Lecture 20 Water Treatment and Drinking Water Quality · 1 GG4/625 Wk 11 L20-21, S'18 Lecture 20 Water Treatment and Drinking Water Quality Please read Manahan Chapter 7 Today –

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Lecture 20

Water Treatment and Drinking

Water QualityPlease read Manahan Chapter 7

Today –

1. Water Quality Standards

2. Water Treatment - input water

Next time –

3. Water Treatment - wastewater

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Drinking Water QualityWater quality standards vary around the world but are based upon the same criteria:

What is an "acceptable" level of pollutant or natural constituent in the water based upon

toxicity and what are the costs to remove it. There are many related issues.

► Chemical - what are concentrations of dissolved solutes

► Environmental Science - where did they come from? How much will it cost to remediate?

► Biology - what is the LC50 of a chemical and at what concentrations do non-lethal effects show up.

► Politics/Social Science - what are "acceptable risks" to different individuals and groups within an ecosystem. Balancing risks with remediation costs are an important component of effective water quality standards.

some compounds present in a water supply that are known to be a problem

may not be removed if the cost to remove them is deemed too high and the number of people or other organisms affected by it is below some threshold.

$

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Examples of US drinking water standards:

This is a subset of those that a municipality might track in their water supply, particularly if contaminants besides these "common" ones occur in that area.

Typically, contaminants will be monitored at both the source and within the distribution system.

Concentration limits vary for different

pollutants because each has different environmental reactivity and toxicity.

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MCL = maximum allowable concentration level.

Excerpted from the latest guidelines....There are also unregulated standards (i.e., optional) that include contaminants that cause cosmetic or aesthetic affects but that are not known to be pollutants.

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Unregulated Drinking Water Contaminants

(by the EPA): Secondary Drinking Water Standards

• National Secondary Drinking Water Regulations are non-enforceable guidelines regulating contaminants in drinking water that may cause cosmetic effects

– e.g. skin or tooth discoloration.

• or aesthetic effects

– e.g., taste, odor, or color.

• EPA recommends secondary standards to water systems but does not require systems to comply.

• States may choose to adopt them as enforceable standards.

• MTBE (gasoline additive), Dieldrin (found in some Oahu wells), and perchlorates are included in this list.

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In general water quality on Oahu is very good - meaning that very limited treatment is required for may sources. But water from parts of the island do contain multiple contaminants that require removal.

HOW ABOUT DRINKING WATER ON OAHU?

http://www.hbws.org/ea_wat_resource/ea01_ct01_mainpage.htm

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Contaminants in Oahu Water:visit

http://www.boardofwatersupply.com/cssweb/display.cfm?sid=1081

for info about water quality.

• Herbicides & Pesticides:

Atrazine, Dibromochloropropane, 1,2-Dichloropropane, Dieldrin, Ethylene dibromide, 1,2,3-Trichloropropane

• Solvents:

Carbon tetrachloride , Isophorone, Tetrachloroethylene, Trichloroethylene

• Inorganics:

Barium, Bromide, Chromium, Copper, Fluorides, Nitrate

• Others:

α & β activity, Coliform bacteria, Radon, Trihalomethanes

typically isolated findings:

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Water Treatment:

We have spent much time this semester discussing how natural waters acquire their compositions and how they become contaminated by human activities.

water purification is essentially the opposite, using many of the same types of processes to effect the reversals

Water treatment is a complex combination of processes used to achieve specific desired results.

� chemical

� physical

� biological

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Primary functions of water treatment:

1. Input water treatment - water to be used for human consumption, irrigation or industrial uses.

� drinking water needs to be clean primarily of known toxins and secondarily of other attributes.

� industrial water, needs to have reduced TDS and hardness as the primary concern.

2. Output (waste) water treatment - water to be re-introduced into the environment from the above sources and from other municipal wastes (sewage, other wastewater).

♻ In some cases, output water is re-used as input water, requiring extra precautions to ensure that it is safe for its second input use.

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Techniques for input and output water treatment:

This week we discuss chemical aspects of water treatment to understand something about how they work, their natural analogues, and their potential drawbacks.

