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Granular sludge batch sequential reactors for water purification

21st February 2013


Delft University of Technology researchers have discovered a method that within few years could drastically change the way water is purified.

In partnership with DHV engineering bureauthe university has developed a compact and environmentally-friendly purification methodin which aerobic bacteria form granules that sink quickly.

With the new aerobic granular sludge technologyknown as Neredaaerobic bacterial granules are formed in the water that is to be purified.

The great advantage of these granules is that they sink quickly and that all the required biological purifying processes occur within these granules (Fig.1).


Important advantages

According to Delftthe technology therefore offers important advantages when compared to conventional water purification processes.

For exampleall the processes can occur in one reactor – a granular sludge sequential batch reactor (GSBR). Moreoverthere is no need to use large re-sinking tankssuch as those used for conventional purification.

Such large tanks are needed for this because the bacteria clusters that are formed take

much longer to sink than the aerobic granule sludge.

According to Delft PhD researcher Merle de Kreukwho has just received her PhD degree based on this researcha Nereda purification installation needs only a quarter of the space required by conventional installations. MoreoverNereda uses 30percent less energy than the normal purification process. In additionit is suitable for both domestic and industrial wastewater.

Delft University of Technology has a long tradition in researching the possibilities of water purification with aerobic granular sludge.

Technological bottlenecks

The maturation of the technology is largely due to the research conducted by De Kreuk. During her PhD research with Professor Mark van LoosdrechtDe Kreuk – working together with DHV engineering bureau and supported by STOWA and STW grants – solved various technological bottlenecks and expanded the capacity of the test installation from three litres per hour to 1500litres per hour.

DHV now has the final designwhich is ready for practical implementation.

The aerobic granular sludge technology is very promisingand has been nominated for the Dutch Process Innovation Award 2006. The technology is now in the commercialisation phase.

In the coming yearsDe Kreuk will continue to contribute to the project’s trajectory as a Delft researcher. DHV is currently negotiating with water purification companies to test this purification method on a larger scale. The first installations are already in use in the industrial sector.”

The case for GSBR

According De Kreuks’s PhD thesisthe experimental results obtained at laboratory scale were used to design two full-scale GSBR-based sewage treatment plants.

Beforehanda desk study was carried out using two different types of GSBR – one with pre-treatment and one with post-treatment.

The study was based on a population of 119000 and compared the new technology with conventional treatment based on activated sludge technology and biological phosphate removal.

In terms of total annual coststhe GSBR options were more attractive by between 7 and 17percent. Alsoin a sensitivity analysisthe GSBR technology appeared to be even more attractive at low ratios of rain weather/dry weather flow and where land prices are high.

Because of its high volumetric loadthe GSBR footprint is only 25percent of a conventional plant. As the granular sludge is cultivated in a one-reactor systemthere is no need for recycling flows of wastewater and sludge.

Less energy

In additiondepending on the design of aeration used in a GSBRthe system requires about 30percent less energy compared with activated sludge systems.

“A growing number of sewage treatment plants in the Netherlands are now faced with more stringent effluent standards.

“In generalactivated sludge plants will have to be extended with a post treatment step such as sand filtration or transformed into a membrane bioreactor.

“In this case a GSBR variant with primary treatment as well as post treatment will be an attractive alternative” she concluded.

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