Reuse System Features PAC Treatment Step

Sept. 1, 2000
In the early 1980s, the residents of El Paso, Texas, watched water levels in their aquifer decline year after year, threatening long-term drinking water supplies to El Paso and neighboring communities. To help counter the problem, the city constructed the Fred Hervey Water Reclamation Plant to provide treated wastewater for water reuse and aquifer-recharge programs.

In the early 1980s, the residents of El Paso, Texas, watched water levels in their aquifer decline year after year, threatening long-term drinking water supplies to El Paso and neighboring communities. To help counter the problem, the city constructed the Fred Hervey Water Reclamation Plant to provide treated wastewater for water reuse and aquifer-recharge programs.

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The 10 mgd plant uses USFilter's Zimpro Products technologies. The reclaimed water is used for non-potable industrial purposes as well as for aquifer re-charge to extend the life of the city's drinking water wells. The plant began operation in 1985. It takes about two years for injected water to migrate to potable water wellheads. For every 10 years the plant performs at its capacity, the life of the city's aquifer is extended by one year.

The wastewater treatment process includes screening, degritting, primary clarification, flow equalization, two-stage PACTRegistered system treatment, lime treatment, two-stage recarbonization, sand filtration, and granular activated carbon filtration.

The PACT® (Powdered Activated Carbon Treatment) system is at the heart of the reuse system, using a combination of powdered activated carbon and bacteria to synergistically treat the wastewater. A Wet Air Regeneration (WAR) system allows for the recovery of the powdered activated carbon and the destruction of the bacteria and sorbed organics.

Treatment of the city's wastewater into a reusable water begins at the Fred Hervey plant by going through a primary treatment step. From here, the primary effluent flows to the first stage aeration basin of the PACT system where regenerated carbon from the WAR system and wasted, partially spent carbon from the second stage are added. The mixed liquor, a mixture of powdered activated carbon and biomass, is kept aerated to treat the organics in the incoming wastewater via adsorption and biostabilization. The ammonia in the wastewater is also treated in this stage and converted to nitrate biologically.

After aeration, the mixed liquor flows to a clarifier where solids separate from the liquid. Most of the solids are recycled back to the aeration tank. Excess solids (biomass and spent carbon) are wasted out of the system and thickened in a gravity thickener to more than six percent solids in order to be regenerated.

The first stage clarifier overflows to the anoxic tank of the PACT system's second stage to which virgin powdered activated carbon is added. Here, trace organics are treated further, and the nitrate formed in the first stage is converted to nitrogen gas with the aid of methanol as a carbon source. After a short anoxic contact period, the mixed liquor is taken to an aeration step that helps strip off the N2 and stabilizes any residual methanol that may be over-added during the denitrification operation. The mixed liquor then flows to the second stage clarifier where the solids are separated from the liquid with most being recycled to the anoxic basin. Excess solids are wasted back to the first stage. Clear water overflows to the lime softening step.

The wasted/spent solids from the PACT system are taken to the Wet Air Regeneration system where simultaneous destruction of adsorbed organics and biomass and recovery of carbon occur. Carbon recovery rates of 90 percent are common. The organic sludge associated with the spent carbon is reduced to a small amount of sterile ash that is disposed in a landfill.

The WAR system is operated on a gravity thickened, spent carbon/biomass slurry. The slurry is mixed with compressed air and pumped into the WAR heat exchangers at approximately 800 psig. The air/slurry mixture passes into the regeneration reactor. Here an exothermic reaction takes place, releasing heat when the organics are oxidized, renewing the carbon surfaces. Heat is recovered as the oxidized slurry passes back through the heat exchangers, warming the incoming slurry. The regenerated carbon slurry then is returned to the PACT system.

The WAR system typically is designed to be self-sustaining (requiring no auxiliary fuel) at a feed solids concentration of six to seven percent.

The Fred Hervey plant has a perfect record of environmental compliance.

"We have always produced a product water from this facility that has met state and federal drinking water standards," said Javier Hernandez, El Paso's Plant Superintendent for the Fred Hervey Water Reclamation Plant.

The plant has met or exceeded the design parameters since startup. The current unit cost for treatment runs about $1.60/1000 gallons, according to published reports.

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