COAGULATION, FLOCCULATION,

AND SEDIMENTATION

J.D. DouglasSt. Albans District Office Supervisor

August 21, 2018

Nature of Particulate impurities in water

• Suspended solids include settleable solids and non-settleable or colloidal mater

• Likely to include suspended and dissolved organic & inorganic matter, and biological forms such as bacteria

Coagulation, Flocculation, & Sedimentation

Need for Coagulation and Flocculation

• Remove particulate impurities, especially non-settleable solids, and color from water being treated

• Coagulation chemicals cause particles to destabilize and clump together forming floc

Coagulation, Flocculation, & Sedimentation

Coagulation

Process Description

• The term describes the effect produced when certain chemicals are added to raw water containing slowly settling or non-settling particles

• Definition of flash mixing is the addition of coagulants to raw water

Primary Coagulants cause particles to destabilize & clump together….while coagulant aids add density to slow settling floc and add toughness to reduce shear floc

• Alum (aluminum sulfate), Al2(SO4)3 • 14H20 common

• Polyaluminum chloride, Al(OH)x(Cl)y less pH adjustment

• Ferric Chloride, FeCl3 more effective than alum in some applications

• Ferric Sulfate, Fe2(SO4)3 effective & economical in some waters

• Cationic Polymers primary coagulant with alum/iron coagulants

Metallic salts may be more effective in

removing natural organic matter

Coagulation

• Coagulation is a physical and Chemical reaction occurring between the alkalinity of the water and the coagulant added to the water which results in the formation of insoluble floc

• For a specific coagulant the pH of the water determines which hydrolysis species predominate

• The best pH for coagulation usually falls in the range of 5 to 7

Basic Coagulation Chemistry

Process Performance Considerations

Mixing Method (Rapid Mix Stage)

○ Mechanical mixing○ In-line mixing○ Hydraulic mixing using

flow energy in the system○ Diffusers and grid systems○ Pumped blenders

WV-BPH design Standards require 10 to 30 seconds of mix (detention) time

Process performance considerations

• Agitation of water at low velocities for a long period of time. The process provides contact between particles to promote their gathering together of floc.

• Floc formation is controlled by the rate at which collisions occur between particles and by the effectiveness of these collisions in promoting attachment between particles.

Flocculation

Flocculation

• Best floc size ranges from 0.1 mm to about 3 mm depending on filtration process being used

• WV-BPH-EED design standards ,

Flow through velocity:

0.5 ft/minute < velocity < 1.5 ft/minute

• Detention time for floc formation ≥ 30 minutes

• Paddle speed: 0.5 ft/sec < velocity < 3.0 ft/sec

Flocculation

Process Performance Considerations

An efficient flocculation process involves the selection of the right stirring (detention) time, the proper stirring intensity, a properly shaped basin for uniform mixing, and proper mechanical mixing equipment.

Flocculation

Types of Flocculators

▪ Horizontal paddle wheels

▪ Vertical propeller, paddles, or turbines (less maintenance)

▪ Flocculator Basins (compartmentalized basins)

Flocculation

Interaction with other treatment processes

▪ Filter operations

▪ TTHM & HAA5 formation

▪ Enhanced Coagulation required? Raw vs Finished TOC & Alkalinity levels.

▪ Is proper type and amount of coagulant being used?

Sedimentation

• Zone after Coagulation & Flocculation

• Zone with no mixing or agitation

• Can provide plates for added surface to settle particles on

• Can use gravity to our advantage by forcing flow down and then back up by use of baffles

• Need to remove sludge periodically

• Normal operating condition indicators

periodic monitoring for turbidity, pH, temperature,

chlorine demand, & floc quality

• Process Actions

monitor process performance

evaluate water quality conditions (raw/tr.)

Check/adjust process controls/equipment

Visually inspect facilities

Speed adjustment of flocculators

Operating Procedures Associated with Normal Process Conditions

Should take into account:

Volume of floc to be formed: low turbidity – slower floc speeds, high turbidity – near maximum floc speeds.

Visual observations: Short circuiting may indicate insufficient mixing intensity. Floc break-up/shearing may indicate too high intensity.

Temperature: Low temps require higher mixing turbulence (so increase speed)

Operating Procedures Associated with Normal Process Conditions

The operator can approximate the correct coagulant dosage for plant use when varying amounts of turbidity, color, or other factors indicate raw water quality changes. The Jar Test is also a useful tool in evaluating new coagulants or polymers being considered for use on a plant scale.

The primary intent of the Jar Test is to define the “optimum coagulant dosage”

The Jar Test Procedurereference: California Manual, Volume 1(1989) chapter 11 lesson 5.6 &

Kawamura, “Integrated Design & Operation of Water Treatment Facilities, 2nd

edition (2000) appendix 10

The Jar Test Procedure

Other Jar Testing applications

• Determine KMnO4 feed rate

• Determine Chlorine Demand

• Enhanced Coagulation to determine optimum

Total Organic Carbon (TOC) removal

• Determine optimum lime feed rate for softening plants

• Confirm existing coagulant feed rates by placing a sample entering the plant flocculator and visually observe floc formation.

• A Stirring Machine with 6 paddles capable of variable speeds from 0 – 100 rpm

• An illuminator under the machine (optional)

• Beakers (100 mL)

• Pipets (10 mL)

• Flask, volumetric (1000 mL)

• Balance, analytical.

• Reagents

Apparatus

Clarksburg Water Board - enhanced coagulation jar test, 2002

Questions