MBR Process Overview
Conventional Activated Sludge (CAS) is widely used in various sewage treatments. Since gravity sedimentation is used as a means of solid-liquid separation, it brings many problems, such as low solid-liquid separation efficiency, low volume load of treatment equipment, large floor space, unstable effluent quality, low oxygen transfer efficiency, high energy consumption, and large residual sludge output.
MBR refers to a new sewage treatment device that combines ultrafiltration and microfiltration membrane separation technology with bioreactors in sewage treatment. This reactor combines the advantages of membrane treatment technology and biological treatment technology. Ultrafiltration and microfiltration membrane components can completely replace secondary sedimentation tanks as mud-water separation units. Ultrafiltration and microfiltration membranes intercept microbial flocs and larger molecular organic matter in the activated sludge mixed liquor, allowing them to remain in the reactor, so that the reactor can obtain a high biological concentration, and prolong the retention time of organic solids, greatly improving the oxidation rate of organic matter by microorganisms. At the same time, after being treated with ultrafiltration and microfiltration membranes, the effluent quality is high and can be directly used for non-drinking water reuse. The system almost does not discharge residual sludge and has a high impact resistance. In particular, in 1989, Yamamoto applied hollow fiber membranes to activated sludge treatment, which greatly reduced the process operation cost and had broad prospects for practical application.
MBR Types
From the overall structure point of view, MBR is composed of two parts: membrane components and bioreactors. According to the combination of these two operating units, membrane bioreactors can be divided into two types: split type and integrated type (immersed type). Split type MBR means that the membrane components and bioreactors are set separately, while immersed type MBR means that the membrane components are placed inside the bioreactor.
The main features of the MBR process are as follows:
1. Good water quality
Due to the use of membrane separation technology, there is no need to set up other solid-liquid separation equipment such as filtration. The efficient solid-liquid separation separates the suspended matter, colloidal matter, and microbial flora lost from the biological unit from the purified water, which can be directly reused without tertiary treatment, and has a higher water quality safety.
- Small footprint
The microorganisms in the biological treatment unit of the membrane bioreactor are maintained at a high concentration, which greatly increases the volume load. The high efficiency of membrane separation greatly shortens the hydraulic retention time of the treatment unit and reduces the floor space. At the same time, since the membrane bioreactor uses membrane components, it does not require a sedimentation tank and a special filtration workshop, and the system occupies only 60% of the traditional method.
3. Save energy
Due to the high oxygen utilization efficiency of MBR and its unique intermittent operation mode, the operation time and power consumption of aeration equipment are greatly reduced, thus saving electricity consumption.
The main disadvantages of the MBR process are as follows:
1. The removal rate of NH3-N is not ideal
Since the essence of the MBR process is still the AO process, its biological treatment capacity is also close to that of the AO process. Judging from the current influent water quality, the C/N of this project is relatively low, so the AO process cannot remove NH3-N to the target water quality. The subsequent nanofiltration has a better interception effect on BOD, SS and TP, but the removal rate of NH3-N is not ideal.
2. Low water flux
Due to the strong interception capacity of the membrane, the water flux per unit membrane area is low. Therefore, the MBR process is mostly used in projects with smaller water volumes. For large-scale sewage projects, the membrane groups are equipped with a large number of units, so the investment is higher.
3. High maintenance costs
Since the membrane assembly is a consumable material, the life of a set of membrane assemblies is about 2-3 years, and the cost of replacing a set is relatively high, resulting in higher maintenance costs for MBR than other processes. In addition, since domestic sewage contains more impurities, the membrane is easily scratched by various sharp substances (such as sand, bamboo pieces, etc.), and the replacement frequency is higher than that of foreign countries, resulting in further increase in operating costs.
