Wetlands for water pollution control tủ tài liệu bách khoa

His sustainable flood retention basin SFRB concept assesses the functionality of all large water bodies, with particular reference to their flood Integrated Constructed Wetlands Ponds De

Wetlands for Water Pollution Control

Second Edition

Miklas Scholz

The University of Salford, Salford, UK

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Knowledge and best practice in this field are constantly changing As new research andexperience broaden our understanding, changes in research methods, professionalpractices, or medical treatment may become necessary

Practitioners and researchers must always rely on their own experience and knowledge

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To the fullest extent of the law, neither the Publisher nor the authors, contributors, oreditors assume any liability for any injury and/or damage to persons or property as amatter of products liability, negligence or otherwise, or from any use or operation ofany methods, products, instructions, or ideas contained in the material herein.ISBN: 978-0-444-63607-2

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Prof Miklas Scholz, Cand Ing, BEng (equiv), PgC, MSc, PhD, CWEM, CEnv,CSci, CEng, FHEA, FIEMA, FCIWEM, FICE, Fellow of IWA, holds the Chair

in Civil Engineering at The University of Salford (Figure 1) He is the Head ofthe Civil Engineering Research Group Prof Scholz has shown individualexcellence evidenced by world-leading publications, postgraduate supervision,and research impact His main research areas (Figure 2) in terms of publicationoutput are as follows: treatment wetlands (20%), integrated constructed wet-lands (ICW; 15%), sustainable flood retention basins (SFRB; 5%), permeablepavement systems (5%), decision support systems (5%), ponds (5%), andcapillary suction time (5%) About 45% and 40% of his research are in waterresources management and wastewater treatment, respectively The remaining10% is in capillary processes and water treatment

He has published four books and more than 176 journal articles covering

a wide range of topics (Figure 2) Between 2009 and 2015, he topped thepublication list in terms of numbers for all members of the staff atThe University of Salford Prof Scholz’s total journal article publications in

FIGURE 1 Miklas Scholz on top of a sustainable flood retention basin near Perth, Scotland, UK (Picture taken by  Asa Hedmark.)

xix

recent years are as follows: 2009, 13 articles; 2010, 19 articles; 2011, 13articles; 2012, 21 articles; 2013, 17 articles; and 2014, 15 articles.

He publishes regularly in the following journals with high impact factors:Bioresource Technology, Building and Environment, Construction and BuildingMaterials, Desalination, Ecological Engineering, Environmental Modelling &Software, Environmental Pollution, Industrial & Engineering ChemistryResearch, Journal of Chemical Technology and Biotechnology, Journal ofEnvironmental Management, Landscape and Urban Planning, Science of theTotal Environment and Water Research

Prof Scholz has total citations of more than 2845 (above 2122 citationssince 2010), resulting in an h-index of 28 and an i10-Index of 64 Prof Scholz

is Editor-in-Chief of 13 journals, including the Web of Science-listed journalWater (impact factors for 2014: 1.428) He has membership experience on 35influential editorial boards Prof Scholz was a member of the Institute ofEnvironmental Management and Assessment (IEMA) Council between 2008and 2015

Miklas has a currently active (on-going) grant income of usually£270,000.His grant income over any past six years is typically£1,500,000 These figuresinclude research and other grants, as well as consultancy

His sustainable flood retention basin (SFRB) concept assesses the functionality of all large water bodies, with particular reference to their flood

Integrated Constructed Wetlands

Ponds Detention Tanks

Silt Traps

Decision Support Systems

Retention BasinsSustainable Flood Retention Basins

Dam Risk Failure Measurement

Capillary Suction Time

Sustainable Drainage Systems

FIGURE 2 Overview of research areas and their corresponding relative importance and linkages between them.

and diffuse pollution control potential A novel and unbiased classificationsystem allows all stakeholders to clearly define the purpose of a water bodythat can be classed as an SFRB Communication among stakeholdersregarding the most appropriate management of SFRB is greatly enhanced.Moreover, the SFRB concept addresses the need to assess the flood controlpotential of all European water bodies as part of new legislation.

His research has led to the incorporation of findings into national andinternational guidelines on wetland and sustainable drainage systems (SuDS).The greatest impact has been made in the area of integrated constructedwetlands (ICW) in Ireland, Northern Ireland, Scotland, and England Prof.Scholz contributed to the design guidelines of wetland systems as a researchconsultant The guidelines assist designers and managers in all aspects of ICWplanning, design, construction, maintenance, and management Moreover,specific guidelines were written for ICW and used by farmers to treat farmyardrunoff in Scotland and Northern Ireland and in Ireland These guidelines arespecifically mentioned in national legislation

The new guidelines on SFRB and ICW have led to the international uptake

of both the SFRB and ICW concepts and the researched hybrid SuDS Thiswork has particularly benefited the British Isles and Central and NorthernEurope For example, ICW are now being constructed in Belgium, Germany,the United States, and China

The first edition of this work, entitled Wetland Systems to Control UrbanRunoff, was published by Elsevier in 2006 It follows that the released material isnow at least nine years old This is not a major problem for most of the material,which has a long shelf-life However, about 30% of the book required updating

to make it more relevant for today’s market

This revised edition has both a more detailed and a broader view of thesubject area More detail has been added to some chapters to account for tech-nological advances in treatment units and scientific progress in areas such asmolecular microbiology Furthermore, the subject area has been broadened toaccount for more multidisciplinary approaches, such as the ecosystem servicesconcept, to solve engineering science challenges with a holistic angle In order torealize this new approach, both updating and expansion (nine new chapters) ofthe current content were required The second edition has therefore beenexpanded by about 40%, making it more competitive in a market where readershave more choice and flexibility due to advances in technology and the openaccess policy

Because the second edition has a much broader focus, it is therefore entitledWetland Systems to Control Pollution, attracting a wider audience of academicsand practitioners The revised and expanded book covers broad water andenvironmental engineering aspects relevant for the drainage and treatment ofstorm water and wastewater, providing a descriptive overview of the complex

“black box” treatment systems and general design issues involved

The fundamental science and engineering principles are explained to addressthe student and the professional market Standard and novel design recom-mendations for, predominately, constructed wetlands and related sustainabledrainage systems are provided to account for the interests of professionalengineers and environmental scientists The latest research findings in waste-water treatment and runoff control are discussed to attract academics and seniorconsultants, who should recommend the proposed textbook to final year andpostgraduate students and graduate engineers, respectively

The revised book deals comprehensively not only with the design, operation,maintenance, and water quality monitoring of traditional and novel wetlandsystems but also with the analysis of asset performance and modeling of treat-ment processes and performances of existing infrastructuredpredominantly indeveloped but also in developing countriesdand the sustainability and economicissues involved

xxiii

The textbook is essential for undergraduate and postgraduate students, turers, and researchers in the civil and environmental engineering, environmentalscience, agriculture, and ecological fields of sustainable water management Itshould be used as a reference for the design, operation, and management ofwetlands by engineers and scientists working for the water industry, local au-thorities, nongovernmental organizations, and governmental bodies Moreover,consulting engineers should be able to apply practical design recommendationsand to refer to a large variety of practical international case studies, includinglarge-scale field studies.

lec-The basic scientific principles outlined in the revised edition should be ofinterest to all concerned with the built environment, including town planners,developers, engineering technicians, agricultural engineers, and public healthworkers The book is written for a wide readership, but sufficient hot researchtopics are also addressed in nine completely new chapters to guarantee a longshelf-life for the book

