OBRARY!

FOR COMMUNITY V W . Ï L ^ ' S Î J P P L ' Y ANO¡jf?

DESIGN MANUAL D3

WATER QUALITY AND TREATMENT

Mí^í.

March 1989"

MAXr;AL D :]

WATÏ.:R QUALITY ANO TREATMENT

T.'.xlijr- oí. C o ri*:.orit. .s

Table oí" Content,s it.)List of Annexes < i i. )List of Tables (Hi)List of F i gures (i v)

1. INTRODUCTION 1

2. WATER QUALITY CHARACTERISTICS & STANDARDS 2

2.1 Drinking Water Quality 22.2 Water Quality Characteristics 22.3 Water Quality Standards 62.4 Water Related Diseases 9

3. SAMPLING AND TESTING 11

3.1 Sanitary Survey 113.2 Types of Analysis 123.3 Sampling Procedures 143.4 Interpretation of Chemical Analyses 163.5 Interprétât ion of Bacteriological Analyses 28

4. SELECTION OF TREATMENT PROCESSES 29

5. UNIT PROCESSES 35

5, 1 Aeration 355. 2 Screening 395, 3 Coagulation 405. 4 Sedimentation 485. 5 Rapid Filtration 605. 6 Slow Sand Filtration 765, 7 Horizontal - Flow Roughing Filtration 875. Ô Disinfection 99

6. CHEMICAL FEEDING 106

6.1 Chemical Préparât i*

6.2 Chemical Dosing

7. OTHER PROCESSES 108

7.1 Manganese Control 1087.2 Fluoride Removal and Fluor idat ion 1.107.3 Removal of Colours, Tastes & Odours 1117.4 Algae Control 114.,5 Desa1 i nat i on 1147.6 Softening 115

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8 ,&.8 ,8 .8 .

123

5

y .9 .9 .9 .9 ,9 ,9 ,9 .9 .

123456789

0. CORROSION CONTROL i 17

Effects oí Corrosion i 1 «'Processes of Corrosion 1 1¡ •'Assessing Corros iv it y of Wat.fjr 120Assessing Corro s i. v i t y o i S o il 12 üPrevention of Corrosion 'I 2'2

9. PLANT DESIGN AND LAYOUTS 125

General Consideration 125Site Layout 126Raw Water Intake 137Flow Measurement 137Instrumentation & Control 138Facility of Opieration 139Laboratory 139Lessons from Existing Plants 141Spec i f i cat i ons 143

9.10 Post—Project Appraisal 144

REFERENCES 145

A N N E X E S

A. Design Example — Cascade Aerator

B. Design Example — Hydraulic Flocculator

C. Design Example — Orifice Plate for Metering/Control ofRapid Gravity Filter with DecliningRate Flow.

D Filter Paper Test

E Preparation of Sand for Filters

F Hydraulics of Under-Drain Systems

G Practical Guide for Dosage oif Bleaching Powder inDisinfection of Public Drinking Water Supplies

H Information to be Included in the Tender Specificationsfor a Water Treatment Works

-ii-

L i st of Tab les •\ o

2.1 Specif leaf ion for Potable WaU-r SLS 614 (1983) -Part 1 Physical and Chemical Reisuire«ieiii.s. 7

2.2 Specification for- Potable Wafer- SLS 614 -P a r t 2 B a c t e r i o 1 o g i c a 1 R e q u i r e in en t s 8

3.1 Additional Sampling Data 173.2 Guide to Chemical analyses of Water- 19

4.1 Effectiveness of Water- Treatment. Processesin Removing Various Impurities 30

4.2 Recommended Treatment Process 314.3 Treatment of Various Sources 32

5.1 Characteristics of Sand Filters 615.2 HRF - Tentative Design Guidelines 915.3 Chlorine Use for Various Treatment Other

than Disinfection 1015.4 Chlorine Cylinder Inventory 1035.5 Chemicals and Water Requirements for Chlorine

Absorption in Alkaline Solution 1035.6 Bleaching Powder Requirements or Disinfection

of Mains and Storage Tanks 105

8.1 Electrochemical Series for Metals in Sea Water 1198.2 Values for Determination of Langelier Index 121

9,1 Typical Design Criteria 127

- H i —

4.1 Flow Di a g r a «i S h o w t n g P o s s i. b 1 e '! ' r e a t MI e n t SI. a g e K •' i 3A. 2 Approximate Operational Ranges íor Treatment

P ro cesses. 34

5. 1 Aerators 385, 2 Chemical Feed Arrangement for Alum 425. 3 Coagulant Mixing 425, 4 Coagulant Diffuser with Dilution Line and

Sampling Tap 445, 5. Combined Floceulation and Sed i wentat i cm Tank 475, 6 Flocculators 495. 7 Horizontal Flow Sedimentation Tank 515. 8 Radial and Vertical Flow Sedimentation Tanks 525. 9 Sedimentation Tank — Typical Design 545.10 Sedimentation Tank - Simple Inlet & Outlet

Arrangements 575.11 Tilted Plate Clarifier 595.12 Rapid Sand Filter - Main Features 625.13 Rapid Filter - Typical Design 635.14 Declining Rate Operation 685.15 Low Pressure Backwashing 715.16 Low Pressure Backwash Filters — Arrangement and

Underdrain Details 725.17 Simple Headloss Gauge 755.18 Slow Sand Filter - Features 795.19 Alternative Underdrain and Filter Support

Arrangements 835.20 Outlet Structures 845.21 Horizontal Flow Roughing Filter — General Layout 895.22 Inlet and Outlet Structures of HRF 925.23 Drainage Systems 945.24 Mechanical Flow Rate Devices 945.25 Effect of Intermittent Operation on Size of Filters 955.26 Typical Gravel Washing & Seiving Installations 975.27 Water Sampling Points 98

9.1 Examples of Flow Diagram 1309.2 " " " " 1319 3 » . . . . « 1329.4 " « H » 133

9.5 Typical Schematic Flow....|D|.ij|g^^^^^^.^^ iS^fe:^^.^**9.6 T y p i c a l H y d r a u l i c P r o f i ÏM^:~'~-^".~\^r~ ... .• -Ty'. ;.'™-" "'."--" .~J.-' , •.••••--13S9.7 Hydraulic Profile and Process Flow Diagram 136

— IV—

I. INTRODUCTION

This Manual is for the use of \'WSDB Engineers in ¿assessing water

quality, identifying the need for water treatment,' art d in

planning, designing, rehabilitating and, to some extent,

operating water treatment facilities. It is a development of the

Design Manual on Small Community Water Supplies, prepared for

NWSDB with WHO assistance in 1982 (Ref.l), and includes all

relevant material from this earlier manual, which in some cases

has been reviewed or updated.

The Manual is substantially «ore comprehensive than the earlier

manual, and includes sections on water quality characteristics

and standards, water borne diseases, sampling and testing,

interpretation of chemical analyses, and other specific unit

processes not covered previously.

The manual was put together by G. A. Bridger, Environmental/

Sanitary Engineer, USAID Project fro» a variety of source

materials, .the most important of which are listed in the

References.

^ E t í f r . i i . ^ ;

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• •.•'¥?

2. WATER QUALITY CHARACTERISTICSAND STANDARDS

QU AL I.T

A potable water supply is essential for the prevention ofwaterborne diseases. Water that is contam inate

o Turbidity — colloidal solids give water a cloudyappearance , aesthet i cal ly uriattract. ivea r í d possíbly harmful. May be due to to

o Solids —

o

e 1 a y a ri d silt p a r t i c les, sewage o rindustrial waste, or micro—organis»s.

may be organic/inorganic matter in.suspension and/or solution. Totaldissolved solids (TDS) are due tosoluble «ateríais; suspended solids(SS) are discrete particles which may befiltered out. Settleabie solids arethose which settle out in a graduatedcone after standing for 2 hours (usefulfor assessing sedimentation tankperformance).

Electricalconductivity— depends on quantity of dissolved salts,

and approximately proportional to TDScontent,

Chemical Characteristics

o pH -

o Alkalinity —

Acidity —

o Hardness —

the intensity of acidity or alkalinityof a sample, actually the concentrationof hydrogen ions present.

due to the presence of bicarbonate,carbonate or hydroxide. It is useful asit provides buffering to resist changesin pH. Expressed in mg/L of CaCO-j,

CO2 acidity is in the pH range8.2—4.5, and most natural waters are inthis range. Mineral acidity, below pH4.5 due to H2SO4, HC1 or HNO3would normally be due to industrialwastes. Expressed in mg/L of CaCC>3.

the property which - prevents latherformation with soap arid produces scalein hot water systems. Due to metallicions of Ca, Mg, Fe, Sr.

-| are two forms —i) Carbonate hardness (metals

associated with HCO3) and

ii) Non—carbonate hardness (metalsassociated with SO^t Cl,

Total hardness —carbonate hardness

Expressed in mg/L of

-3-

alkalinity = non

DissoIvedOxygen

Batter ia

Pungi -

o Algae —

basic units of plant Life, being sirigiecell organisms of various shape» rangingin size from 0.5 to 5. UBI. May be aerobicor anaerobic, aad are responsible. í orwaterborne diseases. Important for wastew a i e r s t a b i 1 i z a t i o n.

«i u 1 i i c e 11 u 1 a r a e r« b i c p i a rt t s , m o r etolerant of acid conditions and a drierenvironment than bacteria. Exist inpolluted rivers, and responsible tortastes and odours.

Mult i—eellular photosyntnetic (energyfrom sun1 i ght) piant s, incorporât i ngchlorophyll. Are usually microscopic infreshwater, and utilize inorganiccompounds such as CO9, NH3, PO4 toproduce wore cells and oxygen. Growthof large algal blooms' in lakes can causesevere taste and odour)problems.

