Activated Sludge Process - 8...

Condition of Wastewaters...

The extent and nature of the bacterial decomposition of solids in wastewater has given rise to certain terms which describe the condition of the wastewater. Some of these are summarized as follows.

Fresh Wastewater...

Is, as the name indicates, the first stage after waste solids have been added to water to produce sewage. These waters contain dissolved oxygen, and are thus said to be aerobic. Fresh wastewater remains fresh only as long as there is sufficient oxygen to maintain aerobic decomposition. Such sewage is turbid with solids in suspension or floating, grayish in color, and has a musty, though not unpleasant odor. Wastewater just prior to entering the treatment plant is called raw wastewater.

Septic...

Is a term which describes a wastewater in which the dissolved oxygen has been completely exhausted. A water in which no dissolved oxygen is present is called anaerobic. In a septic system anaerobic decomposition has taken place with the production of hydrogen sulfide and other gases. Such wastewater is characterized by a blackish color, foul and unpleasant odor and with black floating and suspended solids.

Stable Wastewater...

Is one in which the solids have decomposed to relatively inert end products and which are no longer subject to further decomposition or are only slowly decomposable. When stability has been achieved in a wastewater, dissolved oxygen again becomes present by absorption from the atmosphere and there is little or no odor and few suspended solids.

Physical and Chemical Changes in Wastewater Composition...

Wastewaters are treated by a variety of ways, ranging from sedimentation to biological, chemical or physical methods. For example, the activities of biological life in wastewaters produce many changes in the physical appearance and the chemical composition. In most wastewater treatment processes some form of sedimentation is used - that is the use of settling to reduce the solids. For example, in preliminary wastewater treatment, grit, the heavier, more rapidly settling inorganic components of a wastewater, are removed. This is often followed by the settling of lighter organic solids as primary sludges. Organic components are stabilized by biological reaction (secondary treatment). After secondary treatment is carried out, biological sludges are also settled. As a result of aerobic treatment, certain obvious chemical changes also occur in wastewaters. The wastewater becomes less turbid and less offensive and the solid component, when treatment has been satisfactory, will settle more easily. Changes in pH, dissolved oxygen, ammonia and other characteristics also occur as treatment proceeds. As a result of this physical removal process, the concentrated solids are separated.

The process of biochemical changes is well illustrated by the nitrogen, carbon and sulfur cycles, which show the steps or stages through which organic matter containing nitrogen, carbon or sulfur pass from dead organic matter through decomposition to products used by plant life upon which animal life depends. Animal life in turn, through waste products, death and decay, ends up as dead organic matter to start the cycles over again.

In all three cycles, the left half of the figures pertain to living matter while the right half of the figures is concerned with dead or waste material. Wastewater treatment and disposal lie in the right half. Aerobic biological waste treatment processes (i.e. activated sludge) are capable of converting organic nitrogenous compounds and ammonia nitrogen to nitrate nitrogen. This biological oxidation process is called nitrification.