Sean Mulligan, Peter Leonard, Alan Carty and Eoghan Clifford explain the costs of aeration in wastewater treatment and how energy efficiency technologies and measures can enhance sustainability to boost the bottom line.
The effects of population growth, economic development, climate change, increasing energy costs coupled and more stringent environmental regulations is placing a heavy strain on the wastewater treatment industry.
The treatment of wastewater is a highly energy intensive process which can account for as much as three per cent of a developed country’s electricity usage (1).
The main contributor to this energy intensity is the aeration process which can account for up to 75 per cent of a treatment plant's overall energy expenditure (2).
This energy ‘elephant in the room’ generally results from a collection of aspects such as outdated and inefficient technologies, over- (or under) designed infrastructure, treatment conservatism, poor ability or understanding of control and monitoring and a lack of real-time data providing transparency of operation.
The research group at NUI Galway, funded by the Sustainable Energy Authority of Ireland (SEAI) and Enterprise Ireland, are investigating the aeration in-use factor of the aeration process and aim to address these problems by adopting an interdisciplinary approach to develop novel methods, analysis and technologies to provide new insights to improve overall energy efficiency in wastewater treatment plants.
Why aeration?
The key pollutants in wastewater to be removed are organic matter and nutrients. These pollutants are effectively removed in aerobic biological treatment through the activated sludge process, a 100-year-old technique utilising biomass consisting of bacteria and protozoa which feed on the incoming waste.
The biomass consume oxygen during feeding and thus, a certain quantity of pollutants are removed (or oxidised) in the process. Oxygen is therefore supplied through the process of aeration, where atmospheric air is forced into the biomass and waste mixture to allow oxygen to transfer into the fluid.
Figure 1: Subsurface image of fine bubble aerated flows in a new wastewater aeration technology being tested at NUI Galway.
The overall objectives of an aeration system is then to A/ effectively transfer oxygen from the air into the activated sludge, B/ mix and disperse the oxygen uniformly throughout the tank and C/ do so at the lowest energy cost while not negatively impacting subsequent settlement of activated sludge during clarification.
Mechanical technologies such as surface aeration or fine bubble diffused aeration (FBDA) technology comprise the majority of the current aeration technology market (3).
Cost of wastewater aeration
Wastewater treatment can cost up to 100 kWhs (or €10) per person per year (4). With aeration accounting for up to 75 per cent of a treatment plants energy, costs of aeration can amount to an annual electricity cost of between €15,000 to €21,000 for a small Irish town or €0.65 to €1.3 million for an Irish city such as Cork.
On a global scale, the processes associated with aeration for municipal and industrial wastewater account for up to ~one per cent of global electricity usage. This equates to roughly 209 TW.h of electrical energy (roughly €20 billion per annum) generating circa 111 million tonnes of CO2 every year.
It is stated that this electricity consumption could grow by an additional 300 to 420 TWhs by 2030 if global urban wastewater treatment targets are to be met (5).
Figure 2: Power usage of aeration compared to all other energy users in an industrial wastewater treatment plant.
Why does it cost so much? It comes down to the demand for oxygen in a reactor and how efficiently oxygen is transferred to meet this demand.
For example, to aerobically treat wastewater, about 58 kgO2 per person is required every year (based on typical BOD and ammonium values of municipal wastewater (6).
The first level of inefficiency to supply this demand is derived from the method in which oxygen is transferred to the water – that is, the type of aeration technology being employed.
Aeration technologies are then rated by the so-called standard aeration efficiency (SAE), which is essentially the amount of oxygen introduced per unit energy consumed (kg O2/kWh).
Thus, the higher the SAE, the lower the cost of treatment and vice versa. This is analogous with a car's performance through the 'litres/100km' or 'miles/gallon' metric and is stated by the aeration equipment supplier based on ideal clean water tests under standard conditions.
