Where applicable, content Copyright 2007,
City of Martinsville
55 W. Church St.,
Martinsville, VA 24114
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WASTEWATER TREATMENT PLANT
GENERAL
The City of Martinsville is authorized by the Virginia Department of Environmental Quality to discharge treated wastewater to the Smith River. In accordance with State Water Control Laws and Federal Water Pollution Act Amendments, the quality and quantity of effluent discharged from the wastewater treatment plant is regulated by a state issued discharge permit (VPDES permit).
This permit establishes specific effluent limitations that are based on state and EPA established water quality standards. The discharge permit also establishes certain monitoring and operational requirements.
Based on established criteria with respect to type and size of treatment works, the Martinsville Wastewater Treatment Plant is classified by the VA Department of Environmental Quality (DEQ) and the VA State Department of Health (VDH) as a Class I Plant. The operator in responsible charge of this facility is required to have a Class I operator license issued by the State of Virginia.
TYPE OF TREATMENT AND EFFLUENT REQUIREMENTS
Treatment of wastewater at the Martinsville Wastewater Treatment Plant is based on an activated sludge process that is designed to provide secondary treatment for a wastewater flow of 8.0 million gallons per day (MGD). The plant is hydraulically capable of handling peak flows of 15 MGD. The plant is designed to satisfy the requirements of the discharge permit which limits the average effluent concentration of biochemical oxygen demand (BOD) and suspended solids (SS) to 22.5 milligrams per liter (mg/L) and 30 mg/L respectively for any one calendar month. To achieve these effluent limitations, the plant must be capable of providing approximately a 95 percent reduction of influent BOD and an 85 percent reduction in SS concentrations.
General Definitions
Influent - Incoming waste stream to the plant or a process within the plant, including domestic sewerage, industrial waste and infiltration from storm water runoff.
Effluent - Outgoing stream from plant or processes within the plant.
Pollutant - Any matter which causes air, soil or water to be unfit or harmful to living things.
pH - A measure of the concentration of hydrogen ions [H+] indicating acidity or alkalinity of water. pH is one of the most important and frequently used tests in water chemistry. Every phase of wastewater treatment is pH dependent.
DO - "Dissolved Oxygen" levels in wastewaters depend on the physical and biochemical activities in the water body. The analysis for dissolved oxygen is a key test in water pollution and waste treatment process control.
BOD - "Biochemical Oxygen Demand" indicates the rate of oxygen utilized by wastewater under controlled conditions of temperature and time. The BOD test measures the oxygen required for the biochemical degradation of organic material and the oxygen used to oxidize inorganic material.
Solids - Solids refers to the matter suspended or dissolved in water or wastewater. Solids may affect water or effluent quality adversely. Solids analyses are important in the control of biological and physical wastewater processes and for assessing compliance with wastewater effluent limitations. "'Total Solids" is the term applied to the material residue left in a vessel after evaporation and drying of the sample. Total solids include suspended solids, the amount of residual left on a filter after the sample has been passed through it and "Dissolved solids", that portion of solids which will pass through the filter. “Settleable solids" is the term applied to the material which settles out of suspension within a defined period. Measurements of solids are expressed in mg/L (milligrams per liter).
Aeration - The process of adding oxygen to wastewater by means of mechanical equipment such as aerators or diffusers.
MLSS - "Mixed Liquor Suspended Solids", refers to the biomass consisting of solids and microorganisms that are in suspension and under aeration in the secondary system.
Sludge - The settleable solids separated from liquids during processing or deposits on bottoms of streams or other bodies of water.
COLLECTION SYSTEM
Generally, wastewater flows from domestic, commercial, institutional and industrial sources located in the City of Martinsville and Henry County. These flows are discharged to the sanitary sewer system and transported by gravity to the wastewater plant.
PRETREATMENT
When the wastewater enters the City’s treatment system., it passes through the pretreatment units. These units are designed to remove material that is likely to produce nuisances and cause problems in the proper operation of pumps, mechanical equipment, and treatment processes. Pretreatment consists of an “Aqua Guard” self-cleaning bar screen. The screen removes large objects such as sticks, rags, cans, and other debris. There are two comminutors that can be used as a back up for the bar screen. These units grind and shred materials in the wastewater so that process equipment and wastewater flow will not be obstructed. All materials removed in the pretreatment process are regularly removed to the city's solid waste disposal facilities.
INFLUENT & EFFLUENT METERING
The wastewater from the pretreatment facilities enters the influent Parshall Flume where plant influent wastewater flow is measured and recorded by an ultrasonic flow measurement device. A “Parshall Flume” is an open channel flow measurement device that restricts flow and causes a rise of upstream liquid level so that the rate of flow can be measured in the restricted section. The effluent Parshall Flume is located near the end of the treatment process and measures the plant effluent that is discharged to the Smith River.
