The wastewater treatment sector has used both aerobic and anaerobic treatment for many years.
However, in terms of the generation of energy from anaerobic digestion (AD), New Zealand’s wastewater sector is underperforming compared with other countries such as the United Kingdom and the USA: New Zealand currently has just 10 wastewater AD plants1, contributing to the 2.71 petajoules (PJ) of biogas energy produced each year.
By contrast, in the UK there are 163 wastewater AD plants, from a total of 635 biogas facilities nationwide, which account for 28 percent of the country’s total AD capacity.
Thanks to efficiency increases, the sector delivered a 25 percent increase in power production from a 12 percent increase in capacity between 2010 and 2015.
In the United States, 48 percent of all wastewater is treated by anaerobic digestion, with a third of water treatment plants producing biogas.
Plenty of potential
Many commentators overlook the potential of the wastewater and sewage sectors in terms of increased energy generation, but with 135 wastewater treatment plants nationally, there is a huge opportunity to increase energy production from New Zealand’s wastewater sector. In addition, there is the potential to increase the efficiency of those plants which already use AD, not least as some of the oldest wastewater AD facilities are now in a position to upgrade and take advantage of the latest technology.
Take the heat
In many cases, the economic decision about whether to invest in AD will depend on the overall efficiency of the final project.
Techniques such as heat recovery using heat exchangers enable a resource that would otherwise go to waste, to be used to pre-heat feedstock or digesters to improve gas production efficiency. Equally, the heat can be used for pasteurisation and concentration processes, increasing the value of the digestate co-product produced.
A well-designed system could recover and utilise 40 percent of the heat produced by a wastewater AD plant. This free heat provides a useful bonus when evaluating any investment in AD, and is something that potential developers should consider from the beginning of their planning process.
1 Revenue from Biogas: https://www.bioenergy.org.nz/documents/resource/Information-Sheets/IS24-Revenue-from-biogas.pdf
