Stages of GW treatment
GW from wells –> Rapid Mix –> Flocculation Basin –> Sedimentation Basin –> Recarbonation –> Disinfection –> storage –> Distribution system
GW treatment Objectives
Primary objectives are to remove hardness and other mineralsm eliminate pathogenic organisms. Largely based on precipitation
- Remove iron, which leaves rust coloured stains on clothing, sinks and tubs
- Reduce hardness, or dissolved minerals, which decrease the effectiveness of soap and cause scale in water heaters and boilers etc.
- Remove dissolved gases, such as hydrogen sulfide, they contribute to tast and obour problems.
- Eliminate pathogenic organisms.
Aeration, oxidation of reduced metals, stripping of dissolved gases.
4Fe 2+, + O2 + 10H2O = 4Fe(OH3)(s) + 8H+
2Mn 2+, + O2 +2H2O = 2MnO2(s) + 4H+
H2S(aq) = H2S(g)
Characterises the ability of water to cause soap scum, increase the amount of soap needed, cause scaling on pipes, cause valves to tick due to formation of calcium carbonate crystals, leaves stains on plumbing fixtures.
Total Hardness = TH
Technically - the sum of all polyvalent cations
Practically - the amount of calcium and magnesium ions
Divided into carbonate and noncarbonate hardness.
Soft = 0 - 75 mg/L at CaCO3
Moderately hard = 75 - 100 mg/L
Hard = 100 - 300 mg/L
Very hard = > 300 mg/L
Carbonate and Noncarbonate hardness
Carbonate Hardness CH, called temporary hardness because heating water will remove it, insoluble carbonates precipate when heated and from deposits in water heaters.
Noncarbonate Hardness NCH, called permanent hardness because it is not removed by heating. Much more expensive to remove, NCH = TH - CH
Lime combines with hardness minerals to form solid particles.
neutralisation of carbonic acid:
CO2 + Ca(OH)2 = CaCO3(s) + H2O
Precipitation of CH due to Calcium
Ca 2+, + 2HCO3 -, + Ca(OH)2 = 2CaCO3(s) + 2H2O
Precipitation of CH due to magnesium
Mg 2+, + 2HCO3 -, + Ca(OH)2 = Mg 2+, + CO3 2-, + CaCO3(s) + 2H2O
Mg 2+, + CO3 2-, + Ca(OH)2 = Mg 2+, + CaCO3(s) + H2O
Recarbonation of softened water, purpose is to reduce pH following softening, pH> 11 required for Mg removal.
Sodium hydroxide addition for surface water, coagulant chemicals reduce pH, increase pH to reduce corrosivity
Oxidation, improved disinfection, oxidise synthetic organic chemicals, taste and odour control.
Activated carbon adsorptionm removes SOCs, THMs, taste and odour compounds, concerns bacterial growth problems
Membrane processes, discriminates on both size and chemistry, selective removal including desalination.
Activated Carbon Adsorption
Manufactured from cabonaceous material ( eg coal, wood, coconut shell.)
Huge internal surface area (500 - 1500 m^2/g)
Activation consists of two stage thermal treatment (at > 600 degrees celcius)
Favours the removal of VOCs, SOCs, taste and odour, natural organic compounds.
Granular or Powdered Activated Carbon, GAC / PAC.
small particle size promotes rapid adsorption
GAC preferred for persistent tast and odour, SOC, VOS contamination, can be thermally reactivated to original capactiy. Volume required V = Qt
Ozone / GAC treatment
industry standard for pesticide removal, O3 breaks down large pesticide molecules into smaller ones which are directly adsorbed onto activated carbon and metabolised by bacteria growing on the surface of the PAC/GAC
Ozonation upstream of carbon increases life of AC, O3 also effects oxidation of organics, removal or THM precursors, and disinfection.
subset of filtration, only very few pollutants found in water that cannot be removed economically by membrane technology.
Thin layer of material containing pores, allow the flow of water but retains suspended, colloidal and dissolved species. Seperation based on size, diffusivity or affinity for specific contaminants.
Cost increases as the size of pollutant to be filtered decreases, no commercial membranes exist to remove uncharged inorganic molecules (eg hydrogen sulphide) and small uncharged organics
Microfiltration - 0.1 micrometres
Ultrafiltration - 0.01 micrometres
used to remove larger pollutants such as turbidity, pathogens and particles, act like sieves, mechanical seperation.
Nanofiltration - 0.001 micrometres
used for removal of disinfection by product precursors such as natural organic matter.
