Sedimentation Principles and Design of Facilities Part 1 SUDIPTA SARKAR PRADEEP KUMAR Sedimentation, or clarification, is the process of separation ofsuspended particles heavier than water by using the force of gravity. Suspended materials may be of different types and specific gravities, may or may not be biodegradable or can be of different shapes and sizes. The basic principles of sedimentation is applied to design many different units in a wastewater treatment plant: a) Grit Removal Facilities b) Primary Sedimentation or Settling Tanks c) Secondary Sedimentation tank d) Sludge Thickeners e) Chemical Precipitation Tanks. The most prevalent use is as clarification or settling tanks to produce a clarified effluent. Another use can be for solids concentration to produce a sludge with high solids content for easy handling and removal of sludge INTRODUCTION DIFFERENT TYPES OF SETTLING UNITS CIRCULAR RECTANGULAR PLATE SETTLER OR LAMELLA CLARIFIER SLUDGE THICKENER Particle Settling Basic Principles B W Force of BuoyancyForce of Gravity D Drag Force W = force of gravity = weight of the particle Vgs =g rs)34(3t =g d gds s)6( )8 34(33t t = =B = Force of Buoyancy= Weight of displaced water Vg =g d gd)6( )8 34(33t t = =D = Frictional Drag Force on moving sphere 221v A CD =B W Force of BuoyancyForce of Gravity D Drag Force Vertical momentum of spheres, from Newtons Law D B Wtvds+ + =cc)6(3tMass Acceleration Net Forces The particle accelerates for a short while, until it reaches a terminal velocity and then it maintains the velocity through out the time it falls through the liquid media 0 =cctvWhen Terminal Velocity is attained, there is no acceleration. Thus, D B Wtvds+ + =cc)6(3t0 = + + D B Wg ds)6(3t 0212= + v A CD g d)6(3t +((

= DstCd gv) (34221) (34((

= DstCd gvRe24NCD =4 . 0 =DC34 . 03 24ReRe+ + =NNCDq d v d vNt t= =Re = viscosity of water, = kinematic viscosity of water = q d v d vNt t= =ReIn laminar zone, Re24NCD =(((((

=(((((

=((

= d vd gNd gCd gvts sDst18) (24) (34 ) (34Re2((

= 18) (2d gvstFind out the terminal velocity of a sand particle of size 10 m and density of 2.6 g/cc. Consider the dynamic viscosity of water to 10-3 kg/m.s and density to be 1g/cc.First, consider that the system is in laminar zone, ((

= 18) (2d gvst((

=32 610 18) 10 10 )( 1000 2600 ( * 81 . 9XXs m X / 10 72 . 85 =Check: Whether in laminar zone ) . /( 1010 10 / 10 72 . 8 / 100036 5 3Res m kgm X sX m X X m kgN = d vNt=Re410 72 . 8= X< 1, so in the laminar zone. Hence, ok. If the diameter of the particle is 1 mm what will be the terminal velocity? Assume that it is in the laminar zone ((

= 18) (2d gvst((

=32 310 18) 10 )( 1000 2600 ( * 81 . 9Xs m X / 10 72 . 83 =) . /( 1010 / 10 72 . 8 / 100033 3 3Res m kgm sX m X X m kgN =72 . 8 =>1, Not in the laminar zone. It is not in the turbulent zone as well. It is then in the intermittent zone 107 . 4 34 . 072 . 8372 . 82434 . 03 24ReRe= + + = + + =NNCDs mCd gvDst/ 071 . 0) (342 / 1=((

= ) . /( 1010 / 071 . 0 / 100033 3Res m kgm sX m X m kgN=71 =03 . 1 34 . 0713712434 . 03 24ReRe= + + = + + =NNCDs mCd gvDst/ 14 . 0) (342 / 1=((

= ) . /( 1010 / 14 . 0 / 100033 3Res m kgm sX m X m kgN=140 =76 . 0 34 . 014031402434 . 03 24ReRe= + + = + + =NNCDs mCd gvDst/ 165 . 0) (342 / 1=((

= ) . /( 1010 / 165 . 0 / 100033 3Res m kgm sX m X m kgN= 165 =72 . 0 34 . 016531652434 . 03 24ReRe= + + = + + =NNCDs mCd gvDst/ 17 . 0) (342 / 1=((

