Danish water and wastewater utility VCS Denmark is already a net energy producer, but projects and innovations are set to bring further progress. By Ivan Vølund and Mette Teilmann.
Ejby Mølle, the main water resources recovery facility (WRRF) of water and wastewater utility VCS Denmark, produces 188% of the energy that it consumes. In fact, the energy produced amounts to more than the total consumption of the wastewater and drinking water divisions combined, making the utility, which serves the city of Odense, a net producer of energy.
The process of becoming a net energy producer has included energy savings across the entire company, combined with energy production (electricity and heat) based on wastewater sludge from our WRRFs. As the largest WRRF, Ejby Mølle is our main energy producing facility and a key contributor to the balancing of our corporate energy accounts.
Energy production at Ejby Mølle is based on the widely used approach of using the biogas produced in our digesters to produce electricity and heat. We are now in a position where we can offer the surplus to energy utilities as green energy.
Use of recuperative thickening to reduce the amount of water in our digesters is one contributor to the high performance. The concentration of biomass is high, which optimises the efficiency of the digesters. With less water present, we prolong retention times, increase the amount of biogas produced, and reduce the amount of residual biosolids.
The additional energy consumption needed for recuperative thickening is insignificant. The thickening process is running anyway, and we are not able to measure any difference. Half of the water is removed from the recirculated sludge. The hydraulic retention time in the digesters varies depending on the load; in August 2020 it was 31 days, while the retention time for solids was 34 days. On average, solids retention times are prolonged by 10%.
Despite already having become a net producer of energy, there are a number of ongoing process optimisations and renovation steps that will improve carbon concentrations in the wastewater sludge.
The inlet at Ejby Mølle WRRF is being refurbished. This features new and more energy efficient blowers in the grit removal, combined with a new control strategy of grit removal to match the actual inlet flow, along with optimised aeration. Optimisation of the grit removal will result in a reduction of 31,000 kWh/year (from 2021). In ongoing work, replacement of a screening press with a screening washing press will improve our carbon distribution from screening material to the digesters.
A project started in the summer of 2020 to test the effects of adding ferric chloride to the sludge dewatering process to reduce the amount of resultant biosolids and decrease precipitation of struvite. Removal of struvite deposits is a costly and problematic factor in the treatment process.
In 2020, we also started testing of new polymers in our primary clarifiers combined with targeted analyses of our treatment plant to gain more insight into carbon distribution, and to find ways of optimising the distribution of carbon between our digesters and the treatment process. These tests are exploratory, and we do not know if – or how much – such initiatives could contribute to improving our energy balance further.
Wastewater treatment processes require a lot of energy, and this makes these processes a key focus point for energy savings. At Ejby Mølle, we have analysed our energy consumption and our treatment processes and wastewater and sludge transport systems to detect areas with potential for energy savings. Continual renovation and changing of pumps and equipment are a major factor in reducing our energy consumption.
At the same time, we are not prepared to make any qualitative compromises in our treatment processes, nor in relation to the effluent from the treatment plant. Our framework for energy savings is clear: there should be no increase in nutrients discharged to our aquatic environment and no increase in emissions of greenhouse gases to the atmosphere.
Changes and renovations carried out to date include renovation of main pump station at Ejby Mølle WRRF, undertaken in 2018-2019. Pumps from the 1950s have been replaced with new energy efficient pumps, and processes for distribution of water at the WRRF have been optimised.
The pipe capacity from the digesters has been increased, also reducing losses and emissions of methane through safety valves. Before the pipe work was carried out, safety valves emitted methane for several hours every week. Now they stay closed, even though biogas production increased by 16% from 2019 to 2020.
During 2020, the truck reception area for wastewater has been refitted with a pump with larger capacity, improving utilisation of the carbon source. Based on calculations of our digester capacity, we have increased the hourly intake of our digesters to 5m3/hour from a former maximum of 3m3/hour, above which the rest was pumped into processing tanks.
Another major focus area has been control of the aerobic wastewater treatment processes. We run nitrification and denitrification simultaneously. Aeration is controlled by ammonia sensors and runs with a dissolved oxygen (DO) usually of less than 0.1 mg/l. The low DO makes the energy consumption in our aeration tanks low compared with that seen in many other treatment plants with surface aeration. Surface aeration is an outdated technology. However, with our control strategy, combined with the fact that tanks are only 3.5m deep, upgrading to diffused aeration is not financially viable. In 2014, we installed sidestream treatment with anammox bacteria to save energy and lower the load in the aeration tanks.
We are committed to further progress also. With this in mind, we engage actively in innovation projects and partnerships with the aim of supporting and contributing to the development of new methods and technologies. We are currently facilitating a PhD project testing Membrane Aerated Biofilm Reactor (MABR) technology at Ejby Mølle. Today, aerobic processes account for 37% of the total electricity consumption at Ejby Mølle. With diffused aeration, we are only able to utilise 15% of the oxygen – at best. With MABR, it is theoretically possible to utilise 90% of the oxygen in the membranes. The potential for energy savings / optimisation of the use of oxygen is clearly very significant – and it also carries with it potential for reduction of the physical footprint of future treatment plants.
Other innovation projects currently being planned include implementation of pyrolysis at a smaller wastewater treatment plant (WWTP), scheduled for 2020-2021. This will turn the plant into a WRRF, producing heat and biochar, which is a much cleaner fertiliser product than the biosolids that we produce today. Plastics and micropollutants are degraded, and some heavy metals are removed.
Construction is to start in January 2021 on installation of a new sludge storage tank, which will be mounted after the digesters. With the new tank, emissions of methane are controlled and converted into a resource for energy production. Currently, 111,000 m3 of biogas (methane content: 65%) are vented from our storage tank every year. Utilising the full amount of biogas as fuel in the gas engines, the future power production will gain 260,000 kWh/year.
