CRAFT presents a novel idea

Chris Webb reports on the CRAFT researchers' zeoReactor WwTW technology programme

AWwTW, developed with the help of funding from the European Union's (EU) co-operative research action for technology (CRAFT) programme, could pave the way for better sanitation for smaller communities in Europe.

As UK water companies warn prices must increase to meet urgent infrastructural demands, WWT looks at a turnkey project that is due to reach its climax at the end of this year. The fruits of its labours could reach our shores as early as 2004.

The aim of the Urban Waste Water Treatment Directive, (UWWTD) is that by the year 2005 the majority of communities in the EU with a population of more than 2,000 should have access to adequate sewerage and a WwTW.

But, it is a fact that many smaller communities still do not yet have a WwTW, indeed, it is estimated some 40,000 plants will need to be built or renewed by the end of 2005 if the EU's water companies are to meet the new European water pollution limit values.

In addition to this, almost a third of the WwTWs that were built before 1992 will have to be renovated or replaced, according to Elmar Schmid, a technical director at the German Technologie-Transfer-Zentrum Bremerhaven (TTZ Bremerhaven), an organisation established in 1987 that brings under one roof no less than six established centres of excellence in applied research and development.

Together, these CRAFT researchers have developed the so-called zeoReactor project, which is designed to deliver a technologically ultra-modern, innovative and environmentally-friendly compact plant.

A prototype has already been installed at TTZ Bremerhaven and tests on this are reaching their climax. The next step for the consortium is to develop the model to meet the needs of factories and industrial sites. After which, the world is its oyster. The zeoRactor's innovative features include the ability to carry out nitrification and de-nitrification continuously. This is carried out using zeolites and highly effective bio-catalysts. Anja Degernhardt, TTZ's project leader, explains: "Also crucial, of course, is the fact the zeoReactor can be installed without huge constructional complexity and extremely cost effectively - it is therefore suited to the renovation of existing plants."

After an initial four-month test phase, the equipment has been installed in a food factory in Spain. The project reaches its climax in December and, following this, the zeoReactor partners expect to market it together "for the good of the European environment", says Degenhardt. So, it could soon be coming to a site near you. The increasing number of WwTWs will have an impact on the generation of sludge. According to an EC survey in 1994, 6.5M tonnes of sludge is produced each year in the EU. Some 40% of the sludge produced in the EU comes from just 1.3% of the WwTWs from large urban conurbations.

The EC expects an increase of 50-60% in the sludge generated over the next ten years.

The zeoReactor project is designed to address some of these problems. It is based on the combination of a solid bed, a liquid fluidised bed and a sequential batch system with zeolites as biological carriers for effective nitrification and de-nitrification. The system's originators say the concept can be used to retrofit existing WwTWs just as effectively as it can in new, compact and integrated small-scale systems.

They say the development of the zeoReactor has the potential to provide substantial savings in capital costs and life-cycle costs for nutrient removal for new and existing WwTWs.

The advantages of using zeolites as a bio-film carrier is due to the large specific surface area on which the micro-organisms can settle are relatively well-known.

Significantly higher biomass concentrations compared to traditionally activated sludge processes are achieved, leading to a reduction in sludge generation. Typical values of the settleability index for WwTWs are greater than 200ml/g. CRAFT researchers say that with the zeoReactor, the average value can be less than 100ml/g. The higher the microbial activity, the more oxygen is necessary for the degradation processes.

In conventional systems the aeration would have to be increased, which would result in high energy costs because the oxygen concentration of the air is only 21%. In the zeoReactor concept the supplied air is oxygen enriched by a pressure swing adsorption (PSA) unit. Researchers have achieved an enrichment of up to 50% oxygen, which cuts in half the amount of air needed and reduces energy necessary to undertake the process.

Moreover, the diffusion velocity of the oxygen into the liquid phase is much higher with concentrations above 50%. So far, researchers have been able to design a bio-reactor suitable for equalising strong fluctuating inflows and concentrations and able to obtain very high degradation results in an extremely compact unit for small or remote communities or for applications where space is at a premium, such as on board cruise ships or ferries. Researchers say the advantages of the zeoReactor are:

  • better sludge settleability, enabling the plant to operate at a higher solids retention time and thereby improve effluent quality and/or increase the plant capacity,
  • during fluidised bed operation, the micro-organisms are equally loaded and aerated so that the greatest possible technological use of the biology is realised,
  • gas/liquid phase crossover is improved and energy is saved by using oxygen-enriched air for the aeration - due to the high ion exchange-capacity contamination peaks can be easily buffered,
  • through the solid zeolite particles, which are in continuous motion and rub and grate on each other, the activated sludge flakes are reduced to smaller particles. The specific surface of the sludge flake increases and the biological activity is improved significantly,
  • through the continuous motion of the carrier material, injected air bubbles are reduced in size, leading to a substantially higher specific surface and better gas/liquid phase crossover. In parallel, the air bubbles are kept longer in the reactor since the fluidised bed prevents the injected air bubbles from rising quickly. The oxygen intake is improved and operational costs are reduced,
  • nitrification occurs in a miniature reactor. The nitrification organisms settle on the zeolites and are kept in the activated sludge reactor. Through the nature of the zeolites, de-nitrification occurs simultaneously. In this way, discharged nutrients are reduced,
  • significant reduction in sludge generation thro- ugh increased dry subst- ance contents.

Several wastewater treatment systems already use zeolites as a support material, especially in Japan and the USA, where zeolites are used in adsorption as a material for heavy metals or for anaerobic treatment of wastewater. But CRAFT researchers claim there is no similar aerobic system that uses pure natural zeolites in aerobic conditions in connection with oxygen enriched aeration. The research partners have divided their efforts into eight individual work tasks. The first of these relates to dealing with issues related to the environment where the zeoReactor will operate.

It was always accepted, for example, that factors such as the changing composition and amounts of the wastewater would seriously influence the design of the system. The consortium has focused primarily on keeping its solution inexpensive and simple.

In this way, the major goals of the project have leant towards the development of a versatile means of process control and the utilisation of easily affordable materials.

Consequently, the prototype takes into account the requirements of end-users, its characteristics having been agreed with all of the project's partners involved in the first stage of the research, in accordance with ISO 14000 environmental management standards.

TTZ was initially involved in simulating the performance of the prototype using a computational fluid dynamics (CFD) model to optimise design and performance prior to the unit's construction.

Several design options were tested with the aim of overcoming the many drawbacks of previous processes. In addition, TTZ conducted a workshop on the CAD/CFD - design of the wastewater treatment reactor, on the one hand to ensure the technology transfer into SMEs and on the other hand to collect a technical feedback of the SME engineers.

To date, the zeoReactor system has been tested under real operating conditions. The latest news is that, based on these tests, TTZ has developed the basics for an upgrade kit for existing WwTWs. This upgrade kit will be used for WwTWs that cannot fulfil the increasingly stringent legislation on discharge quality.

Researchers believe the fruits of their labours will have a significant impact on wastewater treatment in the years to come, particularly for smaller outlying rural communities



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