195

Rein Munter

19.INDUSTRIAL WASTEWATER TREATMENT

Table 19.1. Unit processes and operations used in wastewater reclamation and potential for contaminant removal (adaptedfrom Metcalf & Eddy, 1991)

BOD COD TOC Turbid. Color Coli NH3-N

Primary treatment x x x x o oActivated sludge + + + + x + +Nitrification + + + + x + +Denitrification o o o o xTrickling filter + + x x o oCoag.-floc.-sed. + + + + + + oFiltration A/S x x x x x xGAC adsorption + x + + + + xIon exchange x x o o +Chlorination + +Reverse osmosis + + + + + +Ozone o + x +

Symbols: o = 25 % removal of influent concentration, x = 25 - 50 %, + = > 50 %. Blank denotes no data

Pathways of industrial effluents treatment

There are many different ways of treating indus-trial effluent (Figure 19.1). It is often possible andadvisable for an industry to discharge its wastedirectly at a municipal treatment plant, where acertain portion of the pollution can be removed. Amunicipal sewage-treatment plant, if designed andoperated properly, can handle a major part of theindustrial waste, with the exception of toxic sub-stances like cyanides, phenols in high concentra-tions and heavy metals. The wastewater pH shouldbe close to neutral. Generally, the industrial ef-fluent should be free of substances that could im-pair the treatment process or cause violation ofwater quality standards of the receiving waters.Hence, one possibility that should be given seri-ous consideration is for industry and municipali-ties to co-operate in the joint construction and op-eration of a municipal wastewater treatment plant.In many cases, the joint treatment of industrial andmunicipal wastewaters can be economically andenvironmentally beneficial to both the industryand the municipality.

There is a fine line of distinction between avolume of wholly untreatable waste and one con-taining only certain components that are

untreatable if combined with domestic sewage. Forexample, wastewater from a plating plant canprobably be treated together with domestic sew-age if its pH value is first stabilised at about 7.0and the cyanides and the major part of the chro-mium is eliminated at a local wastewater treat-ment unit. The shortest pathway discharge ofuntreated industrial waste directly into a nearbystream should be permitted only after a detailedsurvey, by competent and certified sanitary engi-neers, of the existing conditions and future usesof the receiving stream. Nowadays this case shouldcertainly be considered as an exception. Completetreatment of industrial wastes prior to direct dis-charge or reuse at the plant is gradually receivingmore and more consideration. In fact, completetreatment (or at least, advanced, tertiary treatment)has already been implemented in many large in-dustries, for example, textile, pulp and paper, steeland chemical industries. The methods used forwastewater treatment can be classified as physi-cal unit operations and chemical and biologicalunit processes.

A summary of the unit operations and processescommonly used in wastewater reclamation and theprincipal contaminants removed are presented inTable 19.1.

Readily degradable Moderately degradable Recalcitrant

Gasoline Crude oil TCEJet fuel Lubricating oils PCEDiesel fuel Coal tars VinylchlorideToluene Cresotes PCBBenzene Pentachlorophenol DDTIsopropylalcohol Nitrophenol ChlordaneMethanol Aniline HeptachlorAcetone Long-chain aliphaticsPhenol PhthalatesKetone

Table 19.2. Biodegradability of organic hazardous waste(Leachy & Brown, 1994)

196

Rawwastewater

Preliminarytreatment

Primarytreatment

Aerationbasin

Secondsed.

Lime Rapidmix

Flocc.basin

Primary sludgeto gravity thickener

Clarifier

Waste sludge to gravity thickener ( < 5 Mgal/d )or flotation thickener ( >5 Mgal /d )

CO2 CO2H2SO4Alternative pH

adjustment( < 5 Mgal /d )

Equal.basin

Polymer

Lime sludgeto gravity thickener

Lime sludgeto gravity thickener

Supply pumpingSelect-ionexch.

Media regenerationBreakpoint Cl2

Granular activatedcarbon Spent carbon to regeneration ( > 5 Mgal/d )

or disposal ( < 5 Mgal/d )Supply pumping

O3 Cl2Reverse osmosis Ozone contactchamber

Chlorine contactchamber

Effluent

Recarb.clarifier

Brine to disposal

Figure 19.2. Conceptual flow diagram of advanced wastewater treatment system capable of producing potable-qualitywater supply (Metcalf & Eddy, 1991).

Levels of treatment

Treatment levels of wastewater are often identifiedas primary, secondary and tertiary (the latter alsoknown as advanced).

Primary treatment involves separating a portionof the suspended solids from the wastewater. Screen-ing and sedimentation usually accomplish this sepa-ration process. The effluent from primary treatmentwill ordinarily contain considerable organic mate-rial and will have a relatively high BOD.

Secondary treatment involves further treatmentof the effluent. Biological processes generally accom-plish the removal of the organic matter and the residualsuspended solids. The effluent from secondary treat-ment usually has little BOD5 (30 mg/l as average) anda low suspended solids value (30 mg/l as average).

Most of the readily biologically degradable or-ganic material is removed during biological treatment,but from 40 to 100 mg/l of dissolved and biologicallyresistant or refractory organic material remains in theeffluent. These materials may be end products of nor-mal biological decomposition or artificial products, suchas synthetic detergents, pesticides, oils, chlorophenols,nitro-compounds, TCE, PCE, PCB, etc.

The biodegradability of some organic compoundsis characterised in Table 19.2.

Unit operations

Both treatment levels, primary and secondary, havebeen discussed previously. The task of the present

chapter is to give an overview of the third, advancedlevel of wastewater treatment. The united operationsand processes included here (coagulation, filtration,activated carbon adsorption, electrodialysis, reverseosmosis, ozonation, advanced oxidation processesetc.) are at the same time often used for industrialeffluents local treatment. A conceptual flow diagramof an advanced wastewater treatment process com-bination capable of producing potable water frommunicipal wastewater is illustrated in Figure 19.2.

