Water Treatment

Domestic Wastewater Treatment

• It is the process of removing contaminants from household wastewater. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants.

Screening: • Screening is the first step employed in wastewater

treatment. • The influent sewage water is screened to remove coarse

solid like sticks, boards and other large objects carried in the sewage stream.

• This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants a manually cleaned screen may be used.

• Grit Removal: Grit chamber is provided to remove the inorganic solids (specific gravity) about 2.5, diameter (less than 0.2 mm) like sand, ash, glass, metal fragment and inert (non-putrescible) materials which may create operational problems.

Sedimentation

• Sedimentation is a physical water treatment process used to settle out suspended solids in water under the influence of gravity. Sedimentation is often used as a primary stage in modern waste water treatment plant, reducing the content of suspended solids as well as the pollutant embedded in the suspended solids.

• In sedimentation water is at rest or in slow horizontal motion in huge rectangular sedimentation tanks .

Circular sedimentation Basin

• The settled solid are removed periodically.• This process can remove only about 75% of

suspended solids.• Smaller particles such as finely divided silica,

clay and organic matter do not settle down because of their colloidal nature.

Coagulation

• Coagulation involves the use of chemicals which form ions on dissolving in water and neutralize the oppositely charged colloidal particles.

• It facilitate their agglomeration to large size aggregates or flocs which settle down to the bottom of a tank.

• The commonly used coagulants are aluminum sulphate, ferrous sulphate, ammonia alum, potash alum and sodium aluminate.

Al2SO4 +3Ca(HCO3)2 CaSO4+CO2+2Al(OH)3

The coagulation with alum is successful only between pH 5-7.5 and at sufficient alkalinity.

The required dose of alum may range from 30-100 mg/L depending on the quality of raw water

filtration-slow sand filtration • Filtration is the step carried out after sedimentation, for removing

particles of solid matter usually by passing the water through a bed of sand or other porous media.

• The aim of filtration is to remove the small floc particles and microorganism still remained in water after sedimentation.

• Following are the important filtration technologies used for large and small systems:

(i) Rapid sand filters(ii) Slow sand filters(iii) Diatomaceous earth filters(iv) Membrane filter(v) Cartridge filter(vi) Package Plants

Slow sand filtration

• Slow sand filters are suitable only for high quality raw water with usually a turbidity below10 NTU(Nephelometric Turbidity Units) .

• Raw water containing high color or algae are not suitable for slow sand filtration

• They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water.

• The length and breadth of the tanks are determined by the flow rate desired by the filters, which typically have a loading rate of 0.1 to 0.2 metres per hour.

• Slow sand filtration is a simple and reliable process.

• They are relatively inexpensive to build, but do require highly skilled operators.

• The process percolates untreated water slowly through a bed of porous sand, with the influent water introduced over the surface of the filter, and then drained from the bottom.

• Properly constructed, the filter consists of a tank, a bed of fine sand, a layer of gravel to support the sand, a system of under drains to collect the filtered water, and a flow regulator to control the filtration rate.

• No chemicals are added to aid the filtration process.

Extended Airation

Primary Sedimentation

Aeration Tank

SecondarySedimentation

Raw Sewage

Biochemical oxidation of organic matter

Removal of excess aerobic bacteria Treated

effluent

Sludge return line Sludge

Excess sludge to sludge disposal facility

Chlorination• Chlorine is a powerful disinfectant which

usually takes the form of HOCl, chloramines or remains as Cl2 in water depending upon the pH and the presence of ammonium salts.

• Chlorine or any of its derivatives such as sodium hypochlorite (NaOCl) and bleaching powder [Ca(OCl)2] reacts with water to form hypochlorous acid (HOCl).

Cl2 + H2O HCl + HOCl

NaOCl + H2O HOCl + NaOH

Ca(OCl)2 + 2H2O 2HOCl + Ca(OH)2

HOCl H+ + ClO-

• The concentration of hypochlorous acid and hypochlorite ions in chlorinated water will depend on the water's pH.

• A higher pH facilitates the formation of more hypochlorite ions and results in less hypochlorous acid in the water.

• This is an important reaction to understand because hypochlorous acid is the most effective form of free chlorine residual, meaning that it is chlorine available to kill microorganisms in the water.

• Hypochlorite ions are much less efficient disinfectants. So disinfection is more efficient at a low pH (with large quantities of hypochlorous acid in the water) than at a high pH (with large quantities of hypochlorite ions in the water.)

Break Point Chlorination

• It involves passing chlorine gas through water allowing sufficient time for uniform distribution untill a slight excess of 0.1 to 0.2 ppm of residual or free chlorine is present ensuring complete disinfection.

• chlorination generates hypochlorous acid which along with free chlorine is a powerful bactericidal agent due to its oxidizing power.

• Hypochlorite also rupture the cell walls of microorganisms killing them.

• The generated HOCl reacts with ammonia likely to be present in water forming various chloramines.

• Cl2 + H2O HOCl + HCl

• HOCl + NH3 H2O + NH2Cl(monochloramine)

• HOCl + NH2Cl H2O + NHCl2 (dichloramine)

• HOCl + NHCl2 H2O+ NCl3 (nitrogen trichloride)

• The chloramines formed act as a chlorine reserve killing any microorganisms that remain after the initial chlorination.

