mm or less, for example. System suppliers will ensure that the screening structures are robust, reliable and provide no opportunity for bypass. Hollow fibre membrane systems will also often specify further side stream screens at <1mm to minimise the accumulation of fibrous material known to cause matting.
Potential fouling based on influent pollutants such as grit, grease and salinity are often controlled by specification of inlet composition. Typically fats, oils and greases are limited to < 50mg/l and rates of change of salinity at <200mg Cl-/l.hr. The Membright system, developed by my own company, Brightwater, is an example of a modern MBR that has built upon all the operational experience to date. Operating evidence available with this system shows that where membrane performance is reduced by 'slime' accumulation, the MBR process and system naturally attempts to recover, utilising increased populations of surface-grazing protozoa. Inorganic pollutants also require careful management and perhaps enhanced chemical cleaning to maintain membrane performance.
Improvements in membrane technology have had a significant impact on the robustness of MBR systems. Membranes that offer true ultrafiltration capability, improved resistance to drying out, enhanced mechanical strength and tolerance to recovery chemicals have been developed. These improve operation and give greater confidence in membrane life. Stronger membranes, along with improved methods of backing plate bonding, have reduced the occurrence of fatigue failures which caused reliability issues with earlier designs.
It has also been shown that the capital investment in plate systems can be protected and enhanced by re-skinning with new replacement membrane materials as they may become available. Current membrane enhancements are giving improved resistance to fouling and the ability to deal with emerging pollutants such as endocrine disrupters.
Developments in system design and operational set-point definitions have also significantly enhanced the stability and operability of MBR solutions. A feature of MBRs is the close association of membrane performance with that of the biological process. Sludge-balancing, especially in relation to mixed liquor levels, has been the subject of considerable investigation. Typically mixed liquors now operate at 12-15,000mg/l, compared to early MBR specified concentrations of up to 25,000mg/l. These reduced levels have been introduced to maintain adequate oxygen transfer for microbial activity and sufficient hydraulic circulation over the membrane surface, which will minimise the potential for caking that can occur in any system. Control of mixed liquors at these elevated values is no more onerous than in conventional settlement layouts in the region of 2-3,500mg/l.
Sludge imbalances can be further managed through system design. In the Membright system, for example, the combination of 10mm plate spacing and a patented air distribution system with a managed air supply ensures that issues of caking are minimised, while also maintaining reliable operation of the air supply. This eliminates the need for further flushing systems, thus reducing capital and operational costs whilst improving reliability.
Realistic flux rates, or critical flux, are used for design and are typically in the region of 24-25 l/m2/hr. Peak fluxes in the region of 40-50 l/m2/hr can only be tolerated for short durations and must then be followed by a relaxation period. The enhanced air scour regime of the Membright system, for example, plays a key part in maintaining stable flux rates and improves operational stability.
Chemical cleaning as a means of extending and restoring membrane performance is required in various degrees of complexity. In its simplest form the membranes are soaked, at least annually, in solutions of hypochlorite or citric acid. The frequency of any chemical cleaning is a function of the system design, operation and influent composition. Careful design of the air scour can minimise this requirement and prove to be the most cost-effective solution. Some systems require more complex cleaning, especially hollow fibre plants, which operate with lower membrane footprints. These cleaning requirements, whilst enhancing the operation of the membranes, add complexity along with operational and capital costs. This is often unviable for smaller systems. Selection of a membrane configuration will often be governed by size, where plants with a PE of <10-15,000 (approximately 5-10,000m3/d) favour the simpler plate systems, while larger schemes will absorb the greater system requirements for hollow fibre systems.
Meeting the needs of industry
Environmental and resource drivers continue to enhance the role for membranes, especially in municipal wastewater treatment. The industry has been concentrating upon system and product developments, which, similar to RO membrane systems development, has led to modern MBR systems being able to meet industry needs. The application of membranes in wastewater treatment has now been established for more than 10 years. Modern MBR systems have built upon industry experience and specific developments to deliver a proven robustness which ensures routine production of effluent with parameter concentrations that challenge the limits of traditional analytical methods.
Where scheme drivers such as small footprint or high effluent standards are required, a modern MBR system is a cost-effective and robust treatment solution capable of routinely producing effluents significantly better than BOD 5mg/l, SS 5mg/l, NH3 1mg/l, and standard faecal contaminants of <10/100ml (e.g. T. coliforms, E.coli and Salmonella). The successful delivery of these schemes is dependant upon the MBR supplier understanding the nuances and demands of linking the physical filtration capabilities of the membranes with the demands of the process, while also providing backup support for all system and treatment aspects when required. Where this is implemented successfully, operational standards can be further enhanced to meet tight nutrient standards such as total nitrogen of <5mg/l and total phosphorus <0.5mg/l whilst still maintaining excellent membrane throughput. Modern MBR systems certainly prove that membrane technology has come of age for wastewater treatment.
Thanks to Mr Tony Bodle, CEO Water Services Corporation, for the
historical input on reverse osmosis membrane systems.
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