Coagulation

Coagulation may be used for the clarification of in-dustrial wastes containing colloidal and suspendedsolids. Paperboard wastes can be coagulated effec-tively with low dosages of alum. Silica or polyelec-trolyte aids in the formation of a rapid-settling flock.Typical data are summarised in Table 19.3.

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Influent Influent Effluent Effluent Effluent Coagul. Coagul.

Waste BOD SS BOD SS pH Alum. Silica(ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Board 350-450 15-60 3 5Board 140-420 10-40 1 -Board 240-600 35-85 - -Board 127 593 68 44 6.7 10-12 10Tissue 140 720 36 10-15 - 2 4Tissue 208 - 33 - 6.6 - 4

Table 19.3. Chemical treatment of paper and paperboard wastes (adapted from Eckenfelder, 1989)

Colour* Colour* COD COD

Coagulant Dose pH Influent Removal Influent Removal(mg/l) (mg/l) (%) (mg/l) (%)

Ferrisulf. 250 7.5-11 0.25 90 584 33Alum 300 5-9 86 39Lime 1 200 > 11 68 30

Ferrisulf. 1 000 9-11 4.60 87 1 570 31Alum 750 5-6 89 44Lime 2 500 > 11 87 44

* Colour sum of absorbances at wavelengths of 450, 550, and 650 nm.

Table 19.4. Coagulation of textile wastewaters (adapted from Eckenfelder, 1989)

Color Color COD CODCoagulant Dose, pH Influent Removal Influent Removal

(mg/l) (units) (%) (mg/l) (%)

Ferrisulf. 500 3.5-4.5 2 250 92% 776 60%Alum 400 4.0-5.0 92% 53%Lime 1 500 > 11 92% 38%

Ferrisulf. 275 3.5-4.5 1 470 91% 480 53%Alum 250 4.0-5.5 93% 48%Lime 1 000 > 11 85% 45%

Table 19.5. Colour removal from pulp and paper-mill effluents (Eckenfelder, 1989)

Wastes containing emulsified oils can be clarifiedby coagulation. The oil droplets in water are approxi-mately 10-5 cm and are stabilised by adsorbed ions.Emulsifying agents include soaps and anion-activeagents. The emulsion can be broken by salting it outwith the addition of salts, such as CaCl2. A lowering ofthe pH of the waste solution can also frequently breakan emulsion. For example, polymer waste from latexmanufacture has been coagulated with 500 mg/l ferricchloride and 200 mg/l lime at pH 9.6. COD and BODreductions of 75 and 94 %, respectively, were achievedfrom initial values of 1 000 mg/l and 120 mg/l.

The resulting sludge contained 1.2 % solids byweight. Results from the coagulation of textile

wastewaters are shown in Table 19.4 and colour re-moval from pulp and paper-mill effluents in Table 19.5.

It can be seen from Tables 19.4 and 19.5 that whilecoagulation is very efficient for industrial effluentscolour reduction (the removal is normally more than85 % and often reaches 92-95 %), it is much lesseffective for COD reduction (only 30-60 % removal).

Precipitation heavy metals removal

Precipitation is employed for removal of heavy met-als from industrial effluents. Heavy metals are gen-erally precipitated as hydroxide through the additionof lime or caustic (NaOH) to a pH of minimum solu-

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Metal Achievable effluent concentration, mg/l Technology

Arsenic 0.05 Sulphide precipitation with filtration0.06 Carbon adsorption0.005 Ferric hydroxide co-precipitation

Barium 0.5 Sulphate precipitationCadmium 0.05 Hydroxide precipitation at pH 10-11

0.05 Co-precipitation with ferric hydroxide0.008 Sulphide precipitation

Copper 0.02-0.07 Hydroxide precipitation0.01-0.02 Sulphide precipitation

Mercury 0.01-0.02 Sulphide precipitation0.001-0.01 Alum co-precipitation0.0005-0.005 Ferric hydroxide co-precipitation0.001-0.005 Ion exchange

Nickel 0.12 Hydroxide precipitation at pH 10Selenium 0.05 Sulphide precipitationZinc 0.1 Hydroxide precipitation at pH 11

Table 19.6. Effluent levels achievable in heavy metal removals (adapted from Eckenfelder, 1989)

bility. The pH of minimum solubility varies with themetal in question. For example, the solubilities ofchromium and zinc are minimal at pH 7.5 and 10.2,respectively.

When treating industrial wastewater that containsmetals, it is necessary to pre-treat the effluents toremove substances that will interfere with the pre-cipitation of the metals.

Cyanide and ammonia form complexes with manymetals, limiting the removal of them. For many met-als such as arsenic and cadmium, co-precipitationwith iron or aluminium is highly effective for removalto low residual levels. In order to meet low effluentrequirements, it may be necessary to provide filtra-tion to remove flock carried over from the precipita-tion process. Filtration should reduce effluent con-centrations to 0.5 mg/l or less.

For chromium wastes treatment hexavalent chro-mium must first be reduced to the trivalent state Cr3+and then precipitated with lime. The reducing agentscommonly used for chromium wastes are ferroussulphate, sodium meta-bisulphite, or sulphur diox-ide. Metals removal is summarised in Table 19.6.

Flotation (oil separation)In oil separation, free oil is floated to the surface of atank and then skimmed off. The design of a gravityseparator is based on the removal of all free oil glob-ules larger than 0.15 mm. The Reynolds number isless than 0.5, so Stokes law applies. Typically, ef-fluent oil concentrations in the order of 50 mg/l areachieved.

