Water Treatment Flashcards
Objectives of Water Treatment
” provide potable water that is chemically and biologically safe for human consumption. It should be free from unpleasant tastes and odours”
Potable
water that can be consumed in any desired amount without any concern for adverse health effects
Palatable
water that is pleasing to drink but not necessarily safe
Physical Characteristics of Water
- turbidity (or suspended solids)
- colour
- taste and odour
- temperature
Chemical Characteristics
dissolved chemicals of concern
Microbiological CHaracteristics
pathogens: viruses, bacteria, protozoa, helminths
Groundwater
- constant composition
- high mineral content
- low turbidity
- low colour
- low or no D.O
- high hardness
Surface Water
- variable composition
- low mineral content
- high turbidity
- coloured
- DO present
- low hardness
Three major water contaminants in NZ
- pathogens ( human and animal wastes)
- sediments (from soil erosion)
- nutrients (from farming and agriculture)
Types of impurities in water
- suspended solids (organic and inorganic) (settleable and non-settleable)
- colloidal solids (organic and inorganic)
- dissolved solids (organic and inorganic)
Unit Operations
removal of contaminants by physical forces such as gravity and screening
Unit Processes
removal of contaminants by chemical or biological reactions
Water Treatment Schemes
- Simple disinfection
- Filtration plants
3, Softening plants
Simple Disinfection
for groundwater: direct pumping and chlorine injection operation, used to treat high quality water
Filtration plants
for surface water: removes colour, turbidity, taste, odour and bacteria
- if the source water has better quality with lower solids, floculation and sedimentation can be ommited: this modification is called direct filtration
Softening Plants
for groundwater: removes hardness ions
Coagulation
destabilisation of colloids from their stable suspension
- neutralise charges
- failitate colloids together
- settle out
Colloid Classification
Hydrophobic
Hydrophilic
Hydrophobic Colloids
“water depressing” - thermodynamically unstable or “irreversible”: if enough time is allowed the particles will aggregate (although very slowly)
We deal primarily with hydrophobic colloids in water and wastewater
Hydrophilic Colloids
“water loving” - thermodynamically stable or “reversible”: they will react spontaneously to form colloids in water
Isoelectric Point
- colloidal particles have become electrically neutral
- highest potential for agglomeration
- zeta potential at isoelectric point is zero
zeta potential
- electric potential difference between shear plane of a colloidal partical and the bulk of the solution
- indirect measure of the electric charge of the colloidal particle
Net Repulsive force
- higher the net repulsive force, the less effective the coagulation
- basic goal of coagulation is to reduce net repulsion force (easier to get colloids together)
Removal of Colloids (2 Steps)
- Destabilisation
2, Flocculation
Destabilisation
(or Coagulation) - reduce the forces acting to keep the particles apart after they contact each other
Flocculation
process of bringing destabilised colloidal particles together to allow them to aggregate to a size where they will settle by gravity
Methods to Destabilise Colloids
- Double Layer Compression
- Adsorption and Charge Neutralisation
- Adsorption and Inter-Particle Bridging
- Enmeshment in a Precipitate (Sweep Floc)
Characteristics of Double Layer Compression
- addition of an indifferent electrolyte (increases ionic strength of solution which has effect of compressing the EDL)
- no risk of overdosing
- no relationship between colloid concentration and optimum dosage of coagulant
Characteristic of Adsorption and Charge Neutralisation
- adsorption of charged (+) counter-ions reduces the primary charge of the colloid
1. destabilise at lower concentrations than indifferent electrolytes
2. larger ions not hydrated as easily as smaller ions so they are more easily adsorbed
3, polymerised particles are more easily adsorbed than non-polymerised species
4. can result in overdosing
5. optimal dosage proportional to colloid concentration
Characteristics of Adsorption and Inter-Particle Bridging
- polymers (metal, salt or synthetic organics) specifically adsorb to surface, forming a polymer bridge
1. dosage of coagulant is proportional to colloid concentration
2. overdosing is possible
Characteristics of sweep floc coagulation
- if metal salts added in sufficient quantities to exceed solubility products a sweep floc will form. Colloids become enmeshed in settling sweep floc and are removed from suspension
1. relationship between optimum coagulant dosage and colloid concentration is often inverse
2. probably some primary charge neutralisation and polymer bridging occuring simultaneously
