Gases improve drinkingwater quality

Water is the most important and themost strictly controlled foodstuff

The implementation of the Euro-pean Council directive 98/83/ECin the national legislation ofmember states, is again invokingnew or lower limit values for un-desirable substances containedin the raw water, e. g. specificheavy metals or organic halogens.

Also, water suppliers have tomake sure that drinking watercomplies to these limit values upto the tap (Figure 1).

This means that for exampleplumbosolvency in privatehousehold installations must beprevented by appropriatelyadjusted water chemistry.

Technical gases provide crucialcontributions to technologiesachieving this (Figure 2): as anatural component of healthydrinking water, they can be usedas environmental friendly agentsleaving no undesirable by-products or contamination andimproving economics in watertreatment. This paper presentsthe most important applicationsof gases in drinking waterprocessing.

pH control is crucial in water treatmentA good quality drinking water should be neithercorrosive nor scaling. To this end, the pH-value ofthe water has to be in balance with the degree ofhardness (Figure 3).

Hardness is a natural property of water and iscaused mainly by calcium and magnesium ions.While a certain degree of hardness is healthy andto some extent necessary for corrosion protec-tion, a high calcium-hardness is inconvenient toall users. Hard tap water requires e.g. frequentdescaling of all warm water equipment and waterheaters in household equipment and alsoincreases the usage of soaps and surfactants inwashing and cleaning. Therefore, water with amiddle hardness level is generally regarded asideally suited for drinking water.

Figure 1: Highest standards apply for drinking water, our mostimportant foodstuff. Gases help at achieving those.

Many national recommendations contain a lowerlimit value for hardness. Soft water with lowerhardness is always mineralized, to prevent corro-sion in pipelines. However, an ever increasingnumber of water works also perform softening ofhard and very hard raw waters in order to pro-duce tap water that is ideally suited for allhousehold uses.

Most softening is done by decarbonization influidized bed reactors.

At reactor entrance, lime or sodium hydroxide isinjected into the water to increase the pH-value(Figure 4). As a result, dissolved calcium hard-ness is converted to insoluble hardness, whichprecipitates on the fluidized sand particles. Thesereactions also reverse the initial pH increase.Often, however, residual hardness and pH arenot yet balanced at the reactor exit. Then calcium-carbonate precipitation can continue outside thereactor.

Figure 2: Gases are used in many processes for drinking water production

Figure 3: Influence of pH-value on drinking water quality Figure 4: Softening and mineralization of drinking waterwith lime and carbon dioxide

corrosion

equilibrium curve equilibrium curve

1. mechanical deacidification2. lime or caustic soda addition

scaling 3. softening in reactor4. pH control5. CO2 addition6. lime addition

softening

mineralization

calcium hardnesscalcium hardness

river

bank filtration

ozone

filtration

activatedcarbon

oxidationand

filtration

lime

CO2

CO2

supply area

O2

ultra-filtration

CO2

reverseosmosis

oxidationand

filtration

partialdesalination

CARIX®

     CO2

regeneration

CO2

filtrationdecarbonization

flocculator

lime

CO2

O2

         mineralization

setting pH value

dam groundwater

desired hardness

This results in scaling of downstream pipelinesand valves and shortened runtimes from down-stream filters. To prevent this, pH control withacid is required. For this application the use ofcarbonic acid is advantageous. Carbonic acid isformed when carbon dioxide is introduced intowater, where an equilibrium is reached betweenphysically dissolved carbon dioxide and theproducts carbonic acid, hydrogen-carbonate andcarbonate:

All these forms of carbonic acid are naturalcomponents of water and do not change thequality of drinking water.

Messer installs CO2-dosing and injection equip-ments, which dissolve CO2 shortly before or inthe exit of the fluidized bed reactor. With this,further precipitation is stopped immediatelywhich protects downstream equipment againstscaling (Figure 5).

Carbon dioxide is the acid of choiceIn drinking water processing, carbon dioxide isthe acid of choice as it has many advantages overmineral acids:

Salting up of water is prevented as sulfate andchloride concentrations are not increased. Thisis important for the corrosion-chemistry of thewater

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Carbon dioxide is more economic than mineralacids in drinking water quality

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Storage and handling of carbon dioxide issimple and safe and causes no corrosion ofnearby located equipment

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Carbon dioxide gives more precise pH controlat less investment (Figure 6).

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Figure 6 shows a schematic neutralization curvewith the weak acid carbon dioxide, compared tothat with a strong mineral acid.

