Characteristics of Sewage

Decomposition of sewage:

Physical characteristics:

• Turbidity 
• Colour 
• Odour 
• Temperature

Colour:         

Indicates the freshness of sewage

Yellowish, grey or light brown - fresh sewage

Black or dark brown - stale and septic sewage

Temperature:

Has an effect on

• biological activity 
• solubility of gases in sewage

Chemical characteristics:

Organic matter: 

• Carbohydrates such as cellulose, fibre, starch, sugar. etc..
• Fats and oils
• Wastes from animals, urea fatty acids, hydrocarbons.

Determination of solids:

Total solid &(in mg/7):

• Determined by evaporating a known volume of sewage sample, and weighing the dry residue left.
• The mass of the residue divided by the volume of the sample evaporated ,will represent total solids in mg/l  

Suspended solids: 

• Those solids retained by a filter of 1 
• glass fibre felter appratus is used

Dissolved and Colloidal solids:

• Difference between the total solids and suspended solids

Volatile and fixed solids:

Total suspended solids = Volatile + fixed

• Volatile solids:

1. Suspended solids are burnt and ignited at about 550°C in an electric muffle furnace for about 15 to 20 min.
2. Loss of weight due to ignition represents volatile solids.

• The difference between the suspended solids and volatile solids gives fixed solids.

Settleable solids:

• Sewage is allowed to stand in this imhoff cone for a period of 2 hours
• The quantity of solids settled in tile bottom of the cone can be directly read as settle able solids.

Dissolved oxygen (D.O):

• To know the extent of pollution of sewage 
• At least 4 ppm of DO should be ensured while discharging sewage into river stream.

Types of Organic matter:

• Biologically active (means bacteria can stabilize this organic matter)
• Biologically inactive (means bacteria can not act on this organic matter)

Oxygen Demand: 

• It is the amount of of oxygen consumed by readily oxidizable organic matter for oxidation.

Chemical Oxygen Demand(C.O.D) :

• It is chemical oxidation with Potassium Permanganate or Potassium dichromate in an acid solution.

Theoretical oxygen Demand: 

• It is the oxygen demand that can be worked out theoretically.

Biochemical Oxygen Demand (B.O.D): It is the amount of oxygen required for biological decomposition of biodegradable organic matter under aerobic conditions at a specified temperature and for a specified duration.

• Standard B.O.D. is reported at 20°C for 5 days period.
• B.O.D. -5 days at 20oC - 68% of the total demand
• C.O.D represent both biologically active and inactive organic matter
• Where as D.O.D. gives biologically active organic matter only
• Therefore C.O.D. > B.O.D

Test method:

• Initial D.O measured in the beginning for the sample diluted with water.
• Sample Incubated for 5 days at 20°C
• Final D.O of the sample measured after 5 days.
• B.O.D. (Initial D.O - Final D.O) x Dilution Factor

Dilution factor: Number of times sewage diluted with distilled water

Eg: For 2 % solution, Dilution

Factor =100/2=50

• Rate of change of Organic matter with time is directly proportional to the organic matter present in the sewage at the time

Where Lt=Amount of organic matter present at time "t"

K = Rate constant, KD = deoxygenation constant

Carbonaceous Demand: The first demand that occurs during the first 20 days due to

the Oxidation of Organic matter is called Carbonaceous demand (or) I stage demand.

• The term B.O.D. usually mean I stage demand.
• Represented by "OAB” in the above figure

Nitrogenous demand:

• The latter demand that occurs due to biological Oxidation of Ammonia.  
• Represented by “AC” in the above figure.

B.O.D/C.O.D. Ratio:

• If BODu/COD lies between 0.92 and I, then the waste water can be considered to fully biodegradable.
• If BOD5 /COD) vary between 0.63 and 0.68 then the waste water can be considered to b fully biodegradable wastes.

Relative Stability:

• Ratio of amount of oxygen available in the effluent (D.O) to the total oxygen required to satisfy the first stage BOD demand.

Population Equivalent:

• Industrial waste waters arc generally compared with per capita normal domestic waste water so as to charge industries properly.

Population equivalent =

Total standard BOD5 of industrial sewage per day/Standard BOD5 of domestic sewage per person per day

• Average standard BOD5 of domestic sewage is about 0.08 kg/day/person.