What is a membrane bioreactor?
In a wastewater treatment system, a membrane bioreactor (MBR) removes solids via the membranes resulting in pathogen-free water. Basically, an MBR consists of membrane filtration for the separation of solids from the water. The use of membranes in the MBR not only eliminates the need for secondary clarifiers but also removes microorganisms, including harmful pathogens, from the water.
How does membrane bioreactor (MBR) treatment work?
The MBR in wastewater treatment could be a submerged MBR or a lateral set-up MBR. In the submerged design, the MBR is a tank or in one of the series of the tanks. The wastewater enters the tank or flows through the series of tanks where the MBR act as a bioreactor.
In the latter design, the MBR is outside of the tank. In this case, the wastewater enters the tank and eventually flows through the MBR.
In both designs, the permeate is produced (i.e., pathogen-free water). Sludge is also produced, and the quantity of sludge depends on the additional features of the wastewater treatment plant.
The membrane processes in the MBRs are integral for achieving the separation of the solids from the water. There are two types of these processes.
Two common membrane processes used in MBRs
1. Pressure driven processes
Microfiltration and ultrafiltration membranes are pressure-driven because they rely on hydraulic pressure to achieve the separation of the solids from the water. Membranes used for microfiltration have relatively smaller pore sizes of molecular weight cut off 100-500 kiloDaltons that retain bacteria along with microparticles compared to membranes used for ultrafiltration.
Different combinations of pressure driven membranes are applied in wastewater treatment plants for treating different composition of wastewater. Examples include the use of ultrafiltration with reverse osmosis for treating oily wastewater and microfiltration also with reverse osmosis for treating urban wastewater.
2. Non-pressure driven processes
These processes include forward osmosis but could also be electrically driven such as by electro-dialysis. Forward osmosis has been applied for treating raw municipal wastewater and primary sewage effluent. Electrodialysis has been applied, in addition to treating municipal wastewater, for the removal cadmium and selenium which are heavy metals, for wastewater originating from the electroplating industries.
Why should a wastewater treatment utility consider using membrane bioreactors (MBRs)?
The MBR basically acts as a separation device to separate the solid particles from the water. The solids are retained on the surface of the membrane in the MBR. This feature is beneficial for a wastewater treatment utility to consider because MBRs retain solids including harmful microbes while also producing less sludge.
Additionally, the flexibility in the types of membranes — pressure driven or non-pressure driven — afford utilities the option to select a system that is most effective for treating the wastewater common to their influent characteristics. This makes MBRs feasible for use in varied wastewater treatment applications. There are also other advantages of MBRs, discussed in the following section.
What are the advantages and disadvantages of membrane technology treatment?
Advantages
- The use of filters eliminates the need for secondary clarifiers and thus results in a shorter hydraulic retention time and overall, less space requirement for operation.
- MBRs often prroduce less sludge.
Disadvantages
- Membrane systems are prone to fouling problems.
- Membranes must be replaced regularly for optimal effectiveness.
Membrane fouling significantly reduces the lifespan and the performance of the membrane. It occurs when suspended particulates that include microorganisms, colloids, solutes, and sludge flocs deposit on the surface of the membrane.
Over time, the deposit grows bigger and clogs the membrane pores leading to a decline in the permeability of the membrane. This requires the need to replace the clogged membrane with a new membrane and the replacement could be costly.
IUPAC definition of membrane fouling
According to the International Union of Pure and Applied Chemistry (IUPAC), membrane fouling is defined as “the process resulting in loss of performance of a membrane due to the deposition of suspended or dissolved substances on its external surfaces, at its pore openings, or within its pores.”
Types of foulants for membranes
Organic foulants
Extracellular polymeric substances (EPS) are an example of organic foulants. These are high-molecular weight mixture of polymers and are composed of humic acid in sludge from the processes in biological wastewater treatment reactors.
Biopolymer clusters are another example of organic foulants that result from the clustering of loose EPS and soluble products from microbial metabolism. These are large in size such that they are retained on the membrane in the early stages of membrane fouling.
In a research study, biopolymers extracted from aerobic granular sludge were constituents of EPS, making up more than 70%, indicating that biopolymers could be a constituent of EPS in wastewater treatment plants.
Inorganic foulants
Cations and anions such as calcium ions, magnesium ions, ferric ions, sulfate, phosphate, and carbonate are examples of inorganic foulants. While some of these ions are beneficial for controlling fouling, high concentrations of these can cause fouling because they can precipitate on the surface of the membranes.
Basic mechanism of membrane fouling in a wastewater MBR
In a MBR, sludge floc, EPS, and fine colloids deposit on the side of the membrane where the influent wastewater is in contact with the membrane. This leads to pore narrowing and over time, pore clogging, and eventually cake formation on the side of the membrane where the influent wastewater is in contact with the membrane.
Operative consequence of membrane fouling in a wastewater MBR
Membrane fouling decreases the permeate flux when the MBR is operated at constant transmembrane pressure. There are several factors that causes this as follows.
Factors that affect membrane fouling in MBRs
- Feed and biomass characteristics including the composition of the mixed liquor suspended solids, floc size, alkalinity and pH of the influent water
- Operating conditions including the rate of aeration, food to microorganism ratio, solids retention time, and the chemical oxygen demand to the nitrogen ratio
- Material type of the membrane including pore size and the water affinity of the membrane
