Primary sedimentation
- 1. Primary Sedimentation Dr. Akepati S. Reddy School of Energy and Environment Thapar University, Patiala PUNJAB, INDIA
- 2. Flocculent Particle Settling • Particles do not settle as discrete particles – instead during sedimentation they coalesce and flocculate • Extent of coalescence or flocculation depend on – Overflow rate – Depth of settling basin – Velocity gradients in the system – Concentration of particles – Range of particle sizes • Settling characteristics of a suspension of flocculent particles can be obtained by a settling column test • Settling column – Height: equal to the depth of the proposed settling tank – Diameter can be anything – influences of the wall should be taken care off – sample required should be small enough – Provided with sampling ports
- 3. • Settling column test – To begin with ensure uniform distribution of particle size from top to bottom of the column – Duration of test should be equal to the settling time in the proposed tank – Draw samples from the ports at various time intervals, analyze for TSS and compute % removal – Plot % removal against time and depth and construct curves of equal % removal – Overflow rates for various settling are determined by noting where the curve intersect the x-axis Flocculent Particle Settling 2 (%) 1 1 nn n h n RR H h R
- 4. Settling Column Settling Profile Graph
- 6. Primary Sedimentation Tank • Purpose is removal of readily settleable solids and floating material from wastewater • Usually give 50-70% suspended solids removal efficiency and 25-40% BOD removal (for municipal sewage) • Rectangular or circular sedimentation tanks are used – Two or more cylindrical or rectangular sedimentation tanks may be used for allowing maintenance and repair – Multiple rectangular tanks require less land area than multiple circular tanks and common walls reduce construction cost • Sedimentation tanks should include – Inlet section and Outlet section – Facilities and provisions for the settled sludge scrapping, collection and removal – Facilities and provisions for the floating material entrapment, skimming, collection and removal • Sedimentation tank can be considered to include Settling zone and Sludge zone
- 7. Flow pattern is radial with central feed arrangement • Has a center well of 15-20% of the tank diameter and 1.0 to 2.5 m depth is used • Center well has a tangential energy dissipating inlet • wastewater is discharged into the well within the upper 0.5-0.7 m at 0.3 to 0.45 m/sec flow velocity Solids removal mechanisms • Smaller tanks have hopper bottom with >45 inclination with the horizontal • 3.6-9.0 m diameter tanks have solids removal equipment supported on beams spanning the tank • Larger diameter tanks have a central pier supporting the mechanism and is reached by a walkway or bridge • Bottom of the tank is sloped at 1 in 12 to 64 or lesser • Solids are scrapped to the center and removed Clarified liquid is skimmed off over weirs into a collection trough Circular Primary Sedimentation Tank
- 9. Clarified sewage Collection trough Scum retaining baffle Distribution baffle of central well Scrapper arm Skimmer bladeCentral well Central pier Influent pipe Sludge draw-off pipe Central rotating equipment Sludge trough Tangential openings in the pier for the distribution of the influent Slope 1 in 12 Side wall of clarifier 0.5m 0.3m 0.3m 0.3m 0.6m Central pier Sludge trough 0.5 m 1.5 m 0.5 m Sludge draw-off pipe Influent pipe Primary clarifier Section to show side wall with clarified sewage collection trough Bottom sludge trough and outlet 3.0 m
- 10. Inlet • Full width inlet channels with inlet weirs – can introduce vertical velocity component spreading into the sludge hoppers (provided at the inlet end) • Inlets with submerged ports/orifices (velocities in the ports/orifices are in the range of 3-9 m/min.) • Inlet channels with wide gates and slotted baffles • Inlet baffles can be used to reduce high initial velocities and distribute flow width wise – usually extend from 150 mm below and 300 mm above the water surface Outlet • Clarified effluent is collected into a collection trough through an overflow weir • Floating material or settled suspended solids should not be washed out with the clarified effluent • Floating material retaining or entrapping baffle is provided • Weir length is designed to avoid washout out of sludge Rectangular Primary Sedimentation Tanks
- 11. Facilities and provisions for the settled sludge scrapping, collection and removal • Settled sludge is collected at the inlet end • Chain and flight solids collectors – Have a pair of endless conveyor chains – Scraper flights of wood or fiberglass, running full width of the tank and attached to the chains at 3 m interval – The settled solids are scrapped • to solids hoppers In case of small tanks – multiple hoppers are used in larger tanks • to a transverse trough in case of large tanks - troughs in turn have cross collectors Rectangular Primary Sedimentation Tank
