This Global Largest MABR System Will Change Your Understanding of Aeration Process!
Currently, in most traditional wastewater treatment plants, the energy consumption of the aeration process accounts for more than 50% of the total energy usage of the plant, leaving a large potential for energy savings. As a biological membrane wastewater treatment technology that uses selective oxygen transmission through membranes to supply oxygen and serve as a biofilm carrier, MABR technology can effectively reduce wastewater treatment energy consumption, increase facility load, and enhance the removal of nitrogen and phosphorus nutrients. It has significant technical advantages in treating high oxygen-demand wastewater, volatile organic compounds wastewater, and high ammonia-nitrogen wastewater.
Hespeler Wastewater Plant: The World's Largest Membrane Surface Area MABR System
Located in Cambridge, Ontario, Canada, the Hespeler Wastewater Treatment Plant is one of thirteen plants serving the Waterloo region's population of over 600,000 people.
Originally built in 1973, the plant was upgraded in 1988 and 1992, but these upgrades were insufficient to meet the increasing population demand and rising wastewater treatment efficiency requirements. Therefore, the Waterloo Regional Government decided to overhaul the secondary treatment process at the Hespeler plant.
Hespeler Wastewater Treatment Plant
In 2017 and 2018, the regional government investigated the design plan, long-term performance, and economic feasibility of installing MABR technology, ultimately deciding to upgrade its aerobic activated sludge system to an MABR/AO (anoxic-oxic) combined process system. When the project went live in 2021, it became the world's largest MABR system in terms of membrane surface area.
The decision to choose MABR technology for the Hespeler plant has proven to be highly beneficial.
1. From an energy-saving perspective:
In regard to the original aerobic activated sludge method at Hespeler, this section consumed 60% of the total energy as oxygen was required from micro bubble aeration. During the upgrade, a 40% reduction in the biochemical energy consumption of the plant resulted.
In a typical wastewater treatment plant, the conventional aeration method uses blower or mechanical aeration in which air or pure oxygen is mechanically forced into the wastewater. Although these methods are effective and easy to control, they have many drawbacks: The large number of bubbles aeration produces high energy consumption, operational cost and low oxygen transfer efficiency.
In contrast to the MABR technology, oxygen content in the fiber membrane is driven by the difference between inside and outside oxygen content after aeration starts. The membrane material dissolves and diffuses oxygen as a single molecule so that it can diffuse through the membrane. Compared to conventional aeration methods, MABR has several advantages:
However, oxygen can be directly delivered to the biofilm, which can greatly decrease oxygen transfer resistance via the liquid phase and covers an oxygen utilization of more than 100%. Using traditional methods, the oxygen transfer efficiency only reaches 1.5 kg/(kW·h), whereas it can go up to 6 kg/(kW·h).
◎ It is a stable environment for microbial growth and reproduction. The MABR aeration intensity is mild, producing almost no damage to the microorganisms attached to the membrane fibers, which stabilized microbial growth.
◎ This aeration is bubble free, preventing volatile components being carried into the air usually through bubbles to avoid secondary pollution. Moreover, it suppresses foam generation from microbial metabolism.
◎ By easily adjusting the oxygen supply, gas waste resulting from the minimum oxygen demand for reaction is avoided.
2. From a capacity expansion perspective:
The original processing capacity of the Hespeler plant was 6,600 m3/day. After the upgrade, the treatment capacity increased to 9,320 m3/day, a 40% increase.
MABR equipment is compact, occupying little space, and can be installed directly in existing tanks. Oxygen is selectively transmitted through the membrane for bubble-free aeration, with high oxygen transfer efficiency. The oxygen provided is fully utilized by the biofilm, resulting in high oxygen utilization and greatly increasing the system's biomass, achieving plant capacity expansion without physical expansion.
MABR upgrades can increase the load of existing wastewater treatment plants by 20%-40%, or even higher.
The oxygen transfer direction in MABR is countercurrent to the transfer direction of ammonia nitrogen and organic matter. Nitrifying bacteria form dominant growth near the membrane surface and are protected by the outer biofilm, which not only increases the nitrification rate but also ensures the stability of nitrification. This advantage is especially evident under shock loads or during cold winter months.