Keep in mind that:� not all processes are needed in all cases� the order in which treatment steps are used can be

varied to return a desired result�many of the chemical treatments have alternatives

that can be applied depending on the conditions of the water.

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Physical and chemical specifics of water treatment:

Physical treatment is basically solids removal.

required for:

• most Output water and • many Input water sources

�e.g., surface water can have high levels of suspended solids

�ground waters have much less.

Chemical treatment depends on the water composition.

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chemical specifics of water treatment:

Input water needs to have ~ neutral pH and have relatively low TDS.

Also removed are:

� hardness components (dissolved Ca, Mg, Al salts, and silica).

� volatile solutes such as H2S, CO2, bacterial DOC� reduced metals (Fe+2, Mn+2)� living organisms (bacteria, viruses)� other inorganic constituents such as heavy metals�DOC (e.g., natural and pollutant)

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chemical specifics of water treatment:

Output water almost always contains far higher concentrations of problematic chemicals and/or microbes.

Contaminants are industry-specific in some wastewaters.

Before environmental release Urban wastewater (sewage) and Industrial wastewater needs to have acceptable levels of:

� pH and pE:� living pathogens organisms (bacteria, viruses)� DOC and nutrients to an acceptable BOD� toxic heavy metal and organic compounds� chelated and sorbed heavy metals

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Input water treatment - Overview:This is a schematic of a water treatment plant. The steps involve:

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Loss of volatiles (H2S, CO2) and addition of oxygen. Can oxidize Fe(II) to Fe(III) & precipitate HFO (hydrous ferric oxides). These have very high surface area and effectively scavenge many ionic species (e.g., arsenate & phosphate).

Aeration

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CaO makes water very basic so that calcium carbonate can precipitate and settle out.

Lime Addition

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Addition of coagulant (salts of Al or Fe, sometimes activated silica or organic coagulants).

Enhances particle settling or flotation.

Flocculation

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Separation pond

Low density floc

Purified water

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Used to adjust pH downward from high values associated with lime addition.

Acidification

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Further particle settling. If the solubility of particles is pH dependent, the timing of this relative to CO2 treatment may vary.

Particle removal

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Removal of DOC and “other” dissolved inorganics (this

step does not appear in this schematic), but can include activated C for DOC removal, and ion exchange/electrodialysis/ reverse osmosis to removal soluble ions (such as Na+, Cl-, sulfate)

Other chemical

purification

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Disinfection: treatment with an oxidant - an important drinking water treatment on Oahu.

Disinfection

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Input water treatment -

Removal of dissolved inorganic components.

Low inorganic TDS is important in all water treatment, although the target composition of a specific water depends upon its intended use.

Reduction of hardness components is important for all input

water

Ca and Mg removal are a prime concern because …

precipitation minerals can:

� clog pipes and boilers� reduce the efficiency of soaps and detergents.

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Input water treatment -Removal of dissolved inorganic components.

Ca(OH)2, lime, is added to raise the pH, convert some HCO3

- to CO32, and thereby precipitate CaCO3.

Na2CO3, soda-ash, is also added to the mixture when the water does not contain enough HCO3

- to remove the Ca2+

(e.g., if water hardness is from CaSO4, as is common in surface water of arid regions).

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Input water treatment -

Removal of dissolved inorganic components.

Some Mg2+ may be removed with CaCO3, but problematic Mg levels are more effectively attacked by Mg(OH)2 removal at fairly basic conditions, which are usually attained by Ca(OH)2 and Na2CO3

addition.

A second benefit of Ca(OH)2 and Mg(OH)2 removal is that some heavy metals (e.g., Pb) are also coprecipitated as hydroxides or carbonates at high pH (“cementation”).

Some treatment plants remove CaCO3 in a separate settling tank so that it may be dried, converted to CaO in a furnace, and reacted with H2O to regenerate Ca(OH)2.

reacidificationAfter Ca and Mg are removed at basic pH, the water is reacidified by aeration with CO2. CO2 + OH- ⇆ HCO3

-

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Input water treatment - Disinfection.

