Primary Treatment – Mechanical Treatment
The mechanical treatment of wastewater is normally achieved in the first chamber or tank of a treatment plant. The function of this treatment step is to separate the solids.
Mechanical treatment, therefore, simply means that solids, due to its heavier weight than water, sink to the bottom of the chamber. This results in a so-called sludge in the first chamber. It is necessary to remove the accumulated sludge when its level reaches 50% of the chambers hydraulic depth. The lighter solids, commonly referred to as suspended solids, bypass the first chamber and enter the next treatment step, typically the second chamber.
The Biological Treatment Stage
In this stage the organic content of the wastewater, such as food remains, laundry wash water, human waste and other organic pollutants are biologically reduced, known as bioremediation. This second stage of the water treatment is achieved with the use of microbes, which break down the waste into its basic compounds. In simple terms the microbes feed and multiply using the organic waste contained in the wastewater. Additionally oxygen is required for the microbes to do their work. In the first step the microbes process present carbon into carbon dioxide and water. This process powers their metabolism. This is similar to our own body, which burns carbon (our food) in our muscles, extracts energy and produces the waste product carbon dioxide (which we exhale). This process is known in wastewater technology as “biological carbon reduction”. Packaged treatment plants for biological carbon reduction are characterized as Treatment Class C.
Microorganisms in the wastewater are found either freely floating forming an activated sludge or in a solid form on the surface of specific materials. The solid form of microorganisms is called biofilm found in nature on the surface of rocks and plants. As a result treatment systems exist where the microbe organisms are free floating such as in SBR or systems using biofilms such as trickle filters, fixed bed reactors etc. Oxygen for the microbes is provided by either direct injection of air into the wastewater or by achieving large surface area that are in direct contact with ambient air, such as trickle filters where the wastewater trickles over lava rocks or other suitable media.
Residual sludge and biofilm fragments that have been sheared off (a normal occurrence) should not be discharged into laterals, leach fields or the ground water. It is desirable that wastewater treatment is completed as much as possible before discharge occurs. For this reason the clarifying stage collects the biomass, activated sludge and biofilm residues. This is achieved by simple settlement to the bottom of the tank. Most systems transfer the settled sludge back into the pretreatment chamber.The cleaned water can now flow from the surface of the clarifier to the preferred disposal method, such as laterals, leach fields or Title 22 reclaimed water irrigation systems.
For larger applications a 2 tank system is needed. The system is capable of handling up to a 52 person household equivalent.
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