A deep-shaft activated sludge system helps the Homer treatment plant produce consistently good effluent despite Alaska’s chilly climate.
The mountains, forests and clear waters of Kachemak Bay make Homer a favorite spot for Alaska’s summer tourists. Year-round residents enjoy those features, too, along with a municipal water and wastewater infrastructure that protects the natural resources, while delivering reliable and efficient service.
The Homer Wastewater Treatment Plant is a case in point. Situated near the shore of the bay, the plant uses an innovative deep-shaft aeration system that provides effective biological treatment in this cold climate and lowers the plant’s profile so it doesn’t interfere with the town’s travel-brochure views.
The deep shafts are the central part of a treatment train that treats a daily average flow of 0.3 to 0.5 million gallons and returns clean water through a 2,100-foot outfall to the bay. “We have some of the most picturesque views you can get,” says Todd Cook, wastewater superintendent for his hometown. “Visually, it’s an awesome place.” Another reason deep-shaft technology was the choice for Homer is that U.S. EPA innovative technology funding was available for it.
A step up
That was in 1991, when the Homer plant was upgraded from an old sewage lagoon system that Cook says wasn’t cutting it anymore. “The quality of the effluent coming out of the ponds was not what the regulators wanted,” he says. “We could only get so much treatment out of the lagoons. Besides the beaches and fishing, there are also shellfish here. We needed to increase treatment and get better-quality effluent.”
In the upgraded treatment scheme, wastewater enters the plant through an influent pump station powered by four Flygt pumps, two in operation at any one time, controlled by an automatic level control sensor (Siemens). Two pumps are rated at 700-800 gpm and the other two at 1,000 gpm.
An old bar screen (John Meunier) removes rags, and a conical T-Cup Eutek centrifuge (Hydro International) takes care of grit, which is deposited in one of the old treatment ponds. Sharps and plastics pass through a grinder. Then the flow heads down the hatch.
Homer has a twin deep-shaft system — a splitter box directs flow to one or both shafts depending on volume. Each shaft extends 500 feet below the surface. The raw wastewater and return activated sludge (RAS) enter the system through an 18-inch inner pipe, passing to the bottom where the flow injected with 40 cfm of air at 80 psi from a 60 hp rotary screw compressor (Rogers Machinery). In a 5-foot-deep space at the bottom of the shaft, the flow transfers to the outer pipe and returns to the surface. The main pipe casing is 30 inches in diameter on each shaft.
The mean cell residence time at normal flow rates is about two days, Cook says. “Things run so steadily, it’s almost boring,” he says. “But sometimes boring is nice. Typically, plants use deep-shaft technology because they need a smaller footprint. The systems were first used in Europe, but when funding became available, we went for it.”
Homer’s northern location was an important factor in the decision. “I’ve worked in other activated sludge plants up here,” says Cook. “The weather wreaked havoc. By having the shaft in the ground, the temperature stays stable, and that helps the biology.” Keeping the plant running along with Cook are Jerry Lawver, lead operator; Joe Young and Dave Welty, operators; and Paul McBride and Bob Kosiorek, maintenance technicians.
Maintenance is minimal: “We really don’t have to clean the shafts as long as nobody drops anything in them. There are a few items down there, but nothing worth going after,” Cook says. The crew takes the head tank down periodically to remove rags and some grit and clean off the concrete to prevent deterioration from hydrogen sulfide.
After treatment, a pair of rectangular flotation clarifiers separate mixed liquor from the treated effluent. Between the shafts and the clarifier, the Homer team adds cationic polymer (Hydrofloc 1665 by Russell Technologies) to promote solids coagulation. “Because of all the air entrained in the mixed liquor, our solids float, rather than settle,” says Lawver.
Both clarifiers discharge to a common effluent channel, which directs the water to a UV disinfection system (Ozonia North America) consisting of two banks, each with 12 racks of four bulbs (SunRay or UV Doctor). After disinfection, the flow passes to Kachemak Bay. “The Bay has good tidal action, from negative 3 feet to plus 16 feet, so we get good mixing and flushing,” says Cook.
The system produces about 10,000 gallons a day of waste activated sludge (WAS), which is transported by Moyno pumps to two 50,000-gallon aerobic digesters. Cook and his staff run the digesters in series; WAS enters the first digester and decants to the second digester, which in turn decants to one of the former treatment ponds.
