Characteristics of Wastewater
The Characteristics properties of wastewater can be classified under the following three heads-
- Physical Characteristics
- Chemical Characteristics
- Biological Characteristics
1. Physical Characteristics
- Total Solids – This is the most important physical characteristics of wastewater is its total solids content, consisting of floating matter, matter in suspension, colloidal matter and matter in solution.
- Colour – Fresh domestic sewage is grey, however resembling a weak solution of soap. With the passage of time it begins to get black as soon as putrefaction starts. Therefore, colour of septic sewage is more or less black or dark in colour.
- Temperature – Generally the temperature of wastewater is higher than that of water supply, due to addition of warm water from household and industries. Due to raised temperature the bacterial activities increases. In India, it is nearly 20 deg. celcius. As the temperature increases, thus viscosity increases and solubility of gases decreases. Thus separation of DO and other gases occur. Increase in DO results in increased bacterial activities and it also consumes surface oxygen. This causes lots of depletion of oxygen in water which is harmful for aquatic life.
- Turbidity – It is measure of light emitting properties of wastewater and indicates the quality of waste discharge with respect to colloidal matter.
- Smell and Odor – Normal fresh sewage has musty odor which is not offensive. But when the time passes, it gets stale and begins to give offensive smell. Within 3 to 4 hours, all the oxygen present in it gets exhausted and it starts emitting hydrogen sulphide gas and other sulphur compounds. It is however harmful in many ways (i) reduction in appetite for food (ii) lowering in water consumption (iii) nausea (iv) vomiting.
2. Chemical Characteristics
Sewage contains generally complex organic matters derived from urine, faeces etc. and inorganic chemicals. Fresh domestic sewage is slightly alkaline but tends towards acidic as it becomes stale. Important chemical characteristics of sewage are
- PH value – Fresh sewage has pH between 7.3 to 7.5
- Chloride content
- Nitrogen content (free ammonia, organic nitrogen, albuminiod nitrogen, nitrites, nitrates)
- Fat, grease and oil content –
- Sulphides, sulphates snd H2S gas –
- Dissolved oxygen (DO)
- Chemical oxygen demand (COD)
- Biological oxygen demand (BOD)
3. Biological Characteristics
Biological Characteristics relate to various micro organisms found in wastewater, some of which may be pathogenic. However, all bacteria present in wastewater are not harmful; some of these help to treat the wastewater and reduce the cost of treatment plants.
The organic matter in sewage consists of urea from urine, proteins, carbohydrates, fats and soaps. These undergo continuous decomposition and in that process, pass through several stages, resolving into simpler elements such as nitrogen, hydrogen, carbon and oxygen with small quantities of sulphur and phosporous. These elements ultimately combine by means of chemical and biological actions to form inorganic substance.