Water Sources, Quality and Treatment

Introduction

Safe water is fundamental to public health, domestic life, industry and environmental protection. Civil draftsmen contribute to water-supply projects by preparing source-development plans, intake details, treatment-plant layouts, hydraulic profiles, pipework drawings and construction documentation. Sound drafting depends on understanding what water contains, how demand is estimated, how samples are collected, and how treatment units operate.

Water occurs in the hydrological cycle as precipitation, surface runoff, groundwater, soil moisture, ice and atmospheric vapour. Although water is abundant, only a small proportion is readily available and suitable for human use without treatment. A reliable public supply must provide sufficient quantity, acceptable quality, continuity and reasonable accessibility.

Learning Objectives

After studying this chapter, the learner should be able to:

1. Identify and compare surface-water, groundwater and rainwater sources.
2. Estimate basic water demand and distinguish average, maximum-day and peak-hour demand.
3. Explain physical, chemical, microbiological and radiological water-quality parameters.
4. Prepare representative water samples and describe common field and laboratory tests.
5. Explain screening, aeration, coagulation, flocculation, sedimentation, filtration and disinfection.
6. Interpret treatment-process drawings and apply basic professional and public-health controls.

1. Water Sources

1.1 Surface water

Surface water includes rivers, streams, lakes, reservoirs, ponds and canals. It is often available in large quantities and can support towns and industries. However, it is exposed to soil erosion, agricultural runoff, wastewater, pathogens, algae and seasonal changes in colour and turbidity. Surface-water treatment commonly requires several sequential processes.

Reservoirs can improve raw-water quality by providing storage and allowing suspended matter to settle. They may also create problems such as algal blooms, taste and odour, thermal stratification and increased treatment requirements.

1.2 Groundwater

Groundwater occupies pores and fractures beneath the ground. Wells, tube wells, infiltration galleries and springs are common means of abstraction. Soil and rock provide natural filtration, so groundwater is often clear and relatively free from suspended solids. Nevertheless, it may contain dissolved iron, manganese, hardness, fluoride, arsenic, nitrate, salinity or naturally occurring radioactive substances. Groundwater can also be contaminated by leaking septic tanks, landfills, fertilizers and industrial chemicals.

A protected aquifer is not automatically safe. Microbial contamination can enter through poorly sealed wells, flooding or rapid movement through fractured rock. Sanitary well construction, a protective apron, drainage away from the well and regular testing are essential.

1.3 Rainwater

Rainwater is usually low in dissolved salts but may acquire atmospheric pollutants and contamination from roofs, gutters and storage tanks. It is useful for household non-potable purposes and, after suitable treatment and protection, may serve as a drinking-water source. The first runoff from a roof is commonly diverted because it contains accumulated dust, bird droppings and other debris.

1.4 Source selection

Source selection considers yield, reliability during drought, elevation and pumping energy, catchment protection, existing pollution, treatment complexity, land availability, construction cost and future expansion. A source investigation should include hydrology, geology, land use, water-quality records and a sanitary survey. Source protection is generally more economical and safer than relying entirely on treatment.

2. Water Demand and Supply Planning

Water demand includes domestic consumption, institutional and commercial use, industrial use, public uses such as street washing and firefighting, losses from leakage and unauthorized consumption, and water used within the treatment plant.

2.1 Basic demand equation

If the design population is P persons and the per-capita demand is q litres per person per day, the average daily demand is:

where Q_avg is in megalitres per day (ML/d), P is in persons, and q is in L/person·d. The factor 1,000,000 converts litres to megalitres. The equation assumes that q represents the selected average consumption and that all relevant demand categories have been included.

For a design period, population may be estimated using an approved method. A simple arithmetic projection is:

where P_n is the projected population after n years, P_0 is the present population, and I is the average annual increase in persons per year. This method is appropriate only where historical growth is reasonably constant. It should not be used blindly in rapidly expanding or declining settlements.

2.2 Demand variation

Average demand does not describe the flow that pipes and treatment units must always accommodate. Let K_d be the maximum-day factor and K_h be the peak-hour factor relative to average demand:

The values of K_d and K_h must be selected from local standards or reliable operating data. They vary with population size, climate, industrial activity, metering, storage and service level. Factors must not be combined unless their definitions are compatible.

Worked Example: Average and Maximum-Day Demand

A settlement has a design population of 18,000 and an average demand of 120 L/person·d. The maximum-day factor is 1.8. Find the average and maximum-day demands.

