Green Gas from Sewage Sludge

Can sludge from sewage treatment plants be ‘upgraded’ to green gas via supercritical water gasification technology?
SCW Systems from Alkmaar is the first company in the world with an industrial-scale water gasification plant.SCW will conduct extensive research into the possibilities on behalf of knowledge centre Stowa (Stichting Toegepast Onderzoek Waterbeheer) and eight water boards.

Hoogheemraadschap Hollands Noorderkwartier (HHNK), the water board for North Holland above the North Sea Canal, is also among the group of clients. Indeed, HHNK - headquartered in Heerhugowaard - has long been committed to getting the study off the ground. According to Marjan Leijen, member of HHNK's executive committee and portfolio holder for the water chain, the technology is ‘a radical innovation’. ‘’If successful, this is going to mean a lot to society and how it can deal with different waste streams,‘’ she writes enthusiastically in a memo to other water board members.
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Supercritical water gasification can produce green gas from organic materials such as vegetable, fruit and garden (GFT) waste, plant and animal residues, agricultural biomass, but also sewage sludge, for example. By bringing water in these residual/waste streams to high temperatures (above 375 degrees Celsius) and high pressure (over 221 bars), rapid chemical reactions are caused that produce so-called syngas. This can be treated and upgraded to green gas.

‘’This offers the opportunity for final processing of sludge without incineration, the production of significantly more green gas and the simpler and more environmentally friendly design of the treatment process,‘’ Leijen said.

‘’In fact, it mimics the natural process, where once organic material deep in the ground is converted into natural gas. This takes millions of years, but with supercritical water gasification, it happens within minutes.‘’

The method has been known for some time and is being experimented with on a laboratory scale worldwide. SCW has started a plant in Alkmaar. The construction was partly achieved through 280 million euros of (European) subsidy for stimulating renewable energy. The sewage sludge research is expected to take two years and cost over a million euros. Almost half will be paid by SCW, with the rest coming from Stowa and the water boards. HVC in Alkmaar, the waste processing and energy company, is also contributing.

Sewage is treated by water boards in plants using bacteria. The bacterial mass is called sewage sludge. This is continuously removed from the process and thickened (dewatered).

HHNK produces about 100 million kilograms of this dewatered sludge annually. A small part of this is converted into biogas via digestion tanks, from which electricity and heat are used on site or supplied to the natural gas grid. The rest is dried in Beverwijk, producing 20 million kilos of sludge granules (pellets), which are burned at HVC as biogas feedstock for the heat grid and cement industry. With supercritical water gasification, sludge may no longer need to be dried and incinerated in the future. The residual product will then be fully decomposed and converted into energy.

Source: Noordhollands Dagblad/Alkmaarsche Courant
Photo: HHNK/Chris Pennarts

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