Effluent treatment and its re-use for the Kriel Power Station

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Effluent treatment and its re-use for the Kriel

Power Station

P T Mkabane

21968942

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Declaration

I, Palesa Mkaba n e hereby certify that the work done on, “Effluent T reatment and its re -use f or Kriel power station”, in this document is my o wn orig in a l study, e xcept wher e other wise stated a s ref erences or ack no wledg ement. Ne ither the substa n ce nor a n y p art of this re port has been s ubmitte d f or any other co urse at th is or an y other un ive rsit y.

Sig nature: _ ___ ___ _ ____ ___ ___ . Date: _1 6 October 2 015 _ ___ ___ .

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Abstract

Pales a Mkaba ne, Gerhard Ge ric ke (Me ntor) and Pr of. Fran s Waand ers (Super vi sor)

Chem ica l En gineer in g department, North W est Univers ity Fac ulty of Eng ine ering

Krie l P o wer Station g enerates eff luent of about 5 ML per d a y f rom diff erent eff luent streams of the pre -treatment p la nt. Currently t he water is reco vered into the eff luent sum p and p umped to the hig h le vel ash water sumps wher e a percent ag e is use d f or the ashing syste m wh ile most water remain s within the sump. T here is a contin uou s int ak e of ra w water to the c o oling water (CW ) system due to water losses thr oug h evaporat io n, eff luen t, leak s etc. Due to water scarc it y in S outh Af rica, Es k om embark ed on a drive to s a ve and prote ct water re sources . A propo sa l is made thro ug h this research to reco ver most ef f luent into the CW system i n order to s a ve water and to reduce th e dispos al t o the en vironme nt. CW system is chos en as the best optio n f or eff luent re co ver y be caus e ther e is more co ntrol o v er it on chemistr y in terms of operation .

Dif f erent technolog ie s are dis cuss ed as o ption s on h o w to rec over eff luent water; treat it up to accepta ble Esk om Cooling W ater Chemis try Sta ndard s and re-use it into the station’s cooling water (CW ) system. The study revea led s a ving s of about R14.6 M per year if eff luent could be reuse d in the CW instead of f resh ra w water intak e f or mak e -up; wh ich can be in vested into in itiat in g eff luent reco ver y p roject.

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Acknow ledgements

T he inf ormation pre sented in th is d iss ertation is b ased o n th e researc h supporte d b y the So uth Af rican Rese arc h Cha irs In itiative (SARChI) of the Department of Sc ien ce a nd T ech no log y and Nat ion al Resear ch F ound ation of South Af rica (Coa l Resear ch Ch a ir Grant No. 868 80).

An y op in io n, f inding or conclus ion or r ecomme ndat io n e xp ressed in this materia l is that of the author(s) and the NRF d oes not acc ep t any lia bilit y in this reg ard.

A messag e of g ratitude e xpress ed to th e f ollo wing pe op le f or work done on this project:

 T o my heavenly father; to God be the glory, I’m h umbled.

 Prof essor F ran s W aanders, m y sup er vis or at North W est Universit y (Potchef stroom); wit hout yo u I wo uld h ave ne ver mad e it, you are such a p atie nt be ing .

 Mr G Gericke “Mentor GG”, your passion in doing things right has made me th e pers o n that I am, w ith out your h elp o n th is r esearch I wou ld h a ve g ive n up .

 My h usba nd Sibus is o and m y daug hter s T hozama an d Hlu band isa, yo ur lo ve sust ains me.

 My co lleag ue s in Esk om research at Rosh er ville; chemic al eng ine ering an d che mistry.

 T he sen ior ch emists f or laborat or y and c ooling water s ystem s at Kr ie l po wer stat io n.

 T he po wer station manag er at Kriel, Mr Jabu lan e Ma vimbe la f or his support and f or allo wing the res earch on the stat ion throug h his vision on so lutions to water challe ng es.

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List of Figures

Fig ure 1-1 Globa l W ater a va ilab ilit y tren ds ([1]) ... ... 7 Fig ure 1-2 W ater usag e in So uth Af rica ( [2] ) ... ... 9 Fig ure 1-3 Nett litres of water use d per u nit g enerate d of elec tricit y in

No vemb er 201 2, Kriel po wer sta t ion ([5]) ... ... 10 Fig ure 2-1 Rank in e Cyc le P o wer Plant e xam ple ([11]) ... 16 Fig ure 2-2 Ion E xch a ng e Units u sed in th e remova l of anions a nd cat ion s ([18]) ... ... ... . 26 Fig ure 3-1 S chemat ic f low d iag ram of ion chromatog raph y an alys is ([29]). 38 Fig ure 4-1 Kr ie l P o wer Statio n wa t er con sumptio n a verag e Ja nuar y to

No vemb er 201 4 ([30] ). ... ... ... 49 Fig ure 4-2 Kr ie l P o wer Statio n CW raw water mak e up averag e Janu ar y 2012 to No vemb er 2 014 ([30]). ... ... 51 Fig ure 4-3 CW north sid e chem istr y f rom Januar y 201 4 to Feb ruar y 201 5 ([31] [32]) ... ... ... 53 Fig ure 4-4 Illu stratio n of the CW circuit diag ram ... .... 69 Fig ure 5-1 Illu stratio n of cooling water lime treatment pla nt at Krie l po wer station ... ... ... 71

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Chapter 1 1. Problem Statement

1.1. Introduction

W ater is a sc are c o mmodit y g lo ba lly an d So uth Af rica is no e xce ptio n. In f act South Af rica is r eg arded as a water scarce c ountr y a nd h ence th ere is a need to pre s er ve and re -use. T he countr y is f acing challeng es in deterioration of wat er q ualit y des pit e its q uantity lim itatio ns . Access to water is cru cial to e nsure eco nom ic g rowt h and su stainab ilit y . It is en visag e that more press ure in the f uture will b e p lac ed on the country’s limited water resource s .

F i g u r e 1 - 1 G l o b a l W a t e r a v a i l a b i l i t y t re n d s ( [ 1 ] )

An illustrat io n in Fig ure 1.1 s ho ws water a vaila bilit y t rends g lo ba lly spec if ically f or de ve loping cou ntries inc l uding So uth Af rica. I t can be note d that f rom the beg inning of the 20t h c entur y th ere ha s be en a s erio us de clin e in water a va ilab ilit y main ly due to soc io -econ omic g ro wth factors in the se countries. T he Nat io nal W ater Bill states that the ma in obje ct of th e bill is to provide for the management of the nation’s water resources ; so as to enable the ach ie vement of sustainab le use of water f or the bene f it of all water

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8 users ( [1] ). T o that end it is ne cess ar y t o pro vid e f or the pr ote ctio n of the q ualit y of water re sources and f or the integ rated ma nag ement of wate r resource s with de leg ation of po wers to in stitutions at reg io na l or catc hment le vel s o as to ena b le ever yon e to particip ate in th e proces ses.

T he Bill ac cord ing ly seek s to pro vid e f or the prote ctio n, us e, de ve lo pment, conservation, management and control of the nation’s water resources, tak ing into acco unt the nee d to :

(a) meet the bas ic huma n need s of present a nd f uture g eneratio ns;

(b) promote eq uitab le access to water;

(c) redress the resu lts of past racial a nd g ender dis crim inat io n; (d) promote the eff icient, sust ainab le a nd benef icial use of wa ter in the p ub lic int erest ;

(e) f acilitate s oc ia l a nd eco nomic de ve lo pment; (f ) provide f or g rowing demands f or wate r use;

(g ) protec t aq uatic and asso ciated ec os ystems and the ir b io lo g ical diversit y;

(h) reduce a nd pre ve nt pollut ion and d eg radation of water resource s;

(i) meet intern ationa l oblig atio ns (j) promote dam saf ety; an d (k ) manag e f loods a nd droug hts.

It is throug h this Act t hat the ava ilab le wate r reser ves can b e protected f or current and f uture generat ion a nd ens ure that string ent measures ca n be app lied to thos e wh o exploit water reso urces.