Before the upgrade, the winter effluent ammonia nitrogen (with a minimum water temperature of 10°C) exceeded the 5 mg/L discharge standard; after the upgrade, the effluent met the standard (winter discharge limit < 5 mg/L, summer limit < 2 mg/L).
Additionally, installing MABR reactors in anaerobic or anoxic tanks allows for simultaneous nitrification and denitrification. Under the same effluent total nitrogen conditions, compared to other nitrogen removal processes like A2O, it reduces internal recirculation ratios, improving denitrification efficiency while saving carbon sources and energy.
Three Keys Influencing Factors for Membrane Aerated Biofilm Reactor (MABR)
1. Aeration Pressure
Since the operating pressure in MABR has to stay below the membrane's bubble point, we get bubble-free aeration.
◎ At too low pressure, there will not be enough dissolved oxygen in the inner biofilm, and thus the activity of aerobically nitrifying and heterotrophic bacteria will suffer.
◎ At very high pressure, the entire biofilm will be aerobic, encouraging anaerobic denitrifying bacteria and other anaerobes to not grow and adversely affect the denitrification process.
In fact, the treatment effect should be achieved in practice under appropriate water quality intensity according to specific water quality.
2. Water Flow Velocity
At the microbial attachment stage, the excessive flow velocity will hinder the microbial growth and adhesion, therefore, the flow rate is not too high at this stage.
Once the biofilm is formed, an increase in water flow velocity decreases the liquid boundary layer thickness. As the biofilm thickness stabilizes during stable operation, increasing flow velocity reduces a boundary layer thickness of liquid phase and promotes biofilm renewal, which reduces the biofilm thickness excessively, increasing the oxygen and pollutant transfer efficiency.
Effluent flow velocity is one of the factors affecting microbial growth and biofilm thickness, according to the research.
◎ The thinner the boundary layer is due to the higher flow velocity and the corresponding stable biofilm thickness.
◎ Increasing pollutant degradation rate requires decreasing flow velocity, thicker stable biofilm, and a thicker stable biofilm at lower flow velocity.
3. Carbon, Nitrogen, and Phosphorus Ratio in Wastewater
A suitable C:N:P ratio promotes microbial growth in the MABR biofilm, facilitating simultaneous nitrification and denitrification in the reactor.
◎ In cases of low C:N ratio, the organic carbon concentration is insufficient to meet the carbon source demand for denitrification, affecting total nitrogen removal efficiency.
◎ When the C:N ratio is too high, aerobic heterotrophic bacteria will proliferate and consume a large amount of oxygen, lowering the dissolved oxygen concentration and hindering nitrification.
Four Common Application Scenarios for Membrane Aerated Biofilm Reactor (MABR)
- Treatment of high ammonia-nitrogen wastewater
- Integrated devices for rural domestic sewage treatment
- Biological restoration of urban river waters
- Upgrading wastewater treatment plants for better performance
In fact, in recent years, MABR has been increasingly applied in wastewater treatment plants worldwide.
For example, at the YBSD Wastewater Treatment Plant in Illinois, USA, 10 original aerobic bioreactors were upgraded to 2 anaerobic tanks, 2 MABR anoxic tanks, and 6 aerobic tanks, with 12 MABR modules installed in the anoxic tanks. This converted the previous aerobic process into a nitrogen and phosphorus removal process, achieving the goal of increasing treatment capacity while enhancing biological nitrogen and phosphorus removal.
The MABR system's biofilm acclimation period was only 3 weeks, and after the wastewater treatment system was fully operational, the influent BOD5 load increased to 0.60 kg/(m3·day), a 47% increase compared to pre-upgrade levels.
The plant's final effluent met all design expectations, with effluent BOD5<10 mg/L, total suspended solids (TSS) < 10 mg/L, NH3-N < 1.5 mg/L, and TP < 1.0 mg/L. The average oxygen transfer rate (OTR) and oxygen transfer efficiency (OTE) were 10.8 g/(m2·day) and 33.3%, respectively.
Microbial population analysis of the biofilm showed that ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) accounted for 40% of the microbial population, more than four times higher than in conventional activated sludge processes.
Moreover, compared to the CAS process for a plant of the same scale (with an investment cost of $25 million and a construction time of 2.5 years), the MABR process had an investment cost of only $5 million and a construction time of 1 year, significantly reducing costs and construction time.