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

Joe Walsh, global director of aviation, Hatch, Melbourne, Australia.

His roles ranged from operational airport management to project, design, and construction management of major airfield infrastructure projects across the globe, including the UK and Ireland, Hong Kong, and Australia.

He holds a BE in civil engineering from NUI Galway and is a fellow, CEng, FIEI with Engineers Ireland. He is also a fellow, chartered professional engineer, and engineering executive of Engineers Australia.

1) When did you first become interested in engineering?
Growing up in a home with a retail (grocery) and farming (dairy) business, my parents had us involved from an early stage dealing with customers and working on the farm.

With a keen interest in maths, accountancy and an outdoor life, civil engineering – and specifically construction-related activities – seemed to tick all the right boxes.

NUI Galway also had an excellent reputation for civil engineering and it was on my doorstep – home being Portumna in east Galway. My brother had also completed construction management in Galway RTC (now the Galway-Mayo Institute of Technology), so looking over his shoulder had an influence also.

2) Who were the mentors who helped you on your way?
I met my first mentor during a summer break working as a site engineer on a ground works project in the Isle of Dogs, London. Barry Smiley, the resident engineer for the project, gave me great encouragement in what I was doing and also provided me with a vision of what I wanted to become as a professional civil engineer.

On finishing university, I joined the BAA graduate programme (formerly the British Airports Authority). It had seven UK airports within its group: Heathrow, Gatwick, Stansted, Southampton, Edinburgh, Glasgow and Prestwick.

John Cairns was my supervisor during my civil engineering placement. Our paths have continued to cross over the past 28 years in several countries and I would consider John as being my most influential mentor in the aviation sector.

In my time at Galway airport, our then chairman, Michael Corless, a retired partner with Ernst & Young, was hugely influential and gave a huge amount of time and energy to the business. His approach and focus was on the development of business management and stakeholder engagement skills.

Today my managing director, David Moran, who has spent his career in the contracting environment and is now leading the growth and development of the Hatch infrastructure business in Australasia, is my newest mentor. It is very important for our continuing development as engineers to have mentors and, also, to act as one to the next generation.

3) Your engineer hero?
Bruce Benjamin who was the construction manager for the airfield works contract at the new Hong Kong airport. He was an inspirational leader with major multinational projects experience. Sadly, he passed away before his time.

British industrialist Sir John Egan.

4) An engineer you wish was better known?
All engineers who have ended up leading significant businesses outside of the field of their original degree. I would specifically mention Sir John Egan, former chairman of BAA, and a key player in the development of the New Engineering Contract (NEC).

5) Your idea of project heaven?
A project that has a clearly defined scope, realistic design programme and a collaborative delivery model which is reflected in the contract form and on the ground among the different players and their respective culture and behaviour.

6) And project hell?
Poorly defined scope, inexperienced contractor and adversarial approach to delivery on the ground.

7) What are your favourite engineering feats?
Hong Kong International Airport in Chek Lap Kok. My choice is heavily influenced by my involvement in this project.

Hong Kong International Airport.

Also, the Heathrow Express link because it has created a new underground rail link into one of the busiest airports in the world crossing under existing London Underground routes, and for its use of the New Austrian Tunnelling Method (NATM).

In terms of current projects, I would mention the Metro Tunnel in Melbourne, which will be transformational for the city.

8) What is/are the most important trend/s in engineering right now?
Digital engineering and the progression to 7D (whole of life cycle approach to project delivery driving digital models to the next level).

Resourcing and encouraging the next generation – especially at secondary level – to take up engineering subjects. As engineers and industry there is an obligation on all of us to enhance the role of the engineer in society.

With the evolution of artificial intelligence and machine learning, which will remove a lot of the more mundane/repetitious tasks, our industry has to evolve to create the next generation engineer who enjoys the challenges posed and drives change with technological advancements. Climate change issues across the globe will require engineers to come up with the solutions.

9) If you could, is there any one measure you would introduce to help improve the gender balance within the profession?
Engage with children in primary schools and have very clear role models for both boys and girls to emulate. Industry is taking more focused action on flexible working initiatives and I believe with an enhanced work/life balance that comes through this approach, careers in engineering will become more attractive and improve the gender ratio.

10) What book is on your bedside table?
Atomic Habits by James Clear. It drew my attention while reading the author's blog. It focuses on the formulation of good habits and the concept of the 'one percenters' – taking little steps and making small changes when it comes to habits can have a significant impact on your life.

