Wastewater Characteristics and Sanitation Systems

Introduction

Wastewater is water whose quality has been altered by human use. It may originate from homes, institutions, commercial premises, industries, storm runoff, or combinations of these sources. Public-health sanitation aims to collect, convey, treat, reuse, or safely dispose of wastewater so that disease transmission, environmental pollution, nuisance, and resource loss are minimized.

For a civil draftsman, understanding wastewater is essential because drawings and specifications must reflect the expected flow, pollutant strength, hydraulic levels, pipe gradients, treatment processes, access requirements, and future expansion of a sanitation system. A visually attractive drawing can still be unsafe or uneconomical if it is based on an incorrect wastewater classification or inadequate flow estimate.

Learning Objectives

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

1. Classify domestic, commercial, institutional, industrial, and storm wastewater.
2. Describe the physical, chemical, and biological characteristics of wastewater.
3. Explain average flow, peak flow, infiltration, inflow, and diurnal variation.
4. Interpret BOD, COD, suspended solids, nutrients, pathogens, and related indicators.
5. Compare centralized and decentralized sanitation systems.
6. Apply basic wastewater quantities and pollutant-load calculations.
7. Identify drafting, operation, public-health, environmental, and regulatory requirements.

1. Classification and Sources of Wastewater

1.1 Domestic wastewater

Domestic wastewater, or sewage, is generated in dwellings. It includes:

• Blackwater, containing urine and faeces from toilets; and
• Greywater, from bathing, laundry, kitchens, and wash basins.

Blackwater generally contains higher concentrations of pathogens, nitrogen, phosphorus, and organic matter. Greywater usually has lower pathogen concentrations but may contain detergents, oils, grease, hair, suspended solids, and household chemicals. Mixing the two streams is common in conventional sewers, although separate collection can support water reuse or simplified treatment.

1.2 Commercial and institutional wastewater

Commercial wastewater comes from offices, shops, markets, restaurants, hotels, garages, and similar premises. Institutional wastewater is produced by schools, hospitals, prisons, and public buildings. Its quantity and strength vary greatly:

Commercial wastewater must not automatically be treated as ordinary domestic sewage. A restaurant, for example, may discharge grease that blocks sewers and disrupts biological treatment.

1.3 Industrial wastewater

Industrial effluent may contain acids, alkalis, solvents, metals, toxic compounds, high temperatures, or very high organic concentrations. It should normally be characterized separately and subjected to source control or pretreatment before entering a municipal sewer. Industrial wastewater is outside the scope of ordinary domestic design unless its characteristics have been established.

1.4 Stormwater and combined wastewater

Stormwater is runoff from roofs, roads, and land surfaces. It may carry sediment, litter, oil, faecal matter, and other pollutants. In a separate sewer system, sanitary sewage and stormwater use different networks. In a combined sewer system, both are conveyed in one network. Combined systems can reduce the number of pipes but may produce combined-sewer overflows during storms and require much larger hydraulic capacity.

1.5 Infiltration and inflow

Infiltration is groundwater entering through defective joints, cracks, porous pipe walls, or poorly sealed manholes. Inflow is relatively rapid entry of surface water through illegal connections, open covers, roof drains, or flooded manholes. Both dilute wastewater but increase the volume requiring conveyance and treatment.

2. Wastewater Characteristics

Wastewater characteristics are commonly grouped as physical, chemical, and biological properties.

2.1 Physical characteristics

Important physical observations include colour, odour, temperature, turbidity, and solids. Fresh domestic wastewater is generally grey and becomes darker as it becomes septic. Odour is associated with compounds such as hydrogen sulfide formed under anaerobic conditions. Temperature affects biological reaction rates, oxygen solubility, viscosity, and the performance of treatment units.

Solids are classified by size and behaviour:

• Screenable solids are retained by coarse screens.
• Settleable solids settle under quiescent conditions.
• Suspended solids remain dispersed but can be removed by filtration or settling.
• Dissolved solids pass through standard filtration.
• Volatile solids are lost on ignition and are often used as an approximate indicator of organic or biological content.
• Fixed solids remain as ash after ignition and are associated with inorganic material.

2.2 Chemical characteristics

Chemical indicators include pH, alkalinity, dissolved oxygen, organic matter, nutrients, oil and grease, and toxic substances.

