Water pollution
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Water Pollution. Contents: Water contaminants Water supply Water treatment Wastewater collection Wastewater treatment Sludge treatment. Sources of Water Pollution. industrial pollution: chemicals municipal pollution: combined sewage agriculture sediment erosion petroleum products

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Water Pollution

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Water pollution

Water Pollution

Contents:

  • Water contaminants

  • Water supply

  • Water treatment

  • Wastewater collection

  • Wastewater treatment

  • Sludge treatment


Sources of water pollution

Sources of Water Pollution

  • industrial pollution: chemicals

  • municipal pollution: combined sewage

  • agriculture

  • sediment erosion

  • petroleum products

  • mine leaching


Effect of pollution on rivers

Effect of Pollution on Rivers

  • When high energy organic material is discharged to a river, several changes can occur downstream:

    • decomposition of organics causes drop in dissolved oxygen (DO).

    • If DO>0 can get oxygen sag curve.

    • If DO=0; anaerobic.

    • get changes in biodiversity.


Water supply and treatment

Water Supply and Treatment

Water Supply

  • hydrological cycle

  • groundwater supplies

  • surface water supplies

  • water transmission

    Treatment Methods

  • Coaggulation and flocculation

  • Settling

  • Filtration

  • Disinfection


Source of water 1

Source of Water - 1

  • Potential drinking water sources:

    • groundwater

    • surface water

  • Groundwater comes from underground aquifers, into which wells are bored to recover the water.

  • Water in some aquifers is non-renewable. There is therefore a risk of depleting the aquifer.

  • Also possibly for the city to sink as the water is withdrawn


Source of water 2

Source of Water - 2

  • Surface water is drawn from a lake or a river.

  • For major rivers (e.g. Mississippi River, Rhine, Danube), the water is used by many communities along the river.

  • Groundwater tends to be less contaminated than surface water because organic matter in the water has time to be decomposed by soil bacteria.

  • The soil itself acts as a filter so that less suspended matter is present in groundwater than in surface water.


Contamination of groundwater

Contamination of Groundwater

Inorganic contaminants:

  • The most important inorganic contaminant is nitrate ion, NO3-.

  • Excess nitrate ion in drinking water is a potential health hazard: risk of methemoglobinemia (or “blue-baby” syndrome).

  • Sources of nitrate in groundwater:

    • nitrogen fertilizers

    • atmospheric deposition

    • human sewage deposited in septic systems


Purification of drinking water

Purification of Drinking Water


Conventional water treatment

Water supply

Chemical additions

Flocculation

Removal of suspended materials and precipitation by gravity

Sedimentation

Removal of unsettled hardness precipitate and residual floc

Filtration

Chlorine, to establish residual 0.3 mg/L preventing bacterial growing

Disinfection

Drinking water

Flow diagram

Conventional water treatment


Purification of drinking water1

Purification of Drinking Water

Steps in a typical water treatment plan

  • Coagulation (settling and precipitation)

  • Hardness removal

  • Disinfection


Purification of drinking water2

Purification of Drinking Water


Coagulation 1

Coagulation - 1

  • Coagulation (or settling and precipitation)

  • The finest particles, such as colloidal minerals, bacteria, and pollen do not settle in the raw water.

  • Removal of this colloidal particles is necessary:

    • to give the finished water a clear appearance

    • because they contain viruses and bacteria that are resistant to later disinfection.


Coagulation 2

Coagulation - 2

  • The capture of the fine particles is done by adding to the water either iron(III) sulfate, Fe2(SO4)3, or aluminum sulfate, Al2(SO4)3.

  • In the case of aluminum sulfate, Al(OH)3 is formed (in the pH range 6-8):

  • At this pH values, Al(OH)3 is close to its minimum solubility and at equilibrium very little aluminum is left dissolved in the water.


Coagulation 3

Coagulation - 3

  • Aluminum hydroxide forms a very gelatinous precipitate, which settles very slowly and which incorporate the colloidal particles.

