Coming Up With Air
Unrelenting population growth recently led to deficient oxygenation in the treatment process at the wastewater treatment plant serving Kingsland, Ga.
The latest problem involved deficient oxygenation in a critical phase of the treatment plant’s process.
This community of 15,000, located just across the Florida state line, has experienced a tenfold increase in population in less than a generation. This can be attributed to the quality of life and the employment opportunities at a nearby nuclear submarine base and in nearby Jacksonville. The flat topography and the more than 50 lift stations needed to overcome the lack of gravity flow compound the plant’s process problems by inducing frequent spikes in the influent flow. In the 1960s, the surges eroded the efficiency in the first chamber of the Orbal basin, a three-ring biological oxidation ditch system.
The community’s wastewater treatment facility originated in the 1960s as a 14.2-acre aeration lagoon rated at 400,000 gpd. With the initial upturn in population, the utility added rotating biological contactors and some nominal chlorination in the early 1980s that proved ineffective after only a few years. Subsequent improvements included a 1.6-mgd plant with a 790,000-gal Orbal oxidation basin; a sludge collection system; two 60-ft diameter, 50,000-gal clarifiers; improved chlorine disinfection; and a 40-ft, 198,000-gal aerobic digester for processing solids.
Served similar plants
This process architecture has served many smaller utilities quite well over the years. Engineered as three concentric channels that operate with high concentrations of mixed liquor suspended solids (MLSS), the mature design delivers simultaneous nitrification and denitrification in the outer cell and carbonation in the following two cells. Under ideal operating conditions, the first channel can result in 80% denitrification of influent without internal recycling. Most plants also can achieve effluent phosphorous levels of 1 mg/L without chemical additives.
However, the efficiency depends on a stable influent flow without the recurring surges and steadily increasing volume of the Kingsland service area.
While the utility gained capacity from the Orbal plant to cope with the growth in population, the upgraded volume also exceeded the state’s point discharge limit for the receiving creek near the facility. This demanded a four mile-long, 24-in. line for discharge of treated effluent in the stronger-flowing St. Mary’s River. The line theoretically raised the permit limit to 2.2 mgd, although the plant capacity remained unchanged at 1.6 mgd. The eventual inadequate oxygen level in the first chamber of the Orbal system and the recurring flow spikes cost the facility quality control over the biological process.
The operators coped with the situation by using contact chlorination for suspended solids that sacrificed the beneficial microorganisms essential to the activated sludge process.
Jimmy White, hired four years ago as superintendent of the water utilities, enacted some immediate changes and encouraged more focus on long-range planning.
White brought to the task more than 15 years of experience and an environmentalist’s commitment to “put back into a river water as good or better than what we take out.”
“We have a small group of very talented people at this utility but we faced problems that had the plant running us rather than allowing us to operate the plant,” White said. “The immediate problems were only compounded by the fact that we also were approaching capacity.
“In 2001, our flow, still aggravated somewhat by infiltration, stood above 80% of the permit,” he added. “When you reach 90% of the permit limit, the state of Georgia requires utilities to have plans immediately underway for improvements.”
Stepping up maintenance
Working with a supportive mayor and city council, White and his staff continued a stepped-up proactive maintenance and capital improvement program.
There were evident needs, such as the continuing investment in relining leaking manholes and collection lines, as well as a few obvious oversights such as missing disks in the RBS system. For the first time in memory, the relining program has allowed treated effluent a 1:1 match with the amount drawn from the city’s potable wells.
Carter & Sloop, Inc., the utility’s current consulting engineers, have developed a multi-phase program addressing the plant’s long-term capacity and likely unbridled growth. The goal is to have in place the necessary wastewater treatment infrastructure to handle a population expected to reach 20,000 residents—double that of today—between 2010 to 2015, according to Mark Gatlin, PE, with the Macon-based firm.
The first round of improvements has included enlargement of the headworks and the addition of a bar-screen, defused-air flotation tank to capture grease and grit and a belt press to replace drying beds.
The belt press has proven especially effective. It not only improved the plant’s efficiency but also reduced the amount of sludge hauled to the landfill from 250 to 300 tons per month to 100 tons per month. White would eventually like to see the utility adopt a composting program. The next phase will add three sequential batch reactors (SBRs) adjacent to the existing plant and more discretionary control over the influent streams reaching the two facilities.
Meanwhile, the utility has improved aeration in the critical chamber of the Orbal basin. It normally takes 24 hours for this stage of the treatment, following by 18 to 23 days for the solids to pass through the aerobic digester.
“Our process quality at the front end had steadily declined because of the increasingly stronger spikes in the arriving flow of influent,” White explained. “We began to experience excess ammonia levels in warmer seasonal temperatures. This affected the quality throughout the process. The 15-hp aerators previously serving the first chamber in the Orbal process appeared adequate at the surface but didn’t deliver enough oxygen to the bottom of the 11-ft basin to sustain microorganisms.”
Three violation notices eventually ensued because chronic conditions caused the plant to exceed state compliance limits for the discharge, he added. Any four notices in a six-month period could bring hefty fines.
“We were producing 18.2 mg/L of ammonia versus the state’s allowable average of 17.4 mg/L,” he said. “With our growth, it would have only gotten worse.”
The solution came in adding three 30-hp Jet Aerator units supplied by ITT Flygt. The powerful units share the same clog-resistant “N” impellers as used on high-performance Flygt pumps favored by the utility in new lift stations. White noted that within hours of installation, the aerators had delivered significant results.
“The old aerators just pumped air to the surface but we needed it to reach the bottom of the basin,” White said. “Just four hours after installing the new Flygt aerators last June, our ammonia level dropped and now typically ranges around 8 mg/L or less. Our BOD averages about 350 coming into the plant and used to discharge with an average of 25 mg/L before adding the Flygt aerators.
“The treated effluent now ranges from 11 to 15 BOD level in the discharge and remains the cleanest ever released into the St. Mary’s River,” he said. “The new units are also energy efficient. The 30-hp units actually cost less to operate than the old 15-hp aerators.”