Dr Mamais has been engaged in more than 80 research projects dealing mainly with water, wastewater and sludge treatment, removal and fate of priority substances and emerging pollutants in wastewater treatment plants, nutrients and energy recovery from waste, wastewater disposal and reuse and bioremediation.
He has recently completed a study aiming to assess the energy and carbon footprint of Greek WWTPs and to propose methods to improve energy efficiency and reduce GHG emissions. Data were collected from 31 Greek WWTPs with an average treatment capacity between 250 and 3,650,000 population equivalents. The total population served by the 31 WWTPs was over 6,000,000, which is more than half of the population in Greece with access to WWTPs.
1. You have worked systematically and thoroughly studied the issue of energy management of biological treatment in Greece and Internationally. The latest EU agreement brings significant changes to WWTP energy targets, right? Do you want to tell us a little about it?
The European Union (EU) passed the EU Green Deal in 2019 aiming at achieving zero pollution by 2050. This zero pollution ambition of reducing air, water and soil pollution to levels no longer harmful to health and natural ecosystems is obviously very challenging and requires significant changes in WWTPs. The wastewater industry should be brought under some sort of regulation to become more energy efficient and move towards energy and climate neutrality. For example according to the European Environmental Bureau improving all WWTPs to an average energy efficiency will result to annual energy savings in the order of 5 500 GWh. However, although this need is well acknowledged by the industry, there is a significant lack of energy efficiency indexes in the existing EU legislation regarding WWTP. In my opinion this is an issue that should be addressed in the upcoming revision of the Urban Wastewater Treatment Directive.
2. Could you please tell us a little more about the energy issue of biological treatment. Can you help us identify the points that are critical and equally energetic in WWTP modules?
WWTPs are important energy consumers with an estimated 2–4% share of total electricity consumption attributed to the water industry. According to the literature, the energy consumption of conventional activated sludge treatment systems varies between 0.3 and 1.90 kWh/m3. For example the average annual energy consumption in WWTPs in the USA is approximately 29 kWh/PE, in the UK approximately 38 kWh/PE and in Sweden 42 kWh/PE. These values are quite similar to the German Guide Manual, which sets an objective for annual energy optimization of approximately 20–30 kWh/PE, based on the size of the WWTP. In Greece average annual specific energy consumption ranges between 15 and 86 kWh/PE, with an average value of 38 kWh/PE.
The main energy consumer in a WWTP is aeration. Aeration is needed for most aerobic biological processes: for organic carbon oxidation, nitrification and endogenous respiration and usually accounts for about 40–60% of the total energy consumption in WWTPs. According to a survey we conducted in Greek WWTPs, energy consumption for aeration accounts in most cases for 40–70% of the total energy consumption of the WWTPs.
If we take into account other secondary treatment processes (i.e., bioreactors mixing, sludge recycling, mixed liquor recirculation, etc.), average energy consumption in secondary treatment accounts for approximately 70 % of the total energy consumption in WWTPs.
3.Are there gas emissions that of course contribute to the unit’s footprint, how do we distinguish them and what is their impact in the overall footprint?
It is widely accepted that WWTPs are also a significant source of greenhouse gas emissions. Specifically, WWTPs directly produce several greenhouse gases, such as carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) as a result of the treatment procedure and additionally contribute to CO2 and CH4 emissions through energy consumption. In Greece, according to a previous study focusing on moderate and large WWTPs, the annual GHG emissions ranged between 61 and 161 kgCO2e/PE. The reduction in GHG emissions of WWTPs is crucial, considering that the European Commission has set a target of at least a 55% decrease in GHG emissions by 2030.
The carbon footprint of a WWTP includes both GHG emissions released from a treatment activity directly and indirectly. Direct GHG emissions arise from wastewater collection, treatment and disposal, while indirect GHG emissions are related to electricity supply, transportation of different chemicals or sludge, use of chemicals and additives and disposal of residuals. The average annual GHG emissions from all Greek WWTPs is 73 kgCO2e/PE. Average annual on-site GHG emissions are equal to 56 kgCO2e/PE, while off-site GHG emissions account for 17 kgCO2e/PE. Approximately 30% of the total GHG emissions are related to electrical energy consumption, and thus they can be reduced if renewable energy sources are used.
Moreover, GHG emissions are separated as biogenic and non-biogenic. GHG emissions originating from organic matter degradation from biological wastewater treatment, anaerobic sludge digestion, incineration, landfill and land application are considered biogenic (not derived from fossil-fuel-related activities). According to IPCC (Intergovernmental Panel on Climate Change), biogenic CO2 emissions should not be included in the national GHG emissions inventory. GHG emissions from energy consumption, chemicals and transportation are considered non-biogenic. For Greek WWTPs average annual biogenic emissions are approximately 77% of total GHG and the remaining 23% are non-biogenic emissions.
4. In the studies you have carried out, we observe that aeration system and specifically bioreactors have the largest contribution to the energy consumption of the unit, at rates of 50-60%. How can we fix this without sacrificing the performance of our unit?
Many measures can be taken in order to achieve the goal of energy consumption and GHG emissions reduction in aeration. Because of the high energy use associated with aeration, energy savings can be as high as 50% by designing and operating aeration systems to match, as closely as possible, the actual oxygen demands of the process and by implementing certain operational changes in bioreactors. Implementation of DO automatic control can significantly improve energy consumption and reduce GHG emissions. The use of improved system controls, energy efficient blowers and surface aerators, and energy-efficient diffuser technologies can reduce energy consumption. Furthermore many nitrifying WWTPs have experienced significant gains in energy consumption by switching to an automated DO control system using on line NH4-N measurements to control aeration according to nitrification efficiency. Other simple operational changes that can provide quick gains to energy costs are for example operating at low enough sludge to minimize endogenous respiration, minimize nitrification if not required and provide conditions to maximize anoxic organic carbon removal in nitrogen removal activated sludge plants.
