Trash such as cloth, rags, plastics, and paper, together with some fecal matter and other large objects which may be present in the waste stream, can cause problems in the operation of pumps, valves and other equipment at the plant. It is, therefore, necessary to remove these objects prior to further treatment of the waste stream. The removal of these objects, which are commonly known as screenings, is achieved at the plant by the use of a mechanical bar screen. These objects are removed from the waste stream and emptied onto a screenings conveyor. The conveyor deposits the screenings in a dumpster located outside the grit building. When the dumpster is full it is removed by a truck and the screenings are ultimately disposed of in a landfill.

The grit chamber removes grit, consisting of sand, gravel, cinders, or other heavy solid materials that have settling velocities or specific gravities substantially greater than those of the organic putrescible solids in wastewater. Grit also includes eggshells, bone chips, seeds, coffee grounds and large organic particles, such as food wastes. Grit chambers protect moving mechanical equipment from abrasion and accompanying abnormal wear, and reduce formation of heavy deposits in pipelines, channels, conduits and anaerobic digesters.

Grit removal facilities are designed to slow the flow-through velocity to a point where the heavier grit settles out, while the lighter organic solids are maintained in suspension. Grit is removed from the bottom of the chamber utilizing drag chains with bucket attachments. The grit is removed from the building via a grit screw conveyor and deposited into a dumpster for ultimate disposal in a landfill.

The Pre-Equalization Tanks are used to reduce the peaks in the diurnal flow patterns generated within the sewer system and to provide a constant flow rate to the Sequencing Batch Reactors. The Pre-Equalization Tanks also decreases the daily variations in the concentrations of BOD, suspended solids, and other wastewater constituents. Providing a constant flow rate will enhance the performance of the SBR process. The tanks also reduce the size and volume of downstream treatment facilities. The pre-equalization basins are equipped with a coarse bubble type aeration system to provide mixing of the tank contents and to maintain a minimum of 1.0 mg/l of dissolved oxygen in the basins.

The Sequencing Batch Reactor (SBR) process provides treatment at this facility through carbonaceous removal, ammonia oxidation and nitrate reduction within the same process. The SBR process is a suspended growth, activated sludge type process, that combines and uses a modified version of the activated sludge process with a single sedimentation step.

The SBR process is often called a single-sludge system for this reason. The SBR process uses six (6) sequential phases. The six sequential phases comprise one (1) cycle. During Phase 1 (mixed fill), sewage enters the reactor and "fills" the reactor. While the reactor is being filled, a mixer "mixes" the contents, suspending the biological solids (sludge) in the reactor liquid. During Phase 2 - (react fill), the raw sewage continues to "fill" the reactor (basin). During this phase, the motorized air valve is cycled open and closed such that the biological solids and oxygen "react" with the raw sewage. It is the cycling of the motorized air valve that promotes the nitrification and denitrification biological reactions. During Phase 3 (react), the sewage flow to the basin is shut off and is diverted to another reactor basin. During Phase 4 (settle), liquid-solid separation occurs during the settle phase, analogous to the operation of conventional final clarifiers. The influent valve remains closed, the motorized air valve is closed and the mixer is turned off. This provides a total quiescent state to allow settling. During Phase 5 (decant phase), clarified liquid (supernatant) is drawn down and transferred to the Post-Equalization Basins. During Phase 6 (waste sludge), the sludge is removed from the process tank leaving the correct amount of sludge in the reactor to process another influent batch at the start of the next MIXED-FILL phase. Maintenance of reactor solids is accomplished by varying the duration the sludge wasting pump runs. The total duration of a cycle is approximately 6 hours and is adjustable based on flow and wastewater characteristics.

There are two screw pumps (including one standby unit) which are used to lift the wastewater from the trickling filter recirculation manhole to the Rotating Biological Disc (RBD) units. These screw pumps operate on the same principle as the Archimedean Screw. As the wastewater enters the lower end of the screw it is raised from one spiral of the screw to the next until it flows out the upper end and into the effluent channel to the RBD units.

Screw pumps are capable of pumping large quantities of flow at low lift, and have the ability to pump variable flow rates using constant speed motors.

The Post Equalization Tanks collect the decanted effluent from the Sequencing Batch Reactors and provide a uniform, constant flow to the chlorine contact tank at a rate of 1.8 mgd. The rate of decant from the SBR is high and occurs at set intervals during the day. Therefore, post equalization tanks are needed to reduce the size and to enhance the performance of the chlorine contact and dechlorination systems.

