Question

Compute the most probable number (MPN) for coliform density, expressed as MPN per 100 mL for the three samples below, based on the following test results using the tables and the Thomas equation. At each dilution a total of 5 tubes were tested [30 marks]

	Sample a	Sample b	Sample c
Size of portion (mL)	# of positive tubes	# of positive tubes	# of positive tubes
100	5	5	5
10	5	5	5
1	4	4	4
.1	3	4	2
.01	1	0	-

Answer 1

Using MPN tables (file attached) the Most probable number of coliform density per 100ml sample as deciphered (with 95% confidence) for the samples a, b and c are as follows..

Sample a (5-4-3) - 280

Sample b (5-4-4) - 350

Sample c (5-4-2) - 220

Reference: Standard methods for the examination of water and wastewater (1980), 15th ed, American public health association, New York

Similar MPN values can also be calculated using Thomas Equation :

Sample a :

No. of positive tubes = 13

mL of sample in negative tubes = (0*10+1*1+2*0.1+4*0.01) = 1.24

mL of Sample in all tubes = (5*10+5*1+5*0.1+5*0.01) =55.55

MPN/100mL = 13*100 / (1.24*55.55) = 156

MPN/100mL (sample a) = 156

Similarly MPN values were also calculated for sample b and c using Thomas equation.

MPN / 100mL (Sample b) = 166

MPN /100mL ( sample c) = 129