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F i g u r e 1 - 2 W a t e r u s a g e i n So u t h Af r i c a ( [ 2 ] )

Fig ure 1.2 s ho ws wa ter use b y se ctor in South Af rica, with f o rest pla ntations using an estimated 8% of the 21 billion m3 a va ilab le wa ter ( [2] ). T he amount of water act ually use d b y p lanta tions is co ntro vers ia l, as d iff erent studies report d iff erent resu lts. Scott 1 998 use s f lo w redu ction c ur ves to estimate that the 1.5 million ha (1.18% of land area) reduce total runoff in South Af rica b y a b out 3% (1,41 7 million m3 p er year), whic h eq uates to mean increme nta l water use abo ut 1 00 mm per ye ar. Other researc h, ho we ver, sho ws su b stantia lly d iff erent estimates of water us e b y plantat ion s at about 40 0 millio n m3 p er year ( [3] ).

T he water usag e statistics are on a n increase d ue to po p ulat ion in cr ease and numb er of people who no w ha ve a c cess to cle an water as part of the post -apartheid government’s water policy of providing water for all humans.

T he po wer in dustr y is the sec ond larg est water us er other th an ag ric u lture in th e c ountr y; th eref ore Esk om has e mbark ed on a dr ive to sa ve a nd protect wat er resou rces throug h wat er manag ement in it iative s acros s it s coa l f ired po wer station s.

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F i g u r e 1 - 3 N e t t l i t r e s o f w a t e r u s e d p e r u n i t g e n e r a t e d o f e l e c t ri c i t y i n N o v e m b e r 2 0 1 2 , K ri e l p o w e r s t a t i o n ( [ 5 ] )

.

Fig ure 1.3 illu strate s the measureme nts of water us ed to produce one k ilo watt of electric ity; Esk om set monthly targ ets of wate r usag e whe n produc ing e lectr ic it y and enf orce pen alt ies such as p erf ormance bon uses when thes e are not met. T hese measur e ments sup port the dr ive t o sa ve a nd protect water reso urces as part of the visio n of Esk om and t he g o vernment. It can be o bser ved that there were mor e litres of water us ed t o pro duce ele ctric it y, ab o ve th e targ et until on th e 12t h No vember whe n the water line droppe d be lo w the ta rg et.

Krie l Po wer Stati on g enerates ef f luent o f about 3 ML a nd 4 ML per da y; 5 ML being the maximum f rom diff erent eff luent streams e xclud ing se wag e wh ic h orig in ates f rom the pre-treatment p la nt. Curre ntly the wate r is r eco vered into the eff luent sum p and p umped to the hig h le vel ash water sumps wher e a perce ntag e is us e d f or the ashing s ystem and most water just remains with in th e sump. T here is a c onti nuo us intak e of raw water to the coo ling water (CW ) system due to water lo sses th roug h evaporat ion, e ff luent, leak s, blo w d o wns etc. Alth oug h not muc h can be do ne about t he e va porat ion; it is

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 0 10000 20000 30000 40000 50000 60000 20 12 /1 1/ 01 20 12 /1 1/ 02 20 12 /1 1/ 03 20 12 /1 1/ 04 20 12 /1 1/ 05 20 12 /1 1/ 06 20 12 /1 1/ 07 20 12 /1 1/ 08 20 12 /1 1/ 09 20 12 /1 1/ 10 20 12 /1 1/ 11 20 12 /1 1/ 12 20 12 /1 1/ 13 20 12 /1 1/ 14 20 12 /1 1/ 15 20 12 /1 1/ 16 20 12 /1 1/ 17 20 12 /1 1/ 18 20 12 /1 1/ 19 20 12 /1 1/ 20 20 12 /1 1/ 21 20 12 /1 1/ 22 20 12 /1 1/ 23 20 12 /1 1/ 24 20 12 /1 1/ 25 20 12 /1 1/ 26 20 12 /1 1/ 27 20 12 /1 1/ 28 20 12 /1 1/ 29 20 12 /1 1/ 30

Nett l/USO versus Loading

Total Production

MWhrs

Nett Water Use/ Units Sent Out (l/kWhr) Nett Water Use - Year to Date l/kWhr

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11 assumed that somet hing can be d one wit h the re-treating of the eff luent and blo w do wns a nd the pre vent ion of leak s.

A propos al is made throug h this project to reco ver most eff luent into the CW system in order to s ave water, redu ce the dis pos al to the e nviro nment and achieve Eskom’s Zero Liquid Effluent Discharge (ZLED ) policy. The policy ( [4] ) states that Esk om will ende a vo ur t o ap ply ZL ED po lic y to pre vent the pollut ion of water re sources un le ss the Rece iving W ater Qualit y Object ives of DW AF specif ies other wise. W here pollution pre vention measures d o not suff ice, Esk om will implement a h ier arch y of strateg ies name ly:

1. water/ef f luent re -use and min imisation 2. water/ef f luent treatment, and

3. licenc e app licat ion motivat ing a contro lled discharg e .

T he prolo ng ed stora g e of eff luent into th e ash s ump pos es a risk of seepag e into en vironme nt wh en the sump lin ing start ag ing . ZLE D is def ined as no liq u id discharg e into the e n vironment, in principle. S o of ten zer o d isc harg e and zero liq uid disc harg e are used in t he same mean ing . Practica lly, the conce pt of zero d isc harg e means:

1) reco ver y of reusa ble water or oth er materia ls f rom wa ste water; 2) minim isat ion or, no disch arg e of pollut ing substanc es into the en vironment f rom th e water treatment p lant ( [5] ).

T he CW system is c hosen as the be st o p tion to re co ver water bec ause ther e is more ch emistr y c ontrol in terms of op eratio n. Since the p o wer station is a bus ine ss wh ic h it s sur vival hig h ly d epen ds on c osts; the dec is io n on eff luent reco ver y int o the CW system is depe nde n t on the techno -ec onom ic via bilit y of the pro c ess. B y reco vering and re - using eff luen t water; f resh water intak e will be reduce d at Kr ie l P o wer Station a nd th is will resu lt in better complia nce to the ZLE D po lic y .

Eff luent reco ver y an d re -use can be co mpl e x be caus e the water g enera ll y conta ins hig h le vels of total d isso lve d so lids ( T DS) whic h are g enerally ver y corrosive . T here are dif f erent eff luent s ources f rom water t reatment p la nt with in th e po wer stat ion wit h dif f erent chemica l comp osit io ns , namely:

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12  ra w wat er clar if ier blo w do wns

 CPP’s (Condensate Polishing Plant): regeneration effluents, resin ABRO S (Air Bump Rinse Operat ions) a nd rinse water

 Deminera lisation p lant: reg en eratio n e ff luents, resin b ack wash ing and rins ing .

W ater f rom diff erent eff luent source s will ha ve to be c la ssif ied in terms of their chemica l c ompos itio n in ord er to f ind the b est s olut ion f or treatment and re -us e .

1.2. Ke y Researc h Que stions

1.2.1. W hat are the lim itations whe n re co ver ing eff luent wa ter to the CW system?

1.2.2. Ho w mu ch of fresh ra w wat er int ak e will b e sa ved when reco vering eff luent into th e CW system in vo lume an d in Rand s?

1.3. Problem Sta tement and Objec tive

T he objective of the research is to de sig n a system that will reco ver most eff luent wat e r; treat it u p to accept ab le Esk om Co oling W ater Ch emistr y Stand ards ( [6] ) and re -use it into the station’s cooling water (CW ) system.

1.4. Project Scope

T he scope co vers a literatur e stud y on what has be en do ne on ef f luent reco ver y f or a po wer in dustr y . It als o disc usse s eff luent sample resu lts wh ich were tak en f rom diff erent eff luent s ources at Kriel Po we r Statio n. T he chemistr y c onte nt of these s ample resu lt s will the n be compa red to the CW chemistr y req uirem e nts; and be asse sse d on wh ich reco ver y method will be best su itab le f or eff luent reco ver y .