The primary purpose of using chlorine when treating domestic wastewater is for disinfection. Strictly defined, disinfection is the process of destroying all waterborne pathogenic organisms. The extent and presence of bacteria in plant effluent is indicated by the concentration of coliforms. Coliforms, although not harmful, are found in abundance in human intestines, and their presence, measured by the coliform count, is indicative of the presence of other bacteria. A low coliform count in the plant effluent indicates most harmful bacteria have been destroyed by disinfection. Wastewater varies in its concentration of organic and inorganic material which reacts with and are oxidized by the chlorine. The amount of chlorine not used up in oxidizing these materials after the detention period in the chlorine contact tank is the chlorine residual. The amount of chlorine which must be applied to produce a given residual is an indication of the amount of chlorine demanding material in the wastewater.

Chlorination at the Oyster Bay Sewer District sewage treatment plant is accomplished using sodium hypochlorite solution.

Dechlorination is a process that is used after chlorination and before final discharge to reduce the impact of the chlorine residual concentration on the receiving water body. Dechlorination must occur in a dedicated contact tank, separate from the chlorine contact tank. The dechlorination reaction with free or combined chlorine will generally occur within 15-20 seconds. A contact period of at least 30 seconds has been provided. The contact tank is located adjacent to the effluent channel of the existing chlorine contact tank.

Waste sludge generated by the SBR process is discharged into an aerated sludge holding tank. Aeration is provided to reduce the potential for odors and to keep solids from settling. The tank has the capacity to hold sludge that is generated over several days. Once a sufficient volume is held in the tanks, the sludge is pumped to the gravity belt thickener.

Sludge thickening is used to increase the solids content of the sludge prior to digestion. By reducing the amount of water in the sludge thickening, the digestion process will operate more efficiently. Supernatant, which is the water removed from the sludge during thickening, is returned to the pre-equalization tanks for treatment.

Sludge is pumped from the sludge hoppers of the primary and secondary settling tanks to the primary digester for further treatment. The sewage treatment plant treats the primary and secondary sludge in a primary sludge digester tank where bacteria decomposes the organic solids in the absence of dissolved oxygen.

The purposes of sludge digestion are:

1. A 20 to 30 percent reduction on total, dry solids.
2. The conversion of highly putrescible organic matter to relatively stable compounds.
3. The production of methane gas to be used in heating.

The anaerobic digestion process utilizes anaerobic (living in the absence of oxygen) and facultative (living either with or without oxygen) organisms for stabilization of the organic matter in sludge. Digestion takes place in enclosed tanks.

There are three stages of sludge digestion:

1. Acid fermentation
2. Acid regression
3. Intensive digestion

During each stage the volatile acids and pH go through dramatic changes.

In the acid fermentation stage of digestion, "acid forming" microorganisms attach the soluble solids such as sugar and produce organic acids at a concentration of upwards of several thousands mg/L, causing the pH to drop to 5.1 or lower, carbon dioxide, carbonates, and hydrogen sulfide are also produced by the "acid forming" microorganisms.

The second stage is acid regression, where the organic acids and nitrogenous compounds are liquefied by microorganisms to which an acid environment is favorable. The pH climbs from 5.1 back to 6.8. Small quantities of carbon dioxide and methane are produced in this stage.

Intensive digestion, stabilization, and gasification mark the third stage of digestion. The proteins and amino acids are attached by "methane forming" microorganisms. The pH becomes stabilized in the range of 6.8 to 7.4 and the volatile acid content is down to about 500 mg/L. At this point the solids can be disposed of without causing a nuisance. During this last stage of digestion, a large amount of methane gas is produced which can be burned off or utilized.

In a well-operated digester, all three stages are carried out at the same time, with the acids produced in the first stage being partially neutralized by the ammonia produced in a following stage until they can only be further degraded. The pH of the third stage should predominate and keep the digester in a neutral range. Once good alkaline digestion is established, the acid stages are not apparent unless the normal digestion process become upset by overloading, addition of poisonous chemicals, large temperature swings or poor mixing.

The whole process can be considered to be an "assembly line" process where the products of one stage are utilized and are passed onto the next group of organisms. For this reason, any action which hurts one group of organisms will have an adverse effect on the entire process.