Now in the Russian market you can find the plants for sewage treatment based on multistage circuits. It happens, because classic biological methods without additional blocks of posttreatment do not provide required quality of sewage treatment. So, for example, according to the normative requirements for dumping of the cleared drains to fishery basins, total biological oxygen demand value should be no more than 3,0 mg/l, and biological methods of purification can achieve only high total biological oxygen demand values 10 - 15 mg/l.

The limit of purification quality for biological technology is connected to inevitable loss of active silt together with purified water. Loss of active silt results in secondary pollution of cleared waste water and requires application of special methods of its posttreatment.

Posttreatment of classical technology is made on mechanical and sorption filters with use of various reagents (coagulants, flocculants, alkalis, disinfectants, biogenic additives, biological products, etc.).

The limiting factor at use of posttreatment filters is the low resource of filtering fillings connected to their fast fouling, silting and failure resulting in breakthrough of polluting substances.

The term «multistage posttreatment», presented by suppliers as a synonym of efficiency, as a matter of fact, means the opposite - technological imperfection of the plants, their complexity, unreliability and, finally, inability to provide stable quality of sewage treatment according to the normative requirements of the Russian legislation.

An alternative of biological purification technology with multistage posttreatment and constant introduction of reagents is the modern membrane-biological technology with use of membrane bioreactor (MBR).

The problem of compliance with quality specifications for the cleared waste water for the objects of local water drain was not satisfactorily solved at use of classic flow systems of biological purification. Long-term negative experience of operation of such structures calls for application of essentially new technologies for sewage treatment.

Technological break in this direction has taken place about fifteen years ago when the membrane bioreactors (MBR) for sewage treatment have appeared.

Synthesis of biotechnology and water suspension separation technology is a basis of action of the bioreactor with ultrafiltration polymeric membranes.

MBR system consists of aerotank and membrane module equipped with hollow fibers with ultrafiltration membranes. The treatable sewage goes to aerotank. The silt mix in the aerotank circulates through the membrane module. Ultrafiltration membranes increase concentration of active silt in aerotank and perform deep purification of treatable sewage. Aerotank of MBR system works with high concentration of active silt, therefore its dimensions are 2-3 times less than dimensions of classic flow aerotank.

Aeration is carried out by compressed air with the help of aeration systems (blowers). Depending on required productivity the membrane modules can be combined together into the membrane block. The number of membrane modules in the block can be increased for increase in system productivity.

The tangent filtering of the silt mix used in MBR systems prevents its clogging, i.e. accumulation of deposits (bacteria). Such movement of the silt mix is provided by the circulating pump, which productivity is much higher than the charge of treatable waste water. The opportunity of regulation of charge and pressure in the circulating contour results in full-fledged control of membrane filtering process at its maximum efficiency. Besides the realization of tangent filtering mode has positive consequences for the biology of all the system. Constant washing of the membranes results in dispersion of purification bacteria which do not form dense floccules any more that is why they can considerably increase direct contact to dirt and oxygen. So the ratio of active bacteria and oxidized dirt is higher in MBR system, than usually takes place in classic system with active silt.

Microorganisms of active silt are not taken out from MBR system, therefore the bioreactor works in conditions of high concentration of significant age biomass. Besides constant circulation results in mechanical influence on bacteria environment. For this reason the basic energy consumed by bacteria is used not for reproduction (as it takes place in classic biotechnologies), but is spent for maintenance of life that results in decrease in a gain of superfluous active biomass.

Thus, MBR system makes less (by 20-50 %) superfluous active silt in comparison with classical way of aerobic treatment. And it, in turn, results in essential decrease in operational expenses, because expenses of recycling of superfluous silt made 30-40% of total operational expenses of treatment facilities.

Any membrane filtering requires periodic cleaning of the membrane for restoration of its initial characteristics and removal of possible organic and mineral deposits. The membrane unit of MBR system is located outside of bioreactor and mounted in a separate premise, it is easily accessible for maintenance.

Washing of the membrane block is carried out by circulating pump which provides uniform washing of the membranes along the full length that guarantees identical cleanliness of the surface in any point. Washing of the membrane block is fully-automated. It takes several hours and is carried out several times a year as a preventive measure in an automatic mode.

Application of MBR technology gives an opportunity:

• To make, without inclusion into the manufacturing scheme of additional blocks, deep sewage treatment from polluting substances according to the parameters which meet the requirements for dumping of the cleared drains into natural reservoirs of all the categories;
• To increase the operational resistance of bioreactor to volley of bioresistant substances, typical for industrial objects of local water drain;
• To lower by 20 - 40 % the mass-dimensional characteristics of reservoirs, because necessary amount of active silt occupies smaller volume at higher concentration;
• To reduce by 30 - 70 % the area occupied with the equipment (in view of absence of secondary sediment bowls, posttreatment units, silt platforms);
• To provide high microbiological safety of the cleared drains (due to two-level non-reagent disinfection systems: the membranes are not permeable to the microorganisms, and the ultra-violet irradiation provides additional water disinfection);