Solutions to pressing water quality problems associated with constructedtreatment wetlands, integrated constructed wetlands, farm constructed wetlandsand stormwater ponds, and other sustainable biological filtration and treatmenttechnologies linked to public health engineering are explained Case study topicsare diverse: wetlands, including natural wetlands and constructed treatmentwetlands; sustainable water management, including sustainable drainage sys-tems; and specific applications such as wetlands treating hydrocarbon and pig-gery wastewater The research projects are multidisciplinary, holistic,experimental, and modeling-oriented

The book is predominantly based on experiences gained by the author overthe last 14 years Original material published in articles in more than 170 high-ranking journals and presented in 200 key conference papers has been revisitedand analyzed Experience the author gained as an editorial board member ofmore than 30 relevant peer-reviewed journals guarantees that the textbookcontains sufficient material that fills gaps in knowledge and understanding, andthat it documents the latest cutting-edge research in areas such as sustainabledrainage

The book tries to integrate natural and constructed wetlands and sustainabledrainage techniques into traditional water and wastewater systems used to treatsurface runoff and associated diffuse pollution Chapters 1e4 introduce waterquality management and water and wastewater treatment fundamentals to theinexperienced reader

Chapters 5e9 review preliminary and predominantly primary treatmentunits that can be combined with wetland systems Chapters 10e15 summarizepredominantly secondary but also tertiary treatment technologies that can beused in combination with wetland technologies or as alternatives in caseswhere land availability is restricted due to costs Usually nonessential tradi-tional technologies are briefly presented in Chapters 16 and 17 for the reason ofcompleteness

Microbiological and disinfection issues relevant for treatment wetlands arecovered in Chapters 18 and 19 Chapter 20 introduces wetland science andbiological treatment processes based on microbial biodegradation Furthermore,examples of different wetland types have been presented for readers new to thesubject matter Chapter 21 highlights sludge treatment and disposal options thatshould be considered for sludges obtained from wetland systems.

Chapters 22e38 focus predominantly on a wide variety of timely appliedresearch case studies related to constructed wetlands and associated technologiesfor runoff and diffuse pollution treatment Moreover, wetlands such as sustain-able flood control basins used for both diffuse pollution and flood control pur-poses are introduced These chapters are written for professionals and studentsinterested in design, process, and management details

Miklas Scholz, Salford, October 1, 2015

I would like to thank all current and previous members of my research groups

at The University of Salford, The University of Edinburgh, and the University

of Bradford for their research input, and all institutions that provided fundingfor my research I am also grateful for the support received from the publishingteam at Elsevier

I would like to dedicate this book to my wider family and friends, whosupported me during my studies and career Particular thanks go to my partner

A˚ sa Hedmark, children Philippa Scholz, Jolena Scholz, Felix Hedmark,and Jamie Hedmark, twin-sister Ricarda Lorey and mother GudrunSpiesho¨fer

xxvii

Common Acronyms

and Abbreviations

A Coefficient (unknown function of various variables including

rainfall intensity and infiltration rate)

A Cross-section of flow area (m2)

Al Cross-sectional area of lysimeter (m2)

AEAICAD Aesthetic and educational appreciation and inspiration for

culture, art, and design (%)AFTW Aesthetic flood treatment wetland

ATV-DVWK German abbreviation for German Association for Water,

Wastewater and Waste

B Maximum experimental depth (mm) within the infiltration

basin during an individual storm

BOD Biochemical oxygen demand (mg/l) (usually five days

at 20C)

BP-MLL Back-propagation for multilabel learning

BRE British Research Establishment (company)

C Carbon or combined approach or control or chili

Ce Outflow concentration (of contaminant in wetland cell) (g/m)

Cf Contaminant concentration in infiltration water (g/m3)

C0 Inflow concentration (of contaminant in wetland cell) (g/m)

CIRIA (British) Construction Industry Research and Information

Association

CSS Carbon sequestration and storage (%)

xxix

CST Capillary suction time (s)

D Infiltration basin design depth (mm)

EPMSF Erosion prevention and maintenance of soil fertility (%)

FWS Free water surface (flow wetland)

hwf Average capillary head at the wetting front (m)

HFRB Hydraulic flood retention basin

HSD Honestly significant difference

I Hydraulic gradient or infiltration rate (in wetland cell) (m/d)ICP-OES Inductively coupled plasma optical emission spectrometerICW Integrated constructed wetland

IFRW Integrated flood retention wetland

IR (Empirical) infiltration rate (m/s)

L0 (Contaminant) inflow loading rate (g/m/d)

LCAR Local climate and air quality regulation (%)

MEE Moderation of extreme events (%)

MGD Maintenance of genetic diversity (%)

MLKNN Multilabel k-nearest neighbor

MLSS Mixed liquor suspended solids

MLSVM Multilabel support vector machine

MLVSS Mixed liquor volatile suspended solids (mg/l)

MRP Molybdate reactive phosphate (mg/l)

N Number of entries or nitrogen or north

N Number of instances that are correctly predicted

NFRW Natural flood retention wetland

NTU Nephelometric turbidity unit (similar to FTU)

P Significance level (of a test) (also known as p, p-value,

or P-value) or precipitation rate (m/d)

P Phosphorus (mg/l) or pollination (%) or sweet pepper

PRAST Prevalence Rating Approach for SuDS Techniques

Q Volume of water per unit time (m3/d) or size of the set of

labels or hydraulic loading rate (m/d)

Qf Daily water volume infiltrating beneath a wetland cell (m3/d)

Q0 Inlet wastewater volume flow rate (in wetland cell) (m3/d)

R (Mean product moment) correlation coefficient

RBC Rotating biological contactor

RMPR Recreation and mental and physical health (%)

SESP Spiritual experience and sense of place (%)

SFRB Sustainable flood retention basin

SFRW Sustainable flood retention wetland

SSSI Site of special scientific interest

SVM Support vector machine

T (or t) Infiltration time (s) or temperature (C)

TFRB Traditional flood retention basin

U-matrix Unified distance matrix

WTW Wissenschaftlich Technische Werksta¨tten (company)

Z1 Factor (defined by the BRE method)

Z2 Growth factor (defined by the BRE method)

Chapter 1

Water Quality Standards

1.1 INTRODUCTION AND HISTORICAL ASPECTS

Scientific and public interest in water quality is not new For example, in theUnited Kingdom (UK), it probably had its origins in the mid-eighteenth century

In 1828, the editor of Hansard, Mr John Wright, anonymously published apamphlet attacking the quality of the drinking water in London This led to theestablishment of a Royal Commission, which established the principle thatwater for human consumption should at all times be “wholesome.” The term

“wholesome” has been incorporated into virtually every piece of legislationconcerned with drinking water ever since

The first unequivocal demonstration of water-borne transmission of cholerawas by Snow in 1854 This stimulated great advances in water treatmentpractices, in particular the routine application of slow sand filtration anddisinfection of public water supplies

Although the Royal Commission of 1828 was concerned with waterquality, it had difficulty in defining it precisely, because there were virtually noanalytical techniques available at the time with which to determine eithermicrobial or chemical contamination Consequently, since that time, there hasbeen a continuing and often fierce debate on what constitutes a suitable qualityfor human drinking water Not surprisingly, in the nineteenth and early part ofthe twentieth centuries, the evaluation was largely based on subjective, usuallysensory perception

Many authorities (e.g., Sir Edwin Chadwick) believed that an atmospheric

“miasma” above the water, rather than the water itself, was responsible fordisease transmission As a consequence, great efforts were made to remove thesmell, assuming that this would dispel the disease In 1856, during the “greatstink,” sheets drenched in chemicals were hung from the windows of theHouses of Parliament to exclude the smell This action did at least focus theminds of the politicians on the need to take action to improve the quality ofLondon’s water supply