Average demand:

Q_avg = (18,000 × 120) / 1,000,000 = 2.16 ML/d.

Maximum-day demand:

Q_max-day = 1.8 × 2.16 = 3.888 ML/d, approximately 3.89 ML/d.

In cubic metres per day, the average is 2,160 m³/d and the maximum-day demand is 3,888 m³/d, because 1 ML = 1,000 m³. A designer must also consider fire flow, storage and the difference between source capacity and treatment-plant capacity.

3. Water-Quality Concepts and Parameters

Water quality is judged against a particular use and an applicable standard. Drinking-water criteria protect health and also control acceptability, such as taste, colour and odour. A single test result does not prove that a supply is permanently safe; quality varies with season, rainfall, abstraction rate and pollution events.

3.1 Physical parameters

Temperature affects taste, chemical reaction rates, dissolved oxygen and biological growth. Colour may result from organic matter, metals or industrial pollution. Apparent colour includes suspended particles; true colour is measured after removing turbidity.

Turbidity is the scattering of light by suspended and colloidal particles. It is commonly reported in nephelometric turbidity units (NTU). High turbidity reduces aesthetic quality, interferes with disinfection and may protect microorganisms from disinfectants. Taste and odour can arise from algae, organic decomposition, chemicals or disinfectant by-products.

Total dissolved solids (TDS) represent dissolved mineral and other matter. Excessive TDS may affect taste and scaling. Electrical conductivity provides a rapid indication of ionic content and is useful for detecting changes in salinity or contamination.

3.2 Chemical parameters

Important chemical parameters include pH, alkalinity, hardness, chloride, sulphate, nitrate, fluoride, iron, manganese, arsenic, lead and organic contaminants.

• pH expresses hydrogen-ion activity on a logarithmic scale. It influences corrosion, coagulation and disinfection.
• Alkalinity is the capacity to neutralize acid, mainly due to bicarbonate, carbonate and hydroxide. Adequate alkalinity can support coagulation and stabilize pH.
• Hardness is mainly caused by calcium and magnesium. It produces scale and increases soap consumption, although moderate hardness is not generally a direct health hazard.
• Nitrate at excessive concentrations is associated with infant methaemoglobinaemia and may indicate agricultural or sewage pollution.
• Fluoride has a beneficial range in some contexts but excessive concentration can cause dental or skeletal fluorosis; requirements depend on local conditions and standards.
• Iron and manganese cause staining, taste and deposits and may require oxidation followed by filtration.
• Toxic metals and pesticides require careful source control and validated analytical methods.

3.3 Microbiological parameters

Waterborne pathogens include bacteria, viruses, protozoa and helminths. Direct testing for every pathogen is impractical. Indicator organisms, especially Escherichia coli (E. coli), are used to assess recent faecal contamination. E. coli detection in drinking water signals a serious sanitary risk and requires investigation and corrective action. Total coliforms are useful operational indicators but are less specific for faecal contamination.

Microbial safety depends on treatment, distribution-system integrity, residual disinfectant, pressure management and prevention of cross-connections. A clear sample is not necessarily microbiologically safe.

3.4 Radiological parameters

Radionuclides may occur naturally in groundwater or result from human activities. Screening for gross alpha and beta activity, radium, uranium or other radionuclides is determined by geology, risk assessment and regulations. Radiological testing is specialized and requires accredited laboratories.

4. Sampling and Testing

4.1 Sampling principles

A sample must represent the water being assessed. The sampling plan should define locations, frequency, parameters, preservation, analytical methods, responsible personnel and quality-control procedures. Samples should be taken from raw-water intakes, treatment stages, storage tanks and representative distribution points. Additional sampling is needed after floods, repairs, complaints or unusual test results.

For microbiological samples, use sterile containers supplied or approved by the laboratory. Do not rinse them. Where chlorinated water is sampled, the container normally contains a suitable neutralizing agent, such as sodium thiosulphate, according to laboratory instructions. Avoid touching the cap or inside of the container.

Record the site, date, time, sampler, weather, source condition, residual disinfectant where relevant and any unusual appearance or odour. Keep samples cool, protected from sunlight and deliver them within the prescribed holding time. Chain-of-custody documentation is important when results may support enforcement or investigation.

4.2 Field measurements and laboratory analysis

Field measurements commonly include temperature, pH, conductivity, turbidity and disinfectant residual because these may change during transport. Laboratory analysis may include alkalinity, hardness, nutrients, metals, organic compounds and microbiological indicators.

