North Davis Sewer District Home_Page

Disinfection and effluent

After passing through the biotowers and secondary clarifiers, the treated wastewater needs disinfecting to reduce pathogenic content to acceptable levels.

Chlorine Contact Basin Many disinfecting methods exist in the wastewater treatment industry:

Chlorine was selected by the District as the most cost effective disinfection method. The chlorine used is delivered and stored in a liquid form in large storage vessels. Chlorine is designated as a hazardous material; to insure against dangerous releases of chlorine, the system is specially designed with safeguards and warning alarms. Strict safety policies have been designed to govern the handling of chlorine at the District:

The chlorine alarm systems can detect very small leaks, even down to one part per million. A trained and certified District Emergency Response Team(ERT) can respond to chlorine leak emergencies and is also assigned the duty of off-loading all chlorine deliveries.

Chlorine gas detector

An emergency chlorine gas scrubber facility is an important safety component of the District's disinfection system. The scrubber activates if the chlorine alarm system senses concentrations above acceptable limits within the atmosphere of the facility. A large capacity fan draws all air from within the chlorine facility through a chemical scrubbing system to neutralize the chlorine. The scrubber has the capacity to neutralize the entire contents of the largest storage vessel of liquid chlorine and efficiently removes 99.9 percent of the chlorine from the air flow before release into the atmosphere.

Chlorine contained in the large storage vessels is made up of both the liquid and gaseous forms of chlorine. The gaseous chlorine vaporizes at room temperature and collects in the top of the tank. A pipe at the top of the storage vessel withdraws the gaseous chlorine to begin wastewater disinfection. The rate of chlorine withdrawal and dosing into the treated flow is controlled electronically by a chlorinator valve and meter. The electronic dosage signal is generated by a flow meter and an ORP (oxygen reduction potential) meter that senses the chlorine reaction potential within the treated flow. Chlorine is fed into the flow based on the flow meter readings and/or the ORP meter to maintain a certain reaction level. The greater the concentration of reacting compounds the higher the dosage rate required to maintain the same ORP reaction level the dosing of chlorine is mostly controlled automatically in this manner.

After being metered, the chlorine is drawn by vacuum to the chlorine induction unit located in a collection box just downstream of the secondary clarifiers. The chlorine induction unit performs two important functions in the collection box:

  1. Draws the chlorine to the point of dosing
  2. Efficiently mixes the chlorine into the flow stream

The unit is submerged in the flow and injects the chlorine into the flow just before it enters a pipe that carries the flow to the chlorine contact basins.

Effluent_DesinfectionBefore being released to the environment, chlorine contact basins hold the chlorinated flow throughout disinfection. The chlorine contact basins look like a maze that allows flow to pass slowly and non-turbulently. Upon completed disinfection, flow passes through the utility water pump station and pumped to fulfill all wash-down, lawn irrigation, and pump seal flushing needs throughout the plant. The utility water pump station monitors the residual concentration of chlorine to verify proper dosage and to ensure compliance with the requirements of the District’s discharge permit. Water is tested for the presence of E. Coli bacteria, to determine the effectiveness of disinfection. Reported on a geometric mean basis, the allowable limits for E. Coli are 126/100 mL for a 30-day average and 158/100 mL for a 7 day average.

After passing through the chlorine contact basins and utility water pump station the flow is directed over a weir, through a pipe to a drainage ditch, and released into the Great Salt Lake.

<< Laboratory >>                  << Cogeneration >>

Return to Top