Beyond its obvious importance for local water quality, what is the broader environmental relevance of sewage sludge management – e.g. in reducing carbon emissions?
Dr. Emanuel Grün : Not many people are aware of this, but in many local authorities – both here in Germany and across Europe – sewage treatment plants are the biggest single consumers of energy in municipalities. Treating sewage requires both electricity to drive machinery and heat to dry sludge, so the process is energy-intensive.
Yet sewage also contains lots of potential renewable energy sources such as fermentation gases, which we capture in digesters to produce biogas. Using combined heat and power (CHP) plants previously installed by OSMO, we then convert this gas into electricity and heat; primarily as a result of this, we have already become self-sufficient in terms of our energy consumption. We still, however, have to burn the sludge after draining, and thus far, we have had to add coal in order for it to reach the calorific value required for firing. This is another source of carbon dioxide emissions which we will now be tackling with the new solar-thermal drying facility.
How precisely will the solar-thermal drying facility work, and what environmental effect will it have?
Essentially, you can think of the solar-thermal drying facility as an intelligent greenhouse which uses the energy of the sun (solar) and the addition of heat (thermal) to dry sludge. We have opted for this combined solution because, while, in our northerly latitudes, the sun can only contribute around 20% of the energy required, we can add the other 80% as heat from our own sources to evaporate around 103.000 m³ of water. At present, we produce around 80,000 megawatt hours per year of heat from burning sludge in total of 220.000 tonnes, but leave it to escape into the atmosphere unused. With the new solar-thermal drying facility, we will now be capturing this heat and harnessing it to dry sludge in a circular process which, crucially, cuts out coal.
Currently, we add around 20,000 tonnes per year of coal to our sludge and burning it produces around 70,000 tonnes of CO2 annually. The drying facility will bring these emissions down to zero. What is more, by removing coal from the mix, we will be able to burn more sludge, increasing the energy efficiency in our incinerator – which, of course, also produces low-carbon electricity that, in turn, we can use to power equipment elsewhere in our area of activity (e.g. pumping stations). As you can see, this is a complex system, but one where existing energy can be used more intelligently in the drive for carbon-neutrality.
In this complex system, what do you require from partner companies working on innovative projects such as the solar-thermal drying facility?
Above all, experience! Before it became a part of SPIE in 2019, we had already been working with OSMO for 30 years, so we know that the SPIE OSMO unit has a track record of planning technically challenging projects and that it is able to tailor its approach to our existing infrastructure. With SPIE OSMO, we have a partner who can, beyond simply setting it down on site, actually integrate a CHP plant or an innovative drying solution into our complex systems – and do so on our ambitious timescale (two years from project start to finish!) working within a group of companies with up to 150 members of staff on site during the build and installation phase.
What is more, in this project, SPIE OSMO has shown itself capable of tapping into the full breadth of knowledge as an organisation and contributing optimisations at all stages of the project – right up to and during implementation. Many of these contributions will make running this complex new facility considerably easier during day-to-day operations in the years to come.