FlocculationLecture No. 51. Purpose2. Considerations3. Type and Selection Guide4. Discussion of Alternatives5. Design Criteria(T 3.2.4-2, p.121)6. Operation and Maintenance7. ExampleFlocculationLecture No. 51. Purpose- Flocculation is the gentle mixing phase that follows the rapid dispersion of coagulant by the flash mixing unit.- The purpose of flocculation is to accelerate the pace at which the particles collide, causing the agglomeration of electrolytically destabilized particles into setteable and filterable sizes- The aggregation of particulate matter is a 2-step process- The coagulant is added and reduces the interparticle forces; this is coagulation.- The particles then collide and enmesh into larger particles, floc; this is flocculation.2. Considerations- Raw Water Quality and Flocculation Characteristics. Flocculation characteristics can be evaluated by a jar test. Repeatability of the results must be demonstrated. F3.2.4-1. P.106. The residual turbidity is directly proportional to alum dosage, but not always. Note the effects of alum on kaolinite, topsoil and diatomaceous earth. The initial reaction is a decrease in turbidity, but if too much alum is added, the turbidity increases.- Finished Water Quality Goals. Less than .5TU.- Example:Given: A bentonite soil.Find: What dosage of alum is effective and what is the resulting turbidity.From F3.2.4-1, p.106 An alum dosage of 45 mg/l appears effective. Lesser dosage give markedly poorer results; increased dosages do not markedly effect the turbidity removalThe residual turbidity at an alum dosage is about 5 NTUs.3. Type and Selection Guide- Flocculation can be provided by either mechanical mixers or baffles- F3.2.4-3, p. 111 show the most common types of units. The mixing system include:Mechanical Mixing.- Vertical shaft with turbine or propeller type blades- Paddle type with either horizontal or vertical shafts- Proprietary units such as Walking BeamBaffled Channel Basins.- Horizontally baffled channels- Vertically baffled channelsFlocculation, Page No. 2Others including: Reactor Clarifier Proprietary Systems, Contact Flocculation and Diffused Air Agitation- Selection Criteria: Selection of the flocculation process should be based on the following criteria:- treatment process: conventional, direct, softening or sludge conditioning- raw water: turbidity, color, TDS- Flocculation characteristics in response to mixing characteristics. For example: if the floc is hard, more rigorous mixing intensity can be applied.- The order of preference is:- vertical shaft flocculators in properly compartmentalized, horizontal tanks.- paddle flocculators in properly compartmentalized, horizontal tanks.- baffled channel flocculation for constant flow rate plants4. Discussion of Alternatives- Mechanical Mixing System. The mixer should have the following characteristics:- Must delivered the required G value which may vary by compartment.- The shear must be low at the edge of the blades.- Low maintenance and operation.Regardless of the type of flocculator, tapered mixing across the tank is important. The initial mixing is rigorous, but as the floc grows in size, a more gentle, less disruptive regime is in order.Vertical shaft flocculators are usually the first choice for the following reasons:- minimal maintenance- operational flexibility- very little head loss across the tank- easy control of mixing intensity- Baffled Channel System. Requires a moderate head loss across the tank. Suitable for developing countries that may not be able to afford a mechanical system. 5. Design Criteria(T 3.2.4-2, p.121)- The general design criteria for a basic rectangular flocculation tank are as follows:Energy input: Gt=10,000 to 100,000, t =5x104 s average, G=30 s-1 average, 10-70 rangeDT: 20-30 minutes at Qmax.Depth: 10-15’Stages: 3-4 common, 2-6 range- Among the first considerations are the selection of the mode of mixing and the physical relationship between the flocculators and clarifiers. Subsequent decisions include: the number of tanks, number of mixing stages and their energy level and baffling type- Design usually based on:- DTFlocculation, Page No. 3- mixing energy level- The energy level is the G value or velocity gradient as defined by Camp:G = [ ].5 , units p.117Given: P= 850J/s, and the space influenced by the flocculator 4x6x6m. The temperature is 15-C.Find: 1.)G 2.)What size motor is 850J/s, assume e=70%1.) GFrom App.6, p.632 @15-C, -=1.17x10-3 N.s/m2, N=Newton = kg.m/s2, the units of - are kg/m.sV = 4x6x6mV = 144m3G = [].5 = [].5G = 71s-12.)How many hp is 850J/s1kW=1000J/s850J/s x (1kW / 1000J/s) = .85kw1hp = .746kw.85kw x (1hp/.746kW) = 1.14hpMotor size = 1.14/e = 1.14/.7Motor size = 1.63hpThe velocity gradient indicates the contacts that are being made. The gradients are produced by hydraulic or mechanical mixing. - The number of particle contacts is:N = n1n2(G/6)(d1 + d2)3 units p.117N is the number of contacts between n1and n2 particles. Therefore, the rate of flocculation increases with the number and size of the particles and with the power input but decreases with the viscosity of the fluid.- The mean velocity gradient for baffled systems is:G = ().5 = ().5 , units p.117in which - is the specific weight, 62.4lb/ft3 and -=absolute viscosity, 2.73x10-5 lb.s/ft2 @50-FThe gradient increases with the head loss across the tank and decrease when the viscosity and time increase. In plane English, the more turns and curves the more the mixing; the thicker the liquid and the longer it takes, the less the mixing.Example:Given: Rectangular basin, L=W=4’, D=4.5’, Q=8.7MGD, Water @50-F, -=absolute viscosity, 2.73x10-5 lb.s/ft2Loss = .5V2/2g (typical)Flocculation, Page No. 4Find: Gt=V/Q=4’ x 4’ x 4.5’ / 8.7MGDx1.547cfs/MGD = 72/13.46t= 5.35sh total = no of baffles x loss per bafflev=Q/A = 13.46/(4x1) Note the entire distance is 4’, but there are 4 compartments, therefore each compartment is 4/4=1’wide.v=3.37fpsh = head loss per baffle, see graphic =.5v2/2g = .5(3.37)2/(2x32.2) h = head loss per baffle = .0882fthead total = 3 baffles x .0882fthead total = .264ftG = ().5 = ().5 G = 336.3 fps/ft - For mechanical systems with paddles:G = (CDAv3/2-V).5 units p.117CD is a shape factor, use 1.8 (p.117). A is the cross sectional area of the paddles. G is increases as the area of the paddle increases, the velocity of the paddle increase and G decreases and the fluid gets thicker or the volume of the tank increases. A very important