Even today, taste, smell, and appearance (color and turbidity) are considereduseful criteria for judging water quality However, in addition, there are nowobjective methods for determining the presence and level of many (but by nomeans all) of the microbial contaminants likely to be present in drinking water

Wetlands for Water Pollution Control http://dx.doi.org/10.1016/B978-0-444-63607-2.00001-0

Copyright © 2016 Elsevier B.V All rights reserved. 1

Since the 1960s, the emphasis regarding drinking water quality has shiftedfrom its bacteriological quality to the identification of chemical contaminants.This reflects largely the very considerable success of the water industry inovercoming bacteriological problems, although this victory is not complete(e.g., many viruses and Cryptosporidium cause public health concerns).With the great methodological improvements in analytical chemistry overthe past 50 years, it was recognized that water contains trace amounts ofseveral thousand chemicals and that only the limitations of analytical tech-niques restrict the number of chemicals that can be identified Many of thesechemicals are of natural origin, but pesticides, human and veterinary drugs,industrial and domestic chemicals, and various products arising from thetransport and treatment of water are very commonly found, albeit normally atvery low concentrations.

In addressing the problem of the contribution of water-borne chemicals tothe incidence of human disease, water scientists, whose previous experiencehas typically been confined to microbiological problems, have tended to focus

on acute risks The absence of detectable short-term adverse effects ofdrinking water has been taken by many as conclusive evidence that thepresence of such chemicals is without risk to humans

While information on the acute toxicity of a chemical can be very useful indetermining the response to an emergency situation such as an accidentalspillage or deliberate release of chemicals into a watercourse or even into thewater supply, such information is of little use in predicting the effects of dailyexposure to a chemical over many years

However, low levels of chemicals are much more likely to cause chronic(rather than acute) effects to health Here, direct reliable information is verysparse Some authorities appear to have accepted the “naı¨ve” assumption thatinformation on the acute effects of a chemical, in either humans or experi-mental animals, can be used to predict the effects of being exposed over alifetime In practice, the chronic effects of a chemical have rarely anyresemblance to the acute effects

An evaluation of health risks associated with drinking water is necessaryand timely If we are to obtain a proper assessment of the health risk that couldarise in humans through exposure to chemicals in water over a lifetime,understanding must be developed on the following:

l Identification of the chemicals that are of most concern;

l Data on the effects of long-term exposure in humans and/or animals toeach chemical;

l A measure of the extent and form of exposure to each chemical;

l Identification of particularly at-risk groups; and

l The means of establishing how exposure to other chemicals in the watercan modify the toxicity

1.2 WATER QUALITY STANDARDS AND TREATMENT

OBJECTIVES

It is commonly agreed that there are three basic objectives of water treatment:

1 Production of water that is safe for human consumption;

2 Production of water that is appealing to the customer; and

3 Production of water treatment facilities that can be constructed andoperated at a reasonable cost

The first of these objectives implies that the water is biologically safe forhuman consumption It has already been shown how difficult it is to determinewhat “safe” actually means in practice A properly designed plant is not aguarantee of safety, standards will change, and plant management must beflexible to ensure continued compliance

The second basic objective of water treatment is the production of waterthat is appealing to the customer Ideally, appealing water is clear and color-less, pleasant to taste, odorless, and cool It should be nonstaining, noncor-rosive, non-scale-forming, and reasonably soft The consumer is principallyinterested in the quality of the water delivered to the tap, not the quality at thetreatment plant Therefore, storage and distribution need to be accomplishedwithout affecting the quality of the water; in other words, distribution systemsshould be designed and operated to prevent biological growth, corrosion, andcontamination

The third basic objective of water treatment is that it can be accomplishedusing facilities with reasonable capital and operating costs Various alterna-tives in plant design should be evaluated for cost-effectiveness and waterquality produced

The objectives outlined here need to be converted into standards so thatproper quality control measures can be used There are various drinking waterstandards The key variables are as follows:

l Organoleptic parameters: color, turbidity, odor, and taste;

l Physical and chemical parameters: temperature, pH, conductivity, solved oxygen, dissolved solids, chlorides, sulfate, aluminum, potassium,silica, calcium, magnesium, sodium, alkalinity, hardness, and free carbondioxide (CO2);

dis-l Parameters concerning undesirable substances: nitrate, ammonium, totalorganic carbon (TOC), hydrogen sulfide, phenols, dissolved hydrocarbons,iron, manganese, suspended solids, and chlorinated organic compoundsother than pesticides;

l Parameters concerning toxic substances such as arsenic, mercury, lead, andpesticides; and

l Microbiological parameters: total coliforms, fecal coliforms, fecal tococci, sulfite-reducing clostridium, and total bacterial count

strep-Water Quality Standards Chapter j 1 3

Standards usually give two values: a guide level (GL) and a maximumadmissible concentration (MAC) The GL is the value that is consideredsatisfactory and constitutes a target value The MAC is the value that thecorresponding concentration in the distributed water must not exceed Treat-ment must be provided when the concentration in the raw water exceeds theMAC.

Standards also specify the methods, frequencies, and nature of the analysis.For total hardness and alkalinity, the standards specify minimum values to berespected when water undergoes softening

Most standards group substances into five categories:

l Microbiological;

l Inorganic with consequences on health;

l Organic with consequences on health;

l Appearance; and

l Radioactive components

One of the main sources of confusion regarding water standards and theirinterpretation is the lack of any clear indication as to how the standard wasderived This results in the interpretation of all standards as “health standards”

by the public and, subsequently, in the difficulty of assessing what should bedone by the water supplier if a threshold is exceeded

This is particularly true of drinking water quality directives becauseinsufficient explanation of the derivation of the actual numbers is often given.There are even thresholds for variables regarded as toxic that are based onpolitical or other considerations, and they are therefore only loosely based onscience (e.g., pesticides) The use of such approaches is acceptable as long asthe reasoning behind them is clear to all

International guidelines are usually intended to enable governments to usethem as a basis for standards, taking into account local conditions They areintended to be protective of public health, and they should be absolutely clear,even down to detailed scientific considerations such as the derivation of un-certainty factors and the rounding of numbers It is therefore incumbent on theexpert groups to justify their thinking and present it openly for all to see Such

a discipline avoids the “fudging” of issues while giving the impression ofscientific precision, and it can only be of value in increasing public confidence

in the resulting guidelines

It is clear that, at present, standards for water quality are as follows:

l Loosely based on science (although the situation is improving);

l Not static (the science of monitoring as well as our understanding of thehealth implications of chronic exposure of many contaminants areimproving); and

l Important in the quality control of potable water (for both supplier andconsumer)

Concerning the outflow water quality of most wetland systems, standardseither are unclear or are currently being developed The local environmentregulator usually sets standards for specific wetland system applications.

