Anaerobic Treatment for Water Pollution
The satisfactory disposal of the concentrated organic solids removed from sewage in the primary sedimentation tanks and the excess biological solids from trickling filters or activated sludge is often brought about by anaerobic biological treatment process referred to as anaerobic digestion. The basic problem with these concentrated organic solids is that they cannot be readily dewatered.
In the absence of air, the anaerobic bacteria break down the water-binding organic solids and produce a reduced sludge mass which can be readily dewatered to a stable solid. The basic treatment unit of the anaerobic digestion process is a tank, usually a circular concrete tank with or without a cover. In those tanks without a cover, the sludge mass at the top of the tank will dry out quite quickly, forming a rather solid cover.
Initially, the tanks were designed to hold the sludge solids for several months while the microorganisms slowly brought about digestion. As the volume of solids increased, there was demand for a more rapid process. The raw solids are pumped through external heat exchange to raise the temperature prior to introduction into the digester. The loss of heat from the digester requires that the contents of the digester be slowly recycled through the heat exchanger to keep the digestion tank contents close to 35°.
Originally, it was thought that the use of external heat exchanger with its constant sludge recycle would keep the digester contents completely mixed in the upper layer. This has been shown to be untrue, as a pattern of short-circuiting develops. To keep the digester contents in motion, there is a choice between mechanical mixing and gas mixing. Properly designed mixers of either type will produce the same results.
The major problem with sludge mixers is the lack of well-designed equipment. It has been indicated that gas mixing has a catalytic effect on methane production but there is no scientific basis for this. In single-stage digesters, i.e., only one digester, the mixing is usually confined to the upper volume of the digester.
The lower volume is allowed to remain quiescent so that the denser digester sludge can separate from the lighter undigested sludge. In two-stage digesters, the mixing is usually complete in the first-stage digester with quiescent conditions in the second-stage digester. Once the microorganisms have finished digesting the organic solids, the sludge is removed from the digester and dewatered by gravity or vacuum filtration. The dried sludge is then disposed of on the land where microorganisms break down the small quantity of organic material left.
There are two stages in anaerobic waste treatment: 1) acid formation and 2) methane formation. In the first stage, a heterogeneous group of facultative and anaerobic bacteria, commonly termed the “acid formers,” convert proteins, carbohydrates, and fats, primarily into fatty acids by hydrolysis and fermentation. The methane-producing bacteria then utilize the organic acids in the second stage, converting them into carbon dioxide and methane which are stable organic wastes.