ADEQ Water Treatment 2 Practice Test

In this testing sequence, you screen for coliforms with a presumptive test that uses lactose broth and looks for gas production. Gas appearing in the presumptive tubes at either the 24-hour read or the 48-hour read indicates a potential positive that needs further verification. That’s why the next step is the confirmed test, which uses a more specific condition (often a higher incubation temperature and a fresh inoculum from the presumptive positive) to confirm whether the gas-producing organisms are true coliforms. If gas is not produced by 24 or 48 hours, you typically don’t proceed to confirmation. The other option tries to insert nonstandard times or the completed/final step at this point, but those aren’t the correct next steps in this workflow. So the correct sequence is gas production observed at 24 hours or 48 hours, followed by the confirmed test.