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Chapter 2 2. Literature

2.1. Water scarcit y in South Africa

Access to f resh clea n water is cons id ere d as a bas ic rig ht a s stipu late d in the So uth Af rica n Constitut io n. T he a va ilab ilit y of fresh wate r is a ls o k ey t o ind ustrial and econ omic d e ve lopme nt. South Af rica is, ho we ver a water -stressed cou ntr y an d f resh water is co nsidered a scarc e resource wh ich needs to b e well -ma nag ed an d prot ected ag ainst po llutio n, u ncontro lled use and wastag e. T his a lso in cludes th e prot ectio n of wet lan ds a nd g round water sources. T he Nat io nal W ater Act e xist in th e co untr y to ensure t hat the natio n's water r es ources are protect ed, us ed, de ve lo p ed, co nser ved, manag ed and co ntrolled in wa ys wh ic h tak e into acco unt among st other f actors: meeting the bas ic human nee ds of present a nd f utu re g enerations, promoting eq uit ab le access to water, r edress ing the re su lts of past racial and g ender d iscr im inat ion and pr omoting the eff icient, su stain ab le an d benef ic ia l use of wat er in the p ub lic int erest ( [1] ).

South Af rica is f undamentally a s emi -ar id and water scarce countr y with a mean annu a l rainf all of 490mm, wh ich is half the wor ld a ver ag e, with on ly 9% of that rainf all end ing up as ru n-o ff water to r ivers ( [7] ). Rainf all dis pla ys a d istinct d ecreas ing trend f rom east to we st o ver S outh Af rica an d is h ig hly variab le [8] with in and b et we en ye ars with rec urrent droug hts; this results in h ig hly variable river le vels, da m storag e and g round water storag e over time ( [ 7] ). T he water u se f rom the majorit y of the catc hment area of South Af rica e xcee d s the a va ilab ilit y o n an ann ua l bas is ( [9] ).

In 200 4, 98% of So uth Af rica's s urf ace water yie ld, as we ll as 41% of the annu al us ab le pot en tia l of g round wat er were a llocate d f or use ( [10] ). Of this allo cation, 60% went to ag ric ultur al activities, 2 7% to do mestic de man d, 23% to urban n eeds , 4% to rural, 6% to mining and bu lk industr y, 3% to aff orestation a nd p o wer g ener atio n ut ilised on ly 2% ( [ 9 ] ). T he close interco nne ctedn ess bet we en the c lim ate and the h ydrolog ic al c yc le mea ns that wat er reso urce s will be impacte d on b y climate cha n g e and th is will pla ce increa sed pr es sure on water res our ces an d ult imate ly t hreaten ing the sustainab ilit y of f uture a va ilab ilit y ( [9] ). Schu lze et a l. ( [11] ) state that

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14 water res ources are direct ly impacte d on by current c limate varia bilit y an d it is e xp ected that climate chang e cou ld impact on resource s sig nif icant ly in the f uture. T he m ost s ig nif icant imp acts of climate ch ang e on water resource s ar e the potent ia l ch ang es in the int ens it y a nd seas ona lit y of rainf all. W hile some reg ions ma y rece ive more surf ace wat er f low, f uture prob lems are lik ely to inc lu de water sc a rcit y, increas ed dem and f or water, and water q ualit y det eriorat io n.

T he Department of W ater and Enviro nmental Aff airs outline s the pressure s on South Africa’s natural water resources as well as the need t o manage these reso urces wis ely ( [12] ). In the 19 60s an d 197 0s, Esk om realised th e lim itations in South Africa’s water resources and investigated and tested the use of dr y co oling t echno log y f or its ne w co al -f ired po wer s tations. T od a y, the tota l dr y co ole d insta lled cap acit y is 10 47 7meg a wat ts. Dr y -coo ling techno log y d oes not rely on e vap orative coo ling f or the f unction ing of the main coo ling s yste ms. As a res ult dry co oled po wer stations use appro ximate ly 1 5 times les s wat er tha n con ventiona l wet coo led p o wer stations. Eskom’s leading role in this field is attested to by the fact that it operates the larg es t indir ect dr y coo le d po wer stat ion (K enda l - 4 116 Meg a watts) and the larg est dire ct dr y co ole d po wer station ( Mat imba -3 99 0 meg awatts) in the world ( [8] ). T he in ve stment in dr y -c oo lin g results in an estimated comb ine d sa ving o f over 2 00 meg aliters of water p er da y ( Ml/d a y), or in e xces s of 90 million c ub ic metres per ann um.

T he Nat ion al W ater Reso urce Strateg y ( [12] ) release d b y the Dep artment of W ater and Environm ental Af f airs outline s the pressures on South Africa’s natura l water res ou rces as we ll as the need to manag e these reso urces wis ely. E ver y po wer station, or po wer g eneration en tity, ha s the respons ib ilit y to eff ective ly man ag e their water con sumption and sa ve water wh ile not comprom is ing o n g enerating po wer.

2.2. Importance of Water and Kriel Pow er sta tion

A n eed has be en id entif ied at Kriel Po wer Station to reco ver most of th e eff luent g enerated with in the Po wer Stat ion to treat the eff luent and re -use it with in th e main c ooling water s ystem . Esk om, produc ing 95% of South

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15 Africa’s power, is one of South Africa’s biggest water consumers, and theref ore Esk om is r espon sible f or establish ing eff ective wate r manag ement proced ures and re search init iat ives. E ver y po wer stat ion, or po wer g eneratio n e nt it y, h as the re spo nsibilit y to eff ective ly man a g e their water consumpt ion and s a ve water wh ile not c ompromis ing on g en erating po wer. This must be done in order to contribute to protecting South Africa’s natura l water reso urces and co nser ve water i n lin e with the nation al polic y ( [13] ). T he g oal of water ma nag ement within the p o wer g enerat ion sector is to lim it wa ter consumpt ion a nd to elim inate th e co ntaminat io n of natura l water reso urces ( [8] ). T he prob lem Esk om is curre ntly f acing is t hat old er p o wer stat ions that ar e st ill in o per ation a nd will be in operat ion f or at lea st the ne xt t wen ty years; mak e use of larg e q uantities of water f or coo ling and ash rem ova l purpo ses ( [14] ) .

T here are many tech nolog ies an d proces ses in us e wh ich ha ve b een pro ve n successf ully f or the reco ver ing and treat ing of eff luent water and some ar e disc usse d in th is c hapter. Some tech n olog ies remo ve cert ain sp ec ies of contaminants where by a dd itio na l treat ment techn iq ues st ill ha ve to be app lied do wnstre am to ensure the des ire d spec if ications.

Process choice and identification of a process also depends on the user’s req uirements i.e. po table water produ ction f rom the eff luent wou ld req u ire more string ent a nd c omple x s ystems to e nsure the correct s p ecif icat ions are obtained as it will be f or huma n c ons umption. Other f actors ca n inc lu de materia ls of construction where corros iveness or f ouling f actor of water will pla y a major role an d the q ualit y of the feed wat er to the sys tem will ha ve to match the materia l tolera nces. F ou ling of eq uipment not only res ults in inef f icient heat tra nsf er and incre ase d costs, but in micro -biolog ica ll y ind uced c orros ion ( g enerally a naero bic) is en hanc ed. S ilic e ous sus pen ded matter can a lso g ive rise to eros io n or c orrosion of pump int erna ls, vo lutes, heat e xcha ng er tub ing and pipe - work [6] . Asset s election a nd man ag ement thus p la y an imp orta nt role in d ef ining the best so lution i n ef flue nt reco ver y in terms of eq uipm ent req uireme nts f or a sust ainab le op e ration al s ystem and the e n vironment .

Over the last t wo decade s, Esk om has introduce d a number of inn o vat ive techn olo g ies to save water . T hese include dr y coo ling - both direct and ind irect, desa linat ion of pollu ted min e water f or use at p o wer

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16 station s, use of limited pump ed stor ag e and h ydropo we r potential and techn ica l impro veme nts in treatment reg imes to maximise th e benef icial u se of water. T he compan y ha s tak en a pro -active stanc e in its eff orts to conser ve water, in some cases e ve n pre -empting ne w leg is lation in implement ing eff icie nt and ef f ective water use pract ices. It has con stant ly striven to impro ve on its water use ta rg ets, contin ua lly re search ing a nd implement ing ne w techno log ies to redu ce or lim it water us e, and work ing clos ely wit h the Department of W ater Aff airs and Fore stry (DW AF) to contrib ute to wards long -term water res ource p lan ning and manag ement. This commitment is documented in Eskom’ s W ater Management Policy ( [8] [4] ).