= ) . /( 1010 / 165 . 0 / 100033 3Res m kgm sX m X m kgN= 170 =GRIT CHAMBERS Grit chambers are designed toremove mostly inorganic solids > 0.2 mm and specific gravity of about 2.5 to 2.65Removal is commonly effected using: Settlement Separation using a vortex Settlement in the presence of aeration (to keep the lighter organic particles in suspension) Three Types of Grit Chambers: a) Horizontal constant velocity grit chamber b) Vortex Type c) Aerated Grit chamber This is basically an open channel with a detention time sufficient to allow design particles to settle Horizontal Constant Velocity Grit chamber Additionally, the velocity must be high enough to scour organicmaterials Organic materials should pass through the grit chamber for subsequent biological treatment Bottom scour is an important consideration affecting Grit chambers efficiency. The grit chamber geometry and sizes have to be optimum so that inorganic particles settle down but the organic smaller particles scour back to the flow. The velocity which promotes such a characteristic is called a critical scour velocity. Kc is normally considered to be 3.5 to 4.5 For grit particles of size about0.2 mm the critical scour velocity is 0.15 to 0.3 m/sec. In practice, peak flow rate of about 0.15 to 0.3 m/sec is considered as design flow through velocity. Design recommendation is 0.3 m/sec. Velocity control sections for horizontal grit channels (b) Parabolic channel section and Parshall flume

And the head-discharge relationship is: For this type of weir, the curved portion is defined by the equation:

(a) Rectangular channel section and Proportional weir A A Section A-A The particle will travel vertically from A to B in the same time as it takes to travel horizontally from A to B This is the detention time and is given by h tdvLvHt = =Also, from continuity,BHQvh =h tvLHv =SAQBLQBHQLH= = = .SAQis known as surface loading rate or overflow rateDetention time is about 30-90 seconds for a tank of depth 1 to 1.8 m. Vt Example: Will a grit particle with a diameter of 0.2 mm and a specific gravity of 2.65 be collected in a horizontal grit chamber that is 13.5 m in the length if the average flow in the grit chamber is 0.15 cum/sec, the width of the chamber is 0.56 m and the horizontal velocity is 0.25 m/s. WW temperature is 22 deg C. Viscosity is 0.993 X 10-3 Pa.s at 22 deg C. Density of water at 22 deg C is 997.77 kg/ m3 Q = 0.15 cum/s; L = 13.5 m; W= 0.56 m; vh=0.25 m/s; d=0.2 mm 000993 . 0 18) 10 2 . 0 )( 77 . 997 2650 ( 81 . 918) (2 3 2XX d gvss== s m X / 10 61 . 32 =25 . 7) . /( 10 993 . 010 2 . 0 / 0361 . 0 / 77 . 99733 3Re= =s m kg Xm X sX m X m kgN>1, NOT IN LAMINAR ZONE 76 . 4 34 . 05 2 . 7325 . 72434 . 03 24Re Re= + + = + + =N NCDs mCd gvDst/ 03 . 0) (342 / 1=((

= After another iteration the terminal velocity converges tos m vt/ 0274 . 0 =Ac = C/S area of the tank =26 . 025 . 0 15 . 0m = = m h 07 . 156 . 06 . 0= =Time to reach at the bottom =s 390274 . 007 . 1=Detention time = ss mm54/ 25 . 05 . 13=CAPTURED Volume provided for grit storage depends on cleaning frequency and grit quantities TYPICAL DESIGN GUIDELINE: HORIZONTAL GRIT CHANNEL 0.3 The concentrated grit underflow is pumped or gravity fed to a grit classifier for dewatering. (brown arrow) Flow is introduced into the Grit Chamber via a tangentially positioned inlet causing a rotational flow path around the dip plate. The flow spirals down the wall of the chamber as solids settle out by gravitational and rotational forces. (red arrow) The grit collects in the grit pot as the center cone directs flow away from the base, up and around the center shaft into the inside of the dip plate. (blue arrow) The upward flow rotates at a slower velocity than the outer downward flow. The resulting shear zone scrubs out the finer particles. Vortex Type Grit Chamber Units are usually compact Design is usually proprietary Adjustable rotating paddles maintain the proper circulation within the unit for all flows These paddles may collect rags Highly energy efficient Grit sump can become compacted and clogged May require high-pressure agitation water or air to clean the collected grit -Commonly used in medium to large plants - The introduction of air through a diffuser induces a spiral flow pattern in the sewage as it moves through the tank -The roll velocity is sufficient: To maintain organic particles in suspension while allowing heavier grit particles to settle -Air supply is adjustable to provide optimum roll velocity for different conditions -Sewage is freshened by air, leading to odor reduction - Chamber can be used also for chemical addition, mixing, and flocculation ahead of primary treatment if desired -Grease removal may be achieved with a skimmer Aerated Grit Chamber AERATED GRIT CHAMBERS TYPICAL DESIGN GUIDELINE: AERATED GRIT CHANNEL DESIGN OF A RECTANGULAR GRIT CHANNEL A grit chamber is to be designed for removing particles with a diameter of 0.2 mm, specific gravity 2.65. The wastewater flow rate in the plant is 40,000 cum/day, and has a TSS content of 300 ppm out of which 70% particles have size above 0.2 mm. Also, find out the amount of solid that is separated from the wastewater per day at this facility. STEP 1.Find out the terminal velocity Consider that the flow is in laminar zone, 036 . 010 18) 10 2 . 0 )( 1000 2650 ( * 81 . 918) (32 3 2==((