1.3 BIOCHEMICAL OXYGEN DEMAND

When wastewater, including urban runoff, is discharged into a watercourse, itexerts a polluting load on that water body Microorganisms present in thenatural water and the wastewater break down (stabilize) the organic matter Inpermitting discharges to watercourses, the Environment Agency in the UK, forexample, tries to ensure that the conditions are aerobic so that all other lifeforms in the river (e.g., fish) can continue to survive The early forms ofwastewater treatment developed are aerobic, and so the simplest way ofestimating the biodegradability of a wastewater sample is to estimate theamount of oxygen required to stabilize the waste

To devise an easy and simple method of assessing the oxygen demand, thefollowing constituents of a closed system should be considered:

l Air (in excess);

l A small number of bacteria; and

l A finite amount of substrate (waste representing food)

The following phases of biological growth and decline can be identified insuch a system:

l Lag phase: Bacteria are acclimatizing to system conditions, in particularthe substrate; very little increase in numbers

l Log growth: Bacteria are acclimatized; food is not a limiting factor; rapidlyincreasing population of bacteria

l Declining growth: Food eventually becomes limiting; declining growthrates

l Endogenous respiration: As the substrate concentration becomes depleted,competition increases; bacteria start consuming dead bacterial cells andeventually start consuming live cells

It is a system of this type that is used to assess the oxygen demand ofwastes, including organic matter from urban runoff The test developed fromthis system is the biochemical oxygen demand (BOD) test

The BOD test is carried out as follows: a known quantity of a wastewatersample (suitably diluted with prepared water) is placed in a 300-ml BODbottle The prepared water is saturated with dissolved oxygen (DO), and nu-trients and a buffer are added The bottles are then sealed airtight The bottlesare subsequently incubated at 20C in the dark (Clesceri et al., 1998).Initially, the bacteria break down the carbon-based molecules In practice, asecond oxygen demand is observed In the case of raw sewage, this stageusually becomes apparent after approximately 8 days of incubation at 20C.

Water Quality Standards Chapter j 1 5

This second stage is due to the oxidation of ammonia present in the waste; this

is called nitrification A large percentage of the nitrogen in the wastewateroriginates from proteins; the protein molecules are degraded to releaseammonia The oxidation process is described inEqs (1.3.1) and (1.3.2):

of the waste

Traditionally, the BOD test is carried out for 5 days; the resulting oxygendemand is referred to as the BOD5 The BOD is calculated as follows(Eqs (1.3.3) and (1.3.4)):

BODðmg=lÞ ¼ Initial DO in bottleDilution ratio Final DO in bottle (1.3.3)where:

Dilution ratio¼Volume of wastewater

In practice, the test is often modified slightly in that a quantity of seedmicroorganisms are added to the BOD bottle to overcome the initial lagperiod In this variant, the BOD is calculated fromEq (1.3.5):

BOD ¼ ðD1 D2Þ fðB1 B2Þ

where:

D1¼ dissolved oxygen initially in seed and waste bottle;

D2¼ dissolved oxygen at time T in seed and waste bottle;

B1¼ dissolved oxygen initially in seed-only bottle;

B2¼ dissolved oxygen at time T in seed-only bottle;

f¼ ratio of seed volume in seeded wastewater to seed volume in the BODtest on seed only; and

DR¼ dilution ratio

Additional bottles are incubated These contain only seed microorganismsand dilution water to get the BOD of the seed, which is then removed from theBOD obtained for waste and seed

However, the BOD test has two major disadvantages: it takes 5 days toobtain the standard test result, and the results can be affected by the process ofnitrification (see above) Therefore, a nitrification inhibitor is often used(Chapter 24)

1.4 CHEMICAL OXYGEN DEMAND

The disadvantages of the BOD test have led to the development of a simplerand quicker test This test is known as the chemical oxygen demand (COD)methodology In this test, strong chemical reagents are used to oxidize thewaste Potassium dichromate is used in conjunction with boiling concentratedsulfuric acid and a silver catalyst The waste is refluxed in this mixture for 2 h.The consumption of the chemical oxidant can be related to a correspondingoxygen demand (Clesceri et al., 1998)

The COD test oxidizes material that microorganisms cannot metabolize in

5 days or that are toxic If the COD is much greater than the BOD in rawwastewater, then the waste is not readily biodegradable, and it may be toxic tothe microorganism If the COD is similar to the BOD, then the waste is readilybiodegradable

1.5 OTHER VARIABLES USED FOR THE

CHARACTERIZATION OF WASTEWATER

Most wastewater treatment processes operate best in pH ranges between 6.8and 7.4; indeed, pH> 10 is likely to kill large numbers of bacteria Suspendedsolids (SS) is a measure of the total particulate matter content of wastewater.The nature of the SS is likely to vary considerably depending on the nature ofthe waste

The two most important nutrients in wastewater treatment are nitrogen andphosphorus; both are needed for cell growth Nitrogen (N) is used in proteinsynthesis (e.g., new cell growth) Phosphorus (P) is used for cell energystorage and is usually present as ortho-phosphate (PO4)

Organic nitrogen is associated with cell detritus and volatile SS Freeammoniacal nitrogen (NH3eN) results from the decay of organic nitrogen.Nitriteenitrogen (NO2eN) is formed in the first step in nitrification.Nitrateenitrogen (NO3eN) results from the second and final stage in thenitrification process

For proper microorganism growth, the ratio of C:N:P is important Carbon(C) is measured by BOD5 Nitrogen is measured by organic nitrogen and

NH3eN However, NO3eN is difficult for microorganisms to use in theirgrowth process Phosphorus is measured as acid hydrolysable ortho-phosphate(PO4) To achieve growth, the required minimum values for the C:N:Prelationship are 100:5:1

Water Quality Standards Chapter j 1 7

Water Treatment

2.1 SOURCES OF WATER

The source of raw water has an enormous influence on the water’s chemistryand consequently its treatment Raw water is commonly abstracted from one

of the following four sources:

1 Boreholes extracting groundwater: This water is usually bacteriologicallysafe as well as aesthetically acceptable It may require some treatment such

as aeration or softening

2 Rivers: Water can be abstracted at any point along the length of a river.However, the further downstream it is, the more likely the water is torequire considerable treatment

3 Natural lakes: The degree of treatment required for lake water depends on anumber of factors such as the catchment use in the immediate vicinity of thelake, the lake’s trophic status, and the presence of sewage treatment works

4 Manmade lakes and reservoirs: These are similar to lakes and rivers, butbetter managed The degree of treatment depends on the management ofthe catchment and upstream catchment usage

Water for domestic consumption may also come from other sources such asseawater (via desalination) or treated sewage effluents However, these sourcesare very rare and therefore beyond the scope of this introductory chapter towater treatment

2.2 STANDARD WATER TREATMENT

The purpose of screens is simply to remove solid floating objects (e.g., logsand twigs) from the raw water, which may cause damage or blockage in theplant Sometimes a much finer screening is carried out, called straining This isusually performed on lake and reservoir water to remove algae

A coagulant is added to the raw water to destabilize the colloidal material

in the water (Chapter 7) Commonly used chemicals are as follows:

l Alum (aluminum sulfate) Al2(SO4)3$nH2O;

l Ferric chloride FeCl3;

l Ferrous sulfate (copperas) FeSO4$7H2O;

Wetlands for Water Pollution Control http://dx.doi.org/10.1016/B978-0-444-63607-2.00002-2

Copyright © 2016 Elsevier B.V All rights reserved. 9

l Lime (burnt CaO; slaked Ca(OH)2); and

l Polyelectrolytes (long-chain organic molecules normally used in tion with a conventional coagulant)

conjunc-For the coagulant to function efficiently, it must be rapidly and uniformlymixed through the raw water This usually takes place in a high-shear (turbulent)environment such as one induced by a hydraulic jump (low-cost option), apump, a jet mixer, or a propeller mixer

After the coagulant is uniformly distributed in the water, it requires time toreact with the colloid, and then further time (and gentle agitation) to promotethe growth (agglomeration) of settleable material (flocks) This is generallyaccomplished either in a tank with paddles (mechanical mixing) or through aserpentine baffled tank (hydraulic mixing) Once flows of a settleable size haveformed, they are removed usually by sedimentation (sometimes by flotation)

