REMOVAL OF COLOUR AND TURBIDITY (COAGULATION, FLOCCULATION FILTRATION)

Granch Berhe

CHAPTER TWO

INTRODUCTION

HealthTaste and odourAesthetics (color and turbidity)

The need to clarify water

Turbidity • Turbidity – negative charged particles (sand, silt, clay, bacteria, viruses) in the initial source water that need to be removed to improve treatment. 1. Suspended Solids

2. Colloidal Solids (~0.1 to 1 m)

3. Dissolved Solids (<0.02 m)

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Colloid Stability

------ ------Repulsion

Colloid - A Colloid - B

Colloids have a net negative surface charge

Electrostatic force prevents them from agglomeration

Brownian motion keeps the colloids in suspension

H2OColloid

Colloids – so small: gravity settling not possible

REMOVAL OF COLOUR AND TURBIDITY(COAGULATION/FLOCCULATION PROCESS)

•Common treatment steps used to remove turbidity from the initial source water.

1. Coagulation

2. Flocculation

3. Sedimentation

4. Filtration

Rapid Mixing

Slow Mixing

Settling

Cleaning

The primary purpose of the coagulation/flocculation process is the removal of turbidity from the water.

The process removes many bacteria which are suspended in the water and can be used to remove color from the water.

What is Coagulation? Coagulation is the destabilization of colloids by addition of chemicals that neutralize the negative charges at rapid mixing

The chemicals are known as coagulants, usually higher valence cationic salts (Al3+, Fe3+, Na+, Mg2+, etc.) . 1. Alum- aluminum sulfate

2. Ferric chloride or ferrous sulfate3. Polymers

With destabilization, colloids aggregate in size and start to settle

Mixing Levels:Mixing intensity and residence time determine whether the stated goals will be met.To determine mixing intensity define as the average shear intensity (mean velocity gradient) in the rapid-mix tank.The Camp-Stein equation is often used to compute this , however it is an equation which is based on laminar flow -- a case seldom found in rapid-mix or flocculation basins so it’s an “average” approximation.

G

G

The Camp-Stein equation is :

0.5PG

V

P = power dissipation (mixer power transferred to bulk fluid)

V = volume of reactoru = dynamic viscosity

Large G and small T gives small but dense floc Small G and large T gives big but light flocsWe need big as well as dense flocs which can be obtained by designing flocculator with different G values

Classwork:What horsepower level do we need to supply to a flocculation basin to provide a t value 1000 seconds and a Gt of 100,000 for 438 m3 Reactor? (Given: = 0.89 x 10-3 Pa.s; 1 hp = 745.7 watts)

Retention time, t = Gt/GGt = Camp No.

What is Flocculation?Flocculation is the agglomeration of destabilized particles into a large size particles known as flocs which can be effectively removed by sedimentation or flotation.

Typical layout of a water treatment plant

REMOVAL OF IRON AND MANGANESE

• Pre-aeration, Post-aeration, and Post-filtrationTable 5.2 Initial Iron and Manganese Test Results

Woodmen Water Treatment Facility EvaluationColorado Springs Utilities

SampleDescription Water Quality Parameter (g/L)1

Fetot Fedis Mntot Mndis Turbidity

Plant #1 (2/13/01)

Pre-aeration 2600 250 130 130 6.34

Post-aeration 2000 <10 81 <10 7.90

Finished 39 <10 11 <10 0.29

Plant #2 (1/23/01)

Finished 43 <10 225 218 1.04

Plant #2 (2/13/01)

Pre-aeration 540 <10 48 46 3.37

Post-aeration 460 <10 52 50 3.07

Finished 29 <10 17 15 0.231Values in bold indicate failure to comply with the USEPA Secondary Drinking Water Regulations.

Treatment Alternatives• Pre-Cl2

– Addition at the wellhead– Immediately prior to filters

• Aerated intermediate tank• Backwash System Enhancement• Filter Media Replacement• Filter Media “Cleaning”

– Extended backwashing – Elevated levels of Cl2

• Ion Exchange• Sequestration

WATER SOFTENING AND ANALYSIS

WATER SOFTENING

Water with high concentration of calcium and magnesium ions.Measured in terms of the calcium carbonate equivalentHard if it has 100 mg/L or more as calcium carbonate. 

Hard water

Removal of hardness from water. Not a required part of the water treatment process, hard water does not have health consequences. Is problematic for a variety of reasons

Makes soap precipitate out of water and form a scum, causes bathtub ringsScale formation in boilers, pipes and cooking utensils, if >300 mg/LEncrustation in water supply structureReaction with soap results in excessive use of soaps and detergents. Cause taste problems in drinking waterCathartic and diuretic effectShorten the life of fabrics washed in hard water. Harms many industrial processes,

Softening

Enters groundwater as the water percolates through minerals containing calcium or magnesium.  The most common sources of hardness are

Limestone (which introduces calcium into the water) and Dolomite (which introduces magnesium.) 

