Water Treatment Sedimentation Filtration and Disinfection SedimentationSettling Following

Water Treatment: Sedimentation, Filtration and Disinfection

Sedimentation/Settling • Following flocculation, the water then flows into the settling basins • Water is nearly inactive – low flow with little turbulence • Water resides for at least 3 hours and the flocs settle out and collect at the bottom.

Settling in Treatment Train

Circular Clarifiers

Type I Settling -- Stokes’ Law where νs = settling velocity ρs = density of particle (kg/m 3) ρ = density of fluid (kg/m 3) g = gravitational constant (m/s 2) d = particle diameter (m) μ = dynamic viscosity

Overflow rate where v = overflow rate (m/s) Q = water flow (m 3/s) As = surface area (m 2)

Types of Particle Settling • Type I settling applies to particles that settle with constant velocity -- particles will be removed if v > vs • Type II settling if particles flocculate during settling, velocity generally increases • Type III As particle concentration increases with depth, zone settling occurs • Type IV At bottom of tank compression settling occurs

Filtration • The final step in removing particles is filtration. • Removal of those particles that are too small to be effectively removed during sedimentation • Multiple removal mechanisms depending on design

Filtration • Single media: sand • Dual media: coal and sand • Multimedia: anthracite coal, sand garnet Source: Back to Basics Guide to Surface Water Treatment, American Water Works Association, 1 P-2. 5 M-73026 -11/92 -MG

Filtration

Filter Design where va= face velocity (m/day) or loading rate (m 3/day·m 2) Q = flow rate (m 3/day) As= filter surface area (m 2) • Slow sand filters: va = 2. 9 – 7. 6 m 3/day·m 2 • Rapid sand filters: va = ≥ 120 m 3/day·m 2 • Removal mechanisms are different • Rapid sand widely used in US, slow sand more common in other countries

Rapid Sand Filtration • As particles are removed - filter becomes clogged – head loss increases, turbidity increases • Must backwash (takes about 10 -15 min) done about once per day • Must design to handle flow with one filter out of service

Rapid Sand Filtration • Backwashing is accomplished by forcing water (and sometimes air) up from the clearwell back through the filter. • The particles in the filter become suspended, releasing the trapped particles. • Backwash water retreated or disposed of.

Disinfection • Following filtration, water is disinfected • Chlorine gas is most commonly used • Two design goals – kill majority of organisms in water – provide residual disinfection capability to prevent growth of organisms in distribution system

Chlorine Reactions in Water • Cl 2 (g) + H 2 O = HOCl + H+ + Cl– p. H dependent – essentially complete within a few milliseconds • HOCl = H+ + OCl– HOCl is about 80 - 100 times more effective than is OCl- for E. Coli – [HOCl] + [OCl-] = free available chlorine • HOCl + NH 3 = NH 2 Cl + H 2 O – NH 2 Cl (monochloramine) is less effective but longer lasting – combined chlorine

Other Disinfectants • Hypochlorite salts: Na. OCl and Ca(OCl)2 – more expensive to purchase – easier to handle – more common for small supplies • Chloramines (NH 2 Cl, NHCl 2, NCl 3) – longer contact time if primary disinfectant – used in combination with other disinfectants • Chlorine dioxide (Cl. O 2) – very effective – must be produced on site

Other Disinfectants • Ozone (O 3) – – – very powerful oxidant no taste and odor problems widely used in Europe no residual more expensive than chlorine (produced on-site) • Ultraviolet radiation – effective bactericide and viricide – water must be free of turbidity and lamps free of slime and precipitates – no residual protection

Design of Disinfection Systems • Chick’s Law: where N = number of organisms k = first-order rate constant (day-1) • Design requirements may include – reduction in number of organisms (e. g. 99. 9% kill) – number of organisms allowed in finished water (e. g. < 1/100 m. L) – contact time – residual chlorine • Requirements can be both at plant and at consumer

Disinfection By-products • Trihalomethanes: CHCl 3, CHCl 2 Br, CHCl. Br 2 and CHBr 3 – sound evidence linking THMs to gastrointestinal tract cancers – current regulations require water supplies to limit total THM levels, new rules reduce them • Haloacetic acids – new rules require limits for 5 compounds • Bromate and Chlorite – new rules

Additional Processes - p. H Adjustment • Recarbonation for softened water CO 2 + H 2 O H 2 CO 3 – Purpose is to reduce p. H following softening (p. H > 11 required for Mg removal) • Sodium hydroxide addition for surface water – coagulant chemicals reduce p. H – increase p. H to reduce corrosivity

Additional Processes • Polyphosphate addition – Added for corrosion control as it forms a protective film on pipes – Also helps to control lead levels in tap water as it complexes with lead

Additional Processes • Fluoride addition – Added either as Na. F, Na 2 Si. F 6, H 2 Si. F 6 – React in water to yield fluoride ion (F-) • Well documented that fluoride levels of ~ 1 ppm reduce incidence of dental caries (cavities) • Some controversy remains

Advanced Treatment Processes • Advanced Oxidation Processes – improved disinfection – oxidize synthetic organic chemicals – taste and odor control • Activated carbon adsorption – remove recalcitrant synthetic organic chemicals, THMs, taste and odor compounds – concern with bacterial growth problems • Membrane process – discriminate on both size and chemistry – selective removal including desalinzation

Residuals Management Sludge from clarifiers Finished water

Residuals Management – Dewatering • • Lagoons Sand-dying beds Freeze treatment Centrifugation Vacuum filtration Continuous belt filter press Plate Pressure filters

Residuals Management – Ultimate Disposal • • • On-site storage Land-filling Land application – soil amendment Reclamation/recycling – new products Ocean dumping – banned in most of the countries trough out the world