PRIMARY TREATMENT PROCESS
The City of Martinsville's primary treatment process equipment consists of two circular primary clarifiers that allow those substances in the wastewater that readily settle or float to be separated from the water being treated. Effluent from Parshall Flume No. 1 enters the primary clarifiers where the flow of wastewater is slowed so that heavier suspended solids settle to the bottom of the tanks. Floating material is skimmed from the top of the clarifiers. Here the wastewater is also aerated to aid in solids separation. The sludge that collects on the bottom of the clarifiers is pumped to the sludge handling facilities through a grit separator that removes sand, grit and other inorganic constituents. Wastewater overflows the effluent weirs of the clarifiers and is transported to the secondary treatment process.
SECONDARY TREATMENT PROCESS
GENERAL
A secondary treatment process depends on the activity of microorganisms to convert complex organic matter to simpler, more innocuous forms.
The secondary treatment process at the Martinsville Wastewater Treatment Facility is an extended aeration activated sludge process. The suspended, colloidal, and dissolved solids form a biological floc. This newly formed biological floc, termed activated sludge, is removed from the system through sedimentation, thereby providing a high degree of secondary treatment. The biological floc is developed in the lagoon and settled out in the secondary clarifiers. The components comprising the secondary treatment process at the Martinsville Wastewater Facility are an aerated earthen lagoon, three secondary clarifiers, and a return and waste sludge pumping station.
AERATED LAGOON
The extended aeration activated sludge process provides aeration and complete mixing of activated sludge and primary clarifier effluent wastewater. This process requires a low food-to-microorganism (F/M) ratio. This F/M ratio is accomplished by bringing biological organisms into contact with influent wastewater. This mixture is referred to as activated sludge. The biomass within the activated sludge feeds on the organic constituents of the waste in the presence of dissolved oxygen supplied by the aeration equipment.
The lagoon liquid containing the activated sludge organisms is called mixed liquor, and its concentration, called mixed liquor suspended solids (MLSS) is measured in mg/L. To insure optimum treatment efficiency, excess activated sludge is wasted from the secondary treatment system to maintain the required MLSS concentration in the lagoon for the desired food-to-microorganism ratio. The mixed liquor suspended solids (MLSS) from the lagoon is settled in the three secondary clarifiers. A portion of the settled activated sludge is returned to the lagoon and mixed with the influent wastewater to provide a continuous cycle of introducing biological mass to food and oxygen.
The lagoon is the first unit in the secondary treatment portion of the Martinsville Wastewater Facility. At a design flow of 8.0 MG of wastewater per day the detention time in the lagoon is 24 hours. The lagoon is equipped with nine 75 HP floating aerators, two 40 HP high-speed mixers and four 7.5 HP low-speed mixers.
OPERATION AND CONTROL
To insure a healthy environment for activated sludge to function it is imperative to maintain a biological sludge or MLSS of good physical quality that flocculates well and settles rapidly in the secondary clarifiers. The ability to control and maintain solids settling in the system and at the same time achieve the desired BOD removal is the key to successful activated sludge treatment. The activated sludge organisms will perform best when fed the proper amount of food (BOD). If overfed, upsets will occur resulting in “bulking” sludge that is difficult to handle.
The wasting of sludge is an essential part of any activated sludge process. If sludge were not removed from the activated sludge system, it would eventually overflow the effluent weirs of the secondary clarifiers. Ultimate control of the system, no matter what intermediate decisions are made, is based on solids wasting.
The dissolved oxygen concentration of the lagoon contents is critical in obtaining effective removal of organic matter. In the mixed liquor it is desirable to maintain a minimum D O of 2.0 mg/L while not exceeding 4.0 mg/L. The activated sludge dissolved oxygen demand will vary proportionally to the strength of the incoming waste stream. As the strength of the incoming wastewater increases, the oxygen demand of the activated sludge organisms increases, requiring more dissolved oxygen to be supplied by the aeration equipment and conversely, when the incoming organic concentration decreases, the oxygen demand of the organisms decreases.
A dissolved oxygen sensing probe is located in the lagoon which senses the dissolved oxygen concentration of the wastewater and transmits a signal to a recorder located in the main control panel which records the dissolved oxygen level in mg/L. The operator may use any combination of aerators and mixers to maintain adequate D O levels and mixing in the lagoon.
Effluent from the lagoon flows into three secondary clarifiers, concrete tanks, 80' in diameter with a 13' sidewall depth. The function of the secondary clarifiers is to separate activated sludge and remaining inert material from the wastewater prior to the disinfection process. The operation of the secondary clarifiers is similar to the operation of the primary clarifiers in that quiescent conditions and low velocities are generated in large circular tanks. A floc forms and settles slowly to bottom of the secondary clarifiers and. is either wasted to sludge handling facilities or returned to the lagoon. This must be accomplished quickly to prevent septic conditions. Wastewater overflows the effluent weirs and is transported to the chlorine contact tanks for disinfection.
