Wastewater treatment is utilized to protect the environment and the health of our society. Municipal wastewater contains a variety of wastes. Believe it or not, wastewater is approximately 99.9% drinking or potable water with only 0.1% being solids and waste.
WASTEWATER COLLECTION SYSTEM
The wastewater collection system is composed of a network of sewer pipes, manholes, pumping stations, equipment and appurtenances for the collection, transportation and pumping of wastewater. Generally, flow through such a system is by gravity with the remainder being pumped where topography does not allow gravity flow alone.
There are three principal types of sewer systems. Sanitary, Storm, and Combined. Sewers that receive only wastewater solely from residential, commercial, institutional or industrial sources are called sanitary sewers. Sewers that convey only storm water runoff (such as the road drains on the curb lines of streets) and other drainage are called storm sewers. Combined Sewers convey both sanitary wastes and storm water. NYC has a combined sewer system. The Great Neck Water Pollution Control District operates a sanitary sewer system.
The District’s sanitary sewer system covers 3.2 square miles. It consists of 64 miles of sanitary sewer lines ranging in size from eight (8) inches to thirty-six (36) inches in diameter, 4.4 miles of force mains, 1893 manholes, ten (10) sewage pumping stations and services over 25,000 residents. The District operates and maintains all of these assets, while performing capital repairs in the form of manhole to manhole sewer relining and structural manhole restoration. We are the only sewer District or municipality on Long Island to perform these tasks in house, saving thousands of taxpayer dollars each year.
OBJECTIVES OF WASTEWATER TREATMENT
The primary objective of our District’s wastewater treatment plant is to protect our bay and environment. We do this by consistently meeting all of the permit requirements set forth by the New York State Department of Environmental Conservation (NYSDEC).
Although the characteristics of the influent (or raw wastewater coming into the plant) continually change, the District consistently produces quality effluent (or treated wastewater) from its treatment plant that exceeds the requirements of the NYSDEC.
STAGES OF WASTEWATER TREATMENT
There are four major stages of Wastewater Treatment.
Preliminary treatment of wastewater occurs at the Headworks Building. This stage removes materials that cannot be treated biologically. These items are either inert, or might harm or impair operation of equipment downstream. Substances typically removed include grit, plastics, rags, and other non-organic materials.
Following preliminary treatment, primary treatment is a physical (non-biological) process that removes suspended and floating material. This material is then pumped to the sludge digesters for solids treatment. Along with this material, wastewater often consists of other forms of solids such as colloidal solids, dissolved solids and soluble material that cannot be removed at this stage. Therefore, primary treatment must be followed by secondary and other advanced waste treatment systems to attain the required effluent quality.
Secondary treatment is where biological activity is used to perform most of the work. The microorganisms in the District’s oxidation ditch consume organic material in the waste to sustain their life processes and to reproduce. Reduction of the concentrations of dissolved, colloidal, and soluble organic substances and remaining suspended matter in the wastewater is achieved in this stage. The District’s Oxidation Ditch treatment plant removes these substances almost 10 times better than a typical secondary treatment process, reducing these down to the single digits.
The effluent from the oxidation ditch process contains high concentrations of biological solids (the bacteria that are working for us) in suspension. After leaving the oxidation ditch, the solids must be removed by means of a liquid-solids separation process. Final clarification is achieved by utilizing secondary clarifiers.
ADVANCED WASTEWATER TREATMENT
Advanced wastewater treatment is used to reduce the concentrations of nutrients such as nitrogen, phosphorous, or soluble organic substances to levels below those normally attained through secondary treatment alone. The District’s oxidation ditch is also performs this function by greatly reducing nitrogen levels before they are discharged into Manhasset Bay.
Disinfection of wastewater treatment plant’s effluent inactivates or destroys pathogenic (disease causing) bacteria, viruses and amoebic cysts commonly found in wastewater.
Historically, chemical treatment using chlorine has dominated wastewater disinfection practices. Because of the impacts of chemical disinfection by-products (chlorinated hydrocarbons) on receiving waters and their aquatic life, other means of wastewater disinfection, such as ultraviolet (UV) disinfection are now being used. By utilizing UV disinfection, the District has eliminated approximately 50 lbs of chlorine per day that would have been discharged with the plant’s effluent into Manhasset Bay. Our effluent is both chemical free and very high in dissolved oxygen.
SLUDGE AND RESIDUALS MANAGEMENT
Sludge (Biosolids), the settled solids accumulated and separated from the liquid treatment process, must be treated prior to disposal because raw sludge is unstable and contains pathogenic organisms. The treatment and disposal of sludge is often the most difficult and one of the most costly parts of wastewater treatment.
The District utilizes gravity belt presses for thickening, Multi-Stage Anaerobic Digestion for stabilization, and belt filter presses for dewatering of its biosolids. These processes ensure that we remove all of the water that can be removed from the sludge, allowing us to treat this liquid while reducing the amount it costs to haul the dried sludge.
Sludge thickening is reduction of sludge volume by removing water from the sludge. The concentration of the District’s sludge is increased from 0.25% solids prior to GBT thickening to 5-6% after GBT thickening.
STABILIZATION BY DIGESTION
Digestion, a means of stabilizing sludge, reduces the volatile content and pathogen count, thereby producing a less odorous material. At the District, multi-stage anaerobic digestion is used. Anaerobic digestion results from microbial activity in the absence of free oxygen. Anaerobically digested sludge is relatively nonputrescible (approximately 30 to 60% of volatile solids are destroyed in this process) and the off-gas produced contains about 65% methane. The methane from this process is collected, and used in our Microturbine co-generation system. The Microturbines generate approximately 20% of the plant’s electricity, and almost 80% of its process heating needs.
Dewatering further reduces sludge volume and weight. The District utilizes belt filter presses to dewater its sludge after it is digested. Belt filter presses are used with polymers to increase solids concentrations from 3% up to 25%