Krie l po wer station is a 360 0 MW g enerating coa l f ired f uel p o wer p lant and it con sists of six u nits e ach produ cing 600 MW . About 3 ML p er da y of demin era lized water is su pp lied to s ix boilers to prod uce steam that will drive th e turb ine t hat e ve ntua lly prod uces e lectr ic it y. Kriel is a t yp ic a l examp le of a Rank in e Cyc le p o wer plant where b y steam f rom the bo iler sent to the t urbine, pass e s throug h th e c ond e nser a n d then it is r ec yc led back to the bo iler, (see F ig ure 2.1).

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17 One of the main au xiliarie s at Krie l po wer statio n is the water treatment pla nt (W T P). T he m ain f unct ion of the W TP is to clar if y an d treat ra w water up to req uired ch emistr y spec if icatio ns in order to pr oduce p otab le, demin era lis ed a nd c ooling water ( CW ) for the po wer station . Potab le water is us ed f or human consumpt ion, k itch e n and bathro om f ac ilit ies; where as demin era lized water is use d to g ener ate steam in t he b oile r wh ich is sent throug h to dr ive the turbine f or electric ity produ ctio n. T he CW is used to coo l steam b lee ds f rom the turbin e whic h is the f eed water rec ycle as well as pro vid ing co o ling to other au xiliarie s in the main plants.

W hile the proces s o f treating ra w water is tak ing p lac e at the W T P; there are many “by-products” which are formed and they are not wanted as part of the f inal prod uct and are colle ctive ref erred to as eff luen ts. Eff luent is contaminated and u n want ed water g en erated f rom water t reatment p lant processes that does not meet required “clean water” specifications and also poses an e n vir onm ental impact. Eff lue nt water g enerally conta ins h ig h le vels of total d isso lve d so lids (T DS) wh ich are ver y corros ive. Krie l Po wer Statio n g enerat es ef f luent of appro ximat ely 3 ML to 4 ML p er da y; 5 ML be ing the ma xim um f rom dif f erent eff luent streams e xc lud ing se wag e wh ich orig in ates f rom the pre -treatment pla nt.

T here dif f erent eff l uent sourc es g enerat ed f ro m W T P are reco vered int o the eff luent sump at th e W T P and then pum ped to th e h ig h le ve l ef f luent water sumps wher eb y abo ut 20% is us ed f or the as hing s ystem t o dr ive coars e ash f rom the boiler to the ash d ams.

Most of the ef f luent remain s in the se d ams as the ir ho ld in g capacit y, t o reduce the disposal to the environment and to achieve Eskom’s Zero Liquid Eff luent Discharg e (ZLE D) po lic y. T he Eskom’s W ater Management Policy ( [4] , states that Esk om will e n dea vour to apply ZLE D po lic y to prevent the pollut ion of water re sources un le ss the Rece iving W ater Qualit y Object ives of DW AF specif ies o ther wise. T he pro lon g ed storag e of eff luent into the ash sumps p oses a r isk of seepag e into en vironment when the s ump lin ing start ag ing ; hence proact ive civil pr e ventat ive mainte nanc e ha s b een a pp lied to ins pect th ese dams over a certa in f req uenc y of ye ars to ad dress th is r isk . T here is a wat er h ie rarch y with in Kr ie l p o wer station in terms of costs a nd chemistr y s pec if i cations req uireme nts an d it is a s f ollo ws:

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18 a) Potab le W ater: it is water prod uce d f or human co nsumpt ion and it is guided by Eskom’s Water Quality Stand ard for Potable water ( [15]) to ensure that th e a e sthetic, chem ica l, microb io l og ic al an d ph ys ica l q ualit y is at an ac c eptab le le vel on p otable water chem istr y f or the hea lth of the peop le consum ing the water .

b) Deminera lised W ater: the wat er is prod uc ed f or steam g eneration that will produce electricity and it is guided by Eskom’s Che mistry Stand ard f or Coal Fired Un its with Onc e T hroug h Boilers o pe ratin g at 17 MPa ( [16] ) to ensur e that th e materia l of constru ction f or eq uipment h and ling this water lasts u p to its e nd of lif e as we ll as resin th at produ ce th e water.

c) Coo ling W ater: it is water us ed ma in ly f or the remo va l of he at off the f eed water thro ug h the cond enser as well as sup plying cooling f or au xiliar ies; the wat er spec if icatio ns ar e g uided b y th e Chemistr y Stand ard f or Coo ling W ater f rom Esk om ( [6] ) to ens ure th at t he water is up to req uire d sp ecif icat ions f or the protection of conden ser tube s and au xiliar ies.

Coo ling water allo ws f or a better chemistry c ontro l in terms o f operation as compared to the o ther two, demin e ra lise d and pota ble. It is theref ore chose n as t he b est optio n to re co ver t h e eff luent to. T here is a cont in uous intak e of f resh raw water da ily into the coo ling towers due t o water losse s throug h e va porat ion, eff luent, le ak s, blo w do wns etc. T he main obje ct ive of this proje ct is to discuss b est opt ions of recover ing eff lue nt into th e CW system in order to s ave water, re duc e t he d is posa l t o the e nviro nment (to the holding sump capacity) and achieve Eskom’s Zero Liquid Effluent Dis charg e (ZL ED) p olic y. Esk om ado pte d the ZL ED p olic y d uring 198 7, in terms of wh ich a ll r eason ab le measur es are tak en to preve nt pollut ion of water res ources thr oug h the esta b lis h ment of a hierarc h y of water uses based on q ualit y. Cascad ing the water from hig her q ualit y to lo wer q ual it y uses, e nab les e xten sive r e -use. W here poss ib le, water is lo st only throug h evapor atio n, retaining the accompan yin g dissolved an d sus pend ed so lids. T he net resu lt is t hat there is no de liber ate d isch arg e of polluta nts to a water r esourc e un der norma l o perating con dit io ns an d a verag e climatic c ond itions ( [8] ).

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19 T he Department of W ater Aff airs is currently charg i ng Krie l po wer statio n almost R8 00 0 per ML of raw water. Res e arch has s ho wn that South Af rica is currently f acing cha lleng es in the deter ioratio n of water q ua lit y, des pite its q uantit y lim itations and f urthermore to ens ure s usta inab ilit y on th e econom ic g ro wt h su ccess ( [17] ). T here is a g uarant ee of more press ures in the f uture on lim ited water resourc es he nce the ne ed to ha ve projects suc h as this o ne to sa ve water an d sa ving the en viro nment.

2.3. Cooling w ater standard

T he Esk om Coo ling W ater standar d est ablis hes t arg ets, up per a nd lo wer boun d va lue s f or chemic al parameters in o pen e va porat ive rec ircu lated coo ling systems. S pec if ic modes of operation a pp ly at individua l p o wer station s ac cord ing to the p la nt which is in stalled, in part icu lar, the q ua lit y of the mak e-up water s upp ly. T he o bjective s of this stan dard ar e to f orm ulate ag reed chemica l c ond itio ns whic h will ens ure the most cost eff ective operat ion of a coo lin g water s ystem. T he f ocus is on importan t aspects suc h as o vera ll water m anag ement, cost ef f ective treatment a nd contro l, the ava ilab ilit y and ef f icie nc y of the proc e ss an d plant in co ntact with the coo ling water ( [6] ).

T he limits an d ra ng es ref lect pr actic a l a n d ac hie vab le ch emis try th at c an be mainta ine d thr oug h the app licat ion of sou nd water treat ment an d water manag ement tech niq ues, wh ilst tak ing cog nisa nce of specif ic lim itations. Recommen dat ions f rom such org an isa tions as E PRI (E lectric po wer research institute) and VGB (V erein ig ung Der Grossk raf twerk sbetreiber) with resp ect to mo dern po wer p la nt p ractice s h a ve bee n cons id ered f or implementat ion of su ch techn iq ues.

