Aerobic digestion and nitrification-denitrification work together thanks to an innovative membrane system
The discussion, however, has mainly focused on issues like MABR technology’s low operating expenses, high energy efficiency, low maintenance, scalability, and its neighborly low noise and odor levels.
What’s discussed less is exactly what goes on inside an MABR module. At the most basic level, it’s aerobic digestion, which uses bacteria that consume oxygen to digest wastewater. Sometimes the function of the MABR’s membrane is compared to the human lung‘s. Oxygen at atmospheric pressure diffuses through the membrane wall and feeds the aerobic digestion without bubbles.
Making bubbles with compressed air is not only energy-intensive, but also inefficient: Most of the air in the bubbles rises to the surface before the bacteria even have a chance to use it.
Simultaneous Nitrification and Denitrification (SND)
One interesting thing about MABR is that it allows multiple processes to occur simultaneously in the same tank. According to the Connecticut Water Pollution Abatement Association:
Nitrification is the conversion of ammonia (NH3) to nitrate (NO3). […] This is a two-step process that is done with oxygen and two types of bacteria, Nitrosomonas and Nitrobacter, known collectively as the nitrifiers. […] Denitrification is the conversion of nitrate (NO3) to nitrogen gas (N2) […] Heterotrophic bacteria utilize the nitrate (NO3) as an oxygen source under anoxic conditions to break down organic substances.
These nitrifying bacteria thrive in the biofilm as the membrane continuously delivers oxygen to them at the molecular level without creating bubbles. But how does the membrane do that? Fluence’s self-respiring membrane is made of a polymer that isn’t perforated. Instead, its structure is air-permeable at the molecular scale. This membrane is shaped into a spirally wound envelope, with air flowing within it. This envelope is immersed in a wastewater tank (the reactor). As air flows through the membrane walls and into the wastewater, aerobic bacteria cling to it, form a nitrifying biofilm, and thrive, converting dissolved ammonia into nitrate.
The oxygen levels near the membrane in an MABR are high, but the levels are lower between the membrane sheets and in the rest of the tank, so denitrifying bacteria use oxygen from the nitrate. The nitrate is broken into nitrogen gas, which is discharged to the atmosphere, and into oxygen, which induces further digestion of carbon-based organic contaminants in the wastewater.
This is referred to as the simultaneous nitrification and denitrification (SND) process.
The biofilm-based SND used in MABR is a very stable and resilient process, with high resistance to environmental shocks. It works well in lower temperatures and its reactors are smaller than others that perform similar treatment work. Because no nitrate recirculation is required, energy is saved on pumping.
Air Use Inside an MABR Module
It is important to clarify that although the energy-intensive process of pressurizing air to force it through diffusers is not required with MABR modules as it was in legacy aerobic water treatment facilities, the air must still be pumped into the membrane envelopes. The pressure required to do so, however, is negligible. Because the membrane walls of the envelopes are held separate by a specially designed air spacer, fanning the air through the membrane envelopes is done virtually at atmospheric pressure.
The wastewater within an MABR module must be mixed periodically, and, in fact, bubbles are used in periodic cycles to mix the wastewater to avoid the drawbacks of mechanical mixing.
Take a look inside an MABR module in this short video:
Old Principles, New Application
The MABR module has combined new membrane polymer technology and old principles into an ingenious package that efficiently and simultaneously performs many functions within one small footprint.
Contact Fluence for more information about MABR technology or to learn how it can benefit your wastewater operation.