The hydraulic loading of a cross-flow corrugatedplate separator (Figure 19.3) varies with temperatureand the specific gravity of the oil. Nominal flow ratesare specified for a temperature of 20 C and a specific

gravity of 0.9 for the oil. A hydraulic loading of 0.5 m3/m2/h of actual plate area will usually result in separa-tion of 0.06-mm droplets. Oil emulsions can be bro-ken before separation by acidification, the additionof alum or iron salts, or the use of emulsion-break-ing polymers.

Activated carbon adsorption

Many industrial wastes contain refractory organics,which are difficult or impossible to remove throughconventional biological treatment. These materialscan be removed by adsorption on activated carbonor synthetic active-solid surface. The degree to whichadsorption will occur and the resulting equilibriumrelationships have been correlated according to theempirical relationship of Freundlich or the theoreti-cally derived Langmuir relationship. For practicalapplications more suitable Freundlich isotherm isexpressed as

Figure 19.3. Cross-flow plate separator (from Eckenfelder,1989).

199

Figure 19.4. Continuous carbon column breakthroughcurves (Eckenfelder, 1989).

X/M = kC1/n (6)

whereX = weight of substance adsorbedM = weight of adsorbentC = concentration remaining in solutionk, n = constants depending on temperature, the

adsorbent and the substance to be adsorbed.The values of k for several priority pollutants (ni-

trobenzene, styrene, chlorobenzene, bromoform etc.)are in the range 60 - 360 mg/g and the values of 1/n= 0.12 - 0.98. In granular carbon columns, the car-bon adsorption capacity at breakthrough as relatedto exhaustion is a function of the waste complexity,as shown in Figure 19.4. A single organic substance,such as dichloroethane, will yield a sharp break-through curve so that the column is more than 90 %exhausted when breakthrough occurs. By contrast, amulticomponent petrochemical wastewater shows a

drawn-out breakthrough curve due to varying rates ofsorption and desorption.

Depending on the nature of the wastewater severalmodes of carbon column design may be employed:down flow in series, moving-bed, down flow in par-allel, up flow expanded in series etc. (Figure 19.5).

It is generally feasible to regenerate spent carbonfor economic reasons. The modes of regenerationare thermal, steam, solvent extraction, acid or basetreatment, and chemical oxidation. In mostwastewater cases, however, thermal regenerationis required. Thermal regeneration entails drying,thermal desorption and high-temperature heattreatment (650-980 C) in the presence of limitedquantities of water vapour, flue gas and oxygen.Multiple-hearth furnaces or fluidised-bed furnacescan be used. The regeneration process is a com-promise between burning too much carbon andleaving unburned tars that clog the pores. Depend-ing on the type of carbon and furnace operation,weight losses of carbon during regeneration usu-ally amount to 5-10 % by weight of the carbonregenerated. In practice, the greatest total surfacearea is obtained when about 3-5 % of the originalcarbon is burned off each time.

Lately, powdered activated carbon (PAC) has beenadded to the activated sludge process for enhancedperformance (the PACT process). The flow sheet forthis process is shown in Figure 19.6. The advantagesof the PACT process are decreasing variability in ef-fluent quality and removal by adsorption of non-de-gradable refractory organics and colour. PAC alsohas the advantage of being able to be integrated intoexisting biological treatment facilities at a minimumcapital cost. Since the addition of PAC enhancessludge settleability, conventional secondary clarifi-

Figure 19.5. Types of granulated activated carbon (GAC) column design (Eckenfelder, 1989).

200

ers are often adequate, even with highcarbon dosages (300-500 mg/l PAC).

Ion exchange

Ion exchange can be used for the re-moval of undesirable anions and cati-ons from a wastewater. Cations areexchanged for hydrogen or sodiumand anions for hydroxyl ions. Ion ex-change resins consist of an organic orinorganic network structure with at-tached functional groups. Most ion ex-change resins used in wastewatertreatment are synthetic resins made bythe polymerisation of organic com-pounds into a porous three-dimen-sional structure.

Ion exchange resins are called cati-onic if they exchange positive ions andanionic if they exchange negative ions. Cation ex-change resins are comprised of acidic functionalgroups, such as sulphonic groups, whereas anion ex-change resins have basic functional groups, such asamine. The strength of the acidic or basic characterdepends upon the degree of ionisation of the func-tional groups, similar to the situation with solubleacids or bases. Thus, a resin having sulphonic acidgroups would act as a strong acid cation exchangeresin.

For the other types of ion exchange resins, the mostcommon functional groups are carboxyl (-COOH) forweak acid, quaternary ammonium (R3N+OH-) forstrong base, and amine (-NH2 or -RNH) forweak base.

Cation exchange of the hydrogen cycle can beillustrated by the following reactions:

Ca2+ + H2[KAT] Ca[KAT] + 2H+ (7)

Regeneration with 2 to 10 % H2SO4 yields:

Ca[KAT] + 2H+ H2[KAT] + Ca2+ (8)

Similarly, anion exchange replaces anions withhydroxyl ions:

SO42- + [AN] (OH)2 [AN]SO4 + 2OH- (9)

Regeneration with 5 to 10 % NaOH will renewthe exchange sites:

[AN]SO4 + 2OH- [AN] (OH)2 + SO42- (10)

Since exchange occurs on an equivalent basis, thecapacity of the bed is usually expressed as equiva-lents per litre of bed volume. In some cases capacityis expressed as kg CaCO3 per unit bed volume or asmass of ions per unit bed volume.

Treatment of a wastewater by ion exchange involvesa sequence of operating steps. The wastewater is passedthrough the resin until the available exchange sites arefilled and the contaminant appears in the effluent. Thisprocess is defined as the breakthrough. At this pointtreatment is stopped and the bed is backwashed to re-move dirt and to re-grade the resin. The bed is thenregenerated. After regeneration the bed is rinsed withwater to wash out residual regenerant.