  • With iron(III) sulfate the chemistry is analogous:

    • Fe3+ forms gelatinous iron hydroxide Fe(OH)3.

  • These reactions consume hydroxide

    • pH decreases (neutralize alkaline water)


Hardness removal 1

Hardness Removal - 1

  • Hardness is characterized by the concentration of Ca2+ and Mg2+.

  • Major problem caused by hard water: formation of mineral deposits.

  • Calcium can be removed by addition of phosphate (see later).

  • A more common way is by precipitation and filtering of insoluble CaCO3


Hardness removal 2

Hardness Removal - 2

  • When the calcium is present primarily as “bicarbonate hardness” (intermediate pH), it can be removed by direct addition of Ca(OH)2 alone:

  • When bicarbonate ion is not present at substantial levels, a source of CO3- must be provided at a high pH to prevent conversion of most of the carbonate to bicarbonate.


Hardness removal 3

Hardness Removal - 3

  • Source of carbonate ion: sodium carbonate, Na2CO3.

  • The precipitation of magnesium as the hydroxide requires a higher pH than the precipitation of calcium as the carbonate

  • The high pH required may be provided by the basic carbonate ion from soda ash (Na2CO3)


Hardness removal 4

Hardness Removal - 4

  • There are two main problems:

    • Supersaturation effect: Some CaCO3 and Mg(OH)2 usually remain in solution. They need to be removed.

    • Use of highly basic sodium carbonate, which gives the product water an excessively high pH, up to pH 11.

  • Solution: water is recarbonated by bubbling CO2 into it.

    • The carbon dioxide converts the slightly soluble calcium carbonate and magnesium hydroxide to their soluble bicarbonate forms


Disinfection

Disinfection

  • Disinfection is the most essential part of water treatment

  • Disinfectant used in water treatment:

    • chlorine

    • chlorine dioxide

    • ozone

    • ultraviolet radiation


Disinfection chlorine 1

Disinfection/Chlorine - 1

  • Chlorine dissolves in water:

  • Aqueous chlorine rapidly hydrolyzes to form hypochlorous acid:

  • Hypochlorous acid is a weak acid that dissociates according to the reaction: Ka = 3×108 mol/ L


Disinfection chlorine 2

Disinfection/Chlorine - 2

Speciation of active chlorine as a function of pH


Disinfection chlorine 3

Disinfection/Chlorine - 3

  • Sometimes (e.g. swimming pools) hypochlorite salts, Ca(OCl)2, are substituted for chlorine gas as a disinfectant.

  • The hypochlorite salts are safer to handle than gaseous chlorine.

  • Sodium hypochlorite, NaOCl, can also be used as a substitute for chlorine.

  • The hypochlorite ions is then converted to hypochlorous acid:


Disinfection chlorine 4

Disinfection/Chlorine - 4

  • The two chemical species formed by chlorine in water, HOCl and OCl-, are known as free available chlorine.

  • Free available chlorine is very effective in killing bacteria (in particular HOCl).

  • HOCl(aq) about 10× more effective than ClO-(aq) –result of the more lipophilic HOCl crossing bacterial membranes more easily

  • water with pH > 7.5 requires more chlorine – or longer disinfection time – than water with pH < 7.5

  • In the presence of ammonia chloroamines are formed.

  • Alkaline pH will prevent the formation of these chloroamines.

  • The chloroamines are called combined available chlorine.

  • Breakpoint (Cl:N (wt/wt) =8:1)


Disinfection chlorine 5

Disinfection/Chlorine - 5

  • important terms:

  • chlorine dose = concentration originally used

  • chlorine residual = concentration in the finished water

  • chlorine demand = concentration consumed by oxidizable substances present in the water

  • free available chlorine = sum of concentrations of HOCl(aq) and ClO-(aq)

  • combined available chlorine = the concentration of chloroamines


Chlorine demand curve

Chlorine Demand Curve

applied chlorine

chlorine demand

breakpoint

combined

residual

free residual


Disinfection chlorine 6

Disinfection/Chlorine - 6

Problems with the use of chlorine as a disinfectant:

  • Simultaneous production of some toxic chlorinated organic compounds (e.g. chlorinated phenol).