- 12. Traveling bridge type collectors – Travel up and down the tank on rubber wheels or on rails supported on the sidewalls – Scrapper blades are suspended from the bridge (returning scraper blades lifted free of the solids blanket) Facilities and provisions for floating material entrapment, skimming, collection and removal • Floating scum is collected at the effluent end • Floating scum is skimmed and carried to the effluent end and entrapped for avoiding overflow into the clarified effluent trough – Flights returning at the liquid surface can be used – Water showers can be used to push forward the floating scum – Baffle is provided in front of the overflow weir for the scum entrapment until removed Rectangular Primary Sedimentation Tank
- 13. • In small tanks scum draw-down facility consisting of a horizontal, slotted pipe that can be rotated by a lever or screw can be used – Limitation - creates large volume of scum liquor • Transverse rotating helical wiper attached to a shaft can also be used for scum removal – Scum is removed from the water surface and moved over an inclined apron for discharge to a cross-collecting scum trough Other provisions and facilities • Tanks may sloping bottom towards the influent end • Influent end has sludge hoppers or sludge trench • Scum pit is provided for storing the scum liquid removed • Sludge pump, piping and tank may be there to remove the collected sludge from the sludge hopper or trench Rectangular Primary Sedimentation Tank
- 15. Design of Primary Clarifier • Overflow rate and hydraulic retention time obtained from the settling test and settling profile diagram are used to find surface area and depth of the clarifier • Safety factors of 1.25 is usually used to take into account – Inlet and outlet disturbances – Disturbances created by the rotating mechanism to scrap the settled sludge and to collect the floating scum – Wind blow effects – Temperature variations – Flow short-circuiting • Design also takes into account the following: – Horizontal flow velocity (below the scour velocity) – Weir overflow rate (below a limit to avoid density currents) – Weir overflow balancing to avoid short-circuiting problems – Flooding of collection trough
- 16. Detention time 1.5 to 2.5 hours (2.0 hr.) Overflow rate Average flow 30-50 m3/m2.day (40) – 24-32 (26) Peak flow 80-120 m3/m2.day (100) – 48-70 (60) Weir loading 125-500 m3/m.day (250) Rectangular tanks Depth 3-4.9 (4.3) Length 15-90 (24-40) Width 3-24 (4.9-9.8) Flight speed 0.6-1.2 m/min (0.9) Circular tank Diameter 3-60 (12-45) Bottom slope 1 in 16 to 1 in 6 (1 in 12) Flight speed 0.02-0.05 rpm (0.03) Primary Sedimentation Tank
- 17. Short circuiting and hydraulic stability • Tracer studies can be used for determining short-circuiting problems • Method of influent flow distribution can also affect short circuiting Temperature difference of 1C between incoming wastewater and wastewater of the tank can cause density current Wind blowing across the top of an open sedimentation tank can cause circulation cell to form – this reduces effective volumetric capacity of the tank Surface loading rates (overflow rates) • There can be regulations prescribing limits to surface loading rates • Overflow rates must be set low enough to ensure satisfactory performance at peak rates of flow Primary Sedimentation Tank
- 18. Detention time • Solids reaching the settling tank are susceptible to flocculation • Flocculation is aided by eddying motion of fluid within the tank • Level of flocculation depends on the time elapsed (detention time) • In cold climates because of increase in water viscosity detention time required also increases (1.38 time more HRT for 10C water that when temperature is 20C) Weir loading rates • Have little effect on efficiency of primary settlign tanks Primary sludge scum characteristics and quantities • Factors influencing are – Characteristics of untreated water (strength and freshness) – Period of sedimentation – Conditions of the deposited solids – Period between solids removal operations Primary Sedimentation Tank
- 19. Primary Sludge • Primary clarifier is designed for a specified % removal of TSS • From knowing the TSS and flow rate of the influent sewage primary sludge generated can be quantified • TSS of the sewage is comprised of inorganic TSS, VSS and Biodegradable VSS – Biodegradable VSS contributes BOD and this is 50-60% of the total BOD for municipal sewage • Primary clarifier through removing the Biodegradable VSS contributes to BOD removal – BOD removal in the primary clarifier is typically 35-45% • If BOD removal in the primary clarifier is known, one can find out the biodegradable fraction of the primary sludge – BOD removal can be expressed as BODu removal and BODu removal as TSS equivalent removal – The fraction of biodegradable VSS in the TSS is assumed to remain the same even in the clarified effluent • If consistency of the primary sludge is known even volume of the primary sludge can be calculated