Types of Primary Sedimentation Tanks

Primary Sedimentation takes place in the sedimentation tanks with the objective to remove readily settleable solids and floating materials and thus reduce the suspended solids content. The removal rate is 50-70% of suspended solids and 25-40% of BOD whereas, generally more than two rectangular or circular tanks are used.

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.
• Length ranges 15 to 100m an width from 3 to 24m (length/ width ratio 3:1 to 5:1)
• 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.
• Some 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.

Circular Radial Flow Tanks

These are also used for primary sedimentation.

• Most common-have diameters from 3 to 60m (side water depth range from 3 to 5m)

• 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.

Up Flow Tanks:

• 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.

• 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.

Square sedimentation tank

They may be flat bottomed or hopper bottomed. Wastewater enters the tanks, usually at the center, through a well or diffusion box. The tank is sized so that retention time is about 24 (range 20 minutes to 3h). In the quiescent period, the suspended part ides settle to the bottom as sludge and are raked towards a central hopper from where the sludge is withdrawn.

Primary sedimentation is among the oldest wastewater treatment process. Traditionally the design criteria for sizing setting tanks are:

Average overflow rate: 30 - 50 m³/m²/d (Typical 40 m³/m²/d) [800-1200 gal/ft²-d (Typical 1000 gal/ft²-d]

Peak hourly overflow rate: 50 - 120 m³/m²/d (Typical 100 m³/m²/d) [2000-3000 gal/ft²-d (Typical 2500 gal/ft²-d]

Weir loading rate: 1.5 - 2.5h (Typical 2.0 h) [1.5 - 2.5 h (Typical 2.0h)]

• Always provide minimum of 2 sedimentation tanks.
• Sludge accumulation is same for both.
• Sludgy accumulation 2.5kg of wet solids per m³ of flow.