An i ma1 s — requireform,

only organic food, usually in solid

o Protozoa — single cells 10—100 ijm long and live byeating large quantities of bacteria, andare important in ^biological wastetreatment.

o Rotifers — mult icellular animals ? whichconditions with high DO,

like stablehigh DO, therefore

indicators of good quality water.

o Crustaceans — hardshelled multicelfular animals, animportant fish food, 4nd found in stableconditions. Some may ; be seen with thenaked eye. r

o Worms andLarvae — rot ifiers

deposits.which scavenge in bottom

Measurement ojT bacteriological quality

It is noCf^r^etSfièmble to test the water for all organismsit might contain, therefore bacteriological quality ismeasured using certain specific types of bacteria whichare indicative of contamination, Coliform bacteria are theprédominant organisms that are found in the intestinaltract of man and other animals. The coliform groupincludes faecal and non-faecal bacteria, such as are foundin soil. Presence in drinking water of significantnumbers oí coliform bacteria indicates that the source isnot adequately protected. E—coli (Escherchia poli) is aspecific typ>e of coliform organism which ' indicatescontamination by faecal matter. Generally, testi

The standards tor water quality in Sri Lanka are based onthe SLS 614, 1983 - Part 1 Physical and ChemicalRequirements, and Part 2, Bacteriological Requireidenis ,which are similar to the WHO Standards (See Table 2.1,2.2).

These standards should always be applied with. commonsense, particularly for small community and rural watersupplies where. the choice of source, and the opportunitiesfor treatment are limited.

The criteria should not in themselves be the basis forrejection of a groundwater source having somewhat highervalues for iron, manganese, sulphates or* nitrates than inthe Table. Care must be exercised in respect of toxic-substances such as heavy metals, which should be allowedonly after expert opinion of the health authorities hasbeen obtained.

For small supplies which frequently axe to be providedfrom individual wells, boreholes or springs, the water-quality criteria may have to be relaxed. Obviously, inall instances, everything possible should- be done to limitthe hazards of contamination of water.j Using relativelysimple measures such as the lining and covering of a well,it should be possible to reduce the bacterial content ofwater (measured as coliform count) to less than 10 per 100nL, even for water from a shallow well, persistent failureto achieve this (particularly if E. doli is repeatedlyfound) should as a general rule lead to condemnation ofthe supply. \

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Table 2.1

SLS 614 (1963)

PART 1 - PHYSICAL AND CHEMICAL REQUIREMENTS

Characterist ieMaximum Desirable

LevelMaximum Permissible

Level

PHColourOdourTasteTurbidityElect.Conduct iv ityChloride (Cl)Chlorine—Free resid(Cl'Alkalinity (as CaCO

7.0 ~ 8.5 units5 un itsUnobjectionable

rr

2-JTU750 uS/cm200 mg/L

200 "

&.5 - 9.0 units30; unitsUnobjectionable

8-ïJTU35pO1200

400

uS/emmg/L

Ammon i a—FreeAmmon i a—A1bum ino idNitrate (as N)

{ Nitrite (as N>Fluoride (as F)Phosphates-Total (PO )Total SolidsHardness Total

(as Ca Co )! Iron-Total (as Fe>i Sulphate| Calcium| Magnesium' Copper' Manganesei Zinc, AluminiumI Arsenic| Cadmium! Cyanide\ LeadI Mercury 'MÊÊÈÉÊ&Í

ChromiumAnionic Detergents(as HBAS-LAS)Phenolic Compounds

: (as Phenolic OH)Oil & Grease

| Pesticide Residue\ Chem. OKvsen Demand! (COD)i

_---

0.6 "—

500 "

250 "0.3 "200 "100 "30 to 150 *0.050.05 "5.0 •;

————

0.2 mg/L

0.001 "-

(Refer to WHO &

0.060.Í1510ÎO.Pl1.52.-02000

60D

40024b

- 15D1,50.515>00.20.050 .i)050.Ò5O.p5o.poiCh '"tí 10.05

1.0

0.0021.0

H

ti

«

ti

H

it

H

fi

H

H

ii

H

H

II

FAO requirements)

10 m8/L

• !

;

:

i!

!!•

!

•

i¡

Depending on sulphate content, i.e. for 250 me/h sulphate,max Mg. is 30 mg/L; for less sulphate, wore Mg is allowed

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Table 2.2

SLS 614 (1963)

PART 2 - BACTERIOLOGICAL REQUIREMENTS

2,

Pipe—borne water supplies:

o Throughout any year, 95 per cent of the samples shallnot contain any col i for» organists in 100 mL.

o None of the samples examined shall contain «tore than10 coliforw organisms per 100 iL.

o Coliform organisms shall not be detectable in 100 mLof any two consecutive samples.

o None of the samples examined shall contain E.coli in100 mL. (Faecal coliform).

Individual or small community supplies:

o None of the samples examined shall contain more than20 coliform organisms per 100 mL on repeatedexam i nat i on.

o No sample shall contain E. coli in 100 mL, (Faecalcoliform) •

NOTE: Individual or sinal1 community supplies include Wei Is „bores and springs * ::

1 •

Populat ionServed

tnt erv a 1piSuccessiveSamples

Minimum Number ofSamples;to be TakenFrom Whole Distrib.System Each Month

Less than 20,00020,000 to 50,00050,000 to 100,000More than 100,000

1 Month2 WeeksA Days1 Day

i • :

1 Sample-per 5000 popn.1 - d o -

- do -1 Sample per 10000 popn.

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2.4 WATER RELATED DISEASES

Writer related diseases are tho.se traceable to water supplyand excreta disposal. The most important of these diseaseslav be classified as iollows;

Category Trans m i s s i on Specific Diseases

Water—s ite-insect carried

Water contact

Water qualitymicrobiological

Sanitation—! related/water! hygiene

D i sease—carry i rtginsects breednear water.

•African tripanosomiasisin or I (sleeping sickness)

SOnchocerciasis (river! blindness)¡Halaría :

!Arboviruses(yellow feverland dengue)JFilariasis (including

e1ephant i as i s)Japanese eiíicephalit is

] Disease transmitted Schistosowiiasis| by direct contact ' (hi lhar^ziasis )I with water. ! • -,

Disease transmittedby consumption ofm i crobio logicallycont am i nat edwater.

Disease transmittedby inadequate useof water.

Cholera îTyphoid feverDiarrheaDy s ent ery _(amoeb i c,

bacillar;y)Guinea worm

( dr acorit i a s i s )

Shigellosiis( bac i 1 lary^ dysentery ) .,•Trachoma and

conjuct iiy it isEscarias i s (roundworm)Scabies -

The

o

various categories are:

Water—site%in

o Water contact. - where contact with water such asswimming ¡w sufficient to produce infection in people,usually through a disease-eausine orgím isn penetratingthe feet, legs, hands or other part of;the body.

o Water quality — microbiological — where consumption ot«¿•ater causes disease by ingestion of a geriu, or toxicsubstance. These diseases are endemic in areas whereexcreta disposal is haphazard and can achieve? epidemicproportions in serious cases of contamination.

o Sanitation-related water hygiene — infection is mostcommon due to faecal contamination of water, food, andfingers and might have been eliminated through properwashing or cleaning.

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3. SAMPLING AND TESTING

A sanitary survey is essential for the adequate inter-pretation of chemical and bacteriological analyses ofdrinking water supplies. It consists of an extensivefield inspection and evaluation of local environmental aridhealth conditions by assessing current and potentialhazards to the water supply or existing water system.

Sanitary surveys are important as backup information wheninterpret ing the results of water analyses. If analysesindicate contamination of the water supply, a sanitarysurvey should be conducted to identify the source of thecontamination.

Sanitary surveys should be conducted for:

o development of new water sources; ""

o rehabii itat ion/inspect ion of existing schemes.

Simple observation is the major parti of all sanitarysurveys. Inspections of existing waiter supply systemsinvolve checking facilities and operational practices forsigns of contamination. For example, * «ater from a wellshould be considered as possibly contaminated if the wellis uncovered or unprotected, or when the, collection vesselis not kept clean. Leaking pipes «ay- draw in sewage orother contamination from the soil? particularly inintermittent systems. -,

If a sanitary survey reveals possible bacteriologicalcontamination, further investigation shoXild should be doneto determine the source and the level of contamination anddetermine the treatment needed, or. steps taken toeliminate the source of contamination^- \

Sanitary surveys of existing systems can reveal physicaland chemical impurities. Water that isfturbid may not besafe or aesthetically acceptable. If pipelines, pumps orother system components are corroded, thje water supply maybe chemica5Í|^^^pâ|ia»inated, Hard, scaly! deposits on pipesor pumps ^ ̂ítffifiííSt̂ water with high mirteral content whichmay be unsuitable for many domestic purposes. Hiñeraiscan encrust pipes and pots. If red stains appear onplumbing the water may be high in iron;. If the commun i tybeliev«s a water source is harmful even thoughcontamination is not évident, the water should be testedfor harmful toxic chemicals.

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It. .i sanitary survey reveals possible physical or- chemicalcontamination, a complete laboratory art.ilysis should bedone to verity conditions. It analysis reveals that thewater system is seriously contaminated either physical]yor chemically, the problem should be solved iiuuted » ate-1 y oranother water source should be developed.

3 - 2 I£PES_OF_ANALYSI.S

The following types of analysis are listed in Reference 4as being carried out by NWSDB Central Laboratory.

o Partial Analysis (Short)

The parameters assessed for a partial analysisinclude: temperature (°C), residual chlorine (RCl)thardness (EDTA), alkalinity (ALK), : chloride

o Organic Analysis

Chemical. oxygen demand (COD), biological oxygen dewiand(BOD), extractable oil arid grease, •and total organiccarbon (TOC) are included in this group,

o Microbiological Analysis

This analysis of water includes tests for col i form andfaecal col i for» organisms. In addition, specialstraining techniques such as the gram stain are usedwhen applicable.

o Biological Analysis ;

This analysis of water from surface sources andreservoirs includes the identification and enumerationof observable organises, Included in the grouping arethe green and blue—green algae, and

In addition, it should he noted that the Laboratoryconducts analyses to measure the characteristics oítreatment chemicals and the characteristics of sludge.

3,3 SAMPL¿NG_PROCEDfJRES

Accurate analysis of a water sample depends on propercollection of the sample. Samples must be taken fromseveral locations at different times so that they arerepresentative of the entire water supply. Samples forspecific analysis are those collected when a new watersource is being developed, when an outbreak of water-related disea.se occurs, when a scheme is being rehabil-itated, or when water pollution is suspected. Samplescollected regularly to monitor an existing water systemare for routine analysis.

