IAWWTF Treatment Process
Conventional and Non-Conventional Pollutants
All municipal wastewater treatment plants have one, overriding purpose. They prevent the spread of waterborne pathogens including botulism, cholera, E. coli, dysentery, typhoid fever, hepatitis A, and polio. The simplest way to remove pathogens from wastewater is to remove solids. Raw wastewater is a mixture of more than 99% clean water and less than 1% solids.
Thew IAWWTF was designed to remove "conventional" pollutants which are found in dissolved and suspended solids. They include phosphorus, biochemical oxygen demand, total suspended solids, pH, oil & grease, and fecal coliform bacteria. The plant also removes many non-conventional pollutants such as metals, volatile, and semi-volatile organics.
The goal of the federally mandated industrial pretreatment program is to prevent non-conventional pollutants from entering the wastewater system in amounts greater than the plant can remove.
IAWWTF Process Summary
Wastewater treatment at the IAWWTF incorporates preliminary screening, grit removal, primary clarification, biological treatment, chemical phosphorus removal, disinfection using bleach (chlorine), and de-chlorination. the time needed to do these steps is roughly 18 hours. The solids removed during primary, secondary, and tertiary treatment receive additional treatment.
- Large solids (>6mm (1/4") in all dimensions) are removed by the bar screens in the preliminary treatment step. The removed solids are used as a soil amendment to cover landfills.
- Primary settling tanks remove about 30% of the solids in wastewater. Solids heavier than water settle out and those lighter than water float. The settled and floating solids are then removed.
- Grit is removed from the settled solids using centripetal force in "cyclones". Note: once the 2020-2021 plant upgrade is complete, grit will be located in primary settling (#2).
- The activated sludge process is where the clarified primary wastewater is fed to aerobic microorganisms (who need oxygen to survive) under constant mixing and 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.
- The secondary clarifiers are where the floc is removed by gravity settling. To keep a consistent, healthy ratio of biomass (organisms) in the aeration tanks and incoming organic loading (their food), much of the activated sludge that removed (wasted) to the thickeners for further treatment.
- Tertiary treatment went online in the middle of 2006. Using 3 additives: polymer, ferric chloride, and engineered sand (for ballast), additional suspended solids are made to clump together in larger particles heavy enough to settle and be removed. This system, optimized for phosphorus removal, takes out roughly 80% of the remaining dissolved phosphorus. It also removes about half of the other remaining conventional pollutants. After separating out the sand for reuse, the waste sludge from tertiary treatment is sent to the thickeners.
- The thickeners remove excess water form a mixture of waste sludges coming from the primary settling tanks, secondary clarifiers, and tertiary clarified. The thickened sludge is then pumped into the primary digester.
- In the sealed digester tank, anaerobic (without oxygen) bacteria bio-degrade (eat) organic matter, further reducing the remaining biomass. The digester is heated to 98o F, so the anaerobic bacteria have ideal conditions to thrive. The primary digester overflows into a secondary digester where anaerobic bacteria continue their work while the solids settle by gravity. The total time for digestion is about 28 days (vs. about 18 hours from wastewater entering the plant to discharging clean effluent into Cayuga Lake).
- The thickened, fully digested sludge consists primarily of dead bacteria. It also includes materials that do not easily biodegrade, such as plastics, hair, and seeds. Before it can be used as a soil amendment to cap a landfill, it must be turned into a solid (>20% solids). Two different technologies are used, in parallel, to dewater the digested sludge. Both the belt press and rotary screw presses use a polymer to help remove water from the sludge. The digested sludge starts around 3.5% solids and the presses create a "cake" that is 20-30% solids. Typically, 6,000-12,000 dry lbs/day of solids are trucked to a landfill where it is used as a soil amendment to cap trash cells.
- During anaerobic digestion, a biogas containing 65-70% methane is generated. The biogas produced before the IAWWTF had an $8M energy performance upgrade (see below), generated 25-30% of the total energy (heat and electricity combined) requirements of the plant. now, after that upgrade, biogas produces about 60% of the plant's combined energy needs. The IAWWTF is a renewable energy factory!
- The final treatment step is to disinfect the effluent, just in case any human pathogens remain. Chlorine bleach (sodium hypochlorite) is added to the effluent water for this purpose. After mixing with the effluent for enough contact time to allow for pathogen destruction, the remaining residual chlorine is removed using sulfur dioxide gas.
- The final effluent is then discharged 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 that final section of pipe.