In countries such as the UK and Ireland, developments in the 1940s led tothe introduction of the sludge blanket clarifier (Chapter 8) This is a single unitthat encompasses rapid mixing, flocculation, and settling

To remove either solids carried over from settling tanks and/or anyuncoagulated material (organic or inorganic), a sand bed filter is provided Thewater flows downwards through the bed, and the impurities are removed byattachment to the sand grains The sand grains therefore require periodiccleaning The frequency of cleaning depends on the type of filter used Thetwo commonly used types are the following (Chapters 10 and 11):

l Slow sand filter (slow loading rate: approximately one-tenth of that for arapid gravity sand filter) and

l Rapid gravity sand filter (high loading rate)

Sometimes fluoride is added to the water to reduce the incidence of dentalcaries This is a process that provokes public debate In the United States and

UK, chlorine is usually added to the water to disinfect it (Chapter 19) Itfollows that the water is bacteriologically safe when it leaves the treatmentworks, and excess chlorine is added to protect the water from contaminationduring the distribution process

There are several other commonly used processes Their usage depends onthe nature of the raw water Air can be introduced to the water to oxidizeimpurities (e.g., iron, manganese, or chemical compounds affecting the taste ofwater) pH control is a common process since many of the chemical treatmentprocesses are pH dependent Softening reduces the hardness and/or alkalinity

of water to improve its aesthetic acceptability This is a complex chemicalprocess depending on the nature of both the anionðHCO

3 ; CO2 

3 ; or OHÞand the cation (Ca2þor Mg2þ

2.3 BASIC WATER CHEMISTRY

The most important chemical variables of raw water are usually taken as pHand alkalinity Alkalinity consists of those chemical species that can neutralize

10 Wetlands for Water Pollution Control

acid In other words, these species allow the water to resist changes and providebuffering capacity The major constituents of alkalinity are the hydroxyl (OHcarbonate ðCO2 

3 Þ, and bicarbonate ðHCO

3 Þ ions The relative quantities ofeach are a function of pH

No significant concentration of hydroxyl ions exists below pH 10, and nosignificant carbonate concentration can be detected below pH 8.5 For mostwaters, alkalinity thus consists of the bicarbonate ion The other species may

be formed in the treatment process The bicarbonate and carbonate ions in thewater result from the dissolution of carbonate rocks

The pH is a measure of the free hydrogen ion concentration in water.Water, and other chemicals in solution, will ionize to a greater or lesser degree.The ionization reaction is given inEq (2.3.1)

In neutral solutions, the [OH] activity is equal to the [Hþ] activity Hence,the pH and pOH (a measure of alkalinity) are both equal and have thenumerical value of 7 An increase in acidity, for example, leads to highervalues of [Hþ], thus lowering the pH

The various chemical reactions that occur in natural waters and in processedwater are generally considered to occur in dilute solutions This permits the use

of simplified equilibrium equations in which molar concentrations are ered to be equal to chemical activities

consid-The assumption of dilute conditions is not always justified, but the errorintroduced by the simplification is no greater than the error that might beintroduced by competing reactions with species that are not normally measured

in water treatment

Concentrations of different chemical species in water may be expressed inmoles per liter, in equivalents per liter, or in mass per unit volume (typically,mg/l) The equivalent of a species is its molecular weight divided by the netvalence or by the net change in valence in the case of oxidation and reductionreactions

The number of equivalents per liter (normality) is the concentrationdivided by the equivalent weight The number of moles per liter is called themolarity

In a traditionally combined sewer, all sewage, both foul and surface water, isconveyed in a single pipe A foul sewer conveys the “nasties” (i.e., contami-nants) A surface water sewer conveys the runoff from roofs and paved areas.Concerning separate systems, two pipes are laid in the trench for thesewerage system: one for the foul sewer, and the second for the surface water.This book is concerned with the treatment of both wastewater and urban runoff.The flow in a sewer can be estimated withEq (3.1.1) The mean domesticwater consumption is typically 140 l/h/day for rural and 230 l/h/day for urbanareas.

E¼ industrial effluent discharge to the pipe; and

QT¼ total volume of flow in a 24-h period

3.2 DESIGN FLOW RATES

Normally, at sewage treatment works, flows up to three DWF are given fulltreatment;>6 DWF (since they are diluted by the surface water) require onlypreliminary treatment Flows between three and six DWF are stored tempo-rarily and given full treatment

However, care needs to be taken in the design of overflow structures,particularly for flows>6 DWF These must be designed such that the outflowWetlands for Water Pollution Control http://dx.doi.org/10.1016/B978-0-444-63607-2.00003-4

Copyright © 2016 Elsevier B.V All rights reserved. 13

from them has a minimum impact on the receiving water; in particular, caremust be taken with the solid material, which occurs in the so-called first foulflush or simply the first flush (i.e., immediately after the rainfall stormcommences, accumulated material in the sewer is likely to be flushed out ofthe system).

3.3 TREATMENT PRINCIPLES

Typically, raw sewage contains 99.9% water and 0.1% solids The sewagetreatment process is fundamentally about separating solids from the water Thetreatment of solids and sludge forms an important and costly area of sewagetreatment The impurities in the sewage can be categorized as follows:

l Floating or suspended solids (e.g., paper, rags, grit, and fecal solids);

l Colloidal solids (e.g., organics and microorganisms);

l Dissolved solids (e.g., organics and inorganic salts); and

l Dissolved gases (e.g., hydrogen sulfide and carbon dioxide)

These impurities are removed from the sewage using operations or cesses that are physical, chemical, or biological in nature Physical operationsdepend on the physical properties of the impurity for efficient removal (e.g.,screening, filtration, and sedimentation) Chemical operations depend on thechemical properties of the impurity and use the chemical properties of addi-tives for efficient removal (e.g., coagulation, precipitation, and ion exchange).Biological processes comprise biochemical and/or biological reactions toremove soluble or colloidal organic impurities (e.g., percolating filters andactivated sludge)

pro-3.4 ENGINEERING CLASSIFICATION OF SEWAGE

TREATMENT STAGES

Wastewater engineers tend to describe the sewage treatment process in terms

of the stages of treatment:

l Preliminary treatment (physical): for example, screening and grit removal;

l Primary treatment (physical and/or chemical): for example, sedimentationand flotation;

l Secondary treatment (biological and/or chemical): for example, constructedwetlands, biological filters, and the activated sludge process; and

l Tertiary treatment (physical and/or chemical and/or biological): for example,polishing wetlands, microstraining, grass plots, and lime precipitation

At the secondary treatment stage, either percolating filters or activatedsludge treatment is usually present, but certainly not both in parallel Onoccasions, when treating industrial wastes, they may both be used, but always inseries It should be noted that sludge is produced at the majority of the treatment

stages However, in normal practice, the works are organized such that all sludge

is collected centrally

Wetland systems can be designed for each engineering stage and for sludgetreatment However, constructed treatment wetlands (for definitions, refer toSection 20.2) are usually applied for secondary or tertiary treatment stages.Wetlands integrated in sustainable drainage systems (SuDS; previously calledsustainable urban drainage systems (SUDS)) are frequently used for pre-liminary and primary treatment purposes Urban runoff requires full treatment,which is usually not the case in practice, unless for combined sewer systemsand minor storms

Sewage Treatment Chapter j 3 15

Stream Pollution and Effluent Standards

4.1 ORGANIC STREAM POLLUTION

Since most effluents, including storm runoff, are discharged to a receivingwatercourse, it is important that the concentration of the effluent is such thatthe receiving water can assimilate the waste and break it down The receivingwater should also remain in a condition appropriate to its use (usuallyaerobic)