Groundwater, although filtered by natural processes, is often susceptible to microbial contamination, requiring disinfection.

Drinking water disinfection has played a critical role in

improving public health.

We will discuss disinfection methods next lecture. Chlorine is commonly used because it is effective, inexpensive, and relatively safe.

Chlorine was adopted as a disinfectant by most water treatment plants in the United States and Canada After its initial introduction in 1908 in New Jersey. More than 200 million Americans and Canadians receive chlorine-disinfected drinking water every day. http://www.waterandhealth.org/drinkingwater/groundwater.html

1900 2000

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Input water treatment -

Removal of dissolved inorganic components.

ion exchange is an effective means toremove the many other inorganic solids can

not be precipitated by altering the pH or

Flocculated (i.e., salts that are soluble in bothbasic and acidic media, such as Na+ and Cl-).

Synthetic cation and anion exchangers are typically employed to remove soluble ions.

� polymeric resins are most common� synthetic analogues to zeolite minerals are also used

These materials have CEC and ECS defined in the same way as natural solids such as clays, but behave in more predictably. This makes them easier to optimize for given conditions:

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� Some resins remove cations and replace them with H+ ions.

� Some resins remove anions and replacie them with OH- ions

If Na+ and/or Cl-

concentrations in a water are low, the "Na" and "Cl" forms of the cation and anion resins, respectively, can be used instead of the hydrogen and hydroxide forms.

In exchange: electrical neutrality is maintained by desorption of a positive or negative ion from the resin as an ion is removed from aqueous solution.

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Strong acid and strong base resins work at extreme pH conditions. Weak acid and base resins work nearer to neutral pH. Each has advantages for certain applications (see Chapter 8).

A schematic of an ion-exchange water treatment set up.

Notice that through use of different reagents and valving, the resin can be backwashed and regenerated for reuse.

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Naturally, the attainment of chemical equilibrium on these exchangers is a function of flow rate.

There is a threshold above which adsorption drops below ideal.

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Ion exchange:

� A relatively expensive water purification method.� Typically used in conjunction with other techniques.� Commonly employed to reduce the concentration of Ca2+, Mg2+,

K+, NH4+, SO4

2-, and some trace metal ions.

Note the reduction in TDS, but also the preservation (with a time lag) of some of the peaks and valleys in the feed water composition with time in the output of an ion-exchange wastewater treatment system

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Input water treatment -Removal of Fe, Mn and other reduced inorganic solutes:

This is an issue particularly for low pe groundwater being used as drinking water or certain industrial effluents used in the chemical and electroplating industries.

Reduced metals are typically oxidized at high pH and then precipitated as hydroxides (possibly along with Ca and Mg). Oxidation is usually done with:

� MnO2 (which slowly catalyzes the Mn+2 to Mn+4 transition on its particle surfaces).

� KMnO4.� Chlorine gas (which reacts with water to make oxidative

HOCl, plus H+ and Cl-) . Chlorine is particularly useful in cases of high DOC because it destroys DOC-Fe2+ chelates to allow easier Fe3+ oxidization.

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Electrodialysis removing soluble inorganic ions by preferentially "forcing" ions or water molecules through membranes:

Electrodialysis

works by applying a potential field to water masses that are separated by anion or cation permeable membranes. Ions migrate towards the regions of opposite charge along the imposed electric potential gradient, leaving purified water behind.

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Reverse osmosis uses pressure to force water through an ion-impermeable membrane.

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Boiler water treatment includes chemicals used for the following applications:

• oxygen scavenging

• alkalinity control

• inhibition of scale (i.e., precipitation of CaCO3 , CaSO4)

• inhibition of corrosion (i.e., dissolution of metals – one way to prevent corrosion is to use oxidizing anions, such as chromate, nitrite and nitrate, or non-oxidizing ions such as phosphate and molybdate.

