In the WaterLab, biotechnologist Steef de Valk is carrying out experiments with tubifex worms. These threadlike creatures are capable of reducing by half the quantity of sewage sludge from water treatment. No-one really understands how this works.
When Steef de Valk removes the worms from their container, it looks like he’s holding a ball of dark-coloured slime. “They always do that when they’re stressed”, he says. “They all clump together in a ball.” Changes in temperature, movement, being picked up – these are all things that upset tubifex worms.
But there is one thing that these threadlike creatures can handle very well: chemical pollution. Insecticides and heavy metals leave them completely unfazed, within certain limits of course. These particular worms come from river sludge in Poland, because our rivers are too clean for tubifex; they are forced out by other creatures.
PhD candidate De Valk puts the clump of worms in a glass jar and stirs the water carefully. “From a chemical point of view they are very strong, but they have a fragile body.” And indeed – the worms emerging from the clump are no more than a millimetre thick and a centimetre long. They are red in colour because, just like humans, their blood contains a sort of haemoglobin. They extract oxygen from the water by waving their tails, while ingesting sewer sludge at the other end. Yum. After a few minutes the worms begin to clump together again, forming three balls. “I don’t know why, but they just seem to like being together.”
De Valk isn’t the only person with a fascination for worms; he is following the example of Charles Darwin. In the book on worms that Darwin wrote in 1881, he asked himself: ‘It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures.’
The rectangular glass tank in which De Valk conducts his experiments is a scale model of a 125 m3 tank that was situated at the sewage treatment plant in Wolvega from 2007 to 2013. TU Delft alumnus Jelmer Tamis was involved in this as a biotechnologist. In his final report for STOWA, the Foundation for Applied Water Research, in 2010, he reported that in two years’ time, worms had broken down 193 tonnes of sewage sludge (measured as dry product) of the 303 tonnes offered. Breaking down the sludge is of great economic importance – transporting sludge to incineration plants accounts for around half of the operating costs of wastewater treatment companies.
Tamis summarised the results of this practical experiment: a worm reactor can easily be added to the treatment process (see chart) and can achieve breakdown rates of 60 to 70 percent. Thanks to improved anaerobic digestion and biogas production, the process produces more energy than it uses. This all sounds very positive, but the fact that no-one really understands how the worms can break down so much material continues to nag us. Because if you don’t precisely understand the process, it is hard to optimise it.
Research into sludge breakdown by worms was prompted over thirty years ago by a chance observation during a graduation research project at Wageningen University. This research began with free-swimming worms and later included sessile worms that behave in the same way as tubifex. The researchers reported a huge rate of sludge breakdown, up to 75 percent, but the stability of the population was a recurrent problem. During the last ten years a great deal of research has been carried out by Professor Yuang-song Wei (Research Center for Eco-Environmental Science) who became familiar with the technology in 2001/2002 while working as a postdoc at TNO.
In Wolvega the researchers also experienced problems with the worm populations initially. In the first weeks of 2007 the colony (Aulophorus) grew rapidly and successfully, only to collapse in week 7. The Aulophorus was also supplanted by another worm species (Lumbriculus).
It took some time for the researchers to work out how to manage the worm populations in order to reverse the cycle of growth and collapse and to achieve a stable population. For this the population must be managed in terms of density, age and the worm/sludge ratio. And to make things even more difficult: sudden changes in temperature, nutrients and current can also upset the worms.
The researchers were finally able to create a stable worm reactor for the Wetterskip (water board) that remained operational until 2013. In 2007 the sludge breakdown rate was 66 percent, and in 2008 61 percent. After 2013 the reactor was dismantled to make way for an expansion of the plant at Wolvega.
Back at the Delft WaterLab, Steef de Valk is very familiar with the Wolvega reactor – as he did an internship at Tamis. The worms used in Delft (tubifex) live on grids suspended in the wastewater on one side of the test reactor. De Valk gets their ‘food’ from the Harnaschpolder wastewater treatment plant. The blackwater is kept in a tightly closed jerrycan. A dividing panel separates the test reactor into two halves which are completely identical, apart from the worms. This enables him to precisely measure the worms’ influence on their environment, because the experiment in Wolvega had already led to a number of questions, such as: were the worms responsible for the unexplained breakdown, was it the
bacteria in their gut or was it an enzyme produced by one of the parties? De Valk: “We know that worms accelerate the breakdown process and that during the digestion process they raise the production of biogas as they themselves disintegrate in the digester. But what is the explanation in terms of enzymes? If we knew that, we would be able to create the enzyme using a modified bacteria. If you then added the enzyme during the digestion process, you would achieve the desired results.”
It is not yet clear who or what is producing the enzyme in question. It may be produced by the worm, or possibly bacteria in the worm’s gut. And it could be that a whole series of enzymes is responsible for the breakdown process. “The whole research is a puzzle”, says De Valk, who wants to spend the coming time carrying out genetic tests in order to gain more insight into the bacteria involved. He hopes that comparing RNA fragments from the wastewater with a database will enable him to find out which bacteria are characteristic of the worm reactor.
“That sounds pretty desperate”, says Jelmer Tamis, who is currently also a doctoral candidate at TU Delft and is in the process of completing his dissertation on bioplastics. “I don’t see much point in it. Just compare the result with and without worms and you can see the breakdown with worms is ten times as fast. The worms are the catalyst for the breakdown. And even if the mechanism for this is not known, you can see something is happening. If you want to achieve something useful, you need to build a large reactor. If you want to carry out fundamental research, you can investigate how it all works, but for myself, I’m more interested in using it in practice.”
His report for STOWA includes a new design for a full scale worm reactor (600 m3) in the form of a ring around the sedimentation tank. This would require an estimated investment of 700,000 euros as well as the installation of a digester. When questioned, spokesman for the Wetterskip, Michiel Zijlstra said that this was a bridge too far for the water board. “All in all it was just an experiment. If some other businesses had also been willing to invest, we might have considered it. But this didn’t work out, and we thought it was just too big an investment to be paid for by public money.”
Tamis still thinks it is a pity that things turned out the way they did. “We’re just too penny-pinching here in the Netherlands. Our calculations showed that the investment would repay itself within three years through the savings made on sludge transport, and energy production. The Chinese don’t have to think twice about it. They’ll soon be the ones doing it.”
Read the STOWA report (in Dutch): Slibafbraak door Oligochaeten, 2010. Freely available on internet.
Worms are a rich source of protein, but if they have been raised on sewage sludge they are not allowed to enter the food chain. This is not the case for the worm species (Lumbriculus) raised on products from the food industry. These worms form an excellent alternative to the relatively unsustainable fish meal presently used as fish and livestock feed. Dr Hellen Elissen used worm-breeding technology to launch her startup company TailTec from Wetsus. She develops customised breeding reactors that provide optimum conditions for breeding worms using organic waste from the food industry. This could be an alternative to insect breeding. Hellen Elissen (environmental technology WUR, 2007) is presently in discussion with customers to realise a first