  • Production of trihalomethanes (THMs), CHX3. Of particular concern is the formation of chloroform, CHCl3 (carcinogen, suspected to affect reproductive system)

  • chlorination byproducts, notably trihalomethanes.

  • Example: chloroform CHCl3 which is often present at 10ppb or more. Source is natural substances (humic acids)

  • >C(=O)CH3 + 3HOCl —> –CO2-+ CHCl3 + 2H2O


Disinfection chlorine dioxide 1

Disinfection/Chlorine Dioxide - 1

  • Chlorine dioxide, ClO2, is an effective water disinfectant

  • In the absence of impurity Cl2, it does not produce THMs in water treatment.

  • Chlorine dioxide oxidize organic molecules by extracting electrons from them.

  • Chlorine dioxide is a gas that is violently reactive with organic matter and explosive when exposed to light.


Disinfection chlorine dioxide 2

Disinfection/Chlorine Dioxide - 2

  • Chlorine dioxide is generated on-site, for example by the reaction of chlorine gas with solid sodium hypochlorite:

  • unstable, must be made in situ

  • an oxidizing agent, not a chlorinating agent — no taste and odour problems

  • no residual effect – rapidly decomposes – must add Cl2 afterward

  • Some concern has been raised over possible health effects of its main degradation byproducts, ClO2- and ClO3- (chlorate) ions.


Water pollution

Disinfection/Chlorine Dioxide - 3


Disinfection ozone 1

Disinfection/Ozone - 1

  • Ozone (stronger oxidizer than O2) is sometimes used as a disinfectant instead of chlorine, particularly in Europe.

  • unstable, must be made in situ – by electric discharge on dry O2 (air)

  • Process: air is filtered, cooled, dried, and pressurized, then subjected to an electrical discharge of approximately 20,000 volts.

  • 3O2—> 2O3 formed as a dilute mixture in air

  • The ozone produced is then pumped into a contact chamber where water contacts the ozone for 10-15 minutes.


Disinfection ozone 2

Disinfection/Ozone - 2

  • ozonation equipment is expensive, only economic on a large scale

  • an oxidizing agent, not a chlorinating agent – no taste and odour problems, but cannot be used like ClO2 as a temporary replacement for chlorine

  • no residual in the water, decomposition is pH dependent (also faster at higher water temperature)

  • rate  [OH-]0.55•[O3]2


Disinfection ozone 21

Disinfection/Ozone - 2


A typical ozone water treatment system

A typical ozone water-treatment system

air

ozone

generator

corona

discharge

air

dried

air

filter

water

desiccator

contact

chamber

pump

20,000 volts

cooled

air

oxygen

compressed air

purified water

refrigeration


Disinfection ozone 3

Disinfection/Ozone - 3

  • Interest in ozonation arises from the possible production of toxic organochlorine compounds by water chlorination.

  • ozone is more destructive to viruses than is chlorine.

  • Unfortunately, the solubility of ozone in water is relatively low, which limits its disinfective power.


Disinfection ultraviolet light 1

Disinfection/Ultraviolet light - 1

  • Ultraviolet radiation having wavelenghts below 300 nm is very damaging to life, including microorganisms by photochemical cross-linking of DNA, which absorbs strongly at this wavelength

  • Mercury lamps (germicidal lamps) are available, having their output radiation principally at 254 nm (UV-C at 254 nm – major output of a low pressure)

    Advantage of UV method:

  • Short contact time: 1-10 s. Ozone and chlorine both require contact of 10-50 minutes, therefore construction of a large reaction tank. UV disinfection can be run on a simple “flow-through” system (no holding tank)


Disinfection ultraviolet light 2

Disinfection/Ultraviolet light - 2

Advantage of UV method (cont’d):

  • Low installation costs. Ozone generators are complex and expensive to install; chlorine equipment is less so.

