500°C and potassium Methods

There are several reasons why Swiss farmers are not allowed to put sewage sludge on their fields: it is contaminated with persistent chemicals, pharmaceutical residues and toxic heavy metals, which seep into the ground and threaten water habitats. Therefore, thermal sewage sludge recycling via incineration is now standard. The disadvantage of this is that the phosphorus contained in the sludge, which has great agricultural value as a fertiliser, is lost. The Environmental Technology Group at the HLS Institute for Ecopreneurship, in cooperation with industry and research groups, has developed a technology that makes sewage sludge suitable for agricultural use. “The project aims to integrate the process as a component in existing sewage sludge treatment systems in order to achieve the broadest possible implementation,” says Thomas Wintgens, head of the group. The companies and research institutions involved in the project have practical experience covering the entire value chain from the sewage treatment plant to agricultural fertiliser use.

High temperatures destroy organic pollutants
Central to this innovative process is pyrolysis. Like combustion, pyrolysis is a method of breaking down organic compounds at high temperatures but with an important difference, as project manager Anders Nättorp explains: “During combustion, a surplus of oxygen is added so that carbon is completely converted into CO2. In contrast, with pyrolysis we add very little oxygen in order to obtain different thermal transformation products. This is a reductive process.” The new process also has a com- bustion stage that completely converts every single organic component. Operating temperatures above 500 degrees Celsius not only destroy the organic pollutants but also evaporate heavy metals, which can then be isolated from the exhaust gases. The research consortium already possesses experience using this technology with industrial and municipal waste. Although preliminary studies showed that the concept is also well suited for treating dried sewage sludge, the scientists had to overcome another challenge: plants could not absorb phosphorus from the pyro- lysis ash very well. “Pyro- lysis produces different mineral phosphorus compounds, some of which are so stable that even citric acid secreted by some plant roots cannot dissolve them,” says Nättorp.

Potassium turns pyrolysis sludge into fertiliser
The researchers overcame this by adding potassium compounds to the pyrolysis, leading to the formation of mineral compounds more easily absorbed by plants. “With pyrolysis fertiliser we achieved 90 per cent agronomic efficiency in experiments,” reports Nättorp. “The plants grew almost as well with the new fertiliser as those that received the same amount of conventional phosphorus fertiliser. The research association has patented the process and has already given a name to the fertiliser: Pyrophos. Before the sewage sludge ash can be marketed as a fertiliser, a final process step is needed for effective agricultural use. This concerns both the exact composition of the fertiliser and its form. Fertilisers for dry spreading have standard granule sizes and contain different components depending on the crop, especially potassium, nitrogen and phosphorus. With potassium as an additional pyrolysis reagent, the Pyrophos fertiliser provides two of the three most important plant nutrient requirements. If additional nitrogen is needed, it can simply be added.

Circular economy at the sewage treatment plant
As a test site for the Pyrophos process the researchers used the Altenrhein wastewater treatment plant, where sewage sludge from 300,000 inhabitants and food waste is digested and dried. It currently supplies the nearby cement industry with dry sludge as fuel, so the phosphorus is embedded in cement and lost. The Pyrophos consortium is now analysing whether this plant is suitable for a first implementation of the new process. “So far we have produced on a kilogram scale,” says Wintgens, “but we are now increasing to the tonne scale. Our aim is to show that our process can recover phosphorus economically and at a significantly smaller plant size than conventional sewage sludge treatment.”