Quality assurance includes calibrated instruments, reagent blanks, duplicate samples, control samples, method detection limits and competent analysts. Results should be compared with the applicable national drinking-water standard or regulatory requirement, not with an arbitrary universal value.

4.3 Sampling errors

Common errors include sampling from a dirty tap, collecting stagnant water, using a non-sterile bottle, inadequate preservation, incorrect labelling, excessive transport time and contamination during handling. A technically accurate laboratory analysis cannot correct a non-representative sample.

5. Conventional Water Treatment

Conventional treatment is a process train. A typical sequence is:

Raw water → screening → aeration or pre-treatment (where required) → coagulation → flocculation → sedimentation → filtration → disinfection → storage and distribution.

Not every plant uses every process, and advanced processes may be added for special contaminants. Treatment units must be designed as an integrated system: the performance of one unit affects the next.

5.1 Screening

Screens remove large floating and suspended materials such as sticks, rags, plastics and weeds. Coarse screens protect pumps and downstream equipment; finer screens provide greater removal but create higher head loss and cleaning requirements. Screenings must be safely removed, stored and disposed of. A draftsman should show approach channels, screen inclination, access platforms, bypass arrangements and safe maintenance clearances.

5.2 Aeration and pre-treatment

Aeration brings water into contact with air. It may remove volatile substances, improve taste and odour, add oxygen, and oxidize dissolved iron or manganese before filtration. Pre-treatment may also include pre-sedimentation, chemical oxidation, activated carbon or pH adjustment. Selection depends on raw-water analysis and treatability testing.

5.3 Coagulation

Many fine particles remain suspended because they are small and electrically stabilized. Coagulation involves rapid mixing of a chemical coagulant, such as an aluminium or iron salt, to destabilize particles. The correct dose depends on turbidity, organic matter, pH, temperature and alkalinity. Jar testing is commonly used to compare doses and mixing conditions.

Coagulant addition must be controlled. Under-dosing leaves particles stable; over-dosing can increase residual chemicals, sludge production or operating cost. Chemical storage areas require bunding, ventilation, protective equipment and clear hazard labelling.

5.4 Flocculation

After rapid mixing, gentle and controlled mixing causes destabilized particles to collide and form larger flocs. Excessive turbulence can break flocs, while insufficient mixing produces weak or small flocs. Flocculators may be mechanical or hydraulic and are commonly divided into stages with decreasing mixing intensity.

5.5 Sedimentation

In sedimentation tanks, flocs settle under gravity. Good hydraulic distribution, adequate detention, sludge removal and prevention of short-circuiting are essential. Surface overflow rate is a useful preliminary design concept:

where v_o is the surface overflow rate, Q is the flow rate and A_s is the plan surface area. Units must be consistent, commonly m³/m²·d, which is equivalent to m/d. This relationship is a design aid, not a guarantee of performance; particle characteristics, temperature, inlet and outlet design also matter.

Settled sludge may be thickened, dewatered and disposed of in accordance with environmental requirements. Sludge handling is part of the treatment process, not an afterthought.

5.6 Filtration

Filtration removes remaining suspended particles, floc and many microorganisms as water passes through a granular or other porous medium. Rapid sand filters generally contain graded media, support gravel, underdrains and a backwash system. Removal occurs through straining, sedimentation within pores, interception and attachment.

Filter performance is affected by media size and depth, filtration rate, temperature, particle loading and preceding coagulation. When head loss rises or filtered-water quality deteriorates, the filter is backwashed. Backwash water and spent washwater require suitable recovery or disposal.

Slow sand filters operate at much lower rates and rely significantly on a biologically active surface layer. They can produce high-quality water but require larger areas and careful protection from excessive turbidity.

5.7 Disinfection

Disinfection inactivates pathogenic organisms. Chlorine, chloramine, ozone and ultraviolet radiation are common methods. Chlorination is widely used because it is comparatively economical and can maintain a residual in the distribution system.

A simplified chlorine-contact relationship is:

where C is the disinfectant concentration, usually in mg/L, and t is the effective contact time, usually in minutes; CT is therefore expressed as mg·min/L. The relationship is only a simplified indicator. Actual inactivation depends on organism type, temperature, pH, disinfectant form, mixing and baffling. Effective contact time is not always equal to the tank’s theoretical detention time.