  • Not influenced by pH or temperature. Chlorination and ozonation work best at lower pH, chlorine because more of it is in the HOCl rather than the OCl- form, ozone because it decomposes more rapidly at high pH.

  • applicable to large and small scale installations, even domestic use

  • No toxic residues.

  • water must be clear and free of absorbing solutes


Disinfection ultraviolet light 3

Disinfection/Ultraviolet light - 3

  • Cost comparison between the various disinfectants:

• small installations: UV is cheapest, then chlorine

• large installations: chlorine is cheapest by a wide margin


The treatment of wastewater and sewage

The Treatment of Wastewater and Sewage


Pollutants in sewage 1

Pollutants in Sewage - 1

  • Typical municipal sewage contains oxygen-demanding materials, sediments, grease, oil, scum, pathogenic bacteria, viruses, salts, algal nutrients, pesticides, refractory organic compounds and heavy metals.

  • Major disposal problem with sewage: the sludge produced as a product of the sewage treatment process.


Pollutants in sewage 2

Pollutants in Sewage - 2


Sewage treatment 1

Sewage Treatment - 1

  • Three main categories:

  • primary treatment: primary settling, mechanical treatment

  • secondary treatment: biological treatment, include the related problem of disposal of sewage sludge

  • tertiary treatment: include advanced treatment.


Sewage treatment 2

Sewage Treatment - 2


Primary treatment 1

Primary Treatment - 1

  • Primary treatment of waste water consists of the removal of insoluble matter such as grit, grease, and scum from water.

    • First step: screening to remove or reduce the size of trash and large solids that get into the sewage system. The solids are collected on screens and scraped off for subsequent disposal.

    • Second step: Grit removal.


Primary treatment 2

Primary Treatment - 2

  • In the second step the sewage enters a large lagoon and moves through slowly enough that any solid particles settle out.

  • Some materials float at the surface of the sewage (Those materials are called grease). They are removed by a skimming device.

  • The effluent form the primary settler is almost clear, but has a high BOD (several hundred milligrams per liter).


Secondary treatment

Secondary Treatment

  • Objective of the secondary treatment: to reduce the BOD to acceptable level (below 100 mg/L).

  • The basic principle consists of the action of microorganisms provided with added oxygen degrading organic material in solution or in suspension.

  • Two main systems:

    • Trickling filter

    • Activated sludge


Trickling filter

Trickling Filter

  • The simplest biological waste treatment.

  • The trickling filter is a large round bed of sand and gravel.

  • A rotating boom sprinkle sprays the wastewater over rocks or other solid support material covered with microorganisms.

  • Main threat: presence of toxic substances, which would kill the microorganisms.

  • Disadvantage: require large space

  • Advantage: low energy consumption


Activated sludge reactor 1

Activated Sludge Reactor - 1

  • Very effective wastewater treatment.

  • Require less land and, being enclosed, can be maintained at the optimum temperatures for biological activity.

  • The reactor is a large tank in which the wastewater is agitated and aerated to provide the oxygen required by the microorganisms.

  • A portion of the sludge of microorganisms is removed from the exit stream of the reactor and recycled into the influent stream.


Activated sludge reactor 2

Activated Sludge Reactor - 2

  • Schematic of activated sludge reactor:


Activate sludge reactor 3

Activate Sludge Reactor - 3

  • BOD may be removed by:

    • oxidation of organic matter to provide energy for the metabolic process.

    • Synthesis, incorporation of the organic matter into cell mass.


Activated sludge reactor 4

Activated Sludge Reactor - 4

  • The water content in the sludge may be removed by some drying process and the resulting dewatered sludge may be incinerated or used as landfill.

  • To a certain extent, sewage sludge may be digested in the absence of oxygen by methane-producing anaerobic bacteria to produce methane and carbon dioxide

  • A well-designed plant may produce enough methane to provide for all of its power needs.