The book Joe Walsh has by his bedside.

The concept of 'habit stacking' where you build on existing good habits using them as a trigger for new habit formation. The importance of the environmental setting when establishing new habits to be successful was also enlightening.

11) What is the one piece of advice you would give to somebody starting out in the profession?
Decide on your pathway as early as possible – be it a technical or management orientated role – and link it to key experience. Seek out your mentors to better understand decisions and, ultimately, to inform your choice.

12) What is your favourite film?
The Shawshank Redemption. I’ve watched it many times and still enjoy it. I admire the determination and focus of the lead character, Andy Dufresne, and his desire to help others despite his own challenging circumstances. Willpower and determination trumps all.

13) If you weren’t an engineer, what might you have become?
Having spent my formative years working on our family farm and having developed a strong work ethic from my parents, a role in the agricultural sector would certainly have been a serious option. It would probably have involved managing a large-scale agricultural operation – crops or dairy.

14) What is a typical day for you?
I get up at 4.50am to take the dog for her morning walk, then head to the gym (twice a week) for a 30-minute strengthening session (one-on-one).

If I have calls to North America then I try to co-ordinate these for 5am or 6am to catch the teams in the latter half of their day in Vancouver or Toronto.

Later, I will drive or take the train to our main office in Melbourne, spend the day with the infrastructure team and carry out general management activities, project delivery and business development.

The end of day can then involve calls to South Africa to catch the team at the start of their day. Being in Australia we have the ability to operate in all three regions in the day.

Cape Town, South Africa.

Hatch put a huge focus on flexible working and I’m actively co-ordinating working from home to engage with our teams across the various areas to drive the growth and development of our aviation business.

15) What’s the best piece of advice you’ve ever been given?
In a presentation to the group chief executive of Manchester Airports Group, Geoff Muirhead, who was also a civil engineer, as director of planning and development at Bournemouth airport I had a number of points to cover. The broader team were pushing to move on with the meeting but I persisted in covering the points. At the end of the meeting, Geoff noted to me on leaving the room that, “if you have something to say then say it”, and closed the remark by saying “well done”. This comment/moment has stuck with me ever since. For me, it’s all about adding value and encouraging engagement and contribution from the entire team.

16) What do you do to relax?
Life outside of work evolves around our two Irish Water Spaniels. They are a fantastic and engaging breed and hold a very special place in our hearts in Australia. My original dog, Tory, made the trip to Australia with us and was a huge part of getting established so far from home.

From a sporting perspective, rugby is a passion and I now proudly follow Ireland and Australia. Australian football is slowly taking its place in my sporting diary – it's an incredible game.

Speaking at the launch, Gerard Murray, network manager, Next Level Skillnet, said: “The launch of this programme comes at a critical point for the environment where sustained damage to our planet may occur unless we change our policies.

"This programme can help us make Irish business world leaders in the area of sustainable energy utilisation and environmental protection.”

Skillnet Ireland awarded funding to Next Level Skillnet last year, under its Future Skills programme, to develop this programme in collaboration with industry.

The objective of the Skillnet Ireland Future Skills programme is to encourage collaborations between enterprise, academic institutions, and industry training providers to develop innovative programmes that speak directly to the future skills needs of business and that address gaps in existing provision.

Programme delivery

The programme will be delivered by lecturers with extensive experience as practitioners and academics, part time over one year, by a combination of Saturday on-campus workshops, online lectures, guest speakers and learning activities, eBooks and other digital resources.

The course includes the core modules Environmental Management for Organisations, Environmental Leadership in Organisations, Environmental Science, The Lean Organisation and Technology, Energy Management and Environmental Legislation and Compliance.

Each module will be individually assessed by assignments completed throughout the semester. The major assignment for each module will be the development of a plan for the participant’s workplace.

Application closing date and fee subsidies

There are a few places still available on this course. Delivery starts on Saturday, October 19, 2019.

Fee subsidies are available to students registering for the Diploma in Corporate Environmental Planning, through membership of Skillnet (free membership of Skillnet is available to private sector companies).

For this pilot launch a fee subsidy is available through Skillnet for employed and self-employed applicants.

Further information

Further details of fee subsidies are available on www.nextlevel.ie, email: sue@nextlevel.ie or call 061 363 418.

Interested applicants are encouraged to apply immediately at: http://www.nuigalway.ie/courses/adult-and-continuing-education-courses/corporate-environmental-planning.html or contact: sciencetech@nuigalway.ie for further information.