• antifoaming (chemically inert materials used to inhibit bubble formation, such as dissolved silica)

Input water treatment – considerations for industrial water

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Lecture 21

Water Treatment -WastewaterPlease read Manahan Chapter 7

http://www.waterandwastewater.com/

http://www.waterencyclopedia.com/

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� primary: removes sediment, suspended solids such as grease; flocculants are added to promote solids formation from colloidal suspension

� secondary: removes organic matter, reduces BOD

� tertiary: removes other constituents as necessary to make the water suitable for human consumption

Output water treatment - Overview:

Treatment of sewage (municipal) wastewater falls into three categories:

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Output water treatment - Overview:

All output water treatment schemes are designed to clean water for release into the environment or for re-use (i.e., recycling).

The specifics depend on:

• where the water will go after release

• how the water will be reused

All output water should be disinfected, relatively low in BOD and nutrients, and free of known toxins.

R0 Process: High Quality Water for Industry

http://www.hbws.org/WaterRecycle/how_recycled_water_works.htm

An example of a water reuse treatment schematic using reverse osmosis to reduce tds of a secondary effluent

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R-1 Process: High Quality Water for Irrigation

http://www.hbws.org/WaterRecycle/how_recycled_water_works.htm

This water reuse treatment schematic reduces floccable POC and metals, and includes disinfection so that the water can be safely used for irrigation.

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Output water treatment - Overview:

Secondary treatment facilities have been required since the mid 1980s large US municipalities have been required to have.

Lawsuits: After that some cities (e.g., San Diego, Honolulu) spent a great deal of money to argue that secondary treatment wasn't necessary for them. • Advanced primary effluents continued to be put into the

environment for decades.

Accidental release of primary effluents into near shore environments are still a real threat for humans swimming in and consuming organisms from many coastal waters: • leaky wastewater transfer pipes, • accidents at the sewage treatment plants, • sewage system influx of storm runoff during heavy rain

events

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Coastal Sewage Outfalls:

Most coastal cities pipe sewage off-shore into waters below the marine thermocline.

High BOD of the waste stream will cause localized anoxia or hypoxia.

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Coastal Sewage Outfalls:

High BOD can be detrimental to marine fauna near the outfall and the regional environment (particularly continental shelf areas with poor circulation).

The plot at left illustrates the decline of a normal benthic species and the rise of a low oxygen tolerant species near an outfall.

http://www.marlin.ac.uk/species/

Abra alba is a bivalve mollusk inhabitant of inshore muddy fine sand or mud substrates.

Nucula nitidosa is a primitive bivalve mollusk lacking typical bivalve gills, and instead feeds on a variety of organic and inorganic matter, and microbes including microzooplankton, bacteria and fungi.

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Trace metals associated with sewage outfalls can also be problematic

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Diffuser Cap: Initial dilution of the effluent from the new diffusers is about 1 part treated effluent to 100 parts seawater.

Coastal Sewage Outfalls:

The impacts of sewage outfalls can be reduced in 2 ways:

a. Reduction of BOD

b. Dilution of effluent stream

http://www.mwra.state.ma.us/harbor/html/outfall_update.htm

Outfalls into Boston Harbor were recently improved from 14:1 dilution to about 100:1, significantly reducing the impact of nutrients(which are not specifically targeted for removal by secondary treatment).

The bay discharge is through a diffuser, which consists of more than fifty pipes that rise to the seabed over the last 6,600 feet of the tunnel's length. Each pipe connects to a diffuser cap which splits the flow into several streams, each issuing from a small port. More than 400 diffuser ports disperse the effluent into the 100-foot-deep waters in the Bay, where it is diluted in large volumes of seawater.

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Output water treatment - Solids removal

Typical processes include siltation and filtration.

Bar Screen - Incoming wastewater (raw sewage) passes through mechanically-cleaned bar screens to remove rags, sticks and large objects.

Grit Removal - Detrition and grit cyclones are used to remove sand, gravel and debris.

http://www.lincolncity.org/CityDepartments/PublicWorks/WasteWaterTreatmentCollections/

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Output water treatment - Solids removal by Flocculation.

Many types of flocculants or floccable salts can be added to the water at various stages to remove other dissolved or colloidal constituents.

