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Packaged iron/manganese removal system treats Iowa source water
The town of Grand Junction, Iowa, uses a groundwater source to provide water to 850 people. The water quality from a 365-ft-deep well is generally good, with iron concentrations of about 0.3 mg/L and hardness of about 273 mg/L. In the early 1930s, the town installed a vertical pressure filtration system with a softener to treat the water. The filter media, however, eventually became completely clogged and stopped operating properly. Water quality became poor, and there were issues with maintenance of the distribution system. It was difficult to find parts to repair the equipment.
When Grand Junction received funding from the state revolving loan program to build a new water treatment plant, it hired Fox Eng. to design the new plant. Even with low-interest loans, the city required a system that was both cost-effective and simple to operate with low maintenance.
The engineer selected an 11-ft-diameter Aeralater system from Evoqua to treat the iron in the raw water. In addition, the engineer selected two 66-in.-diameter-by-84-in. straight side height ion-exchange pressure vessels for hardness removal. The Aeralater system is a packaged groundwater treatment system that combines aeration, detention and filtration in a single unit to remove common groundwater contaminants (e.g., iron, manganese, radium and arsenic). Ion-exchange softening uses a cationic resin to remove other contaminants typically found in groundwater (e.g., calcium and magnesium hardness and radium).
How It Works
The packaged system uses an induced draft aerator to saturate the raw water with oxygen. Raw water is introduced in the top and distributed evenly using a distribution tray. Target nozzles spray the water across the top of the media. This design eliminates the need for high-pressure spray nozzles, reducing the pressure required for the system and the overall operating cost. The media is made of thin-wall PVC pipes spread on 2-in. centers with 2-in. spacing. The pipes promote air contact with the water, saturating it with oxygen.
The tray slats are removable to allow cleaning of iron that precipitates in the aerator. An induced draft blower pulls atmospheric air through a screened opening designed to allow air flow with minimal pressure drop. The distribution tray has air stacks to allow the air to flow freely out of the unit without entraining water and to eliminate the pressure drop associated with tray systems. The bottom of the aerator is open to the detention tank. If chemical addition is required, a static mixer can be added under the aerator.
The detention tank is designed to have a minimum of 30 minutes of detention time prior to the filter media. At typical pH and temperature values, this is adequate time for the oxidation and precipitation of iron using oxygen. Inside the detention tank are manways for accessing the filters below. The level in the detention tank is controlled by a valve connected to a float assembly. As the water level in the tank rises, the valve closes, increasing the pressure to the well pump and reducing the flow. If the water level in the detention tank drops, the valve opens, allowing more flow to enter the system. Maintaining the water level in the detention tank ensures that there is always adequate head pressure to backwash the filters. Using a mechanical device simplifies the operation and eliminates the need to buy and maintain modulating actuators and level transmitters.
The water then flows through a set of face piping and into a four-cell filtration system. The filter uses 0.6- to 0.8-mm anthracite media. The filters remove the precipitated iron and any other particulates. Filter backwashing is accomplished by closing the inlet valve from the detention tank and opening the backwash waste valve. Pressure from the detention tank forces water through the remaining filter cells and into the common underdrain. The water is forced up through the filter being backwashed, where the backwash water is collected in an overdrain, out the backwash waste valve and then to waste. The cells are backwashed in sequence until all have been backwashed. The entire filter then is brought back online.
Because the system is backwashed manually, this eliminates the need for backwash supply pumps and tanks or to bring system water back for backwashing. This makes the overall system more efficient, controlling operating costs for Grand Junction.
The system is constructed of high-quality aluminum, which eliminates corrosion and reduces maintenance at the facility. The unit is manufactured in three primary sections: the aeration section, the detention/filtration section and the filter face piping. Thus, the installation consists only of attaching the aerator section, attaching the face piping and valves, installing the filter media and hooking up the power to the blower. This reduces the overall installation cost, saving money for Grand Junction and its ratepayers.
High-service pumps take the water from the underdrain to the softener tanks. Part of the water is bypassed for aesthetic reasons, with the rest sent to the softeners. The water to each softener is measured by magnetic flowmeters. The ion-exchange resin removes calcium and magnesium ions by exchanging them for sodium ions, which do not contribute to hardness. Filtered water is used to backwash the ion-exchange units, and an existing brine system is used to regenerate the resin. The treated water is combined with bypass water to meet the target hardness for the system. After softening, the water is chlorinated and sent to the distribution system.
The system was installed in 2007. Since then, the town’s water quality has improved dramatically, particularly during semi-annual hydrant flushes. Before the new system was installed, multiple hydrants had experienced significant fouling. Now only two or fewer hydrants per flush cycle show any fouling. The system also is easier to maintain and operate.
“It was like going from a horse to a car,” said Plant Superintendent Dean Lyons. “The raw water iron was reduced from levels close to 0.3 mg/L to nondetectable levels. With the low levels of iron, the filter run lengths are very long. We backwash the filters every Friday, whether they need it or not.”
The packaged treatment system operates in a manual mode due to the low backwash frequency. The softeners regenerate automatically based on actual usage. The control system monitors the amount of water treated by each unit and regenerates the system once the amount of treated water reaches a set point.
The water treatment system has operated so successfully that Lyons gives tours of the installation. The system has met or exceeded the town’s requirement for high-quality water, while offering simple operation and maintaining the overall economy expected of a packaged groundwater treatment system.