A clump of bacteria

Coming clean � the future of sewage

Breakthroughs in modern sewage-treatment technologies are creating economies of scale and environmental benefits. We talk to Professor Mark van Loosdrecht, whose brainchild 'Nereda' is helping sewage to clean up its act.

Back in 2005, environmental biotechnologist Professor Mark van Loosdrecht unveiled a technology that appeared to have the potential to change the face of modern sewage treatment. Named after the Nereids – water nymphs from Greek mythology – the Nereda technology, according to its inventor, could make a huge difference to the impact of sewage treatment on the environment. Plant sizes would be reduced by three-quarters, while energy consumption could be cut by as much as 40 per cent.

Currently applied to a dozen industrial and urban wastewater treatment plants, the technology is being taken up quickly, with at least another 40 plants planned or proposed for sites as globally distributed as Australia, the Middle East and Brazil. Van Loosdrecht is confident that a decade after his novel sewage treatment design was announced, it will soon be cheap enough to move into developing nations. "The costs are strongly reduced, energy consumption is strongly reduced and, most importantly, the amount of mechanical equipment is minimised. This is so important for countries that have problems with their national budget," he adds.

Sludge discovery

The first great breakthrough for sewage treatment came in the early 20th century when UK engineers Edward Ardern and WT Lockett discovered activated sludge. Now forming the backbone of modern sewage treatment, this sludge is a mixture of inert solids from sewage and microbes, and is used to treat sewage and industrial wastewaters.

But while the original activated sludge defined sanitation in the Western world, engineers and scientists have since been working to refine the process to boost sewage-treatment plant efficiencies and cut costs. Van Loosdrecht is among a handful of researchers that have led the pack.

It all started in the late 1960s, when Professor Gatze Lettinga from Wageningen University in the Netherlands noticed that under certain conditions the bacteria in activated sludge would spontaneously clump together to form granules rather than flakes, as had previously been observed. Such a phenomenon was considered important, as this sludge would sink more rapidly than conventionally dispersed or flocculent sludge – known as 'floc' – during wastewater treatment, speeding up water purification.

Crucially, this fast-sinking granular sludge also meant gravity-based solids-liquid separation could take place in a single treatment reactor, removing the need for a separate sedimentation tank and more than halving the plant footprint. In the following decades, Prof Lettinga developed his discovery, pioneering the so-called upflow anaerobic sludge blanket reactor. Today, 3,000 reactors are in operation, making up around 80 per cent of all anaerobic-based treatment systems worldwide.

But van Loosdrecht was keen to explore a different route. While the bacteria in anaerobic granular sludge remove organic contaminants in wastewater without oxygen – reducing energy consumption – and also produce biogases such as methane for potential electricity generation, hitches have always existed.

First, the process is less effective at low temperatures. And second, after treatment, algae-growing nutrients including phosphates and nitrogen compounds are left behind in the wastewater, which have to be removed using further complex biological treatment.

As van Loosdrecht puts it: "Nitrogen removal is one of the more complex parts of wastewater treatment, and if you do not remove it, your water quality will be poor due to algae growth and possible toxic algae."

The rise of granular sludge

With this in mind, van Loosdrecht decided to cultivate granular sludge under aerobic conditions and produce a more streamlined and effective process. His first challenge was to tease aerobic bacteria into a granular structure; the anaerobic microorganisms from Prof Lettinga's granular sludge tend to naturally form granules, but these bacteria and protozoa do not.

By experimenting with different bacteria treatments, turbulent flow conditions and more within aerobic reactors, van Loosdrecht proved his aerobic bacteria could form a granular sludge, and the race was on to develop a reliable process. In the late 1990s, his team joined forces with Netherlands-based engineering consultancy Royal HaskoningDHV. By 2002 they had established stable laboratory-scale granulation – the aerobic sludge granule.

What makes the aerobic granule special is that they tend to settle faster than conventional flake-like flocs, removing the need for extra sedimentation tanks and reducing overall plant footprint. Crucially, while the various bacteria within a conventional floc are scattered randomly across the aggregate, the bacteria within these granules organise in such a way that the key biological water treatment reactions can take place at the same time. For example, oxygen-hungry nitrifiers concentrate around the outside of the granule ready to oxidise ammonia to nitrogen gas.

Meanwhile nitrate-reducing bacteria, known as denitrifiers, and phosphate-accumulating species – that don't require oxygen to function – are sited towards the centre of the granule core. This clever distribution of different microorganisms removes the need for additional sludge recirculation steps to ensure all reactions have taken place while reducing the number of water clarification tanks.

