ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS potx

Stander and Van Vuuren 1969 investigated the treat-ment of raw wastewater in a pilot plant where solids removal was achieved by primary sedimentation and chemical coagu-lation with lime,

INTRODUCTION

The substances in domestic and industrial wastewater having

significance in water-pollution control, disposal, and reuse

are (1) dissolved decomposable organic substances

result-ing in dissolved oxygen depletion in streams and estuaries

and/or causing taste and odor; (2) suspended organic solids

resulting in dissolved oxygen depletion; (3) inert suspended

solids (SS) causing turbidity and resulting in bottom

sedi-ment deposits; (4) toxic synthetic organic substances and

heavy metals; (5) oil, grease, and floating materials; (6) acids

and alkalis; and (7) dissolved salts, including nutrients like

phosphorus and nitrogen

Conventional wastewater-treatment practices have been

oriented to the removal of grit and floating matter followed

by the removal of suspended and dissolved organic matter

The removal of suspended matter has been achieved by

sedimentation, and the bulk of the soluble organic matter

is removed by biological oxidation and flocculation

These processes, when carried out in combination, have

proved to be economical and effective means for

remov-ing organic matter from wastewaters However, there are

certain disadvantages associated with them These include

the following:

1 Biological process require considerable operating

control and often generate operating problems of

a complex nature

2 Biological processes are easily upset by shock loads

and require time to regain efficient operation

3 Biological processes are unable to remove certain

nutrients, heavy metals, and inorganic salts, when-ever there is a requirement for their removal

4 Many waste streams contain certain compounds

that do not respond to biological treatment or, alternatively, require extensive pretreatment

In the last decades, physical-chemical treatment of

waste-water has been studied both on laboratory and pilot-plant

scales with important industrial and municipal

wastewater-treatment applications This type of wastewater-treatment is used either as

a pretreatment, tertiary treatment, or advanced treatment given

to the effluent from secondary treatment, or as a substitute for

conventional biological treatment In the latter case, it is found

to produce effluent of a quality at least equal to that produced

by conventional biological treatment

The first study on the treatability of raw wastewater

by physical-chemical processes was reported by Rudolfs and Trubnick in 1935 In this study, solids were removed

by chemical coagulation with ferric chloride followed by absorption of dissolved impurities with activated carbon

Stander and Van Vuuren (1969) investigated the treat-ment of raw wastewater in a pilot plant where solids removal was achieved by primary sedimentation and chemical coagu-lation with lime, and adsorption with activated carbon Rizzo and Schade (1969) have also reported results on the pilot-plant treatment of raw wastewater with chemical coagulation and anionic polymer and adsorption with activated carbon

Zuckerman and Molof (1970) studied the efficiency of a treatment system in which raw wastewater was lime-clarified

at high pH and then activated-carbon-treated: their results showed that the chemical oxygen demand (COD) values of the final effluent were significantly lower than those associ-ated with good conventional treatment Moreover, they con-cluded that the removal of soluble organics with activated carbon was enhanced because of the hydrolytic breakdown

of high-molecular-weight organic compounds, at a higher

pH value, which are absorbed more readily by activated carbon

Weber et al (1970) investigated the chemical clarifi-cation of primary effluent with ferric chloride followed

by activated-carbon adsorption Their results showed that 65% of the organic matter present in primary effluent was removed by chemical treatment with ferric chloride

Overall removal of biochemical oxygen demand (BOD) was reported as being consistently in the range of 95 to 97% Final effluent from the system contained approxi-mately 5 mg/l BOD as compared to 30 mg/l for the same wastewater treated conventionally

In another study, Villiers et al (1971) showed that the treatment of primary effluent by lime clarification and acti-vated carbon, in a steady flow system, produced an efflu-ent with total organic carbon (TOC) averaging 11 mg/l and turbidity averaging less than 2 Phosphates and SS removal were consistently 90% or better These product characteris-tics are comparable to those associated with products from well-operated conventional treatment plants

Shuckrow (1971) had developed a sewage-treatment process involving chemical coagulation for SS removal, fol-lowed by adsorption of soluble organics on powdered carbon

The advantages cited for this process were (1) a total treat-ment time of less than one hour, (2) a high-quality effluent,

(3) lower initial plant cost, (4) ability to remove nitrogen and

phosphorus, and (5) a final sludge reduction to sterile ash in

a centrifuge-incineration process combined with a chemical

regeneration step to recover both the coagulant and the carbon

While the estimated operational costs were high, the overall

costs during a 20-year plant life were considered to be

signifi-cantly less than costs for comparable biological facilities

Ecodyne Corporation’s first complete physical-chemical

treatment plant in Rosemount, Minnesota, with 0.04 to

0.08 m3/sec peak capacity, consisted of bar screening,

phos-phate removal with sludge recirculation, dual media filtration,

carbon absorption to remove dissolved organics, secondary

filtration, and ammonia removal by ion exchange with

zeo-lite The plant included facilities for regenerating the carbon,

recovering ammonia, and regenerating brine from the

ion-exchange system (Ecodyne, 1972)

Examples of more recent research include an exhaustive

review on the treatment of pulp- and paper-mill

wastewa-ter published by Pokhrel and Viraraghavan in 2004 This

includes the different processes involved with their effluents,

the different methods for treatment of these effluents, the

integration of biological and physico-chemical processes,

a comparison of them, and conclusions from this review

An article by Van Hulle and Vanrolleghem (2004) presents

the development, calibration, and application of a model for

the simulation and optimization of a wastewater-treatment

plant The constructed model proved to be able to predict

large variations in influent composition This could be an

important tool for production scheduling when applied to

industrial wastewater-treatment plants Recent research in

specific areas is included in for each process

The number of water-treatment facilities in the United

States by treatment capacities is presented in Table 1 Many

of these facilities include some sort of physical-chemical

treatment technology A diagram of alternative technologies

for wastewater treatment is shown in Figure 1; it includes

most of the processes to be discussed in the next section As

environmental regulations, space availability, and cost

fac-tors affect the treatment of waste streams, more and more

physical-chemical treatment will be needed to meet these

constraints This is an important research area that will

con-tinue to grow in the next years

PHYSICAL AND CHEMICAL PROCESSES USED IN WASTEWATER TREATMENT

The following important unit operations and unit processes involved in the physical and chemical treatment of wastewater are discussed in detail:

Flow equalization and neutralization Chemical coagulation, flocculation, and sedimentation Filtration

Gas stripping Ion exchange Adsorption Flotation Chemical processes Oxidative, photochemical, and electron-beam processes

Flow Equalization and Neutralization

Both domestic and industrial wastewater flows show con-siderable diurnal variation, and it is considered necessary

to significantly dampen these variations in inflow to relieve hydraulic overload on both biological and physical-chemical plants This process will also smooth out variations in influent characteristics

Flow-equalization basins are basically flow-through or side-line holding tanks, and their capacity is determined by plotting inflow and outflow mass curves These tanks are generally located after preliminary treatment and should be designed as completely mixed basins, using either diffused air or mechanical surface aerators, to prevent settling of sus-pended impurities If decomposable organic matter is present

in the wastewater, aeration will prevent septicity The pre-aeration can also reduce the BOD on subsequent treatment units

Flow-equalization basins can also be used to neutral-ize the acidity or alkalinity in incoming wastewater The neutralizing chemicals are added to the inflow wastewater stream before entering the flow-equalization basins, and the retention period in these basins provides sufficient time for reaction Any precipitates produced during neutralization are separated in subsequent sedimentation basins

Chemical Coagulation, Flocculation, and Sedimentation

The use of chemical treatment appeared early in the devel-opment of sewage- and wastewater-treatment technology

Aluminum sulfate, lime, and ferrous sulfate, when used in the manner usually adopted for water clarification, were successful in producing an effluent of quality better than that obtained by plain sedimentation An effluent that is generally fairly clear, with only very fine suspended or col-loidal solids but with practically all of the dissolved solids remaining, can be produced Under most favorable condi-tions and with skilled operation, SS may be reduced in an amount of up to 90% and BOD up to 85% However, the

TABLE 1 Number of wastewater-treatment facilities in the United States (1996)

Flow ranges m 3 /s Number of facilities Total existing flowrate m 3 /s

0–0.00438 6444 12.57

0.0044–0.0438 6476 101.78

0.044–0.438 2573 340.87

0.44–4.38 446 511.12

4.38 47 443.34

Total 16204 1409.68

Source: Adapted from Tchobanoglous et al., 2003.

cost of the coagulants and the difficulty of disposing of the

larger amount of the sludge produced by this process caused

it to be abandoned The revival of chemical treatment can be

attributed to a number of factors that accumulated as a result

of continuous investigations and reevaluation of the process

These are (1) the decrease in cost of chemicals; (2) better

understanding of floc formation and the factors affecting

it; (3) the development of methods of sludge filtration and

processing that overcome, in part, the difficulty of greater

sludge bulk; and (4) the establishment of the relationship

between eutrophication in streams and nutrients, particularly

phosphorus, nitrogen oxides, and organic matter This

rela-tion establishes the need of final effluent wastes free of such

pollutants regardless of the cost of additional treatment

The settling velocities of finely divided and colloidal

particles in wastewaters are so small that removing them

in a settling tank under ordinary conditions is impossible

unless very long detention periods are provided Therefore,

it has been necessary to devise means to coagulate these

very small particles into larger ones that will have higher

settling velocities The aggregation of dispersed particles in

wastewater is induced by addition of chemical coagulants to

decrease the effects of stabilizing factors such as hydration

and zeta potential, and by agitation of the medium to

encour-age collisions between particles

Because of the greater amount of suspended matter in

sewage, the doses for chemical coagulants are generally

con-siderably greater Therefore, in order to keep costs down, it

is important that the chemical reaction involved with each

coagulant should be known and enhanced and that optimum

pH values be obtained by adjustment with acid or base to get more efficient coagulation and clarification with least sludge production

are added to an aqueous system for the purpose of creat-ing rapid-settlcreat-ing aggregates out of finely divided, dispersed matter with slow or negligible setting velocities The poten-tial applications of this process in treating wastewater are:

(1) direct coagulation of organic matter present mostly as colloidal particles in wastewater; (2) the removal of colloi-dal substances prior to such tertiary treatment processes as ion exchange, carbon adsorption, and sand filtration; (3) the removal of colloidal precipitates formed in phosphate pre-cipitation processes; and (4) the removal of dispersed micro-organisms after a brief biooxidation process

The majority of colloids in domestic wastewater or in organic wastes are of a hydrophilic nature; that is, they have

an affinity for water The affinity of hydrophilic particles for water results from the presence of certain polar groups such

as −COOH and −NH2 on the surface of the particles These groups are water-soluble and, as such, attract and hold a sheath

of water firmly around the particle The primary charge on hydrophilic colloidal particles may arise from ionization of the chemical groups present at the surface of the particles, e.g., car-boxyl, amino, sulfate, and hydroxyl This charge is dependent upon the extent to which these surface groups ionize, and thus the particle charge depends upon the pH

Equalization Raw

wastewater

Spill pond

Filtration Precipi-tation Oxidation/

reduction Heavy metals

Process wastewaters chemicalsOrganic

Organics, ammonia In-plant treatment

Centrifugation

Drying

Sludge disposal Incineration

Lagooning

Land disposal Sludge digestion

Filtration

Gravity thickening Dissolved-airflotation Sludge dewatering

Air or steam stripping

To discharge or POTW GAC

adsorption

Oxi-dation

Neutralization Coagulation

Filtration

Anaerobic treatment Activated

sludge

Ozonation PAC coagulant

PACT Nitrification/

denitrification

RBC

Aerated lagoon Trickling

filter

To publicly owned treatment works (POTW) Primary treatment

Acid or alkali Chemicals

Flotation Sedi-mentation

Filtration adsorptionGAC

Discharge to receiving water

Secondary treatment

Tertiary treatment Biological

roughing

Wastewater Return flows Sludge GAC and PAC: granular and powdered activated carbonRBC: rotating biological contractor FIGURE 1 Alternative technologies for wastewater treatment (From Eckenfelder, 2000 Reprinted with permission from McGraw-Hill.)

The precise zeta potential that yields optimum

coagu-lation must be determined for a given wastewater by actual

correlation with jar test or plant performance The control

point has been reported to be in the range of 0 to −10 mV

when raw sewage is coagulated by alum It is important that

coagulants contribute polyvalent ions of charge opposite to

the zeta potential of the dispersion On a molar basis,

biva-lent ions seem to be about 10 to 50 times and trivabiva-lent ions

about 300 to 700 times as effective as monovalent ions for

destabilization of dispersion in wastewater (Rich, 1963) The

zeta potential is unaffected by pH in the range of 5.5 to 9.5

(Eckenfelder, 2000)

Since most dispersions encountered in wastewaters

are stabilized by negative charges, coagulants required are

polyvalent cations such as aluminum, ferric, ferrous, or

cal-cium Organic polyelectrolytes are also effective coagulants

Dispersions stabilized principally by electrostatic force are

in general amenable to coagulation inasmuch as addition of

small doses of suitable electrolytes may effect a significant

change in zeta potential of the particles The most widely

used chemicals for coagulation of wastewater are the salts

of aluminum and iron Lime alone has also been used for

precipitation of phosphates

requires the presence of alkalinity, which, if naturally present

in wastewater in the form of bicarbonate, would lead to the

following reaction:

Al2(SO4)3.xH2O  3Ca(HCO3)2 →

In case of insufficient alkalinity in the wastewater, lime is

generally added, and the reaction with alum becomes:

Al2(SO4)3.xH2O  3Ca(OH)2 →

In the presence of phosphate, the following reaction also

occurs:

Al2(SO4)3.xH2O  2PO4-2 →

Aluminum hydroxide flocs are least soluble at a pH of

approximately 7.0 The floc charge is positive below pH 7.6

and negative above pH 8.2 (Eckenfelder, 1966) The

solubil-ity of AlPO4 is related to the pH and the equilibrium

con-stant for the salt Stumm and Morgan (1970) state that the

solubility of aluminum phosphate is pH-dependent, and the

optimum pH for phosphorus removal lies in the range of 5.5

to 6.5 Generally, at pH above 6.3, the phosphate removal

occurs either by incorporation in a complex with aluminum

or adsorption on the aluminum hydroxide flocs According to

Yuan and Hsu (1970), the reaction mechanism for

precipita-tion of phosphates by aluminum hydroxide is very complex

They have proposed that the positively charged

hydroxy-aluminum polymers are the species that accounts for the

precipitation of phosphates and that effective phosphate pre-cipitation can occur only when the positive charges on the polymers are completely neutralized It is also reported that the effectiveness of aluminum is related to the nature and concentration of the foreign components present and to the ratio of phosphate to aluminum

Alum has been used extensively for phosphate removal

in raw wastewaters Bench-scale tests of alum addition were conducted at Springfield, Ohio, and Two Rivers, Wisconsin (Harriger and Hoffman, 1971 and 1970, respectively) Raw wastewater at Springfield required an average Al:P mass ratio

of 1.9:1 to achieve 80% removal, while at Two Rivers the average mass ratio was 0.93:1 to obtain phosphate removal

of 85% The stoichiometric equation (3) indicates that each kilogram of phosphorus requires 0.87 kg of aluminum for complete precipitation

wastewater coagulation, addition of a small amount of base, usually sodium hydroxide or lime, is essential The required dosage is related to the alkalinity of water Ferrous sulfate reacts with calcium bicarbonate in water, but this reaction

is much delayed and therefore cannot be relied on (Steel, 1960) Caustic alkalinity, due to the addition of lime to the wastewater, produces a speedy reaction The lime is added first, and the following reaction takes place:

FeSO4.7H2O  Ca(OH)2 → Fe(OH)2  CaSO4  7H2O (4) The ferrous hydroxide is not an efficient floc, but it can soon

be oxidized by the dissolved oxygen in wastewater as ferric hydroxide:

An insoluble hydrous ferric oxide is produced over a pH range of 3 to 13 The floc charge is positive in the acid range and negative in the alkaline range, with a mixed charge over

a pH range of 6.5 to 8.0 This process is usually cheaper than the use of alum but needs greater skill to dose with the two chemicals

for wastewater coagulation because it works well in a wide

pH range (Steel, 1960; Wuhrmann, 1968) The reactions

of ferric chloride with bicarbonate alkalinity and lime are, respectively:

2FeCl3  3Ca(HCO3)2 → 2Fe(OH)3 ↓  3CaCl2

Wuhrmann (1968) was successful in removing phosphates from sewage effluent by precipitation with a mixture of ferric salt and lime The ferric dosages varied between 10 and 20 mg/l and the lime dosages from 300 to 350 mg/l

in order to raise the pH to values between 8 and 8.3 The actual lime dosage required is related to the alkalinity of

the water According to Wuhrmann, the dominant reaction

product between the phosphate ion and the ferric ion at pH

above 7 is believed to be FePO4, with a solubility product of

about 1023 at 25°C The colloidal particle size of the FePO4

requires a sufficient excess of ferric ion for the formation of

a well-flocculating hydroxide precipitate, which includes the

FePO4 particles and acts as an efficient adsorbent for other

phosphorous compounds

It has been reported that for efficient phosphorus

removal (85 to 95%), the stoichiometric amount of 1.8 mg/l

Fe required per mg/l P should be supplemented by at least

10 mg/l of iron for hydroxide formation Also, the use of

anionic polymer is considered desirable in order to produce

a clear supernatant (Wukash, 1968)

waste-water to form calcium carbonate, which precipitates, under

normal conditions:

Ca(OH)2  Ca(HCO3)2 → 2CaCO3 ↓  2H2O (8) Normally, 70 to 90% of the phosphorus in domestic sewage

is in the form of orthophosphates or polyphosphates that may

hydrolyze orthophosphates The remaining phosphorus is

present in the form of organic-bound phosphorus The removal

of phosphorus can be achieved by direct adsorption on the

surface of calcium carbonate particles Orthophosphates can

also be precipitated in the alkaline range by reaction with

cal-cium salts to form hydroxyapatite, according to the following

reaction:

10Ca(OH)2  6H3PO4 → 10Ca  (PO4)6(OH)2 ↓

Schmid and McKinney (1969) observed that hydroxyapatite

was present in soluble form at a pH value above 9.5 They

also found that at pH values of 9.5 or less, phosphorus was

adsorbed onto the growing faces of calcium-carbonate

par-ticles, thereby inhibiting their growth Buzzell and Sawyer

(1967) have shown that at pH levels of 10 to 11 in the primary

sedimentation tanks, BOD removal of 55 to 70%, nitrogen

removal of 25%, phosphate removal of 80 to 90%, and

coli-form removal of 99% can be expected Bishop et al (1972)

have reported that precipitation of domestic wastewater with

lime removed approximately 80% of the TOC, BOD, and

COD; 91% of the SS; 97% of the total phosphorus; and 31%

of the total nitrogen Phosphates from secondary effluent have

been removed successfully at Lake Tahoe by precipitation

with lime (Slechta and Culp, 1967) Albertson and Sherwood

(1967) found that by recirculating calcium-phosphate solids,

previously formed due to the addition of lime, it was possible

to reduce the lime dosage by about 50%

Galarneau and Gehr (1997) present experimental results

of their studies on phosphorous using aluminum hydroxide

de-Bashan and Bashan (2004) present an extensive review of

recent advances in phosphorous removal from wastewaters

and its separation for use as a fertilizer or as an ingredient in

other products

In wastewater-treatment practices, it is detrimental to form large floc particles immediately in the flocculation step because it reduces the available floc surface area for adsorp-tion of phosphorus Therefore, it is essential to maintain fine pinpoint flocs in order to get a maximum phosphate removal

by surface adsorption, and this can be achieved by minimizing the time of their flocculation This is not the case if the goal is one of colloidal-solids removal, as is often the case in water treatment

The process of coagulation and flocculation in wastewater treatment can be summarized in the following three steps:

1 As the coagulant dissolves, positive aluminum and ferric ions become available to neutralize the negative charges on the colloidal particles includ-ing organic matter These ions may also react with constituents in solution such as hydroxides, car-bonates, phosphates, sulfides, or organic matter to form complex gelatinous precipitates of colloidal dimensions that are termed “microflocs.” This is the first stage of coagulation, and for greatest effi-ciency a rapid and intimate mixing is necessary before a second reaction takes place

2 After the positively charged ions have neutralized

a large part of the colloidal particles and the zeta potential has been reduced, the resulting flocs are still too small to be seen or to settle by gravity The treatment, therefore, should be flocculation, slow stirring so that very small flocs may agglomerate and grow in size until they are in proper condition for sedimentation Some evidence suggests that aggregation of microflocs with dispersed waste con-stituents is the most important mechanism affecting coagulation in water treatment (Riddick, 1961)

3 During the third phase, surface adsorption of parti-cles takes place on the large surface area provided

by the floc particles Some of the bacteria present will also become entangled in the floc and carried

to the bottom of the tank

the coagulating ions are produced by electrolytic oxidation of sacrificial electrodes This technique has been successfully used in the removal of metals, suspended particles, colloids, organic dyes, and oils An interesting review of this technique

is presented by Mollah et al (2001) In it the advantages and disadvantages of electrocoagulation are presented as well as

a description and comparison with chemical coagulation His group studied its use in the treatment of a synthetic-dye solu-tion with a removal of 99% under optimal condisolu-tions (Mollah, Morkovsky, et al., 2004) Another publication (Mollah, Pathak,

et al., 2004) presents the fundamentals of electrocoagulation and the outlook for the use of this process in wastewater treat-ment Lai and Lin (2003) studied the use of electrocoagulation for the treatment of chemical mechanical polishing wastewater, obtaining a 99% copper removal and 96.5% turbidity reduction

in less than 100 minutes

Sedimentation Sedimentation basins are important

compo-nents in water- and wastewater-treatment systems, and their

performance greatly depends upon proper design In chemical

treatment of wastewater, the separation of chemically

coagu-lated floc depends on the characteristics of the floc in addition

to the factors normally considered in the design of

conven-tional primary and secondary clarifiers Field experience

indi-cates that the usual values for surface overflow rates used in

separating chemical floc in water-treatment plants must be

reduced in order to obtain a good efficiency in the removal

of floc from chemically coagulated wastewater (Weber et al.,

1970; Convery, 1968; Rose, 1968; Kalinske and Shell, 1968)

In wastewater-treatment practices, the recommended overflow

rate for removal of alum floc is 30 m/day, while with use of

lime or iron salts, it can be increased up to 40 m/day

Filtration

Filtration of wastewater can be accomplished by the use of

(1) microscreens, (2) diatomaceous earth filters, (3) sand

fil-ters, (4) mixed media filfil-ters, or (5) membranes The filtration

of sludges, on the other hand, is achieved by sand beds or

vacuum filters

The filtration characteristics of the solids found in a

bio-logical treatment plant effluent are greatly different from

those of the floc formed during chemical coagulation for the

removal of organic matter and phosphates Tchobanoglous

and Eliassen (1970) have noted that the strength of the

bio-logical floc is much greater than that of the flocs resulting

from chemical coagulation

Accordingly, biological flocs can be removed with a

coarser filter medium at higher filtration rates than can the

weaker chemical flocs, which may shear and penetrate through

the filter more readily Lynam et al (1969) had observed that

the chemical floc strength can be controlled, to some degree,

with the use of polymers as coagulant aids Their

experi-ments yielded higher SS removal by filtration when 1 mg/l of

anionic polymer A-21 was used along with alum

The filterability of solids in a conventional biological

plant effluent is dependent upon the degree of flocculation

achieved in the biological process For example, filtration of

the effluent from a trickling-filter plant normally cannot yield

more than 50% removal of the SS due to the poor degree of

biological flocculation in trickling filters On the other hand,

the activated sludge process is capable of a much higher

degree of biological flocculation than the trickling-filter

pro-cess The degree of biological flocculation achieved in an

activated sludge plant was found to be directly proportional

to the aeration time and inversely proportional to the ratio of

the amount of organic material added per day to the amount

of SS present in the aeration chamber (Culp and Hansen,

1967a) It has also been reported that up to 98% of the SS

found in the effluent from a domestic sewage-treatment plant

after a 24-hour aeration time could be removed by filtration

without the use of coagulants (Culp and Hansen, 1967b)

in which flow is passed through a special metallic filter

fabric placed around a drum The filter traps the solids and rotates with the drum to bring the fabric under backwash water sprays fitted to the top of the machine, in order to wash the solids to a hopper for gravity removal to disposal

The rate of flow through the microscreen is determined

by the applied head, normally limited to about 150 mm

or less, and the concentration and nature of the SS in the effluent

Extensive tests at the Chicago Sanitary District showed that microscreens with a 23 µm aperture could reduce the

SS and BOD of a good-quality activated sludge effluent, 20–35 mg/l SS and 15–20 mg/l BOD, to 6–8 mg/l and 3.5–

5 mg/l, respectively (Lynam et al., 1969) It was noted that the microscreens were more responsive to SS loading than

to hydraulic loading and that the maximum capacity of the microscreens was reached at the loading of 4.3 kg/m2/day

at 0.27 m/min

widest application in the production of potable waters, where the raw water supply was already of a relatively good quality, i.e., of low turbidity Operating characteristics of diatomite filters can now be predicted under a wide range of operat-ing conditions by utilizoperat-ing several mathematical models (Dillingham et al., 1966, 1967) Several investigators have studied the filtration of secondary effluent by diatomite filters whose ability to produce an excellent-quality effluent is well established (Shatto, 1960; R Eliassen and Bennett, 1967;

Baumann and Oulman, 1970) However, the extremely high cost and their inability to tolerate significant variations in SS concentration limit the usage of diatomite filters in sewage-treatment practices

sand filters or as rapid sand filters made with one or more media A slow sand filter consists of a 150- to 400-mm-thick layer of 0.4-mm sand supported on a layer of a coarser mate-rial of approximately the same thickness The underdrainage system under the coarser material collects the filtrate The rate of flow through the filter is controlled at about 3 m/day

This rate is continued until the head loss through the bed becomes excessive Then the filter is thrown out of service and allowed to partially dry, and 25 to 50 mm of the sand layer, which includes the surface layer of sludge, is manually scraped from the top for washing Disadvantages of slow sand filtration system are: (1) the filters may become inoper-ative during the cold winter weather, unless properly housed;

(2) slow sand filters may not be effective due to the rapid clogging of filters (the normal frequency of cleaning filters varies from once to twice a month; Truesdale and Birkbeck, 1996); (3) the cost of slow sand filtration is three times the cost of rapid sand filters and twice the cost of microscreens (anonymous, 1967); and (4) the large space requirement

Rapid sand filters consist of about a 400-mm-thick layer

of 0.5- to 0.65-mm sand supported on coarser gravel The rate of filtration ranges between 80 and 120 mm/min At this high filtration rate, the filter beds need backwashing when the head loss becomes excessive

Lynam et al (1969) reported results of detailed tests

con-ducted on filtration of secondary effluent from an activated

sludge plant of the Chicago Sanitary District They used a

filter bed of 0.85-mm-effective-size sand in a 280-mm depth

and a filtration rate of 120 mm/min, and analyzed the data in

terms of both hydraulic and SS loadings Poor correlations

were obtained between effluent quality and hydraulic

load-ing, effluent quality and solids loadload-ing, and solid removal

and hydraulic loading However, an excellent correlation

existed between SS loading and SS removals It was also

observed that the sand filtration of alum-coagulated solids

was no better than that of uncoagulated solids, and the

opti-mum SS removal was obtained by alum and polymer

coagu-lation in combination with sand filtration

A review of the retention of pathogenic bacteria in

porous media is presented by Stevik et al (2004) The

review includes the factor affecting bacteria retention and

the factors that effect elimination of bacteria from porous

media The authors also suggest priority areas of research

in this field

rapid sand filter follows from its behavior as a surface

filtra-tion device During filter backwashing, the sand is graded

hydraulically, with the finest particles rising to the top of the

bed As a result, most of the material removed by the filter

is retained at or very near the surface of the bed When the

secondary effluent contains relatively high solids

concen-trations, the head loss increases very rapidly, and SS clog

the surface in only a few minutes One approach to increase

the effective filter depth is to use dual-media beds consisting

of a discrete layer of coarse coal placed above a layer of

fine sand

More recently, the concept of mixed-media filters has

been introduced in order to achieve a filter performance that

very closely approaches an ideal one In this case a third

layer of a very heavy and fine material, garnet (with specific

gravity of 4.2) or illmenite (with specific gravity of 4.5), is

placed beneath the coal and sand Conley and Hsiung (1965)

have suggested the optimum design values for these filters

The selection of media for any filtration application should

be based on the floc characteristics An example of a typical

dual-media filter is shown in Figure 2

market by the Johns-Manville Corporation in the late 1960s

It is a continuous sand filter in which influent wastewater

passes through the bed and becomes product water Solids

trapped on the filter face and within the bed move with the

filter media, countercurrent to the liquid Solids and small

amounts of filter media regularly removed from the filter

face are educted to the filter media tower without stopping

operations Solids are scrubbed from the media and

dis-charged as a waste sludge, while the washed media is fed

back into the bed

The filter medium usually used is 0.6- to 0.8-mm sand

with a maximum sand-feed rate of 5 mm/min and

maxi-mum filtration rate of 85 m/day (2100 U.S gal/day/ft2) The

advantages claimed for this system are (1) automatic and continuous operation, (2) that the filter allows much higher and variable solids loadings than is permissible with a sand bed, and (3) that through an efficient use of coagulant chemi-cals, the system has the flexibility to reduce turbidity, phos-phorus, SS, and BOD to the desired level (Johns-Manville Corporation, 1972)

more extensively as membrane materials are becoming more resistant and affordable Fane (1996) presents a description of membrane technology and its possible applications in water and wastewater treatment An extensive study on microfiltra-tion performance of membranes with constant flux for the treatment of secondary effluent was published by his research group in 2001 (Parameshwaran et al., 2001) Kentish and Stevens (2001) present a review of technologies for the recy-cling and reuse of valuable chemicals from wastewater, par-ticularly from solvent-extraction processes

A feasibility study on the use of a physico-chemical treatment that includes nanofiltration for water reuse from printing, dyeing, and finishing textile industries was per-formed by Bes-Pia et al (2003) In this work jar tests were conducted for flocculation using commercial polymers fol-lowed by nanofiltration Their results show that the combina-tion reduces COD from 700 to 100 mg/l Another treatment approach by the same authors (2004) uses ozonation as a pretreatment for a biological reactor with nanofiltration as

a final step A combined approach is presented by Wyffels

et al (2003) In this case a membrane-assisted bioreactor for the treatment of ammonium-rich wastewater was used, showing this to be a reliable technology for these effluents

Galambos et al (2004) studied the use of nanofiltration and reverse osmosis for the treatment of two different waste-waters For their particular case the use of reverse osmosis was more convenient due to the high quality of the effluent, but the permeate of the nanofiltration can only be released into

a sewer line or would have to be treated, resulting in an eco-nomic compromise A comparison between a membrane bio-reactor and hybrid conventional wastewater-treatment systems

at the pilot-plant level is presented by Yoon et al (2004)

The removal of volatile organic compounds (VOCs) using a stripper-membrane system was studied by Roizard

et al (2004) Their results show that this hybrid system can

be used for the removal of toluene or chloromethane with a global efficiency of about 85%

Vildiz et al (2005) investigated the use of a coupled jet loop reactor and a membrane for the treatment of high-organic-matter-content wastewater The main function of the membrane is the filtration of the effluent and the recycle of the biomass to the reactor One advantage of the system is its reduced size as compared with traditional treatment systems,

as well as a better-quality effluent

A comprehensive review on the use of nanofiltration membranes in water and wastewater use, fouling of these membranes, mechanisms of separation, modeling, and the use of atomic force microscopy for the study of surface mor-phology is presented by Hilal et al (2004) The future of

membranes and membrane reactors in green technology and

water reuse was published by Howell (2004) In it, water

problems in different regions of the world are discussed,

different membrane systems are presented, and different

approaches for new research are introduced

cur-rently in use to produce freshwater from seawater With recent

improvements in membranes, this process is also being used

for purification of wastewater Substantial removal of BOD,

COD, total dissolved solids, phosphate, and ammonia by this

process has been reported (Robinson and Maltson, 1967)

In a reverse-osmosis process, wastewater containing

dis-solved materials is placed in contact with a suitable

semi-permeable membrane in one of the two compartments of

the tank The pressure on this compartment is increased to

exceed the osmotic pressure for that particular waste in order

to cause the water to penetrate the membrane, carrying with

it only a small amount of dissolved materials Therefore, the

dissolved material in the wastewater gets concentrated

con-tinuously, while highly purified water collects in the other

compartment

The performance of the reverse-osmosis process depends

mainly on (1) the membrane semipermeability or its

effi-ciency to separate dissolved material from the wastewater,

and (2) the membrane permeability or the total amount of

water that can be produced with appropriate efficiency for

the removal of dissolved materials It has been reported that

the conventional cellulose-acetate membranes give adequate

separation efficiency, but the flow rate of water is too small

to be of practical interest However, cellulose-acetate mem-branes allow a much higher flow rate of product water, at the same separation efficiency, which makes it applicable in wastewater-treatment practices (Goff and Gloyne, 1970)

The operating pressure, as well as the rejection perfor-mance of the membrane, is dependent on the membrane porosity Rejection performances of three graded mem-branes with secondary sewage effluent were investigated by Bray et al (1969)

Merten et al (1968) evaluated the performance of an 18.9 m3/day pilot reverse-osmosis unit in removing small amounts of organic material found in the effluent of carbon columns treating secondary effluent With a feed pressure

of 2760 kPa and water recovery of 80 to 85%, 84% removal

of COD present in the carbon column effluent, averaging 10.8 mg/l, was achieved Problems of clogging have occurred when operating with waters containing high concentrations

of bicarbonate, and as such, adjustment of pH to prevent calcium-carbonate precipitation is normally required

Sadr Ghayeni et al (1996) discuss issues such as flux control and transmission in microfiltration membranes and biofouling in reverse-osmosis membranes in their use for the reclamation of secondary effluents The process used for the study consisted of a microfiltration membrane fol-lowed by a reverse-osmosis membrane The performance

of this combined system was evaluated by Sadr Ghayeni et

al (1998b), as was the study of the adhesion of bacteria to reverse-osmosis membranes (1998a)

Effluent

Transfer pipe

Storage compartment

Filter compartment

Collection chamber

Air Underdrain nozzles

Recycle

Backwash

Equalization tank

Drain Sump

Coal Sand

Filter backwash

Polymer

Three-way valve

Water level

Alum

Influent

FIGURE 2 Typical dual-media filter (From Eckenfelder, 2000 Reprinted with permission from McGraw-Hill.)

A study on the processing of composite industrial effluent

by reverse osmosis was published by Sridhar et al in 2003

The effluent used in the study was from combined bulk drug

and pharmaceutical companies, obtaining a removal of 88%

of dissolved solids, COD, and BOD, with reasonable water

recovery They also present a comparison between aerobic

and reverse-osmosis treatment for this effluent

A physical-chemical process for the treatment of

chemi-cal mechanichemi-cal polishing process wastewater is presented by

Lin and Yang (2004) In it the authors used chemical

coagula-tion using different coagulants followed by reverse osmosis,

obtaining water capable of being reused in the process due to

its characteristics

inorganic ions from water by creating an electrical potential

across two electrodes dipped in water One of the two strips

serves as a cathode and the other as an anode The treatments

that can be achieved by electrodialysis include:

1 Removal of inorganic ions: Under the effect of

applied potential, cations and anions migrate to the cathode and anode, respectively By alternat-ing membranes, a series of concentratalternat-ing and diluting compartments can be created For a long run and better efficiency, it is essential that turbid-ity, SS, colloids, and trace organics are removed from the wastewater before it enters the electrodi-alysis unit

2 Effective bacteria reduction in wastewater: Most

of the municipal wastewaters contain a high con-centration of chloride ions Oxidation of chloride

at the anode produces chlorine, hypochlorite,

or chloramines, depending on the nature of the wastewater Chlorine in these forms is a good dis-infectant and also provides an effective means of reducing soluble BOD

In order to reduce the operating cost of the electrodialysis

pro-cess, the eroding anodes made of aluminum or iron are now

being replaced by nonconsumable noble anodes, which appear

to have more potential in wastewater treatment (Culp and Culp,

1971) The cost of disinfection by electrodialysis is reported to

be 0.053 $/m3 of wastewater as compared to 0.095 $/m3 for

the conventional chlorination (unpublished proposal, 1970)

However, some other sources have reported that the cost

of electrolytic treatment of wastewater was too high for the

removal of a large percentage of secondary effluent COD

Grimm et al (1998) present a review of electro-assisted

methods for water purification, including electrodialysis

Fukumoto and Haga (2004) applied this technique for the

treatment of swine wastewater with removal rates for NO3− and

PO4−3 ions of 99% and an average color reduction of 58%

Gas Stripping

In domestic wastewaters, most of the nitrogen that gets

converted to ammonia during biological degradation is

present either as ammonia or in organic form When the carbon concentration in wastewater becomes low and the nitrifying bacteria are populous, this ammonia can be oxi-dized by bacteria to nitrites and nitrates in the presence of dissolved oxygen The stripping process can be employed either before or after secondary treatment for removing high levels of nitrogen that is present as ammonia If it

is to be used as pretreatment prior to a biological system, enough nitrogen, N:BOD  5:150, must be left in the efflu-ent to satisfy the nutritional requiremefflu-ent (Eckenfelder and Barnhart, 1963)

In wastewater, ammonium ions exist in equilibrium with ammonia and hydrogen ions:

At pH levels of 6 to 8, ammonia nitrogen is mostly present

in the ionized form NH4 Increasing the pH to above 10 changes all the nitrogen to ammonia gas, which is remov-able by agitation The stripping of ammonia from wastewater

is carried out with air In this operation, wastewater is agi-tated vigorously in a forced-draft countercurrent air-stripping tower when the ammonia is driven out from the solution and leaves with the air exhausted from the tower The efficiency

of ammonia removal in the stripping process depends upon the pH, airflow rate, tower depth, and hydraulic loading to the tower

Slechta and Culp (1967) have shown experimentally that the efficiency of the ammonia-stripping process is dependent

on the pH of the wastewater for pH values up to 10.8 However,

no significant increase in ammonia removals was achieved

by elevating the pH above 10 Kuhn (1956) had come to the same conclusion It has also been reported that the efficiency

of the ammonia-stripping process depends on maximizing the air–water contact within the stripping tower Higher ammo-nia removals and lower air requirements were obtained with

a 40  50-mm packing than with a 100  100-mm packing

Increased tower depth, which provides additional air–water contact, results in greater ammonia removals and lower air requirements Ammonia removals of 90%, 95%, and 98%

were obtained at airflow rates of 1875, 3000, and 6000 m3 per cubic meter of wastewater, respectively

Gas stripping is also used for removal of H2S and VOCs from wastewater

Ion Exchange

The ion-exchange process has been adopted successfully

in wastewater-treatment practice for removing most of the inorganic dissolved salts However, the cost of this method for wastewater treatment cannot be justified unless the efflu-ent water is required for multiple industrial municipal reuse

One of the major applications of this technique is the treat-ment of plating-industry wastewater, where the recovery of chrome and the reuse of water make it an attractive choice (Eckenfelder, 2000)

Gaffney et al (1970) have reported that the modified DESAL process, developed for treating acid mine drainage

waters, can be applied successfully to the treatment of

sec-ondary sewage effluent This process consists of passing

secondary sewage-plant effluent upflow through an

ion-exchange unit filled with a weak base anion ion-exchange resin,

Amberlite IRA-68, operated on a bicarbonate cycle The

effluent with a pH of 6.0 is then treated with a small

quan-tity of bentonite and cationic flocculant, Prima floc C-7,

fol-lowed first by aeration to drive out carbon dioxide, and then

lime softening in proportion to its hardness concentration

A dosage of 30 mg/l of bentonite, 3 to 5 mg/l of

polyelec-trolyte, and normal lime levels are required The effluent

that is partially desalinated and essentially free of nitrates,

phosphates, chlorides, alkyl benzene sulfonate (ABS), and

COD can be produced If the salinity is too high, it may be

reduced further by passing a portion of the effluent through

a weak acid cation, Amberlite IRC-84 It has been observed

that IRA-68 can remove much of the organic contents and

COD, thereby eliminating or markedly reducing the need for

carbon treatment

Slechta and Culp (1967) tested a cationic resin, Duolite

C-25, for the removal of ammonia nitrogen from the carbon

column effluent that was containing ammonia nitrogen in

the range of 18 to 28 mg/l as nitrogen A 100-mm-diameter

Plexiglas cylinder filled to a depth of 700 mm with the resin

served as the pilot ion-exchange column The rate of

appli-cation of influent waste to the ion-exchange column was

0.4 m3/min per cubic meter of resin Following breakthrough

of the ammonia nitrogen to 1 mg/l, the bed was backwashed

and the resin was regenerated On the average, about 400 bed

volumes of carbon column effluent had been passed through

the ion-exchange resin prior to a breakthrough to 1 mg/l

ammonia nitrogen However, considering the operating and

capital costs, they concluded that the ammonia-stripping

process was more efficient

Nitrate nitrogen, present in the effluent from the

acti-vated sludge process, has been removed by anion exchange

regenerated with brine by R Eliassen and Bennett (1967)

This ion-exchange process also removes phosphates and

some other ions; however, pretreatment by filtration is

essential The resin is restored by treatment with acid and

methanol

The removal of heavy metals with Mexican

clinoptilo-lite was studied by Vaca Mier et al (2001) In this study the

interactions of lead, cadmium, and chromium competed for

the ion-exchange sites in the zeolite The authors also

stud-ied the influence of such factors as the presence of phenol

and the pH of the solution to be treated

Adsorption

Application of adsorption on granular active carbon, in

columns of counterflow fluidized beds, for the removal of

traces or organic pollutants, detergents, pesticides, and other

substances in wastewater that are resistant to biological

deg-radation has become firmly established as a practical, reliable,

and economical treatment (Slechta and Culp, 1967; Weber,

1967; Parkhurst et al., 1967; Stevens and Peters, 1966;

Presecan et al., 1972)

Adsorption can also be accomplished with powdered carbon (Davies and Kaplan, 1964; Beebe and Stevens, 1967), which is mixed in wastewater, flocculated, and ultimately settled However, there are certain problems associated with the use of powdered carbon These are: (1) that large quanti-ties of activated carbon are needed in wastewater treatment, because it is used only on a once-through basis, and han-dling of such large quantities of carbon also creates a dust problem, and (2) problems in disposal of precipitated carbon unless it is incinerated along with the sewage sludge

includ-ing soluble organic pollutants, are removed by adsorption

on a large surface area provided by the activated carbon

Smaller carbon particles enhance the rate of pollutant removal by providing more total surface area for adsorption, partial deposition of colloidal pollutants, and filtration of larger particles However, it is almost always necessary to remove finely divided suspended matter from wastewater by pretreatment prior to its application on a carbon bed

Depending on the direction of flow, the granular carbon beds are either of the downflow-bed type or upflow-bed type

Downflow carbon beds provide the removal of suspended and flocculated materials by filtration beside the absorption

of organic pollutants As the wastewater passes through the bed, the carbon nearest the feed point eventually becomes sat-urated and must be replaced with fresh or reactivated carbon

A countercurrent flow using multiple columns in series is considered more efficient The first column is replaced when exhausted, and the direction of flow is changed to make that column the last in the series Full countercurrent operation can best be obtained in upflow beds (Culp and Culp, 1971)

Upflow carbon columns for full countercurrent opera-tions may be either of the packed-bed type or expanded-bed type Packed expanded-beds are well suited to treatment of wastes that contain little or no SS, i.e., turbidity less than 2.5 JTU

However, the SS invariably present in municipal and indus-trial wastewaters lead to progressive clogging of the carbon beds Therefore, expanded-bed upflow columns have certain potential advantages in operation of packed-bed adsorbers for treating wastes that contain SS In expanded-bed-type adsorbers, water must be passed with a velocity sufficient

to expand the bed by about 10%, so that the bed will be self-cleaned

Experiments conducted by Weber et al (1970) have shown that expanded-bed and packed-bed adsorption sys-tems have nearly the same efficiency with regard to the removal of soluble organic materials from trickling-filter effluent, under otherwise similar conditions The packed-bed system was found to be more effective for removal of

SS, but the clogging that resulted from these solids required higher pumping pressure and more frequent cleaning of the carbon beds Because of the time elapsed in cleaning, the expanded-bed production was about 9% more than the packed-bed production

The Lake Tahoe Water Reclamation Facilities, described

by Slechta and Culp (1967), included pretreatment of second-ary effluent by chemical clarification and filtration, thereby