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Sep 1, 2003 -
Much interest and ongoing development work is being carried out around
the world regarding a wastewater treatment process known as Membrane
Bioreactor (MBR). The process was developed in the late 1970s in North
America, Japan, and South Africa concurrently. The process combines
mixed liquor activated sludge, operating at high concentrations, with
ultrafilter membranes for solids separation. In the beginning, the
process was limited to small wastewater flows where high-quality
effluent was required, due to the high capital and operating costs
when compared to other processes. With the ever increasing demands for
higher effluent quality, as well as the advent of the immersed hollow
fiber and flat sheet membrane for the process, MBR’s are now very much
in demand and are being applied in larger municipal projects.
Recently,
a new tubular membrane arrangement has been developed in Germany,
Holland, and the United States for use in the MBR process. This is due
to the desire for an economical MBR that uses external tubular
membranes to compete with the hollow fiber and flat sheet type of
membranes. Conventional use of cross-flow tubular membranes requires
significant pumping power to create high turbulence inside the tubes in
order to minimize fouling. Tubular membranes are more robust than
hollow fiber and flat sheet membranes, and can be placed outside of a
tank for easy maintenance. Their longer life has driven the market to
find a way to use the external tubular product.
Unique Membrane Arrangement
 By
placing the tubular modules vertically, and limiting the amount of
permeate withdrawn from the membrane, it has been proven that membrane
flux is stable and fouling is manageable using much less pumping power
than in a conventional cross-flow system. Air is injected at the bottom
of the module, creating an airlift effect that increases the turbulence
inside the tubes. A cyclical backwash of the membrane modules maintains
consistent
flux.
The flux is less than a standard cross-flow
membrane arrangement and the power required is only about 10%-20% of a
cross-flow system. This power requirement is very competitive with
immersed product systems. Low applied pressure and the extremely low
trans-membrane pressure (TMP) minimizes the amount and the density of
the solids boundary layer on the membrane surface. This produces very
stable flux and low rates of fouling.
Flux
stability is enhanced by a periodic backwash. Cleaning is achieved by
either injecting cleaning solutions with the backwash water, or by
circulating or soaking cleaning solutions through the membrane modules.
This effectively keeps the membrane flux at the required levels for
long periods of time.
System Description
System Operation
The
system operates on a continuous basis by controlling the rate of
permeate flow from the membrane modules. A recirculation pump feeds
mixed liquor from the bioreactor to the bottom of each module where air
is injected. This, in effect, acts as an airlift pump increasing the
velocity that aids scouring inside the membranes. The scouring mixture
discharges from the top of each module and is returned to the
bioreactor (or the anoxic zone, if de-nitrification is taking place).
Back-flushing with permeate water is initiated on a timed cycle to each
bank of modules. This removes any cake formed on the inside of the
membrane tubes, thus maintaining flux rates.
Advantages
- Robust
tubular membranes are more durable and require less frequent
replacement. Membranes are double supported by a rigid backing that
ensures no breakage or rupture.
- Effluent integrity is assured, since membrane rupture or breakage is not an issue.
- Low-power requirements comparable with immersed hollow fiber and flat sheet systems.
-
Safer working environment for the operator since maintenance cleaning
does not require moving the membranes with hoists and trolleys.
- Minimizes operator exposure to wastewater and potential health impacts.
- Lower installed capital cost since no additional tanks, sludge, recycle or permeate pumps, hoists or trolleys are required.
- Lower lifetime operating cost due to lower power costs, fewer membrane replacements and lower labor costs.
- Compact footprint does not require additional tanks.
- Better control of fouling since 100% of the membrane surface area is
continuously exposed to the circulating fluid without dead spots.
- Thirty-year proven reliability of the tubular product.
- Resistant to chemical degradation due to the use of high strength PVDF membrane chemistry.
- Efficient cleaning in place without membrane removal being required.
Contacts.
Dynatec Systems Inc.
909 Jacksonville Road
Burlington, NJ 08016
609-387-0330 voice
609-397-2060 fax
www.dynatecsystems.com |
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