Membrane and Desalination Technologies - TCE Moodle Website

Treatment of Industrial Effluents, Municipal Wastes, and Potable Water 207
Cote et al. (27) discovered that when a submerged membrane was placed in an aeration
tank for municipal WWT with an anoxic–aerobic process, total Kjeldahl nitrogen (TKN)
removal efficiencies were greater than 69 and 94% at mixed liquor suspended solids (MLSS)
concentrations of 15,000 and 25,000 mg/L, respectively. Further studies on aeration strategies
to optimize nitrogen removal designs are needed. The application of membranes to biological
WWT is limited by membrane fouling and high energy consumption. Back-flushing with
permeate or air in a crossflow membrane coupled to a biological reactor has been used to
reduce membrane fouling (22, 28, 29). An improvement in flux rates compared to that for
operations without back-flushing was reported.
Air contaminated with trichloroethylene (TCE) was passed through microporous hollow
fibers in a hollow-fiber MBR, whereas an oxygen-free nutrient solution was recirculated
through the shell side of the membrane module. A removal efficiency of 30% was achieved at
inlet TCE concentrations of 20 ppmv and a 36-s gas phase residence time (30).
A bioreactor was developed by Clapp et al. (31) using silicone tubing with an attached
methanotrophic biofilm to treat TCE-contaminated waste streams. The reactor was developed
to overcome the low solubility of methane, competitions between methane and TCE, the lack
of NADH regeneration in the presence of TCE only, and the cytotoxic products of TCE
metabolism.
Many other techniques such as formation of a dynamic membrane, precoat, or hydrophobic
skin layers atop the membrane have been introduced to reduce fouling in crossflow
MBRs, but these are still in an early stage of evaluation. Some researchers using crossflow
MBRs have reported that the pumping shear stress caused biological floc break-up, leading to
severe flux decline in long-term operations caused by the small flocs forming a denser
biomass cake layer on the membrane. Additionally, continuous recycling of mixed liquor
in a crossflow MBR requires a relatively large amount of energy (32–35).
Yamamoto et al. (36) studied an alternative to a crossflow membrane operation using a
submerged membrane with permeate removal by vacuum suction. Power consumption per
unit volume of treated water was greatly reduced by eliminating the circulation pump, but the
permeate flux was reduced to an impractical low level of less than 2 L/m 2 h. The energy
consumption associated with filtration in these new submerged membrane reactors was at a
substantially low level of 0.2–0.4 kW h/m 3 compared to the relatively high energy consumption
(2–10 kW h/m 3 ) with circulation loops (27).
Performance of an SBR using a membrane for effluent filtration was investigated by Choo
and Stensel (37) in terms of chemical oxygen demand (COD) removal, nitrogen removal, and
membrane permeability during long-term continuous operation treating synthetic wastewater.
The reactor was operated with six 4-h cycles per day consisting of 0.2, 2.0, and 1.5 h for
fill, aeration, and effluent filtration-idle, respectively. Minimal solids wasting occurred for the
first 10 months of operation, followed by an 8-day solids retention time (SRT) for the final 1.5
months. Membrane fouling was controlled by backwashing with aeration for 10 min during
each cycle. A stable permeate flux of approximately 0.34 (L/m 2 h)/kPa or 34 (L/m 2 h)/bar was
achieved and was independent of MLSS concentrations of 700–10,000 mg/L. The reactor
effluent turbidity averaged less than 0.20 NTU and more than 98% COD removal occurred.
Nitrogen removal efficiency ranged from 87 to 93% through biological nitrification and