3. Design Criteria for Primary Treatment Plants
Traditionally, the design criteria were
- Basic overflow rate (surface loading m3/m2-d)
- 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.
3.2 Circular Radial Flow Tanks
These are also used for primary sedimentation.
Careful design of the inlet stilling well is needed to active a stable radial flow pattern without causing excessive turbulence in the vicinity of the central sludge hopper.
The weir length aroid the perimeter of the tank is usually sufficient to give a sates factory weir loading rate at maximum flow, but at low flows, very low flow depths may result.
To overcome the sensitivity of these tanks to slight errors in weir level and wind effects, it is common to provide v-much wares.
Sludge removal is effected by means of a rotary sludge scrapper who moves the sludge into a central hopper, form which it is with drown.
Scum removal is carried out by surface skimming board attached to the sludge scrapper mechanism and positioned so that scum is moved towards a collecting hopper at the surface.
3.3 Up Flow Tanks:
In a horizontal flow tank assuming that such particles were uniformly distributed to the flow, particle with Vp=Q/A still be removed in proportion.
up flow tanks, usually square in plan and with deep hopper bottoms, are common in small treatment plants.
Their main advantage is that sludge removal is cared out entirely by activity and no mechanical parts are required for cleaning them.
The steeply sloping sides usually to to horizontal concentrate the sludge at the bottom of the hopper.
Weir loading rate is a problem only at low flows. So that v-match weirs are desirable.
The required up flow pattern is maintained by weir troughs.
True up flow tanks have an disadvantage on that hydraulic over loading may have more serious effects than in horizontal flow tanks.
Any practical with a velocity lower than VP = Q/A will not removed in an up flow tank, but will escape in the effluent.