The waste water sector is increasingly moving away from a role as major electricity consumer to become producers of green energy.
In Denmark, it has especially been the larger plants which have begun to incorporate the energy paradigm shift, but a project with NIRAS as the lead engineer pulls the smaller plants in the same, positive direction.
In May 2017, the project selected three waste water treatment plants, located in cities of Randers, Naestved and Svendborg. These cities are geographically spread across Denmark and have populations between 58.000 and 98.000. The waste water treatment plants were analysed crosswise by experts and suppliers. The purpose was to assess how the three plants could reduce electricity consumption and at the same time increase heat energy production.
The technologies of the future
As part of the project, a list of relevant solutions and technologies was drafted. The technologies on the list are all capable of contributing to a better energy balance at the treatment plants. An example of the technologies that can be used to reduce electricity consumption would be the usage of sensors that measure the waste water’s content of oxygen and nutrients. The sensors allow you to adjust the cleaning to save energy on pumps and aeration – a process that sends air through the waste water to increase the oxygen level.
In relation to energy production, a relevant technology can be to extract biogas from the decaying sewage sludge or using heat pumps to utilise the excess heat in the waste water, if the temperature is sufficiently high.
However, the possibilities depend on the specific plant, its surroundings and the local conditions. For example, the scope of industry in the area can play a role, just as the quantity and quality of waste water and access to the district heating network affect the possibilities.
A self-sufficient sector
At the national level, the energy potential is significant. Today, many of the resources in the waste water are not utilised to a sufficient extent and in addition, the opportunities for energy savings are lost.
Analyses by NIRAS show, that Denmark's 674 sewage treatment plants are expected to be able to supply themselves with energy and also produce a net supply of 130 GWh (443500 mmBTU) as heat or electricity for consumers. This corresponds roughly to the electricity consumption of 32,500 households (single-family homes).
Small investments matters
While the large treatment plants have the economy and the organization for a smooth and gradual transition, even small investments can go a long way for the smaller plants. One solution is to upgrade gradually and think long-term and strategically. This is one of the conclusions in NIRAS’ analysis. This includes reviewing all processes within the waste water companies looking for possible synergies to exploit.
The Danish Eco-Innovation Program (MUDP) supported the project.