• The peak of the curve occurs when all of the free ammonia is used up forming chloramines. With excess chlorine due to higher dosages, the chloramines are unstable and destruction occurs due to one or both of the following reactions:

• 2NH2CI + HOCI -----> N2 + H2O + 3HCI

• NH2CI + NHCI2 -----> N2 + 3HCI• This accounts for the downward sloping portion of the curve

on the right side of the peak. When the dosage reaches approximately 8 to 10 times the ammonia concentration, the “breakpoint “ is reached indicating that all the ammonia compounds have been destroyed.

• When the point is reached where the chloramines are completely destroyed, this is called the breakpoint. Any further addition of chlorine will be measured as free chlorine.

• This can be seen in the following image.

e

• Uses of breakpoint chlorination • Breakpoint chlorination provides the powerful disinfecting action

of hypochlorous acid.• Breakpoint chlorination is used in treating public swimming pools.

A properly chlorinated swimming pool provides users with water that is sanitary and pleasant for swimming. When eye irritation occurs, or the so-called chlorine smell is noticed, it is probably due to insufficient chlorine being used. These problems are normally caused by chloroamines, which can be destroyed by the addition of more chlorine. These phenomena are commonly misinterpreted by the general population, who believe that detection of a chlorine smell or eye irritation are due to excessive chlorine being used.

Industrial Water Treatment

Zeolite or Permutit Process: It is Na2O. Al2O3.xSiO2.yH2O where x=2 and y=2• It is hydrated alumino silicate, which are able to exchaging

reversibly its sodium ions for hardness producing Ca2+ and Mg2+ ions in water. It can remove both temporary and permanent hardness.

• Zeolites are of two types:• Natural Zeolites: Non porous green sand, is Na2O.

Al2O3.xSiO2.yH2O, These are more durable• Synthetic Zeolites: These are porous and possess gel

structure

Zeolite

Zeolite Softner• A zeolite softners consists of a steel tank packed

with a thick layer of Zeolite.• The water enters at the top and passes through

the bed of zeolite.• It involves the alternate cycles of softening and

regeneration• Softening: When water passes trough the zeolite

bed, Ca2+ and Mg2+ ion removed from the water by zeolite and simultaneously released equivalent amount of Na+ in exchange.

• The cemical reaction takes place in Zeolite softner

• Ca(HCO3)2+Na2Z CaZ+2NaHCO3

• Mg(HCO3)2+Na2Z MgZ+2NaHCO3

• CaSO4+Na2Z CaZ+Na2SO4

• MgCl2+Na2Z MgZ+2NaCl

Regeneration: After some time when zeolite is completely changed in to calcium and magnesium zeolites, then it gets exhausted and it ceases to soften water. It can be regenerated and reused by treating it with a 10 % brine solution.• CaZ+2NaCl Na2Z + CaCl2

• MgZ+2NaCl Na2Z + MgCl2

Advantages of Zeolite Process

• It removes the hardness almost completely. • The equipment used is compact, occupying a small

space.• The process automatically adjust itself for variation in

hardness of incoming water.• The process does not involve any type of precipitation,

thus no problem of sludge formation occurs.• The plant can be installed in the water supply line itself,

decreasing the cost of pumping.• It require less time for softening.

Disadvantages of Zeolite Process

• The treated water contains more sodium salts.• This method only replaces Ca2+ and Mg2+ ions

by Na+ ions.• High turbidity water cannot be softened

efficiently by zeolite process.

Limitations of Zeolite Process

• The water must be free from turbidity and suspended matter.

• Hot water should not be used as the zeolite tend to dissolve in it.

• Water containing excess of acidity and alkalinity should not be used as mineral acids may destroy the zeolite bed.

Numerical

• A zeolite softner was 90% exhausted, when 10000L of hard water was passed through it. The softner required 200L of NaCl solution ( 50 g NaCl/L of solution). What is the hardness of water?

• 10000L of hard water= 200L of NaCl solution• = 200L X 50 g of NaCl/L• = 10000g NaCl• = 10,000 X(100/117) of CaCO3 eq.• = 8457 g of CaCO3 eq.• 1 L of hard water = 8457g of CaCO3/10000• =0.8457 g of CaCO3 eq.• Thus the hardness of water= 854.7 mg/L

• An exhausted zeolite softner required 500 litres of sodium chloride soluion containing 100 g/l of NaCl for regeneration. If the hardness of water is 600 ppm, calculate the volume of water softened by this softner.

Sodium chloride solution for regeneration contain100 g NaCl per litre= 100 X500

= 50000g of NaCl50000 NaCl= 50000X (100/117) g CaCO3 equivalent50000 NaCl= 50000X (100/117)X1000 mg CaCO3

equivalentHardness for V liter water= 50000X (100/117)X1000

mg CaCO3 equivalent

Hardness=600 mg/lVX600= 50000X (100/117)X1000 V= (50000X (100/117)X1000 )/600=7.116X104