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This has been achieved without adding external carbon to the wastewater facility or using any external energy sources.
In recent years a few wastewater treatment facilities have started to achieve energy neutrality. The most common approach is to digest the sludge and use the gas from the digester to generate electricity and heat. The electricity and heat are typically used to meet the needs of the wastewater facility itself and the surplus, if any, is sent into the electricity grid or district heating system. If this is not possible, the gas can be cleaned and exported to the natural gas network or used as fuel for buses and other road vehicles.
There are good reasons to adopt this sort of approach, as wastewater facilities are highly energy intensive as is the whole water industry. According to the Environmental Protection Agency (EPA), between 3% and 4% of the total electricity consumed in the USA is used by water and wastewater handling facilities. The UN claims that the corresponding figure worldwide is nearer 8% and the EPA also states that around 35% of the electricity bills of local government relate to water and wastewater operations.
Aarhus Water Ltd is a water service company in Denmark with operations that cover both water and wastewater handling. Five years ago, it started a process aimed at optimising the energy efficiency of its treatment facilities, with a special focus on the Marselisborg catchment area. This is a relatively flat geographic region in the centre of Aarhus, which has around 200,000 inhabitants. It is a traditional city area where water is obtained from a groundwater source and is, on average, pumped from a depth of 35m.
On the water supply side, energy savings have been obtained by reducing leakage to between 6% and 8%, from more than 14%, and by splitting the city into pressure zones. It is anticipated that additional energy savings will be possible by optimising the pumping for groundwater, and by even more effective pressure zone management.
The Marselisborg wastewater facility, which deals with household wastewater from the city, is a conventional activated sludge treatment plant with mesophilic digestion. No FOG (fats, oil and grease) or external carbon is added to the process, and no solar or wind energy sources are used. The typical outlet value for biochemical oxygen demand (BOD5 modified) is around 2.4, for total nitrogen (TN) around 7.1 mg/l and for total phosphorus (TP) around 0.53 mg/l.
The wastewater facility has been upgraded with more energy efficient equipment and, equally important, a comprehensive computer control system, which makes extensive use of on-line sensors. Key elements of the upgrade include:
• Securing energy-efficient bottom aeration by, among other things, ensuring regular maintenance of the aeration system
• Installing high speed turbo blowers
• Installing a high-efficiency CHP (combined heat and power) plant
• Fitting Danfoss AC variable speed drives (VSDs) on all rotating equipment in the catchment area – a total of 290 VSDs.
Fitting the VSDs was an important measure as wastewater treatment processes are characterised by large load variations over the 24-hour daily cycle and also seasonally throughout the year. Using VSDs to control blowers, pumps and other motorised equipment enables their operation to be accurately matched to these load variations, ensuring that their energy efficiency is optimised under all conditions.
In addition, the key for real-time control of the biological stages is to use the VSDs to control the operation of the blowers in accordance with a load estimate calculated from the on-line ammonium sensors and the incoming flow. This results in typical dissolved oxygen (DO) values between 0.3 and 0.5, and simultaneous nitrification and de-nitrification. Automatic control of sludge age, based on VSD control of the return activated sludge (RAS) pumps, as well as effective carbon harvesting in the primary sedimentation tanks, ensure that the maximum amount of carbon is secured for digesting and energy production.
In 2014, these measures resulted in the production of 130% electricity – that is, 30% more than used for operating the process – and 2.1 GWh of heat, which is used in the local district heating network. Overall, this equates to a total energy production of 192% – 92% more than the energy used by the wastewater facility. This energy covers 94% of all energy used for water supply production, water distribution, wastewater pumping and wastewater treatment in the 200,000 inhabitant catchment area.
Toward the end of 2015, the Anammox (anaerobic ammonium oxidation) process was implemented for treating the reject water and an additional highly efficient CHP generator was installed. These steps are expected to increase energy production to the level where it will be sufficient to cater for the entire energy needs of the complete water cycle in the Marselisborg catchment area. In the first eleven months of 2016, the wastewater treatment plant produced around 234%, equal to 134% more energy than needed for its own operation, which means that the catchment (water production and distribution, wastewater pumping and treatment) area was now energy neutral, in fact with an energy surplus of 7%.
Aarhus water is now in the process of upgrading the next catchment area, Egaa. This is a smaller area with 120,000 inhabitants. Performance similar to that achieved in Marselisborg is expected, even though the Egaa facility is only half the size of the Aarhus one.
In conclusion, it has been proven, that based only on traditional processes and household wastewater, it is possible to make the whole water cycle in a catchment area completely energy neutral, without adding external carbon or using wind or solar energy. A further bonus is that the return on investment period has been found to be less than five years and so Aarhus has been able to reduce water prices rather than, as is generally seen, increasing water prices for end customers.
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