Groundwater generally has a greater hardness than surface water. 

Sources of Hardness

Most widespread and troublesome ions, Ca2+ and Mg2+ Other dissolved metal like Sr2+, Fe2+, and Mn2+  Hardness-causing ions are divalent cations, Na+ and K+ do not cause hardness. 

Caused by a variety of divalent cations, primarily calcium and magnesium.  Cations have a tendency to combine with anions in the water to form stable compoundsType of anion found in these salts

Carbonate and Non-carbonate hardness.

Types of Hardness

Carbonate hardness compounds

Noncarbonate hardness compounds

Calcium carbonate (CaCO3) Calcium sulfate (CaSO4)Magnesium carbonate (MgCO3) Magnesium sulfate (MgSO4)Calcium bicarbonate (Ca(HCO3)2)

Calcium chloride (CaCl2)

Magnesium bicarbonate (Mg(HCO3)2)

Magnesium chloride (MgCl2

Calcium hydroxide (Ca(OH)2)Magnesium hydroxide (Mg(OH)2)

Carbonate hardness is caused by metals combined with a form of alkalinity. Alkalinity is the capacity of water to neutralize acids and is caused by compounds such as carbonate, bicarbonate, hydroxide, and sometimes borate, silicate, and phosphate.  Noncarbonate hardness forms when metals combine with anything other than alkalinity.

Carbonate hardness - temporary hardness - can be removed by boiling water  Noncarbonate hardness - cannot be broken down by boiling - permanent   When measuring hardness, we typically consider total hardness

Calcium bicarbonate → Calcium carbonate + Water + Carbon dioxide

Ca(HCO3)2 → CaCO3 + H2O + CO2

Deposition of calcium carbonate scale in pipes and equipmentCO2 can combine with water to give carbonic acid - corrosion of iron or steel equipment

Calcium sulfate + Sodium carbonate → Calcium carbonate + Sodium sulfate

CaSO4 + NaCO3 → CaCO3 + Na2SO4

Noncarbonate hardness is the culprit in forming soap scum

Types of TreatmentSoftened water - a hardness of about 80 to 90 mg/L as calcium carbonate.Excessively soft water can be problematic - causes corrosion of pipes.

 

Chemical PrecipitationSimilar to removal of turbidity by coagulation/flocculation

Adding lime to raise the pH of water until it is high enough for reactions to occur which prompt hardness compounds to settle out of the water.

Chemical precipitation using lime will remove carbonate hardness. Soda ash and lime, both carbonate and noncarbonate hardness may be removed. In either case, chemical precipitation does not remove all hardness from water. Reduced as low as 30 to 40 mg/L, although the typical goal is 80 to 90 mg/L.

Chemical precipitation is an effective softening process, Disadvantages

Requires a lot of operator control High pH used in lime softening can set colors in water and make them difficult to removeProduces large quantities of sludge which can create disposal problems.

Ion Exchange Ion exchange softening, also known as zeolite softening, Passes water through a filter containing resin granules.  Calcium and magnesium are exchanged for sodium from the resin granules. The resulting water has a hardness of 0 mg/L Must be mixed with hard water to prevent softness problems

Does not require flash mixer Does not require as much operator timeEffective at removing both carbonate and noncarbonateOften used for waters high in noncarbonate hardness With a total hardness less than 350 mg/L.Disadvantages   Sodium ions, health problems for those not supposed to eat any salt.  Softeners have to be backwashed similar to a filter, and the recharge water, brine, can cause disposal problems.

Other Softening Processes

Other processes can be used to soften water, but they are generally expensive and only used in rare circumstances

RO water is forced through a semi-permeable membrane.  Calcium, magnesium, and dissolved solids are captured

Electrodialysis - passing water between two plates with opposite electrical charges.  The metals attracted to negative charge Non-metals are attracted to positive charge.  Both types of ions can be removedElectrodialysis is used on very hard water, more than 500 mg/L as calcium carbonate.

Distillation involves the evaporation of water.  The evaporated water leaves behind all hardness compounds,

Advantages of Softening

Softening will deal with the problems caused by hard water - excessive soap use and scaling being the most troublesome.  Softening often removes iron and manganese, reduces tastes and odors, reduces total solids content, and removes radioactivity.  The high pH associated with lime softening can aid in disinfection.  When water is stabilized using recarbonation at the end of the lime softening process, corrosion in the distribution system is avoided. 

Disadvantages of SofteningCarry a certain monetary expense.  High pH associated with lime softening favor the formation of hypochlorite as the dominant free chlorine residual, and hypochlorite is a less powerful disinfectant than other free chlorine residuals.  The high pH may also increase trihalomethane levels in the water.  If the water is not properly stabilized after treatment, then corrosive water may be produced which will corrode the distribution system.  Ion exchange softening, as noted above, can also cause problems due to the high levels of sodium in the treated water.  Both lime softening and ion exchange softening create waste disposal problems. 

