BEIRUT ARAB UNIVERSITY FACULTY OF ENGINEERING CIVIL & ENVIRONMENT ENVI RONMENTAL AL ENGINEERING ENGINEERI NG DEPT DE PT..
FALL 2016-2017 2016 -2017
Coagulation turbidity, and • Raw water contains suspended particles of color, turbidity, bacteria that are too small to settle in a reasonable time time period and cannot be removed by simple filtra-tion.
• The object of coagulation is to alter these particles in such a way as to allow them to adhere to each other. Thus they can grow to a size that will allow removal by sedi-mentation and filtration. • Coagulation is considered to be a chemical treatment process process as opposed to the physical treatment operations operations of flocculation, sedimentation, and fil-tration that follow. follow.
Coagulation
Coagulation • The coagulants must be added to raw water and perfectly distributed into the liquid. This can be done through RAPID MIXING or AGITATION. • Coagulation results from adding salts of iron or aluminum to the water. Common coagulants: o
Alum (Aluminum Sulfate)
o
Sodium sulfate
o
Ferric sulfate
o
Polymers
Detention time in the rapid mix tank is typically less than one-half minute.
Coagulation • Coagulation is the reaction between one of these salts and water. The simplest coagulation process occurs between alum and water. When alum is added to water, a chemical reaction occurs that produces positively charged aluminum ions. The overall result is the reduction of electrical charges and formation of sticky substance called “Floc”. • Liquid alum is preferred in water treatment because: o
Ease of handling
o
Lower costs
o
Less labor required
o
Less storage space
o
Greater accuracy in measurement
o
Elimination of nuisance
o
Easier maintenance
Factors Influencing Coagulation • pH • Alkalinity • Temperature • Time • velocity • Zeta potential
Jar Test • Optimum pH for coagulation and other factors must be considered before deciding which chemical to be use and at what levels. • Jar or Coagulation test is performed. A laboratory procedure to determine the optimum pH and the optimum coagulant dose
Jar Test Determination of opt imum p H
•
Fill the jars with raw water sample (250 or 1000 mL) – usually 6 jars
•
Add the coagulant and start mixing for 1 min at a speed of 100 rpm
• •
Reduce the stirring speed to gentle 30 rpm and continue mixing for 20 min
•
This slower mixing speed helps promote floc formation by enhancing particle collisions, which lead to larger flocs
•
Turn off the mixers and allow flocs to settle for 20 to 30 min
Jar Test •
Measure the final residual turbidity in each jar
•
Plot residual turbidity against pH
Jar Test • Repeat all the previous steps • This time adjust pH of all jars at optimum (6.3 found from first test) •
Add different doses of the selected coagulant (Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L)
• Rapid mix each jar then, Reduce the stirring speed • Turn off the mixers and allow flocs to settle, then measure the final residual turbidity in each jar • Plot residual turbidity against coagulant dose. The coagulant dose with lower residual turbidity will be the
Optimum coagulant dose: 12.5 mg/L
Rapid Mixing Tank Design • • • •
Detention Time : 20 to 60 seconds The tanks are usually, baffled horizontally into two or three compartments inorder to provide sufficient residence time. Tanks should also be vertically baffled to minimize vortexing. Chemicals should be added below the impeller, point of most mixing.
Mixing i mpeller t ypes us ed i n w ater treatment
SEDIMENTATION
Flocculation •
Flocculation is the physical process of slowly mixing the coagulated water.
•
It’s a gentle mixing process
•
The primary objective of flocculation is to bring particles in contact with one another so they will collide, stick together, and grow to a size that will easily settle.
•
Detention Time : 20 to 40 minutes
Flocculation
Sedimentation
Sedimentation
It is the process of removin g solid particles heavier than water
Sedimentation Sedimentation t anks are either rectangular or circular tanks.
Rectangular Basin
Sedimentation Sedimentation t anks are either rectangular or circular tanks.
Circular Basin
Sedimentation
-A layer of sludge is formed on the floor of the tank
Typical detention times range from 1 to 10 hours. The longer the detention time, the more expensive the tank is, the better will be the tank's performance.
Design Criteria of Sedimentation Tank - OFR = 20 – 40 (m3/m2/day)
- Detention time is 4 hours - Tank Depth 3 to 6 meters - For rectangular tanks; maximum tank length 40m, Preferable 30m - L:W = 4:1 - For circular tanks, maximum diameter 40m including the gentle mixing - Average water flow at weir = 450 m3/m/day
Sedimentation • Solids collected from the bottom of the tank will be removed manually by periodically shutting down the tank and washing out the collected sludge, or the tank may be continuously and mechanically cleaned using a bottom scraper. The effluent from the tank is then filtered.
Circul ar Sludge Scraper
Weir Overflow • The water velocity within the sedimentation tank is slow in order to allow for sedimentation.
• To remove water from the basin quickly, it is desirable to direct the water into a pipe or small channel for easy transport, which will produce a significantly higher velocity.
Sedimentation Concept • There are 2 important terms to understand in sedimentation zone design:
settling velocity, Vs : The rate at which the particle is settling downward Velocity at which the tank is designed to operate, called the overflow rate, Vo, the velocity of the liquid rising is the overflow rate. Overflow rate (Vo) must be less than or equal th e settling velocity Vs
Sedimentation Concept Critical settling velocity is the settling velocity of particles which are 100% removed in the basin.
Since smaller particles have lower settling velocities, if you want to remove smaller particles in the settling basin you have to have a lower overflow rate.
•The percentage of particles removed in an ideal horizontal sedimentation tank:
V
÷
v
v
=
L ÷ H ←→ ∴ v
Q =
.
H
B.H L
Q =
B.L
Q =
A
V.H =
V
L
Q =
B.H 3
Overflow Rate
Flow Rate (m / s ) =
2
settling surface area (m )
Example 1
For flow rate of 120000 m3/d, design the following: - Flash mixing tank. - Circular sedimentation tank. - Rectangular Sedimentation tank & gentle mixing - Aluminum sulfate for coagulation for one year if dose = 30ppm and cost of one ton = 250 $ - Volume of sludge m 3/d if turbidity = 60ppm
SOLUTION: Flash mixing: Hydraulic Retention Time (HRT) = 20-60 seconds, take=40sec. Volume = Q * HRT = 120000*40/(24*60*60) = 55 m3 For circular tank (Assume D=H); then D=4.2m
Circular Sedimentation zone:
Over Flow Rate (OFR) = 30 m 3/m2/d Area surface = 120000 / 30 = 4000 m2 HRT = 3 hours Volume = 120000 *3/24 = 15000 m3 Depth = volume/area = 15000/4000 = 3.75 m
Gentle mixing zone: HRT = 30 min Volume = 120000*30/24*60 = 2500 m3 Depth = 3.75 – 0.50 = 3.25 m Area = 2500/3.25 = 769 m2
Total area of sedimentation and gentle mixing zones • Total area of sedimentation and gentle mixing zones = • Area of
sedimentation tank = 4000+769 = 4769 m2
• we take 6 (or 8 or 10) tanks, the area of tank = 4769/6 = 795m2 • Diameter of sedimentation tank = 32 m less than 40ms … OK • Area of one gentle mixing tank = 769/6 • Diameter of gentle mixing zone = 12.8m
