civil engineering-->>Primary Treatment of Wastewater & Types of Primary Sedimentation Tanks

Primary Treatment of Wastewater & Types of Primary Sedimentation Tanks
	Primary treatment is often called clarification sedimentation or setting. This ‘unit operation’ where the wastewater is a allowed to settle for a period (≈2h) in a setting tank and so produce a somewhat clarified liquid effluent in one stream and a liquid-solid sludge (called primary sludge) in a second steam. 1. Objectives of Primary Treatment of Wastewater To produce a liquid effluent of suitably improved quality for the next treatment stage (i.e.) secondary biological treatment. To active a solids separation resulting in a primary sludge that can be conveniently treated and disposed of. 2. Benefits of Primary Treatment The benefits of primary treatment include Reduction in suspension solids Reduction in BOD Reduction in the amount of waste activated sludge (WAS) in the activated-sludge plant. Removal of floating materials (oil and geese). partial equalization of flow rates and organic load.
3. Design Criteria for Primary Treatment Plants Traditionally, the design criteria were Basic overflow rate (surface loading m3/m2-d) Depth Surface geometry Hydraulic retention time Weir rate (m3/d-m) The above criteria are physical and while they may be adequate for design of the tank they sues nothing about the performance and operation of the sedimentation process. Therefore, additional parameters called performance criteria were established to monitor and improve the day-to-day performance and operation of the sedimentation process. Influents flow rates and their variation (daily variation) influent waste strength rates and its variation. Recycle influent streams. From activated – sludge or Septic. Supernatants form sludge de watering. Washings from tertiary filter processes.  They check efficiency of removal. Septic may have a BOD's value 30 times greater than municipal raw wastewater. Supernatants form anaerobic digestion process or filtrate back washing may also be very high in waste strength. As such the performance of a primary clarification is not solely dependent on influent flow variations. Types of Sedimentation Tanks 3.1 Typical Primary Sedimentation Tank 3.1.1 Rectangular Horizontal Flow Tanks These are most commonly used for primary sedimentation, since they Occupy less space than circular tanks. They can be economically built side-by-side with common walls. The maximum forward velocity to avoid the risk of scouring settled sludge is 10 to 15 mm/s (06 to 09m/min or 2 to 3 ft/ min), indicating that the ratio of length to width l/w should referrals be about. The maximum weir loading rate, to limit the influence of draw-down currents, is preferably about 300 m3/d-m, this figure is sometime increased where the design flow is great then 3 ADWF. Inlets should be baffled to dissipate the momentum of the incoming flow and to assist in establishing uniform forward flow. Sludge is removed by scraping it into collecting hoppers at the inlet end of the tank. Sum removal is essential in primary sedimentation tanks because of the grease and other floating matter which is present in wastewater. The sludge serapes can return along the length of the tank a the water surface. As they move towards the outlet end of the bank, the flights then move the sum towards a skimmer located just upstream of the effluent weirs.