Membrane bioreactors

On June 24, 2009, Professor Gatze Lettinga from Wageningen in the Netherlands was presented with the Singapore Water Prize for outstanding contributions in the field of water with a cash prize of SGD300,000 (£127,500), an award certificate and a gold medallion. His revolutionary treatment concept for anaerobic wastewater treatment, which stood out among 39 international nominations, enables effluents to be purified cost-effectively and produces renewable energy, fertilizers and soil conditioners.

Professor Lettinga had chosen not to patent this invention so his water treatment technology can be universally available and, as a result, this technology is in use in almost 3,000 reactors, representing about 80% of all anaerobic treatment systems in the world.

Professor Lettinga’s success is a typical example of the Dutch know-how of water

management, earned since approximately the 9th century, in a country where two thirds of the territory would regularly be flooded if there were no dikes.

Today, the Dutch water sector consists of 1,450 companies with annual sales of

US$12B (£7.46B – 2% of GDP) with 75% raised domestically; 222 companies (16%) are producers of water purification technologies. The overall activity is less than the water business of France’s Veolia, which realised a turnover of €12.5B (£10.8B) in 2008.

Because of the small size of the companies, cooperation is crucial. Dutch niche players look for collaboration with a limited number of parties, such as water companies, water agencies, and research and education institutions. Two well-known universities, Wageningen with almost 5,000 students, half of which are graduate and doctorate level, and TU Delft, about three times larger, help the clustering of technological know-how.

One example of the collaborative effort is the application of membrane bioreactors (MBR). The Water Board Hollands Noorderkwartier and the Foundation for Applied Water Research (STOWA) first commissioned the engineering firm DHV in 2000 to carry out pilot plant research in Beverwijk, 20km NW of Amsterdam. Using four different types of MBR pilot installations, the aim was to investigate the possibilities of the MBR technology for Dutch municipal wastewater with low temperature in the winter and dynamic influent patterns. With the support of the membrane suppliers, research was carried out on 12 different configurations of seven suppliers between 2000 and 2004. Initially, four pilots by Kubota, Mitsubishi, XFlow and Zenon were run, which were later extended with units from Memfis, Seghers-Keppel and Huber-VRM.

In parallel, the Rhine and IJssel Water Board, responsible for water management and 14wastewater treatment plants (WWTP) in Eastern Netherlands around Apeldoorn, studied the options to upgrade an existing WWTP in Varsseveld, hometown of football coach Guus Hiddink. A feasibility study led to the decision to

construct the first MBR installation for the treatment of domestic wastewater in the Netherlands. The plant was started up using sludge from the existing wastewater treatment plant in Varsseveld.

Improvement of the effluent quality was necessary because Varsseveld discharges into a small and ecologically vulnerable inland waterway.

While conventional technology with tertiary filtration could have achieved a similar aim, the innovative character of MBR technology played an important role in the decision-making process.

The cost of the entire installation amounted to approximately €10M. Because the investment and operating costs for an MBR are higher than for a conventional WWTP, the water board sought additional funding. To cover the unusual costs and risks, STOWA created an Innovation Fund with support from all Dutch water boards. Further support, around €1.6M, was granted by the EU through their LIFE program. The Dutch government qualified the project for incentive schemes for green investment, and subsidies for energy supplies in the non-profit and special

sectors were allocated.

The load of the Varsseveld WWTP was planned for the population equivalent of 23,150 and a dry weather flow of 3,500m3/d in 2015.

Originally, a load of about 30% was integrated from a nearby cheese factory, but it had to be disconnected during the start-up phase to avoid excessive membrane clogging.

Because the membranes are vulnerable to contamination and damage, pre-treatment to remove leaves, plastic, sand, grease, hairs and the like consists first of a 6mm screen, followed by an aerated sand and grease trap and finally of microsieves with 0.8mm perforations. In addition, to keep the activated sludge clean, it is continually recirculated over the microsieves.

Following a European tender procedure, the contract for membrane filtration was awarded to the membrane supplier Zenon. A demonstration pilot was also installed on site, which showed that a nitrogen content below 5mg/l could be consistently achieved. During a period of a few weeks, a value of 2.2mg Ntotal/l could be met without dosage from a carbon source. The phosphate content of the effluent was largely dependent on the iron dosage, and with a level close to 0.8mol Fe/mol P, a residual between 0.1 to 0.3mg Ptotal/l was obtained.

While it is difficult to compare MBRs to other options, the specific utilisation costs including amortisation remain high with the Varsseveld MBR at €64 per year per PE (volume of waste water produced by one person) and €1.07 per m3 treated.

An important part of the cost is the energy consumption, around 0.9 kwh/m3, as

well as the depreciation of the plant that contributes almost half to the yearly cost (49%), and the replacement of the membranes at a little more than 10%.

The Netherlands has the highest nitrogen application for agriculture in Europe, with an organic fertiliser use per surface four times larger than EU average. At the same time, the country has a relatively larger area of aquatic and wetland habitats than other countries in the EU.

Consequently, nitrogen control technologies were developed early to control at least the point discharges, for instance:

Anammox technology: Anammox (Anoxic ammonium oxidation) is a new method of nitrogen removal from wastewater with partial nitrification that cuts short the traditional oxidation of ammonia in nitrite then nitrate. Compared to conventional nitrification/ denitrification, this method saves 100% of the required synthetic carbon source (methanol) and 50% of the required oxygen. This can lead to a considerable reduction of operational costs and energy demand, and a positive CO2 balance as the process actually consumes CO2. The process is most applicable on wastewater streams (or gases) high in ammonium (>0.2g/l) and low in organic carbon (C:N ratio lower than 0.15), such as sludge reject waters (sludge liquor) and industrial wastewater or leachates. The technology was first discovered by Gist Brocades on a pilot in Delft in 1988, and then developed at TU Delft University and applied by Paques for the first time at full-scale in Rotterdam, the Netherlands (Waterschap Hollandse Delta) in 2002.