“We operate our digester at 8,000 to 15,000 ppm TSS,” says Lawver, noting that the organic loading on the plant is much higher in the summer. “We see a reduction of 2,000 to 4,000 parts in TSS from digester to digester.”
From the pond, solids are pumped to drying beds, which are covered against wet weather. According to Lawver, the biosolids dry to about 35 to 40 percent solids, resulting in 400 to 500 cubic yards of cake per year, hauled to a landfill and used as landfill cover.
Cook and his staff also operate the Homer water treatment facility, so they split duty between the two plants. “Generally, we have an operator and a mechanic at both plants most of the day,” says Cook. “If we have a big project at either plant, then it’s all hands on deck. We flip flop just to keep things fresh.”
The crews work overlapping schedules, half Monday through Thursday and the other half Tuesday through Friday. To fill in for the operator who is off-duty, Lawver covers one of the plants on Mondays, as does Cook on Fridays. “It gets our hands back into the operation,” Cook says. “This paperwork stuff is for the birds.” A SCADA system (S&B Controls with Siemens controllers) provides automatic control and monitors the operation.
While it’s generally “steady as she goes” at Homer, Cook and his staff have faced their share of challenges. One issue involved the recycle of return activated sludge. “The original design used head pressure to get solids to recycle off the bottom of the clarifier,” says Lawver. “But we were getting more liquids than solids and that was throwing off our polymer injection rates, because those are based on flow. Our sludge was not coagulating as well as it should have, and our fecals were going up.”
Now, “Homer homemade” airlift pumps have been installed in the clarifiers to pull RAS off the bottom, says Cook. While that has solved the polymer feed issues, it also added to maintenance because the pumps get jammed with rags from time to time.
Another issue has been algae growth in the decant ponds after the aerobic digesters, but a new solar-powered floating mixer (SolarBee) may have taken care of the problem. “We used to get long, stringy green algae,” says Lawver. “It didn’t inhibit the treatment process, but once it started, we couldn’t get rid of it.”
Homer was using UV inhibitor chemicals to counter the algae but since has switched to the surface mixer. The mixing impeller is 30 inches in diameter and shears the water molecules, throwing them back across the surface of the water. One impeller covers the 1.4-acre pond, keeping dissolved oxygen up to the desired level of 1.0 mg/L. Solar powered, the unit offsets about 30 hp that normally would be required for mixing.
Due to infiltration and inflow, the Homer plant tends to get high flows in springtime. “The seasonal change makes things a bit challenging for us,” says Lawver. The spring breakup of ice and snow from connected roof drains and basement sump pumps add to the volume of water. “We chlorinate with 12 percent sodium hypochlorite as a backup during these high flows, and dechlorinate with sodium bisulfate,” Lawver says.
Other staff-driven changes are adding to treatment efficiency. Homer will replace its old bar screen with a rotary drum screen later this year, and that will help greatly with rag removal.
Improvements have been made to the polymer system, as well. “We replaced our polymer system with a new dry feed system from Fluid Dynamics,” Lawver says. “We’re happy with it. We couldn’t get parts anymore for the old system.”
Energy conservation is also paying dividends. According to the U.S. Energy Information Administration, Alaska has the fifth highest electricity rates in the country — 14 to 16 cents per KWh — so conservation can save significant money. “We’ve replaced all our ballasts and installed motion-sensored lighting throughout the plant,” Cook says. The team has also installed new transformers in the UV system, and has replaced mercury vapor lighting with LED lights.
Finally, the plant’s deep-shaft system requires just one of the pair of compressors to provide the air needed for biological treatment.
The energy program has won a state award. The product of a citywide energy audit and upgrade plan developed by Siemens and Sylvania, with local electrical contractors, Homer’s conservation measures were funded by a state grant and received recognition in the Great Alaska Energy Challenge in 2011. Other awards for the plant include:
- 1993 Outstanding Plant of the Year, Alaska Water Wastewater Management Association, Southeast Region
- 1993 Large System Plant of the Year, AWWMA statewide
- 2011 Wastewater Treatment Plant of the Year, Alaska Rural Water Association
Cook has used the honors to boost the image of his plant and operators in the community: “It gave us some bragging rights. We received a proclamation from the city council, and our staff received awards. We’ve been on the local radio station.”
The recognition has made the energy conservation measures known and has also boosted public confidence in the plant while giving its operators due credit, Cook believes. That’s especially important in Homer where the wastewater treatment facilities themselves are nearly out of sight.