5. According to the studies, the units that serve a large equivalent population are observed to have low energy costs per capita equivalent or CO2 emissions, while we would expect the opposite, ie small units to be more energy efficient. Why do you think this happens?
Operational data illustrate that larger WWTPs with conventional activated sludge treatment and anaerobic digestion consume less energy per population equivalent (PE) than smaller ones with extended aeration activated sludge treatment. In Greek WWTPs average daily energy consumption for small WWTPs is 0.37 kWh/PE and for large WWTPs 0.09 kWh/PE. The primary reason for the significantly higher energy consumption obtained in small WWTPs is that all small WWTPs operate as extended aeration treatment systems and do not perform any energy recovery from anaerobic sludge treatment. In addition there is the economy of scale that favours larger WWTPs. However smaller WWTP can easier apply Natural Based Systems like constructed wetlands or high rate algal systems that have some advantages as compared to the Activated Sludge Systems among which lower biomass production and less energy consumption.
6. In the new WWTP units that are being designed, the parameter of energy consumption and the environmental footprint is now entering, do you want to tell us a few words?
The Urban Wastewater Treatment Directive (UWWTD) is a relatively old directive (adopted in 1991) that is mainly focusing on removal of nutrient and organic pollution and does not address issues related to energy consumption. The requirements of the UWWTD were developed before European Green Deal and Circular Economy policy agendas were developed and do not encourage or discourage efficient energy use. As I mentioned previously there is a significant lack of energy efficiency indexes in the existing EU legislation regarding WWTPs. Given this fact it is up to each member State to develop incentives for new WWTPs to decrease energy consumption and GHG emissions. It is most likely that green public procurement criteria are currently the most availably means of encouraging energy savings and energy neutrality of WWTPs. Currently, these are not applied uniformly in the EU and depend on each country environmental policy. For example in Greece according to the latest climate policy there is a need to decrease GHG emissions by 30% by 2030 in all WWTPs. However efficient treatment options/approaches to making WWTPs energy neutral or energy generating are not specified.
7.Existing treatment plants in order to achieve the EU’s objectives for the rational management of energy resources and GHG emissions, what are the key steps they need to take?
After manpower, energy is the highest operating cost item for most WWTPs. Pumping and aeration are usually the main energy consumers but equipment upgrades and operational modifications should not be limited only to these and not be a onetime event, but should be incorporated into a comprehensive energy management strategy.
In order to optimize energy efficiency gains and reduce GHG emissions, existing WWTPs should develop energy management strategies and implement energy conservation measures. In my opinion, some of the actions that should be included in such an energy management strategy are:
Ø Collect data on energy use and develop a baseline of energy performance indicators against which you can measure improvements over time.
Ø Conduct energy audits and determine the energy use of various treatment processes and identify opportunities for energy use conservation.
Ø Incorporate energy efficient practices into daily operations such as operating at lower sludge age, limiting nitrification if not needed, maximizing anoxic carbon removal in biological nitrogen removal WWTPs.
Ø Improve efficiency of aeration equipment by using automated DO controls, energy efficient blowers and surface aerators, and energy-efficient diffuser technologies.
Ø Automatic control of the blowers based on the NH4-N concentration in the final effluent.
Ø Improve pumping and motor efficiency by ensuring that pumps are sized appropriately and installing variable frequency drives, whose speed varies according to flow.
Ø Produce thermal and electrical energy from biogas recovered from anaerobic sludge digesters that can be used in Combined Heat and Power (CHP) systems.
Ø Improve efficiency of operations by installing a SCADA that can significantly improve the automated control, monitoring and efficiency of the processes.
Ø Establish a long-term maintenance program to ensure that all pumps and motors are operating efficiently.
Ø Optimize energy conservation in buildings (reducing energy needs for lighting, cooling and heating.
Ø Reducing energy cost by transferring, if possible, some of the electricity consumption during off-peak periods when electricity price is lower.
Ø Evaluation of the possibility of employing renewable energy sources such as hydroelectric power generation, wind turbines, photovoltaic panels.
Ø Evaluation of the possibility of employing innovative energy conservation technologies for selected wastewater and sludge treatment processes.
8. What is the future trend of wastewater treatment regarding energy conservation and GHG emissions reduction according to your perspective?
Minimizing energy consumption and reducing GHG emissions to mitigate the climate crisis will become increasingly important in the water industry in the coming years. It should be underlined that the potential chemical energy in municipal wastewater that can be harvested through treatment exceeds the energy consumption of a conventional activated sludge plant by at least a factor of 5. Therefore, in my opinion innovative wastewater treatment technologies will be developed and applied in a wider scale that can achieve net energy-neutral or even energy-positive wastewater treatment. Some of these novel processes are natural based systems especially suitable for decentralized systems, chemically enhanced primary treatment, high rate activated sludge process the so called A/B process, partial nitritation/anammox, anaerobic membrane bioreactors, microbial fuel cells and anaerobic co-digestion of bio-solids.
