Wastewater is the flow of used water from a community. The characteristics of the wastewater discharges will vary from location to location depending upon the population and industrial sector served, land uses, groundwater levels, and degree of separation between storm water and sanitary wastes. Domestic wastewater includes typical wastes from the kitchen, bathroom, and laundry, as well as any other wastes that people may accidentally or intentionally pour down the drain. Sanitary wastewater consist of domestic wastewater as well as those discharged from commercial, institutional, and similar facilities. In general, the volume of sanitary wastewater generated is about 400 liters per capita. However, the range of flow usually varies from a minimum of about 20% to a maximum of about 400% of the average dry weather flow for small communities and about 200% for larger communities. Industrial wastes will be as varied as the industries that generate the wastes. The quantities of storm water that combines with the domestic wastewater will vary with the degree of separation that exists between the storm sewers and the sanitary sewers. Most new sewerage systems are separate, collect sanitary wastewater and storm wastes, whereas older combined systems collect both sanitary wastewater and storm water.

Physically, wastewater is usually characterized by a grey colour, musty odour, a solids content of about 0.1%, and a 99.9% water content. The solids can be suspended (about 30%) as well as dissolved (about 70%). Dissolved solids can be precipitated by chemical and biological processes. From a physical point of view the suspended solids can lead to the development of sludge deposits and anaerobic conditions when discharged into the receiving environment.

Chemically, wastewater is composed of organic and inorganic compounds as well as various gases. Organic components may consist of carbohydrates, proteins, fats and greases, surfactants, oils, pesticides, phenols, etc.. Inorganic components may consist of heavy metals, nitrogen, phosphorus, pH, sulfur, chlorides, alkalinity, toxic compounds, etc.. In domestic wastewater, the organic and inorganic portion is approximately 50% respectively. However, since wastewater contains a higher portion of dissolved solids than suspended, about 85 to 90% of the total inorganic component is dissolved and about 55 to 60% of the total organic component is dissolved. Gases commonly dissolved in wastewater are hydrogen sulfide, methane, ammonia, oxygen, carbon dioxide and nitrogen. The first three gases result from the decomposition of organic matter present in the wastewater.

Biologically, wastewater contains various microorganisms but the ones that are of concern are those classified as protista, plants, and animals. The category of protista includes bacteria, fungi, protozoa, and algae. Plants include ferns, mosses, seed plants and liverworts. Invertebrates and vertebrates are included in the animal category. In terms of wastewater treatment, the most important category are the protista, especially the bacteria, algae, and protozoa. Also, wastewater contains many pathogenic organisms which generally originate from humans who are infected with disease or who are carriers of a particular disease. Since the identification of pathogenic organisms in water and wastewater is very time consuming and difficult, the coliform group of organisms which are more numerous and more easily tested for, is used as an indicator of the presence of pathogenic organisms. However, this test does not accurately reflect the presence or absence of all pathogens that may be found in the treated effluent, ie. viruses. Typically, the concentration of fecal coliforms found in raw wastewater is about several hundred thousand to tens of million per 100 mL of sample.

STORMWATER AND COMBINED WASTEWATER

Stormwater runoff is precipitation that finds its way across and surfaces into receiving waters. Urban storm runoff is collected and transported in storm or combined sewers. Storm sewers carry stormwater only; combined sewers also carry sanitary wastewater.

A composition of stormwater reflects the composition of precipitation and the surfaces with which it becomes in contact. Solids concentrations are high, BOD values are relatively low, bacterial values are significant because of animal sources, and nutrient concentrations may be significant. Storm sewers are subject to illicite connections of sanitary wastewater, which can increase concentrations of all of these parameters.

Combined wastewater in dry weather is composed of sanitary wastewater. Since the pipes are sized to carry high flows in wet weather, in dry weather they may allow solids to settle out. In wet weather combined wastewater composition at any time depends on the extent to which sanitary wastewater is diluted by stormwater, and is augmented by contaminants in stormwater and in the solids deposited in dry weather and scoured in wet weather flows.

TOXICS

Toxics found in wastewater pass through wastewater treatment facilities that have not been designed to remove them and can interfere with their operation. In a biological treatment process toxic materials can upset a treatment process or even kill the biological community and make the process ineffective. To remove the toxic pollutants at the treatment facility can be very costly since they can very be numerous and varied in a larger community and therefore it is generally advantages to remove them at the source. Source control can be achieved by the use of municipal by laws limiting pollutant discharges to the sewerage system. The removal of toxic pollutants at the source can be achieved by requiring treatment prior to discharge, recycling of waste by products, manufacturing process changes, and the substitution of raw materials.