The effective management of any waste-water flow requires a reasonably accurate knowledge of its characteristics. This is particularly true for waste-water flows from rural residential dwellings, commercial establishments and other facilities where individual water-using activities create an intermittent flow of waste-water that can vary widely in volume and degree of pollution.
Detailed characterization data regarding these flows are necessary not only to facilitate the effective design of waste-water treatment and disposal systems, but also to enable the development and application of water conservation and waste load reduction strategies.
Characteristics of Waste-water in Terms of Content
A number of chemical and physical characteristics are used to describe waste-water. The most common are:
Biochemical Oxygen Demand (BOD) is a measure of the amount of unstable organic matter in the water. It measures how much oxygen is required by the available micro-organisms to break down the readily available organic matter into simpler forms, such as carbon dioxide, ammonia and water.
Total Nitrogen (TN) and Total Phosphorus (TP) are the sum of all forms of nitrogen and phosphorus in the water, respectively. Both nitrogen and phosphorus are nutrients essential for plant growth.
Read Also: Types and Sources of Waste-Water
A shortage of either one, or both, may limit the rate of plant growth in a water body. If the supply of the limiting nutrient is increased (and other circumstances such as appropriate amount of sunlight and appropriate temperature are favorable) blooms of algae or waterweeds may occur.
Faecal Microbes (which includes viruses, bacteria and protozoans) are found in waste-water and may cause disease. Faecal coliforms and faecal streptococci are bacteria that are found in the digestive tracts of humans and animals, including birds. Their presence in a water body indicates faecal contamination.
Components of Waste-water
Effluent quality is the physical, biological, and chemical characteristics of a liquid flowing from a component or device. The components of waste-water may be divided into four categories:
- Biochemical oxygen demand, total suspended solids and fats, oils and grease (BOD5 , TSS, FOG)
- Pathogens (fecal coliform, viruses)
- Nutrients (nitrogen, phosphorus), and
- Other chemicals.
Biochemical Oxygen Demands (BOD5) – Biochemical oxygen demand (BOD5) is the most widely used parameter applied to waste-water.
BOD5 is a measure of the dissolved oxygen required by micro- organisms to oxidize or decompose the organic matter in waste-water. A typical BOD5 value for septic tank effluent is 150 milligrams per liter. For a Type I system, the BOD5 limit is 170 milligrams per liter.
When the dissolved oxygen (DO) contained in septic tank effluent is measured, it is usually very low, typically one milligram per liter. While DO in water can be as high as 12 milligrams per liter, the micro- organisms in the septic tank normally use up any available oxygen to break down organic matter.
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