The Way of Bremen-Seehausen to an Energy Neutral Plant

During the 6^th edition of Ask The Experts series themed: “Valorising the end-products of domestic and industrial wastewater treatment” organized on April 25^th, 2023, by the African Water and Sanitation Association (AfWASA) with the German- African Partnership for Water & Sanitation (GAPWAS), the collaboration between the cities of Windhoek in Namibia and Bremen in Germany was highlighted.

Indeed, Windhoek and Bremen cities began a collaboration in year 2000 from a long historical relationship including the support from Namibia struggle for independence. In 2013, the two partners joined the municipal climate partnership project, continuing the tradition of knowledge sharing. The project mainly prioritized the solid waste management, wastewater management and provision of basic sanitation services for informal settlements in a quest to contribute to the United Nations Sustainable Development Goals. The city of Windhoek and Seehausen started collaborating in 2018. This knowledge- based collaboration focused on issues mainly pertaining to wastewater treatment. The team usually meets on a monthly basis to discuss and analyze different topics, seeking for solutions and improvements.

The collaboration approach over the past couple of years focused on exchangeable visits in Nambia and Bremen on topics of common interest. For example, the two charts on variation in influent flowrate (see Attachment, Page 3) show the hourly influent volumes of Seehausen and Gammams plants, and the hourly organic meta concentrations. For both plants, the factor between the minimum and maximum daily figures is about 0.5. But there are some differences between the trends or the hourly patterns per day, which could be due to the travel time of water to the plant, which can differ. In Bremen, there is also a storage capacity on the pipelines. The patterns could also be different per hour of a day due to industrial and domestic waste. Indeed, Gammams in Whindhoek only takes water of domestic origin, while Bremen takes wastewater from domestic and industrial origin, considering that industrial waste is very hard to predict. Also, in Namibia there is only a separate sewer system, meaning that most of the infiltration is diverted into rivers, while Seehausen in Bremen has both the combined and a separate sewer. These are all important factors, among others that can be used to troubleshoot or rectify faults towards improvement and to plan optimization to ensure process efficiency.

This gives few insights into the actual cooperation which aim to get more knowledge on the whereabout of our carbon and what it can be used for. In one of the Bremen treatment plant balances (see Attachment, Page 4), we can see the quantity of carbon which is transferred to the Bio reactor, and the quantity which is brought to the digestion. Biogas is derived from that, and with a combined heat and the power plant from which energy can be produced. So, every optimization of a treatment plant, can change the future. The results at the end of the treatment process should be the possibility to produce more energy and gas, or the use of carbon for denitrification to get a better affluent quality of treatment plant. Exchanges with laboratories that make the analyzes and other partners of the wastewater sector revolves around similarities in operations and special tasks as well to identify what can learned from each other. For example, Windhoek has a 50 years’ experience in removal of micropollutants and climate change adaptation. Bremen can learn this know-how from Windhoek, especially since Bremen is getting more and more dry; natural water bodies get increasingly smaller, and the city has to think over how to use water and what for. For example, Bremen uses semi-purified water for gardens or public places just like Windhoek. On another realm, Bremen has been preparing for rainfall events for the last 30 years, and this is something Windhoek may learn from Bremen.

Bremen’s biggest wastewater treatment plant is Seehausen, and there is a process to get an energy neutral plant. Bremen is a city in the northern part of Germany, and the responsible for city sanitation is hanseWasser, operating on a public- private partnership model. The Bremen area is very flat; consequently, more than 200 pumping stations are needed to pump every drop of wastewater against gravity to the treatment plant in Bremen Seehausen located in the highest (1012 meters higher than the rest) region of the city.

The treatment plant in Seehausen had about 1 million inhabitants connected, and the wastewater treatment plant in the northern part of Bremen had about 160,000 inhabitants connected. The presentation (see Attachment, Page 8) highlights a very good development of the self-production of energy, with two (2) big steps between 2010, 2011 and in 2013. The first step was the introduction of a new wind turbine, and in 2013, the combined heat and power plant station was renewed; this allowed the generation of more energy from the gas available. In 2022, the self-production stood at 130%, with about 100% from the combined heat and power plants and 28% from wind turbines. It rains very often at Bremen, and the city can only generate around 1 to 2% of energy from self-production. The city didn’t only focus on the production, but also on reduction of the total energy consumption of the plant, with a drop of about 25% over the years with optimization and a new aggregate.

The specific energy consumption per inhabitant is also an indicator for the reduction. Bremen’s way to energy neutrality is based on three pillars:

• The first one is the repowering. Three (03) combined heat and power units were renewed. Every unit is around 1.4 megawatts electric per unit, and a wind turbine (see Attachment, Page 9). What comes from the combined heat and power plant can be used to improve the gas production.
• Some projects are also ongoing to have a higher gas production, including a demand reduction in a technical way by specific reinvestment. It was also economically viable to get new aggregates with the lower specific demand. For example, in this case a compressor hall with seven (7) Compressors for the appropriation of in-house processes.
• The third pillar is optimization. There is a digital twin of Bremen’s treatment plan for all biological processes. This allows for optimized process, and especially the aeration, the quantity of oxygen needed for the microbiology part. Some set of DWA A- 216 rules described in the presentation (see Attachment, Page 11) can also be used to view how much energy is needed, and if there may be a possibility to reduce it a little bit more (how to do an energy check, an energy analysis for wastewater treatment plant in Germany, with guidelines through the whole calculation process). An energy analyzes enables to see the best value that can be reached for specific energy demand for the treatment plant, with a highlight on the quantity of energy that can possibly be reduced in future projects.

The presentation (see Attachment, Page 12) showcases parts of set rules, with specific energy consumption of the whole treatment plant. The total energy demand of the plant is known, as well as the number of inhabitants connected to the plant; this allows to calculate the specific energy demand in kw/hours per inhabitant and per year; getting inside the benchmarking system of this set of rules allows to find the frequency of lower deviation and understand the self-monitoring system of energy of a plant. The presentation (see Attachment, Page 13) also highlights the results of energy analyzers from the plant, with the best values that the plant can reach over the year. Bremen-Fargo is a bit far away from this added value and must figure out how to make the treatment plant better in energy consumption.

To sum up, the project started at a good point because of lot of aggregates for energy production, and high demand of energy had to be renewed. A company-wide goal was set to get energy neutral for the whole company; this allowed to reduce the specific demand of aggregates and raised the production efficiency.