McDonald Group International, Inc

Water, Wastewater, Environmental Engineering 


Types of Wastewater and Wastewater Treatment

Wastewater is generally thought of in two types: domestic and industrial. Domestic wastewater is sewer water from homes and shops. Industrial wastewater is sewer water from factories, meat packing houses, laundries, vegetable canneries, poultry farms, metal plating shops, chemical dye plants, and so forth. Typically domestic wastewater contains a collection of suspended matter, ammonia, biological organisms and chemicals, oil and grease. Industrial wastewater can contain all of this plus be either acidic or alkaline, contain heavy metals, complex hydrocarbons, and many other potentially harmful pollutants.

Environmental Effects From the Discharge of Untreated Wastewater

When discharged to streams or lakes , raw wastewater can introduce disease carrying organisms, deplete the oxygen in the water, or introduce chemical changes rendering it unsafe for drinking or bathing and harming fish life. When discharged untreated on the ground, untreated wastewater seeps downward where it can contaminate drinking well water.

Levels of Treatment

Several levels of treatment can be provided. The level of treatment required depends on local environmental protection standards.
Primary Treatment:
Primary treatment consists of allowing the wastewater to settle in a tank where heavy solid matter can be separated from the liquid. Screening the raw wastewater of large objects is also considered primary treatment. Floating material, such as oil and grease are partially removed by skimming the surface of the liquid. Depending on the type of wastewater, prolonged retention in the tank results in a natural biodegradation of the solid matter in the wastewater called anaerobic digestion. Primary treatment removes roughly 30 to 50% of the primary constituents of concern, namely the oxygen depleting and solid matter in the water.
Secondary Treatment
Secondary treatment is the removal of at least 90% of the oxygen depleting and solid matter in the sewer water. This may include adjustment of the pH of the water and aerating the raw wastewater. The treated water is also disinfected at this level of treatment. Secondary treated water in some areas can be discharged to surface water, but preferably is discharged to land application systems. Secondary water can be reused by irrigation of a hay crop for farm animals.
Advanced Treatment
Advanced treatment can consist of removing ammonia, nitrate, phosphorus, and most of the remaining suspended matter left over from secondary treatment. Treatment of industrial wastewater beyond primary levels will probably require an advanced treatment processes. Advanced treated water is often recycled for use as irrigation water in parks, golf courses, toilet flushing, vehicle washing, etc.

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