A Clean River Runs Through It
An Alaskan water treatment plant switched to a filter press to dewater its mineral byproducts and avoid discharging into the Chena River
All drinking water treatment plants (WTPs) must dispose of their potable water residuals. One privately owned water utility in Fairbanks, Alaska, uses a filter press to dewater its water treatment mineral byproducts. By doing so, the utility achieves zero discharge to the nearby Chena River, which serves as a recreational area and spawning ground for several fish species threatened with possible extinction.
The filter press handles up to 8,000 gallons per day (gpd) of mineral slurry from clarifiers at the Golden Heart Utilities (GHU) water treatment plant, which delivers 3 million gallons per day (mgd) of water to 40,000 residents in Fairbanks and its immediate surroundings. The press helps GHU capture for landfill disposal some 3 million to 4 million pounds per year of manganese, iron and calcium slurry that formerly discharged directly into the river.
Although the discharge was regarded as more aesthetically displeasing than environmentally detrimental (it was never considered harmful to humans), GHU agreed to work with the Alaska Department of Environmental Conservation (ADEC) to find an alternative to pumping slurry into the river. The solution, a J-Press® filter press from USFilter, went into service in December 2000, and allows GHU to operate without a state water discharge permit.
The filter press produces about 68,000 pounds of filter cake per week, with the filtrate water being pumped back to the head end of the plant for re-treatment.
Protecting a Resource
In 1997, the city of Fairbanks sold its public electric and water utilities to two private entities -- Usibelli Coal Mine and Golden Heart Utilities, respectively. Usibelli purchased a 30-megawatt (MW), coal-fired electric power generating station and distribution system, from which emerged its Aurora Energy electric utility. Likewise, then-newly formed GHU, a private corporation owned by local shareholders, became owners of the WTP and related infrastructure.
The GHU WTP was built in 1953 and expanded in 1962 and again in 1990. It has an 8-million-gallon-per-day (mgd) peak design capacity and delivers 3 mgd of water to 40,000 residents in Fairbanks and its immediate surroundings.
"The plant's mineral sludge discharge became an issue about the same time the utilities went private," explains Sam Fleury, GHU WTP foreman. "For years, we shared a state discharge permit with the electric utility, pumping our mineral slurry into the same outfall the power plant used for its non-contact cooling water."
At the time the city sold the utilities, the discharge permit had lapsed. Based on comments submitted by the Alaska Department of Fish and Game to ADEC, it was noted that Arctic grayling spawning and juvenile chum and chinook salmon rearing occurred in the discharge area. ADEC water quality regulations prohibit mixing zones within anadromous and high-value resident fish spawning areas.
Additionally, both departments also agreed that the mineral discharge was aesthetically altering the river (although it was never considered harmful to humans).
"Before we installed the filter press, we used timer-activated sludge pumps to discharge precipitates from the bottom of the clarifiers to the power plant outfall," recalls Fleury. At three minutes on, 10 minutes off, the sludge pumps discharged about 30 gallons per minute of slurry into the river, creating an unsightly rust-colored plume that extended 200 yards to 300 yards downstream. Filters were also backwashed nightly, processing 100,000 gallons in 20 minutes.
While GHU contemplated ways to remedy the discharge, the utility operated under a compliance order issued by the ADEC. The obvious alternative to discharging the slurry was to dewater it and transport it to the Fairbanks North Star Borough Landfill, about three miles away. GHU management evaluated several dewatering technologies, including filter presses, belt presses, centrifuges, pressure filters and vacuum filters.
"We chose the filter press after assessing capital and operating costs, chemical inputs, labor for operations and maintenance and quality of the final product," says Fleury. "With respect to chemical inputs, we should be able to save several thousand dollars over the equipment's life because the filter press doesn't require polymer treatment to the sludge prior to dewatering.
"In addition, we found that filter press technology would achieve the highest mineral capture efficiency and would also consistently yield the highest cake solids content, thus minimizing hauling costs."
The J-Press model 1200 MM filter press, supplied by USFilter, came online in December 2000. Installed as part of a treatment plant upgrade for approximately $200,000, the filter press handles up to 8,000 gallons per day (gpd) of mineral slurry from the clarifiers and produces about 68,000 pounds of filter cake per week.
