Membrane Bioreactors

What is an MBR?

• MBR means membrane bioreactor. It joins a living treatment step with a fine filter.
• The living step uses microbes, mostly bacteria, to eat biodegradable waste. The tank where this happens is the bioreactor.
• The mix of water, microbes, and fine solids is called activated sludge. Air helps the microbes work.
• The membrane then separates clean water from the sludge. The clean water is the permeate. The microbes stay in the tank.

One drop’s journey

1. Incoming wastewater: used water arrives from homes, hotels, or industry.
2. Fine screen: rags and grit are removed before they harm the membrane.
3. Biological tank: air and microbes break down biodegradable waste.
4. Membrane: water passes through. Sludge and most bacteria stay behind.
5. Treated water: operators test it. Some uses need disinfection or more filtering.

• Extra sludge leaves the tank for separate treatment and disposal.
• Flow based on the US EPA MBR fact sheet.

What changes from a conventional plant?

• The membrane does not replace the microbes. It replaces the settling step.

What do the tiny pores do?

• MBRs commonly use microfiltration or ultrafiltration membranes. Their tiny holes keep out suspended solids and many microbes.
• The biological tank removes much of the biodegradable waste. It can also remove nitrogen if the plant has the right treatment zones.
• The membrane alone does not remove dissolved salts.
• Usually reduced well: suspended solids, many bacteria, and waste that microbes can break down.
• May remain: dissolved salts, some small chemicals, and some viruses. Removal varies by design.
• MBR water is not automatically drinking water. Extra steps may include disinfection or reverse osmosis. See this virus-removal review.

Why can the tank be smaller?

• A settling tank struggles when too much sludge is mixed in. A membrane holds that sludge back.
• Some MBRs run with about 8,000–12,000 mg/L of mixed solids. A conventional system often uses about 2,000–4,000 mg/L.
• The unit mg/L means milligrams of material in one litre of water.
• These are example ranges, not a rule for every plant. A stronger mix treats more waste in less space.
• Source: California State Water Resources Control Board.

Where can the membrane sit?

• Submerged MBR: the membrane sits in the tank or a nearby membrane tank. A gentle pump draws water through it. Air bubbles sweep its surface.
• Side-stream MBR: a pump sends the sludge mixture through a separate membrane unit. It is easier to service. Pumping can use more energy.

What is fouling?

• Fouling is a layer of sludge or other material on the membrane. It slows the flow of water.
• Operators first screen the incoming water and scour the membrane with air.
• Many systems also use backwashing. Clean water briefly flows backward through the membrane.
• Periodic chemical cleaning removes what these steps cannot lift.
• Blowers, pumps, cleaning, and membrane replacement cost money. They often use more energy than a settling tank.

Where does India use this idea?

• Building reuse: a case study describes a 300-kilolitre-per-day MBR at a Bengaluru hotel. It reused water for flushing, landscaping, and cooling. Invest India case study.
• Textile industry: a 2022 Ahmedabad plant paired MBR with reverse osmosis for reuse. The MBR removed biological waste and solids. Reverse osmosis handled dissolved material. Vendor case study.
• Pharmaceutical cluster: a vendor case study describes a 750-kilolitre-per-day MBR near Ahmedabad. It treats shared wastewater from pharmaceutical sites. Toray case study.
• Municipal planning: a NITI Aayog report proposed an MBR-based reuse scheme for Ballari. A design report is a plan, not proof the plant was built. Ballari design report.

When is MBR a good choice?

• MBR helps when land is scarce or treated water will be reused. Its clear permeate can make disinfection easier.
• The trade-off is more equipment, energy, and membrane care. Thick sludge must still be removed.
• A poor screen or a sudden change in incoming wastewater can disrupt treatment.
• The right choice depends on the water, the reuse goal, the land, and operating skill. MBR is not a cure for every pollutant.