However, a ‘use at your own peril’ type disclaimer is often recommended by experienced wastewater engineers and scientists (3) when adopting such values as the question is also prompted “what happens when the equipment is put in real ‘in-use’ conditions?”.
Getting technical: Ideal costs versus real costs
Typical aeration technologies have an SAE of 1.8 to 3.0 kg O2/kW.h which means that the cost of aeration treatment ideally should be about 19 to 32 kWhs per person per year (based on the typical oxygen required to treat a person's wastewater per year).
This is a far cry from the 70 kW.hs per person per year that is often measured in the field. The difference between measured clean-water and in-situ aeration efficiencies are down to three main issues: A/ the initial interpretation of total energy costs, B/ effects of real ‘in-use’ wastewater conditions and C/ limited aeration control.
Usually the choice of aeration technology at the design stage is only based on the SAE value without interpreting the total energy costs of the project.
For example, diffused aeration in many cases require supplementary mixers working continuously alongside blowers.
Because aeration is essentially the process of oxygen transfer and mixing, the additional mixing energy should always be included during the initial design and opex considerations as it effects the bottom line aeration energy usage.
Adopting a less efficient technology that provides suitable mixing may be a preferred route if the total energy costs are considered.
Regarding the second aspect, the performance of many aeration technologies tend to reduce substantially in real process conditions primarily due to the effect of what is known as the 'alpha factor'.
The alpha factor has the effect of reducing the SAE and is mainly due to the presence of surfactants (resulting from detergents, soaps and foaming agents) in industrial and domestic wastewaters.
These surfactants have the effect of creating a barrier for oxygen to transfer from the bubble to the wastewater and can severely diminish the oxygen transfer efficiency.
The alpha factor - which can result in up to an 80 per cent reduction in performance (3) - varies per technology and tends to have less of an effect in technologies that have inherently higher circulation, turbulence and mixing effects.
In addition to this, some technology’s performance varies over time due to fouling. For example, diffuser heads in an FBDA system can accumulate a biofilm over time which can in turn increase required blower pressure and power.
Fouling can also result in larger bubbles which are less efficient in transferring oxygen, thus calling for more airflow and blower power which work even harder against fouled diffuser heads.
The combined effects of the alpha and fouling factors are time dependent and should be interpreted carefully and accounted for during the design stage in order to understand the whole life cycle costs of a proposed aeration system along with maintenance schedules for diffuser cleaning.
Finally, the lack of control over an aeration system can have a significant impact on the energy intensity whereby oxygen is often supplied when it is not needed.
The challenge is to match the supply and demand of oxygen through the ability of turning up or down the equipment or by switching off the aerator and resorting to mixers when no oxygen is required.
Often, aeration equipment has a limited turn-up or torn-down capability and by doing so often results in equipment working outside of its best efficiency point or the inability to supply air against the necessary water pressure in the tank.
Technology and approaches
In summary, there remains to be considerable inefficiencies present in the process of aeration today which is the result of a combination of factors such as choice of technology, total energy considerations, effects of process conditions and lack of control.
Figure 3b: High-speed surface aeration at test sites.
The research team at NUI Galway are working actively to develop new monitoring techniques, analytics and metrics through research sites which will enable industry to optimise the design and control of aeration systems as well as improved modelling of long-term costs of various technologies.
As part of this project, the team is currently monitoring one municipal wastewater treatment plant and three industrial wastewater treatment plants in the dairy and meat industry.
These plants offer the team access to various aeration equipment including: low speed surface aerators; high-speed surface aerators; horizontal brush aerators; coarse bubble diffused aeration; fine bubble diffused aeration; sub-surface mixers; and a novel cyclonic aeration technology which was developed at NUI Galway.
Figure 4a: Oxygen uptake rate analysis.
The team are also investigating the application of novel monitoring techniques such as off-gas monitoring. This approach involves placing a floating ‘hood’ in the tank and measuring the properties of gas that is released from the water surface such as oxygen and carbon dioxide - see example in Figure 4 (7).
