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Flashcards in Drinking Watta Treatment Deck (17)
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1

Surface water treatment

Screen --> Rapid mix --> flocculation --> sedimentation --> rapid sand filter --> disinfection --> storage --> distribution

lake and reservoirs are more uniform in quality and require less treatment than rivers.

2

coagulation and flocculation theory

alters the surface charge of particles contributing to colour and turbidity so they adhere to one another and will readily settle via gravity. (removal of colloidal and dissolved material)

colloidal particles (usually negative charge) 0.001 - 1 micrometre diameter.
floc of particles 1 - 100 micro metres.

3

coagulation and flocculation method 1

must control the energy input G to promote mixing but to avoid shearing the flocs apart.
-rapid mixing tanks
-flocculation tanks
-settling tanks

G decreases throughout the process, average G is the design value.

4

coagulation and flocculation method 2

Mixing chamber uses a vertical shaft impeller, retention time 10 - 30 seconds

Flocculation chamber either uses slow paddles rotating at less than 1 rpm and water velocity between 0.15 and 0.45 m/s. retention time over 20 minutes.

Type 2 simulates plug flow, flows through a long chamber without mixing. baffles limit short circuiting.

Retention time T = V/Q or L/v

5

Coagulant chemistry

metals, non toxic and inexpensive, insoluble in neutral pH

Alum sulphate Al2(SO4)3. 14H2O - by far the most widely used

turns into 2Al 3+, + 3SO4 2-, + 14H2O

Alum neutralises negative colloid charge (6HCO3 -)

turns into 2Al(OH)3 (solid) + 6(CO2)

Optimum pH 5.5 -6.5, operating 5 to 8.

Alternatives:
Ferric Chloride FeCl3
Ferric Sulphate FeSO4

sensitive to nature of the turbidity substances, type and dosage of coagulant and most importantly pH.

Jar tests used, worst case scenario tests.

6

Coagulant aids

-difficulties often due to slow settling precipitates or fragile flocs.
-aids benefit flocculation by improving settlinf and toughness of flocs.
- synthetic polymers are long chain, high molecular weight organic chemicals
- anionic and nonionic polymers used with metal coagulants to develop larger tougher floc growth.

-requires reduced alum dosages thus reducing waste sludges. Cationic polymers can be used as primary coagulants, polymer sludges are relatively dense and easier to dewater and dispose of.

Jar tests and plant operation used to determine the effectiveness of polymers.

Acids and Alkalies used to adjust for optimum pH.

7

sedimentation

basins are rectangular (most) or circlular with upward or radial flow. design variables include retention time, overflow rate and horizontal velocity.

Following flocculation, water flows gently into settling basins with little turbulence, water resides for at least 4 hours and the flocs settle out and collect at the bottom.

8

settling velocity

Vs = (d^3.g.(density of particle - density of fluid)) Over (18.dynamic viscosity)

Design Vs > Vo particles will settle

75% of settable solids may settle in the first 1/5 of tank

Tank should be 2m deep near the inlet and 0.3m near the outlet, with a 1 to 10% slope.

9

Filtration 1

removal of particles that are too small to be removed during sedimentation. Mechanism dependant on design.

Most common design is Granular-Media gravity filter, design variables include:
-loading rate, headloss, backwash rate, filter depth.

Va = Q / As

Slow sand filters: Va = 2.9 - 7.6 m3/day.m2
Rapid sand filters: Va = > 120 m3/day.m2

10

Filtration 2

filter becomes clogged, head loss increases, turbidity increases. Backwash takes about 10-15 mins, done about once a day when head loss or tubidity hits a prescribed value.

Treatment plants must be able to handle flow with one filter out of service.

Typical depth of 3m filter bed, granular media = 0.6m sand = 2m course gravel = 0.3m. With a surface area of

11

Disinfection

Chlorine gas most commonly used following filtration. 2 design goals, kill majority of organisms in water, provide residual disinfection capability to prevent growth in the distribution system.

Must destroy pathogens within reasonable time and varied temperatures
Must meet fluctuations in water quality
Must be non toxic and palatable
Must be non-dispensible and storable
Must be inexpensive
Must provide a residual concentration
Right option can depend on the scale of the facility and the original state of the water.

12

Disinfection agents

Chemical agents:
Chlorine and its compounds (common)
Bromine and Iodine
Ozone (no residual effect and expensive, but tasty as fuk)
Heavy metals
Hydrogen peroxide

Physical agents:
Heat (boil to kill)
Light (UV, zap to wap)
Microfiltration (separation station)

13

Chlorine reactions in water

Popular compounds, Cl2, NaOCl, Ca(OCl)2.

Cl2(g) + H2O = HCl + HOCl

HOCL = H+, + OCL-

pH dependant (HOCL: 4 to 7.5, OCL - : over 7.5 ,Cl2: pH under 1

14

Chlorine reactions in water 2

Essentially the reaction is complete in a few milliseconds

HOCL is about 80 - 100 times more effective than OCL- for E.coli.

HOCL + NH3 = NH2Cl + H2O

NH2Cl (monochloramine) is less effective but lasts longer for large distribution systems.

Dichloramine and Trichloramine by adding more HOCL

15

Disinfectant strength

C*T concept, the product of the disinfection concentration and time (mg/L * mins)

Design pH between 6.5 - 7.5 to optimise disinfection
Less effective without proper coagulation and flocculatio

Hypochlorite salts: NaOCL and Ca(OCL)2 more expensive but easier to handle. common for small supply

Chloramines, longer contact time, used in combination with other disinfectants

Chlorine dioxide (ClO2), very effective but must be produced on site.

Ozone (O3) very powerful oxidant, kills cysts, no taste or odour problems, widely used in europe, no residual, expensive.

Ultraviolet radiation, effective bactericide and viricide, water must be free of turbidity and lamps free of slime and precipitates, no residual protection.

Rank:
O3 > ClO2 > Free chlorines > Chloramines

16

Chicks Law

-dN/dt = kN
N = number of organisms
k = first order rate constant (day^-1)

requirements include reduction in number of organisms (eg 99.9% kill) or number of organisms allowed in finished water (eg less than 1/100 mg/L) or based on contact time or residual chlorine.

requirements can be at plant and consumer side.

17

Disinfection By-Products

Trihalomethanes:
CHCl3, CHCl2Br, CHClBr2, CHBr3

-links with THMs and gastrointestinal tract cancers
-alreadt limits on THM levels in water, new rules will reduce them.