Apr 11, 2010

Primary Treatment

Primary treatment is designed to remove organic and inorganic solids by the physical processes of sedimentation and flotation.  Primary treatment devices reduce the velocity and disperse the flow of wastewater.  In primary treatment the velocity of flow is reduced to 1 to 2 feet per minute to maintain a quiescent condition so that the material denser than water will settle out and material less dense than water will float to the surface.  Approximately 40 to 60 percent of the suspended solids are removed from the waste stream (25 - 35% BOD reduction).  The solids that remain in suspension as well as dissolved solids will usually be biochemically treated in subsequent processes for physical separation and removal in the final (secondary) settling tanks. 
The size and number of primary tanks is dependent on the estimated wastewater flow and the design detention time.  Generally, a detention time of 2 to 3 hours will provide a sufficient time period for most particles to settle out.  Further, the settling rate of a particle depends on the strength and freshness of the wastewater being treated, the weight of the solid compared to the specific gravity of water, the size and shape of the solid and the temperature of the water.  Water is more dense at lower temperatures;  therefore, the required settling time increases.  As the temperatures of the water increases, the required settling time decreases.  Equal distribution of flow throughout the tank is critical.  The greater the velocity in one area, the less the actual detention time.  Solids not having sufficient time to settle out will be discharged in the effluent. 
Principle primary treatment devices are referred to as sedimentation tanks, primary tanks, primary clarifiers or primary settling tanks, some of which have the further function of providing an additional compartment for the decomposition of settled organic solids which is known as sludge digestion.  There are several types of primary tanks in use. 

Septic Tanks
The septic tank was one of the earliest treatment devices developed.  Currently, septic tanks provide wastewater treatment for small populations, such as individual residences, small institutions, schools, etc. 
They are designed to hold wastewater at low velocity, under anaerobic conditions for minimum detention time of 36 hours.  During this period, a high removal of settleable solids is achieved.  These solids decompose in the bottom of the tank with the formation of gas which, entrained in the solids, causes them to rise through the wastewater to the surface and lie as a scum layer until the gas has escaped, after which the solids settle again.  This continual flotation and resettling of solids carries some of them in a current toward the outlet to be discharged with the effluent.  The final effluent disposal occurs by subsurface methods.  The effectiveness of this method is dependent on the leaching ability of the soil. 
These primary type units require a minimum of attention which involves an annual inspection and the periodic (3 - 5 years) removal of sludge and scum accumulations. 


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