Flocculation

Flocculation is a physical process used to promote the growth of the floc under slow mixing conditions. This process is a mixing stage to allow particles and small agglomerates (microfloc) formed during destabilization to collide and bond. The purpose of this process is to provide the number of chance of contacts between coagulated particles in water by gentle and prolonged agitation.

The efficiency of the flocculation process is largely determined by the number of collisions between the minute coagulated particles per unit of time. The electrostatic repulsion between colloidal particles effectively cancels out the electronic attraction forces (Van der Waals’ forces) that would attach the particles together. Certain chemicals (called coagulating agents, coagulants) have the capacity to compress the double layer of ions around the colloidal particles. They reduce the range of the electrostatic repulsion, and thus enable the particles to flocculate, i.e. to form flocs. These flocs can grow to a sufficient size and specific weight to allow their removal by settling, flotation or filtration.

Flocculation requires slow and gentle agitation which will not create turbulence to split up the floc particles that already formed from coagulation process. The substances that frequently are to be removed by coagulation and flocculation are those that cause turbidity and color. Turbidity may result from soil erosion, algal growth or animal/vegetable debris carried by surface run-off. Substances leached from decomposed organic matter, leaves, or soil such as peat may impart color. Both turbidity and color are mostly present as colloidal particles.

There are two types of flocculator that can be used in water treatment plant which is hydraulic method and mechanical method. In our designing we prefer to use hydraulic method. The reason of choosing this method is it simple to construct and operate, less charge of short circuiting, no motor power, electric cables switchgear, etc. to maintain and general maintenance is easier. The flocculation system is effective and capable of producing good floc if the flow rate is constant. However, by using this method, there have also involve the problem for the treatment where it cannot be easily adjusted and will increases head loss.

Baffled type mixing basinsBaffle type mixing basins is the example of Hydraulic Flocculator that we

designed. Baffles are provide in the basins, the required velocity gradient is induced through the baffle for achieving floc formation. In baffle type mixing basins, the flow of the water is so influenced by small hydraulic structures that a stirring action results. The main factors in determining the applicability of the baffled channel flocculation process are local conditions and the presence of a moderate amount of allowable head loss across the tank. Since the baffled type flocculation process does not have any

mechanical parts, it is considered seriously in this type of situation. The main shortcomings of this hydraulic flocculator are no adjustment is possible to changes of raw water composition, no adjustment is possible to the water production rate of the treatment plant, the head loss is often appreciable and it is difficult to be clean.

For horizontal-flow baffled flocculation channels (Figure 6.0), the design water velocity usually is in the 0.10-0.30 m/s range. Width of the channel should be minimum of 45 cm. Depth of flow should not be less than 1.0 m. Detention time normally is 20-50 minutes. Loss of head is range in 15-60 cm. This flocculator can be used for our large treatment plants (Q=16298m3/day > 10000 m3/day) where the flow rates can maintain sufficient head losses in the channels for slow mixing without requiring that baffles be spaced too close together.

Fig. 6.0. Horizontal-flow baffled channel flocculator (plan)Sufficient time and proper mixing intensity shall be provided in flocculation basins to produce settle able floc under varying raw water characteristics and flow rates. For our plant where the capacity is greater than 3.0 mgd, so we designed two flocculation tank to operate in parallel.

Design ParametersDesign flow, Qpeak = 16298 m 3 /day Convert day to minutes: 16298 m3 /day × 1 d / 24 hr × 1 hr / 60 min = 11.32 m 3 / min Detention time, t = 20 minGeometry information:Assume the basins with a ratio of Length : Width: Depth = 1 : 1: 1Number of proposed unit: 2 units basin are used, each basin is evenly divided into three stages.

Design CalculationFlow for each basin, Q’= (11.32 m3 / min ) / 2= 5.66 m3 / min

Volume required for each basin,V req = Q’ x t req = 5.66 m3 / min x 20 min = 113.2 m3

Volume required for each stage,

V stage ,req = 113.2m33 stage

= 37.73 m3 / stage

Based on the assumed basin geometry information :A ratio of Length : Width : Depth = 1 : 1: 1

( L : W : D = 1: 1: 1 )Volume ,V stage ,req = L× W ×D

= W × W × W =W3

Solve the basin width, WW = 3√ 37.73 m3

= 3.35 m= 3.4 m

So the dimension of the basin is:D = 3.4 mW = 3.4 mL = 3.4 m

Actual volume provided, Vact (for each basin)

Volume ,V stage , act = 3.4 m x 3.4 m x 3,4 m = 39.3 m3

Volume ,V basin , act = 3 stage x 39.3 m 3stage

= 117.9 m3

Actual detention time provided, tact (for each basin)

Detention time ,t basin , act = V basin ,actQ'

= 117.9m3

5.66m 3min

= 20.8 min

Equipment DesignWater temperature, T = 5˚CAgitation requirements:1st tank, G = 90 s-1, 2nd tank, G = 60 s-1

μ = 1.518 x 10-3 N.s/m2 at 5°C

Power required for slow mixing in the first stage from desired G;

P=G2μ∀

= (90 s-1 )2 x (1.518 x 10-3 N.s/m2 ) x 117.9 m3

= 1450 N.m / s(W) = 1.5 kN.m / s (kW)

Assume the efficiency is 90% and 70% for gearbox and motor, respectively.Use the following equation to calculate wire power:

Wire power required for the first stage,2100%)(

mg

ae

E E

PP

Pe=1.5kW90 x70

x(100 %)2 = 2.38kW

Power required for slow mixing in the second stage from desired G;

P=G2μ∀

= (60 s-1 )2 x (1.518 x 10-3 N.s/m2 ) x 117.9 m3

= 644 N.m / s(W) = 0.6 kN.m / s (kW)

Assume the efficiency is 90% and 70% for gearbox and motor, respectively.Use the following equation to calculate wire power:

Wire power required for the first stage,2100%)(

mg

ae

E E

PP

Pe=0.6kW90 x70

x (100 %)2 = 0.95kW

Design of Baffle

Flow rate for each basin, Q' = 0.094 m3/sAverage Velocity Gradient, G = 90 s-1

Total Flocculation Time, t = 1248 secondsDepth of the each basin, D = 3.4 mLength of the each basin, L = 3.4 mWidth of the each basin, W = 3.4 mDynamic Viscosity of Water at 5 °C, µ

= 1.518 x 10-3 N.s/m2

Density of water, ρ = 1000 kg/m3

Coefficient of friction timber baffle, f = 0.3

The number of baffles required for horizontal flow baffled flocculator :-Number of baffles in each basin, n =

[( 2 µtρ (1.44+ f ) )(DLGQ' )]

13

= ¿¿= 2.89≈ 3