Tertiary treatment 1

Tertiary Treatment - 1

  • In some cases a portion of the drinking water is actually water that has been discharged from a municipal sewage treatment.

  • Tertiary waste treatment (also called advanced waste treatment): term used to describe a variety of processes performed on the effluent from the secondary waste treatment.


Tertiary treatment 2

Tertiary Treatment - 2

  • The contaminants removed by tertiary treatment fall into three general categories:

    • suspended solids: responsible for residual biological oxygen demand in secondary sewage effluent waters.

    • dissolved organic compounds: they are potentially the most toxic

    • dissolved inorganic materials: the major problem: nitrates and phosphates (nutrient for algae). Also, potentially hazardous toxic metals may be found among the dissolved inorganics.


Removal of solids

Removal of Solids

  • Some of the colloidal particles are removed using aluminum salt which forms Al(OH)3.

  • Similar to the process described during the purification of the drinking water.


Removal of dissolved organics

Removal of Dissolved Organics

  • The standard method for removal of dissolved organic material is by adsorption on activated carbon.

  • Activated carbon is characterized by a very large surface area.

  • The carbon is regenerated by heating it to 950C in a steam-air atmosphere.

  • Adsorbent synthetic polymers can also be used instead of activated carbon. They are regenerated by using solvent such as isopropanol and acetone.


Removal of dissolved inorganics

Removal of Dissolved Inorganics

  • The effluent of secondary waste treatment generally contains 300-400 mg/L more dissolved inorganic material than does the municipal water supply.

  • The most cost-effective methods of removing inorganic material from water is currently membrane processes.

  • Methods considered: reverse osmosis, electrodialysis, and ion exchange.


Reverse osmosis

Reverse Osmosis

  • Basic principle: force water through a semipermeable membrane that allows the passage of water but not other material.


Electrodialysis 1

Electrodialysis - 1

  • Basic principle: apply a direct current across a body of water separated into vertical layers by membranes alternately permeable to cations and anions.

  • Cations migrate toward the cathode and anions toward the anode.

  • Layers of water enriched in salts alternate with those from which salts have been removed.


Electrodialysis 2

Electrodialysis - 2


Ion exchange

Ion Exchange

  • Basic principle: passing the water successively over a solid cation exchanger and a solid anion exchanger, which replace cations and anions by hydrogen ion and hydroxide ion, respectively.

  • The cation exchanger is regenerated with strong acid and the anion exchanger with strong base.


Phosphorus removal 1

Phosphorus Removal - 1

  • Even when water is not destined for immediate reuse, the removal of the inorganic nutrients phosphorus and nitrogen is highly desirable to reduce eutrophication downstream.

  • Organic phosphorus is converted to orthophosphate (H3PO4, H2PO4-,HPO42-,PO43-).

  • The main sources of phosphate are polyphosphates in detergents, raw sewage, and the runoff from farms that used phosphate fertilizers.


Phosphorus removal 2

Phosphorus Removal - 2

  • Algae may grow at PO43- (phosphate ions) levels as low as 0.05 mg/L. Growth inhibition requires levels well below 0.5 mg/L.

  • Municipal wastes typically contain approximately 25 mg/L of phosphate (as orthophosphates, polyphosphates, and insoluble phosphate)

  • The efficiency of phosphate removal must be quite high to prevent algal growth.


Phosphorus removal 3

Phosphorus Removal - 3

  • This removal may occur in the sewage treatment process

    • in the primary settler

    • in the aeration chamber of the activated sludge unit. Activated sludge treatment removes about 20% of the phosphorus from sewage.

    • after secondary waste treatment.

  • Phosphorus is most commonly removed by precipitation, which are capable of at least 90-95% phosphorus removal at reasonable cost.


