MBR (Membrane Bioreactor)
Membrane biological reactors for waste water treatment
The enforcement of Legislative Decree 258/00 in Italy, assimilating EU Directives 271/91 and 676/91, and the recent Decree issued by the Ministry of the Environment and the Protection of Land and Sea No 185 of 12 June 2003, establishing technical standards for the re-use of wastewater, has led to the introduction of more stringent regulatory limits regarding the removal of nutrients from effluents. In particular, the concept of a 'sensitive area', conceived as a receiving body of water exposed to the risk of eutrophication, has established the need for action resulting in the adaptation of existing waste water treatment plants, often characterized solely by compartments for the biological oxidation of organic compounds. However, this need for development tends to clash with the decreasing availability of surface areas for the execution of conventional activated-sludge processes, resulting in a growing interest of operators in certain types of treatment capable of rendering plants more compact. Membrane biological reactors (MBR) derive from the coupling of traditional suspended biomass processes with filtration processes on microporous or ultraporous membranes, depending on the nominal size of the pores. The major advantages deriving from this technology are to be found in the possibility of eliminating the sedimentation unit downstream of the biological compartment and all management and operational constraints related to it.
The replacement of the sedimentation compartment with a membrane filtration compartment involves:
- A considerable reduction in the layout area of the purification plant, due to both the disappearance of the settling tank and an increase in the concentration of suspended solids
in the biological reactor; - The possibility of managing the biological process in a way that is totally independent from fluctuations of the hydraulic load (sludge residence time and hydraulic retention time are entirely unrelated);
- The reduction of excess sludge associated with the highest values of sludge age;
- The elimination of sludge sedimentation issues common in traditional, activated-sludge systems;
- The significant improvement of qualitative characteristics of the effluent, compatible with the potential re-use of purified water.
Against these advantages of the MBR systems, there are still many aspects to be clarified, in relation to both the kinetics of the biological process and the control of the phenomena of membrane fouling.
MBRs remove between 96-99% of COD, compared to 95% in conventional activated sludge processes. MBRs can be operated at higher MLSS concentrations of up to 30 g/L with TSS < 1 mg and turbidity <0.2 NTU*. In addition, typical submerge systems can not operate at more than 10 – 12 g/L therefore tubular UF membranes solve this limitation.
Traditional wastewater treatment plants consist of pre-treatment systems to capture large floating particles and coarse suspensions (e.g. wood, textile, plastic and stones), followed by biological aerobic digestion that converts organic waste into carbon dioxide, nitrogen gas and water.
However, traditional submerged MBRs are highly susceptible to fouling, resulting in extensive maintenance requirements and lower flux rates.
Cf. “I reattori biologici a membrana per il trattamento delle acque reflue. Principi ed applicazioni”, Gianni Andreottola, Martina Ferrai, Giuseppe Guglielmi, Giuliano Ziglio. University of Trento (Italy), Department of Civil and Environmental Engineering.
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