Consider the discharge of an organic effluent into a river In the receivingwater, there are two processes taking place: oxidation of the organic waste andre-aeration, and the introduction of oxygen into the water Before the effluent

is discharged, the river contains dissolved oxygen (DO) The effluent reducesthe initial DO concentration progressively to satisfy the BOD

In the zone of degradation, re-aeration is smaller than the rate of position In this section, the decomposition of the effluent dominates, and sothe DO concentration drops rapidly The sediment accumulation in theimmediate vicinity may be large due to the settling of suspended material inthe effluent

decom-In the zone of active decomposition, re-aeration is approximately equal tothe rate of decomposition The water is likely to contain little diversity of lifeforms; the bottom sediment may possibly be anaerobic

In the zone of recovery, the rate of re-aeration is larger than the rate ofdecomposition Since the effluent oxygen demand is dropping and the DOdeficit is large, then atmospheric oxygen will diffuse into the water body at agreater rate; thus, the DO begins to increase Nitrification is also likely tobegin to take place, and the life forms present in the river increase indiversity

In the zone of clear water, the DO has now returned to its original value,and the BOD has been virtually eliminated; only a background levelremains However, the river has been permanently changed: there areincreased levels of nutrients in the water, and this may lead eventually toeutrophication

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Copyright © 2016 Elsevier B.V All rights reserved. 17

4.2 PREDICTION OF ORGANIC STREAM POLLUTION

In the absence of re-aeration, Eq (4.2.1)should be considered:

dD

where:

D¼ dissolved oxygen deficit (DOsat  DOactual);

K1¼ reaction rate coefficient; and

Lt¼ ultimate BOD remaining at time T

In water bodies, re-aeration usually occurs as a result of a difference inpartial pressures and the turbulence in the river flow Re-aeration can beexpressed with Eq (4.2.2):

D0¼ dissolved oxygen deficit at time t ¼ 0;

L0¼ ultimate BOD initially (i.e., t ¼ 0);

Lt¼ L010k 1 t;

k1and k2¼ reaction rate and re-aeration coefficient (base 10), respectively;and

Dt¼ dissolved oxygen deficit at time t

The critical point on the DO sag curve is the minimum value of DO Theoccurrence of the critical point can be defined withEqs (4.2.5)e(4.2.7)

k1L k2Dc¼ 0 ¼dD

18 Wetlands for Water Pollution Control

Dc¼ critical deficit reached at time tc.

The StreeterePhelps equation is valid only when no change in the tion load or dilution occurs Complex discharge and river problems require astepwise process to be solved

pollu-4.3 EFFLUENT DISCHARGE STANDARD PRINCIPLES

Water quality standards should achieve the following:

l Safeguard public health;

l Protect water so that it is suitable for abstraction and subsequent use indomestic, agricultural, or industrial circumstances;

l Cater for the needs of commercial, game, and course fisheries; and

l Cater for relevant water-based amenities and recreational requirements.The limits or standards placed on effluent discharges have been tradi-tionally specified in terms of effluent volume, BOD, and SS Standards arebased traditionally on the fact that a pollution-free stream would have aBOD5¼ 2 mg/l, and if the BOD5> 4 mg/l, the stream may become a nuisance(i.e., occasionally anaerobic) However, these standards are now superseded byvarious national standards and international directives Nevertheless, standardsfor urban water runoff are currently being developed

treat-The second operation constituting preliminary treatment is grit removal.Grit includes sand, dust, and cinders These are nonputrescible materials with aspecific gravity greater than that of organic matter It is necessary to removethese materials in order to:

l Protect moving mechanical equipment and pumps from unnecessary wearand abrasion;

l Prevent clogging in pipes and heavy deposits in channels;

l Prevent cementing effects on the bottom of sludge digesters and primarysedimentation tanks; and

l Reduce accumulation of inert material in aeration basins and sludge gesters, which would result in the loss of usable volume

di-5.2 DESIGN OF SCREENING UNITS

For preliminary treatment, screens normally comprise vertical or inclined barswith openings between 20 and 60 mm for coarse screens and 10 and 20 mm formedium screens The bars are usually made from steel, and they are between

50 and 75 mm wide and between 10 and 15 mm thick The spacings used arenormally approximately 20 mm wide for mechanically raked screens andbetween 25 and 40 mm wide for manually cleaned screens

The hydraulic design of screening units must include the calculation ofscreen area and the head loss through the screen The area of submerged screensurface is based on the velocity of flow through the clean openings Thecorresponding velocity is 600 mm/s for average flows and 900 mm/s for themaximum rate of flow

Wetlands for Water Pollution Control http://dx.doi.org/10.1016/B978-0-444-63607-2.00005-8

Copyright © 2016 Elsevier B.V All rights reserved. 21

The largest area is the controlling area For small and manually cleanedunits, the minimum working width is 450 mm However, the usual width isbetween 600 and 900 mm for mechanically cleaned units In general, theeffective screen area should be about two times the cross-sectional area of theincoming pipe, such as a sewer The head loss through the bars is calculatedusingEqs (5.2.1)e(5.2.3).

hL¼ head loss through bars (m);

V¼ velocity through bars and in the channel upstream of unit (m/s);

W¼ maximum cross-sectional width of bars facing the direction offlow (m);

b¼ minimum clear spacing of bars (m);

hv¼ velocity head of flow approaching the bars (m);

q ¼ angle of bars with horizontal ();

A¼ effective submerged open area (m2);

C¼ coefficient of discharge (i.e., 0.6 for a clean screen); and

b ¼ bar shape factor with different b values for different shapes edged rectangular, 2.42; rectangular with semicircular upstream face, 1.83;circular, 1.79; rectangular with semicircular faces upstream and down-stream, 1.67; and teardrop shape, 0.76)

(sharp-The width of the screen channel is usually calculated by applying Eq.(5.2.4):

W ¼ Bþ b

where:

W¼ width of the channel excluding supports (m);

B¼ width of the screen bars (m);

b¼ spacing of the screen bars (m);

Q¼ maximum flow rate (m3/s);

V¼ maximum velocity of flow through screens (m/s); and

D¼ depth of flow at screens (m)

Screens can be either manually cleaned (not popular but common forwetland systems) or mechanically cleaned In mechanically cleaned screens,cleaning either is continuous or is initiated when the differential head throughthe screen reaches 150 mm.

5.3 DESIGN DETAILS FOR SCREENING UNITS

Good access is required, because screening units need frequent inspection andmaintenance Ample working space must be provided for screens in deepchannels Protective structures such as guards are traditionally omitted,although trends are changing Good corrosion protection must be written intothe specifications

Cleaning mechanisms such as front or back cleaning with front or backdischarge are usually designed by the manufacturer Back-cleaning devices areprotected from damage by the screen itself Front discharge by maceratedscreenings is preferable to back discharge

The following types of operating controls are used for mechanicallycleaned screens, sometimes in combination: manual stop-start, automatic stop-start by time control, high-level alarm, differential head-actuated automaticstop-start, and overload switch and alarm

5.4 COMMINUTORS

Comminutors are combined screen and macerator units They consist of anelectrically rotated drum with horizontal slots that form a screen The sewagegravitates from the upstream channel into a spiral flow channel, through theslots and open bottom of the drum, and into the downstream channel via aninverted siphon Suspended and floating solids are held by the liquid flowagainst the outside of the drum, and they are macerated by stationary cutters

It is a great advantage of comminutors that screenings do not have to beremoved from the flow A disadvantage is the tendency for “stringing” or