WATER ANALYSIS

DETERMINATION OF BOD, COD, TDS AND TOXICITY

Parameters that determine water qualityDissolved oxygenBiochemical Oxygen DemandChemical Oxygen DemandDissolved SolidsNitrogen – ammoniaBacteriological quality

Dissolved oxygen – amount of actual oxygen dissolved in a water sample. Higher number = purer waterBOD5 – Actual amount of dissolved oxygen metabolised over 5 days. Higher number = dirtierBOD – Extrapolated amount of theoretical oxygen that would be needed to completely metabolise organic waste. Higher number = dirtierCOD – Actual amount of oxygen required to completely oxidize organic waste CHEMICALLY. Higher number = dirtier.TDS – Concentration of dissolved chemicals in water

BOD (Biochemical Oxygen Demand) is a measure of the polluting efficiency of water.Oxygen is demanded in effluent for the oxidation of inorganic and organic matter. BOD is defined as the amount of oxygen required to carry out the biological decomposition of dissolved solids under aerobic conditions at standard temperature.

Where Si = Sample initial in mg/l, Sf = Sample final in mg/l, Bi= Blank initial in mg/l and Bf = Blank final in mg/l, f = Dilution factor

ti f

sb

BOD = DO - DOVV

WhereBODt = biochemical oxygen demand at t days, [mg/L]DOi = initial dissolved oxygen in the sample bottle, [mg/L]DOf = final dissolved oxygen in the sample bottle, [mg/L]Vb = sample bottle volume, usually 300 or 250 mL, [mL]Vs = sample volume, [mL]

Biochemical Oxygen Demand

BOD - loss of biodegradable organic matter (oxygen demand)

Lo

Lt

L or

BO

D re

mai

ning

Time

Lo-Lt = BODt

BODBottle

BODBottle

BODBottle

BODBottle

BODBottle

COD (Chemical Oxygen Demand) is measure of oxygen required to oxidize unstable materials in a sample by means of dichromate in acid solution.

Where B = Blank value, N= Normality, V = Volume of Sample, f = Dilution factor

Where N1 = Normality of FAS, V1 = Volume of FAS, N2 = Normality of KCr2O7 and V2=Volume of KCr2O7

• COD test is faster than BOD analysis: used for quick assessment of wastewater strength and treatment performance

• Like the BOD, it does not measure oxidant demand due to nitrogeneous species

• It does not distinguish between biodegradable and non-biodegradable organic matter. As a result COD's are always higher than BOD's.

Chemical Oxygen Demand

to calculate the normality (N) of FAS

The chemical oxygen demand (COD) of a waste is measured in terms of the amount of potassium dichromate (K2Cr2O7) reduced by the sample during 2 hr of reflux in a medium of boiling, 50% H2SO4 and in the presence of a Ag2SO4 catalyst.

Total Dissolved Solids

Where W1 = weight of dish in gram, W2 = Weight of dish plus sample after treatment in gram and V = volume of effluent in ml.

Total Dissolved Solids (TDS)" is the concentration of the dissolved chemicals in a sample of water. Before dissolving, these chemicals could have been a solid or a liquid.

An initial DO (dissolving oxygen) of 6.39 mg/l for Blank, 6.45 mg/l for conventional, 6.45 mg/l for cationized; a final DO of 5.85 mg/l for blank, 4.47 mg/l for conventional and 4.91 mg/l for cationized were used to calculate the BOD.

To calculate the COD, use 5 ml of 0.25N KCr207. Quantity of sample was 1ml each for conventional and cationized; quantity of titrant (FAS) was 11 ml for blank, 3.7 ml for conventional and 4.6 ml for cationized.

For TDS; use 50 ml effluent sample each for conventional and cationized. Weight of dish was 41.9294g for conventional and 42.9124g for cationized. Weight of sample was 43.9275g for dish plus conventional and 43.4103g for dish plus cationized.

Homework:Determine the % improvement in BOD, COD and TDS after cationization

Alkalinity

Acute Toxicity

Single or multiple exposures in a short space of time (usually less than 24 hours).

Skin Irritation VomitDiarrhea

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Toxicity

Chronic Toxicity

Non Genotoxicity1.Water solubilityWater-soluble molecules are generally excreted rapidly by a living organism2.Water InsolubilityDue to the insolubility, toxicants gets large size particles [0.1 to 3 mm] in the body which are not transported across cell membranes

Genotoxicity Mutagens Carcinogens Teratogens

Toxicants reach and interact with the DNA. Carcinogens attack a nucleophilic site in DNA, which may be a

carbon, nitrogen or oxygen atom, to form a covalent chemical bond E + [DNA] or E–[DNA]

The high alkalinity and traces of heavy metals (chromium) adversely affect the aquatic life and human health.

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Other worse Effects Brain function, decreased attention, retardationReproduction including miscarriage, untill births

Most sensitive are children <7; immature blood-brain barrier; effects mental development etc……