3 ETE discoveries of the past 50 years: 2. Cost-effective sulfur recovery from waste water

In the early 1990’s Cees Buisman, professor at Environmental Technology, and Paques BV developed a revolutionary cleaning device, called the Thiopaq. It makes wastewater cleaning more efficient and cost-effective. As a result, the principle is applied at world-wide scale.

Wastewater cleaning is a complicated process, involving several steps, like bacterial conversions Wastewater cleaning plant Industriewater Eerbeek BV, in The Netherlands, purifies wastewater from three paper mills in the area using the UASB reactor principle in combination with another ETE invention: the Thiopaq. ‘First, we remove the paper sludge by allowing it to sink to the bottom. The sludge is subsequently burned’, Jan Moorman, Head Technology explains. ‘Then millions of microorganisms clean the water under anaerobic conditions.’ After this first cleaning step the water is oxygenated by aeration and aerobic microorganisms take on the remaining chemicals. Using these principles, the company daily purifies 12 million liters of industrial wastewater with 99 percent efficiency.     

Dangerous gas
Anaerobic microorganisms play a dominant role in the water-cleaning process and are responsible for the conversion of three quarters of the contamination into biogas. A smelly substance, containing methane (CH4) and the notorious and highly toxic hydrogen sulfide (H2S). Biogas is a perfect energy source, but removal of H2S is needed. This can be achieved using sodium caustic gaswashers, since this dangerous gas easily dissolves in liquids with high pH, such as a sodium hydroxide solution. By guiding biogas through this solution, H2S is converted into sodium sulfide, that is subsequently converted into sodium sulfate in the final oxidation step. This salt is a potential environmental hazard, because it leads to eutrophication and salinization. In addition, the method is expensive: on a yearly basis it costs about 150.000 euro worth sodium hydroxide for Industriewater Eerbeek BV.

High-quality sulfur

In 1993 the company had a world premier: it installed a so-called Thiopaq, a cleaning device for biogas, developed by ETE professor Cees Buisman who worked at the company Paques BVat that time.This was the first commercial application of this new device. Soon it became clear that the Thiopaq beat all expectations. Due to an efficient bacterial conversion, it removed H2S more efficient and more cost-effective than the old method using sodium hydroxide solution. ‘In one single step the bacteria convert 99 percent of the H2S into pure, high-quality sulfur’. Cees Buisman says. ‘Such a high quality is not easy to produce using other methods. In contrast with sodium sulfate recovered by using the old method, the pure sulfur has quite substantial value.’ In addition to environmental gain, costs of sodium hydroxide dropped by more than 90 percent.


Today, over 350 Thiopaqs have been sold world-wide. The company Paqell, a joint venture between Shell and Paques BV, sells a large version of the Thiopaq that removes sulfur from natural gas at a huge scale. ‘I think it is remarkable and quite spectacular that the chemical world decides to use a technology that is based on a bacterial conversion process from Wageningen’, Buisman says.
A Thiopaq installation
A Thiopaq installation