The treatment and regeneration cycle is shown inFigure 19.7. In this figure, the area ABHG is the quan-tity of ions in the volume of solution treated beforebreakthrough. The area ABC is the quantity of ionsleaking through the column and the area ACHG isthe quantity of ions removed by the exchange resin.The resin utilisation is therefore area ACHG/K, where

Figure 19.6. General process diagram of a powdered activated carbon(PACT) wastewater treatment system (Eckenfelder, 1989).

Figure 19.7. Treatment and regeneration cycle of ionexchange resin (from Eckenfelder, 1989).

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Material to be removed Approximate size, nm Process

Ion removal 1-20 Diffusion or reverse osmosisRemoval of organics in true solution 5-200 DiffusionRemoval of subcolloidalorganics 200-10 000 Pore flowRemoval of colloidal matter 75 000 Pore flow

Table 19.7. Membrane processes (Eckenfelder, 1989)

K is the ultimate capacity of the resin.The area BCDEF is the quantity of ionsremoved from the bed during regen-eration. The regeneration efficiency istherefore area BCDEF/R, where R isequal to the concentration of theregenerant times its volume.

To ensure contact of liquid with theresin and to minimise leakage, the mini-mum bed depth is 61 to 76 cm. The treat-ment flow rate can vary between 0.27 to0.67 m3/(minm3). The regenerant flowrate is 0.13 to 0.27 m3/(minm3).

One of the major applications of ionexchange in industrial waste treatmenthas been in the plating industry, wherechromium recovery and water reusehave resulted in considerable savings.

For recovery of the spent chromicacid in plating baths, the chromic acidis passed through a cation exchange resin to removeother ions (Fe3+, Cr3+, Al3+ etc.). The effluent can bereturned to the plating bath or to storage. The recov-ered chromic acid from the spent regenerant will havean average concentration of 4 to 6 %. The spentregenerant from the cation exchanger will requireneutralisation and possibly precipitation of metallicions before it is discharged to the sewer. A flow dia-gram of an ion exchange process for a plating plantis shown in Figure 19.8.

Membrane processes

The membrane processes can be classified accord-ing to the size range of the separated species: Reverse osmosis is used to separate dissolved salts

and small organics (size under 1 nm). Example:production of drinking water from seawater orseawater desalination.

Nanofiltration is used to separate antibiotics (sizeunder 10 nm). Example: selective demineralisationof water or concentration of organic solutions.

Ultrafiltration is used to separate emulsions, colloids,macromolecules or proteins (size under 100 nm).Example: treatment of pulp and paper industryseffluents.

Micro-filtration is used to separate small particles,large colloids and microbial cells (size under 10 mm).Example: removal of microorganisms from thefermentation products.

Gas and vapour separation is used to isolate a gasfrom a mixture of gases or vapours. Example: recoveryof ammonia or hydrogen from industrial gases.

Electrodialysis is used to separate anions and cationsby means of two charged membranes (anode andcathode). Example: production of pure water.

Reverse Osmosis

As can be seen from the list above, membrane filtra-tion includes a broad range of separation processes,ranging from filtration and ultrafiltration to reverseosmosis. Generally, those processes defined as fil-tration refer to systems in which discrete holes orpores exist in the filter media, generally in the orderof 102 to 104 nm or larger. The efficiency of thistype of filtration depends entirely on the differencein size between the pore and the particle to be re-moved. The various filtration processes relative tomolecular size are shown in Table 19.4.

In normal osmosis, a semipermeable membraneseparates a salt solution from a pure solvent of a lessconcentrated solution. This membrane is permeable

Figure 19.8. Ion exchange system for chromate removal and reuse(Eckenfelder, 1989).

202

to the solvent and impermeable to the solution (Fig-ure 19.9). The chemical potential of the pure solventis greater than that of the solvent in solution, andtherefore brings the system to equilibrium. For twosolutions with different concentrations, separated bya semipermeable membrane, the osmotic phenom-enon will be a water flux from the most diluted solu-tion to the concentrated one. To stop this water flux,a pressure on the concentrated solution has to be ap-plied. By increasing the pressure above the osmoticpressure, a water flux from the concentrated solu-tion to the diluted one is obtained. This phenomenonis called reverse osmosis.

The equation of osmotic pressure, the so-calledVant Hoffs equation, is:

P = C RT

where C is the concentration of the solute, R is thegas constant and T is the temperature in K. Itshould be noted however that this relation shouldbe applied to dilute solutions, i.e. to solutions underlow osmotic pressures.

The permeate flux can be expressed as follows:

J = e d2 P / 32 m t z

where e is the membrane porosity, d is the pore di-ameter, t is the tortuosity factor, m is the solvent vis-cosity, P is the pressure difference across the mem-

brane and z is the membrane thick-ness.

The selectivity of the mem-brane is defined by the solute re-jection coefficient, which is givenby the following expression:

R = 1 - Cp/Cf

where Cp is the solute concentra-tion in the permeate and Cf, thesolute concentration in the feed.

There are three different typesof membrane structures: homo-geneous, asymmetric and com-posite membrane. The homoge-neous membrane is not used veryoften because of its considerablylow permeate flow rate. It is onlyused for micro-filtration. Asym-metric membranes are made oftwo layers: a 1-2 mm thick top layerof finest pore size supported by a 100mm thick, more porous matrix. Withsuch an asymmetric structure, rea-sonable rates of permeate flux are

Figure 19.9. Osmosis and reverse osmosis (Eckenfelder,1989).

Figure 19.10. Membrane filtration system. Tubular reverse osmosis unit(Eckenfelder, 1989).