Methods of sample collection depend on whether theanalysis to be made is bacteriological or physical andchemical, and on whether the sampling and analysis areroutine or specific. The main considerations in samplingare to collect representative samples; of the water andtransport them quickly to the laboratory. Where theequipment is available, field testing m&y also be carriedout. I

\Samples must be collected irt the correct volume, with theright device and in the right container. They must bemarked properly showing the date, time |md exact locationof the sampling point, and be accompanied by &n analysisrequest form showing the required tests. In addition,samples should be adequately preserved^ if required, andtransported to the laboratory with a;minimum of delay.Analyses only represent the water quality at the time ofsampling, and it will often be necessary to take a seriesof samples at different times or seasons in order toobtain a more complete picture of the water qualityvariations. This is particularly true for bacteriologicalsamples — analysis of a single sample is not reliable.For routine bacteriological sampling, i the frequency ofsampling is established in SLS 614. (See Table 2.2)

Samples fo^^feaet:eiráological analysis must be collected insterile. -^ia^-^Jliíles with sterile stoppers. A paper orfoil hood over the stopper and bottle neck is necessary.Stoppers can be made of ground glass or rubber. Normally,fre-steri 1 i sed s amp* le bottles are obtained from theCentral Laboratory. In an emergency, well—washed bottlesfan be sterilized in the field by boiling them for fiveminutes. If sample bottles are not sterile, any bacteriapresent on them will wake the analysis useless. Thesample bottle must be kept sealed, and the stoppers andbattle neck should be covered with parchment paper or thinaluminium foil. The covering should be kept in placeafter sampling. Each sample should be at leastf 100 mL inorder to perform all tests required for bacteriologicalcontamination, ,.

Sampler for physical and chemical aria lys is must becollected in chie if¡ Í ca 1 ly clean plastic or f;lass bottles.At least 2 litres of water need to be collected for acomplete physical and chemical analysis,

Samples drawn from a stream or river that is beingdeveloped as a new source should not be collected tooclose to the bank nor too deep under the surface. Areasof stagnation should be avoided. Samples should not bedrawn from the same spot every time, but should be takenf rom d i f fereiit po i nt s .

In piped water systems, samples should be collected at allpoints where water enters the distribution system. Inaddition, samples should be drawn from taps connecteddirectly to the water main and not from storage tanks.

Special samples need to be drawn from known problem areas,such as areas with low pressure, areas with high leakage,and areas far from the treatment plant,1 When collectingbacteriological samples, great care must ïj>e taken to avoidcontamination of the sample by touching the inside of thestopper or bottle neck or contamination from other sourcesapart from the water being tested. Bottles should belabelled immediately after sampling.

The following procedures should be used for sampling fromdifferent sources: •

o Stream or river — collect fro» midstream, if possible,holding base of bottle and scoop water in an upstreamdirection beneath the surface,

o Lake or tank — collect from alternative intakelocations, plunging bottle neck down, into the waterand filling the bottle horizontally, beneath thesurface.

o Shallow well — use a weighted bottle and lower torequired depth, until full.

o Handpump well — pump to waste for at least 1 minutebefore sampling.

o Tap — ̂ •^ÈÏ^SÊ'"^1^-'run to waste for at least 1 minutebefore sampling.

Sample labels should give the following information:

o Sample number;o Reason for sampling;o Exact place of .sampling and type of source;o Temperature of sample source;o Date and time of sampling;o Sample depth (if required) ; -,

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Additional ¡informai ion, as shown itt Table 3,1 should be.kepi for each sample, to aid in the in.terprM.at ion ofresults. It is also useful to sketch on the reverse sideof the. form the example location.

Storage of s£W>Eles Changes occur in the bacteriologicalcontent of water when it is stored. To get an accurateanalysis, bacteriological testing of the water sample isbest begun within one hour of collection. Samples forbacteriological analysis must, be tested within 24 hours ofcollection. When a field kit is used to analyse samples,testing within one hour of collection is possible. Ifsamples must be transported to another site, they must becarefully stored and transported so they remainrepresentative of the water supply at the time the samplewas collected. Temperature of samples during storageshould remain as close as possible to the temperature ofthe source from which they were drawn. The length andtemperature of storage of all samples .should be recordedand considered in interpreting the analysis.

For physical and chemical samples, analysis should be doneas soon as possible, and should not be delayed, over 72hours. Whenever possible, samples should be kept betweenO°C and 10°C during storage and transportation.

Interpreting the results of a chemical analysis requires aknowledge of three factors;

a) What is the water to be used for?

b) What are the values of the test results?

c) What are the consequences of using the water for itsintended use, taking into account the test results?

Some substances, although dangerous in excessive amounts,may be tolerable in trace amounts. "Çhese among othersinclude lead, arsenic, fluoride, selenium, and hexavalentchromium. They are not normally determined unless thereis reason to believe the elements are present, since theyseldom occu^^^j^^a^f^tj^r supply.

Engineers involved with interpretation of chemicalanalysis of water should know, for instance, that nitrateand sodium make water unfit only in specific cases;chlorides and sulphates over 500 mg/L will give water asalty taste; sulphates have a Laxative effect; iron causesstaining; and calcium and magnesium cause heavy use ofsoap and leave residues.

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Table 3.1 Additional Sampling Data

e of person requesting sample: Sample No:

for sample: | [ Routine | for bacteriologicalanalysis

Specific (explain) | or physical/chemical

analysis

l.i.-jt.e and hour of sampling j [ for other analysis

Sample location:Scheme/Project: , .

Simple source: | |Tap | | Cistern [ ) Stream | | Pond | [ Well

I ISprirnt CZl Rain catchment | lother (specify)

Source name, if any .....,,-..,. ,,. u. , •

Exact spot from where sample was drawn: ; i _̂ '••-.:.,••_•>. s p o t . r

When a chemical analysis is, required, en;;< ineers should beaware of the objectives and the available tests, (refer toSection 3.2) and be selective regarding tests to becarried out, A #xiide to intepretat ion of cheinicalanalyses is given in Table 3,2.

The "partial analysis" is designed to give enough basicinformation about a proposed water supply to be able todetermine whether the supply is suitable as a potentialwater supply. The tests are not too cumbersome, timeconsulting or costly and include the following;

L'H ~

Values are given in units varying from O to 14, All pHvalues below 7.0 denote acidity. Values above 7,0 denotealkalinity. The closer the value is to 7,0 (neutral) themore neutral is the substance, thus a pH value of 5.5indicates a greater degree of acidity than a value of6.7. Similarly 7.6 is slightly alkaline while 9,5indicates a considerable degree of alkalinity. Thefurther one gets from 7.0 the mare seviere is the acidityand alkalinity. '.

K pH of 6 is ten times the acidity of neutral water, a pHof 5 is 100 times, a pH of 4 is 1000 times, and a pH of 3is 10,000 times the acidity of neutral water. Below pH6.5, corrosion and objectionable tastes may occur.

Colour — :

Colour in water is generally derived from leaves, peat,logs and other similar organic substances. Sometimes itis due to iron or manganese that has combined with organicmatter. Swamp water is the source ]of most colouredwater. Colour is not injurious to health but isaesthetically objectionable.

True colour of water is due to material" in solution whilethe apparent colour is due to the effect of' particles ofsuspension. Only true colours are important, thereforesubstances in suspension must be removed before colourdetermination is Made. Water that is highly turbid isseldom highly coloured because materials.causing turbiditytend to absar^tUose. caus ing colour. ?

The unit of colour used as a standard is that produced by1 »g of platinum in a litre of water. Colour should notexceed 30 and preferably be less than 5. Colour of lessthan 3 vmits will not be noticed, even in a filled bathtub, whereas colour of 5 units, although scarcelyperceptible, will be noted by many.

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TabJ, G . : .it

N.~im

Water is turbid when i i carries in suspension so manyfinely divided particles of clay,. loans, sand oroccasionally micro organisms that it presents a muddyappearance. There is evidence that freedom from diseaseorganisms is associated with freedom fro» turbidity.Turbidity may cause gastro-intest irial irritation, but ispr i mar i 1 y ob j ect i onab 1 e i rom a s t atidpo i nt o f appearance.

Turbidity is «easured photometrically by determining thepercentage of light of a given intensity that is eitherabsorbed or scattered. Host turbidity meters work on thescattering principle, although turbidity caused by darksubstances that absorb rather than reflect light should bemeasured by the absorption technique., Fornazin is thestandard reference chemical with meter readings expressedas formazin turbidity units (FTUs). The term nephelometryturbidity units (NTUs) is used to indicate that the testwas carried out on the scattering principle. Turbidity ofa good water supply should not exceed 5 and it ispreferable to maintain it at 1 or lefes. At 10, slightcloudiness can be observed in a glass' tumbler. At 500,water is practically opaque, \

The specific conductance of a water is a measure of theability of the water to conduct an electric current. Itprovides an accurate measurement of stream purity: purewater is highly resistant to the passage of an electriccurrent and therefore has a low specific ¡resistance.

Chlorides are combinations of chlorine with other elementsand are usually found in natural waters.; Where the normalchloride content of water is known, the determination ofchloride-s can be of value in judging the sanitary qualityof water. Chlorides in excess of the normal*"content maybe considered an indication of contamination by sewage,since chloride is a constituent of urine, or intrusion ofsalt water if the supply is located near the coast.Chlorides increase the corrosive character of water.

Where conÉ^^^^^i^n is suspected, the source of chloridesmay be due €o sewage, manufacturing wastes, mineraldeposits, oil fuel wastes or salt water. The chlorideupper limit of 250 mg/L is not set for health reasons, andwater containing greater concentrât ions can be usedwithout harm.

Chlorides will impart a salty or brackish taste to waterwhich will be noticeable around 250 mg/L. At 500 to 1,000mg/L content, water will be unpalatable. Water containingmore than 4,000 »g/L is unsafe. Chloride tesjts are notessential in determining safe water supplies, put may beused as a general indication and guide. *

-20-

Water can be aeid, alkaline or neutral , Alkalinity i susually caused by the presence of alkaline salts inwater. Various degrees of alkalinity are typical withdi f f ei-ertt forms of the salts,

o Bicarbonate alkalinity is derived from bicarbonates ofcalcium and magnesium.

o Carbonate alkalinity is derived from carbonates ofsodium, potassium, calcium and magnesium.

ò Hydroxide or caustic alkalinity is derived fromhydroxides of sodium, potassium., calcium andmagnesium.

