Backyard Biogas or: Putting the P(ee) in Power

Biomass can provide energy in a million different ways – some more advantageous than others. Locally produced biogas from sewage and farming waste is a biofuel that I see the largest potential in because it combines many advantages. Briefly put these are the short transport distances in all steps of the process, the improvement of circularity due to efficient use of a waste product and the low impact on arable land.

Compared to fossil fuels, the energy density of any biomass is lower (Cheng, 2018), so minimal transport is one key factor in making energy production from biomass viable. Modern towns and cities have municipal sewage treatment plants and an extension to include biogas production completely erases transport. A vehicle fuel station can be built just outside where the gas is produced which is often just on the edge of a city, a reasonable distance for people to drive to fill up their cars. Alternatively, biogas can be produced at farms as Eliasson describes (2018) or farm waste could be collected and added to the sewage sludge to increase the amount of biogas that can be produced. This would of course mean that some transport is necessary but compared to the transport footprint of fossil fuels this would still be minimal.

Using a by-product of agriculture or a waste product like sewage sludge as a biomass feedstock means it can be classified as a second-generation resource. Second-generation resources don’t require arable land like first-generation resources, but this comes at a cost. Unlike first- and third-generation resources, second-generation feedstocks are not produced for the purpose of energy conversion which means that suitability and energy content can be lower. Nevertheless, using a waste product enhances circularity of agricultural and water treatment systems by decreasing the amount of virgin biomass or other external inputs required (e.g. fossil fuels) and thereby promoting positive climate impact.

Fig. 1: Biogas stations across Sweden (Fordonsgas.se, 2020)

The potential to expand biogas production in Sweden is great because there are many incentives from policies that encourage the investment in fossil-free fuels as well as tax exemptions (Lönnqvist et al, 2015). However, biogas plants are still a heavy investment and they may be difficult to shoulder for smaller municipalities. The map illustrates the spread of vehicle gas stations across Sweden and shows that – unsurprisingly – the majority are found in the south. Nonetheless, I have experienced that many people north of Stockholm frequently drive much longer distances than what’s common in the south, which makes investment in biogas plants in remote areas just as valuable as plants in the well populated south.

An issue that has been identified in several studies has been the fact that manure and sewage sludge have relatively low energy contents and the degradation and gas production can be slow. Co-digestion has therefore been proposed where energy crops (Eliasson, 2018) or microalgae (Thorin et al, 2017) are added to provide higher gas yields.

Altogether, locally produced biogas from sewage sludge is a viable biofuel that is a step towards making a city independent from fossil fuel deliveries from half-way across the world.

References:

Cheng, J. (2018). Biomass to renewable energy processes (second edition). CRC Press.

Eliasson, K. A. (2018). Swedish farm-scale biogas production-substrates and operating parameters. (Doctoral Thesis, SLU, Uppsala). Retrieved from https://pub.epsilon.slu.se/15821/8/ahlberg_eliasson_k_190107.pdf

Hitta tankstation | Här tankar du biogas och fordonsgas i Västsverige. (n.d.). FordonsGas. Retrieved 20 September 2020, from https://fordonsgas.se/tanka-gas/gasstationer/

Lönnqvist, T., Sanches-Pereira, A., & Sandberg, T. (2015). Biogas potential for sustainable transport – a Swedish regional case. Journal of Cleaner Production, 108, 1105–1114. https://doi.org/10.1016/j.jclepro.2015.07.036

Thorin, E., Olsson, J., Schwede, S., & Nehrenheim, E. (2017). Biogas from Co-digestion of Sewage Sludge and Microalgae. Energy Procedia, 105, 1037–1042. https://doi.org/10.1016/j.egypro.2017.03.449