Aquafer is a sludge consisting of ferric oxide/ferric hydroxide, which is a residual of drinking water treatment processes. Reststoffenunie (now AquaMinerals) looks after the disposals of the aquafer for the water companies. Of the annual production of about 50,000 tons of liquid aquafer, approximately two-thirds is sold to biogas energy plants and one-third to WWTPs. Of the annual production of about 20,000 tons of dewatered aquafer, approximately three-quarters is consumed in biogas reactors – this sector’s demand for the residual is growing rapidly.
Liquid iron(hydr)oxide can be transformed into granular iron(hydr)oxide through granulation. These granuals can be used as filter medium for gas or air cleaning at biogas plants. The application involves binding hydrogen sulphide in the gas phase. This reduces the corrosive properties of the gas, which means that there is no longer a need for over-dosing iron in the fermentation phase; after all, only in the relevant gas phase the sulphide does need binding. This also means that phosphorus remains available for recovery in the digestate. The granular ferric oxide is also recoverable.
These advantages give granular iron(hydr)oxide added value, lead to higher-value sales and significantly increase the application possibilities and market value of biogas. On the other hand, the production of the granular iron(hydr)oxide involves additional costs. These additional costs and the revenues from the treated biogas are key considerations.
The project began with a technical literature review. This showed that granular iron(hydr)oxide can be used to directly remove hydrogen sulphide (H2S) from biogas, and is particularly suitable for application in low flows of digestor gas, such as those from co-digestors and WWTPs. It is possible that granular iron(hydr)oxide can remove siloxanes as well.
The binding of H2S in the gas phase by granular iron(hydr)oxide was researched on a lab scale. The exploratory tests were successfully completed and showed the potential of granular iron(hydr)oxide for binding H2S. The follow-up experiments were mainly focused on determining a reliable decision curve. Supplementary experiments might be conducted into the impact of loading levels, siloxane and/or moisture content in the gas, as well as the size of the granuals on the H2S removal.
Parallel work was done on the development of the logistics value chain and market strategy, so as to take the interest expressed by the market into account.
At the conclusion of the project, the use of granular iron(hydr)oxide for the removal of hydrogen sulphide from biogas on a semi-technical scale had been researched, and a sustainable logistics value chain had been developed.