203

Parameter Range Typical

Pressure (MPa gage) 3-7 4Temperature (C) 15-40 21Packing density (m2/m3) 50-500 -Flux m3/(d m2) 0.5-4 0.6-1.7Recovery factor (%) 75-95 80Rejection factor (%) 85-99.5 95Membrane life (years) - 2PH 3-8 4.5-5.5Turbidity (JTU) - 1Feed-water velocity (m/s) 0.01-0.75 -Power utilization (kW h/ 103 l) 2-3.7 -

Table 19.8. Summary of system operational parameters(adapted from Eckenfelder, 1989)

obtained. Composite membranes aremade of two layers as well: an ex-tremely thin layer (1 mm) of finestpore structure deposited on a porousmatrix. The thin layer can be madeof inorganic materials.

Membranes are most commonlymade of polymeric materials such aspolyamide, polysulfone and poly-car-bonate. The method of packing themembrane leads to a module designof a membrane separation unit. Thereare four types of modules commer-cially available: tubular modules, flat-sheet modules, spiral-wound modulesand hollow-fibre modules.

A typical tubular membrane typeis shown in Figure 19.10. Manufac-tured from ceramics, carbon, or po-rous plastics, these tubes have an inside diameterranging from 1/8 inch (3.2 mm) up to 1 inch (2.54cm). The membrane is typically coated on the insideof the tube, and the feed solution flows through theinterior from one end to the other, with the perme-ate or filtrate passing through the wall. Other mem-brane systems used are spiral-wound hollow-fibre andplate or frame.

The water (permeate) flux is a function of the pres-sure differential between the applied pressure and theosmotic pressure across the membrane. Flux rates ofapproximately 0.1 m3/m2/day are typical for tubular andspiral-wound systems. The maximum pressure is gen-erally taken to be 1 000 lb/in2 (6 900 kPa) gage. Thewater flux increases with increasing feed water tem-perature. A standard of 21 C is generally assumedand temperatures of up to 29 C are acceptable.

Salt rejection depends on the type and characterof the selected membrane and the salt concentrationgradient. Rejection values of 85 to 99.5 % are gener-ally obtainable, with 95 % commonly being used.

Membranes consisting of cellulose acetate aresubject to hydrolysis at high and low pH values. Theoptimum pH is approximately 4.7, with operatingranges between 4.5 and 5.5. Generally, membranes willlast up to two years with some loss in flux efficiency.The pre-treatment of water to a membrane system isvital, to maintain productivity and to obtain longmembrane life. In particular, the level of fine andcolloidal solids must be controlled (Figure 19.11).

An industrial standard method called the siltdensity index (SDI) is used. The SDI test entailspassing the feed water through a fine-pore membranefilter (0.45 _m) under standard conditions and meas-uring the rate at which the flow falls through themembrane. It is recommended not to have an SDIhigher than 0.3. Membrane systems are normally

turned off periodically for chemical cleaning, to allowdeposits to be removed. Cleaning methods include pe-riodic depressurisations, high-velocity water flushing,backwashing, cleaning with enzyme detergents, ethyl-ene diamine, tetraacetic acid etc. A summary of opera-tional parameters is given in Table 19.5.

Reverse osmosis systems can operate at 90 % ef-ficiency or better with respect to total dissolved sol-ids. In addition to inorganic ions, the membranes alsoremove residual organic molecules, turbidity, bacte-ria and viruses.

In Figure 19.12 the application of ultrafiltrationfor oily wastewaters treatment is illustrated. The per-meate is recycled as rinse water and the concentrateis hauled or incinerated.

Demands for lower freshwater consumption to-gether with stringent environmental legislationwithin the pulp and paper industry have created aneed for process water and effluent recycling, andthus totally or partly closed water circuits havebecome a reality. The membrane processes (ultra-and nanofiltration) are already being used in pa-

Figure 19.11. Reverse osmosis general system (Newton & Solt, 1994).

204

Oxidation species Oxidation power*

Chlorine 1.0Hypochlorous acid 1.10Permanganate 1.24Hydrogen peroxide 1.31Ozone 1.52Atomic oxygen 1.78Hydroxyl radical 2.05

* Based on chlorine as reference point (=1.00)

Table 19.9. Relative oxidation power of some oxidising spe-cies (EPRI, 1996)

per machine white-water treatment and the expe-rience has been good. The development of mem-brane materials like ceramics will prolong mem-brane lifetime and thus reduce cost. New modules arealso needed, because e.g. the nanofiltration spiral-wound module needs a very heavy pre-treatment, usu-ally by ultrafiltration. During the last decade, as a re-sult of the development of a new filter, called the CR(cross-rotational), as well as of new membrane materi-als, membrane applications have proved to be com-petitive processes for the concentration of valuable com-pounds or purification of water.

Electrodialysis

The electrodialysis process uses a series of mem-branes made from ion-exchange resins. These mem-branes will selectively transfer ions. They are basedon polymers of materials such as styrene orpolyethylene incorporating fixed and mobile chargedgroups. In, for example, cation-exchange membranes,negatively charged groups are fixed and positivelycharged groups mobile. Then, under the influence ofa driving force (an electric field or a concentrationgradient), positive and negative ions are attracted tothe membrane, and the membrane is permeable forpositive ions while rejecting negative ions. In theelectrodialysis process the transport through themembrane is promoted by an electric field across themembrane. Diffusion is negligible by comparison andion transport is directly related to the electric cur-rent. According to Faradays law an electric currentof 26.8 Ah is needed to transport 1 gram equivalent ofa salt. In practice the electric current is higher, due toinefficiencies caused by electrical resistance. Gener-ally in electrodialysis a package of membranes is usedwith alternate cation and anion membranes arranged ina so-called membrane-stack (Figure 19.13). Mem-branes in an electrodialysis unit are approximately

0.5 mm thick, separated by porousspacers about 1 mm thick. Water flowsthrough the porous spacers. Anions andcations pass through the membranes sothat the original feed is desalinated anda concentrate forms on the other sideof the membranes. A contact time of10 to 20 s is required to obtain a re-moval efficiency of about 25-60 %. Asreverse osmosis, electrodialysis also re-quires a high degree of pre-treatmentprior to the process. Suspended solidsremoval is absolutely necessary, anddissolved organics should be removedto prevent fouling.