Although some of these compounds can cause hardness, thetotal amounts of alkalinity and hardness are the same2Eíi^ when calcium and magnesium carbonates are presentalone. A criterion for minimum and maximum alkalinity ina public water supply is related to the' relative amountsof bicarbonate, carbonate and hydroxide alkalinity.Generally alkalinity should not be less than 30 mg/L norhigher than 400 mg/L - \

Albuminoid Nitrogen (called albuminoid ammonia) isnitrogen present in organic matter before anydecomposition occurs. The formation of*ammonia nitrogenor free nitrogen marks the first step in the decompositionof organic matter. Ammonia combines with oxygen to formnitrates ultimately.

—_N i_trat es — •

Nitrates in water should be regarded w^th suspicion, asthey may be due to pollution by organic wastes or due tomineral deposits. The Nitrite ion is the substanceresponsible for causing methemoglobinemia, a blood diseasein babies. Having considered this aspect, nitrates over10 mg/L may be safe providing they are bacteriologicallysafe. :

Spot testst^pïr- íretirâtes show whether 'or not there1 arenitrates in the sample and give only a rough approximationof the content. The purpose of this test, which is verysimple to perform, is to indicate the presence or absencef>t nitrates, thus indicating whether further tests shouldbe carried out.

-21-

jr

The process of forming nitrafes is briefly aw follows;

Organic matter — (albmrt-inoid nitrogen, a Ibviia i no i d ammonia)decomposes to ammonia nitrogen (free ammonia, freenitrogen) then decomposes to unstable nitrites(characteristic of fresh sewage) then oxidizes tonitrates — a relatively stable form.

The relative amounts of various nitrogen compounds ratherthan the actual amounts indicate the condition of thecontaminating material. When ammonia nitrogen is greaterthan the albuminoid nitrogen, it is a fairly definiteindication that the water has been contaminated by sewageand that decomposition has already commenced. If theorganic matter in the. water is old, nitrites will havebeen changed to nitrates. It is seen therefore, that wherenitrites are found continuously, it may be concluded that:

there is continuous contamination in close proximity tothe sampling point, .

Additional tests may be required to give further specificinformation for specific problems.

Fluoride occurs naturally in groundwaters in certain areasof Sri Lanka. The element fluorine". may be found involcanic gases, fluorite or fluorspar in sedimentaryrocks, or triolite in igneous rocks.. Disfigurement ormottling of teeth occurs when the fluoride content of apotable water exceeds approximately 1.5 mg/L and becomespronounced when the content exceeds 3 to 6 mg/L.

However, fluoride in the range of 0.6 to 1.5 mg/L isbeneficial in prevention of tooth decay in children, andis often added to water supplies when naturally absent.

Total solids in water is the weight of all mineral andorganic matter in the water — either in solution orsuspension. It is not a measure of any particularingredient but rather the sum of all the various elementsand compounds dissolved and suspended in the water. Itincludes, .theggefãĵ ê _̂ ron and other chemical constituents.

The SLSI recommends the total solids content should notexceed 2,000 mg/L and preferably should be below 500 mg/L.

-22-

The suitability oí: a water for drinking purposes isindicated in a general way by the sol id.s content. Highsolids content indicates that a water is highlycontaminated or contains excessive amounts of mineralmatter.

Filtering a sample of water will remove all suspendedsolids and a sample that is evaporated will leave behindall total solids. The difference between total solids ands us pended solids is the d i s s o1ved solids. If the to t a1solids are burned and the resulting ash weighed, this isan indication of the fixed solids.

Hardness —

Evidence of hardness in water can easily be detected inthe field. Water is hard if it requires m c h soap toproduce a lather and soft if it lathers freely. Hardnessis due to salts of calcium and magnesium, and is expressedin mg/L of CaCO-j. Hardness is not dangerous to health.

The following are examples of mineral compounds related tohardness in water: ?

o Calcium bicarbonate — solution of; CaCÛ3 in waterscontaining CX>2- =

o Magnesium bicarbonate — together with above are themain constituents of temporary or carbonate hardness.

o Calcium sulphate. ~

o Magnesium sulphate — permanent hardness or non-carbonate hardness. Í

Also present sometimes are calcium and magnesium chloride,and calcium and magnesium nitrate. *-•-

The objections to hardness are:

o It wastes soap and affects the skin. ;. ¿aï», -gfej.^kiri..-- - • Ï

o Soaps àná Salt form a precipitate or curd which clingsto fabrics in laundry work and textile manufacturing.

o Hardness modifies colours in dying work.

-23-

o Carbonate hardness forms boiler scale when heated thatadheres to the boiler shell reducing the capacity oí:the boiler and causes tubes to leak or burn out.

o Calcium deposits choke water heaters.

Since hardness may be due to several different compounds,results are normally expressed so that we can see at aglance the relationship between different waters.Hardness, ratings are;

0 - 50 mg/L - soft50 - 100 - moderately soft100 ~ 200 - moderaly hard200 - 400 - hard400 + over — very hard

Water can contain up to 800 wg/L and be satisfactory,though over 200 mg/L softening may be considered. Somewells, however, containing 5,000 mg/L hardness have beenused without harm.

Iron, although it has no bearing on health, is importantwith respect to appearance, paiatability and suitabilityfor domestic and industrial use. It stains glass,porcelain fixtures, clothes when laundered, and uniteswith tanic acid in tea and coffee changing their flavourand appearance. It may impart an inky or metallic tasteto water. Water which contains excessive amounts of ironwill develop a brown precipitate or finé brown suspensiondue to iron oxide (iron rust) following exposure to air.An iron content exceeding 0,05 »g/L may develop someodour, stain fixtures or form precipitates.

The SLSI set limits of iron content at 0.3 mg/L. . Thislimit was not set for reasons of health but rather taste,odour, laundering, etc. Condemning a water because ofhigh iron content is therefore not justified for healthreasons although a water with higher iron content is notreadily accepted because of a reddish appearance andstaining characteristics.

Manganese in concentrations of a few hundredths milligramsper 1 itr.ê . ,j3ggjá|LjLaf.fe;- cause a build up Î of coatings indistribut ioîï ̂ q̂ p̂ ÏKJÉfli which slough off. It can causelaundry staining or formation of precipitates.

-24-

Sulphate content, is not too siguií icarit when assessing thepot. ability of a dt inking water,

Excessive suiphat.es may;

o Cause bad tasteo Have a laxative effect when used for- the first titite.

The SLSI specifies that a potable water should not exceed400 lag/L sulphates. Elsewhere, water with sulphates up to6,000 »g/L is being consumed without any apparent harm.

Calcium and Magnesium are the constituents which producehardness in water. They waste soap, affect skin, are lesssatisfactory for cooking and washing and for» scales inboilers and pipes. Calcium and magnesium bicarbonates arethe main constituents of temporary hard water. Thesulphates and chlorides constitute permanent hardness, andremoval is more difficult, :

Calciui compounds cause hardness, alkalinity and salinityin water and although their presence; in most naturalwaters has no sanitary significance, they may have adetrimental effect on the use of water for domestic andindustrial purposes.

Except for traces, copper salts do not occur in naturalwaters. Their presence in potable water is attributableto corrosive action on piping, 1

Concentrations large enough to be significant from ahealth standpoint would render water supplies completelyunpalatable. However, there is little danger of copperpoisoning from this source. Tastes in the water supplywill occur with concentrations of 3 to íT mg/L.Concetrations of 0.5 mg/L or less in some soft waters willcause staining of porcelain.

The SLSI specifies a limit of 1.5 mg/L.

Zinc -

Zinc has been foundzinc mining areas.limited to amountsgalvanized piping,s i grt i i i c:nrice unt i 1met allí c t as tes i n

in concentrations up to 50 mg/L inHowever, zinc content is normally

incidental to corrosion of brass,and thoMy amounts aré not of practicalthe corrosion is sufficient to cause

water. The limit set by the SLSI is 15mg/L. However 5may a i d c orro s i on

mg/L may cause a disagreeable taste and

-25-

Arse¡ri_i_c —

Arsenic may be present in water from hot springs or in thewastes from softie industries. The maximum' limit set by theSLSÏ is 0.05 Mg/L.

Lead ~

Lead is not present in natural waters and would not bepresent if not for direct contamination.

Lead contamination results when corrosive waters areallowed to stand for a length of time in lead piping or tocome in contact with lead bearing jointing compounds, andthe problem is considered limited. Lead poisoning usuallyresults from the cumulative toxic effect of lead afterprotracted consumption of water containing significantconcentrations, rather than from occasional presence ofconcentrât ions in excess of 0. 1 mg/L.

The limit set by SSI is 0,5 »g/L.

Sodium content of water may be deleterious to thosesuffering from high blood pressure, where 200 mg/L ofsodium way be significant.

Sodium and potassium chloride can be tested. Sodiumcontent around 200 mg/L is reasonable except for sodiumfree diet consumers where content above 100 mg/L is bad.

Silica -

Silica in water can be present in colloidal form or insolution. Its presence has advantages such as assistingcoagulation with aluminium sulphate and helping build up agood coating that protects iron from corrosion. Itsdisadvantage is in high pressure boilers and turbineswhere it forms a scale which is hard and troublesome.

ABS is short for Aklyl Benzene Sulphonate and it is asurface active agent or surfactant, as they are called.They are X̂JS&JJ.JfeQ:/-.. substances which, at usage concen-trations, --"^^^IB^^inrfe ly lower the surface tension ofw a t e r . • • ~" ' •

The ease with which a surfactant is degraded or dischargedby biochemical weans is called biodegradab i 1ity, Thoseresistant to biodegradability are called hard detergentsand conversely those detergents that contain surfactantsthat are easily degraded are called soft detergents.

-26-

When wash waters and w;.̂ t.p which contain syntheticdetergents go down the drain they will containapproximately G.l£ to 0.5% of finishsd detergent product.This is 1,000 mg/L to 5,000 mg/L respectively and isequivalent to 200 rog/L to 1,000 isg/L of either ABS, LAS,or other surfactant. This LAS is a linear .a Iky late whichwhen sulphonated becomes linear alky late sulphona+e henceLAS.

When ABS goes down the drain into the sever it becomesdiluted by other wastes 100 times or so, so that rawsewage contains typically about 10 mg/L ABS,

Traces of ABS which find their way into drinking water,almost always come from sewage. ABS therefore is art indexof pollution of sewage origin.

While a level of 5 ng/L ABS might be encountered as anunusually rare circumstance on a temporary basis, ABSlevels in drinking water are almost always below 1.0 rag/Lwhich is the recommended limit and are usually well under0.2 mg/L.

