ENG421 (7c) – Coagulation and Flocculation Flocculation – design considerations Flocculation – design example

General Water Treatment Technologies (Week 4) Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

Flocculation Flocculation - gentle mixing of stabilised colloids to speed agglomeration some mixing brings colloids together too much (too vigorous) mixing breaks up flocs - forms flocs - physical process that brings particles in contact with one another - three mechanisms due to range of particle sizes in natural water : Brownian diffusion rapid, random motion of particles aka perikinetic flocculation particle size < 1μm Differential settling particles and flocs of larger size and density settle faster than small, less dense particles relative velocity results in particle-particle contact → particle size grows → particle settles faster, strikes more particles Fluid shear during slow mixing, particles follow motion of fluid different fluid layer velocity causes particle-particle contact aka orthokinetic flocculation particle size > 1μm

Flocculation – design considerations (1 of 12) factors to consider : raw water characteristics turbidity, particle size distribution, organic and algae content pH, alkalinity, temperature plant layout coagulation, other treatments to follow headloss type of flocculator flow variations

Flocculation – design considerations (2 of 12) factors to consider (cont) : no chemical addition mixing intensity is lower, G = 5 - 100 s-1 mixing time is longer, t = 15 - 45 min as flocculation proceeds → mixing must become gentler (promote floc growth) → tapered mixing across flocculation unit → flocculation unit is compartmentalised → decreasing velocity gradient occurs in steps → use of baffle walls to avoid short-circuiting hydraulic flocculators higher headloss mechanical flocculators preferred where flow variations are high (up to 50%)

Flocculation – design considerations (3 of 12) Types of flocculators Mechanical mixers most common very flexible paddle flocculators low to medium mixing energy applications preferred in conventional treatment system sedimentation tanks high turbidity water velocity gradient 10 – 50 s-1 aka horizontal shaft flocculators power requirements CD = 1.1 – 1.5

Flocculation – design considerations (4 of 12) Types of flocculators Mechanical mixers propeller or power turbine flocculators medium to high mixing energy applications preferred in direct filtration treatment system strong flocs of small size velocity gradient 50 – 100 s-1 aka vertical shaft flocculators power requirements

Flocculation – design considerations (5 of 12) Types of flocculators Baffled Channel Flocculators baffles are vertical (over and under) horizontal (around the ends)

Flocculation – design considerations (6 of 12) Types of flocculators Baffled Channel Quick Turn Flocculators

Flocculation – design considerations (7 of 12) Types of flocculators Baffled Channel Flocculators advantages minimal short-circuit problems inexpensive produce good flocs ideal for fairly constant flow conditions disadvantages higher headloss inflexible mixing arrangements to taper mixing energy baffles adjusted to slow velocity near outlet velocity gradient 20 – 30 s-1

Flocculation – design considerations (8 of 12) Types of flocculators Baffled Channel Flocculators - calculations

Flocculation – design considerations (9 of 12) Types of flocculators Baffled Channel Flocculators - calculations

Flocculation – design considerations (10 of 12) Types of flocculators Baffled Channel Flocculators – calculations Manning’s number is a measure of the roughness of the channel

Flocculation – design considerations (10 of 12) Types of flocculators Contact Flocculators water passes through a gravel bed in the contact flocculator → a velocity gradient velocity gradient is a function of : gravel size flow rate cross-sectional area of bed flow may be upwards or downwards gravel size is uniformly increased along the flow → velocity decreases (greater interstitial spaces) simple and inexpensive system used in small water supply systems (<60 L/s)

Flocculation – design considerations (11 of 12) Types of flocculators (cont) - typical design criteria mixing time mixing intensity also : number of separate basins number of stages (tapering) energy levels

Flocculation – design considerations (12 of 12) Types of flocculators (cont) - design criteria for different types of flocculators

Flocculation – design example (1 of 4)

Flocculation – design example (2 of 4)

Flocculation – design example (3 of 4)

Flocculation – design example (4 of 4)

References Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D76 1997), pages 384 – 415 Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H46 2006) , pages 160 - 165, 277 - 391, 481 – 527 Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K38 2000), pages 74 – 138 MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W375 2005), pages 643 – 777 Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E58 2009), pages 147 - 149 Parsons, S.A. and Jefferson, B., 2006, Introduction to Potable Water Treatment Systems, Blackwell, Oxford (TD430 .P37 2006), pages 26 – 42 Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages 324 – 330 Vigneswaran, S., and Visvanathan, C., 1995, Water Treatment process : Simple Options, CRC Press, Boca Raton