Sewage Treatment Applications

As shown in Figure 7.33.4, the effluent from the activated-sludge reactor is continuously withdrawn through a screening device to the recirculation pump of a membrane loop. The membranes separate the large molecules, and the retained solids are returned to the activated-sludge reactor. Because only small molecules can pass through the membrane separation device, a high concentration of biological solids develops in the activated-sludge reactor. Wastewater treatment facilities characteristically run this type of activated-sludge process with biological solids concentrations from 15,000 to 40,000 mg/l. Operating with such high solids content has two advantages: (1) the large biomass quickly degrades organics that can enter with the

FIG. 7.33.4 Sewage treatment system using a combination of membrane filtration and biodegradation by activated sludge. aThe influent flow range is 3000 to 30,000 gpd at BOD concentrations of 200 to 600 mg/l. bThe loop is purged when the solids concentration reaches 4% by weight and the total volume is only a few gallons. Total volume removed is well below 1% of the total influent volume.

FIG. 7.33.4 Sewage treatment system using a combination of membrane filtration and biodegradation by activated sludge. aThe influent flow range is 3000 to 30,000 gpd at BOD concentrations of 200 to 600 mg/l. bThe loop is purged when the solids concentration reaches 4% by weight and the total volume is only a few gallons. Total volume removed is well below 1% of the total influent volume.

feed and prevents fouling of the membrane surfaces, and (2) the activated-sludge reactor size is much reduced.

The membrane filter in the system guarantees a practically infinite detention time for the slow biodegradable components in the sewage because they cannot exist from the system. The biological solids are also totally contained by the membranes. Such a sewage treatment system operates with almost no discharge of excess activated-sludge solids. Practically all feed materials are converted to carbon dioxide, water, and inorganic salts. Some inert materials do accumulate in the reactor, and purging (see Figure 7.33.4) of the contents of the reaction system is periodically necessary.

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