Examination of Sewage and Sewage Effluents

Quantitative.—

Collection of the Sample.—As only small quantities of material are needed, the samples should be collected in a manner similar to that described under water for quantitative examination and transmitted in the ice apparatus used in packing those samples.

Apparatus Required.—As for water (vide page 420).

Method.—

1. Arrange four sterile capsules in a row and number them I, II, III, IV.

2. Pipette 9 c.c. sterile bouillon into capsule No. I.

3. Pipette 9.9 c.c. sterile bouillon into capsules II, III, and IV.

4. Add 1 c.c. of the sewage to capsule No. I by means of a sterile pipette, and mix thoroughly.

5. Take a fresh sterile pipette and transfer 0.1 c.c. of the mixture from No. I to No. II and mix thoroughly.

6. In like manner transfer 0.1 c.c. from No. II to No. III, and then 0.1 c.c. from No. III to No. IV.

Now 1 c.c. of dilution No. I contains 0.1 c.c. of the original sewage.
1 c.c. of dilution No. II contains 0.001 c.c. of the original sewage.
1 c.c. of dilution No. III contains 0.00001 c.c. of the original sewage.
1 c.c. of dilution No. IV contains 0.0000001 c.c. of the original sewage.

7. Pour a set of gelatine plates from the contents of each capsule, three plates in a set, and containing respectively 0.2, 0.3, and 0.5 c.c. of the dilution. Label carefully; incubate at 20° C. for three, four, or five days.

8. Enumerate the organisms present in those sets of plates which have not liquefied, probably those from dilution III or IV, and calculate therefrom the number present per cubic centimetre of the original sample of sewage.

Qualitative.—The qualitative examination of sewage is concerned with the identification and enumeration of the same bacteria dealt with under the corresponding section of water examination; it is consequently conducted on precisely similar lines to those already indicated (vide pages 426 to 441).