disposal into lakes and rivers

Wastewater disposal to water environments

Disposal into a lake, stream or ocean needs to take into account the ability of the receiving water to assimilate wastewater. The natural purification capacity of the environment is limited. Even when wastewater is disposed to the ocean, the area surrounding the outfall can be sufficiently polluted and the pollutants (including pathogens) can be washed towards the beaches. Nutrients (nitrogen and phosphorus) promote the growth of algae in the receiving water. In lakes and sensitive water environments the removal of nutrients may be required. Furthermore if the wastewater contains high levels of heavy metals and toxic chemicals, these may have to be removed before wastewater disposal. Over the years the requirement for disposal into water environments has become stricter.

Wastewater Disposal

There are three methods by which final disposal of wastewater can be accomplished. The general problem areas that are of concern in final disposal are pathogenic microorganisms (viruses, etc.), heavy metals and the presence of biologically resistant organic compounds, such as pesticides or insecticides which can find their way into water supplies. 


Surface Disposal

Generally this is disposal by irrigation. This involves spreading the wastewater over the surface of the ground, generally by irrigation ditches. There is some evaporation, but most of the wastewater soaks into the ground and supplies moisture with small amounts of fertilizing ingredients for plant life. This method is largely restricted to small volumes of wastewater from a relatively small population where land area is available and where nuisance problems will not be created. It has its best use in arid or semi-arid areas where the moisture added to the soil is of special value. If crops are cultivated on the disposal area, the growth of vegetation, often must be excluded from wastewater. Because untreated wastewater will also contain pathogenic organisms, the production of foods for human consumption which may be eaten without cooking is not desirable.

Subsurface Disposal

By this method wastewater is introduced into the ground below its surface through pits or tile fields. It is commonly used for disposal of settled wastewater from residences or institutions where there is only a limited volume of wastewater.
                                                              Disposal by Dilution

Disposal by dilution is the simple method of discharging wastewater into a surface water such as a river, lake, ocean, estuaries or wetlands. This results in the pollution of the receiving water. The degree of pollution depends on the dilution, volume and composition of the wastewater as compared to the volume and quality of the water with which it is mixed. When the volume and organic content of the wastewater is small, compared with the volume of the receiving water, the dissolved oxygen present in the receiving water is adequate to provide for aerobic decomposition of the organic solids in the wastewater so that nuisance conditions do not develop. 

Where the dissolved oxygen in the receiving water is inadequate to maintain aerobic decomposition, anaerobic decomposition takes place and putrefaction with objectionable conditions results. A volume of of wastewater that has been treated to remove or reduce this organic matter can be discharged to a natural surface water. The dissolved oxygen in the receiving water is an important factor for aerobic decomposition of organic waste.

The problems associated with this type of disposal are the effects of toxic or potentially toxic compounds found in domestic and industrial wastewater. These may involve immediate toxic effects such as heavy metals in fish and the "concentration" of certain biologically resistant compounds in the food chain. An example would be the accumulation of certain pesticides by microorganisms that are consumed by higher organisms to include fish, birds, and even man. Another environmental effect of concern due to disposal of untreated wastewater by dilution is the enrichment of receiving waters by the introduction of plant nutrients such as nitrogen and phosphorous. The presence of excessive amounts of these nutrients can stimulate plant and algae growth in the receiving waters.

Need for Wastewater Treatment

The problem of wastewater disposal developed with man's use of water as a vehicle for carrying away the waste products of human life. Prior to that the volume of wastes, without the water vehicle, was small and disposal was largely restricted to the individual's or family's excreta. The earliest practice was simply to leave body waste and garbage on the surface of the ground where it was gradually decayed by bacteria, mostly the saprophytic anaerobic type. This caused the production of foul odors. Later, experience showed that if these wastes were promptly buried the odors could no longer be detected. Burial of human waste is a very ancient practice and even has biblical references. The next logical step was the development of the earth privy or outhouse, a method for the disposal of excremental wastes which is still widely used.

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.

Disposal

Satisfactory disposal of wastewater, whether by surface, subsurface methods or dilution, is dependent on its treatment prior to disposal. Adequate treatment is necessary to prevent contamination of receiving waters to a degree which might interfere with their best or intended use, whether it be for water supply, recreation, or any other required purpose.

Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater. Usually wastewater treatment will involve collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes. Most often, since large volumes of wastewater are involved, treatment processes are carried out on continuously flowing wastewaters (continuous flow or "open" systems) rather than as "batch" or a series of periodic treatment processes in which treatment is carried out on parcels or "batches" of wastewaters. While most wastewater treatment processes are continuous flow, certain operations, such as vacuum filtration, involving as it does, storage of sludge, the addition of chemicals, filtration and removal or disposal of the treated sludge, are routinely handled as periodic batch operations.