Wastewater Treatment

Ithaca Area Wastewater Treatment Facility
The Ithaca Area Wastewater Treatment Facility (IAWWTF) went into service in October 1987. Prior to 1987, the major portion of the wastewater plant was located in what is now the Sciencenter. Along with wastewater from City of Ithaca, the Town of Ithaca and the Town of Dryden, the IAWWTF also treats peak flows diverted from the Cayuga Heights Wastewater Treatment Plant and trucked wastes including: septage, landfill leachate, municipal sludge, alkaline hydrolysis liquid waste from the College of Veterinary Medicine, whey and other dairy processing wastes. Wastewater from well drilling and hydraulic fracturing (fracking) operations is not treated at our facility. The IAWWTF discharges into Cayuga Lake through a half-mile long, 48-inch diameter line that reduces to 36 inches for the last 240 feet. The effluent is diffused into the lake through 6-inch riser pipes, located 10 feet apart along the last 240 feet of the line.

Cayuga Lake is part of the Great Lakes Basin (Oswego-Seneca-Oneida Drainage Basin). The Great Lakes Water Quality Initiative sets the effluent limits for the IAWWTF. The State Department of Environmental Conservation (DEC), the Environmental Protection Agency (EPA), the New York State Department of Health (NYS DOH), and the National Environmental Laboratory Program (NELAP) are responsible for regulating different aspects of the operations at the IAWWTF.

The IAWWTF was designed to remove phosphorus, biological oxygen demand and solids. The plant also removes most non-conventional pollutants as well. A federally mandated industrial pretreatment program prevents non-conventional pollutants from entering the wastewater system in amounts greater than the plant can treat.

Wastewater treatment incorporates: preliminary screening, primary clarification, biological treatment, chemical phosphorus removal, chlorination for disinfection and de-chlorination. Anaerobic digestion is used to stabilize biosolids, which are then dewatered and land-filled. The anaerobic digestion process produces biogas (65-70 % methane) that is purified and then burned to generate electricity and hot water.

Operational Parameters
The design capacity of the plant is 13.1 million gallons per day (MGD), monthly average. The average flow of sewage treated daily at the IAWWTF is approximately 6.5 MGD. Peak flows in excess of 30 MGD have been recorded.

Phosphorus is a nutrient that, in combination with other nutrients, promotes vigorous plant growth. Phosphorus is the limiting nutrient in Cayuga Lake. To avoid eutrophication, the amount of phosphorus going to the lake needs to be minimized. The average load of total phosphorus to the IAWWTF is more than 200 pounds per day (lbs/day). From this the primary and secondary systems remove 170-180 lbs/day. The tertiary system removes 75-80% of the remaining phosphorus.

The IAWWTF currently discharges an average of less than 10 lbs/day of phosphorus; well below the allowed limit of 40 lbs/day. At this level, the IAWWTF is contributing approximately 10-15% of the phosphorus load to the south end of Cayuga Lake. Non-point sources, such as farm runoff, are the largest contributor of phosphorus to the Lake.

Biochemical Oxygen Demand
The biochemical oxygen demand (BOD) is a standard measure of the amount of conventional contaminants in wastewater. Basically, the microbes present consume oxygen dissolved in the water in proportion to the amount of organic and chemical food present. The design capacity of the IAWWTF is 15,100 Lb of BOD/day in any one month and 11,300 lbs/day in any one year. The average amount received is 7,200 lbs/day. The BOD is removed during treatment with an efficiency of 92 to 97% , well above the required 85%. The maximum allowed BOD in the effluent is 30 milligrams per liter (mg/L). The average BOD of the treated water released into Cayuga Lake is approximately 11 mg/L.

Solids are composed of biodegradable and non-biodegradable matter. The non-biodegradable portion is composed of soil, sand, salts and organic matter resistant to biodegradation such as lignin and cellulose.

The solids are removed during treatment with 97% efficiency, far surpassing the required 85% removal. The allowed limit for solids in the effluent is 30 mg/L. The typical average concentration of solids in the effluent is 3.3 mg/L.

The total average daily load of suspended solids to Cayuga Lake from the IAWWTF effluent is 178 lbs/day. As a comparison, the 1997 average suspended solids load from Six Mile Creek was 94,600 lbs/day.

We are required to reduce the amount of volatile solids by at least 37%. The IAWWTF accomplishes 57% volatiles reduction (a 43% overall destruction of solids).
IAWWTF Process Summary
View a process schematic illustration.
  1. Large solids are removed by the bar screens in the preliminary treatment step. The removed solids are land-filled.
  2. Primary settling tanks are where most of the solids are removed. Those heavier than water settle out and those lighter float. Approximately 30% of the incoming organic load is removed here.
  3. Settled solids are degritted in the cyclones and then dewatered in the thickeners before going to the digester.
  4. The activated sludge process is where the clarified primary wastewater is fed to aerobic microorganisms under constant aeration. The microorganisms in the aeration tanks agglomerate to and assimilate organics in the wastewater. The clumps of microbes and food that are formed are called "floc" particles.
  5. The secondary clarifiers are where the floc is removed by gravity settling. To keep a constant, healthy ratio between the amounts of biomass (organisms) in the aeration tanks and the amount of incoming organic loading (their food), much of the activated sludge that settles in the secondary clarifiers is returned to the aeration tanks. The excess activated sludge is removed (wasted) to the thickeners for further treatment.
  6. Tertiary treatment went online in the middle of 2006. Using 3 additives: polymer, ferric chloride and engineered sand (added as a ballast) additional suspended solids are made to clump together in larger particles that are heavy enough to settle and be removed. This system is optimized for phosphorus removal, removing roughly 80% of the remaining dissolved phosphorous. It also removes about half of the other remaining conventional pollutants. Waste sludge from this system is sent to the thickeners.
  7. Excess water is removed from all the waste sludges (which come from the primary settling tanks, the secondary clarifiers, and the tertiary clarifier) in the thickeners. The thickened sludge is then pumped to the primary digester. There anaerobic digestion occurs which further reduces the total biomass. The biomass is digested for approximately 28 days at 98° F, and overflows to the secondary digester. After further stabilization there, the sludge is dewatered using a chemical polymer and a belt press. The dried cake (at 20-26% solids) is transported to a landfill for disposal (6,000-12,000 dry lbs/day).

    During anaerobic digestion, a biogas rich in methane is generated. The biogas produced before the energy upgrade was utilized to generate 25-30% of the total energy (heat and electricity combined) requirements of the IAWWTF. As different aspects of the energy performance upgrade are completed, the total energy requirements of the plant will decrease while the amount of energy produced from biogas will increase. The guaranteed result of these improvements is that the plant will generate 60% percent of its energy requirements using this renewable fuel.
  8. The final step of cleaning wastewater is disinfection for pathogen removal. Chlorine bleach (sodium hypochlorite) is added to the effluent water for this purpose. After mixing with the effluent for enough contact time to provide good pathogen destruction, the remaining residual chlorine is removed using sulfur dioxide gas. The clean effluent is then discharged into Cayuga Lake.