2.3.1 Turbidit y

T urbid it y in coo ling water resu lts f rom the prese nce of silt, cla y, pu lveris ed f uel ash, co al du st prec ip itated sa lts and alg ae. T he s o urce of thes e suspe nde d particles is the mak e -up wat er, inter na l microb io log ic al g ro wth and/or dust part ic le s scrubbe d f rom the air be ing supp lied to the cooling to wer. T urbidit y is a g eneral term use d to descr ib e the opt ical opa cit y of water cont aining an y f orm of insolub le su spend ed so lids matt er.

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20 Susp end ed ma tter is resp ons ib le f or f ouling of pipe - work , heat e xcha ng er surf aces, cooling to wer pack ing s and d rif t elimin ators, part icu lar ly where the ve loc ities ar e lo w or h a ve b een r ed uced inte ntiona lly. F ouling of heat excha ng er surf aces not only re su lts in ineff i cient h eat trans f er and hence incre ased c ost bu t, in add itio n, microbiolog ica lly in du ced corros io n (g enerally ana erob ic) is en han ced. S uspen ded mater ia ls can a lso co -precipit ate with ca lc ium carbonat e and c a lc ium su lph ate.

Alg ae, f ung i and bac teria l g ro wt hs f louris h in the c oo ling tower en vir onmen t and co nd itio ns suc h as the prese nce or absenc e of sunlig ht and adeq uat e f ood supply susta in such g ro wth.

The limit for turbidity has be en spe cified as <100 For mazin turbidity units (FT Us ). Ho we ver, th is must n ot b e tak en as a mandat e to operate at the maximum le vels. Generally, the turbidit y sho uld be contr olled as lo w a s cost -eff ectively pos sib le. During f lood ing condit ions th e turbidit y of raw water sup plie s can rise dramat ica lly o ver a perio d of a f ew d a ys. Dur ing these per iod s, statio ns may well de pen d on their reser voir s to supp ly water until the t urbidit y s ubs ides. E xten ded operat ion of conden sers ab o ve 1 00 FT U thresh olds h as pro ve n to b e d etrimenta l to t he c on denser tubes. Se vere thinn ing of cond enser tu b es has b een e xper ie n ced where th e thresho ld was reg ula rly e xc eede d.

2.3.2 Co ndu cti vit y at 25°C

T he condu ctivit y at 2 5°C is a f unction of the tota l d isso lved io nic s olid s. For the raw water currently being supplied to Eskom’s coal fired plant, the limit for the con centrat ed c ooling water system has be en set at < 400 0 µS/cm at 2 5°C . T h is is an ar bitrar y limit and has bee n s et to min imise g alvan ic corros io n in hig hly sa line/o xyg enated water, to pre ve nt damag e to adjacent veg etatio n by sa lts re leas ed b y the coo ling to wer dr if t, and to a llo w suff icient hig h c yc le s of conce ntratio n t o be obta in ed in or der to ma inta in zer o liq uid eff luent disc harg e. Con duct ivit y le ve ls of Esk om cooling water samples rang e bet ween 96 0 to 305 0 µS/ cm.

It must be clearly u n derstood th at the co nduct ivity limit must not be vie we d in iso lat ion as some ions are s ig nif icant ly more c orros ive th an others, e.g . chlorid e ion s are more corro sive th an sulphate with r espe ct to meta llic compone nts.

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21 2.3.3 Alkalinit y, p H, calcium hard nes s, con d uct i vit y an d ca lcium

carbo nate (pre cipit ation pote ntial and sulphat e con centr a tion) All f ive paramet ers are inter -re lat ed an d , as such, the lim its spec if ied must be vie wed in c onjun ction wit h o ne a noth er. T he objective in controlling this g roup of parameters is to:

 minim ise c orros ion b y ma inta in ing adeq u ate protect ive a lk alin it y,  pre vent/minim ise th e le ach ing of calc iu m f rom asbestos ce ment drif t

elimin ators or co olin g tower pa ck ing s,

 avo id prec ip itat ion o f calcium carbo nate scale on h eat e xch a ng er surf aces a nd on th e coo ling tower pack in g s,

 allo w eff ective remo va l of alk alin it y b y pr ecipitat ion proces se s, and  minim ise the us e of sulphuric acid as a neutra lis ing ag ent f or

alk alin it y.

A prob lem wh ic h has arisen more f req uently is the lo w c a lcium conce ntratio n of the cooling water in t h e so -ca lled b ottom -end pro blems, wh ich is pr imarily as a res ult of the so dium alk a linit y in th e mak e -up water. Water contains sodium alkalinity when the “M” alkalinity > total hardness. M-a lk alinit y is a me asure of the amou nt of acid it tak es to drop th e p H to appro ximate ly 4.3. Alk alin it y is def ined as the acid a bsorb ing propert y of water. T here is a ls o P -alk alin it y which is a measure of the amount of acid req uired to drop th e pH to a ppro ximat ely 8.3. T he major acid ab sorb ing constit uents that ar e typ ica lly dea lt wit h are hydro xide (O H-), b icarb onate (HCO3 -) an d carb on ate (CO3) ions ( [1 8] ). T he sodium a lk alinit y co n verts to

soda ash d uring the lime treatment process and prec ip ita te the ca lc ium assoc iate d with s ulp hates a s ca lc ium car bonate. T he paramet ers f or cooling water at se lecte d Es k om power stat io ns are prese nted in T ab le 1.

Table 1: S ummar y o f cooling w ater qua lit y at Es kom pow er stations

Parameters Rang e

Alk alin it y T o tal (a s CaCO3) Bet we en 9 0 and 2 00 mg /l

pH @ 25 0C 8.1 to 8.6

Ca lcium hardn ess (a s CaCO3)

Bet we en 2 00 an d 75 0 mg /l Cond uctivit y @ 25 0C

(µS/cm)

Bet we en 9 60 an d 30 50 Ca lcium (as Ca CO3) Bet we en 2 00 an d 50 mg /kg

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22 2.3.4 Calcium p hos phate

T he ortho -ph osph ate that comes f rom se wag e is ver y r eact ive with ca lc ium and th e limit is set at 0.5 mg /l as PO4 so as to a vo id ca lcium phos phate,

particu larly at e le vat ed coo ling water tem peratures e xper ien c ed at the out let of the condensers and co olers. It is recommend e d that treated se wag e water be introduc ed via cold lime sof tening clar if iers. T his will pre vent the accumu lat ion of pho sphate in the c oo ling water s ystem.

2.3.5 Nitrate a nd Ni trite

W hilst no limits are spec if ied as yet f or nitrate an d nitr ite it is nece ss ar y f or these parameters to be measur ed as discharg e of b lo w - do wn to th e en vironment ma y b e req uire d. Nitrate and nitr ite are ind icators of both ind ustrial an d d ome stic po llutio n of wat er sup plie s ma in ly f rom se wag e a nd se wag e eff luent. Nit rate le ve ls in t he E sk om cooling water rang e bet ween 0.1 and 1 1 mg /l, wh ile that of nitrite rang es bet ween 0.0 2 an d 0.42 mg /l. 2.3.6 Alga e

Alg ae, lik e f ung i, are relatively larg e or g anisms a nd ar e no rmally co lo ured g reen or blue g reen by the pres ence of chloro ph yll. S u n lig h t is necess ar y f or g rowth and the most eff ective co ntrol method is th e exclusion of sun lig ht. Dep os its of dead a lg ae pro vide f ood f or bacteria a nd f ung i, since the y act a s f ilters a n d catch other org an isms. Alg ae are not k no wn to ca use corrosion d irec tly, e xc ept f or occ as ion al occurr ence un der a lg ae d epo sits. Alg ae g ro wth is main ly resp ons ib le for the b lock ag e of coo ling to wer screens a nd mass ive build -u p in th e clarif ier laun ders.

2.3.7 Bact eria

E xcess ive biof ilm ca n be seen wit h the nak ed e yes when la rg e colon ies of bacteria are prese nt. Species are id entif ied with the a id of a microscop e at a mag nif ication of 800 times or more. E ach type of bacteria has a spec if ic actio n and of ten is ref erred to by its eff ect on materia ls. Some class es of bacteria c au se s lim e, corrosion an d g as product ion. Bacte ria common in coo ling water s yst ems, are the slim e -f ormers, they pr oduce a s limy, g elatin ous dep osit t hat can c log he at excha ng er tubes, in crease f riction loss es an d sh ie ld h eat e xch ang er surf aces f rom inhib itors and the c oo ling medium. In re verse osmosis, the ba ck diff usion of the con c entrated s alt is

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23 retarded b y s lim y layers o n the memb rane surf aces, thus aff ecting the osmotic pre ssure a n d the permeate q ua lity.

