Pathways to energy neutrality: a German case study of the Ruhrverband

© Ruhrverband

Germany’s Ruhrverband is taking a pioneering role in sustainable utility operations. Norbert Jardin describes how the co-organiser of next year’s IWA LET conference succeeded in becoming energy neutral in 2023.

German utility Ruhrverband will host IWA’s Leading Edge Conference on Water and Wastewater Technologies (LET) 2024, to be held in Essen in Germany on 24-28 June. The main host of the event, together with IWA’s German National Committee and the other co-organisers Emschergenossenschaft / Lippeverband and Gelsenwasser, the Ruhrverband is one of the most established and largest water management companies in Germany, and has been setting technological standards for more than 100 years, focusing on the most current issues of the sector.

The Ruhrverband operates 65 wastewater treatment plants in the catchment area of the Ruhr, a low mountain range in the federal state North Rhine-Westphalia and namesake of the Ruhr region, which is the largest urban agglomeration in Germany. As a water association under public law, the Ruhrverband strives to be financially prudent and 15 years ago established goals in its corporate strategy to improve the energy efficiency of its wastewater treatment plants, and to increase its generation of renewable energy.

Since climate change has made these goals increasingly important, the Ruhrverband went one step further in 2022, when it revised its corporate strategy and formulated a key objective to achieve energy neutrality by 2024.

Sustainability as a strategic corporate goal

Implementing such a goal goes far beyond any technical challenges it poses. It is crucial that such an effort is anchored to the organisation’s corporate mission statement to ensure that acceptance among the management team and workforce is achieved, and resources are allocated appropriately. In 2021 and 2022, the Ruhrverband revised its corporate mission statement to comprise five fields of action, one of which is dedicated to the goal of sustainability.

The energy targets are also anchored in the target principles found in the field of action ‘Sustainability’. Further goals in this field are to continuously improve the ecology of the region’s waters, to effectively manage forests to protect water resources, and maintain the Ruhrverband’s forests in a climate-friendly manner. This field will also safeguard jobs and provide young people with career opportunities and training.

As a largely new and future-oriented goal in this field of action, the Ruhrverband has committed to achieving a balanced climate footprint and to drawing up a climate protection plan. It was helpful that the Ruhrverband had already established an energy management system along ISO 50001 guidelines some years ago. However, this only covered some aspects of the company’s greenhouse gas (GHG) emissions.

Consistent reduction of GHG emissions

A climate protection plan creates transparency about a company’s emissions inventory and climate-related performance, promotes climate awareness within the company, and helps it to identify potential savings. The Ruhrverband’s GHG inventory shows that it has reduced emissions from 156,100 to 121,200 t CO2,eq/a in the past 11 years. This was largely because of increased renewable energy generation and use, improved nitrogen elimination, and reduced use of carbon sources.

The Ruhrverband’s GHG inventory identifies potential opportunities for reducing GHG emissions. This includes the construction of a solar sewage sludge drying plant at the site of the company’s fluidised bed combustion plant, which enables the company to use dried sewage sludge instead of coal in the auxiliary firing, reducing GHG emissions by almost 30%. The company has identified that nitrous oxide emissions are highly dependent on the plant’s ability to remove nitrogen. Ruhrverband will use measurement techniques to verify the correlation between a shutdown and emissions to assess this further.

In the case of sludge treatment, residual gas must not be activated immediately after anaerobic treatment. In addition to capturing methane, the retention times in secondary thickeners must be kept as short as possible. Interim sludge storage should also be avoided by effective sewage sludge logistics with immediate dewatering and transport to an incineration plant. Stationary dewatering is preferable to campaign dewatering with mobile centrifuges as far as GHG emissions are concerned.

According to manufacturers, vacuum degassing technologies enable the methane dissolved in the sludge water to be removed effectively and promise to improve dewatering performance significantly. The Ruhrverband is currently determining how much methane can be recovered from all its wastewater treatment plants with laboratory tests and plans to test vacuum degassing in a technical plant. Last but not least, by further increasing its own electricity generation, the company should be able to substantially reduce its emissions (currently 20,100 t CO2-eq/a because of external electricity purchases).

Since the Ruhrverband set itself the goal of increasing energy efficiency and generating renewable energy in-house 15 years ago, the company’s electricity consumption has decreased from 110 to 95 GWh/a. The main driver of this success has been improved energy efficiency in operating the wastewater treatment plants, the electricity consumption of which has fallen from 93.5 GWh/a in 2006 to 80 GWh/a in 2021.

Increasing in-house renewable energy production

With diverse water quality and quantity operations, the Ruhrverband has a number of ways to increase the amount of energy it produces in-house. For example, in almost half of its wastewater treatment plants, the digester gas produced is used in combined heat and power plants, which have a total capacity of 9.8 MW, with annual electricity generation of 43-45 GWh. To increase digester gas production, kitchen and food waste is accepted at six of the company’s plants.

So far, 13 photovoltaic (PV) plants with a total capacity of 1.2 MWp have been erected. Currently, the company is preparing a pre-planning study in order to erect further PV plants, with the aim of generating an additional 8-10 GWh/a of electricity, which will probably require an area of 11-14 ha.

