Waste toilet paper (WTP) is not often considered an asset. In fact, most people usually prefer not to think about it at all. Yet it is a rich source of carbon, containing 70–80 wt% of cellulose on a dry basis. On average, people in Western Europe produce 10–14 kg waste toilet paper per person per year. Accumulating in municipal sewage filters, it is a modest yet significant part of municipal waste. The ultimate waste has a negative cost

Waste toilet paper from sewage treatment plants contains up to 80 wt% cellulose. Photo: UvA/HIMS.

At the same time, waste toilet paper is a businessman’s dream because it is one of the few raw materials with a negative cost. While this may vary across countries and regions, in the Netherlands wastewater treatment facilities pay around 70 €/ton to get rid of WTP. It is therefore an extremely attractive resource since people will actually pay you to take it off their hands. Being such ‘true waste’, WTP offers a great opportunity for closing loops, increasing resource efficiency and creating a truly circular economy. For the UvA’s chemists, using WTP as a resource for generating electricity therefore ‘the ultimate waste recycling concept’. The Amsterdam region alone generates some 10.000 tons of WTP per year, enough to power 6400 homes.

In Amsterdam the sewage treatment facility and the waste-to-energy company are located right next to each other. Photo: AEB website.

What's more, since the cellulose in WTP comes from trees, the electricity produced is renewable. This offers a great opportunity for matching society's demand for renewable energy. Renewable resources often show discontinuous peaks. Plant stems can be recycled, but only after the harvest; sunlight is available in the daytime (and depends on cloud cover); and wind supply is as predictable as the weather. Waste toilet paper, however, is a continually available resource. Two-step process The Amsterdam-Utrecht research project, led by UvA professors Gadi Rothenberg and Bob van der Zwaan of the UvA’s Van ‘t Hoff Institute for Molecular Sciences, proposed a simple two-step process for the conversion of WTP, creating a direct route from unwanted waste to a useful product.

Master's student Els van der Roest examined the possibility of combining devices for the gasification of WTP (step 1) with high-temperature solid oxide fuel cells (SOFCs) able to directly convert the WTP-gas into electricity (step 2; see Figure below).

Waste toilet paper can be converted to electricity by gasification followed by reaction with air in a high-temperature fuel cell. Image: UvA/HIMS.

The project's goal was to assess the feasibility of such a WTP-to-electricity system at a scale of 10.000 ton WTP per year, based on real-life parameter values. Using techno-economic analysis methods, the team presented a basic process design, an overall energy balance and an economic study for this concept. Data for the experiments and calculations were obtained in collaboration with the Amsterdam waste-to-energy company (afvalenergiebedrijf, AEB). Efficiency of a natural gas power plant In an open-access paper published in the international peer-reviewed journal Energy Technology, the researchers present the basic system design, as well as its electricity yield and overall efficiency, based on detailed mass and energy balance calculations.

Els van der Roest. Photo: KWR Watercycle Research Institute.