CORTLAND WASTE WATER PROCESS DESCRIPTION

The following process description was written by the Biology department of SUNY Cortland, in cooperation with the staff of OYW Cortland

The City of Cortland and portions of the nearby villages of Cortlandville, Homer and McGraw generate the wastewater which enters the Cortland Waste Water Treatment Plant (CWWTP). The greater volume (about 80%) is from residential homes, apartment buildings and the dorms on the SUNY campus; wastewater from these sources comes from toilets, sinks, garbage disposals, and bathtubs. A lesser, but significant, portion of the wastewater (about 20%) is generated by local industries. The wastewater from industries is periodically tested for the presence of toxic materials such as heavy metals (nickel, chromium, and mercury), solvents, etc. If an industry produces wastes that are particularly toxic to the microorganisms involved in wastewater treatment at the CWWTP, then the industry must have a pretreatment program to remove the toxic chemicals before their wastewater is discharged into the sewer.
Wastewater coming into the CWWTP arrives not just in sewer pipes. Rainwater and other fluids that percolate down through the garbage at the Cortland County Landfill is collected at the bottom and trucked to the CWWTP for treatment. This liquid is called leachate. The volume of leachate processed by the plant is relatively small, but the concentration of organic material in it is extremely high. The city of Cortland does not charge the landfill for treating the leachate, and, in return, the county landfill accepts treated, dewatered sludge from the CWWTP.
Wastewater entering a treatment plant is referred to as the INFLUENT. Most of the influent is just plain water, but about one percent is solids and dissolved materials. These consist of organic materials such as those from toilets and kitchen disposals, and inorganic compounds such as sediment and industrial chemicals. Some of this material floats and must be skimmed off the top of the water, while other is heavy enough to be "settleable." Some particles are too small or lightweight to settle out and remain in suspension, and some substances simply stay dissolved in the water. The CWWTP treats an average of 6-7 million gallons of wastewater per day (MGD), but flow may exceed 30 MGD during wet times such as spring snow melt coupled with heavy rainfall. An elevated water table allows ground water to infiltrate the sewer pipes.
The purpose of wastewater treatment is to remove as many of the wastes from the water as possible so that the treated water (EFFLUENT) will have few, if any, detrimental effects when it is returned to the environment, which in this case is the Tioughnioga River. One of the biggest tasks of the CWWTP is to reduce the BIOLOGICAL OXYGEN DEMAND (BOD) in the wastewater. BOD is a measure of the organic pollution present. It is defined as the amount of oxygen needed by bacteria to break down any organic material present in the wastewater. The influent has a high BOD due to the high levels of organic wastes in the water. Bacteria utilize this organic material as a source of food, and as they break down the organic material, the bacteria use up the oxygen from the water, leaving less oxygen available for other forms of life. By removing the organic material, and hence the BOD, in the wastewater, the environmental impact of the effluent on the Tioughnioga River is minimized. The CWWTP operates under a permit from the New York Department of Environmental Conservation and must submit a monthly report on its effluents’ BOD (biochemical oxygen demand), total solids, suspended solids, fecal coliform count, and ammonia concentration. Reports on heavy metals and pesticides are submitted quarterly. The plant removes 95% of the BOD and 90% of the suspended solids during normal plant operation. This compares favorably with their NYDEC required removal rate of 85% for both BOD and suspended solids.

To reduce the BOD and amount of suspended solids in the wastewater, the CWWTP utilizes two types of processes:

1. PHYSICAL - physical actions include screening the influent to remove large objects, settlement of suspended particles by gravity, and aeration.

2. BIOLOGICAL - favorable conditions are created for the growth of microorganisms which use the organic matter in the wastewater as food and convert it into gases such as carbon dioxide, hydrogen sulfide, or methane. This occurs both in the aeration chambers and in the sludge digesters (see below), where the microorganisms multiply rapidly. The microorganisms are primarily various species of bacteria and protists, and possibly nematodes and fungi.

TREATMENT OF WASTEWATER (Influent to Effluent)

1. Pretreatment of wastewater
BAR SCREENS - Water enters the plant through a 48-inch pipe fitted with a measuring device (flume) to determine the volume of the influent. A mechanical bar screen removes solids greater than one inch across. These solids may include stones, rags, bottles, cans, lumber, etc. It is important to remove these to prevent damage or clogging of the plant's equipment.

2. Raw Sewage Pumping
Three variable speed centrifugal raw sewage pumps convey the screened raw sewage from the raw sewage wet wells (holding tanks) to the grit removal chamber. These pumps are capable of pumping up to 14,000 gallons of wastewater per minute (gpm).

3. Grit Removal Chamber
Water is pumped into the grit removal chamber at an angle which generates a circular flow of water around the tank. This motion results in the movement of heavier inorganic solids toward the center and bottom of the chamber. The "grit" is removed by truck to the Cortland County Landfill. The water with the lighter suspended solids is transferred to tanks for primary clarification.

4. Primary Clarifiers
Water moves into two circular tanks, each 80 feet in diameter. Here the water circulates for about two hours while surface skimmers remove the "floatables," and sludge scrapers on the bottom of the tank remove the primary sludge. Sludge is a collective term which refers to all of the solids (mostly organic) which have settled to the bottom of the tanks. The primary sludge is transferred directly to the anaerobic digesters, while the floatables are collected and stored for later removal to the digesters. Water from the clarifiers (containing dissolved and suspended solids) is moved into the aeration tanks.

