Treatment and disposal of wastewater is a necessary function in communities where waste is transported via water. Domestic wastewater is composed of waterborne solids and liquids as well as many bacteria and other microorganisms. Because of this, using proper removal and disposal techniques is a fundamental part of protecting domestic water supply sources as well as preventing diseases, protecting the environment, conserving and protecting water supplies for agricultural and industrial purposes, and maintaining navigable water channels. In urban centers, water is most often carried through the sewers to a wastewater treatment facility, but roughly 20 percent of U.S. homes still rely on on-site wastewater systems, such as septic tanks, that range from individual to cluster treatment systems that process sewage.
There is a difference between wastewater disposal and treatment. All wastewater must be properly disposed, but in many cases, wastewater must also be treated to completely or at least partially remove solids. Treatment will often break down the highly putrescible organic solids to more stable organic solids, the extent of which depends on the treatment processes involved. The main goals behind treatment are to mitigate the public health hazards and reduce negative impacts on the environment and water quality.
Wastewater Disposal Methods
Disposal methods for wastewater have evolved over the years into a complex body of knowledge that involves applied engineering and environmental sciences and are carried out by professionals in the area. Three primary methods for proper disposal are generally used for final disposal of wastewater.
Surface disposal of wastewater is an irrigation process that spreads wastewaters over land areas, normally through irrigation ditches. Though evaporation plays a small part, the majority of the wastewater is soaked into the ground, serving as moisture and, to a certain extent, fertilizer for the vegetation. For safety purposes, surface disposal is only suitable for small quantities of wastewater and, thus, only effective for relatively small population centers where plenty of land is available and disposal in the area won’t create any problems. This disposal technique is most effective in arid and semi-arid regions where the wastewater ultimately serves as extra moisture and is an added value to otherwise dry land. Generally speaking, production of foods for human consumption without cooking on land being used for wastewater disposal is ill-advised.
Wastewater disposal through the subsurface disposal method is most commonly implemented for settled wastewater from residences or institutions which have a limited amount of wastewater. Subsurface discharge takes place at the treatment site by discharging septic tank liquid waste and introducing it into the ground where it then leaches through pits or tile fields (drain field). Subsurface disposal is limited to small-scale disposal efforts and has little to no application for larger population centers.
The dilution process for disposal is carried out by injecting the wastewater into a larger volume of surface water such as a river, lake, ocean, estuary, or wetland region. Unfortunately, this disposal method results to a certain degree of pollution, which enters the receiving body of water. The amount of pollution, of course, is dependent on the volume and composition of the wastewater in relation to the quality of water it is being mixed with. If the volume and organic content of the wastewater is relatively much smaller than the water it is being diluted into, there is normally enough dissolved oxygen in the receiving water to break down harmful substances. Microbial pollution, of course, still remains and any solids not removed will remain in the water, which may cause a potential health hazard if left unchecked.