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The ultimate return of used water to the environment. Disposal points distribute the used water either to aquatic bodies such as oceans, rivers, lakes, ponds, or lagoons or to land by absorption systems, groundwater recharge, and irrigation. Wastewaters must be mixed, diluted, and absorbed so that receiving environments retain their beneficial use, be it for drinking, bathing, recreation, aquaculture, silviculture, irrigation, groundwater recharge, or industry.
Wastewater is treated to remove contaminants or pollutants that affect water quality and use. Discharge to the environment must be accomplished without transmitting diseases, endangering aquatic organisms, impairing the soil, or causing unsightly or malodorous conditions. The type and degree of treatment are dependent upon the absorption capability or dilution capacity at the point of ultimate disposal. See Sewage, Sewage treatment
Discharges into any aquatic system cannot contravene the standards set for the most beneficial use of that water body. Water quality standards are used to measure an aquatic ecosystem after the discharge has entered and mixed with it. Water quality standards relate to the esthetics and use of the receiving environment for public water supply, recreation, maintenance of aquatic life and wildlife, or agriculture. The parameters of water quality, which define the physical, chemical, and biological limits, include floating and settleable solids, turbidity, color, temperature, pH, dissolved oxygen, biochemical oxygen demand (BOD), numbers of coliform organisms, toxic materials, heavy metals, and nutrients.
Effluent standards define what is allowed within the wastewaters discharged into the aquatic environment. Effluent standards specify the allowed biochemical oxygen demand, suspended solids, temperature, pH, heavy metals, certain organic chemicals, pesticides, and nutrients in the discharge. Point-source wastewater effluent discharge standards, established for ease of sampling, simplicity of repetitive testing, and clarity for enforcement, are more likely to be used by regulatory agencies. See Environmental engineering