3, evidence that possibility of charge reversal is mitigated by sulfate ions
Primary metal salts used as coagulants
Al or Fe sulphates
Effect of pH on Coagulation
- lower pH = higher positive charge
- higher pH = larger dominant species and greater tendency to form polymers
- low pH = primary charge reduction (however higher positive charge per Al or Fe means increased risk of overdosing to get charge reversal)
- high pH = more adsorption and bridging ad finally sweep floc (if saturation concentration is reached)
very low surface area (effect of colloid concentration)
not enough particle-particle interaction to produce good destabilisation = need higher dosages
very high surface area (effect of colloid concentration)
need lots of coagulant because of large surface area = almost impossible to overdose
as surface are increases (effect of colloid concentration)
sufficient colloid concentration to effect destabilisation before sweep floc is reached
High Colloid Concentration, low alkalinity
- add coagulant and dont worry about pH
- no concern with overdosing because colloid surface area is too large
High Colloid Concentration, high alkalinity
choices are:
- destabilise by adsorption/charge neutralisation at neutral pH
- elutriate the sludge to lower alkalinity
or add accid to lower pH
Low colloid concentration, high alkalinity
either:
- destabilise by high dosage to give sweep floc
or
- add coagulant aid to get destabilisation at lower dosage
- increases turbidity
Low Colloid concentration, low alkalinity
- most difficult case, generally requires added alkalinity and coagulant aid
- sweep floc difficult to form and easy to overdose
Ability of a chemical additive to produce coagulation determined by:
- electric charge (larger the charge the more effective the coagulant)
- size (larger the molecule the more effective the coagulant)
Electrolysis of Alum in Water
- produces hydrolysis of sulfate with consequent formation of insoluble aluminium hydroxide
- insoluble aluminium hydroxide forms a floc precipitate
Reaction of Alum with Ca and Mg alkalinity
- forms aluminium hydroxide that precipitates forming sweeping floc
Hydrolysis of Ferric Chloride in Water
- produces hydrolysis of ferric chloride and subsequent formation of insoluble ferric hydroxide
- insoluble ferric hydroxide forms a gelatinous sweeping floc
Reaction of Ferric Chloride with Ca and Mg Alkalinity
- forms ferric hydroxide that precipitates as before forming a sweeping floc responsible for colloid removal
Reaction of Ferric Sulphate with Alkalinity or LIme
- forms ferric hydroxide which precipitates and forms sweeping floc
Reaction of Ferrous sulphate with alkalinity or lime
- forms ferrous hydroxide, which is converted to ferric hydroxide by the oxygen in the water
Perikinetic Flocculation
thermal activity or Brownian motion is responsible for colloid collision
Orthokinetic Flocculation
external mixing source which promotes particle-particle contact
Softening
process in which hardness causing ions (Ca^2+ and Mg^2+) are removed either completely or partially
- can be achieved using lime-soda ash method or by ion exchange
Chemistry of Lime-Soda Ash Softening
- Addition of lime (Ca(OH)2) to neutralise any carbonic acid (CO2)
- Addition of lime for Ca hardness (1:1 ratio)
- Addition of lime for Mg hardness (2:1 ratio for CH and 1:1 for NCH)
- Addition of soda ash for Ca non-carbonate hardness (NCH) (1:1 ratio)
- Addition of soda ash for Mg NCH (1:1 ratio)
amount of Mg hardness required to consider removal
Mg2+ in excess of 40 mg/L as CaCO3
- as it is expensive to remove
Ca 2+ hardness remaining
30 mg/L as CaCO3
or
0.6 meq/L
Mg 2+ hardness remaining
10 mg/L
or
0.2 meq/L
amount of lime required
lime (meq/L) = carbon dioxide (meq/L) + carbonate hardness due to calcium (meq/L) + magnesium ions (meq/L) + 1,25 (meq/l)
amount of soda ash required
soda ash (meq/L) = non-carbonate hardness (meq/L)
Three schemes for lime-soda ash softening
- Excess Lime Treatment
- Selective Calcium Removal
- Split Treatment
Excess lime required for removal of Mg2+ hardness
62.5 mg/L as CaCO3
or
1.25 meq/L
Ion Exchange Softening
- exchange of ions in solution for other ions in a medium
- effective for CH and NCH
- does not require operator time
- produces lowest levels of hardness in the final water
CH
carbonate hardness
NCH
non carbonate hardness
Clarification
- removing solid particles denser than water by gravity force
- particles that settle in a reasonable period of time can be removed using a sedimentation tank (clarifier)
- particles settle faster in warmer water
Filtration
- solid/liquid separation process in which the liquid passes through a porous medium to remove as much fine suspended solids as possible
Alkalinity
Alkalinity = (HCO3^-)+(CO3^2-)+(OH-)-(H+)
