Testbed Ellinge: Preliminary conclusions to date
Last week, VA SYD held its seventeenth steering group meeting as part of the Testbed Ellinge project. Ahead of the meeting, preliminary conclusions to date were summarised.
Production
- Mercury is separated from the sludge biochar at all temperatures (500–900 °C).
- A temperature of 650 °C produces sludge bicohar with a cadmium content that meets Revaq’s long-term target of 21.4 g Cd/kg P. At 750 °C, very little cadmium remains.
- A limited proportion of the mercury and cadmium (20–30%) is separated out with the scrubber water.
- Other heavy metals are concentrated in the sludge biochar compared with the sludge itself, but as the phosphorus remains, the amounts of heavy metals on the arable land do not increase, since the application rate is determined by the phosphorus content. However, there are two exceptions: chromium and nickel, the levels of which increase in the sludge biochar due to erosion caused by machinery.
- This concentration and the resulting inputs mean that some treatment works may find it difficult to meet the concentration limits for certain heavy metals.
- Undigested sludge generates more carbon that remains after sludge treatment compared with anaerobically digested sludge.
- Only 10–20% of the sludge’s nitrogen is found in the sludge biochar, and 10–20% of the nitrogen ends up in the drying condensate. The rest is carried away with the flue gases.
- The concentrations of various substances in the sludge biochar vary depending on whether the sludge is anaerobically digested or undigested, as well as on the operating temperature in the pyrolysis furnace and the amount of precipitation chemicals used in the wastewater treatment process.
- The pipe between the pyrolysis furnace and the gas burner – the so-called pyrolysis gas pipe – becomes blocked daily, and the design and operation of this pipe are important factors to consider in full-scale applications.
- When the pyrolysis gas pipe begins to become blocked, there is a risk of pyrolysis gas escaping into the operating area if the system is not completely airtight. During normal operation, there is sufficient negative pressure in the system.
- When the pyrolysis gas pipe begins to become blocked, the PAH content in the sludge biochar increases and may exceed the limit values.
- Concentrations of brominated flame retardants, polychlorinated biphenyls (PCB7) and per- and polyfluoroalkyl substances (PFAS22) are almost always below the detection limit.
- The Revaq certification system should take into account the removal of mercury and cadmium, as well as organic micropollutants, in its regulations concerning sludge biochar.
- The operating settings for sludge drying are important to ensure that the sludge does not become too dry – so that it does not burn or undergo pyrolysis whilst still in the dryer – and that it does not become too wet, which could cause the dryer outlet to become blocked or make it difficult to maintain the pyrolysis temperature. It is therefore important, at full scale, to have the option of drying the sludge only. In our pilot, wetter dewatered sludge requires a longer drying time and a higher drying temperature.
- At full-scale, heat exchanger blockage appears to be a major problem. We are glad that we did not choose heat exchangers in our pilot.
Climate impact
- The carbon sink provided by sludge biochar is of the same order of magnitude as the reduction achieved by avoiding sludge storage.
Market, business models and financing opportunities
- There are few pyrolysis furnace suppliers on the market, and they have not supplied very many units each.
- Sludge biochar creates value as a fertiliser and soil improver. There is greater value in its fertilising effect than in its role as a soil improver.
- The climate benefits (‘negative emissions’) also confer value.
- Pyrolysis/sludge biochar also brings societal benefits – primarily through detoxification, acting as a carbon sink, increased self-sufficiency and improved preparedness.
- For the wastewater sector, there are also other benefits – such as reduced storage space, less odour and less transportation.
- Sludge biochar is likely to compete in the fertiliser market. There is a shortfall in phosphorus supply in Swedish agriculture, so the market is not saturated.
- For sludge biochar to generate revenue, it needs to be approved as a fertiliser with ‘end-of-waste’ status, and will likely need to be registered as a product.
- Some form of certification or guaranteed quality control is desirable, but certain established certification schemes are very costly. The Revaq module, in its developed form, appears to be the most attractive option.
- Carbon credits have a value and can be traded on a market (there are several platforms). However, carbon credits should not be sold as long as wastewater treatment plant itself has a carbon footprint to reduce.
- It may be possible to obtain support from, for example, the Climate Leap for investment in the technology.
- From a financial perspective, choosing pyrolysis/sludge biochar as a sludge treatment method still entails increased costs, unless some form of funding, grant or co-financing is put in place.
We’ll be seeing a lot more results in the coming years!