Dairy Plant Effluent
The increase in the size of dairy plants produces mounting problems with the discharge of the rising quantities of effluent. In addition, new dairy plants are placed outside city limits where the existence of effluent processing facilities is the exception rather than the rule.
Discharge fees necessitated the knowledge of both the quantities of effluent as well as the content of pollutants. The pollutants in dairy plant effluent are primary the organic substances fat, protein, and lactose.
Two methods are used to determine the content of organic materials in effluent: BOD and COD.
BOD (Biological Oxygen Demand) is determined by examining the quantity of the soluble oxygen from an aqueous sample by leaving it for 5 days at 20 degree Celsius. The oxygen quantity is established by letting oxygen oxidize iodine into iodide and establish the released quantity of iodine by titration with thiosulphate. The result is expressed in mg oxygen per liter.
COD (Chemical Oxygen Demand) is determined by treating a sample with potassium dichromate solution and neutralizes excess dichromate by titration with ferrous ammonium sulphate. This is done simultaneously with a blind test for control of the chemicals. The quantity of dichromate used in the treatment is an expression of the chemical oxygen demand. The result is stated in mg oxygen used for each liter of sample.
The below table states COD values and thus the “pollution degree” of whole milk, skim milk and whey (rewritten after IDF Bulletin 184-1983):
|
Substance
|
Whole Milk
|
|
Skim milk
|
|
Whey
|
|
|
|
Content mg/L.
|
mg COD/kg
|
Content mg/L.
|
mg COD/kg
|
Content mg/L.
|
mg COD/kg
|
|
Fat
|
40,000
|
120,000
|
400
|
1,200
|
400
|
1,200
|
|
Protein
|
34,000
|
46,000
|
34.000
|
46,240
|
10,000
|
13,600
|
|
Lactose
|
46,000
|
52,000
|
47.000
|
53,110
|
47,000
|
53,110
|
|
|
|
|
|
|
|
|
|
Total
|
|
218,000
|
|
100,550
|
|
67,910
|
A term often used to describe the “pollution degree” is “person equivalent” (p.e. correspond to 250 L. of water polluted to a COD figure of 600. In other words, 1 P.E. correspond to 250 x 600 = 150,000 mg COD.
Example: A dairy plant receives a daily quantity of 300,000 liter of milk. The loss is estimated to be 1 % e.g. 3,000 L/day. COD: 3,000 x 218,000/150,000=4,360 P.e., an effluent pollution equal to the pollution from 4,360 people.
Insert your numbers and see how you might reduce pollution.