Chlorine demand is the amount consumed by reactions with organic and inorganic substances. The residual after contact is approximately:

where all concentrations use the same units. This is a mass-balance approximation and should be confirmed by operational testing. Excessive chlorine can cause taste and odour and may increase disinfection by-products when natural organic matter is present.

Ultraviolet treatment does not normally provide a lasting distribution residual, so it must be supported by secure storage and distribution. Ozone is a strong disinfectant and oxidant but requires more complex equipment and may form by-products under some conditions.

6. Treatment-Plant Layout and Drafting Considerations

A treatment-plant drawing should show process direction, hydraulic levels, equipment, chemical-feed points, sampling locations, drains, overflow routes, access, ventilation, electrical and safety provisions. Sections and profiles are especially important because treatment units depend on gravity flow and controlled water levels.

Drawings should distinguish raw water, settled water, filtered water, treated water, sludge, backwash water and chemical lines. Valves must be accessible, flow arrows must be unambiguous, and equipment schedules should identify capacity and duty/standby arrangements. As-built drawings must record actual levels, pipe sizes, materials, valves and later modifications.

7. Applied Case: Illustrative Small-Town Supply

Background

A fictional town of 18,000 people obtains water from a river. During the rainy season, turbidity rises sharply and residents report intermittent colour and taste. The existing plant has screens, a sedimentation tank, rapid sand filters and chlorination, but the distribution system has no reliable residual monitoring.

Problem and analysis

Raw-water tests show high turbidity after storms, variable pH and occasional E. coli detection. Inspection identifies livestock access to the catchment, inadequate chemical storage, poor sludge removal and a chlorine-contact tank with short-circuiting. Sampling at the treatment outlet is infrequent and does not represent distant distribution points.

Approach and solution

The town establishes a catchment-protection programme, repairs the intake screen and provides safe chemical storage. Jar tests are introduced to select coagulant dose during changing raw-water conditions. The flocculator and sedimentation tank are hydraulically assessed, sludge removal is improved, and filter backwash procedures are documented. Baffles are installed in the contact tank, and chlorine residual is measured at the outlet and selected network points. A sampling schedule includes raw water, post-filtration water, treated water and remote consumers.

Outcome and learning points

The improved process reduces turbidity and provides more dependable microbial protection. The principal lesson is that compliance cannot be achieved by chlorination alone. Catchment protection, representative sampling, process control, maintenance, operator competence and distribution management must work together.

8. Limitations, Governance and Contemporary Practice

No treatment process removes every contaminant. Conventional treatment is effective for turbidity and many microorganisms but may not adequately remove dissolved salts, nitrate, arsenic, pesticides or some industrial chemicals. Treatment selection must follow risk assessment and source characterization.

Public-health protection requires adherence to applicable legislation, drinking-water standards, worker-safety rules, chemical-management requirements and environmental-discharge permits. Operators have a duty to report abnormal results and protect consumers rather than conceal failures. Water-quality records should be traceable and retained according to regulatory requirements.

Online sensors, supervisory control and data acquisition systems, automatic coagulant control and remote alarms can improve response time. Artificial intelligence may assist anomaly detection and demand forecasting, but it cannot replace calibrated instruments, laboratory confirmation, competent operators or accountable decisions. Cybersecurity is increasingly important where treatment plants use networked control systems.

Best practice follows a source-to-consumer, risk-based approach: protect the catchment, use multiple treatment barriers, monitor critical points, maintain infrastructure, verify results and prepare emergency response plans. The most important professional habit is to connect every drawing and operational decision to the protection of public health.

Chapter Summary

Water supplies may come from surface water, groundwater or rainwater, each with different yield, contamination and treatment risks. Demand estimation requires population, per-capita use, demand categories, losses and peak factors. Water quality includes physical, chemical, microbiological and radiological characteristics; E. coli is an important indicator of faecal contamination.

Representative sampling, correct preservation, chain of custody, calibrated instruments and quality assurance are essential for credible results. Conventional treatment generally combines screening, coagulation, flocculation, sedimentation, filtration and disinfection, although the exact process train depends on source-water conditions. Coagulation destabilizes particles, sedimentation removes flocs, filtration polishes the water, and disinfection controls pathogens. Effective plant design also requires sludge management, chemical safety, hydraulic control, monitoring, maintenance and regulatory compliance.

Key Terms

Source water: Water used as the input to a treatment system.

Surface water: Water occurring in rivers, lakes, reservoirs or other open bodies.

Groundwater: Water stored beneath the ground in pores and fractures.