Figure 5: Rapid decarbonization in a fluidized bed with carbon dioxide injection at reactor exit

Figure 6: Comparison of neutralization curves with carbondioxide and mineral acids

allowed range

CO2-dosage

Phase I Phase II Phase III

mineral acid

The flatter curve form with carbon dioxideshows, that also near to neutrality, carbon dioxideinjection only results in small pH changes at atime, which practically excludes any over-acidification. Therefore, carbon dioxide does notrequire an elaborate control technique. Also,continuous dosing of small and varying amountsof acid is easier with a gas than with a liquid. Thisis particularly important when pH control is per-formed in a pipeline with plug flow characteristic.

Mineralization and remineralization:the classic carbon dioxide applicationRaw water from dams or from wells in granite,sandstone or basalt areas can be very soft.Hardness below 0.5 mmol/l is not unknown. Alsothe steadily increasing amounts of drinking waterdesalinated by reverse osmosis or distillation arecharacterized by very low alkalinity, and there-fore, without further treatment, they would bevery aggressive. To prevent corrosion of pipelinesand equipment, a mineralization step is requiredto achieve a hardness and buffer capacity of atleast 0.5 mmol/l. As often drinking waters frommore than one source enter a distribution net-work, hardness is generally set at values be-tween 0.7 – 1.4 mmol/l. Also higher values occur.

The most economic process for mineralization isthe dissolution of lime with the help of balanced

amounts of carbon dioxide. Carbon dioxideassures that all of the added lime reacts to formthe soluble calcium-bicarbonate (Figure 4).

Apart from softening, pH control with carbondioxide is also advantageous in other treatmentprocesses found in water works:

In nano-filtration or reversed-osmosis withmembranes, acidification of feed water withcarbon dioxide prevents scaling of membranes– even for very hard raw waters with hardnessof e.g. 7.5 mmol/l calcium – thereby maintain-ing constant productivity. As carbon dioxidepermeates even reverse osmosis membranes,the treated water in the product stream alreadycontains most of the carbon dioxide requiredfor remineralization.

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pH control with carbon dioxide is also used atflocculation stages. Raw water from rivers anddams is often flocculated to remove small andcolloidal particles before further treatment. Inwarm, southern countries, the pH of surfacewaters is sometimes observed to rise above avalue of 9 during the summer, which is a resultof algal growth. Addition of aluminate as aflocculant would then result in an undesirable,partial dissolution of aluminum into the water.Therefore, during these periods, pH controlwith carbon dioxide is used to prevent alumi-nium dissolution and to optimize flocculation.

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Partial desalination with carbon dioxideA combination of elevated hardness, high nitratecontent or high sulfate or chloride concentrationsin raw water asks for treatment with the Carix®*process. The Carix®* process (Figure 7) is basedon the combined use of a weak acid cationexchanger (adsorbing hardness) and an anionexchanger (adsorbing nitrate, chloride andsulfate). Both exchangers are applied together inone mixed bed. When both ion-exchangers areloaded they are regenerated jointly with carbondioxide (Figure 8).

Favorable characteristics of the Carix®* processare:

Regeneration of the flushing water only needscarbon dioxide and no additional salts. Thismeans salting up is prevented and only thosesalts separated from the raw water aredischarged. Most Carix® plants are allowed todischarge their back-flushing water to surfacewater.

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Hardness, sulfate, chloride and nitrate contentare reduced to the desired levels in one singleprocess step. This makes the process simpleand economic.

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The bulk of carbon dioxide is recycled duringthe process, which improves economics of theprocess even more.

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Last but not least, partial desalination withCarix® is beneficial to corrosion index (Larsonindex), as not only bicarbonate is reduced (as inlime-decarbonisation) but also sulfate andchloride. Depending on the composition of theraw water, the ratio between anion exchangerand cation exchanger even can be set at such alevel, that the emphasis is shifted fromsoftening to anion removal.

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Figure 7: Schematic representation of the Carix® process*

Figure 8: Carix® plant with a capacity of 3000 m3/day *Carix® is a registered trademark of VA TECH WABAG

raw water drinking water

flushingwater

CO2+H2O

softening, partial desalination, nitrate removal

regeneration

Oxidation with oxygenOxidation reactions are used in several purifi-cation steps. The separation of iron and manga-nese certainly is the most widespread. Waterworks using groundwater mostly have to removeiron and manganese in order to prevent incrus-tation in distribution pipelines. As groundwater isoxygen depleted, it contains iron and manganesein a reduced, soluble form. After enrichment ofgroundwater with oxygen, iron(II) oxidizes veryfast to iron(III)-oxide particles, which are heldback on the filters. Under appropriate (chemical)conditions also oxidation of manganese andretention of manganese(IV)-oxide is performed inthese filters.