=XX d gvst m/sec 2 . 7) . /( 1010 2 . 0 / 036 . 0 / 100033 3Re= =s m kgm X sX m X m kgN>1 and hence, not in laminar zone 79 . 4 34 . 02 . 732 . 72434 . 03 24ReRe= + + = + + =NNCDs mCd gvDst/ 03 . 0) (342 / 1=((

= After another iteration the terminal velocity converges tos m vt/ 0278 . 0 =100% of the particles of size equal or above size 0.2 mm will be settled in the grit chamber sp tAQv v = =sAday hr hrd mm/ 24 * sec/ 3600/ 40000sec / 0278 . 03=264 . 16 m As =Recommended horizontal velocity is 0.3 m/s Depth of a grit tank is generally kept at 1.5 m. Time of detention required =ss mmvHttd54/ 0278 . 05 . 1= = =The length of the channel required = 0.3m/s *54 s = 16.2 m Width of the channel =mmLAs8 . 02164 . 162= =Take 25% more length in account of providing of inlet and outlet zones = 20.25 m (21 m) So, the dimensions are 21 m L X 0.8 m B X 1.5m D Amount of solid to be removed per day = 40,000 m3/d *300 mg/L *0.7=8400 kg STEP 2. SIZING The particle will travel vertically from A to B in the same time as it takes to travel horizontally from A to B This is the detention time and is given by h tdvLvHt = =Also, from continuity,BHQvh =h tvLHv =SAQBLQBHQLH= = = .SAQis known as surface loading rate or overflow rateRectangular Sedimentation Tank Functionally, a rectangular sedimentation tank is similar to a grit chamber. However, in primary settling tank we intend to remove particles with size in the range of 0.05 mm or higher. PST is used to reduce the load on the downstream biological treatment facilities The detention time for a primary settling tank usually ranges between 1 to 2 hours. The settling velocity of the particles with size 0.05mm are in the range of 0.3 mm/s or 1.2 m/hr . Hence, a detention time of 2 hrs (2.4 m depth) is adequate Efficiently designed PSTs can reduce 50-70% of influent suspended solids and 25-40% influent BOD loading Vt Analysis of a Circular Sedimentation Tank H dh dr r vr vs R1 At radius r, rHQvrt 2=R2 D1 D2 Slope of the curve = drdhrsvv=Q rH vst 2 *=rdrQH vdhs.2t=} }=2120RRsHrdrQH vdhtQA H vQR R H vR RQH vHs s s s. ) ( .) .(21.221222122== =t tSsAQv =In case of circular settling tank also, the settling efficiency depend on the surface area and flow rate through the settling tank Circular settling tanks are generally called clarifiers d st v H =Where td is the detention time of the tank Wastewater is introduced through the central shaft and then it spreads horizontally. Radial velocity changes as the liquid spreads out towards the periphery. Design Parameters for a Primary Settling Basin ItemValue RangeTypical Primary Settling tank followed by secondary treatment - Residence or detention time, hr1.5 -2.52.0 - Overflow rate, m3/m2/d - average flow33- 50 - peak hourly flow80-120100 - Weir Loading Rate, m3/m/d125-500250 Rectangular Tanks - Depth, m 3-4.53.6 - Length, m15-100 m25-40 - Width, m3 -245-10 - scraper speed, m/min0.6-1.21.0 Circular - Depth, m 3 4.53.6 - Diameter, m3 6012-45 - Bottom slope, mm/m65-16585 Detention time lower than the optimum time shall lead to less effective removal of suspended solids. Higher detention time shall induce septic situation and therefore may cause foul odors. Scour Velocity In sewers scour forces were intentionally induced in the wastewater by maintaining high enough velocity so that no particles settle down.In case of primary settling tanks,horizontal velocity should be kept very low so that the settled particles are not scoured from the bottom of the basin. The critical velocity is given as: 2 11 8/shf)gd k(Sv((

=Horizontal velocity that will just produce scour Constant, for inorganic sand like material k= 0.04; k=0.06 for organic matter Specific gravity of the particle Darcy-Weisbach friction factor 0.02-0.03 Size /diameter of the particle Outlet Channel Settling Zone Overflow weir Short-circuiting in a Sedimentation tank For efficient removal of sediment in the settling tanks, it is required that the flow is uniformly distributed through out the cross-section of the tank.If a situation arises when the particles along with a stream of flow do not get uniformly distributed, rather directly goes to the outlet, it tends to stay for shorter duration inside the tank compared to the uniformly distributed flow. This is called short-circuiting. Short-circuiting decreases the efficiency of the tank for the solids removal. Short-circuiting is caused by improper design of inlet and outlet arrangements inside a settling basin.Properly designed inlet and outlet arrangements help to distribute the flows evenly within the tank, causing less short-circuiting.V1 V2 Baffle Type Inlet V1