By measuring the change in oxygen concentration in this off-gas, it enables the determination of how much oxygen has been transferred to the water by the aeration technology.
This method enables the real-time determination of the in-use aeration efficiency, quantification of the alpha and fouling factors which can help inform diffuser cleaning schedules.
Figure 4b: Off-gas module(7).
With the nexus between water and energy becoming ever more apparent, the team are also exploring characteristics of energy supply and demand to propose further areas for energy use optimisation.
For example, the power factor is an important parameter to be determined for equipment which represents the ratio of the real power used by the equipment to do work to the actual (apparent) power consumed which is paid for by the consumer.
The lower the power factor, the more power is essentially wasted. In one study, a power factor was determined resulting in 7.5 per cent additional energy being consumed than required, which could be reduced by power factor correction equipment.
Additionally, the team have identified that demand-side flexibilities may exist in aeration systems where load shifting could be applied in a demand-response programme in order to contribute to grid stability without threatening treatment system performance.
Figure 4c: In-use oxygen transfer rate measurement.
As part of their study, the research team have developed an aeration energy audit (AEA) programme which involves a one/two-day survey to investigate the previous aspects in a WwTP, specifically in the aeration process, focusing on:
- Energy baseline analysis
- Steady state and non-steady state oxygen transfer testing
- Oxygen uptake rate determination
- Aeration off-gas analysis
- Dynamic alpha factor monitoring
- In-situ aeration efficiency assessments
- Real time monitoring of aeration KPIs
- Contextual and comparable KPIs of overall system performance (for example, energy consumption per kg BOD5 (or NH4) removed or per m3 treated)
- Reactive power assessment and power factor correction
- Computational fluid dynamics
- Aeration process optimisation
In one study, the research team determined that implementation of proposed energy efficiency measures could save 53 per cent of aeration energy costs (€13,283 per annum), and 27 per cent of total WWTP energy costs with a saving of 53 metric tonnes per annum in CO2 emissions.
Acknowledgements
The research team would like to thank the Sustainable Energy Authority of Ireland (RDD/377) and Enterprise Ireland for their support in this project.
The authors would also like to extend a special thanks to Ward and Burke Construction Ltd, ESB, Irish Water and ABP Food Group for facilitating the research along with their constructive feedback on the project.
References
1.) USEPA (United States Environmental Protection Agency), 2010. Evaluation of Energy Conservation Measures for Wastewater Treatment Facilities.
2.) Rosso, D and Stenstrom, MK, 2006. Surfactant effects on α-factors in aeration systems. Water research, 40(7), pp.1397-1404.
3.) Rosso, D, 2018. Aeration, Mixing, and Energy: Bubbles and Sparks. IWA Publishing
4.) Smyth, M, 2018. Wastewater treatment and anaerobic digestion – bridging the skills gap. Engineers Ireland Journal http://www.engineersjournal.ie/2018/02/06/wastewater-treatment-anaerobic-digestion-bridging-skills-gap/
5.) International Energy Agency (2018), World Energy Outlook 2018 Gold standard of long-term energy analysis
6.) Zanoni, AE and Rutkowski, RJ, 1972. Per capita loadings of domestic wastewater. Journal (Water Pollution Control Federation), pp.1756-1762.
7.) Muszyński-Huhajło, M and Janiak, K, 2017. Accurate oxygen transfer efficiency measurements by off-gas method-tank coverage dilemma. Proceedings of ECOpole, 11.
Authors: Dr Sean Mulligan, lead inventor and co-principal investigator, Department of Civil Engineering, College of Engineering and Informatics; Peter Leonard, PhD candidate, Department of Civil Engineering, College of Engineering and Informatics; Alan Carty, research associate, Department of Civil Engineering, College of Engineering and Informatics; Dr Eoghan Clifford, senior lecturer and co-principal investigator, Department of Civil Engineering, College of Engineering and Informatics