Membrane Bioreactors and Nutrient Removal

To achieve nitrogen removal with a MBR system, aerobic-anoxic operation conditions are necessary as in the conventional process (see Section 10.4.2). A MBR offers good nitrification conditions because of the high retention of sludge allowed. Therefore, even slowly growing nitrifying bacteria have an excellent chance to establish themselves in the aerated tank. Nitrogen removal with preliminary denit-rification (see Fig. 10.9a) and with post-denitrification (see Fig. 12.11b) is in operation with MBRs (Brepols et al. 2005). Investigations have also been performed in small-scale applications with post-denitrification without dosage of a carbon source (Gnirg 2005; Maas 2005).

The high oxygen transfer associated with submerged membranes creates a need for a larger denitrification zone (Engelhardt 2003; Gnir£ et al. 2003; MUNLV 2003). In the case of post-denitrification, the membrane modules are located in a separate container behind the anoxic zone with optiomal aeration to avoid sludge cake formation (Adam 2004; Drews et al. 2005).

Phosphorus elimination by simultaneous precipitation is a well known technique in conventional wastewater treatment and is also used in MBR. The high MLSS retention efficiency of the membrane process significantly reduces the discharge of coagulant chemicals and leads, therefore, to less loading of rivers. In surface water processing, Dietze (2004) achieved low phosphate concentrations (<15 pg L-1 PO4-P) in permeate using phosphorus flocculation and membrane filtration while reducing the coagulant and salts discharge, compared to traditional flocculation and sand filtration.

Fig. 12.11 MBR and nutrient removal with submerged membranes with: (a) preliminary denitrification, (b) post-denitrification without substrate dosage and with biological phosphorus removel.

Investigations have been conducted to combine biological phosphorus elimination with an anaerobic zone, not only in WWTP (Section 10.4.3, Fig. 10.9a) but also in the MBR process to reduce the use of chemicals like FeCl3. The question remains whether the high sludge ages used in MBR yield higher phosphorus elimination capacities. The high retention of MLSS and the prevention of phosphorus release caused by the high oxygen concentration in the MBR system lead to a high efficiency (Gnirfi et al. 2003). Orthophosphates from release cannot be retained by ultrafiltration membranes (Adam 2004).

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