Pioneering wastewater technology trialed by eThekwini Municipality.
Traditional aeration technology has low oxygen transfer efficiency (between 10% and 15%) and high energy consumption (approximately 50% of the overall operating expenditure of the plant) and requires regular maintenance (from fouling and clogging of membranes). MABR (membrane aerated biofilm reactor) is a solution to these challenges.
The technology models the functioning of the human lung. Lungs absorb oxygen and pass it onto the blood stream. Similarly, MABR provides the effective treatment of wastewater by passively supplying dissolved oxygen through a semi permeable membrane onto an aerobic biofilm.
“This is a fabulous aeration system where only oxygen can pass through the membrane to the biofilm – without the formation of bubbles. The biofilm performs as an active barrier that consumes oxygen, supporting anoxic conditions in the bulk liquid. It constantly removes contaminants from the wastewater,” says Lunga Patso, process engineer, MEB Energy SA.
Counter grade diffusion of the substrate
MABRs are especially effective for total nitrogen removal, due to the counter gradient diffusion of substrates.
On the one side of the membrane, there is a low stream of air that is continuously supplied by using a standard blower that works at very low air pressure. Oxygen passes through the semi permeable membrane to the biofilm (where micro-organisms are active) on the other side of the membrane. Here, the nutrients and chemical oxygen demand (COD) diffuses from the bulk liquid to the biofilm where nutrients are reduced and COD is almost fully absorbed (bio absorption).
The dissolved oxygen concentration is the highest at the attachment surface and drops to low levels in the bulk liquid as it is consumed by the biofilm.
“Nitrifying bacteria are exposed to high dissolved oxygen levels, leading to higher nitrification rates. At the same time, the outer biofilm and bulk liquid are anoxic, allowing the heterotrophic bacteria to reduce nitrates with influent biological oxygen demand (BOD) as an electron donor. This makes MABR technology unique as there is simultaneous nitrification and de-nitrification and BOD removal. The nitrifying biofilm oxidises nitrogen compounds (ammonia) to nitrates in the oxygen rich, low (BOD) layer. The denitrifying biomass breaks down nitrates to nitrogen gas as well as the remaining BOD,” explains Patso.
This process typically requires multiple chambers but MABR performs the action in a single tank, due to the high removal efficiency of nitrogen and phosphorous. This smaller footprint is especially useful in urban areas.
The MABR module is a spiral wound sleeve of oxygen permeable material. An air spacer is inside the sleeve that allows low air pressure flow. There is also an external water spacer that controls the water volume that is in contact with the membrane.
It is submerged in the biological reactor where intermittent mixing from diffusers causes the wastewater to circulate through the spiral. A short mixing duration is sufficient to homogenise the contents of each stage of the spiral reactor. In a four-stage spiral reactor, nutrients are removed in a one pass process without internal circulation.
The aerobic biofilm develops on the surface of the membrane and its thickness is well controlled, it cannot grow beyond 300 microns.
Patso adds that it is important to note that the wastewater is not filtered through the membrane. “The membrane provides optimum conditions for biology in wastewater. Therefore, there is no membrane clogging and fouling, no need to replace membranes every few years and no need for regular chemical maintenance. This reduces the operating expenditure of the plant. The MABR module is constructed from durable materials, ensuring more than a 20-year life expectancy.”
MABR can be used in packaged plants and as a retrofit in existing wastewater treatment plants.
Furthermore, MABR technology is extremely energy efficient due to its passive aeration. By using low air pressure (40 millibars), the bubbleless system reduces energy consumption by up to 90% . An average of 0,25 kwh of energy is consumed per m3 of wastewater treatment.
“Its effective oxygen permeability lead to a high eutrophication rate. Wastewater is treated to irrigation standards without the need for tertiary treatment. BOD is usually removed within a single attempt and this reduces the need for mixers required for the anoxic stage in the conventional activated sludge process. The MABR module therefore has less electro-mechanical components, making the entire system easy to operate,” says Patso.
MABR’s low odour and noise levels makes the technology ‘neighbourhood friendly’. The technology also uses less chemicals – it needs a disinfectant and a carbon source. This is because MABR can achieve up to 80% phosphorous removal without a dedicated anaerobic zone, driving down the operating expenditure on chemicals and manpower.
“This is not a new technology. MEB is one of three suppliers of MABR and has installed over 400 systems worldwide. We decided to trial the technology in eThekwini to showcase the effectiveness of MABR under local conditions,” states Patso.
eThekwini Municipality Trial
The Aspiral S1 System – a full packaged plant containing one MABR module, fine screening, aerobic volume and disinfection in a 20 ft container – was installed at a wastewater treatment plant in 2020.
“This is the first phase of a wastewater to drinking water pilot that is intended to be scaled up for comprehensive reuse for potable water quality applications. Currently, the effluent produced can be used for non-potable uses, such as the flushing of toilets or for agricultural purposes,” adds Patso.
Due to the limited amount of civil work (only concrete slabs), the entire system was installed within one week.
As a fully automated system, the Aspiral S1 can be accessed from any location. This feature provided value during the Covid-19 pandemic and when there was civil unrest in Kwa-Zulu Natal where the site could not be physically accessed for a period of two weeks and the system could be optimised remotely.
Pre-treatment is done through screening.
There is also an equalization tank.
“MEB is working with an independent, accredited laboratory that samples the treated effluent and provides results. As seen in the results below, the biofilm can protect the process against hydraulic shocks, low temperatures and even power disruptions,” states Patso.
After being treated by the MABR zone, the effluent travels to secondary clarifier where the effluent and sludge are separated. In the second phase of the project, MEB will be treating the effluent to Sans 241 standards where tertiary treatment will be added, comprising multimedia filtration and advanced oxidation processes.
MEB is a leading supplier with over 20 years of experience in decentralised water and wastewater treatment solutions.
The company has head offices in Umhlanga, KwaZulu Natal as well as Maputo, Mozambique.
The company has provided engineering, procurement and construction services, as well as the maintenance, management, operation and support to water and wastewater projects.