Phenols —

Phenols result from oily or tarry wastes from coke ovensand wood installation plants. The limit of .001 mg/L willproduce tastes with the addition of chlorine.

The amounts of phenols would not be significant if not forthe fact that objectionable tastes and odours result whenphenol bearing wastes are subjected to ehlorination, i

Biodegradeable organic matter in waste discharges isbroken down by aerobic microorganisms to provide food andenergy. The dissolved oxygen used by the microorganismsin this metabolic process is referred to as the bio-chemical oxygen demand (BOD).

The test for BOD is an important one for determining thedegree of pollution of streams and the oxygen requirementof sewage and trade wastes,

The BOD ...,i:SL̂ |̂ ii6asure of the amount of organic matterpresent oir^r^ihier" of the oxygen required to oxidize theorganic matter. In raw sewage the five day BOD isgenerally above 100 mg/L and for industrial wastes the BODmay range from 1,000 to 50,000 mg/L.

-27-

This test should be included where there is thepossibility oí industrial- pollution or agriculturalrunoff, since pesticides m;*y create health problems.Refer to local WHO or FAO office for guidelines.

Hay cause tastes and odours alter chlorination.

3. 5 INÏIE£E§ÏAÏI9^_QE_iAÇTERI,OLgGIÇAL_ANALYSES

If the water samples taken do not meet the prescribedstandard, the source should be considered unsuitable foruse without treatment. If any col i form organisms arefound, further investigations way be necessary todetermine their source, if this is not evident.Disinfected water supplies should be completely free ofany col i foras, however polluted the raw water may havebeen.

-28-

4, SELECTION OF TREATMENT PROCESSES

The object of water treatment is to obtain water of approvedquality standards. Treatment must be both economical andreliable, requiring the least capital investment and operationand maintenance. The. relative merits of sources requiringtreatment against the cost of long pipelines from a distantsource not receiving treatment should be weighed carefully inview of the constant attention which would be required for theoperation and maintenance of a treatment, plant.

The methods of treatment depend on the nature of the source andits quality. Treatment may comprise only fhlorination fordisinfection where raw water quality is within the recommendedlimits or it may involve certain specific or full treatmentincluding aeration, sedimentation (either plain or flocculationand coagulation), rapid or slow sand filtration followed bychlorination. Ground waters generally require the least amountof treatment, chlorination usually being sufficient. In the caseof surface water and iron—bearing ground waters, part or all ofthe full treatment processes may be required. It is important*therefore, that water quality of the source be thoroughlyevaluated to select, first of all, the source which requires theleast treatment and secondly, the most economical treatmentprocess (es). Data on design of the various unit processes aregiven in Chapter 5.

For small community water supplies, complicated treatment schemesare not suited and in such cases a better solution may be todevelop an alternative unpolluted source of water, even when thiscan only be found at a greater distance.

Important design considerations are:

o Low costo Minimum use of mechanical equipmento Avoidance of use of chemicals, when possibleo Ease of operation and maintenance.

The possible treatment stages including unit processes, chemicaldosing points, washwater recovery and sludge disposal are shownin Figure 4.1. The effectiveness of treatment processes inremoval of various impurities is shown in Table 4.1.

Table 4.2 presenter^^Es¿p^Ípnt options in a slightly differentformat with recoïiaendëâ™ processes for various levels ofundesirable characteristics. Both Tables 4.1 & 4.2 should beused with caution since treatment effectiveness usually dependson a combination of unit processes.

Figure 4.2 shows the approximate operational ranges of treatmentprocesses with respect to removal of particles of differentsizes, and Table 4.3 shows unit processes required for varioustypical types of source waters.

-29-

Table 4.1Ef f eci i veni-s s o t: Water Treat Miont Fc-ij

v ing Various Impurities

Water1Quality

Chewi i cal ! 'Coagula—; ;tion and i ¡Rapid

Aera— '. Floccu- ' Sed i wen— i F i It ra-;Characteristic! tion Iat ionj_ ____„_ L___ i - ~__

¡Dissolved ! jOxygen Content + ' o

I ! • !

jCarbort Dioxide! ;'Removal ! + I o

', tat i

o

o

Í t ion

iSlow Sand ''Filtra- iChlori-i•Ition 'nation í

!Turbidity! Reduet ioni

¡Colour!Reduet i on o

o

¡Taste and•Odour Removal

¡Bacteria¡RemovalI|Iron and|Manganesej Removal

'.Organic(Matter¡Removal!! Hardness

!Corrosiveness ; +++I I

o

o

o

o

o

Legend

-M-+ increasing positiveo no effect— negative effect

-30-

e 4.2 Recommended Treatment Processes5OTS

Water QualityCharacteristic

to beReduced

Floating debris

Algae

Turbidity:0 - 5 TU5 - 30 TU

3 0 - 100 TU100 - 7 5 0 TU750-1000 TU

> 1000 TU

Colour:< 30 Hazen> 30 Hazen

T a s t e s & odours

Calcium carbonate>200 mg/L

Iron and manganese: 1 »g/£.

Chlorides:0-500 «g/1,> 500 ag/L

Col iform bacteria,HPN per 100 «L:

0- 2020- 100

100-5000 ••> 5000

Toxic Chemicals

M

aS

Raw

-

-

•411R

|—-

-

-

---

--

--- ; •

R-!í

2oto

i:

í1-1

•

¡iu

m

een

ipe

r

E

-

RRRR

- RR

_-

-

-

---

--

--

-:

wuo

1 COHMSJ

R

R

__----

_-

-

-

---

-—

• " ' • ' - ' •

-

-

c0

•H

VI

-

-

_

_

-

-

-

-

-

-

R

-

RRE

-—

---

•t " : Î

•HOC

•-IO

?2sca.-

R

_—----

-

R

-

R-E

-—

l''~ *".:• O

-

• ^

a a"g i•H 4Ja *JM 0P- W

-

-

_

_

-

R

E

-

-

-

-

---

-—

iff ï í,'t"!

-

•î'Rr:

tí

a ao oO -Ho u

-

- .

-RRRRR

E

-

---

-—

-

--

': :• r

U. HD ¿1

(4a tnH -S

Coa

and

Set

-

E

REEEEE

E

-

-EE

-—

: reat-

r E

.3 E,"! E ':

1 Ci«R ^et! ^ : : *•

i.n r; -!ïf;

E

i ^"CE,¡1EË

• r |

OC 1-H 03-HOC OCHOHOGM OUIDC0C

-

-

-

-

-

--

-

-

-

R

-

---

-

-

1 ^-.• fi fri ...A

•0

oV) Cn1 C

t) «

5?a «-

-

-.---

-

--

-

E

---

-—

-

L % -\ Vi Cri'. ; c:

; c

•O

_> j

a s--_-------R

-

---

-

-! - ; •

-

] fi J '

ena•H

(B

Des

,

-

-

-

-

-

-

-

-

-

-

-

-

-

—

-

-

• - • • _

-

; • " —

; i

Legend E » Éssént£áÍL7!1 : Ht tf. i Oí

T a b l e4-3 Treatment Systems foJ^^/ater from Various Sources

S o u r c e

for Pre-Inereas i ng R e d u c i n g l C h l o r i -

nati on

Inf i 1-trati onGa I t ery

PlainSedimentai i on/

Rough!ng Til ter

CoaguI at i onand

Ft occuI at i onSediraen-tat ion

Rap i di Itrat ion F i

S lowSandI trati on

PO5 tChlor i -nation

? C ? '2 0 'Iw 3 Í. Ç CAerobic,not corrosive

Aerobic,corrosive

Ancrobic,not corrosive;no Fe and Mn

Anaerobic,not corros i ve,with Te and MH

Ana«rob ic, corrosiveno Fe and Hn

Anaerobic, corrosiveHIth Te & Hn

-L-EZ3-

BYPASS OPTIONS

r 1•CZ>

Surface HatersPrõtêcte*2~hTTT

country streamsTurbidity 10

Streams polluted,frequent highturbidity andcolour

Tanks or lakes,cana Is s!tghtlypo1 luted, lowturbidity anda 1 gae

Tanks or lakes,canals, lowlandrivors,pollutedhigh turbidityand algaetastes & odours

BYPASS OPTIONSi ?

STPASS OPTIONSI f

- H H^—

BYPASS OPTIONS

BYPASS OPTIONS

HZZhBYPASS

OPTIONSI t : . : t. \ d i';

i •: ti i ii :! Ï' i ! t •> r ~ i '•

To supply

High lift pumps

Dechlorination

Pure water tank

Chlorine

pH correction

Filters

Main settling basins

Flocculation

Mixing chamber

Coagulants

Pre—settling tanks

Pre—chlorination

Aeration

Microstrainers

Fine screens

Raw water storage

Raw Water pumps

Coarse

Source

LZWashwater

Sludge

Sludge

Washwder

Washwater

Supernatant)Water i

F ; H

Hair

Pre -

Sluiige Treatment

Sludge

ÍjTo

DryingBeds

Flow Diagram Showing Possible Treatment Stages

Source: Smethurst,G., 1979 "Basic water treatment for applicatiorworldwide". Thomas Telford, London. j

Figure A.1

-33-

ib3 ib1PARTICLE SIZE

1 t> 10 10 10

MOLECULAR SIZESTRUE SOLUTIONS

COLLOIDALSUSPENSIONS SUSPENDED AND FLOATING SOLIDS

H-

C

5. UNIT PROCESSES

5.1 AERATION

Aeration may be used on surface waters to help removetastes and odours due to dissolved gases, or on groundwater supplies to reduce or remove objectionable amountsof carbon dioxide, hydrogen sulphide or wet harte and tointroduce oxygen to assist in iron and manganese removal.In surface waters with a high organic content (such asKalatuwawa) aeration may also assist in colour removal.