“balling up” of material, and the head loss is higher than that with screens.These comminutors are designed to take the maximum flow rate Abnormaloverload is diverted through a hand-raked bar screen via an overflow weir Thehydraulic head required depends upon the following:

l Flow rate;

l Machine capacity and other characteristics; and

l Upstream and downstream conduit widths and flow rates

The presence of the comminutor in the channel will only affect upstreamflow depths The calculation of head losses includes three principle parts:

1 Head required to force the desired flow rates through the comminutor Thedifferential head varies with the depth of flow, and it may be obtained fromthe manufacturer’s data and literature;

Preliminary Treatment Chapter j 5 23

2 Open channel friction losses in the approach and tail channels; and

3 The drop in water surface due to the relative elevations of comminutor, andapproach and tail conduits Careful design can minimize these drops.Selection of the proper comminutor size and the calculations of the overallhead loss involve an iterative approach In general, a head loss between 150and 750 mm is involved, depending on the capacity of the unit

The following are the major areas that must be considered in the detaileddesign of comminutor units:

l Protection from damage by large floating objects;

l Provision of a flow bypass and emergency manual screening in the event ofpower failure;

l Standby capacity must be provided;

l Arrangements for isolation of the unit from the flow for maintenancepurposes; and

l Provision must be made for drawing the siphon pipe (together with thespiral chamber); this can be obtained prefabricated, thus avoiding complex

in situ construction

5.5 GRIT REMOVAL

These units are used to remove inorganic grit from sewage The typical organiccontent of the retained grit is usually<15% of the total Since inorganic grithas a higher density than organic solids (typically 2.6 times), all grit removalunits are velocity-regulating devices that maintain the flow velocity to acritical figure at which only the inorganic grit will settle out The choice of gritremoval methods is dictated by the quantity and quality of the grit, head loss,space requirements, and the type of equipment used in other parts of the plant.Constant-velocity grit chambers and channels are common in both largeand small schemes The theoretical section for constant velocity at all flows isparabolic Problems arise at minimum flows because of a high width-to-depthratio A trapezoidal section is normally adopted for medium-sized schemes.The side slopes should be a minimum of 45to the horizontal (preferably 60).

In the absence of data from settling tests on sewage or storm runoffsamples, a velocity of 300 mm/s should be maintained throughout the tank.The velocity must be between 200 and 400 mm/s under all conditions.The capacity of the channel is normally six DWF At least two channelsshould be provided Standby provision must be made to allow for maintenanceand grit removal without decreasing the efficiency of the unit

Flow regulation is normally undertaken by a standing wave flume Whenthe velocity of flow is 300 mm/s, the settling velocity is about 30 mm/s Thisshould be modified by tests in specific case studies

Theoretically, the length of the channel should be approximately 10 timesthe depth of flow However, the length of the channel is usually taken as the

depth of flow multiplied by 20 to allow for turbulence effects and varyingsettling velocities.

Automatic grit removal equipment is available and may include suction orbucket dredgers and conveyor types mounted on rails Where manual gritremoval is employed, the bottom width of the grit channel should not

be<300 mm

The other grit removal systems are all proprietary systems They areusually rectangular or circular tanks with mechanical grit removal and washingequipment, and they are designed to maintain a constant velocity across,throughout, or around the tank

For example, the Pista Grit Trap has no moving parts in contact with grit.Paddles and peripheral drag keep the horizontal velocity of the sewagereasonably constant throughout the circular tank at varying flow rates Gritsettles in the sump, where it is washed by a counter-flow of air and water underpressure in the sump, before it is air-lifted at regular intervals

In comparison, spiral-flow aerated tanks are used at larger water treatmentworks where space is limited They are provided in duplicate, with each tank

or series of tanks capable of treating the maximum rate of flow The units areapproximately rectangular in shape Air diffusers located at the sides of thetank give vertical movement to the flow The tanks discharge over weirs orthrough penstocks The air induces a spiral motion and consequently pre-aerates the sewage Grit is removed by gravity, airlift, or pump action.The design of most spiral-flow aerated tanks is empirical The forwardvelocity is usually 0.1 m/s (maximum discharge) with a retention period

of60 s The air supply is approximately 0.05 l/s per l/s of sewage

Preliminary Treatment Chapter j 5 25

Primary Treatment

6.1 INTRODUCTION

A sedimentation tank or at least a silt trap is usually located just before aconstructed treatment wetland Historically, sedimentation tanks have beendesigned on the basis of three simple principles:

1 Overflow rate;

2 Weir loading rate; and

3 Hydraulic retention time

However, this has led to an empirical basis for designing efficient settlingtanks The general principles in any sedimentation tank design method are asfollows:

1 The objective is to remove settleable solids (SS) and to reduce the SScontent of the sewage, thereby reducing the corresponding biochemicaloxygen demand (BOD)

2 The principle of gravity settlement in relatively quiescent conditions isemployed in the design of all sedimentation tanks

3 On a typical sewage treatment works, sedimentation units are provided inthree stages of treatment: stormwater detention and treatment, primarysedimentation, and final or secondary sedimentation

4 Sedimentation tanks are designed to operate on a continuous-flow basis.They are usually rectangular or circular in shape and are equipped withmechanical sludge-collecting devices With the exception of tanksdesigned for continuous sludge removal (final sedimentation tanks inactivated sludge plants), the bottom of sedimentation tanks is essentiallyflat (up to 15
) and has sludge hoppers with relatively steep sides Settledsludge on the tank floor is moved by mechanical scrapers into the hoppersfor subsequent withdrawal

5 Imhoff or two-story tanks provide both sedimentation and digestion in asingle tank The use of such devices is limited to small plants servingpopulations<5000

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6.2 LOADING RATE METHODS

Concerning the traditional loading rate methods, primary sedimentation tanksare designed using the data shown below:

l Overflow rate (surface loading rate): 16e32 m3

/m2/day (normally imately 24 m3/m2/day)

approx-l Length-to-width ratio: between 3:1 and 5:1, with a bottom slope of one in40

l Weir loading rate: 450 m3/m/day (normally approximately 225 m3/m/day);

if >225 m3/m/day, use a double-sided weir

l Retention time: 2 h at three dry weather flow (DWF)

Tanks designed using these figures will, typically, give between 30% and40% BOD removal for domestic sewage For circular tanks, diameters areusually between 10 and 40 m The actual depth depends on the type of sludgebeing removed (i.e., a batch settling test to determine the required depth isnecessary)

In general, for mechanically cleaned tanks, the minimum depth is usuallybetween 2.5 and 3.5 m As the tank depth increases, so does the constructioncost, as does the difficulty of keeping the tank in the ground

Concerning the traditional loading rate methods, secondary tanks aredesigned using the following data:

l Overflow rate: 32 m3/m2/day

l Side water depth (minimum): 2.1 m (always circular)

l Weir loading rate: 120 m3/m/day (the lower the better)

Where activated sludge is the secondary treatment process, the actualfigures depend on the variant of the process being used, but the majorconsideration is that there should be a continuous removal of sludge The tanksare usually designed as settling and thickening tanks

6.3 TANK DESIGN

Many factors impact on the sedimentation efficiency in a continuous-flowtank, including particle size and density, liquid density, variations in liquidtemperature (viscosity), average velocity through the tank, and the influence

of the inlet, outlet, and sludge removal arrangements In a vertical-flow tank,the suspended particles will be eliminated if their settling velocity is equal

to the upward velocity of the water For a horizontal-flow tank, suspendedparticles are assumed to be retained in the tank, if they can settle through thefull depth of the tank in the time it takes for them to pass through the tank.The design settling velocities are the same for vertical-flow or horizontal-flow tanks and are independent of the tank depth The flow velocity issometimes termed the overflow rate or surface loading rate

