Types of Sewage Treatment Plants
Sewage treatment is one of those things that is easily identified (most everyone knows what a plant looks like) and essential to modern life but not widely understood. Many believe that the core of sewage treatment is chemical, when in fact it is mostly based on useful microorganisms. How these microorganisms are put to work in treating wastewater is the main element in categorizing a sewage treatment plant.
Identification
The more precise description for sewage treatment is domestic wastewater treatment. This is the process of cleaning up sewage from domestic and common commercial sources, but it does not usually include cleanup of wastewater from heavy industry, agricultural sources (such as factory farming) or radioactive sources such as nuclear reactor coolant water.
Function
Modern, effective sewage treatment plants all attempt to process wastewater through three individual stages: primary, secondary and tertiary treatment. The usual way of distinguishing between types of treatment plants is through the means used to achieve secondary treatment.
Primary treatment is the first stage, in which the substances that are easy to take out of wastewater are removed. These include fluids like fat and oil (which can be skimmed from the surface), rocks and grit (which can be strained), and trash (which can be raked).
Secondary treatment is intended to decrease the biological contaminants in the sewage. As these are the usual means of determining a treatment plant's "type," they will be dealt with individually and in detail below.
Tertiary treatment is meant to improve water quality before putting the water back in the environment. This often involves the use of either artificial filters or manmade, natural filters such as the construction of a lagoon or artificial wetland with reeds. There may also be treatments for the removal of excess nitrogen and phosphorus. There is usually a final disinfection, typically either through chlorination or UV treatment.
Types
Secondary treatment is the type in which the greatest and most substantial technical differences in sewage treatment take place. There are many ways of going about reducing the biological contaminants in wastewater, but a common feature is the promotion of microorganisms to eat or eliminate undesired elements of wastewater. Activated sludge uses dissolved oxygen to promote aerobic processes that remove organic wastes. A variant on this type combines activated sludge with a membrane separation system.
Biological aerated filters work by using a filer medium (either suspended in solution or attached to a layer of gravel at the bottom of the tank) to achieve the removal of organics and denitrification.
Filter beds put the sewage in a tank over a bed of coke or limestone chips that support the growth of microorganisms feeding on the organic matter in the sewage. These microorganisms, in turn, are fed upon by worms and the like.
Rotating disks work by placing the active component on the face of a disk. It's then slowly rotated through the tank, constantly re-exposing it to the open air and providing the extra oxygen the active microorganisms need to work.
For treatment types that have open tanks at this stage, surface aerators may be used to promote the right environment for the growth of the active microorganisms.
The final stage of secondary treatment, regardless of type, is to feed the wastewater into a tank and allow sediments to settle.
Considerations
The sludges that result from wastewater must be dealt with separately from the water that is put back into the environment, adding an extra wrinkle to the problems of sewage treatment. This is usually achieved by combined anaerobic and aerobic digestion techniques (which use non-oxygen and oxygen-using bacteria), and possibly composting to degrade the biological contaminants of this solid waste byproduct.
Misconceptions
Contrary to what many believe, the water produced by a wastewater treatment plant is cleaner, rather than clean. It is in no way safe to drink or considered "potable," although it may be safe enough for direct use in purposes such as irrigation (as is done in Israel). The idea behind wastewater treatment is to mimic the processes that go on in nature on an accelerated basis. Thus, the water that is poured by a treatment plant into a river is safe enough to be put there, as it will be heavily diluted and the river itself will serve as a fourth stage of treatment.