There has in the past been over—use of aeration in surfacewater schemes in Sri Lanka. Where aeration is notrequired for the above purposes, it adds to the cost ofpumping (usually 1—3 m head) and increases the oxygencontent making the water more corrosive. This adds to thecost of treatment and makes for a more aggressive waterin the distribution system. Aeration should not,therefore, be used unless required for: ,

o addition of oxygen to anaerobic waters;

o precipitation of iron and manganese in:solution; or

o removal of carbon dioxide, hydrogen sulphide or othersoluble organic and inorganic substances causingtastes, odours and sometimes colour. •

Natural surface waters often contain dissolved complexorganic and inorganic substances which volatilise onaeration and some oxidation of dissolved organic matteralso occurs. This exchange has the effect of removing orreducing obnoxious tastes and odours and, with it, theultimate chlorine demand of the natural water. However,aeration may not remove all tastes and odours caused byless volatile substances. •

Dissolved ferrous and manganous compounds are oxidised byaeration, and precipitated as insoluble ferric andmanganic oxide hydrates which may then be removed bysedimentation or filtration. This process is likely to beless effective in the presence of organic matter, whichmay combiftCĴ f̂rfcfoĝ the iron and manganês^* to form complexcompounds wHich do not precipitate well¿ In such cases,moderate rather than vigorous aeration may work better.

Groutidwaters will release a part of any dissolved gasesand aerate with atmospheric oxygen thus acquiring a moredesirable taste. The release of carbon dioxide increasesthe pH and reduces the corrosivity of the water.

Reducing the carbon dioxide content, however, may shiftthe carbonate—bicarbonate equilibrium in the water so thatdeposits of calcium carbonate are formed which may causeproblems.

-35-

The rate oí aeration is governed by the area of interfacebetween the air and liquid, the thickness of theinter layers and the time of contact,

The type of aerators commonly used are:

Aerators*

These consist of" stationery nozzles, through which wateris sprayed into the air, at about 5-7»/sec. The nozzledesign is i«portant in achieving optimum dispersion.Spray aerators are appealing, but are no more efficientthan other types which require less space.

Typical design criteria are;

Nozzle dia: 10-40 mm @ 0,5 — 3.0 » spacing

Nozzles to be tilted 3° — 5° to the vertical to avoidinterference due to falling water. They should beenclosed by suitable windshields to prevent loss ofvf at er ;

Output per nozzle: 5—10L/s

Head required at nozzle: 2—9 m of water

Velocity of water in the aerator pipe: 1,0—1,5 «/sec.

Aerator area: 0,72 - 2.16 m 2/m 3/day of flow.

The ti«e of exposure of the droplets, the head requiredand the flow fro» each nozzle can be calculated fro» thefollowing formulae;

V =

q = Cd a J2 gh

t = 2 C Y sin x 2hg

h = total head of water at nozzle8 = .. ̂ SEêàS.r^M>» due to gravityV = ±ït3:ï^SlÀ^'felocity of drop emerging from nozzleC — coefficient of velocity (0.9)C = coefficient of discharge (0.9)q = discharge per nozzlea = croSS section area of nozzle openingt = time of travel or exposurex = angle

These are simple, inexpensive and occupy little space (SeeFig 5.I(a). Water is dispersed evenly through perforatedpipes over the upper tray from where it trickles down,

Typi ca1 des i gn cr i teri a are :

Number of perforated trays = 3—5Tray perforations = 10 mio dia,

RAW WATER

AERATED WATER

(a)MULTIPLE TRAY AERATOR

(b ) CASCADE AERATOR

PLATFORMS

PLAN

WATERBASIN

(P) MULTIPLE PLATFORM AERATOR AERATORSFIGURE S.I

- 3 8 -

5, 2 SCREENING

For river and lake intakes some form of protective boom orcoarse screen, about. 75 mm opening, should be used toprevent large floating objects entering the intake. Thevelocity of water through Lhe screen opening should notexceed 0.5 m/ s »

are usually 5—20 mm opening, duplicateFine screensscreens being provided to allow the renoval of one screenfor cleaning manually. Alternatively, provision may bemade for cleaning by raking or water jetting. Maximumvelocity through the screening medium should not exceed1 m/s. Mechanical screens may be necessary on larger

operated continuously or intermittently asby the loss of head across the unit. For ascreen 15 mio/s would be a typical belt speed foroperation and drum and disc type units may

schemes,controlledbelt typecontinuousrotate continuously at 0.2—2

The •icrostrainer is a development of the drum screenusing a fine stainless steel isesh supported on a coarsermaterial. Because of the small aperture size of thefabric, clogging occurs rapidly and the fabric must becontinually cleaned by washwater jets'. The speed ofrotation of the drum depends on the diameter but ingeneral the maximum peripheral speed should not exceed 0.5m/s. Filtering head is normallyoperating head should not exceed 0fabric are in general use: HarkMark I 35 micron aperture and Hark II

40—140 mm and maximum25 m. Three types of0 23 micron aperture,65 micron aperture.

Straining rates are usually in the range 700—2300iu /u^û and backwashing requires about 1% of theinfluent water. Typical sizes and capacities are shownbelow: :

MACHINE SIZE

EFFECTIVE WIDTH MOTOR(kW>

CAPACITY RANGE¡

0.40.61.53.04.0

230-23001360-66003620-1610013600-4536023000-66000

-39-

5.3 COAGULATION

Colloidal particles and fine suspended material cannot beremoved by plain sedimentation because of their extremelylow settling velocities. It is therefore not normallypossible to remove particles of normal specific gravitywith diameters less than 50 aicron by sed intentât ion inconventional tanks. Suitable chemicals can be added towater to form bulky floe pai~ticl.es which will settlereadily removing most of the colloidal and suspendedconstituents of the water.

The most commonly used chemical for coagulation of wateris aluminium sulphate (alum) AI2 (20^)3. IAH2Othe following chemicals may sometimes be used;

Ferrous sulphate (copperas) FeS0¿,. 7H2Û

Chlorinated copperas FeSO^TH^O + Cl 2

Ferric sulphate

Ferric chloride

The substances that frequently are to be removed bycoagulation and f ioeeulat iort are those that causeturbidity and colour. Surface watets in Sri Lanka areoften turbid and contain colouring material. Turbiditymay result fro» soil erosion, algal growth or animaldebris carried by surface runoff. Colour may be impartedby substances leached from decomposed organic matter,leaves, or soil such as peat. Both turbidity and colour-are mostly present as colloidal particles.

Generally, water treatment processes involving the use ofchemicals are not so suitable for small community watersupplies and should be avoided whenever possible. Chemicalcoagulation and flocculation should only be used when therequired water quality cannot be achieved with anothertreatment process using no chemicals. If the turbidityand coloaĤ gcrfMsdbhe-- raw water are not much higher than isper» issib^^^f*^'" drink ing water, it should be possible toavoid chemical coagulation in the treatment of the water.A process such as slow sand filtration would serve both toreduce the turbidity and colour to acceptable levels, andto improve the other water quality characteristics, in asingle unit. A roughing filter can serves to reduce theturbidity load on the slow sand filter, if necessary.

-40-

AlUM is by far the most widely used coagulant but ironsalts have advantages over alum in some circunstances, Oneadvantage is the broader pH range for good coagulation.(For alun the pH zone for optimum coagulation is quitenarrow, ranging from about 6,55 to 7,5 whereas for ferricsulphate it is considerably broader, about 5.5 to 9.0).Thus, in the treatment of soft coloured waters wherecolour removal is best obtained at low pH, iron salts aaybe preferred as coagulants. Iron salts should also beconsidered for coagulation at high pH since ferric-hydroxide is highly insoluble in contrast to aluminiumsalts which for» soluble alumínate irons at high pH.Synthetic organic poly-electrolytes have become availableas coagulants and coagulants aids, but are generally noteconomical for small water supply systems, nor are theyreadily available in Sri Lanka.

The coagulant must be added in the correct dose and mixedthoroughly, for optimal coagulation. The optimal doseshould be determined periodically by carrying out a jartest.

Dosing :

Chemicals are normally added to water as a solution orsuspension so that rap»id dispersion throughout the mainbody of water becomes possible. For accurate continuousdosing the chemical must be carefully measured and thismay be done by either dry or solution feeders.

Dry feeders draw finely ground chemical from agitatedstorage hoppers, measurement being gravimetric orvolumetric. The measured chemical is usually delivered toA solution pot for injection into the main supply as asolution or a slurry.

With solution feeders the chemical is dissolved in largestorage vats holding 12—24 hours .supply of solution withstirrers to a standard concentration (usually 3—7%) whichis then added via a constant head orifice or similarmeasuring device.

In either ease"duplicate feed capacity must be provided incase of breakdown and there should be at least one month'schemical storage capacity adjacent to the feeders. Sincemost of the concentrated chemical solutions are highlycorrosive, solution lines should be of rubber or plasticand tanks should be lined with a plastic coating. Atypical arrangement is shown in Figure 5.2.

Problems have been reported with lime and alum dosingequipment in treatment plants in Sri Lanka, from which thefollowing conclusions may be drawn;

-41-

HANDAGITATO PERFORATEDBASKET

r

SOLUTION

I-.-.,.!

•

FIGURE 5.2CHEMICAL FEED ARRANGEMENT FOR ALUM

ut//////// I//J///////

FEED POINT OF COAGULANT

r :

BAFFLED CHANNEL FOR RAPID MIXING.

FEED POINT OF COAGULANT

NOTE-.THE WEIR CAN ALSO BE USEDTO MEASURE THE FLOW

COAGULANT MIXING

FIGURE 5.3'-

- 4 2 -

o

o

o

proportional feed dosing pumps have been commonlyused, but are unreliable!, and a gravity feed systemthrough pipes of steady gradient should be utilizedwherever poss i b1e;

the amount and concentration of the solution must beknown for optimum coagulation and economy of coagulantuse;

solution tanks are sometimes some distance from thepoint of chemical dosing, requiring long feedingpipes. Solution preparation, dosing and flocculationshould be as close together as possible.

The initial dispersion of the coagulant in the raw wateris of utmost importance, and to ensure even dispersion ofthe chemical throughout the water some form of mixing isessential. The desired degree of mixing may beaccomplished either by mechanical weans or by utilisinghydraulic turbulence.

Hydraulic methods include overflow weirs and baffledchannels as shown in Figure 5.3 or in the discharge pipingof a pump. 5—10 sec. mixing time should be adequate, ifit is really vigorous and continuous.

If a hydraulic jump is used for mixing, the velocity offlow in the flume should be 3—4 m/s and the water leavingthe flume should mix with water moving at about 0.7 m/s,the total mixing time being about 5 sec. The head lossfor such mixing is 0.25—0.50 m.