With a volume of 72m3, the reactor was scaled-up directly from laboratory-size to fullscale and treats up to 750kg-N/d. In the initial phase of the startup, anammox conversions could not be identified by traditional methods and it took about three years to stabilise the reaction, despite a doubling time of 10-12 days.

The experience gained during this first startup in combination with the availability of seed sludge from this reactor, will lead to a faster startup of anammox reactors in the future., of which a handful are already operating. A project for a conversion capacity of 11tonnes of nitrogen per day, almost ten times larger than the largest plant built so far, is currently under construction in China for a large producer of glutamate, starch and amino acids from maize.

Babe technology: with a similar goal to Anammox to reduce the nitrogen returns, the Babe process (Bio-Augmentation Batch Enhanced) comprises a single batch reactor (SBR), which is fed with ammonia-rich wastewater such as reject water from sludge dewatering. The reject water in municipal wastewater treatment can have ammonia concentrations as high as 1,000 to 1,500mg/l, which enhances the development of the slowgrowing nitrifiers, together with the favourable temperatures of the return liquors from anaerobic digestion. The SBR reactor allows long sludge retention times to favour the growth of specialised nitrifying and denitrifying bacteria in the Babe reactor.

Return activated sludge from the main treatment is added to augment nitrifying microorganisms in the activated sludge flocs. About 50% of the ammonium reduction in the effluent is caused by the nitrification of the reject water in the Babe reactor. The other 50% is removed in the aeration tank by nitrifying bacteria washed out from the SBR reactor. This bio-augmentation results in an increased nitrification capacity of the activated sludge process in the mainstream and is an essential feature of the process design. The Babe process was developed by DHV in cooperation with the TU Delft and first tested in a full-scale installation at the WWTP Garmerwolde (NL) in 2002.

Nereda technology: while in the Babe reactor, the nitrifying micro-organisms are encapsulated in activated sludge flocs making them less prone to preferent grazing by higher organisms. TU Delft has discovered a new method to form granules of aerobic bacteria that sink quickly. This new aerobic granular sludge technology allows all processes in one reactor to concentrate, and avoids large clarification tanks. This needs only a quarter of the space required by conventional installations while, at the same time, using 30% less energy than the normal purification process. This process is suitable for both domestic and

industrial waste water, and various technological bottlenecks were solved during upscaling the test installation from 3l/hour to 1,500l/hour. The first installations are already in use in the industrial sector, with a full scale 3,000m3/day plant currently being constructed in South Africa, and final designs are ready for practical implementation for municipal applications.

One of the newest structures in the Dutch water landscape is the Wetsus Centre for Sustainable Water Technology. Aiming to discover sustainable solutions to tackle the impending shortage of fresh water, Wetsus brings together the expertise of top researchers at Dutch universities and knowledge institutions, industry, and government. In the Wetsus laboratory, various universities and institutes cooperate on a multidisciplinary basis. From the interface of separation technology and biotechnology, sustainable solutions for treatment and production of water are developed.

Crystallisation, membrane technology, adsorption and electrochemistry are linked to bioconversions in the research themes, grouped under six topics:

Salt and water (bioconversions at high salt concentrations, development of selective membranes for separation of specific components, etc.): Capacitive deionisation isone example of a new way of removing salts from liquid streams, placing electrodes in the stream to be treated so that Ions migrate to them to be adsorbed, leaving fresh water as a result.

Decentralised preparation of process and drinking water and treatment of waste water (development of efficient and cheap point-of-use and point-of-availability technologies, recycling of slightly polluted household water, etc.): Separation at source is one way to recover resources and save energy to achieve sustainability objectives like water reuse.

Membrane bioreactors (reduction of energy flux, increase of flux by preventing pollution, development of new applications): to make MBR more robust, less energy demanding and more economical, the main bottleneck is membrane fouling, which requires frequent cleaning with chemicals, and high energy consumption. To reduce fouling, its mechanisms need to be understood, and microorganisms, and membrane properties as well as operations need to be adapted to each other.

Biofouling of membranes for drinking and process water preparation (developing new methods of treatment and design rules): to control biofouling in practice, pretreatment can be extended to remove nutrients and micro organisms before the membrane. The cheaper alternative is cleaning the membranes more often, or dosage of biocides or disinfectants, leading to an increased use of chemicals and associated risks. The challenge is to develop membrane systems that are less susceptible to biofouling, or using chemical biofouling inhibitors, adapted operational conditions or new membrane module designs.

Energy from water (reuse of contaminants from water in the form of energy, direct production of hydrogen or electricity): Water treatment can play an important role in sustainable energy production, by producing bioelectricity from wastewater or from advanced mixing of salt and fresh water. Biofuel cells operate by converting organic material by means of bacteria into protons and electrons. The protons diffuse through a selective membrane and the electrons travel through an external circuit, thereby generating electricity.

An even more exotic theme is Blue Energy, where the difference in salt concentration between seawater and river water can be used to generate electricity by separating positive and negative ions by ion specific membranes. Mixing seawater and river water in this way would have no fuel costs and no emissions but brackish water, which is much easier to desalinate.

Sensoring to guarantee the supply of reliable water to customers: remote monitoring will be required for process and quality control. By specialisation, miniaturisation and innovation a growing number of parameters become measurable more easily, quickly and accurately, allowing to verify, control and optimize processes more reliably.

With all the investments into new sustainable water technology, the Netherlands could become serial awardees of the major recognitions in the water world between Stockholm and Singapore, and foster a booming business of green

technologies on the way.