• pH indicates acidity or alkalinity and affects biological treatment and corrosion.
• Alkalinity provides buffering capacity against pH changes, particularly during nitrification.
• Nitrogen occurs as organic nitrogen, ammonia, nitrite, and nitrate. Excess nitrogen can cause eutrophication and ammonia toxicity.
• Phosphorus is a major nutrient that can stimulate excessive algal growth in receiving waters.
• Oil and grease can coat pipes, interfere with oxygen transfer, and obstruct treatment equipment.
• Toxic compounds may inhibit microorganisms and create occupational or environmental hazards.

2.3 Biological characteristics

Wastewater may contain bacteria, viruses, protozoa, helminth eggs, and other pathogens. Indicator organisms such as Escherichia coli are commonly measured because direct testing for every pathogen is impractical. Pathogen control requires containment, treatment, safe sludge handling, personal protection, and prevention of human contact. Disinfection reduces microbial risk but does not replace adequate primary and secondary treatment.

3. Organic Pollution Indicators

3.1 Biochemical oxygen demand

Biochemical oxygen demand (BOD) is the oxygen required by microorganisms to biologically oxidize biodegradable organic matter under specified conditions. The commonly reported measure, BOD₅, is the oxygen consumed during five days of incubation, traditionally at 20 °C.

where DO_initial and DO_final are dissolved oxygen concentrations in the test bottle, corrected for dilution and seed effects when applicable, expressed in mg/L. BOD₅ is a standardized test value, not the complete amount of oxygen ultimately required. Carbonaceous BOD is associated mainly with oxidation of organic carbon; nitrogenous oxygen demand arises from nitrification.

BOD is useful for estimating biodegradable organic loading and biological-treatment requirements. It is relatively slow to measure and can be affected by toxic substances, seed quality, temperature, and nitrification control.

3.2 Chemical oxygen demand

Chemical oxygen demand (COD) is the oxygen equivalent of chemically oxidizable substances in a sample, measured using a strong oxidizing agent under standardized laboratory conditions. COD is usually obtained more quickly than BOD and generally exceeds BOD for municipal wastewater because it includes some substances that microorganisms cannot readily biodegrade.

The ratio BOD₅/COD is a useful screening indicator of biodegradability, but it is not a universal design constant. A high ratio often indicates readily biodegradable wastewater, while a low ratio may indicate refractory compounds, toxic materials, or substantial industrial contribution.

3.3 Suspended solids

Total suspended solids (TSS) are the solids retained when a well-mixed sample is filtered through a specified filter and the residue is dried under standardized conditions. TSS affects settling, sludge production, turbidity, and receiving-water quality. A wastewater plant may remove a large portion of TSS by screening and sedimentation before biological treatment.

3.4 Concentration and mass loading

Concentration alone does not describe the total amount of pollution entering a plant. The pollutant mass load is calculated by:

where L is mass loading, Q is flow, and C is concentration. When Q is in m³/day and C is in mg/L:

The factor 1000 follows from 1 m³ = 1000 L and 1 kg = 1,000,000 mg. This equation assumes that Q and C represent the same period and that the concentration is representative of the flow being considered.

Worked example. A treatment plant receives 1,200 m³/day with a BOD₅ concentration of 250 mg/L. The load is:

L = 1200 × 250 ÷ 1000 = 300 kg/day

Thus, the plant receives approximately 300 kg BOD₅ per day.

4. Wastewater Flow and Its Variation

4.1 Average domestic flow

A preliminary average wastewater flow may be estimated from population and per-capita wastewater production:

where Q_avg is average wastewater flow, P is contributing population, and q is wastewater production per person per day. If q is expressed in L/person·day, Q_avg is obtained in L/day and must be divided by 1000 to obtain m³/day.

The value of q depends on water supply, climate, plumbing, income, commercial activity, leakage, conservation, and local practice. It must be selected from applicable standards or reliable local data rather than assumed universally.

4.2 Peak flow

Sewer pipes and pumping stations must accommodate short-duration high flows. A simplified planning relationship is:

where K_p is a peak factor selected from local design criteria and the size and characteristics of the contributing population. This relationship is a planning approximation; peak factors should not be applied blindly to very large systems, strong industrial flows, or systems with significant infiltration and inflow.

A more complete design flow may be expressed as:

The terms must be defined using compatible time bases. For treatment plants, designers may also examine minimum flow, wet-weather flow, hourly peaks, and seasonal conditions.

4.3 Diurnal and seasonal variation

Domestic flow commonly rises in the morning, falls during the day, and may show another evening peak. Schools and offices have pronounced working-hour patterns. Hotels and hospitals may operate continuously but still exhibit daily variation. Rainfall creates rapid inflow and may introduce dilution, grit, and pollutants. Flow-monitoring data, occupancy records, water-consumption data, rainfall records, and local standards improve estimates.

