Membrane Bioreactor Plant

A MEMBRANE BIOREACTOR (MBR) is a process which combines a microfiltration or ultrafiltration membrane unit with a suspended growth bioreactor, and is now widely used in both municipal and industrial WasteWater Treatment Plants (WWTPs).

In a wastewater treatment process, a bioreactor is specifically-designed chamber to support a biologically active environment, namely where bacteria and protozoa (the so-called biomass) can grow and consume some (or all) the substances within the raw wastewater.

They can be aerobic (to remove organic matter and oxidize ammonia to nitrate), anoxic (to remove nitrogen from nitrates to nitrogen gas) or anaerobic (to remove organic matter), depending on the presence of oxygen and nitrates or their absence. Typically, membranes are installed after aerobic or anaerobic bioreactors (respectively, the MBR and the An MBR processes).

There are three types of bioreactors:

• Suspended growth bioreactors, where the biomass grows into flocs;
• Attached growth (or biofilm) bioreactors, where the biomass grows attached to carriers;
• Hybrid bioreactors, which combines suspended and attached growth.

Typically, suspended growth bioreactors are these ones used for MBR processes. If properly designed, hybrid bioreactors can be used as well.

In the MBR process, membranes act as a solid-liquid separation device, keeping the biomass within the bioreactor before discharging the treated effluent to the nature. Basically, they take the place of clarifiers used in the conventional activated sludge (CAS) process.

Both micro- (MF) and ultrafiltration (UF) membranes can be used in MBR applications. Typically, UF membranes are the preferred choice because of their superior separation characteristics (thus, being able to remove some colloids and viruses as well) and lower fouling tendency (because of the smaller pore size, they have a lower risk of pore clogging).

There are three types of membrane geometries used for MBRs:

1. Hollow fibre (HF);
2. Flat sheet (FS);
3. Tubular (or multi-tubular, MT).

Other configurations, such as spiral-wound (SW), are not suitable to MBR applications because of their sensitivity to suspended solids contents.

The crucial function of membranes is to separate solids from a liquid. In activated sludge facilities, this is traditionally accomplished using secondary clarifiers.

Two process configurations are possible:

• Submerged MBR, in case vacuum-driven membranes are used (like PCI HF-Zmbr2 Series);

• Sidestream (or external) MBR, in case pressure-driven membranes are used (like PCI A-Series).

Usually, pressure-driven membranes are used for smaller installations and/or tough-to-treat industrial wastewaters, while submerged membranes are used for medium and large installations.

Large clarifiers no longer are needed. A smaller often rectangular shaped chamber, fitted with the membrane cassettes replaces the secondary clarifier whose size is governed by hydraulic and solids loading. On top, because of the higher biomass concentrations that can be sustained within the bioreactors, the same total mass of solids is stored in a smaller tank, resulting in up to 50% smaller footprint.

In comparison to the activated sludge (CAS) process, the effluent is free of suspended solids and reduced bacteria and viral content. Therefore, minimum disinfection is required.