Sulph ide - produ cing bacteria produ ce c h emica ls t h at res ults dir ectly in th e corrosion of metals. T hese bacter ia con vert water s olu ble su lph ur compoun ds t o h ydro g en su lph id e. Clo strid ium is a lso a h ydr og en su lp hide produc er b ut not a sulphate redu cer. T his con vers ion usu a lly starts with sulphates that e it her occur n atura lly or c ome f rom the addit ion of sulphur ic acid f or alk alin it y co ntrol.

Hydrog en su lph id e is acid ic a nd ag g ressively attack s met als, principa lly mild stee l, but a ls o s tain les s stee l a nd co pper a llo ys. Ho we ve r, most metals are subject ed to c orrosion un der lo w pH, reduc ing con dit ions and th e presenc e of sulph id e s.

In a rec ircu lat ing co oling - water s ystem, corrosion d ue to th ese org an isms can oc cur at a ra pid rate an d perf oration of a 16 mm mild stee l co upo n with in 60 da ys ha s bee n recor ded. Usi ng chlorine to control thes e org anisms is not ef f ective b ecau se:

 T he org anisms are usua lly co vered b y slime masses that p revent the chlorine f rom reaching the sulp hide pr oducers; a nd

 T he h ydrog en s ulp hite s urroun ding these org an isms rea c ts with chlorin e to f orm chloride sa lts that n eg ate the eff ect of chlorine.  T he eff icienc y of chlorin e at the e le vated pH le ve ls f ound in c ooling

water is limite d.

Spec ia l t o xicants ar e ne cess ar y to co ntrol th ese ba cteria a nd f avoura ble temperature an d hig h ra w water mak e -u p f urther complicate control. T he app lication of bioc id es pro vide s a limit e d protect ion d ue to f actors alread y mention ed. B io -d is p ersants ca pab le of controlling the attac hment of slime produc ing bacter ia h ave pro vide d the most cost eff ective co ntrol f or the se bacteria. B io -d isp er sants ca n pro vid e s uff icient prot ection in areas wher e water f lo w c ond it ion s are suc h that the dis persant c an f unct ion, ar eas s uch as stag nant lo w f lo w and under de pos its may still be su bject e d to microbial corrosion.

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24 Table 2: Cooling w ater standar ds for bi ological q ualit y

C o m p o n e n t U n i t S t a n d a r d T o t a l a e r o b i c b a c t e r i a C F U / m l 1 04 C F U / m l ( t a r g e t ) T o t a l a n a e r o b i c b a c t e r i a C F U / m l 1 03 C F U / m l ( t a r g e t )

2.4. Cooling Water c ontamination

T he q ualit y of the ava ila ble mak e-up wa ter is e valuate d as a crit ica l f actor during the desig n st ag e of a power stat ion. Nee ds suc h as t he use of third party eff luents are of ten not f oreseen dur ing the desig n stag e. South Af rica n surf ace water sour ces (rivers and da ms) are cha lleng ed with dec lining q ualit y an d re cent r eports in dic ate th at at least one th ird of So uth Af rican rivers are eutro ph ic (i.e. contain e xc essive le ve ls of phosph ates an d nitrates), wh ile a not her on e th ird of the rivers are con sid ered b order line eutroph ic. Most of th e river s that f all with in th e ab o ve c ateg ories are with in Esk om po wer stat ion s water sup ply r eg io ns ( [18] ).

Ra w water f rom the Vaa l catc hment are a has h ig h sus pen d ed so lid le ve ls (up to 2 00 p pm), as we ll as h ig h le ve ls of iron, mang anese, alum in ium a nd copper a nd the r is ing sulph ate le ve ls of these water sour ces are a lso a g rowing concern. It is be lie ve d that the q ualit y of process water used b y Esk om has t he hig h est nutr ient le ve ls in the wor ld, as b oth phosp hate an d nitrate le vels are e xces s ive ly h ig h ( [19] ). Hig h nitrog en le ve ls in water tend to attack metals resu lting in a h ig hly corr osive compo und.

T he recover y of the station dra ins to th e cooling water s yst em is of ten a source of contamin a tion of the coo ling water. Co ntaminants that typ ica lly f ind their wa y into th e coo ling water s yst ems alo ng this route are:

 Org anic solvents

 Dis perse d or d isso lved lu bricants, or as f ree oil  Ash part ic les

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25  Soap s and d eter g ent s

 Chem ica l inh ib itors from auxiliar y co oling systems.

T hese produ cts ca n be detr imenta l to the treatment p rocesse s and conde nser p erf ormance. T he y are also e xce llent nutrients f or microorg an isms thu s of ten resulting in microb ia l co ntaminat ion. Spe nt reg enerants will ad versely in creas e the blo w do wn req u irem ents and of ten cause d amag e to the concrete dra in s yste ms.

2.5. Treatme nt options and technical brief

T here are man y t ech nolog ies and proces ses whic h h a ve b een pro ven as th e best opt io ns f or recovering and treat ing eff luent wat er, and some will be disc usse d in the f ollo wing sections. Som e techn olog ies remo ve certa in p arts of contaminants wh ereb y ad dit io na l processe s still h a ve to be app lied do wn stream to ens ure a f urther remova l depe nd ing on the req uired specifications. Chosen processes also depends on the user’s requirements i.e. p otab le g en era tion f rom eff luent wou ld req uire more string ent and comple x s ystems to ensure th at the best specif icat io ns are o btain ed as th e water prod uce d will be f or human con su mption.

2.5.1 Ion Ex cha nge Tech nolog y

Ion e xch ang e is a ch emica l rea ctio n proc ess wh ereb y an ion f rom a solutio n is e xch ang ed f or a sim ilarly ch arg ed ion attac hed to a n immobile so lid partic le. T hes e so lid ion e xc hang e p artic les ar e nat ura lly occ urring , inorg an ic zeo lites o r synth etic ally pro d uced org an ic resins . Ion exch ang e materia ls are in so lu ble su bstanc es cont ain ing loo se ly he ld ions wh ich are able to be e xc hang ed with oth er ion s in so lut ion s wh ich c ome in c ontact with th em. T hese exc hang es tak e pl ac e with out an y ph ys ical a lterat ion to the io n e xch ang e materia l ( [20] ).

Ion e xc hang e is u se d in water treatment proces ses to rep la ce un des irab le ion s present in wat er with more de sira ble ion s. It is achie ve d b y pas sing

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26 water throug h a bed of insolub le s ynthet ic po lymer ic b ead s o f ion e xcha ng e resin s. T here are e s sentially t wo bas ic typ es of ion e xch ang e res ins; cation and a nion e xch ang e resins. T here are two categ or ies of cation e xch ang e resin n amely; strong and weak acid e x ch ang e resins. T he strong acid res in can neutra lise str ong bases and c on vert neutra l sa lts into the ir correspo nd ing ac ids . T he weak cat ion acid res in of cation resin is on ly eff ective in alk a line con dit ion s. W eak acid cat io n res in is on ly ab le to neutra lis e stro ng ba ses. R- be lo w ref ers to the f ixed po lyme r phas e of the catio n e xch ang e res in.