The operation of hydropower plants has always been part of the Ruhrverband’s core business. Until 2021, Ruhrverband and its wholly owned subsidiary LLK (Lister- und Lennekraftwerke) operated around 20 hydropower plants on Ruhr reservoirs and dams. Since May 2022, Ruhrverband has also taken over operational responsibility for three run-of-river power plants on the upper Ruhr reservoirs, which have a total capacity of 11.8 MW (Hengsteysee, 3.3 MW; Stiftsmuhle weir, 1.9 MW; Harkortsee, 6.6 MW). These plants were each leased to the regional power producer after the respective reservoir was constructed.

The power generation of these three run-of-river power plants varies depending on water flow, but, on average, they produce 30-35 GWh/a. This power is now used fully by the Ruhrverband, either via direct cable connection with existing operating facilities or via virtual feed-in to the energy balancing group, displacing power that would otherwise be required to be purchased.

Increasing energy efficiency in wastewater treatment plant operations

Energy efficiency has been improved as plant operations have been consistently optimised. Furthermore, the energy needed for new construction has also been optimised and existing plants have been converted. The Ruhrverband uses cascade denitrification as the preferred option to eliminate nutrients to the greatest possible extent in all major new construction projects. By dispensing as far as possible with the internal recirculation customary in upstream denitrification and installing at least three-stage cascade denitrification, the Ruhrverband is able to achieve high energy efficiency with very extensive purification performance. Such plants generally achieve the optimum value according to the national regulations for energy optimisation of wastewater plants.

An example of how an existing wastewater treatment plant can be optimised is the Bochum-Olbachtal wastewater treatment plant (293,100 p.e.). The Ruhrverband converted the biological wastewater treatment plant, which previously used upstream denitrification, to three-stage cascade denitrification, achieving energy neutrality and an effluent value of less than 5 mg/l Nanorg.

After the three-stage cascade denitrification system was commissioned, it became apparent that there was potential for optimisation with regards to the flow behaviour of the basins and air distribution. This was exploited by further improvements to the control and process technology. Among other things, the aerator elements installed in the course of the rebuild were replaced by silicone tube aerators, a sliding pressure control system was implemented, the control of the air intake was adapted, and the inlet distribution was improved.

In 2022, the electricity consumption of the Bochum-Olbachtal wastewater treatment plant was 4.4 million kWh. With an average influent load of 177,000 p.e., resulting in electricity consumption of 24.9 kWh/p.e., far below the target value for 2030 of 33 kWh/p.e. for all wastewater treatment plants in the Ruhrverband. Hence, the Bochum-Olbachtal wastewater treatment plant not only generates the energy it needs, but a surplus.

In addition to converting existing wastewater treatment plants, Ruhrverband uses innovative process designs to further increase energy efficiency. A typical example is the Altena wastewater treatment plant (35,000 p.e.), which was last expanded between 1982 and 1984 to include more advanced nutrient elimination. As industrial pollution loads have decreased considerably and the population has declined in the catchment area, the plant is now clearly too large. In addition, the mechanical and electrical engineering had to be completely renewed. An economic feasibility study showed that a new plant could be built much more economically and would be better adapted to the future smaller capacity of 20,000 p.e. The patented NEREDA process with aerobically granulated sludge was selected as the process technology – the first of its kind in Germany. Decisive factors were not only its innovative character and the expectation of considerable energy savings (at least 30% compared with the previous plant operation), but also the limited space available. The plant was commissioned in September 2022.

As 70% of the total electricity consumption of wastewater treatment plants is because of aeration and mixing, the Ruhrverband has implemented comprehensive monitoring to closely track the energy efficiency of these process steps and, if necessary, initiate measures to optimise plant operation.

As part of this project, the 20 largest wastewater treatment plants were equipped with extensive measurement technology to enable monitoring. This included separate power measurements for aeration and mixing, pressure measurements at various points in the air intake system, and air volume flow measurements. Key figures for energy efficiency are derived for both aeration and mixing, enabling continuous assessment of energy efficiency as part of operational controls.

Energy self-sufficiency every single day

Thanks to the measures described, the Ruhrverband has been able to increase both the energy efficiency of the wastewater treatment plants and the renewable energy generation so substantially that it achieved energy neutrality in 2023. With the exception of seasonal residual electricity quantities, external electricity purchases can be dispensed with.

However, our goal goes much further. With the addition of further PV systems, we not only want to achieve an annual electricity balance, but also self-sufficiency every single day of the year.

An important prerequisite for achieving this target is the management of a balancing group for electricity. At Ruhrverband, all run-of-river power plants feed into this balancing group (provided they do not physically transfer the electricity to the wastewater treatment plants), as do the combined heat and power plants (if they generate more electricity than can be consumed at their own wastewater treatment plant). As some wastewater treatment plants now generate more power than they consume, this surplus energy can be put to good use elsewhere. In this way, the Ruhrverband can largely compensate for the otherwise expected cost increases from electricity purchases and stabilise the financial contributions that the company’s members have to make in the long term.

For more details on LET 2024, see: