Desalination, 75 (1989) 213-240 Else&r Science Publishers B.V., Amsterdam -Printed in The Netherlands

213

MUNICIPAL WASTEWATER RENOVATION BY REVERSE OSMOSIS

STATE OF THE ART

A. H. Ghabris’, M. Abdel-Jawad and G.S. Alg2

‘Kuwait Institute for Scientific Research, P. 0. &nt 24885, Safat 13109, Kuwait

‘Kuwait University, Chemical Engineering Dept., P. 0. Box 5969, Safat 13060, Kuwait

Reverse Osmosis technology has proven to be technically efficient, cast effective and pollution controlling process for the renovation of different municipal wastewater streams. A critical review on the application of reverse osmosis technology to renovate municipal wastewater will be presented. Spe- cial emphasis is given to recent process developments, flow sheet configurations, membrane efficiency in reducing effluent ‘IDS, microorganisms, organ.&, nut- rients and others. Major problems encountered, pollution aspects, economic feasibility and areas of product water utilization is also discussed.

INTRODUtZ’ION

Desalination of seawater and brackish water has been the principle his-

toric focus of reverse osmosis application. Based on 1987 statistics, RO covers

more than 65% of all desalting plants available in the market, with a capaci-

ty of 100 m3/day and more (ref. 1).

Increasing demand for water sources coupled with improving quality of

the environment necessitates the review of available water resources. The con-

tinuous development and refinement of RO membranes and hardware has now

214

made it possible to implement the technology to municipal sewage effluents.

This separation process is usually used as a “polishing” step for the treatment

of secondary and tertiary effluents. Limited work was done on raw sewage

and primary effluents.

Comprehensive and valuable research efforts, mainly sponsored by the

U.S. Environmental Protection Agency, were carried out during the 1970’s.

Unfortunately, such efforts have slowed down in the 1980’s. However, the

largest commercial reverse osmosis plant (5 mgd) treating secondary effluent is

located at Water Factory 21 (WF21), Orange County, California. The distin-

guished contribution of its research and operation personnel to this field should

be acknowledged.

Experimental pilot plants and some large scale commercial plants using

reverse osmosis in renovating secondary effluents showed remarkable efficien-

cies in reducing total dissolved solids, a broad range of organic contaminants,

microorganisms and nutrients at relatively low cost as presented in the follow-

ing comprehensive review. The utilization of product water for community

reuse, industrial and irrigation purposes gives the process more importance.

PRETREATIkIENT

The objective of pretreatment for reverse osmosis is the reduction or

removal .of hazardous and undesired constituents from the feed. Pretreatment

largely depends on feed water quality, membrane type and configuration, meas-

215

ures arranged for fouling prevention, cleaning frequency, designed recovery

ratio, and product water quality. Several pretreatment techniques, mechanical

and/or chemical, are usually implemented on sewage effluents prior to pump-

ing the feed to the RO membranes. Figure (1) is an informative block dia-

gram showing different pretreatment methods.

MlYSIL cn.CW_.“~

Fig. 1. Pretreatment methods of municipal secondary effluent for reverse osmosis (ref. 2).

Unit operations in common use include chemical clarification or floccula-

tion, multi-media filtration, activated carbon adsorption, chlorination, acid addi-

tion for pH control and sodium hexametaphosphate addition for scale control.

Chemical clarification is the first step in almost any pretreatment sequence.

At WF21, a lime dose of 350-500 ppm as CaO applied to secondary effluent

reduces COD by 40-60% (ref. 3). In fact, a direct lime clarification followed

by RO’was carried out in 1984 at WF21, as described by Argo (ref. 4). This

simple combination proved to be remarkably effective in terms of constituents

216

removal and cost. Wechsler (ref. 5) used 500 ppm alum L412&0~>~18H20>

in the batch clarification of secondary effluent as pretreatment for an RO

pilot facility in Leider, Netherlands. Kalinske et al. (ref. 6) proposed the use

of Lime&da ash (Na2C03) for a wastewater recycling facility for Petromin

refinery at Saudi Arabia, as shown in Fig. 2. Soda ash is added to precipitate

calcium ions associated with non-carbonate hardness. Stenstrom (ref. 7) showed

that the evolutionary development of cleaning and pretreatment techniques can

improve water recovery from an average of less than 30 percent to more than

70 percent. Several coagulants were tested including organic polymers, ferric

chloride, alum (Al(OHj3) and aluminium sulfate. The latest coagulant was

the least efficient of all.

Media filtration, also referred to as rapid sand filtration, is an economi-

cal and widely used pretreatment step for RO. It could remove dissolved

organic-s and suspended solids (> 10 pm>. Typical media are coal, silica sand

and garnet (refs. 3,891 or only sand and anthracite (ref. 6).

Cartridge filters can remove small particles in the 5 to 25 pm range.

It is a preliminary polishing step and is used in almost every RO process.

Dissolved chlorinated organics and refractory organics are usually removed by

activated carbon adsorption. Such organics might not be removed efficiently

by RO, so the use of activated carbon adsorption prior to RO is necessary

(refs 3, 10). At WF21, removal efficiencies of activated carbon adsorption are

6540, 58% and 99% for COD, TOC and phenol, respectively.

217

Ultrafiltration is a low pressure membrane process used when solute

molecules are at least two orders of magnitude larger than solvent molecules.

It is very efficient in removing suspended solids and BOD. Olsen and Haagen-

sen (ref. 11) used ultrafiltration as a pretreatment process. By ultrafiltration,

the colloidal matter will be removed, leaving the salt contents unchanged, thus

producing a clear filtrate suitable for further refinement by reverse osmosis.

Normal RO processes require further injection of chlorine as disinfectant, acid

for pH control and antiscalant such as SHMP as practiced at WF21 and shown

in Fig. 3 (ref. 3).

Ml?MBRANl3 DEVELQPMENT

Membrane preparation and testing received serious attention since 1957

when Reid and Breton (ref. 12) reported that cellulose acetate membranes were

sufficiently permeable to water and sufficiently impermeable to salt. Loeb

and Sourirajan (ref. 13) did remarkable experimental works in developing

asymmetric semipermeable cellulose acetate membranes. The first reported

work on the application of reverse osmosis to wastewater reclamation

sponsored by the U.S. Public Health Service during 1964 (ref. 14).

Cellulose acetate membranes were used to treat secondary sewage 0

WilS

effl-

uent. Ultrathin membranes with thickness from 500 to 6000 A were inves-

tigated .by Rozelle et al. (ref. 15,161 for reverse osmosis treatment of municipal

wastewater. Three polymers, cellulose acetate 0-propyl sulfonic acid

218

Fig. 2. Proposed wastewater reclamation plant in Riyadh(reproduced from ref. 6).

219

(CAOPSA), cellulose acetate methyl sulfonate. and ultrathin cellulose acetate,

were tested. Documented results show general comaminants removal of 95%

and above. The highest average flux of 43 gallons/sq.ft. day (gfd) was

observed for the CAOPSA. After forty hours of operation, the average water

flux for this membrane remained at about 36 gfd which was comparable to

the ultrathin cellulose acetate membrane (34 gfd). Much lower average flux

was observed for the cellulose acetate methyl sulfonate (20 gfd). More recent-

ly (1976). Fang and Chian (ref. 17) conducted an extensive experimental study

on 12 different membranes made of cellulose acetate, cellulose acetate butyrate,

cellulcse triacetate, polysulfone-polyethyleneimine-toluene-2,4-di-isocyanate (NS-

series), poly-2, 2_(m-phenylene) 5, 5-bibenzimidazole (PBI series), sulfonated

polyphenylene oxide (SPPO - series) and aromatic polyamide were tested on

removing 13 low molecular weight polar organic compounds. Rejection of the

13 model compounds ranged between 13-27, 50 and 75% for the cellulose ace-

tate base, aromatic polyamide and the NS-type membranes, respectively. Fluxes

were 6.02 and 7.63 gfd for the cellulose acetate and NSseries membranes com-

pared with 2.45 gfd for the aromatic polyamide flat sheet membranes.

A three years study had been made on the performance of cellulose ace-

tate reverse osmosis membranes using secondary effluent by Winfield (ref. 18).

The study showed that the deterioration in salt rejection performance is less

than what the hydrolysis theory predicts The recent developments of thin

film composite membranes are well summarized by Riley et ah (ref. 19) of

UOP. A new generation of low pressure RO membranes developed by Filmtec

220

Corp. and Desalination System Inc., were tested in a pilot study at Water Fac-

tory 21. The thin film compcsite type, primarily polyamide (PA) materials,

can operate at considerably lower pressure (17 bar vs 31 bar) than the con-

ventional cellulosic membranes as stated by Argo (ref. 20). Flux for these

low pressure TFC membranes ranged between 11-15 gfd, whereas for the cel-

lulose acetate membrane it was only 8 gfd. In general, the use of polyamide

membranes is favoured over the conventional cellulose acetate membrane due to

its chemical stability which allows operation at higher temperatures, wider pH

ranges, better resistance to bacteria attack and better resistance to aggressive

cleaning chemicals (ref. 21). Several comparative pilot plant studies had been

conducted to evaluate and compare the performance of different modular

designs, namely tubular, flat cells, spiral wound and hollow fine fibers (refs.

9, 22, 231, when applied to municipal wastewater streams Since spiral

wound membranes are less prone to fouling, easy to handle, easy to clean and

manufacture, and available at competitive prices they are usually favoured

over other configurations (refs. 3, 21).

REMOVAL OF INORGANICS

Reverse Osmosis is known as a demineraLizing process. Most of the

work done on the renovation of municipal effluents has been concerned with

salt rejections. Salt content, however, is by no means the most important cri-

terion of quality. This is attributed to the fact that total dissolved solids in

221

sewage effluents is relatively low, and the inorganic pollutants usually present

in very small amounts. Membrane rejection of TJX is a function of different

variables, mainly membrane type, ionic charge, feed sour= and recovery

required. Strauss (ref. 24) cited increased interest in RO wastewater separa-

tions. A system started in 1978 treating secondary effluent to produce a per-

meate with 20-30 ppm of TDS. The units had 98.9% solid rejection capability

at 70% water recovery. Feige et al. (ref. 25) concluded that when using

tubular cellulose acetate membranes with different types of sewage streams

namely primary effluent, lime clarified raw wastewater and secondary effl-

uent, TDS rejection in all cases was above 95%. Smith et al. (ref. 26) report-

ed results obtained at Pomona plant, California, using spiral wound ~el.hlo~e

acetate membranes. TJX rejection ranged between 93 to 95%. Sourirajan (ref.

27) presented percentage rejection of various salts by cellulcse acetate mem-

branes, mainly MgS04, Na2S04, CaC12, NaCl, NaN03, and NH4C1. In most

cases, rejection was above 99%. Reverse Osmosis was used to renovate primary

effluent at the city of Corona, California (ref. 28). TDS reduction was

always above 94%. McCarty et al. (ref. 29) showed effectiveness of RO in

removing inorganic pollutants such as heavy and trace elements at WF21.

More than 20 components were tested and removal was almost always above

90%. Feuerstein et al. (ref. 30) presented the reduction of electrical conductiv-

ity, an indirect measure of TDS, for various waste streams Rejections for

carbon. treated secondary effluent, secondary effluent, primary effluent, raw

sewage effluent and digester effluent was reported to be 92%, 91.7%. 87.640,

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82.8% and 77.6%, respectively. ‘IDS rejection by RO at Hemet. California for

a six years period averaged 97% (ref. 87). Wechsler (ref. 5) demonstrated that

as recovery increases, ‘IDS rejection decreases and visa versa. Briefly all

workers in this field reported impressive results in terms of TDS reduction by

RO. Furthermore, manufacturers of the newly developed high rejection thin

film composite membranes guarantee minimal rejection of 98%.

REMOVAL OF MICROORGANISMS

Microorganisms are usually present in enormous numbers, measured as

counts/ml, in sewage effluents. Clarke et al. (ref. 31) reported that about 39

percent of all the samples of chlorinated effluents of conventional treatment

analyzed contained viruses. More than 100 different types of enteroviruses can

occur in domestic sewage (refs. 32, 33). The conventional processes of sewage

treatment do not remove all microorganisms from wastewater. In any waste-

water reclamation process, the removal or inactivation of these microorganisms

is of utmost importance. Viruses, being very active at very low numbera, are

considered the most dangerous among all other microorganisms and pathogens

that might exist in domestic sewage supplies

Because of the size of viruses and the generally accepted membrane

transport theory, manufacturers of reverse osmosis membranes have long insist-

ed that .no viruses should appear in the product water (ref. 34). In an early

study by Hindin et ah (ref. 35). it was found that coliform bacteria and bac-

223

teriphage of approximately the same shape and sire as entric viruses did not

permeate through a porous cellulose acetate membrane.

Early studies of Sorber et al. (ref. 34) showed almost complete removal

of both coliphage T2 and poliovirus by RO using cellulose acetate membranes.

Coliphage T2 is the most structurally complex virus and consists of a

polyhedral-shaped head 95 pm long and 65 pm wide and complex tail of 25

pm in diameter and 100 pm long. Poliovirus is roughly spherical with a

diameter of 25 pm. In all runs Performed in this study, rejection was above

99.99% and the small permeation of viruses, if cccurred, is attributed to ran-

dom phenomena and operational problems.

More recent studies done by Leong et al. (ref. 36) and McCarty et al.

(refs. 37, 38) showed the efficiency of RO in removing enteroviruses and bac-

teria at WF21. In a case study by Wojcik et al. (ref. 39) utilizing the Las

Gallinas trickling filters effluent, the celh&se acetate RO pilot plant removed

99.95% of coliform bacteria and 85.53% of coliphage viruses.

The complete elimination of viruses and bacteria typically present in

wastewater by RO was also discussed by Cooper et al. (ref. 40).

REMOVAL OF ORGANICS

Conventional water treatment methods are not capable of removing all

soluble and colloidal organic matter that might exist in sewage streams (refs.

41, 42). Sewage and many other types of wastewater are considered heteroge-

224

nous mixtures Normally, the hinds and concentrations of specific organic con-

taminant present in the treated sewage effluent might vary according to both

location and time.

Standards for water supplies were provided for some 300 harmful subs-

tances, almost all organics, by the World Health Organization (WI-IO) according

to studies conducted by the Ministry of Health of the USSR (ref. 43).

Even though reverse osmosis was originally developed for demineraliza-

tion of sea and brackish water to obtain potable water, much attention has

been given to study its characteristics of rejecting a wide range of organic

compounds and contaminants that might exist in domestic sewage supplies

(refs. 44-49). Contaminant removal usually depends on the chemical and

physical characteristics of both the membranes and the contaminants (refs. 46,

50, 51). Early investigations (refs. 52-55) showed that the sire, shape and

chemical characteristics of a compound influence its passage rate through the

membrane.

In a series of reverse osmosis tests performed by Duvel and Helfgott

(ref. 461, the organic rejection was found to be semiquantitatively related to

molecular weight and size as determined by steric geometry. It was also

found that chemical characteristics of a molecule, particularly its ability to

form hydrogen bonds are also important in determining permeability. Matsuu-

ra and Sourirajan (refs. 4556.5758) declared that solute separation in reverse

osmosis is a function of the extent of preferential sorption of water by the

membrane material, which in turn is a function of the chemical nature of the

225

organic solutes. Polar effect of the solute molecule which includes both the

functional and the substituent groups also affect the organic permeability.

Should the compound have the same functional groups, increasing the branch-

ing will increase the solute separation (ref. 59).

Typical composition of soluble organics in secondary effluents was

reported by Rebhum and Marka (ref. 60) to consist of 40-50% humic substanc-

es such as humic and fulvic hymathomelamic acids, 8.3% ether extractables,

13.9% detergents, 11.5% carbohydrates, 22.4% proteins and 1.7% tannins Even

though such organics are usually defined in gross parameters such as COD,

TOC and BOD, however, several trials were done by researchers to study indi-

vidual groups.

Ea,$y works in this field were done by Hindin et al. (ref. 441, who

studied the permeation of about 30 different chemical species using cellulose

acetate membranes. Detergents, soaps, motor oil, DDD, DDT, tan& and humic

acids, peptone, 2,4diisopropyl ester, p-chloronitrobenze, lindane, soluble starch,

acetone and cellulose. Rejection was in most cases above 90% except for the

last five species where rejection was below 70%. Chian and Fang (ref. 48)

covered 14 model toxic species belonging to aromatics, acids, alcohols, aldehydes,

ketones, amines, ethers and eaters using both cellulose acetate and polyamide

membranes. Polyamide membranes (Dupont B9) and non-cellulose derivatives

ultrathin membranes (NS-1) showed superiority in terms of rejection over cel-

lulose acetate membranes.

226

McCarty et al. (refs. 29, 61) and Reinhard et al. (refs. 49, 62) did

extensive work on performance and reliability of Water Factory 21, in remov-

ing trace organics and pollutants. The performance of reverse osmosis was

studied in combination of other advanced treatment processes. Removal of a

wide range of organic priority pollutants as identified by Keith and Telliard

(ref. 63) were tested at WF21 (ref. 3). These organ&s were mainly chlorinat-

ed hydrocarbons, high molecular weight organ& and pesticides. Overall rejec-

tion of such species by RO ranged between 45% and 99%. Recent comparable

studies, using cellulose acetate and polyamide membranes, were also done at

WF21 by Reinhard et al. (ref. 49). Volatiles (trihalomethanes), purgeables

(non-chlorinated aromatic hydrocarbons), base neutrals, phenols and acids were

tested. Overall rejection was expressed in terms of TOC (Total organic carbon)

and was 89% for CA membranes and 99% for PA membranes. Hrubec et al.

(ref. 64) used the effluent of a sewage treatment plant as a feed to a system

including reverse osmosis and activated carbon filtration. The majority of the

identified organic micropollutants and toxins were removed by reverse osmosis

and by subsequent activated carbon filtration. Trihalomethanes, dichlorometh-

anes and alkylphenols were not removed by this system.

Organic pesticides were studied by Chian et al. (ref. 65). Pesticide resi-

dues, including insecticides, fungicides, herbicides and others, impart an unplea-

sant odour and taste to water.

Using CA membranes of NS-100, Chian et al. (ref. 65) obtained excellent

performance in removing a wide variety of pesticides, including chlorinated

227

hydrocarbons, organophosphorus, halogeneous cyclodienes, triazinea and mercap-

tams. Johnston and Lim (ref. 66) reported promising results of the efficiency

of RO in complete elimination of organic phosphate pesticides and other chlori-

nated hydrocarbons. Chlorinated hydrocarbons, known to be very toxic and

cancerigenic, which result from the chlorination process can only be removed

by Reverse Osmosis (ref. 67).

REMOVAL OF NUTRIENTS

Studies of nutrient removal (nitrates, phosphates and ammonia) by RO,

especially in pilot or full scale plant operations have been very limited and

received less attention than many other biological, chemical or physical treat-

ment methods. Lim and Johnston (refs. 68, 69) concluded that nutrients, i.e.,

nitrates, phosphates and ammonia, can be separated from raw and secondary

municipal wastewaters by cellulose acetate membranes. Throughout four

months of pilot plant operation, phosphate removals approached 100%. Ammo-

nia removals averaged 85% and nitrate and nitrite removals were about 56%.

Besik (ref. 70) used a process sequence consisting of dual media filtra-

tion, break point chlorination, dechlorination, reverse osmosis, activated carbon

adsorption and ozonation in treating secondary effluents. He found that even

though reverse osmosis was not the prime process in removing ammonia and

phosphor&s, however, it removed the residues by 99% and 80-85%, respective

ly, should they exist after chlorination. Smith et al. (ref. 26). using cellulose

228

acetate membrane in three different configurations namely spiral wound, plate

and frame and tubular, concluded that all three configurations are capable of

rejecting phosphates by 90-99%, ammonia nitrogen by SO-90% and nitrate nitro-

gen by 60-70%.

Olsen et al. (ref. 11) showed excellent removal of ammonia nitrogen,

nitrite nitrogen, nitrate nitrogen and total phosphor when treating secondary

municipal effluent by subsequent ultrafiltration and RO processes.

MAJOR OPERATION PROBLEMS

Fouling and brine disposal are the most challenging problems encountered

in the field of municipal wastewater renovation by reverse osmosis. RO

membrane foulants can be classified broadly into the following categories: spar-

ingly soluble inorganic compounds such as CaS04, colloidal or particulate mat-

ters, dissolved organic compounds and biological growth. Winfield (ref. 71)

showed that gross solids do not significantly affect the rate of membrane foul-

ing on the concentration in which they exist in normal secondary sewage effl-

uents. The major factor controlling the rate of fouling is the dissolved organ-

ic content of the liquid feed. Winfield also recommended that in order to

mimmize the flux decline index and chemical hydrolysis of the membrane, the

sewage effluent feed should have a pH value of 6.0 which corresponds to per-

meate pH of 5.0. Belfort (ref. 2) speculated that reversible, shear sensitive

fouling probably occurs on the surface, and that irreversible fouling probably

229

OCCURS inside the membrane. True dissolved organics include humic acids, pro-

teins, carbohydrates and tannins in addition to biological growth were the

major fouling constituents. Belfort and Marx (ref. 72) also concluded that the

deleterious effect of fouling and compaction are reduced with increased mem-

brane curing temperature and reduced pressure. Based on their experience at

Water Factory 21. Ridgway et al. (ref. 73) quoted: “Demineralization of pre-

treated wastewater by reverse osmosis yields safe, potable supplies, but microor-

ganisms and chemical substances in the feed water rapidly impede membrane

flux, reducing plant efficiency and increasing treatment cost”. Ridgway et ah

(ref. 74) showed that many microorganisms are present in the biofilm attached

to the feed and product water sides of the membranes. The fouling bacteria

(Mycobacterium, Acinetobacter, Flavobacterium, Pseudomonas and Klebsiella) irre-

versibly adhere to the RO cellulose diacetate membranes surfaces by producing

large amounts of extracellular slime, possibly of mucopolysaccharide or glyco-

protein composition leading to plugging of membrane pores and consequently

flux decline.

The lack of adequate methods for waste brine disposal presents a serious

limitation in the use of RO to municipal wastewater effluents. Jennett and

Patterson (ref. 75) proved that the unique waste streams produced by reverse

osmosis processes are treatable by a conventional activated sludge system. In a

pilot plant study at Milwaukee area (ref. 76), it was demonstrated that the

RO concentrates can be successfully treated biologically. TOC removals in the

range of 87 to 93% were obtained in 4 to 10 hours aeration time in an acti-

vated sludge process depending upon the suspended solids maintained.

230

Brine disposal, in general, may be accomplished by evaporation in lined

ponds (ref. 6). underground injection, spreading on unusable arid land, ocean

discharge via pipeline, stream discharge, or abandonment at the operation site

(ref. 77). Van Den Heuvel et al. (ref. 78) recently proposed a genuine method

of optimizing brine stream by feediig it to an anaerobic digester. The overall

process is considered efficient in terms of both energy and environmental

aspects. The combined process RO-anaerobic digestion has several advantages

mainly: production of raw potable water, reduction of plant sire because of

the concentration step and the high loading rate of the anaerobic digestion, cost

reduction by converting the digest product methane into electric energy and

finally reduction of organic load of receiving surface waters measured in terms

of BOD5 Even though the recovery of saleable chemicals or products that

might exist in brine solution as suggested by Buckley et al. (ref. 791 is not

widely practiced, a novel recovery system of lime and magnesium was pro-

posed by designer of Petromin refinery wastewater treatment plant in Saudi

Arabia (ref. 6).

PRODUCT UTILIZATION

Industrial reuse is the major area in which wastewater was treated for

recycle and a gocd review was published by Smith (ref. 80). In their

attempts to provide 3 types of industrial water to Petromin Refinery located

in Riyadh, Saudi Arabia, using secondary effluents, Kalinske et al. (ref. 6) sug-

231

gested two stages reverse osmosis followed by ion exchange. These three types

of water are low grade water needed for utility and fire protection systems,

such water must be low in suspended solids, biologically and chemically stable

and non pathogenic, next is a better quality water with TDS -z 500 ppm used

in cooling towers and crude oil desalters and finally highest quality water for

high pressure boilers.

An application of RO to sewage pr ocessing for boiler feed was carried

out by Pacific Power & Light Co. and Black Hills Power & Light Co. at their

Wyodak Station in Gillette, Wyo, as presented by Gray et ah (ref. 81). The

system has supplied water with 30-40 ppm TDS to its 136 bars boiler, using

reclaimed wastewater from the city’s sewage disposal system.

At Water Factory 21, RO product water is injected into a series of

coastal wells to provide hydraulic barrier against Pacific Ocean water incursion

and to supplement the domestic water supply (ref. 3).

A tertiary water treatment scheme consisting of coagulation and floccula-

tion, settling rapid sand filtration, reverse osmosis and ozonation was evaluated

to produce potable water from the municipal secondary effluent by Roy et al.