Turbidity: Cloudiness caused by suspended or colloidal particles.

TDS: Total dissolved solids present in water.

Alkalinity: Capacity of water to neutralize acid.

Hardness: Mainly calcium and magnesium content of water.

Indicator organism: Organism used to indicate possible contamination.

E. coli: Indicator of recent faecal contamination.

Coagulation: Chemical destabilization of fine particles.

Flocculation: Gentle mixing that forms larger settleable flocs.

Sedimentation: Removal of particles by gravitational settling.

Filtration: Removal through a porous or granular medium.

Disinfection: Inactivation of disease-causing organisms.

Chlorine demand: Chlorine consumed by reactions in the water.

Residual disinfectant: Disinfectant remaining after contact and available for protection.

Surface overflow rate: Flow divided by the plan area of a sedimentation tank.

Chain of custody: Documented control of a sample from collection to analysis.

Catchment protection: Measures that prevent pollution at the source.

Review Questions

Short Answer Questions

1. Name three common sources of public water supply.
2. What is the public-health significance of E. coli in drinking water?
3. State the purpose of coagulation.
4. Why are samples kept cool and protected from sunlight?
5. What is meant by chlorine residual?

Long Answer Questions

1. Compare surface water and groundwater with respect to availability, contamination and treatment requirements.
2. Explain the conventional water-treatment sequence from screening to disinfection.
3. Discuss the major physical, chemical and microbiological water-quality parameters.
4. Describe the requirements for collecting and transporting a representative microbiological sample.
5. Explain the role of a civil draftsman in preparing treatment-plant drawings.

Analytical / Application Questions

1. A town has a design population of 25,000 and a demand of 110 L/person·d. If the maximum-day factor is 1.6, calculate the average and maximum-day demands in ML/d.
2. A treatment plant has increasing filtered-water turbidity despite normal chemical dosing. Identify a logical sequence of investigations involving coagulation, sedimentation, filtration and sampling.
3. A groundwater source is clear but contains excessive iron, manganese and nitrate. Explain why conventional clarification alone may be inadequate and propose the type of additional investigation or treatment assessment required.

Multiple Choice Questions with Answers

Question 1

Which source is generally most exposed to seasonal turbidity and pathogen contamination?

A. Deep confined groundwater

B. River surface water

C. Protected spring

D. Sealed rainwater tank

Correct Answer: B. River surface water

Question 2

A population of 10,000 uses 100 L/person·d. What is the average demand?

A. 0.1 ML/d

B. 1.0 ML/d

C. 10 ML/d

D. 100 ML/d

Correct Answer: B. 1.0 ML/d

Question 3

What is the main purpose of screening?

A. To remove dissolved salts

B. To inactivate viruses

C. To remove large debris

D. To increase water hardness

Correct Answer: C. To remove large debris

Question 4

Which organism is a specific indicator of recent faecal contamination?

A. E. coli

B. Algae

C. Yeast

D. Iron bacteria

Correct Answer: A. E. coli

Question 5

Coagulation primarily causes fine particles to do what?

A. Dissolve completely

B. Become chemically radioactive

C. Become destabilized so they can form flocs

D. Increase water temperature

Correct Answer: C. Become destabilized so they can form flocs

Question 6

If Q = 2,000 m³/d and a sedimentation tank has a surface area of 500 m², what is the surface overflow rate?

A. 0.25 m/d

B. 2 m/d

C. 4 m/d

D. 10 m/d

Correct Answer: C. 4 m/d

Question 7

Which process normally removes remaining floc and suspended particles after sedimentation?

A. Filtration

B. Screening only

C. Population projection

D. Catchment mapping

Correct Answer: A. Filtration

Question 8

In the simplified relationship CT = C × t, what does C represent?

A. Catchment area

B. Disinfectant concentration

C. Coagulant mass only

D. Carbon content

Correct Answer: B. Disinfectant concentration

Question 9

Why is a chlorine residual maintained in many distribution systems?

A. To increase turbidity

B. To provide continuing protection against microbial regrowth and intrusion

C. To remove all hardness

D. To replace source protection

Correct Answer: B. To provide continuing protection against microbial regrowth and intrusion

Question 10

Which practice most directly improves the reliability of laboratory water-quality results?

A. Ignoring sample holding times

B. Using unlabelled containers

C. Calibrating instruments and applying quality-control samples

D. Sampling only when consumers complain

Correct Answer: C. Calibrating instruments and applying quality-control samples