Mechanical rapid mixing may be achieved by high speed(300—900 rev/mirt) propeller type mixers installed in asmall chamber; criteria to follow are;

Detention time of at least 30 s (commonly GO s)Power requirement 2,5—5.0 kW/m3

Head I os sajú ..jya» .6- »Ratio Vrf^^jdfk;í^ã to height 1-3

However, experience indicates that mechanical rapid mixingis not as effective as well designed hydraulic mixing, andParshall flumes or weirs are preferred.

Dilution of tshould be carwater. The dipH adjustmentto measurecoagulant. Ais shown inacross the flow

he coagulant, to •* solution of 0.5 to 1.0?ried out immediately prior to dosing the rawlut ion water should be filtered, but without(before lime addition). It must be possiblethe volumes of both dilution water andsuitable and simple type of diffuser system

Figure 5,A. The feeding point should beat the point of maximum agitation,

-43-

COAGULANT ANDDILUTION WATER

SLOTTED PIPEDIFFUSER

WEIR AND COAGULANT APPLICATION POINT

•

DILUTION WATER

SAMPLING POINT,,

fDIFFUSER

SAWLÏNG

COAGULANT-

COAGULANT DIFFUSER WITH DILUTION LINE ANDSAMPLING TAP

FIGURE 5.4

F 1 occu I at ion

The size of t loe particles and their speed of formationcan be improved by creating velocity gradients in the

increase the chance of collision betweenSuch conditions can be provided by rectangular

flow tanks with slowly rotating paddles or withor by vertical flow tanks with or without

water topart icles,horizontalbaffles,mechanical stirring.

a)

Flocculators are generally of mechanical type inlarger plants and hydraulic or gravitional type insmaller plants. For mechanical type, use thefollowing typical criteria:

Det ent i on per i odAvg. velocity of flowPaddle areaSpeedMax. per i phera1

velocityHead lossDepth of tankPower requirement

20 to 30 minutes0.3 m/s10 to 25% of the area swept2 rpm

0.15-,0.75 m/s0. 15 ml*í~2 paddle diameters2,5-5,0 kW/m3/s. (designfor starting torque)

Operate as many paddles as possible from one shaft andmotor. Many methods of coagulation and floeculationcan be effective as long as the concept of plug flowis maintained rather than uncontrolled or undirectedflow through large open basins. The very simplestmechanically—driven flocculators, either horizontal orvertical, can be effective if the tank is divided intosections or cells requiring a series flow pattern.Even if not provided in the original design, this canoften still be accomplished later by installingha f f 1 eAggĝ i_OT..-i::̂ ..ex i & t i n g tanks to create cells or

j_d_Ç6tttact¿Upf l.ow_SJ.udge_B 1.anket Ç¿ari. f¿

Most existing plants in Sri Lanka use J combination off1occu1at ion and settling within a single structure inupflow sludge blanket type clarifiers. (see Fig. 5.5.>These units operate; best under heavy turbidityconditions so that a dense sludge blanket can beformed; this is not the case in most oí the plants.Furthermore, these basins do not take overloads welland sharp changes in flow adversely affect theirperformance. The sludge rises and overflows at theslightest provocation.

-45-

Most unreliable oí the existing units ar-e Lhe singlecompartment f locculators which allow short circuitingand short f locculat ion times. Since slow ¡agitation iscritical to the formation of sett le.i ble floe, this unitis very important. The time of agitation and amountoí energy input are important parameters. In upi'lowbasins neither is well, controlled.

For the most part the loading of the basins is at thelimit of reasonable overflow rates, A.dd to this thefact that flocculation times are close to or belowreasonable minimums, and it becomes clear that theseclarifier-flocculator units are not easily modifiedand upgraded,

If more clarification capacity is needed in the futureat these plants, new flocculation and settling basinswill be required. There is, however, an alternativesolution: a light coagulant dosage and directfiltration can probably produce excellent waterquality for most of the year and possibly even copewith seasonally high turbidity waters (during therainy season) by more frecjuent filter washing.

c ) !jv _i ç or s

Baffled or hydraulic flocculation basins are simple,economicalThey havemechanicalcircuiting,intensity

alternatives to mechanical flocculators.the distinct advantage of being free fromequipment, O&M problems as well as short-

Their disadvantages are that mixingis dependent on flow rate and that they

require more area.

The velocity gradient G (sec—-) for hydraulicflocculators can be controlled by the spacing of thebaffles. G should be greater than 10 in order topromote flocculation but less than 75 if dis-integration of the floe by shear forces is to beavoided. The value of G can be adjusted to floe sizegrowth requirements by providing a higher initialvalue and decreasing it progressively. The initial Gvalue may be as high as 100 with the final valuesdropped to as low as 10 (typical values would be inthe range 45-90).

basin should be atmist be taken inthe sedimentation

The recommendedthe sedimentationand emphasis must

The minimum^detention time in theleast 20 win (1200 .sec). Careintroducing flocculated water intotank to minimize floe disintegrationvelocity in tho channel or pipe tobasin should be between 0.1—0.3 m/sbe placed on smoothness of flow.

-46-

¡vi::.

Effluent'"

SedimentationZone

/ • . " - • •

ij-^ia?- itiTMti:

COMBI^EDWEDCCULATION AND SEDIMENTATION TANK

FIGURE 5.5

-47-

Two methods of prov;c a »n p i «calculation for a fixed (.ross-.section channel, is g J veni n Annex B,

Typical sketches of horizontal and vertical flowbaffled channel ílocculators are shown in Figure 5.6.

Typical design criteria are as follows;

Ve 1oc i t y grad i ent,Detention time,Channel vel.Head lossNo. of baffles

Velocitygradient G —

GtVH

sgH

u T

45-90 sec l

600-1800 sec.0. 1-0.3 ta/sec.0.5 m12-20 with 100°direction change andwell-rounded corners

where S = special gravityH = headloss per baffle (

ENTRANCE CHANNEL

/ / J' j'77/ //7

¥y . U

BAFFLES OR PARTITIONS FLO.CULATED WATER

iT

/ / / / / / /

I " i n

/ / / / / / / / / / / j

/ 77/JJ/JTf.ll.fIL

HORIZONTAL-FLOW BAFFLED CHANNEL FLOCCULATORP L A N

- L O S S O F *HEAD

.BAFFLE

SMALL OPENINGS

VERTICAL-FLOW BAFFLED CHAMBER FLOCCULATORCROSS SECTION

FLOCCULATORSFIGURE 5.6 -

- 4 9 -

The most common design provides tor the water flowinghorizontally through the tank (Figure 5.7) but thereare also designs for vertical or radial flow (Figure5.8). For si«aJ. I water treatment plants,horizontal—flow, rectangular tanks generally are bothsimple to construct and adequate.

Sedimentation tanks are designed to reduce thevelocity of water entering the tank so that it isretained long enough to permit particles, to settle. Asédimentation tank has four sections: the inlet zone,the settling zone, the outlet zone, and the sludgedeposit zone. (See Figure 5,7)

o

o

The inlet zone is the area where water enters thetank and is distributed evenly throughout itsentire width and depth.

A baffle or wall separates the inlet zone fro» thesettling zone. The work of the sedimentation tankis done in the settling zone, Where the water isheld long enough to permit suspended particles tosettle. The efficiency of the settling processwill be improved if there is no turbulence or crossci rcu1at i on in the t ank.

o In the outletfro» the topseparated fro»controls wateroutlet zone.

zone, clarified Water is collectedlayer of the tank. This area isthe settling zone by a weir whichflow out of the tank and into the

o Finally, thesettled fromzone shoulddrain.

sludge zone collects the materialthe water. The floor in the sludge

have a minimum of 5% slope toward the

Sedimentation tank design is based on the assumption ofideal conditions in the settling zone, such as:

o

o

A uniformhorizontallyzone;

set t --ËT

flow distribution vertically andat the inlet and outlet to the settling

i d s concentration at the inlet to thevert i ca 1 ly and hor i zont ally.

Consider a particle with settling velocity (Vo) which isjust removed

DRAIN VALVEHOLES DISTRIBUTEWATER EVENLY

SLUDGE POCKET

CONCRETE ORWAU.SFLOOR SLOPES

5% TOWARDS DRAIN

SEDIMENTATION BASIN

SETTLING ZONE

LUDGE POCKET AND DRAIN

HORIZONTAL FLOW\SEDIMENTATION TANK

' MAIN FEATURES

.FIGURE'S.?*

-51-

Peripheral Weir Annular Baffle

•

Influent

Sludge Bleea

.

RADIAL FLOW TANK

• •/•

Sludge .*

Scraper

Effluent

* V

Sludge Drain

VERTICAL FLOW TANK

RADIAL AND VERTICAL FLOWSEDIMENTATION TANKS.

FIGURE 5.8

\'O = H ='i

ta.' h r T = A^Q

h r

Then Vo =A i-l

= QA

A - s u r t..i ce ,st(;a o i t .iïikT m

Q = flow r a t e ia : i /hr

2

m/hr which is the sur-face overflowor loading rate

Note that the tank depth, H, is therefore inmaterial.Particles with settling velocity Vs of Vo or greatershould all be removed, but when Vs is lower than Vo:

\ they are removed, but if1 1 1 I _ . . _T

p r o v i d e d H < V s T , ._n.*=j- •-*. >-• *. •-••• % VsT, they are not removed.

Therefore proportion of removed particles is VsVo

where Vo is design velocity.

Settling tank design should be based on a settlingcolumn analysis of the raw water. Some typical settlingvelocities are given below:

~ "- n

Material

• — " —

Partial Diameter

Coarse sand! 1.0\ 0.5i

Fine sandi

Silt

!

F i ne c1ay

0.25

o.io0.50.005

0.0010.0001

— — i

Settling Velocity(m/hr)

365194

97.529.0

10.60. 14

0.005 (5mm/h)0.00005

For plain sedimentation (withoutchemical coagulation and flocculation), lower surfaceloading rates should be used.