Alum, Al2(SO4)3 and Ferric iron sulfate, Fe2(SO4)3 form hexahydrated metal complexes in water that can react with a bicarbonate ion to create metal hydroxides [Al(OH)3 and Fe(OH)3] and CO2.

An additional coagulant, Hydrated ferrous iron sulfate, FeSO4 (also known as copperas), is sometimes used. This becomes oxidized to Fe(III) by oxygen in the water.

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Flocculation: All 3 of these metal flocculants can remove other heavy metals(as we have discussed previously for estuaries) and are also very effective in destroying viral proteins and removing viruses from the water.

Virus removal efficiency is species specific.

Adsorption is fast but pH dependent

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Sludge contains high organic and nutrient content, which is commonly

treated with biological agents to reduce their concentrations.

Aerobic respiration is a key component of (secondary) wastewater treatment. It consumes BOD and destroys some (not all) unwanted microorganisms.

SludgeSludgeSludgeSludgeSolids removed during sewage treatment are known as sludge.

Primary treatment amounts to settling/sedimentation of Sludge .

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Aerobic sludgesludgesludgesludge degradation

This treatment is analogous to

natural DOC/POC degradation in the hydrosphere…

although at much higher

microorganism populations and using tuned micro-organism assemblages.

The process forms new cells, resulting in a substantial increase

of sewage solids that must be dried and disposed of.

Activated sludge treatment

involves constant re-introduction of some of the sludge into a

decomposition tank that is aerated with O2, to produce

● low DOC and nutrient effluent ● high POC waste sludge.

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All wastewater solids are de-watered

Sludge is then disposed of in one way or another (e.g.,

incineration, burial, redistribution onto the landscape) depending on what it contains.

Sludge dehydration pondSludge Digester - Aerobic digestion

stabilizes the excess activated sludge, reducing the amount of volatile solids and harmful pathogens.

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● High heavy metal content (e.g., up to 0.9% Zn, 0.6% Cu)

● Dangerous microorganisms

● toxic organics

limit the extent to which Sludge…

(an otherwise ideal fertilizer, with high DOC and high nutrients)

can be applied into agricultural lands.

Undesireable attributes of wastewater Sludge

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Nutrient element removal.

This is one of the most important for the treatments of output water.

Biological and chemical techniques are used to convert these elements to solid forms or, for some of the nitrogen, to gases like N2

Greater emphasis is placed on P removal since this is commonly

a limiting nutrient in the freshwater hydrosphere.

Releases of excessive P in wastewater into the environment can

cause eutrophication.

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Nutrient element removal.

P removal

A threshold of ≤ 0.5 mg P/L is desirable to prevent over fertilization upon reintroduction into the environment.

20 to 90% of the phosphorous is removed to:

� sludge by biological processes

� the precipitation of metal phosphate salts during the high pH

phase of treatment.

Biological precipitation is difficult: the right combination of chemical conditions must exist for biological transformation of P to insoluble rather than soluble forms.

P solubility is pH dependent; Ca, Zn, Al and Fe phosphates only precipitate at pH > 8.5

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Some plants employ chemical precipitation during lime treatment to lower P. Hydroxy-apatite solid, Ca5OH(PO4)3, is formed.

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Al is traditionally added to improve P removal.

La+3 is an even more effective at the P removal but cost considerations usually limit this application.

It's effectiveness relative to Al is also a function of metal/P ratio.

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Free nitrogen as ammonium ions can be removed during lime treatment and aeration by high temperature conversion to ammonia.

This is not always desirable in populated areas since NH3 is a respiratory irritant gas

Basic N-cycle concepts are also applied to decrease fixed N content of waste waters (see table 8.2 on next slide).

Nitrogen is also partially removed as N2 gas during biomass formation in the biological treatment process (e.g., by denitrifying bacteria from NO3

-; see fig 11.12 on the slide after that).

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Nutrients are not remineralized in Redfield Ratio proportions

during aerobic sludge treatment, but nutrients in effluents can

then be further reduced by biomass (algal) accumulation.

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Removal of DOC from (input and) output water:

� Input water DOC varies widely in composition but it is typically relatively low in concentration. � It is mostly composed of natural compounds.� But, pollutant chemicals can also make their way into the water supply.

