Chemical Constituents of Wastewater

Chemical Constituents of Wastewater, the dissolved and suspended solids in wastewater contain organic and inorganic material.

Organic Matter may include carbohydrates, fats, oils, grease, surfactants, proteins, pesticides and other agricultural chemicals, volatile organic compounds, and other toxic chemicals (household and industrial).

Inorganics may include heavy metals, nutrients (nitrogen and phosphorus), pH, alkalinity, chlorides, sulfur, and other inorganic pollutants.

Gases such as carbon dioxide, nitrogen, oxygen,hydrogen sulfide, and methane may be present in a wastewater.

Normal ranges of nitrogen levels in domestic raw wastewater are 25 to 85 mg/L for total nitrogen (the sum of ammonia, nitrate, nitrite, and organic nitrogen); 12 to 50 mg/L ammonia nitrogen; and 8 to 35 mg/L organic nitrogen

The organic nitrogen concentration is determined by a totalkjeldahl nitrogen (TKN) analysis,

which measures the sum of organic and ammonia nitrogen. Organic nitrogen is then calculated by subtracting ammonia nitrogen from the TKN measurement.

Typical total phosphorus concentrations of raw wastewater range from 2 to 20 mg/L, which includes 1 to 5 mg/L of organic phosphorus and 1 to 15 mg/L of inorganic phosphorus

The strength (organic content) of a wastewater is usually measured as 5-days biochemical oxygen demand (BOD5), chemical oxygen demand, and total organic carbon. The BOD5 test measures the amount of oxygen required to oxidize the organic matter in the sample during 5 days of biological stabilization at 20 Deg C.

This is usually referred to as the first stage of carbonaceous BOD (CBOD),not nitrification (second phase). Secondary wastewater treatment plants are typically designed to remove CBOD, not for nitrogenous BOD (except for advanced treatment).

The BOD5 of raw domestic wastewater  is normally between 100 and 250 mg/L. It is higher in other countries.

The ratio of carbon, nitrogen, and phosphorus in wastewater is very important for biological treatment processes, where there is normally a surplus of nutrients.

The commonly accepted BOD/N/P weight ratio for biological treatment is 100/5/1; i.e. 100 mg/L BOD to 5 mg/L nitrogen to 1 mg/L phosphorus.

Ratio of Raw sanitary wastewater and settled (primary) effluent are 100/17/5 and 100/23/7, respectively.


Both Nitrogen and Phosphorus in wastewater serve as essential elements for biological growth and reproduction during wastewater treatment processes and in the natural water.


Author: T.P.Sivanandan

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