28 Wetlands for Water Pollution Control

The optimal tank design is difficult to achieve in practice due to thefollowing reasons:

l Tanks do not behave in an ideal way;

l The solids in suspension have many different settling velocities;

l The SS content varies from work to work and even from day to day at thesame work; and

l Flocculation can occur, either deliberately induced or arising ously from certain conditions in the tank

spontane-As a result of these complications, design methods have been developed toovercome these practical difficulties The principles used in idealized designmethods are as follows:

l Design for a particular settling velocity;

l Take account of high velocities that tend to cause scour near the tank floor;and

l Design for the effect of an imperfect velocity distribution

6.4 DESIGN PARAMETERS

6.4.1 Design Settling Velocity

Maximum velocities in the tank should not be so great as to prevent the settling

of particles, and they should have a design settling velocity based on settlingtests for specific samples The normal design value for the settling velocity of,for example, domestic sewage is 0.339 mm/s However, note that efficientprimary sedimentation is possible even if a surface loading corresponding to anupward velocity>0.339 mm/s is used Design settling velocities can exceedthis figure in specific cases Many tanks achieve their highest efficiency at toohigh costs They could reasonably be loaded at much higher rates

6.4.2 Horizontal Velocity

The maximum velocity occurring anywhere in the sedimentation tank shouldnot exceed the scour velocity of the previously settled sludge A theoreticaldetermination of scouring velocities involves understanding of cohesion,friction factor, diameter, and specific gravity of the particles

Usually, experimental methods are employed on a sludge sample ally accepted values for the scour velocity of different sludge types are asfollows:

Gener-l Settled primary sewage: 31 mm/s

l Humus sludge: 36 mm/s

l Activated sludge: 20 mm/s

l Alum flocculation: 50 mm/s

6.4.3 Time Ratio

In an ideal tank, the ratio of effective flow-through time to effective settlingtime would be 1 In practice, the effective flow-through time is reducedbecause of an uneven distribution of flow throughout the tank; a value>0.95 isunattainable

In the case of circular tanks, experience shows that the time ratios inexisting tanks vary between 0.50 and 0.90 (compared with 0.30 and 0.95 forrectangular tanks) The highest values of both time ratio and sedimentationefficiency have been calculated for tanks with a 10
floor slope, followed byflat-bottomed tanks, and finally by tanks with floor slopes between 20
and 30
(thus to be avoided)

6.5 ECONOMICS OF CONSTRUCTION

6.5.1 Rectangular Settling Tanks

Tanks should be kept short so that depths and costs can be kept down In somecases, a double row of tanks (back to back) may be convenient to reduce thetank length A large number of tanks also indicate an increase in capital costs

in terms of mechanical and electrical equipment and also increased runningcosts The shape of rectangular tanks should be determined on the grounds ofcost unless this is contrary to overall plant efficiency

A numerical addition should be made to the tank depth to allow for sludgestorage The final decision as to the tank breadth depends on the final timeratio, which in turn depends on the inlet arrangements

6.5.2 Circular Settling Tanks

For tanks constructed below-ground, as the number of tanks increases andthe radius decreases, there will be a saving in volume of excavation, sincedepth is related to the radius As the number of tanks increases, so does thetotal perimeter together with the effluent collection channel Also, for agreater number of tanks, the machinery and support structures, pipework,and maintenance costs rise well above any savings in excavation costs,except in very difficult ground Therefore, the number of tanks should be assmall as the flexibility of the operation allows, and the floor slope should

be 10
.

6.6 DESIGN DETAILS

6.6.1 Rectangular Settling Tanks

The aim is to feed influent uniformly across the tank This is achieved bythe careful design of inlet pipes, channels, and weirs Where multiple tanks

30 Wetlands for Water Pollution Control

are employed, it is important to ensure that each tank is fed with the samedischarge This may be achieved by means of a tapered channel.

In order to prevent bottom scour, experience shows that the inflow should

be located at about half of the tank depth; also, simple plate baffles have beenshown to be very effective when extending to this depth Sludge hoppers atthe inlet end of the tank also help to reduce the maximum velocity near thefloor If these principles are incorporated into the design, the time ratio may

be kept at 0.95; otherwise, a proportionate increase in the tank width should

be made

The outlet weir must be level Castellated or V-notch weirs benefit theefficiency at low flows, but slightly reduce the time ratio at high flows Ashallow scum board should be provided at0.25 m from the weir

6.6.2 Circular Settling Tanks

Research shows that the best time ratio is obtained with a slightly submergedinlet pipe surrounded by a large circular baffle submerged to a depth of0.25 of the tank depth at the inlet However, this leads to high floorvelocities

With the pipe submerged to 0.75 of the tank depth and the baffle omitted,the floor velocities can be reduced despite a reduced time ratio value It isimportant to ensure equal discharge to each tank where multiple tanks areused

The baffle or pipe radius significantly affects the time ratio The small loss

of surface area due to the larger inlet is more than offset by the increase of thetime ratio

6.7 HYDRAULIC LOSSES

Hydraulic loss calculations for sedimentation tanks include the following:

l Control chamber losses;

l Losses due to distribution devices;

l Inlet-conduit losses;

l Velocity head losses at inlets;

l Head on outlet weir and free fall below the weir; and

l Outlet channel losses

The control chamber consists of a small chamber with outlets controlled bysluice gates Head losses can be calculated for the inlet, changes in flow di-rection, and outlet For a system of two or more tank units, equal flow division

is necessary Measuring weirs or flumes are often included The hydraulicdesign of conduits and structures downstream of sedimentation tanks shouldnot permit the occurrence of a backwater level above the hydrostatic criticaldepths at the channel inlets around or across the settling tanks

6.8 GENERAL DESIGN DETAILS

It is a normal design practice to construct at least two tanks The standbycapacity should be sufficient to allow one tank to be out of operation formaintenance without affecting the overall efficiency Tanks should be operated

in parallel rather than in series because inhomogeneous sludge affects thesludge treatment efficiency

Tank inlets and outlets must be designed in a satisfactory manner to preventshort circuiting Inlets should be designed to dissipate the flow velocity in theinlet conduits This is usually achieved by baffle boards

Arrangements for scum retention and removal must be made Scum boardsshould be no nearer than 250 mm from the outlet weir

Settled sludge must be regularly removed at intervals sufficient to preventanaerobic activity Generally, sludge is removed either manually (small works)

or under hydrostatic head Sludge is much more viscous than normal sewage.Multiplying factors for head losses calculated for clean water and used forsludge of varying moisture content (%) are shown below:

6.9 DETAILS OF VARIOUS TYPES OF SEDIMENTATION

TANKS

6.9.1 Storm Tanks

The design is based on the total capacity rather than velocity principles Two

or more tanks are provided with a total capacity equal to a DWF of 6 h Thisallows 2 h at the maximum flow rate of three DWF Normally, storm runofftanks are rectangular with the length between 4 and 5 times the width Nor-mally, the inlet weirs are fixed at different levels while the outlet weirs are atthe same level so that the tanks fill one at a time, with discharge commencingonly when all tanks are full

After a storm, the tank content should be returned to the main inlet of theworks or wetland system The sludge is discharged to the sludge treatmentunit Because the tanks have only intermittent use, the sludge scrapers aregenerally designed as simply as possible Traveling bridge units are often tooexpensive Usually, the scrapers are of the squeegee type: a cable-hauled boomdriven by a trolley-mounted motor

32 Wetlands for Water Pollution Control