The press, with an 88-cubic-feet capacity (expandable to 100 cubic feet), includes feed pumps, an acid wash system and an operating platform that sends filter cake into a receiving container system. The filter press collects virtually all the naturally occurring calcium, iron and manganese (roughly 3 million to 4 million pounds per year) removed from the water during treatment, as well as the lime and ferric sulfate added in the treatment and filtration process.
And with the filter press, GHU does not need a state water discharge permit.
GHU draws Fairbanks' drinking water supply from four groundwater wells along the Chena River. The main objective in treating the water is to remove iron and manganese, which give the water a bad taste and can discolor and stain household fixtures.
Because of the arctic cold, the entire treatment process and water storage are contained indoors. A "basement" clearwell stores some 4.7 million gallons of treated water. From December through April, GHU warms the water with heat purchased from Aurora Energy. This helps protect against localized freezing in the water distribution mains and services, and buys precious time for repairs resulting from contained system outages.
GHU's treatment is essentially a softening process, adding lime to elevate the pH, chlorine to oxidize the iron and manganese, ferric sulfate for coagulation and polymer to accelerate coagulation and settling. The one-step process takes place in two solids contact upflow clarifiers -- a 3.5-mgd unit used in winter and a 6-mgd unit used in summer, when demand for water is higher. Treated water is dosed with chlorine up to 0.7 parts per million (ppm) for disinfection and with fluoride (to 1 ppm) for dental hygiene.
Instead of discharging directly to the river, the plant's sludge pumps now convey the precipitates to two 10,000-gallon storage tanks in the filter press building. When one tank is full, a valve channels the flow into the other tank. Air diaphragm pumps then transfer sludge into the filter press for dewatering.
USFilter's J-Press technology is designed specifically for large-scale dewatering applications. The unit consists of a heavy steel frame engineered to support a pack of filter plates. The filter plates are recessed on both sides and, when placed side by side, form cavities called chambers. The plate pack is sealed by an electrically powered hydraulic cylinder.
At the start of the cycle, the mineral sludge is pumped through the feed inlet into the filter pack chambers. Solids are trapped by filter cloth that lines each chamber, while the liquid escapes through channels on the plate surface. Particle deposition continues until a cake is formed in each chamber.
As the cakes thicken, filtrate flow becomes restricted and pumping pressure rises until it reaches a setpoint at which the feed pump automatically shuts off. The programmable logic controller then activates two press cycles: air blowdown to remove any residual water from the plates, and core-blow to extract the relatively moist feed core. Afterwards the press is opened, and the operator uses a plate shifter to easily discharge the dewatered filter cakes into receiving containers for transport to the landfill. Typical press cycle time is two to four hours, and the press generally goes through two cycles per business day (or 10 cycles per week).
The filter press consistently achieves greater than 99 percent solids capture. It collects roughly 10 tons of filter cake at 70 percent solids every two days. Landfill disposal costs $3,000 to $4,000 per week, including trucking costs and tipping fees. About 5,000 gpd of filtrate water from the press is directed back to GHU's raw water reservoir for retreatment.
According to GHU president and CEO George Gordon, the J-Press filter press is cost efficient not only on the back-end of the process, but also on the front-end, saving GHU in labor costs.
"The J-Press process and equipment are extremely simple to operate and maintain," says Gordon. "The process is conventional yet fully automated, making it pretty hands-off. A sole operator spends on average 1 1/2 hours per day cleaning the filter plates with a pressure washer and approximately 4 hours each week doing general cleaning in the filter press room."
State officials and Golden Heart Utilities management seem satisfied by the results of the filter press installation.
Of the utility's efforts to achieve zero discharge, Tim Wingerter, section manager for domestic wastewater permitting in ADEC's Division of Air and Water Quality, recalls, "They were excellent people to work with, and did everything we asked them to do."
GHU's Gordon says that installing the filter press dewatering system has been a positive step for all concerned. "Filter press technology helps us manage responsibly mineral solids while meeting the expectations of state regulatory officials and environmentally concerned community residents," says Gordon.
And perhaps most important of all, zero discharge also means an end to unsightly discharges and, over time, incremental improvement in water quality in the Chena River.
* J-Press is a registered trademark of the United States Filter Corp. or its affiliates.
This article originally appeared in the 09/01/2003 issue of Environmental Protection.