With urbanization and the development of community water supplies and the use of water to flush or transport wastes from habitations, it became necessary to find disposal methods not only for the wastes themselves, but for the water which carried them. All of the three possible methods - irrigation, subsurface disposal and dilution - were employed.
As urban communities increased in population, with proportional increase in the volume of wastewater and in the amount of organic waste, all methods of disposal resulted in such unsatisfactory conditions that remedial measures became essential and the development of methods of treatment of wastewaters prior to ultimate disposal was started.
The objectives originally sought in wastewater treatment include :
- Protection and maintenance of sources for use as domestic water supplies.
- Prevention of disease and spread of diseases.
- Prevention of nuisance conditions.
- Maintenance of clean waters for bathing and other recreational purposes.
- Protection and maintenance of the environment. For example, maintaining natural waters for the propagation and survival of fish life.
- Conservation and protection of water for industrial and agricultural uses.
- Prevention of silting in navigable channels.
A wastewater treatment plant is designed to remove from the wastewater enough organic and inorganic solids so that it can be disposed of without contravening or affecting the objectives sought.
Treatment devices merely localize and confine these processes to a restricted, controlled, suitable area or environment and provide favorable conditions for the acceleration of the physical and biochemical reactions. The extent or degree of treatment needed varies greatly from place to place and is regulated by law. In general, the following are the determining factors :
- The character and amount of the solids carried by the wastewater, that is; BOD and suspended solids present.
- The objectives sought.
- The ability or capacity of the land (in disposal by irrigation and subsurface disposal) or the receiving water (in disposal by dilution) to handle by self-purification or dispersal the water and solids in the wastewaters.
- Legal aspects and constraints.
The degree of wastewater treatment required to satisfy the first three conditions above is variable and is highly dependent on the local conditions and needs. Simple settling or even the mere removal of floating solids by screens may be adequate for wastewaters under certain conditions, while a very high removal of suspended solids, decomposition of dissolved organic solids and destruction of pathogenic organisms may be required before discharge to a river which is used downstream as a source of public water supply.
After the disposal of the wastewater effluent from a treatment plant, there still remains in the plant the solids and water constituting the sludge which has been removed from the wastewater. This too must be disposed of safely and without nuisance.
The progress of self-purification of a stream can be measured by appropriate physical, chemical and biological laboratory tests. Similar tests are used to measure and control the progress of wastewater treatment plant processes.
The serious problem involving the disposal of wastewaters and other wastes by adequate and effective means that will eliminate nuisances and not violate the rights and welfare of individuals and communities has led to the development of laws and regulations governing such disposal.
It is presumed that in ancient times, customs slowly developed which regulated the disposal of the wastes of the individuals and of the group. As time went on, custom took on the force of law and led, over the years, to the formulation of legal regulations - first as common law and then as statutory law.
