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An on-site wastewater treatment system (OWTS) includes four major stages; identifying the source of wastewater, collection and storage of wastewater, pre-treatment of wastewater, and final treatment and dispersion of wastewater to the soil. The pre-treatment stage forms the third stage of on-site wastewater treatment, after which the wastewater undergoes final treatment.
Image Credits: ossf.tamu.edu
The extent up to which the pre-treatment of the wastewater is needed depends on the environment where it is to be released and the final usage. In the pre-treatment stage, the level of impurities in water is reduced such that it is not harmful to the soil during the final distribution.
The major components of a pre-treatment system of OWTS are a septic tank, trash tank, processing tank, effluent screen, and recirculation tank. This article explains each component of a pre-treatment plant in an OWTS system.
1. Septic Tank
The septic tank is one of the most cost-efficient pre-treatment methods used to treat residential wastewater. A septic tank is a closed watertight container that collects wastewater from the storage and separates solids from the effluents. When the rate of flow wastewater decreases, the settleable solids settle at the bottom, and floatable solids like oil, grease rise to the top.
Image Credits: Biocycle
The time needed for the solids to settle down is called detention time, which must be a minimum of 24 hours.
After pre-treatment, 50% of the solids retained in the septic tank decompose; the rest deposit as sludge at the bottom and are removed by pumping. Pumping must be performed every 5 to 7 years and inspected annually.
It is necessary to choose the right kind of septic tank for the given household size and soil type. A well-maintained septic tank can add durability to the OWTS system.
2. Trash Tank
The wastewater coming from the septic tank is taken to the trash tank for further treatment. It is generally used prior to advanced treatment units. An advanced treatment unit in OWTS is a small biological treatment system used for individual residential buildings to treat wastewater to a higher degree.
The trash tank can serve as a facultative or anaerobic treatment device. It also removes plastic and non-degradable materials from the wastewater stream.
The volume of a trash tank is half the daily design flow volume. The dimensions of the trash tank are generally recommended by the manufacturer of advanced treatment units.
3. Processing Tank
The processing tank is an anaerobic treatment plant used to facilitate nitrogen removal in an OWTS. In some situations, it is used in combination with advanced treatment units.
It combines the septic tank, equalization tank, pump tank, and recirculating tank. The ammonia generated in the processing tank during the anaerobic process is pumped to advanced treatment units and converted to nitrate under aerobic conditions. This aerobic effluent is recirculated back to the processing tank, where nitrite is converted to nitrogen gas (denitrification).
4. Effluent Screen
The effluent screen is placed at the septic tank’s outlet to provide additional filtering of pollutants. The screen easily removes all the debris that clogs the downstream treatment devices. Annual cleaning and maintenance of effluent screens can prevent any biological growth that prevents the flow of sewage through them.
Image Credits: ossf.tamu.edu
5. Recirculation Tank
A recirculation tank is used to remove or decrease the percentage of nitrates, biochemical oxygen demand (BOD), and total suspended solids in the wastewater. The recirculation tanks collect the wastewater from the before-completed pretreatment component and mix it with highly treated wastewater taken out from an advanced pretreatment component. This process dilutes the wastewater and gives a cleaner effluent.
A recirculation tank is a watertight tank that lets the solids settle and permits additional treatment procedures.
Recirculation Ratio = Amount of wastewater that flows through advanced pretreatment component / Amount of wastewater sent to the final treatment.
The recirculation ratio value can vary between 3:1 to 5:1. The value varies based on the desired level of treatment.
FAQs
The major components of a pre-treatment system of OWTS are a septic tank, trash tank, processing tank, effluent screen, and recirculation tank.
The trash tank serves as a facultative or anaerobic treatment device. It also removes plastic and non-degradable materials from the wastewater stream. The wastewater coming from the septic tank is taken to the trash tank for further treatment. It is generally used before advanced treatment units.
Recirculation Ratio = Amount of wastewater that flows through advanced pretreatment component / Amount of wastewater sent to the final treatment.
The recirculation ratio value can vary between 3:1 to 5:1. The value varies based on the desired level of treatment.
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