Chemical oxidation of a waste-water may be employed to oxidise pollutants toterminal end products or to intermediate productsthat are more readily biodegradable or more read-ily removable by adsorption. Common oxidantsare chlorine, hypochlorous acid, potassium per-manganate, hydrogen peroxide and ozone.

The strongest oxidants are hydroxyl radicals formedin various advanced oxidation systems, such as O3/UV,O3/H2O2, H2O2/UV, O3/TiO2, etc. (see Table 19.6).

Ozonation of wastewater compounds

Van Marum, the Dutch chemist who discovered howto lead electric current through oxygen, also discov-ered ozone (trioxygen) in 1785. The name ozone(ozo = to smell in Greek) was given to the gas bythe Swiss chemist Schnbein in 1840.

Under normal pressure and temperature, ozone isa bluish gas with a characteristic intensive odour, abouttwice as heavy as air. Its boiling point is -112 C and itsmelting point -192.7 C at 0.1 MPa. Pure (100 %) ozoneis at -112 C a dark-blue liquid, extremely sensitive tothe smallest organic impurities, shaking, and tempera-ture and pressure changes, and very explosive.

Ozone is produced in the field of a corona dis-charge at voltages of 10-15 kV and frequencies of

Figure 19.12. Treatment of oily wastewaters by ultrafiltration (Eckenfelder,1989).

50-1 000 Hz. Nowadays mediumfrequency (400-600 Hz) is mostcommonly used in producing ozone.

Ozone production with reactionsfrom air or pure oxygen can be sum-marised as

3O2 2O3+298 kJ (11)

This reaction is reversible: part ofthe produced ozone decomposesback to oxygen, liberating 149 kJ ofenergy per mole of ozone decom-posed. The amount of liberated en-ergy makes ozone dangerous in highconcentrations.

In water/wastewater treatment ozone is synthe-sised in tube or plate generators, yielding air-ozoneor oxygen-ozone mixtures with concentrations ofozone less than 20 % (by mass), which is totally safe.

Ozone dissolves in water better than oxygen, butnot as well in chlorine. For the dissolution processHenrys law applies. Dissolved ozone decomposesin water solutions quicker the higher the pH. As aresult several radicals (among of them the very ac-tive OH-radicals) are formed. The main steps of theOH-radicals formation are:

O3 + OH-

O2-

+ HO2 (12)

O3 +HO2 2O2 +OH (13)

Ozone is a very strong oxidant; its normal oxidationpotential at 25 C is 2.07 V in acidic solutions and1.24 V in basic solutions. Only fluorine and OH-radicals have higher oxidation potential.

In its reactions with inorganic or organic com-pounds in water, usually one of three oxygen atomsis active, while the residual two are separated as anoxygen molecule:

CN-+ O3 CNO-

+O2 (14)

RCH2OH + 2O3 RCOOH + 2O2 + H2O(15)

However, with unsaturated hydrocarbons, ozoneforms very unstable intermediates (ozonides) involv-ing all three oxygen atoms:

O

O O

C = C + O3 C C

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Ozonides decompose very quickly resulting in for-mation of polymers or aldehydes, ketones and or-ganic acids.

The reaction rate of molecular ozone with organiccompounds depends on their oxygen saturation de-gree and may range from 0.1-1.0 (carboxylic acids)up to 106-108 (aniline, toluidine) M-1s-1. As a rule,chemical reactions of ozone with organic compoundsare of second order; first order by both compoundsconcentrations.

During the ozonation process, depending on theozone dose consumed, several intermediate oxida-tion products are formed. For example, ozonation ofordinary phenol (C6H5OH) proceeds through the stepsof resorcinols formation (with 2 OH-groups) anddecyclisation to muconic acid, muconic aldehyde andmaleic and fumaric acids.

Higher doses of ozone (7-10 mole/mole) will leadto the formation of glyoxal (HOC-COH), formic acid(HCOOH), oxalic acid ((COOH)2) and glyoxalic acid(HOCCOOH). Theoretically, all organic com-pounds consisting of carbon and hydrogen maybe oxidised to the final inorganic products, CO2 andH2O, but ozone requirements in this case may ex-ceed 15-25 mole/mole, which is not economical.

Ozone has a complex impact on water/wastewaterparameters it reduces colour, improves taste andodour, kills bacteria and viruses (even the chlorine-resistant Polio-virus), oxidises iron, manganese, cya-nide, phenol, benzene, chlorophenol, atrazine, ni-trobenzene and other pollutants. Ozonation is a cost-effective treatment method for many types of indus-trial wastewaters. From the point of view ofwastewater treatment it is very important that ozonecan increase the biodegradability of wastewater, i.e.increase the ratio BOD/COD before the activatedsludge process.

Figure 19.14 (especially the scheme below) il-lustrates the impact of ozonation on the ratio BOD/COD. If the raw effluent is characterised by COD

Figure 19.13. Electrodialysis stack (Newton & Solt, 1994).

Figure 19.14. Effect of oxidation by ozone on COD/BOD content (Von der Mark et al., 1993).

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= 5 000 mg/l and BOD = 200 mg/l; then, afterozonation, the corresponding values are 4 500 and 2000 mg/l. The ratio of BOD/COD and biodegrad-ability was increased 10 times (!) due to ozonation.