From a stoichiometric point of view, oxidation ofiron and manganese requires only small amountsof oxygen. Therefore, the required oxygen enrich-ment could be performed with air, but the use ofMesser’s Oxysolv® process with pure oxygen ismore economic and has many advantages:

Use of oxygen instead of air often leads to adistinct increase in filter throughput betweenback-flushing cycles. This minimizes water lossin back-flushing and also reduces costs fortreatment or discharge of back-flushing water.Aeration often implies an oversaturation of thewater with nitrogen, especially when pressureaeration is applied. During operation, the pres-sure drop in the filters causes degassing ofnitrogen, which accumulates in the filter bed asmicro-bubbles. The accumulated micro-bubblesblock the filter so that early back-flushing isrequired. Operation with pure oxygen insteadof air avoids this and achieves longer filter run-times.

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Oxygen avoids "white water" at the tap. Withintense aeration, nitrogen also degasses in thedistribution system and at the tap. The custo-mer observes this as so-called white water.

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With pure oxygen, oxygen concentrations of 20mg/l and more are easily attained. This is im-portant when raw water also contains ammo-nia, methane and hydrogen sulfide, as oxidationof these compounds requires much moreoxygen, by comparison.

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Oxygen is a clean, hygienic and good qualityingredient. The use of oxygen preventshygienic or sensory (e. g. odor) problems.

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Loss of carbon dioxide from soft water isprevented as only the exact oxygen require-ment is injected and stripping of carbon dioxideby larger air volumes does not occur. In thisway, the carbon dioxide content of raw waterremains available for subsequent mineralization.

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Thus, use of pure oxygen often is more econo-mic than pressure aeration. Low investment andoperational costs and omission of, or consid-erably lower, maintenance and cleaning expendi-ture for compressors and vent valves are in favorof oxygen. These advantages have led to oxygenapplications in many German water works, whereit is regarded as state of the art technology(Figure 9).

Ozone, the universal remedyWhen traditional purification steps such asflocculation, filtration and/or chlorination areinsufficient to ensure the quality and safety ofdrinking water, advanced oxidation with thestrong oxidizing agent ozone is used as anenvironmental friendly and effective remedy.Next to fluorine, ozone is the strongest oxidizingagent available. It reacts, however, to relativelyharmless oxidation products and oxygen andleaves no undesirable by-products or taste in thewater.

Figure 9: Oxygen injection with an oxygenator for iron andmanganese oxidation

Ozone treatment can contribute to drinking waterprocessing in many ways:

Ozone is used for disinfection, often in combi-nation with UV. Compared to chlorine com-pounds, ozone acts faster on bacteria (e.g.legionella), cysts, spores, fungi, parasites,cryptosporidium (types of monads which causediarrhoea) and is more effective againstviruses. Also, use of ozone prevents theformation of chloramines and other chlor-hydrocarbons, connected with chlorination.

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Ozone is used for oxidation of iron andmanganese when these are complexed byorganics such as humic acids and cannot beoxidized by oxygen.

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Ozone inhibits algal growth and preventsformation of biological slimes on surfaces.

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Ozone is used for oxidation of (persistent)organics, thereby improving color, turbidity,odor, and taste. It is often used in combinationwith granular activated carbon (GAC) filters forpesticide control. Ozone also cracks precursorsfor haloforms (CHX3). This is important whensubsequent chlorination at entrance of thedistribution network is performed.

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Ozone improves flocculation.•

Messer HaiphongIndustrial Gases CO., Ltd.

An Duong, HaiphongVietnam

Tel: +84 31 387 1551Fax: +84 31 387 1798info@messer.com.vn

www.messer.com.vn

Ozone, the tri-atomic form of oxygen, is notstable and has to be produced on-site (Figure 10).Especially for medium and large size plants,oxygen as feed gas is more economic than air:oxygen does not require a high capital costpreparation plant to remove any moisture, tracecompounds and particulates which would affectthe lifetime of the ozone generators.Furthermore, oxygen allows for much higherozone concentrations in the product gas (10 – 15% by weight). Therefore ozone generators andozone injectors are more compact, less expen-sive and require less energy. As a result, allmodern ozone installations in e.g. Germany or UKuse pure oxygen. Also many old ozone genera-tors using air are refitted for pure oxygen to savehigh costs for monitoring and maintenance of theair preparation unit.

ConclusionFrom well to tap, carbon dioxide (for pH control,softening, mineralization, partial desalination andso on), oxygen or ozone (oxidations and disinfec-tions) are used in the total process chain fordrinking water production. Messer’s team ofqualified scientists, engineers and technicianshave extensive experience with the applicationsdescribed in this paper and the know-how to usethe gases efficiently in these processes. Theyconsult on location and offer solutions comprisingengineering, hardware and gas supply. As aresults, more than 200 reference installationshave been commissioned at waterworks all overEurope in recent years.

Figure 10: Modern ozone generators like this 3 kg/hinstallation use pure oxygen as feed