DISINFECTION PROCESS
CHLORINATION
The pathogenic microorganisms generally considered of particular significance in wastewater are those that are the causative agents of typhoid fever, gastroenteritis, cholera, dysentery, and infectious hepatitis. At the City of Martinsville’s wastewater treatment facility, chlorine is used for disinfection. The amount of chlorine that must be added to produce the desired results under steady conditions is called the chlorine requirement. This requirement may be based on any number of criteria, such as a stipulated coliform bacteria density, a specified residual chlorine concentration or the destruction of a chemical constituent. Chlorine demand is the difference between the amount of chlorine fed and the residual amount of chlorine after a given contact time. Chlorine demand may change with dosage, time and under specified conditions. Chlorine is added to the wastewater in solution with water. Non-potable water is pumped through injectors that create a vacuum to draw chlorine gas through one of two regulators. The chlorine water solution is applied to the waste stream just prior to the chlorine contact tanks. The function of the contact tanks is to provide sufficient time for the chlorine to contact and destroy pathogenic material. The three contact tanks are circular concrete tanks 40’ in diameter with a 9’ side-wall depth. Solids which collect in the bottom of the tanks are gently pushed toward the sludge hopper located in the center of each tank by a rake arm with steel scrapping blades and are removed and pumped to the sludge thickner. The contact time in the three tanks is approximately 45 minutes. The total residual chlorine concentration after contact time must be equal to or greater than 1.0 mg/L to ensure proper disinfection. A residual chlorine test is performed hourly by the operators on duty.
DECHLORINATION
To reduce effluent chlorine levels as required in the "Water Quality Standard and Policy for Chlorine in Surface Waters," the City of Martinsville uses a sodium bisulfite liquid feed system. Sodium bisulfite solution is a straw colored aqueous solution of sodium metabisulfite (H2SO3) that reduces excess residual chlorine. Sodium bisulfite is applied to the waste stream as it leaves the chlorine contact tanks. A mechanical mixer called a flash mixer insures adequate mixing of the bisulfite and the wastewater. The final effluent chlorine residual after dechlorination shall not exceed 0.1 mg/L. A dechlorination test is performed hourly by the Operator on duty.
A post aeration tank follows the chlorination and dechlorination processes. The function of the post aeration tank is to ensure maintenance of adequate effluent dissolved oxygen concentrations. Dissolved oxygen levels of at least 5.0 mg/L is required by the discharge permit. Post-aeration is accomplished in a circular concrete tank, 40' in diameter with a 9' sidewall depth that is equipped with a 15 hp floating aerator which may be run continuously or as needed to maintain adequate dissolved oxygen levels.
SLUDGE HANDLING FACILITIES
SLUDGE THICKENER
Thickening or concentration of sludge is a process for relieving water from sludge after its initial separation from the wastewater in other plant processes. The basic objective of thickening is to reduce the volume of liquid sludge to be handled in the disposal process. The primary objective is the concentration of sludge rather than the clarity of water. The treatment plant has two thickeners. They receive primary sludge from the primary clarifiers, waste sludge from the secondary clarifiers and sludge from the chlorine contact tanks. After the thickening process concentrated sludge is removed from the thickeners and pumped to the digesters.
AEROBIC DIGESTERS
Aerobic digestion is accomplished by aerating sludge until it is stable and relatively nuisance free in two square concrete digesters with an approximate volume of 573,000 gal. each and two round digesters with an approximate volume of 284,000 gal. each. The total volume is designed to provide a 30-day detention time Four centrifugal blowers provide up to 966 lbs. of oxygen/sq. ft./ per hour through diffusers positioned in a grid pattern on the tank floors to maintain aerobic conditions.
BELT FILTER PRESSES
Two belt filter presses are located in the sludge processing building. Belt filter presses combine gravity drainage and dewatering by compressing the sludge between two belts. The incoming sludge is mixed with polymers to facilitate dewatering. Sludge entering the belt presses will typically be between 2% and 4% solids. After processing, the belt presses will produce a cake of approximately 20% solids. This dewatered sludge is hauled by truck to the City owned and operated land fill.
QUALITY CONTROL TESTING
The purposes of laboratory testing is to evaluate overall treatment plant efficiency and to monitor the performance of the plant's individual processes. Results of laboratory testing provide information regarding required corrective actions to increase unit efficiencies and provide a record of plant operations. This data is required by state and federal regulatory agencies to insure that the plant is in compliance with the provisions of the NPDES permit. The data is also important to design engineers when plant expansion or improvements become necessary.
