Florida Wastewater Practice Test

If the effluent coliform count fails to meet required standards despite normal chlorine residual, what is the probable cause?

1. Insufficient chlorine dosage

2. Contact time too short

3. Faulty monitoring equipment

4. Improper sampling technique

The correct answer is: Contact time too short

The identification of the contact time as a probable cause for the failure to meet effluent coliform standards despite normal chlorine residual is based on the principle of disinfection effectiveness. In chlorination processes, the contact time refers to the duration that chlorine is in contact with the wastewater in order to effectively kill pathogens, including coliform bacteria. Even if the chlorine residual levels seem adequate, if the contact time is too short, there may not be enough time for the chlorine to interact with and eliminate the coliform organisms effectively. This interaction is crucial because chlorine works at a specific rate, and insufficient contact time could mean that the chlorine has not had the opportunity to react with the coliform, thus allowing some to survive. This is why maintaining the correct contact time is essential in ensuring that wastewater meets disinfection standards. While other factors, such as chlorine dosage or monitoring equipment, could theoretically play a role, they do not directly address the relationship between the timing of the disinfection process and the effectiveness of coliform removal, which is central to the observed failure in the effluent count. Therefore, the significance of contact time in relation to achieving effective disinfection makes it the most plausible explanation for the observed results.