R - H+ + Na+ Cl - ↔ R - Na+ + H+ Cl

-T he anion e xch ang e resins a lso ha ve t wo categ ories; strong and weak bas e typ es. L ik e the stro ng acid cat ion res in s, the stro ng ba se a nio n res in s ca n operate o ver a wide pH rang e. T hese res ins c an n eutra lis e st rong acids and con vert n eutra l sa lts into the ir corre spond ing ba ses. T he we ak base excha ng ers are on ly eff ective in ac id ic solutio ns a nd e xch a ng e on ly ion s such as c hlorid e, ni t rate and su lp hate, b ut not silica or b ica r bonate ( [21] ) R+ OH - + Na+ Cl - ↔ R+ Cl - + Na+ OH –

F i g u r e 2 - 2 I o n E x c h a n g e U n i t s u s e d i n t h e r e m o v a l o f a n i o n s a n d c a t i o n s ( [ 1 8 ] )

All nat ural water c o ntain s, in various co ncentrat ions; d iss olv ed sa lts wh ic h diss oc iate in water to f orm charg ed io ns . T hese ions nee d t o be r emo ved f rom the water be ca use of their ad vers e eff ects in the b oiler and turb in e

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27 pla nt, name ly sc alin g and corros ion. T he proc ess work s b y bring ing the solutio n co ntain ing the minera l ions into contact wit h sp ec ia l ion -e xcha ng e materia l (in the f orm of resin bea ds). T hese beads h a ve h ydrog en or hydro xide io ns on th eir surf ace. T hese s urf ace ions are e xch ang ed f or other ion s in the so lut io n; posit ive ly ch arg ed io ns are ca lled cations a nd neg ative ly ch ar g ed ions are ca lled an ion s ( [22] ).

Ion e xch ang e curre ntly rema ins t he p ref erred and eco no mic cho ic e of treating water c ont ain ing lo w tota l d issolved s alts f or the purp ose of produc ing de io nis ed mak eup water. T he t echno log y is wid ely used in Esk o m po wer p lants an d is hig hly re liab le in ac hie ving the e xpecte d spec if ications f or the product io n of the d eminera lised water. A f ully reg en erated re sin i.e. catio n is loa ded with H+ io ns a nd water p assing o ver th e res in bea ds enters the p orous structure an d the ions in th e water disp lace the H+ ions f rom the excha ng ed sig hts wh ere one H+ is e xch an g ed per Na+ or K+ ( [23] ).

2.5.2 Clarification T echn olog y

Susp end ed matter in the ra w water sup p lies is remo ved b y va rious metho ds to pro vide water s uitab le f or domest ic purp oses an d m ost industr ia l req uirements. Clar if icatio n is u sed e xt en sive ly thro ug hout th e Esk om water treatment plants a nd has be en pro ven reliab le a nd af f ordable. T he suspe nde d matter in ra w water ma y c o nsist of larg e solids , settlea ble b y g ravit y a lon e wit hou t an y e xtern al a ids, and n onsett lea ble materia l, of ten collo id al in natur e. T he matter remova l is a clarif icat ion pr ocess which is g enerally accomp lis hed b y a comb in atio n of the f ollo wing pr ocesse s with in clar if ication: c oag ulation, f loccu lat ion, a nd s ed imentat ion. T he c ombination of these three proce sses is ref erred to as con ve ntio na l clarif icatio n.

T he perf ormance of the clar if icatio n de vice is en hanc ed b y red uc ing the number of smaller p artic les thro ug h f loccul atio n. T he result is a h ig her un it capac it y and impr o ved o verf lo w q ualit y becau se of the f ormation of f aster settling , larg er ag g lomerate s f rom the f iner particles. F loc she ar an d break up in the a erat ion s yst em is not ne cessar ily detrime ntal, because th is incre ases th e surf ace area an d decre ase s the dif f usion res ist ance ( [24] ). Coa gulation is the process of destab ilizat io n b y charg e ne utralizat ion a nd once n eutra lized; th e partic le s no lo ng er repe l on e an oth er and c an b e

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28 broug ht tog ether. T he removal of suspen ded part icles wh ich will not settle by g ra vit y alone re q uires the ad dit ion of chemical comp ou nds common ly ref erred to as co ag ulants. Particu late ma teria ls c ompris ing d ispers ions ma y range in size from 0.1 to 100 μm. Materia ls within this particle size range are termed c ollo ids ; the sma ll s ize of collo ids cou pled wit h their surf ace charg e is pr imarily r espon sible f or estab lish ing co nd itio ns f avourab le f or the creatio n of disper sio ns. Stab ilizing f actors associat ed with c ollo i d al dis pers ions are e lectrostat ic c harg e and h ydration; these surf ace phen omena are of g reater relative imp ortance d ue to th e larg e surf ace area to total vo lume rat io of a dispers ion of small partic le s ( [25] ). Coag ulation is necess ar y f or the removal of the collo id a l -sized s uspe nde d matter.

Floccul ation is th e process of bring ing tog ether the destab ilized, or "coag ulate d," particles to f orm a larg er ag g lomeration n ame ly "f loc." W hen two c ollo id al part ic le s join tog ether to f o rm a f loc, the y b eco me chem ica lly bridg ed int o a thr ee - dimen siona l n et work and t he c hemic al br idg ing pro cess is c a lled f locc ulation . Co llo ids are alwa ys neg ative ly c harg ed in n atura l a nd waste waters. Und er a micros cop e the c o lloida l matter ca n be made to mo ve around u nder the a ction of an electr ica l f ield. T he coag ula nt species are theref ore adsor bed onto the surf ace of the turbidit y par ticles and the y become co ated with the coag ulant.

T he mechan ica l f loc culatio n of ra w was te water to impro ve the remo va l of total sus pen ded s olids (T SS) and b ioc h emica l o xyg en d emand f or 5 days (BOD5) has bee n p ractice d f or a lo ng time. Combined f loccu lat ion and

sed imentation ( [ 26] ) was discuss ed wit h the perf ormance of centrally driven mechan ica l f loc cu lat ors in c ircu lar clarif iers. It was f ound th at these un its; with out chem ica l ad dit ion, perf ormed better than con vent io na l clar if iers. T he enha nced perf ormance is re cog nized tod ay t o be the re su lt of optimizing the f locculatio n pote ntia l of the inc oming part icu late s.

Sediment ation ref ers to the p h ys ica l re moval f rom suspe ns ion or sett ling , that occ urs o nce t he p artic les h a ve been co ag ulate d a nd f locc ulated. Sed imentat ion with o ut prior coag ulatio n results in the re moval of only relat ive ly coars e sus p end ed so lids, le a ving behin d some parts of the matter not be ing remo ved. Sed imentat ion is t he f inal step in th e clar if icat ion process. F locc ulated wat er f rom the slo w mixing phas e f lows to the settling zo ne wher e ag g regated f loc p artic les settle out. As th en ag g reg ated or

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29 cong lomerate d f loc settles, c larif ie d (clear) water ris es an d is sep arated f rom the sedime nt. Settle d f loc particle s are remo ved in a thick ened st ate (as s ludg e) f rom the bottom of the se dimentat io n vesse l. Clarif ied water typ ica lly o verf l o ws f rom the surf ace and is treated f urther throug h f iltratio n eq uipment.

Crit ica l to succes s are g ood mixing and f loccu lat ion zo nes bef ore the sed imentation a nd f ilter un its, to g enera te suff icient part ic le size of solids f or remova l. T his a lso will decr e ase th e chem ica l dos ing r eq uirements ( [26] ).

2.5.3 Filtration Te c hnolog y

Filtration is the proc ess of pass ing settled water throug h a porous me dium to remove an y rema in ing matter held in suspe nsion. In wat er purif icat ion, the matter to be re moved is co lloida l in size and inc lu des suspen ded s ilt, cla y, org anic c ollo id s, and microorg an is ms, inc lud ing a lg ae, bacter ia, an d virus es.

Filtration cou ld b e c ons idere d a ver y f in e f orm of separatio n , since the f ilter media is inten ded t o ph ys ica lly remo ve impurit ies. Ho we ve r, f iltration is inten ded t o remo ve ver y small impur itie s that remain af ter the sep aration process. T here are var io us f iltrat ion t echno log ies su ch a s sand f ilt ers, cartridg e f iltration, onlin e f iltratio n and membran e f ilt ration. T hese techno log ies c an t arg et ver y f ine p artic les such as sand an d s ilt, microsco pic part icle s such a s bacter ia and a lg ae, molecu lar const ituents such as virus es a n d ac ids, and e ve n ionic imp urit ies suc h as s alts and metals. T he costs as sociated with f iltration g reatly incre ase as the targ eted impur itie s decre ase in size.

Filters are d ivid ed in to two g enera l class es, depe nd ing on th eir f ilter med ia: g ranular and pref o rmed. Granu lar f ilt ers are de pth -t ype f ilters using ind ivid ua l g rain s in layers , su ch as s and charco a l or comb in ation s of these, as a f ilter me dium. Pref ormed f ilters ca n be s creen, s urf ace, or depth t ype, rang ing in th ick ness f rom a sing le th in membrane element t o a t hick f ilter mat ( [27] ).

