After passing through the biotowers and secondary clarifiers, the treated flow is disinfected to reduce pathogenic content to acceptable levels. Gaseous chlorine is the disinfectant used by the District. Other disinfection alternatives exist in the wastewater treatment industry including sodium hypochlorite, ultraviolet light, and ozone. Chlorine was selected by the District as the most cost effective disinfection alternative.

The chlorine used is delivered and stored in a liquid form in large storage vessels. Chlorine is 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. These policies include written unloading procedures, emergency response procedures, and a Risk Management Plan as required by the EPA. Even very small leaks of chlorine can be detected by the chlorine alarm systems. The alarms can detect chlorine concentrations down to one part per million. The District has a trained and certified Emergency Response Team (ERT) that can respond to chlorine leak emergencies. The ERT is also assigned the duty of off-loading all chlorine deliveries.

Chlorine vaporizes at room temperature. Chlorine contained in the large storage vessels is made up of both the liquid and gaseous forms of chlorine. The gaseous chlorine collects in the top of the tank as it vaporizes. Only the gaseous form is withdrawn for disinfection purposes. A pipe at the top of the storage vessel allows withdrawal of the gaseous chlorine. The rate of chlorine withdrawal and dosing into the treated flow is controlled by an electronic signal to 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 reaction level of  chlorine with compounds in the treated flow. Chlorine is fed into the flow in proportion to the flow meter reading and/or to maintain a certain reaction level as indicated by the ORP meter. The greater the concentration of compounds that will react with chlorine the higher the dosage rate required to maintain the same ORP reaction level. For the most part, dosing of chlorine is controlled automatically in this manner.

Chlorine is drawn by vacuum through the entire dosing system by a chlorine induction unit. After being metered, the chlorine is drawn 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. It draws the chlorine to the point of dosing and it 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.

The chlorine contact  basins are designed to hold the chlorinated flow for a specific period of time necessary to assure the maximum opportunity for the chlorine to fully react and disinfect before  being released to the environment. The chlorine contact basins look like a submerged maze through which the flow must  pass.  The flow passes slowly and non-turbulently as it winds its way through the maze.  At the end of the maze the disinfected flow passes through the utility water pump station. All utility water for wash-down, lawn irrigation, and pump seal flushing throughout the treatment plant site is pumped from this source. At the utility water pump station the residual concentration of chlorine is monitored to verify proper dosage and to comply with the requirements of the District’s discharge permit.

Achievement of adequate disinfection is monitored by analyzing for the presence of Ecoli bacteria..  The allowable limits for Ecoli are 126/100 mL for a 30-day average and 158/100 mL for a 7 day average.  These values are reported on a geometric mean basis. 

After passing through the chlorine contact basins and utility water pump station the flow drops over a weir and enters a pipe which carries it to a drainage ditch which then carries the flow to the Great Salt Lake. 

Emergency Chlorine Scrubber

An emergency chlorine gas scrubber facility is an important component of the District's disinfection system. The purpose of the scrubber is to provide additional safety against the escape of chlorine gas from the District’s site. The scrubber activates if the chlorine alarm system senses concentrations above acceptable limits. A large capacity fan draws all air from within the chlorine facility through a chemical scrubbing system that neutralizes the chlorine contained in the air flow. The scrubber has the capacity to neutralize the entire contents of the largest storage vessel of liquid chlorine. The scrubber is efficient at removing 99.9 percent of the chlorine from the air flow before discharging to the atmosphere.