(ref. 82). Although the process can produce potable water, however, direct

human use is not recommended and is widely unaccepted, as stated by Ikehata

(ref. 83). Application of reverse osmosis product to irrigation, landscaping and

other agricultural purposes is receiving more and more attention especially in

the arid, and developing countries, as discussed by Bowler et al. (ref.&Q). In

such areas, the production cost of fresh water from seawater by distillation is

rather high compared to reclamation of secondary effluents by reverse osmosis

232

COST

The cost of treatment for use, reuse or disposal of municipal sewage

effluents has been the prime concern of many engineers and researchers Cost

of application of reverse osmosis to renovate sewage streams is a function of

the following variables:

1. Plant size and location.

2. Membrane type and configuration

3. Pretreatment and post treatment needed.

4. Product water quality required.

5. Feed quality.

6. Capital amortisation.

7. Membrane cleaning frequency.

8. Local conditions.

Smith (ref. 881, hypothetically, compared the cost of RO with other

treatment processes To treat 1 mgd, 10 mgd and 100 mgd, cost estimates

were $ 0.37, S 0.30 and S 0.276 per 1000 US gallons, respectively. Cruver et

al. (ref. 10) conducted a 3 year pilot plant test at Pomona Water Renovation

Plant, Lcs Angeles, California. A rough economic analysis demonstrated that

reverse osmosis treatment cost ranged between S 0.38, S 0.29 and S 0.26 per

1000 US. gallons for 1 mgd, 10 mgd and 100 mgd plants, respectively.

233

In a pilot plant study (10 gpm) at Hemet, California done by Roen et

al. (ref. 86) using different membrane types and configurations utilizing secon-

dary effluent, an average reverse osmosis cost, excluding brine disposal is esti-

mated to be $ 0.78/1000 gallons for a 0.9 mgd products water facility and

about $ 0.73/1000 gallons for a 9 mgd product water facility. Using the data

collected in a 3 year case study and current cost data compiled by EPA (ref.

871, Stenstrom et al. (ref. 88) developed an economic simulation model.

Reclaimed water containing 500 ppm TDS can be produced at a cost of $

1.6/1VOO gallons. In their continuous efforts to optimize the reclamation cost

at WF21, Argo (ref. 4) proved that the low pressure membranes, recently

developed by Filmtec Corp. and Desalination Inc., in the lime/R0 process can

save up to 44% of the original operating cost reported earlier (ref. 3). This

means that it would cc& only $ 0.84/1000 gallons should the above mem-

branes and combination be used.In Japan, Tsuge et al. (ref. 89) and Murayama

et al. (ref. 90) carried the same experiments on reclamation of secondary effl-

uent streams by reverse osmosis. Costs were equivalent to $ 3.44 - $ 3.9 per

1000 US gallons product water, respectively. It is worth mentioning that

such costs are very high if compared with those obtained in the U.S. The

effect of recovery on reverse osmosis economics was well discussed by Wech-

sler (ref. 5). The study showed, for example, that in high recovery operation

(up to 95%), the total reverse osmosis costs could drop by 6.1%.

234

CONCLUSION

In spite of the rapid growth of reverse osmosis as desalination process,

its application to municipal sewage effluents is limited compared to brackish

and seawater applications.

Reverse osmosis proved to be pollution abatement and cost savings pro-

cess. It is an excellent separation process where high rejection of total dis-

solved soiids, a broad range of organics and micropollutants, microorganisms

and pathogens, nutrients and others can be satisfactorily achieved. The recent

advancements in membrane manufacturing and process hardware made it possi-

ble to produce potable water of quality according to WHO standards using

municipal secondary effluents. However, due to ethical and psychological rea-

sons, the product water which is suitable for many industrial, agricultural and

other reuse purposes is by no means recommended for direct human consump-

tion. The technology proved to be practical and economically feasible when

applied to large scale commercial plants. Water Factory 21, located at Orange

County, California, has been demineralizing 5 mgd of secondary municipal effl-

uent by reverse osmosis since 1977.

Unlike many countries where a surplus of rainfalls and natural water

resources are available, arid areas in particular Kuwait and other Arabian Gulf

states, produce fresh waters needed for survival and development at a very

high cost from the sea.

235

It is a high time to consider the very low salinity municipal effluents

available in these states as major water resources and to reuse them through

reclamation by reverse osmosis.

Although the cost effectiveness of the process to render large quantities

of water (fresh quality) is very encouraging, it requires careful preparation of

the design specifications to suit the particular characteristics of the effluent.

Membrane selection, material of construction, pretreatment of the feed, mem-

brane fouling and brine disposal are the major points to consider before final

decission is made.

REFERENCES

1. 2.

3.

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