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30 Filtration in vo lves t he mech an isms of adsorpt ion (ph ys ic al and c hemica l), strain ing , se dime nta tion, interc ept ion, diff usion an d inert ia l compact ion. It does n ot remove diss olved so lids, b ut may be used t og ether with a sof tening process whic h does re duce th e conce ntr atio n of disso lved so lids. Filtration is used in addit io n to reg ular coag ulat ion and se dimentat io n f or removal of solids f rom surf ace water or wa ste water. It prepares the water f or use as potable, b oiler, or coo ling wat er mak e -up at the po wer p lants.

2.5. 4 Membran e Tec hnolog y

Membra ne proc esse s ha ve b een used incre as ing ly f or the product ion of pure wat er f rom fresh water a nd sea wa ter. T he processes are als o be ing app lied in pro cess and waste water s ys tems. Membran e microf iltratio n is replacing con ve ntio nal clarif icat ion an d f iltratio n pro cess es. Ho we ver, waters with h ig h le vels of suspend ed s olids st ill r eq uire to be treate d b y con ve ntio na l c lar if ic ation tech niq ue s. Over the years membr ane te chn olog y has bee n e xpen sive and re lat ive ly e xpe rimenta l; rec ently th e tech no log y is ad van cing q uick ly a nd b ecom ing less e xp ens ive, impr o ving perf ormance, and e xt end ing lif e expecta nc y. A me mbrane is a p ermeab le or s emi -permeab le, so lid ph ase (po lymer, inorg anic or metal), whic h controls the relat ive rates of transport of certa in spec ies pres ent in the so urce water an d restricts the ir mot io n. Genera lly, memb ranes work by se le ctive ly a llo wing some con stitu ents to pass thr oug h th e membrane wh ile blo ck ing the passag e of others ( [28] ).

Membra ne t ech no lo g ies is a g en era l te rm f or removing con taminants f rom f eed water b y mea ns of a thin, por ou s barrier ca lled a membrane. It is based o n a pro cess k nown a s cro ss-f lo w f iltration th at a llo ws f or continuo us treatment of inp ut liq uid streams. I n th is type of process, t he pre ssur ized ra w water in let f lo ws para llel to a poro us semipermeab le membrane f ilter. T he f act that this system is press uris e d, water is f orced throug h the f ilter membrane. T he c lean p ermeate p as ses throug h th e semip ermeab le membrane, a nd the un wante d p artic le s remain in the conc entrate stre am that is d isch arg ed to the se wer.

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31 Common membrane process es in clude u ltraf iltration (UF), rever se osmos is (RO), electro d ia lysis (ED) an d e le ctro d ia lys is r e vers al (EDR). T hes e process es ( wit h the exce pt ion of UF) reduce most ions; RO a nd UF s ystems als o pro vid e ef f icien t reduction of non -io nis ed org an ics and p articu lates. UF membrane p oros it y is too larg e f or ion r ejectio n; the UF pro cess is us ed to reduce c ontam inants , such as o il and g re ase, and s usp end ed solids. Due to the co ncentrat e s tream cont inu ous ly remo ving cont amina nts; thes e pressure - driven membrane tech no lo g ies req uires on ly occa siona l back wash ing and c le aning ( [29] ).

RO membranes are desig ned f or remova l of dis so lved so lids with h ig h eff icienc y, b ut ad versely aff ected or f ouled b y su spen ded so lids, co lloida l materia l or sca le. Common e xamp le s of such f oul -a nt s are c alcium precipit ates, meta l oxides, co llo ida l s ilica an d var iou s org anics. In dustr ia l sid e-stream waste water t yp ica lly c onta ins most, if not a ll, of these f ouling substan ces. Once f oule d, limited b y its membrane material propert ies, on ly mild c lean ing c hemicals suc h as citr ic acid an d d eterg ent can b e us ed to restore the f lu x. S trong er or more ef fectiv e c lea ning che mica ls s uch as sulphur ic a cid, h ydr o chloric ac id, b le ach, and p ero xide can not be a pp lied as the y will ca use irre ver sible d amag es to the RO membran es. Ap propr iat e pre-treatment must be pr o vided to a chie ve st ab le p erf ormance of RO membranes.

Attentio n has re cen tly be en g ive n to munic ip al waste water t reatment plant eff luents as a s ourc e of ind ustria l and ag ricult ural reus e water throug hout the wor ld. Se vera l recent pu blic atio ns h ave portra yed the us e of membrane techno log ies on s e condar y and ter t ia ry ef f luents as re lative ly rec ent occurren ces. Ho we ver, f acilit ies ha ve b een utilizing such membranes on waste water streams f or many years. O ne of the pio neer in g sites to use reverse osmos is (RO) techno log y f or eff luent reuse in po wer pro duct io n witn ess e d en viab le oper atio na l e xperiences an d membrane lif e expecta nc ies ( [30] ).

In operat io n f or o ver eig ht ye ars, the f ac ilit y utilized the s ec ondar y eff luent f rom a municipa l tre atment f acilit y as th e so le sourc e of f eed wat er to a 49 Meg a watt natura l g a s po wer turb in e f acilit y in Lod i, Ca lif . As e viden ced b y this pr oject's succ e ss and b y the suc cesses of subseq ue nt eff luent -f ed po wer f acilities; ef flue nt reus e has be come a s ig nif icant ad va ntag e in

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32 loc ating ne w p o wer product io n f a cilit ie s. T he po wer comp anies that ha ve embraced th e reuse of treated munic ip a l wast e water h a ve been we lcomed more op en ly in the p ursuit of reg iona l ap pro va ls and rece ive d more p olit ica l endors ements of projects ( [30] ).

EPRI is co llaborat ing with S and ia Nationa l L abor atories an d W EN Eng ine ering to evaluate the f easibilit y of integ rating membrane d istillat ion water treatme nt tec hno log y with coo ling to wer s ystems f or uncon vent ion al typ es of water treatment systems th at could s er ve as alternat ives to f reshwater f or mak eup water. Membra n e d istillat ion ( MD) t echno log y uses latent energ y f rom waste he at to dr ive a membrane s eparat io n proc ess th at removes sa lts a nd ot her tota l d isso lve d s olids f rom an un con ventio na l water typ e such as bra c k ish g round water o r sea water. B y us ing waste h eat sources at a po wer pla nt, MD c ou ld de s alinate water with ou t added e nerg y costs, repres enting a ne w lo w -c ost meth od to treat br ack ish g round water f or mak eup water. T he techno log y ma y a lso represent a n i nno vat ion th at allo ws man y po wer plants in sem i arid area s to mainta in f ull utilizat io n of wet -coo ling systems b y using alternat ive so urces of water ( [3 1] ). Membrane dist illat io n techno log y cuts e nerg y co sts by u sing latent en erg y f rom waste heat to drive a membrane sep aration pr ocess t hat remo ves salts and othe r total d isso lve d so lid s (T DS) f rom an uncon ventiona l water type suc h as brack ish g round wate r or sea water.

Rese arch st ill cont inu es o n th is te ch nolog y t o ass ess the ec on om ic f easibilit y of integ rating MD water treat ment techno log y int o wet coo ling to wer tec hno log y. E conom ic a na lys is will also e va luate a combined wet coo ling plus MD s ystem vers us a dr y co oling system.

2.5.5 Pinc h An al ysi s Tech nolo g y

Pin ch an alysis is a p r ocess integ ration to ol wh ic h was in itia lly used f or heat and mass transf er analog ies ( [32] ). T he techniq ue in vo lves th e conser vatio n of energ y throug h optimisat ion of heat exc hang er net work s. A similar ap proac h is no w ap plie d to the des ig n of water -usin g systems that conf orm to the usage of patterns described in the W ater Research Coun cil (W RC) report (85 1/1/03). T he p inc h conc ept en com passes wa ste minim isat ion and p ollut ion pre ve ntio n b y de velop ing mass e xc hang er

Effluent treatment and its re-use for the Kriel Power Station