With thoughts of smaller, more efficient waste treatment plants in mind, van Loosdrecht and colleagues quickly started to scale-up laboratory operations. In 2003, the first Nereda pilot plant was opened at STP Ede, the Netherlands, and two years later the first industrial full-scale Nereda retrofit was installed.

In 2007, the Dutch Foundation for Applied Water Research and six Dutch Water Boards joined the team to set up the Dutch National Nereda Development Program. Come 2012, the first full-scale system for domestic wastewater purification was opened by the Veluwe Water Board at Epe, the Netherlands. The €15m plant comprises three 4,500m3 Nereda reactors, with a capacity to process 36,000m3 of wastewater a day for some 60,000 people, and nearly doubles the plant it replaced without taking up additional space.

Today, 12 full-scale Nereda plants are in operation: a plant in Garmerwolde services 175,000 people, while more than 20 systems are under development across Brazil, South Africa, Portugal, the Netherlands, Switzerland, Ireland and the UK. All in all, these facilities are as much as four times smaller than the conventional equivalent plant and reduce energy consumption by up to a third.

Andrew Thompson, technical manager of wastewater and sludge at Imtech Water, Waste and Energy, is just one of many in the industry that believes the Nereda process is on the brink of a major international breakthrough. Late last year, his company struck a deal with Royal HaskoningDHV to deliver the technology across the UK and expects to build up to eight treatment plants in the coming years, as conventional wastewater facilities come up for renewal.

"The reactor size of the Nereda process is much smaller than conventional [facilities] and we expect power consumption to be reduced by almost a half," he says. "So when you start thinking that the wastewater industry is responsible for around 1 per cent of the UK's power consumption, of which aeration, secondary treatment, is half of that, you can suddenly start thinking about shutting power stations down."

Thompson will not be drawn on actual reductions in capital expenditure, but reckons figures could fall by up to 20 per cent. He says: "It's the operating costs that are massively lower. You need much less aeration, and half the overall power consumption.

"When I tell potential clients you can combine this with a lower footprint and a better effluent quality, they don't believe me some of the time," he laughs.

Right now, three pilot plants have been set up in the UK, the first at Davyhulme, Manchester – home to the inventors of activated sludge – with the others at Dalmarnock and Daldowie in Scotland. Initial results from the Manchester plant show a very high effluent quality, and as Thompson predicts: "What I expect will happen is we'll get three to four plants built and then UK industry will take a pause to see how these facilities perform, and then the floodgates will open."

But for van Loosdrecht, Nereda has been only the beginning. The researcher has pioneered several other wastewater treatment processes, the most notable called 'Anammox'.

Like Nereda, this is a granular sludge, but the microorganisms are anaerobic, rather than aerobic, and are used to treat high-concentration ammonia sewage liquors, converting ammonia directly into nitrogen gas. This direct conversion vastly reduces the energy consumption of a wastewater treatment, with van Loosdrecht's calculations putting savings at nearly 50 per cent compared to conventional treatment.

"This is a much more efficient way to remove nitrogen and also means you can now use [more bacteria] to generate biogas," he says. "An entire treatment plant can now be energy neutral, or even energy producing."

Van Loosdrecht expects Nereda to become a major municipal wastewater technology in the coming decade and, as he points out, while Anammox can be integrated to this process to further reduce energy demand, it very much has its own market appeal. But he is also looking to take his cheaper, smaller footprint and energy-saving technologies to developing nations.

Helping developing nations

Less than a tenth of sewage in developing nations is treated and, as van Loosdrecht highlights, conventional centralised activated sludge systems often fail as many are simply replicated without taking into account local conditions such as sewage characteristics, temperature and even operator experience. Twinned with the fact that governance from responsible institutions is often weak, for him, smaller, simpler, community-managed, decentralised systems are an answer.

Indeed, successful trials of Anammox in China – nitrogen pollution has created severe environmental problems across the country – bode well with this and Nereda processes becoming established in Brazil. Critically, progress from researchers worldwide means valuable resources such as ammonia, water and even bioplastics could be recovered from these technologies to boost affordability.

"Investments are much lower per unit so these systems are easier to introduce, at least for those people who think it is important to implement," says van Loosdrecht. "Decentralised systems are easy to manage... and give the opportunity for individuals to make decisions."

Thompson also believes that van Loosdrecht's alternative waste treatments are suitable for developing nations. But, as he says: "The technology is quite young and we have to get it established first.

"In the UK, the biggest problem I've been coming up against is the natural cautiousness of the water industry. Most water companies like to be third or fourth in line when adopting a new technology. So somebody's got to jump first," he adds. "But in my mind we should never build another activated sludge plant." *

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