A typical tank layout is shown in Figure 5.9 andsuggested design criteria are as follows:

-53-

GROUND LEVEL-T

DESLUXJE t

Î* •Vtf

• i

nvi j'l'i? n i ljSUJPE

» •

V

' • •

- ,

INLET

V NOTCH

RAW WÂTE

SECTION B B

SECTION A A

IDESLUDCE

PLAN

SEDIMENTATION TANKTYPICAL DESIGN

-S4-

G.L

TO FILTER

FIGURE 5.9

Surface .load, ins rate (s«e table bei O K )Tank depth 2.5-4,0Length/width ratio 3:1. to ft : I

4-6 hrs20 ; 1

D e t e ri t i o n p e r i o dLength/depth rat i o(for basins over 60mWeir overflow rate <Sludge storage (additional depth)Diameter (circular tanks) <Average velocity in basin <Velocity in sludge drains >Velocity in inlet pipe/channel

and out let

16 m3/m.b0, 15-0.3m60 MI0,02 ro/s1.4 m/s0.15-0.4 m/s

Sedimentation after efficient chemical coagulationshould remove almost all of the colloidal and suspendedimpurities in a water. Large floe particles havesettling velocities in the range 0,5—3.5 m/hr and foralum floe 1.0 »/hr is commonly used for design purposes»

Recommended surface overflow rates are given below:

Q/A, » /day p e r TOT y p e

| Normal ! Easy Very bad! conditions I conditions ! conditions

No pretreatmerit !

;With chemical !coagulation i

With coagulant\aids 1

¡

0.

0.

1.

5

75

131

1

1

\

,0

.0

.5

0,

0,

0.

1

4

75

For efficient design most, if not all, of the followingfactors should be considered.

o number of tanks;o dimensions;o velocity of flow;o detention time;o sludge storage and removal;o inlet and outlet arrangements;o particle size of solids to be removed; ando water temperature.

It is very difficult to takefactors, particularly sinceupset by outside influencesview of this, most designsjudgement and experience.

into account all the abovethe whole situation can besuch as wind action. Inare based principally on

-55-

Outlet weirs should be designed to maintain suitable.-settling velocities and to mininise short-circuit ing.The rate of flow over an effluent weir should berestricted to 18 m̂ /ai hr. Vee notch effluent weirs;with not che.-* 50 turn deep at 75—150 nun spacing arepreferable to plates since the effect of subsidence onthe woir would be less acute. Submerged orifices may beused as an alternative to overflow weirs,

For the inlet, distribution of water equally and atuniform velocities requires an inlet arrangement such asbaffles with open or submerged ports. Simple inlet andoutlet arrangements are shown in Figure 5.10. Sludgeremoval may be achieved by mechanical sludge collectionor by a hopper bottom with perforated pipes throughwhich sludge is removed under hydrostatic pressure.Also rectangular tanks with reverse bottom slope areused with high pressure hoses for manual cleaning.'Minimum size of pipe to be used for desludging is 200 mmdia. for non—mechanised units and 100—1.50 mm dia formechan i sed un its.

The following criteria should be used for sludgeremoval ;

For manual cleaning : slope of 1 in 10, hopper

bottom 1,2 in 1 ~ 2 in 1

For mechanical scraping : 1 in 12 slope

Allowable head loss : 0,50 m •:Power required : 0,15 - 20 kK/m3/d.

Rake speed : 0.3 m/min

Manual cleaning has the disadvantage that the tank hasto be taken out of service, but good plgnt designnormally provides more than one settling tank, even withmechanical equipment. Where the solids load in the rawwater is small, a plant can be operated for monthsbetween scheduled sludge removal. Small units can beprovided with hopper bottoms for even simpler cleaning.

Solids contact units are popular in developed countriesand combine in a single basin the various clarificationoperations with flow in an upward vertical directionthrough a layer

INLET PIPE OVERFLOW

eons

HUTCHED OVERFLOWWEIR

20mm holes at 60mm cjcdistribute water evenly

INLET

OUTLET

SEDIMENTATION TANKSIMPLE INLET AND OITT/LET ARRANGEMENTS.

FIGURE 5.10

Commonly used criteria are:

Surface load I tig ~ 1.23—2.5 m/ht-Detention period = 2 hours (not usually used as a

cri ter ion)Weir loading = < 215 m /ó/m of weir length.

These clarifiers require competent operation to maintainan opiittun sludge blanket layer with suitable controlsof sludge withdrawal. The blanket is sensitive to thecoagulant dosage and to a number of other factorsincluding the prevailing difference between ambienttemperature and the influent te»perature.

Experience with this type of clarifier in Sri Lanka hasbeen wixed and often unsatisfactory, and they are nottherefore recommended for small water treatment plants,

et tiers

Tube settlers are now being incorporated in the designof many new water and wastewater treatment plants. Theyare also being used to increase settling capability ofexisting plants. Much remains to be learned about thedesign and performance of tube settlers, but experiencealready indicates they can increase operating rates bytwo to three tines above conventional design criteriaunder proper conditions,

A recent development in high—rate sedimentation is theplate settler. Plate settlers way be made of metal,plastic, or water—proof plywood. Flow through the platesettlers may be either vertical or. horizontal. Thedesign concept is similar to that of tube settlers butis cheaper. The size of structures is kept to aminimum. The basin overflow rates for plate settlers are5—10 times those of conventional rectangular basins.

Tubes may be circular or square in .section, of about 5cm size and inclined at 60° to the horizontal. Water ispassed up through the tubes, and sediment settles outand trickles down, keeping the tubes clean. Tubes areusually prefabricated in packs and,, made of PVC theyare l-i-gÍ|Í̂ jaŜ Jac|¿eap; pipes may also be used. Loading isnormally 9 j*~/tti* per m^ (m/hr) .

When used for upgrading existing sed imentat i or. tanks,improved surface loading rates of 4—5 times may beobtained, and it is important also to upgrade thf sludgeremoval facilities to deal with the extra volume ofsludge. A typical layout of a plate clarifier is shownin Figure 5.11,

-58-

RAW WATER

TILTED PLATE CLARIFIER

FIGURE 5.11

Rapid filtration is recommended only tor medium orlarger schemes where skilled operators «ill be-available. Filtration can be considered as the finalpolishing operation in water treatment. If theproceeding units are operating correctly, about 95^ ofthe impurity load will have been removed from the waterbefore it reaches the filters. Although overallpurification resulting from the filters is small theprocess does ensure that the clarity of the treatedwater is to a high standard ,

For filtration the choice is between slow sand filtersfor (gravity operation) and rapid sand filter (gravityor pressure filters). Rapid filters must be providedwith efficient coagulation and sedimentationpretreatment (for water with turbidity in excess of10-20 units).

A comparison of characteristics of the main types offilters is shown in Table 5.1 and Figure 5.12 shows themain features. A. typical design layout is shown inFigure 5.13,

Common criteria for rapid gravity filters are:

At least 2 filters to be provided.Influent turbidity 20 units after pretreatmentUnit area Up to 140 m 2

Filtration rate 120-3G0 m3/m2dHead loss • Up to 2.5 mFilter run 1—7 daysWashwater consumption 1—(>% ot filtered waterDepth of filter 0^,3-0.6m gravel, 0,6-1.0m stratified

sandGravel graded from 2—50mm over depth of layer.Sand specification;

0.4-1.2 mm Effective Size 1.5-2.0 mm Uniformity Coefficient (UC)

or pa^i^^^pwesh reta i tied ̂30 mesh (0. 5mm )

Minimum depth of water when operating — lmMaximum inlet velocity — 0.6 m/sMinimum ratio underdrain area to filter area - 0.003Backwash velocity — 5—10 mm/sMinimuta backwash time — 5 min.

Wash water tank Provide for 2 or minimum of 1filter washes of 10 mins each.Should be capable Î of beingrefilled in 60 min. j

-60-

: .!•; '. :.. i

i1. '.

INLET

DRAIN

OUTLET'BACKWASH-

JTV/L I WASH WATER TROUfcH

\

' . ' • . ' . /

-. ' ' . - ' « * . ¿ • * , * • - ' : ? * . " . •'. ••• . • • • . . ; % » * > • . * • • • ;

STRATIFIED SAND O.S TO U3m

GRAVEL 0.2 TO 0.5m

UNDER DRAIN

SECTION

n nÜ

i !

li

-WASH WATER TROUGHS

•UNDER DRAINS

GRAVITY TYPE - P L A N

DRAIN

BACK WASH - C = î = -

INLET

PRESSURE VESSEL

• ' I - - - I f '

-STRATIRED SAND

ROTATING HOLLOW-BACKWASH ARMS

-GRAVEL

UNDER DRAINS

OUT LET

PRESSURE TYPE

'RAPID SAND FILTERMAIN FEATURES

FIGURE 5.12

- 6 2 -

S A N D ; ..••.'•"

GRAVEL

. INFLUEN1

SECTION B B

»

PERFORATED LATERALS

SECTION A A

^-BACKWASH CONNEQED10 RISING MAIN

DITCH FORBACKWASHWATER

INFLUENT.

PLAN

RAPID FILTERTYPICAL DESIGN

FIGURE 5.13

-63-

A u >; i 3 i a r y s e o u r - 1 e c 1 i .-i 11 i e a 1rev/ Hiin.

before20-35 kX'/ro

r.:* k e i n s a n d 1 -1 025

for several.backw.nshing,

Surface

minutespressurewash by

additional 1 — 20 mm/s jetted ontosurface of sand at 150—450kN/»2,

Head loss in wash water syste» 8—15 HI

Wash water troughs— Should be clear of expanded .sandwith maximum horizontal spacingof 2m. Top of troughs should belevel and dimensions such thatmaximum flow can be carried with50 HIMI free board.

Velocity in washwateroutlet -2.4-3.6 m/s

Length/width ratio -1.25-1.33Minimum depthControls

—2,5 msampling tapsloss of head gaugerate of flow cjontrolrate indication forlinesand expansion gauge.

washwater

Should consist of a central manifold with lateralseither perforated on their underside or having umbrellatype strainers on top (also other types such as porousplate may be used). Typical criteria, are as follows:

Minimum dia, ofunderdrainsDia. of perforations

— 20 cm— 6 to 12mm (staggered at a

slight angle to verticalaxis of pipe)

fF Îpërïbration along laterals:— 7,5 cm for

6mm ; 20cmof 12mm

perforation offor perforation

Ratio of total area of perforation to total crosssectional area of laterals; 0.25 for 6 mm size;

0,50 for 12 mm size.

Ratio of totalfilter area: .

area of perforation to the entire- 0,003 *

- 6 4 -

Length/d * tiiiieter ratio oí i aterá i-60: 1

Max. spacing of laterals: 30 cmCross sectional area of manifold: 1.5 to 2.0 times

the t ota 1 a r e a o t"latera Is

Velocity of filtered water outlet: 1,0 to 1,8 m/s

1 er s

Until recent years, most rapid sand filters weredesigned for a rate of 120 m /d/m*. Some healt