Chemical Characteristics

Inorganic Chemicals

The principal chemical tests include free ammonia, inorganic nitrogen as nitrate, nitrite, organic phosphorus, and inorganic phosphorus. Nitrogen and phosphorus are important because these two nutrients are responsible for the growth of aquatic plants. Other tests such as chloride, sulphate, pH, and alkalinity are performed to assess the suitability of reusing treated wastewater and in controlling the various treatment processes (Rouessac and Rouessac 2007).

Trace elements, which include some heavy metals, are not determined routinely, but trace elements may be a factor in the biological treatment of wastewater. All living organisms require varying amounts of some trace elements such as iron, copper, zinc, and cobalt for proper growth. Heavy metals can also produce toxic effects; therefore, determination of the amounts of heavy metals is especially important where the further use of treated effluent or sludge is to be evaluated. Many metals are also classified as priority pollutants such as arsenic, cadmium, chromium, mercury, etc.

Measurements of gases such as hydrogen sulphide, oxygen, methane, and carbon dioxide are made to help the system to operate. The presence of hydrogen sulphide needs to be determined not only because it is an odorous and very toxic gas but also because it can affect the maintenance of long sewers on flat slopes, since it can cause corrosion. Measurements of dissolved oxygen are made in order to monitor and control aerobic biological treatment processes. Methane and carbon dioxide measurements are used in connection with the operation of anaerobic digesters.

Organic Chemicals

Over the years, a number of different tests have been developed to determine the organic content of wastewaters. In general, the tests may be divided into those used to measure gross concentrations of organic matter greater than about 1 mg/l and those used to measure trace concentrations in the range of 10⁻¹²–10⁻³ mg/l. Laboratory methods commonly used today to measure gross amounts of organic matter (>1 mg/l) in wastewater include (i) BOD, (ii) COD, and (iii) total organic carbon (TOC). Trace organics in the range of 10⁻¹²–10⁻³ mg/l are determined using instrumental methods including gas mass spectroscopy and chromatography. Specific organic compounds are determined to assess the presence of priority pollutants (Metcalf & Eddy 2003). The BOD, COD, and TOC tests are gross measures of organic content and as such do not reflect the response of the wastewater to various types of biological treatment technologies.