Combining submerged membrane technology with anaerobic and ...

Objectives and summarymore used instead depth filtration as pre-treatment step for the reverse osmosis process.Compared to depth filtration, tertiary membrane treatments produce water of better quality.This should be taken into account when water will be reused or discharged into sensitiveareas.In general, submerged MBR require higher aeration and initial investment costs, withrespect to side-stream membrane configurations. In contrast, pumping and operating costsare lower, requiring lower operating flows and cleaning frequencies. Thus, in the case ofsewage treatment, the selection between submerged and external configurations foraerobic MBRs seems somehow settled, in favour of submerged MBRs. In fact, submergedmembrane systems in municipal applications, represent in practice the totality of theinstalled membrane surface in Europe during the last decade. Although, nowadays most ofthe commercial applications are based on the submerged configuration, due to lowerassociated energy requirements, external configuration is still commonly used for certainindustrial applications as well as for tertiary filtration treatment.The main advantages of the employment of submerged membranes in combinationwith biological wastewater treatments are the total control of sludge retention time, the highstability of the process against peak loads and temperature and the high quality of theobtained effluent, which enable water reuse. Besides, the use of the membranes allowsthe complete retention and development of extremely slow-growth bacteria, such as newlydiscovered denitrifying methanotrophs, avoiding its wash-out from the biological systems.On the contrary, membrane fouling is one of the main drawbacks associated with theapplication of membrane technology for wastewater treatment. Fouling decreases thepermeability of a membrane, limits flux and shortens the life of membrane modules, thusincreasing both the capital and the operating costs of filtration systems. Membrane foulingis caused by the deposition of organic and inorganic substances on the membrane surfaceor within the pores. Organic fouling is mainly caused by colloidal and soluble organicmatter as well as by the sludge itself, which forms the so-called sludge cake layer.Different strategies can be adopted in order to minimize membrane fouling.Reversible fouling can be counteracted by physical means such as backwashing orrelaxation and air scouring (biogas in the case of AnMBR), whereas irreversible fouling canonly be removed by chemical cleaning. Finally, irrecoverable fouling refers to thephenomena which cannot be recovered using either physical or chemical cleaningstrategies.22