Water Pollution TT67
Published on May 8, 2014

Water Pollution - Tech Talk 67

Any physical, biological or chemical change in water quality that adversely affects living organisms or makes water unsuitable for desired uses can be considered pollution.

Common Impurities in Water

Dissolved: Cations, anions, organic compounds such as vegetative dyes and pesticides and gases such as hydrogen sulfide and chlorine.

Suspended: Silt, clay, feces, organic and inorganic colloids and living organisms such as bacteria, viruses, protozoa and algae.

Floating: Oils and grease, feces and debris.

Water Quality Characteristics

Physical: Total suspended and dissolved solids, turbidity, color, odors and temperature.

Chemical: pH, alkalinity, hardness, conductivity, dissolved gases, etc.

Biological: Algae and microbes (bacteria, protozoa, fungi, etc.).

Water Quality Indicator

The amount of oxygen dissolved (D.O.) in water is a good indicator of water quality commonly used in aquaculture. Water with a D.O. of 8 ppm will support game fish and other desirable forms of life. Water with less than 2 ppm oxygen will support only certain fish, worms, bacteria, fungi and other decomposers. Certain organic materials increase oxygen consumption by decomposers. This is measured by the biological oxygen demand (BOD). This is the amount of oxygen used by microbes, mainly bacteria, in the oxidation of organic materials. The standard test is for 5 days at 20°C, and the value it yields is referred to as the 5 day biochemical oxygen demand, commonly shown as BOD5. Another measurement is chemical oxygen demand (COD), which is the measurement of the organic materials in terms of the oxygen required to chemically oxidize them. It is measured by boiling and refluxing the sample with a strong oxidizing agent (potassium dichromate), then determining the amount of agent used.

Typical Treatment Processes for Organic Compounds

Anaerobic Digestion. Biological oxidation of degradable organics by microbes under conditions where there is no molecular oxygen. These are mainly used for swine, poultry and beef industries.

Digestion is done by anaerobic bacteria, organic-acid forming and methane-producing heterotrophs.

Advantages: Low operational costs, no need for aeration and self-sufficiency if managed properly.

Disadvantages: Odors, sludge must be removed periodically, land space needed (long treatment time), and NIMBY (not in my back yard).

Aerobic Digestion.

Biological oxidation of organics under conditions where there is molecular oxygen. Digestion is done by aerobic and facultative bacteria.

Advantages: Less odor, reduction in pollution characteristics of the effluent and low treatment time.

Disadvantages: Higher operational cost (equipment maintenance, electrical cost).