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Potable Water Biotechnology, Membrane Filtration and Biofiltration 495
processes can be reduced. In addition, long denitrifying culture retention times and short
hydraulic retention times can be maintained in the process (22).
The performance of a pilot-scale MBR for denitrification of groundwater was tested as a
function of hydraulic and biological parameters by Buttiglieri et al. (43). Synthetic groundwater
was prepared by taking lake water and adding known amounts of ethanol and sodium
nitrate to study the NO3 – removal capacity of the sludge, to search for an optimum C/N ratio,
and to measure filtering ability for microorganisms through the membrane. The optimum C/N
ratio was 2.2 g C/g N, resulting in an effluent NO3 – concentration within the limits for
drinking water use. The membrane module, Zenon ZW-10, was monitored and performed
well except for a short stress episode due to low airflowthat was rapidly corrected afterwards
and thus putting the membrane back to its previous stable behavior. Total bacterial count for
the treated effluent was lower than influent water, and 100% removal was observed for both
total coliforms and Escherichia coli.
Hollow fiber membrane bioreactor is one of the major configurations widely used in
denitrification from drinking water. Ergas and Rheinheimer (20) have described the working
process of this reactor and developed a mathematical model of NO3 – mass transfer. In their
study, NO3 – contaminated water flows through the lumen of the tubular membranes and NO3 –
diffuses through the membrane pores. A denitrifying population is circulated on the shell side of
the MBR, creating a driving force for NO3 – mass transfer. Nutrients and electron donors are
added to the shell side of the membrane bioreactor to support the microorganisms. The
membranes provide high NO 3 – permeability, while separating the microbial process from the
water being treated (20). The advantages of this system is described by Ergas and Rheinheimer
as: (a) the contamination of the product water with sloughed biomass is eliminated by the
separation of water from biomass using a microporous membrane; (b) a number of low cost
electron donors such as ethanol, methanol, and acetic acid can be used to drive the high rate
denitrification process; and (c) reactors can be constructed of low cost tubular and hollow-fiber
membranes. It was also reported that pressure drops across these modules are low.
A MBR that combines the advantages of biological conversion of NO3 – to nitrogen with
that of hollow fiber ultrafiltration technology to produce high quality drinking water was
studied by Chang et al. (89). The influence of several parameters, both biological and
hydraulic, on the overall performance of the process has been investigated. With adequate
membrane backwashing frequency, a crossflow velocity was maintained for more than two
months with a net permeate flow rate of over 100 L/h/m 2 . A high permeate flow rate was not
detrimental to the overall denitrification process, since permeate NO 3 – and NO2 – concentration
remained below 20 and 0.1 mg/L, respectively, for a NO3 – volumetric loading rate of 2.8
kg/m 3 /day at a hydraulic retention time of either 60 or 30 min.
Li and Chu (90) investigated the performance of a hollow-fiber membrane reactor in
treating raw water supply. A submerged polyethylene hollow-fiber membrane module with
aporesizeof0.4mm and a total surface area of 0.2 m 2 was used for treating a raw water
supply slightly polluted by domestic sewage with a TOC level of 3–5 mg/L and ammonia
nitrogen (NH 3–N) concentration of 3–4 mg/L. The process was highly effective in eliminating
conventional water impurities, as demonstrated by decreases in turbidity, total
coliforms, and UV 254 absorbance. With the MBR treatment, the 3-day trihalomethane