Groundwater Treatment Flashcards
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)
Hardness theory
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 addition
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
Additional processes
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
Advanced Processes
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.
Membrane Processes
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.
Membrane categories
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.
Membrane performance
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
foulants:
suspended and colloidal matter
micro-organisms
organic matter
membrane materials
- organic
- 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!