Phosphorus removal 4

Phosphorus Removal - 4

  • Some chemical precipitants and their products:

  • Lime, Ca(OH)2, is the most commonly chemical used for phosphorus removal

  • The products formed are insoluble calcium phosphate, such as Ca5OH(PO4)3 and Ca3(PO4)2.


Nitrogen removal 1

Nitrogen Removal - 1

  • Organic nitrogen is converted to ammonium ion and nitrate

  • Ammonia is the primary nitrogen product produced by most biological waste treatment processes.

  • One method is to strip ammonia in the form NH3 gas from the water by air.

  • Another method is nitrification followed by denitrification.


Nitrogen removal 2

Nitrogen Removal - 2

Nitrification - denitrification:

  • First step: conversion of ammonia and organic nitrogen to nitrate under strongly aerobic conditions:

    These reactions occur in the aeration tank of the activated sludge plant.


Nitrogen removal 3

Nitrogen Removal - 3

Nitrification-denitrification (cont’d)

  • Second step: reduction of nitrate to nitrogen gas.

    This reaction is also bacterially catalyzed and requires a carbon source and a reducing agent such as methanol, CH3OH:

  • In pilot plant operation, conversions of 95% of the ammonia to nitrate and 86% of the nitrate to nitrogen have been achieved.


Waste water treatment methods

Waste Water Treatment Methods

  • aeration to remove volatile solutes

  • precipitation of divalent cations

  • coagulation and flocculation

  • settling

  • filtration

  • disinfection


A typical municipal water treatment plant

first basin: insoluble

Mg2+ Ca2+

precipitate

A typical municipal water treatment plant

lime

coagulant

Raw water

second basin:

flocculation

aerator

filter

carbon

dioxide

chlorine

Clean water

sludge lagoon


Aeration

Aeration

Reasons for aeration systems:

  • to reduce [dissolved CO2]

  • to reduce [dissolved H2S]

  • to promote oxidation of Fe and Mg

  • to remove volatile organic compounds

    Types of aeration systems:

  • gravity aerators, eg. cascade of steps

  • spray aerators - require large land area


Coagulation and flocculation

Coagulation and Flocculation


Filtration

Filtration

  • Chemical Feed Injections - alum coagulant followed by polymeric flocculent

  • Depth Clarifier - flocs adhere to the sand as water passes through filter

  • Depth Filter - consists of three different media


Primary treatment

Primary Treatment

bar rack

primary clarifier

grit chamber

influent

treated effluent

waste grit

waste sludge


Secondary treatment remove the bod

Secondary Treatment(remove the BOD)


Tertiary treatment

Tertiary Treatment

  • Wastewater receiving tertiary treatment is unable to support microbial growth, and can be of such high quality that it can be pumped directly into the water supply.

  • The most popular means of removing BOD is with biological treatment.

  • Physiochemical processes are used to remove inorganic nutrients, especially phophate and nitrate.


Sources of sludge

Sources of Sludge

Two kinds of sludge are generated in a waste treatment plant: -

  • organic sludge from activated sludge, trickling filter, or rotating biological reactors.

  • inorganic sludge from the additional of chemicals such as those used for phosphorous removal.


Anaerobic sludge digestor

Anaerobic Sludge Digestor

gas outlet

methane

scum layer

scum removal

supernatent

sludge inlet

supernatent

removal

supernatent

activated digesting

sludge

stabilized

sludge

sludge outlet


Sludge thickening

Sludge Thickening

99%

water

80% of water

removed

100%

1% solid

5% solid


Ultimate disposal

Ultimate Disposal

  • Some alternatives for ultimate disposal of sludge have included ocean dumping, land spreading, and incineration.

  • Since 1992, there has been legislation prohibiting ocean dumping of sewage sludge.

  • Accumulation of heavy metals is of concern when sewage sludge is used on cropland. Prior control of heavy metal contamination from industrial sources enables sludge to be used more extensively.

  • Incineration is an expensive alternative to sludge disposal, and there is no benefit from its potential use as a fertilizer.


Water pollution

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