During conventional bleaching of Kraft pulp, asignificant amount of organic material is dissolvedin the first two bleaching stages, chlorination andalkaline extraction. Over 200 chlorinated organiccompounds have been identified in bleach plant ef-fluents. Some of them exhibit acute and chronic tox-icity, while others are persistent and bio-accumulate.Studies on the oxidative treatment of bleaching ef-fluents with ozone are being spread rapidly.Ozonation is particularly effective in removing col-our. In acidic conditions, direct ozonation reactionsare highly selective and only those compounds con-taining functional groups that are easily attacked bythe electrophilic ozone become oxidised. In alka-line conditions, ozone decomposition leads to theformation of OH-radicals and to indirect and non-selective oxidation reactions (AOPs).

The combination of radicals and molecular effectsof ozone is efficient and economical. An 80 % re-duction of AOX (adsorbable organic halides) andover 60 % reduction of COD and BOD can be ob-served. Acute toxicity and chlorophenolic derivativesdisappear. A rough estimation of cost indicates thatthe use of ozone in treating bleaching effluents wasexpensive, but its combination with biological treat-ment is still very attractive.

Advanced oxidation processes (AOPs) are definedas near ambient temperature and pressure watertreatment processes which initiate completeoxidative destruction of organics based on thegeneration of hydroxyl radicals, OH. AOPs usedfor the treatment of process wastewater andgroundwater include:

ozone at elevated pH ( > 8.5)ozone + hydrogen peroxide (H2O2)ozone + granular activated carbonphotooxidation:UV + O3 + H2O2UV + H2O2photocatalysis:UV + TiO2.

Ozone at elevated pH. As pH rises, the decompo-sition rate of ozone in water increases. For example,at pH 10, the half-life of ozone in wastewater can beless than 1 minute. Oxidation of organic species mayoccur due to a combination of reactions with mo-lecular ozone and reactions with OH.

The rate of attack by OH-radicals is typically106 to 109 times faster than the corresponding ratefor molecular ozone. The major operating cost forthe ozone oxidation process is the cost of electricityfor ozone generation. The energy requirement forozonation using air ranges from 22 to 33 kWh/kgO3, including air handling, preparation and ozonecontact with water. The energy requirement for ozoneproduction from pure oxygen is approximately half,but then the cost of the oxygen should be added.

Ozone + H2O2 (peroxone). Addition of H2O2 toozonation processes can initiate the decompositioncycle of ozone, resulting in the formation of OH-radi-cals. H2O2 is a relatively inexpensive, readily availablechemical oxidant. H2O2 is produced by electrolysis ofammonium bisulphate or by oxidation of alkylhydro-anthraquinones. The electrolytic process consumesapproximately 7.7 kWh/kg of H2O2 produced.

The very interesting combination ozone plusgranular activated carbon (GAC) for creating OH-radicals was discovered and described only recentlyby the Swiss scientists Jans & Hoigne (1998). They

Figure 19.15. Proposed structure of activated carbon andcarbon black (Sergides et al., 1987). A variety of differentchemical groups can be seen on the skeletal carbon frame-work of AC or CB able to initiate radical-type chainreactions.

Process Cost of oxidant Cost of UV

O3/UV High MediumO3/H2O2 High N/AH2O2/UV Medium HighPhotocatalysis Very low Medium-high

Table 19.10. Comparative operating costs of advancedoxidation processes (AOPs) (EPRI, 1996)

Figure 19.16. Phenol concentration reduction versus time(Preis et al., 1995).

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established that if a few milligrams per litre of acti-vated carbon or carbon black was added to ozone-containing water, a suspension of the radical-typechain reactions in the aqueous phase would be initi-ated and there would be an acceleration in the trans-formation of ozone into OH-radicals. The authorsexplained this phenomenon as being due to the ex-istence of a variety of chemical groups on the skel-etal carbon framework of activated carbon or carbonblack (Figure 19.15).

Photooxidation. Completion of oxidation reac-tions, as well as oxidative destruction of compoundsimmune to ozone and H2O2 oxidation alone, can beobtained by supplementing the reaction with ultra-violet (UV) radiation. UV radiation accelerates thedecomposition of ozone and hydrogen peroxide mol-ecules. The photolysis of aqueous ozone producesH2O2 as an intermediate product, which then decom-poses to OH. Ozone readily absorbs UV radiationat the 254 nm wavelength. Many organic contami-nants absorb UV energy as well and decompose dueto direct photolysis or become excited and more re-active with chemical oxidants.

Although photochemical cleavage of H2O2 isconceptually the simplest method for hydroxyl radi-cal production, the exceptionally low molecular ab-sorptivity of H2O2 at 254 nm (20 M-1cm-1) limits the

OH yield in the solution (0.09 OH-radicals areformed per photon). Photolysis of dissolved ozone(3 300 M-1cm-1) yields 2.0 OH-radicals formed perphoton. In practice the power requirement for UVlamps in ozone photolysis processes is in the wattsrange, as compared to the kilowatts range for hydro-gen peroxide photolysis.

In photocatalysis, UV radiation is used to excitea solid-state metal catalyst (TiO2), creating positiveand negative charge (electron hole, e-h+ pairs) onthe catalysts surface. These positive and negativecharges promote redox reactions, e.g. oxidation oforganics in the solution by the photogenerated posi-tive charges and reduction of metal ions or oxygenby the photogenerated negative charges.

TiO2 may be introduced in the process as a slurryor fixed catalyst. In the slurry method microfiltrationis a necessary post-treatment step to remove TiO2particles. Photocatalysis can be driven by near-UV(300-400 nm) radiation; therefore, solar energy canbe used to empower the processes. Table 19.7presents a comparison of AOPs operating costs.