5. Aeration and Biological Treatment (Aeration Basins)
The water in these tanks is agitated by giant "propellers," and air is bubbled through aeration diffusers located in the bottom of the tanks. The aeration process assures a maximal supply of oxygen for the microorganisms that come in with the wastewater. These microorganisms are responsible for the biological treatment that occurs at the CWWTP. With an adequate supply of oxygen, the microorganisms are able to act with optimal efficiency in decomposing the organic compounds in the water. In these aeration basins, the microorganisms multiply as they feed on the material in the wastewater. Retention time in these tanks is about six to 8 hours. From here, water flows by gravity to the secondary clarifiers (settling tanks).

6. Secondary Clarifiers
In the secondary clarifiers, heavier solids such as dead microorganisms and undigestible material settle out and accumulate at the bottom of the tanks as sludge. About 50% of the solids settles out within five minutes of reaching the secondary clarifiers. During the five hours that water stays in these tanks, solids are continuously removed from the bottom by mechanical scrapers. The majority of this activated sludge (sludge rich in live microorganisms) is returned to the aeration tanks to help maintain the population of microorganisms needed for biological treatment, and the remainder of the sludge is pumped to the anaerobic digesters. The water from the secondary clarifiers is transferred to chlorine contact tanks.

7. Chlorination/Dechlorination/Aeration
Chlorine is added to the water leaving the secondary clarifiers during the warmer months from May 15th to October 15th. It is added to achieve a "significant pathogen kill" by destroying coliform bacteria and associated pathogens, and should not be equated with sterilization. However, if the added chlorine were discharged into the Tioughnioga, it could kill naturally occurring microbes and possibly larger forms of life. To avoid this, sulfur dioxide (SO2) is added for dechlorination. Final aeration occurs just prior to discharge of the effluent to raise the dissolved oxygen (DO) levels. The influent of the CWWTP has a DO of about 3-7 mg/l, whereas the effluent has a DO of about 10-11 mg/l. This latter figure exceeds the DO of the Tioughnioga, which is normally about 7.5-8.5 mg/l.

8. Discharge of Effluent into the Tioughnioga River
The cleaned wastewater is released into the Tioughnioga River less than 20 hours after it entered the plant.

TREATMENT OF SLUDGE

1. Sludge Pumping
Sludge is transferred from two sources in the plant: the primary clarifiers and the secondary clarifiers (settling tanks) to the anaerobic digesters.

2. Primary Anaerobic Digestion
Anaerobic digestion takes place in two 36-foot-diameter concrete tanks. Here the sludge from the primary and secondary clarifiers is mixed and warmed to 98-100°F. The anaerobic acidophilic (acid-loving) bacteria naturally found in sludge break down the organic materials into hydrogen sulfide gas (H₂S) and volatile acids. Then, methanogens (methane-forming bacteria) produce methane (CH₄), carbon dioxide (CO₂), and inert materials from the volatile acids produced by the acidophilic bacteria. The sludge from the primary anaerobic digesters is pumped into a third tank, where the sludge is neither warmed nor mixed. Here the heavy, digested sludge is allowed to settle to the bottom of the tank for removal to the dewatering process. Methane produced in the digestion process is collected and used to heat the primary digesters and also the plant's main buildings during winter. The process of anaerobic digestion of sludge takes about 15 days.

3. Sludge Dewatering
Because the solid content of the final sludge is only about 3 percent, it is necessary to add a conditioning polymer and pass the sludge through a belt press that concentrates the sludge to about 15-20% dry solids. The "filter cake" is then moved by conveyor to a dump truck for subsequent disposal at the Cortland County Landfill.

STREAM ECOLOGY
Humans use lakes and streams for a variety of reasons, such as recreation, power generation, crop irrigation, or drinking water. However, lakes and streams may also be used as a dump for domestic and industrial wastes, and the water pollution that results can affect its use for recreation, food production or human consumption. An elevated level of heavy metals, fertilizers, pesticides, or organic material from urban sewage and agricultural operations can have serious effects on aquatic ecology.
Detrimental effects may also result from thermal pollution. Thermal pollution - from disposing of heated water in lakes or rivers - is considered to be one of the major hazards facing lakes in the future. A major source of heated water is the modern power plant, which uses water to remove excess heat. The power requirements of modern societies increase at the rate of about 7 percent a year, and there is great concern about the thermal pollution of even the larger lakes. Thermal, like chemical, pollution can damage the ecology of lakes and rivers by changing what can live in the warmer waters.
If humans dump improperly treated wastewater into streams and lakes, these bodies of water may become unsuitable as a source of food or for recreational uses. Onondaga Lake in Syracuse is an excellent example of what can happen when a body of water becomes too polluted. For decades, the lake received untreated industrial and domestic sewage. The levels of organic material and toxic chemicals in the lake rose high enough to kill just about all the fish in the lake and make it literally unhealthy for swimming. Fortunately, the enactment of stiffer controls over the quality of wastewater that is dumped into the lake has helped to improve its status; however, some of the pollutants that have accumulated in the lake's sediment may remain toxic and continue to affect the ecosystem for decades to come.
Untreated sewage and agricultural runoff flowing into a lake or stream may result in eutrophication. A eutrophic body of water is one rich in dissolved nutrients, such as phosphates (-PO4-3) from detergents, fertilizers, and manure. Because of the high concentration of dissolved nutrients, algae will typically proliferate, die, and sink to the bottom. Here, their decomposition by bacteria requires a great deal of oxygen - i.e. the BOD goes up. If oxygen levels drop too low, the oxygen-requiring organisms, like fish, might be killed. In fact, some eutrophic lakes may become anaerobic (with all the aerobic life dying) if the BOD becomes too high.
Recall that the influent at the CWWTP has a BOD of 80-150 mg/l, and the effluent has a BOD of 2 mg/l. Considering the environmental impact of wastewater with a high BOD, you can now appreciate why it is necessary to treat our wastewater before it is discharged into the Tioughnioga River. The benefits of keeping our streams and lakes clean make proper wastewater treatment well worth the cost.