Reverse Osmosis - 0.0001 micrometres
used to remove salts from brackish or salt water, both operate in relation to the physiochemical properties of the permeating components and membrane material.
drivingforce is the pressure differential or transmembrane pressure TMP, between the feed and the permeate sides of the membrane.
the flux and pressure are interrelated and either one may be fixed, process needs to deliver set quantity of water.
fouling is the process of accumulation or rejected material at the membrane surface. increases hydraulic resistance and therefore TMP must increase. caused by physiochemical and biological mechanisms, it is different to clogging.
backwashing: reverse of flow to dislodge material, occurs at 10-30 minute intervals, 5-10% loss of productivity. after a series of backwash cycles, chemical cleaning is required for tightly bound materials.
Membrane foulants and materials
suspended and colloidal matter
- inorganic (ceramic / metallic) lower fouling propensities, greater tolerance to chemical additions and easier to clean, construction much more expensive than polymer systems (£500/m2) or £0.5/m2
Organics Removal (Carbon Hydrogen Oxygen Nitrogen)
Moorland and upland source waters contain natural organic matter, lowland waters are likely to have organic inputs from both agricultural and anthropogenic sources, main groups of concern are disinfection by-products (DBPs), pesticides and other micro-pollutants, algae, taste and odour.
See table of specialist treatments!
conversion of natural organic matter into disinfection by-products (eg trihalomethanes) when chlorine is used. THMs in drinking water shown to cause cancer, limit lowered in UK to 100 micrograms/L.
Optimisation of coagulation process in treatment works used to reduce levels of precursors and the overall DOC.
Ion exchange and advanced oxidation processes (IEX & AOP) are alternatives to coagulation.
current and emerging pollutants, 1/3 of all pharmaceutical products previously identified in surface waters.
4 of the most popular 12 pesticide products used on arable crops.
Endorcrine disrupting compounds (oestrogens enter aquatic environment from contraceptive pill residues)
- problematic algae are colloidal in nature and removed by conventional processes.
- Dissolved Air Floatation more popular than sedimentation due to low density of algae.
- Seasonal algal blooms can impair coagulation and flocculation resulting in, high carryover of algae, increased coagulant demand and increase in coagulant residual in treated water.
- Filter cloggging occurs with large algae
- Prechlorination is unfeasible as algal cells are THM precursors.
- Often offensive taste and odour from algae
- Potential for toxin release.
Taste and Odour
- MIB and geosmin are the two compounds commonly identified as causing taste and odour problems.
- They are earthy, musty odour compounds produced as secondary metabolites by some cyanobacteria
- Conventional treatment is inadequote for removal
- Activated carbon considered the best technology.
Inorganics Removal - nitrate
See table for overview!
Nitrate: Conventional treatment does not remove
Simple solution is to blend high nitrate water with low nitrate water
Ion exchange, reverse osmosis, electrodialysis and biological denitrofication work.
nitrate Removal - ion exchange
ion exchange resins exploit functional groups that are initially bonded to chloride ions
the chloride ion is exchanged for a nitrate ion
when all of the functional groups have been bonded to the resin is saturated and can be regenerated with 2-3 bed volumes of brine solution.
the pressure cells are up to 4m in diameter and contain 0.6-1.5m detpth of resin
nitrate Removal - reverse osmosis
pressure is applied to water to force it through a semi permeable membrane leaving the majority of impuritites behind
85-95% removal of nitrate achievable.
depends on initial quality of water, system pressure, type of membrane and T (temperature?)
nitrate Removal - electrodialysis
elctrochemical process in which ions migrate through an ion-selective membrane as a result of their attraction to the electrically charged membrane surface.
can separate nitrate from cations such as calcium and magnesium.
recieve and concentrate the residuals and dispose of them.
treatment, transportation and disposal of sludge makes up major fraction of the total costs.
sludge consists of SS, algae, viruses, dissolved solids as well as chemicals added that form precipitates including lime, iron, aluminum based coagulants.
UK sludge production is largely alum sludge based, moving away from alum to iron due to alzeihmers link.
lagoons, sand drying beds, freezeing, centrifugation, vacuum filtration, belt filter press, plate pressure filters.
up to 40% of water is chemically bound to particles, conventionaly dewatering processes, increase solids content up to 20%.
freeze thaw conditioning followed by gravity filtration changed sludge to granular material with grains large enough to settle easily under gravity.
on site storage, landfill, Waste water Treatment Works, soil ammendment, incorporation into new products such as bricks, recovery of metal ions and reuse as coagulants ocean dumping (banned in many countries)
Mean disposal cost of £41 / tonne of dry solids.