Problem: Drip-dose method for handling odor in a sanitary sewer wet well strains municipality’s budget.
Solution: Installation of a wet well wastewater recycling system that mixes and disperses odor control chemicals.
Odor from a sanitary sewer wet well in Southern California proved to be an ongoing problem and expense. After receiving odor complaints from a nearby residential area and recognizing the need to address the health, safety, and environmental concerns regarding the hydrogen sulfide emanating from a manhole approximately 300 m (1000 ft) downstream of the wet well, the municipality decided to take action.
The municipality began treating odors with a proprietary blend of calcium nitrate liquid, using a drip-dose method. Even though the chemical successfully reduced odor levels and resident complaints, the cost of treatments and operating expenses amounted to more than $16,000 a year and strained the municipality’s budget.
Installing a new system
To reduce operating expenses while controlling odor, the municipality chose to try a different method of dosing, the EnviroPrep™ (EP) system, manufactured by Anue Water Technologies (Carlsbad; Calif.). The system originally was installed to remediate fats, oils, and grease (FOG) build-up, but the municipality expanded its use to include controlling odor a few months after its installation. The wet well environment reconditioning and washing system recirculates wastewater in the wet well and injects a liquid odor-control chemical, which is mixed and evenly dispersed by the system’s rotating head. The system is intended to reduce chemical use and, thus, costs. The system’s reticulating action aerates, homogenizes, and washes FOG from the lift station.
Putting the system to the test
The municipality agreed to an experiment in which calcium nitrate was dosed through the system in volumes that decreased weekly. Hydrogen sulfide was measured downstream to provide a control measurement. Prior to using the system, the drip dosing rate was 76 L/d (20 gal/d). Rerouting the liquid stream of calcium nitrate into the EP system enabled reducing the dosing rate. During 7 weeks, the dosing rate dropped 39% to a final rate of 49 L/d (13 gal/d) without sacrificing effectiveness.
The experiment also found that hydrogen sulfide concentration peak levels, peak occurrences of 5 minutes, average levels, and medial levels all decreased even though dosing volumes were reduced. In addition, the system reduced the need to vacuum-clean the station from monthly to twice annually, saving 66 hours of labor.
Benefits of the system
The EP system provided automation for wet well maintenance, substantially increased efficiencies, and lowered costs for odor-control chemicals. The system is designed to operate as a “green” approach by reducing — by about 30% — odor-control chemicals. Also, automating FOG cleaning leads to fewer vacuum-truck clean-out visits, reducing the amount of pollution emitted by the 1.5-km/L (3.5-mi/gal) trucks. Automated cleaning eliminates fuel, air pollution, traffic disruption, and water use associated with line cleaning.
The city also benefits from improved consistency of wastewater effluent, because solids are held in a highly emulsified state. When water quality is consistent, pump operation is more efficient, with less variability on loads. As the need to enter lift stations decreases, so do the risks associated with confined-space entry.
Detailing cost and maintenance
The cost to install an EP system depends on each application, with a range of $5900 to $9900. The systems recirculate wet well wastewater by either a tap of the discharge main or from its own dedicated pump. An installation receiving wastewater from its own dedicated pump will have slightly higher equipment costs due to the pump and control. Payback on the system is a function of lower chemical usage. The installation described above, for instance, had a 30% reduction in chemical use.
Maintenance includes routine visual checks during wet well inspections and biannual inspections during which the system is removed, inspected, and returned — a process taking approximately 20 minutes. If the wet well floods, the system should be removed, and the rotary head should be cleaned and the seal bearing assembly should be disassembled, inspected, and cleaned, if necessary, before being reassembled and reinstalled — a process that also takes approximately 20 minutes.
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