Applications of advanced oxidationprocesses

Up to 10 % of the oil shale mined in Estonia (about2.5 million tonnes annually) is treated chemically ata temperature of 500-550 C to produce oil. Phenoliceffluents from crude oil separation and phenols ex-traction together with sewage from the local settle-ments (15 600 m3/day, COD 1 570 mg/l. total phenols

oxidise organic compounds to CO2 and other innocu-ous end products. Carbon is oxidised to CO2, nitro-gen is converted to NH3, NO3, or elemental nitrogen,and halogens and sulphur are converted into inor-ganic halides and sulphates. The lower concentra-tion limit of organic substances for wet air oxidationis about 20 g/l COD.

Chemically, the end results of wet oxidationare similar to the end results of conventional oxi-dation in a furnace or incinerator. However, themechanism of oxidation is different. Instead oftaking place at atmospheric pressure at high tem-perature and on the dry material, wet oxidationtakes place at relatively high pressures, low tem-peratures and in the presence of liquid water.Sometimes different metal catalysts are used aswell. This is why wet air oxidation has often calledburning without a flame. The wet air oxidationprocess presents the following reaction-engineer-ing problems: The air oxygen needed for oxidation must be dissol-

ved in the wastewater. The dissolved oxygen must react with the organic

compounds.Wet oxidation reaction kinetics has been the subjectof numerous studies. Most of the published resultsindicate that wet oxidation proceeds by a free-radi-cal reaction mechanism. Free radicals, in the absenceof initiators, are formed by the reaction of oxygenwith the weakest C-H bond of the oxidised organiccompound:

Figure 19.17. Flow diagram for an advanced oxidation process (O3/H2O2/UV) to destroy organic contaminants (EPRI,1996).

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120 mg/l) are treated biologically at the CentralWastewater Treatment Plant in Kohtla-Jarve.

A significant modern-day reduction of the capac-ity of oil shale semi-coking was made in 1994. Be-cause of this, the concentration of the total phenolsat the outlet of the biological treatment plant was only3.3 mg/l (COD 100 mg/l) in April 1994 instead of95 mg/l (COD 840 mg/l) in 1987.

Ozone and its combinations with H2O2 and UVwere studied for post-treatment of the biologicallytreated phenolic effluent to achieve the MPC = 0.5mg/l of total phenols established by the EstonianMinistry of Environment. Figure 19.16 illustratesclearly the impact of the addition of H2O2 and UV toozone in simple phenol concentration reduction. Fig-ure 19.17 shows the flow diagram for an AOP usedto destroy organic compounds in contaminated wa-ter. This process utilises chemical oxidants, ozoneand hydrogen peroxide, in combination with UV ra-diation, to achieve optimum destruction efficiency.

Wet oxidation

Wet oxidation is an attractive destruction method forthe treatment of waste streams that are too dilute toincinerate but too concentrated or toxic for biologi-cal treatment. Wet air oxidation (WAO) involves liq-uid phase oxidation of organic or oxidisable inorganiccomponents at elevated temperatures (125-300 C) andpressures (0.5-20 MPa) using a gaseous source of oxy-gen (usually air). WAO has been demonstrated to

RH + O2 R + HO2

RH + HO2 R + H2O2

H2O2 + M 2OHwhere M can be either a homogeneous or heterogene-ous species. The thermal decomposition of hydrogenperoxide at WAO temperatures is also significant:

H2O2 H2O + 1/2 O2

Since the OH radical has a large electron affinity(568 kJ), it oxidises not only all organic compoundscontaining hydrogen, but also reacts with most halideions to produce halide radicals.

Despite of the advantages described above, WAOhas the shortcomings of requiring relatively high oxy-gen pressures as well as high temperatures, resultingin high capital and operating costs and incompleteor partial oxidation. In the presence of a catalyst, or-ganic compounds are more rapidly and more thor-oughly oxidised to carbon dioxide and water. Theprocess may be operated at lower temperature, lowerpressure and reduced reaction time. Practically allmetals that change their valence easily and hence are

Figure 19.18. Ultrafiltration-concentrate formation in thesecondary water circuit of a paper machine. Ultrafiltrationconcentrate contains different lipophilic extractives andhas a typical COD of 4 000-8 000 mgO2/l, solids contentof 10-15 g/l, and pH of 5.0-6.5.

Figure 19.19. The effect of wet air oxidation (WAO) reactiontemperature on the COD reduction of the membrane con-centrate (A non-filtered sample, B filtered sample) atoxygen partial pressure of 1 MPa.

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suitable as redox catalysts can be used (Cu, Cu-Zn,Mn, Mn-Zn-Cr, Co, Co-Bi, Fe, Ti, Al etc.).

Wet air oxidation has been successfully appliedin the treatment of sewage sludge, to convert thesludge into a sterile and biologically stable mass withgood settling and dewatering characteristics. Wet airoxidation is a very promising method of treating theblack liquor from pulp and paper mills, since the con-ventional treatment method (evaporation with incin-eration in furnaces) does not yield a satisfactory re-covery efficiency of chemicals and energy. WAO hasalso been used for treatment of the paper machineeffluents membrane separation concentrates (Figure19.18 and 19.19).

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Figure 20.2. Exploitation of waste heat and recovery of condensates from hot effluents in an integrated vacuum flash/falling-film evaporator (Billet, 1989).

Figure 20.1. Flow chart of the Hoechst AG process for the heat treatment of industrial effluents (Billet, 1989). Plantdesigned by Friedrich Uhde GmbH, Dortmund.

19. INDUSTRIAL WASTEWATER TREATMENTPathways of industrial effluents treatmentLevels of treatmentUnit operationsIon exchangeMembrane processesElectrodialysisOzonation of wastewater compoundsApplications of advanced oxidation processes