During 2020-2021, two old boilers will be replaced with a new boiler. The benefits include improved efficiency in operation, less ventilation energy loss, and reduced energy loss from preheating the boilers. This project will provide energy savings amounting to 235,000 kWh/year, resulting from a reduction in consumption as well as increased heat production.
Further projects this year include installation of heat pumps at a minor WWTP to replace electrical heating. This is expected to reduce energy consumption to about 25% of the current heat consumption (annual electricity savings of 24,000 kWh).
Also, in August a feasibility study started concerning the potential for installing Thermal Hydrolysis Processing (THP) to produce more energy and reduce the amount of residual biosolids.
Control of greenhouse gas emissions forms another part of our efforts. In 2010, VCS Denmark decided to become CO2 neutral by 2014 – and we did. As well as CO2 emissions, this balance includes emissions of methane and N2O as well. To reach our target in 2014, we purchased green energy to top up our own direct contribution to the CO2 balance. In our balance, we have included emissions from processes, fossil fuels, power, heat, and composting of biosolids. Our contribution to evening out the emissions comes from biogas production, installation of solar panels, afforestation, and purchase of green energy.
Figure 1 shows that emissions have been reduced, and we are now less reliant on the purchase of green energy.
In further work, this year we have started up a greenhouse gas emission detection project, to pinpoint the most significant geographical emission points. The efforts to further evaluate the potential emissions from the sewer system and from the different process steps at our major WWTP will include liquid measurements, off-gas metering, and advanced air measurements calibrated with a tracer gas. The different data sources are expected to be included for modelling, and for a more holistic understanding of the emissions and the dynamics of emissions of N2O and methane.
Optimised utilisation of resources in wastewater
With the production of energy at Ejby Mølle, we have proved that it is possible to introduce a new business activity, without compromising our core business. We wish to boost national and international awareness of such potential available in wastewater, and we participate actively in networks and innovation projects to this end. Most importantly, we make our experiences and knowhow available, and engage with others to learn more about new and existing technologies and solutions.
Water utilities reflect their specific location and local conditions. Solutions are tailored for each individual utility. Solutions implemented at Ejby Mølle will, consequently, not be applicable in all water utilities. Our knowhow and experience may serve as inspiration in other utilities, and to this end, we offer consultancy and knowledge-sharing both nationally and internationally.
Our main message is that water utilities must be prepared to review and revise processes to include resource recovery in general, and energy production in particular, and to demonstrate that the inclusion of new activities (for example, energy production) does not necessarily require costly construction of entirely new facilities.
A further opportunity we are pursuing highlights this – one involving a mutually beneficial collaboration with local district heating company.
By 2022, the district heating company must have phased out its coal-fired combined heat and power plant. This means that a broad-ranging process to identify and establish alternative energy sources and production methods is currently in progress. Heat pumps have been identified as a potential, and partial, solution.
With this opportunity in mind, the district heating company is currently establishing a major heat pump installation (20 MW in total) at Ejby Mølle. The heat pumps will utilise the energy (heat) in the effluent from our wastewater treatment process. The installation will be operational by the end of 2020.
The project will provide the district heating company with a new production facility based on a free and readily available energy resource. VCS Denmark, meanwhile, benefits from the resultant lower effluent temperature. The receiving water, Odense River, sometimes suffers from low oxygen content, particularly in the summer. The lower effluent temperature contributes are a result of the higher oxygen content in the effluent.
Introduction of energy production in wastewater treatment plants or WRRFs requires willpower and perseverance. As we see it, resource recovery must be prioritised in all major WWTPs and WRRFs. As water utilities, we have an obligation to generate as much benefit to society as we possibly can – and we have not even scratched the surface regarding resource recovery and utilisation of energy in wastewater.
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Figure 1 shows that emissions in grey (process emissions / fossil fuels), yellow (power and heat) and blue (biosolids composting) have been reduced. Due to our increased biogas production, installation of solar panels and continued afforestation projects (shown in green), we now rely less on the purchase of green energy (shown in orange) while maintaining a positive CO2 balance
Energy statement – Ejby Mølle WRRF (2019)
Electricity consumption: 7,964 MWh
Electricity production: 9,518 MWh
Heating consumption: 4,700 MWh
Heating production: 14,234 MWh
Self-sufficiency ratio (energy) for Ejby Mølle WRRF: 188 %
Energy statement – total for VCS Denmark (drinking water and wastewater) (2019)
Energy consumption (in total: heat, power and fuel): 23,873 MWh*
Energy production (in total: heat, and power (biogas and solar panels)): 24,176 MWh
Self-sufficiency ratio (energy) for VCS Denmark (both operations and management of drinking water and wastewater): 101%
* Energy consumption includes the consumption of power and heating, as well as fuel for vehicles and external haulier’s transport of sludge. The energy statement does not include the energy consumption of external contractors in their execution of construction works.
VCS Denmark – core activities
- 233,000 people (municipalities of Odense and Northern Funen)
- Treatment plants: 8
- Treated wastewater (2019): 33 Mm3 (18.5 Mm3 at Ejby Mølle)
- 2,650 km of wastewater pipes
- Production (2019): 22,600 tonnes of biosolids (biological fertiliser)
- Total load (2019): PE 310,000 (of which: PE 218,000 at Ejby Mølle)
- Ejby Mølle capacity: PE 410,000
- 175,000 people (main part of the municipality of Odense, the third-largest city in Denmark
- Water treatment plants: 5
- Distribution of potable water (2019): 9.7 million m3
- 1,100 km of potable water pipes
Ivan Vølund is Head of Department, Wastewater, and Mette Teilmann is Consultant (Web and Communications) at VCS Denmark.