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The MBR Book: Principles and Applications of Membrane

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Figure 4.36 Ejector aerator<br />

Commercial technologies 193<br />

reactor volume for a given sludge loading. However, membrane flux performance<br />

deteriorates with an increase in MLSS, <strong>and</strong> the optimal sludge concentration is on<br />

average 17–20 g/L for the 8 mm tubes used (Fig. 4.36). Bioreactor pressurisation,<br />

up to a pressure <strong>of</strong> 3 bar, <strong>of</strong>fers a number <strong>of</strong> advantages <strong>of</strong>:<br />

● control <strong>of</strong> sludge foaming;<br />

● enhanced oxygen dissolution, thus permitting higher organic loadings <strong>and</strong>/or<br />

reduced tank size;<br />

● reduced risk <strong>of</strong> stripping <strong>of</strong> volatile organic matter, <strong>and</strong> so reducing the size <strong>of</strong><br />

any air scrubbers which might be required for <strong>of</strong>f-gas treatment.<br />

<strong>The</strong> atmospheric bioreactor can be constructed <strong>of</strong> GRP or PP for capacities up to<br />

20 m 3 . At capacities greater than this, it is usual to use glass coated steel. <strong>The</strong> maximum<br />

tank height in either case is 10 m. <strong>The</strong> pressurised system tends to be used for<br />

more recalcitrant feedwaters (COD/BOD ratio �~4). For these reactors, coated steel<br />

is used for capacities up to 200 m 3 <strong>and</strong> the maximum tank height is 15 m. Disc<br />

membrane aerators are used for low loads <strong>and</strong> ejectors (high-shear jet aerator,<br />

Fig. 4.36) for high loads.<br />

<strong>The</strong> use <strong>of</strong> a smaller aeration tank tends to elevate the temperature to between<br />

30°C to 35°C due to energy generated from the exothermic bio-process, aeration<br />

blowers <strong>and</strong> pumps. This process then operates more efficiently due to increased<br />

bioactivity <strong>and</strong> reduced permeate viscosity. On the other h<strong>and</strong>, the specific energy<br />

dem<strong>and</strong> is high compared with an immersed process (Section 2.3.1).<br />

Two other processes have been developed by Wehrle: a vertically mounted airlift<br />

sidestream BIOME<strong>MBR</strong>AT ® Airlift process, which provides a lower flux combined<br />

with a lower energy dem<strong>and</strong> (Fig. 4.37), <strong>and</strong> a low-crossflow process called the<br />

BIOME<strong>MBR</strong>AT ® -LE (Fig. 4.38) where the modules are placed in series. <strong>The</strong><br />

BIOME<strong>MBR</strong>AT ® -LE is designed to allow an adjustable crossflow velocity which permits<br />

a wide range <strong>of</strong> hydraulic loads. This means that peak loads are dealt with by

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