Removal:Most simple organic components in input water (especially drinking water) are removed by sorption onto :� activated charcoal� compound-specific synthetic resins (especially in contaminated sources)

More complicated molecules (e.g., humates) are decomposed during oxidative removal of Fe and Mn.

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Removal of DOC from output water:

DOC concentration in sewage wastewater is much higher and consists of two components:

1. those that entered with the waste water stream 2. those produced within the sludge reactor(s)

Biological treatment of sewage wastewater is sometimes used to convert DOC and remaining N and P into biomass using Redfield ratio algae (see above).

To use this water for drinking, it would require ternary treatment and would be subjected to the same processes as input water.

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More Removal of DOC from input and output water:

A second treatment for some industrial and sewage output waters involves "natural" degradation by bacteria in soils. Wastewaters are applied to soils containing microorganisms that are acclimated to digesting certain types of compounds (see Chapter 8 and parts of Chapter 20 of the text).

http://www.unep.or.jp/ietc/Publications/Freshwater/FMS1/2.asp

Biosolid slurry (left); preparing the land for the incorporation of biosolids on the soil (right)

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Removal of biological agents (disinfection)

Again, input and output waters have different constraints and

compositions.

Every disinfection technique has its specific advantages and its own application area. Some of the advantages and disadvantages are shown in the table below:

Technology Environmentally

friendly

By-products Effectivity Invest-

ment

Operational

costs

Fluids Surfaces

Ozone + + ++ - + ++ ++

UV ++ ++ + +/- ++ + ++

Chlorine

dioxide+/- +/- ++ ++ + ++ --

Chlorine gas -- -- - + ++ +/- --

Hypochlorite -- -- - + ++ +/- --

http://www.lenntech.com/water-treatment-chemicals.htm

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Removal of biological agents (disinfection)

Input water

Drinking water standards vary around the world on this aspect but within the US, biological components are very stringently diminished during potable water treatment.

1. Besides viruses (which are removed by Al flocculation), most living organisms in input water are destroyed using OCl-

("hypochlorous acid anion") introduced from the reaction of Cl2 and H2O (as discussed previously) or from direct addition of Ca(OCl)2.

This works by oxidative destruction of cellular matter.

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More disinfection

OCl- reacts with available NH4+ to produce NH2Cl, NHCl2 and

NCl3 (see the text), thereby minimizing the "free available chlorine" in the form of OCl-.

Chlorine is added in proportion to ammonium to exceed the "breakpoint", at which there is an acceptable "free available chlorine" in the water (something like Cl:N of 8:1).

A drawback of water chlorination using hypochlorous acid anion is the production of toxic low molecular weight chlorohydrocarbons (such as the partial chloromethanes CH2Cl2 and CHCl3). These are not easy to remove with

activated charcoal or resins.

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More disinfection

2. Chlorine dioxide, Cl2O can be used instead since it does not form as many chlorohydrocarbons (aka "halocarbons") during the process, nor does it react with ammonium ions.

Unfortunately, Cl2O is very reactive (it oxidizes explosively in the presence of light and O2), so it is not as safe to work with.

Cl2O can be produced on-site by the reaction of NaClO2 and Cl2, as described in the reading. The potential negative health effects of byproduct ClO2

- and ClO3- have not been

thoroughly investigated.

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More disinfection3. Ozone, O3, that is typically produced on-site by electrical discharge into high pressure O2 vessels is another disinfectant.

O3 is a better disinfectant that chlorine and does not produce halocarbons, but it is expensive and fairly inefficient because O3

will also spontaneously decompose back to O2 soon after production.

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Sewage wastewater disinfection

This is typically not as rigorous as for input water.

Most pathogen and non pathogen bacteria and viruses are transferred to sludge biomass during settling and then treated by anaerobic digestion or basification to pH > 12

Sewage output water may also be chlorinated before reintroduction if pathogen content is still high.

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Sewage wastewater disinfection by UV light

Images from ITT Wedeco Tak45 system