A draftsman should show flow direction, invert levels, manholes, overflow routes, pumping facilities, access points, and hydraulic control structures clearly. Hydraulic drawings must distinguish normal flow from emergency or bypass flow.

5. Centralized and Decentralized Sanitation Systems

5.1 Centralized systems

A centralized system collects wastewater from many properties through a sewer network and conveys it to a large treatment plant. It commonly includes house connections, lateral sewers, branch and trunk sewers, manholes, lift stations, rising mains, treatment units, sludge facilities, and a final discharge or reuse system.

Advantages include professional operation, uniform monitoring, economies of scale, and the possibility of advanced nutrient or pathogen removal. Limitations include high capital cost, extensive excavation, dependence on electricity and pumping, difficult expansion in informal or dispersed settlements, and the risk that a network failure affects many users.

5.2 Decentralized systems

A decentralized system treats wastewater near its source or within a small service area. Examples include septic tanks with soakaways, twin-pit systems, anaerobic baffled reactors, constructed wetlands, package plants, community-scale treatment plants, and separate greywater systems.

Advantages include shorter conveyance distances, phased construction, suitability for dispersed settlements, and potential local reuse. Limitations include the need for regular desludging, competent local management, land availability, groundwater protection, and reliable monitoring. A decentralized system is not automatically low-cost or low-risk; poor emptying practices can expose workers and communities to pathogens.

5.3 Comparative framework

The appropriate choice depends on density, topography, soil permeability, groundwater level, water availability, land, institutional capacity, energy supply, affordability, and regulatory requirements. Hybrid systems may combine simplified sewers with cluster treatment or centralized sludge management.

6. Sanitation System Components and Drafting Practice

A sanitation drawing should communicate both the physical arrangement and the public-health function. Essential information includes:

• site boundary, north arrow, contours, roads, buildings, water bodies, and wells;
• pipe diameter, material, gradient, flow direction, invert and cover levels;
• manhole numbers, spacing, access covers, drop arrangements, and maintenance zones;
• treatment-unit dimensions, liquid levels, freeboard, baffles, vents, screens, and bypass arrangements;
• inlet, outlet, overflow, sludge-removal, sampling, and disinfection facilities;
• setbacks from buildings, wells, property boundaries, drainage channels, and flood-prone areas;
• safe access, handrails, confined-space warnings, and personal-protection requirements.

Gravity sewers should be designed to achieve adequate self-cleansing without excessive velocity or erosion. The hydraulic design itself requires locally approved equations, roughness values, minimum slopes, allowable depths, and surcharge criteria. A draftsman must not substitute visual symmetry for hydraulic verification.

7. Applied Case: Illustrative Community Sanitation Assessment

Background

A fictional settlement of 4,000 residents is located on gently sloping ground. Houses are moderately dense near the main road but scattered on the outskirts. The groundwater table is seasonally high, and electricity supply is unreliable. A small stream passes downstream of the settlement.

Problem

The existing arrangement consists of individual pits and informal discharge points. During the rainy season, pits overflow and wastewater reaches the stream. The local authority must select a sanitation approach that is affordable, maintainable, and protective of groundwater.

Analysis

A preliminary wastewater estimate is made using a locally justified allowance of 80 L/person·day for wastewater generation. The estimated average flow is:

Q_avg = 4000 × 80 = 320000 L/day = 320 m³/day

If a preliminary peak factor of 2.5 is permitted by the applicable design guidance, the domestic peak is:

Q_peak = 2.5 × 320 = 800 m³/day

These figures are planning estimates. Infiltration, commercial wastewater, future population, and measured water use must be evaluated before final design. Because groundwater is high, unsealed soakaways and poorly constructed pits present a contamination risk.

Approach and solution

A hybrid solution is selected: simplified small-bore sewers in the dense central area, grease pretreatment for food businesses, and two community-scale treatment facilities. The scattered outskirts receive improved lined containment systems with scheduled, safe sludge collection. The treatment facilities include screening, primary settling, biological treatment, polishing, and controlled discharge or restricted reuse subject to regulatory approval.

The drawings identify separate stormwater drainage, sealed manholes in flood-prone locations, access for desludging vehicles, sampling points, emergency storage, and safe routes for operators. A monitoring plan includes flow, BOD₅, TSS, pH, and indicator organisms, with additional tests where industrial or healthcare sources exist.

Outcome

The proposed arrangement reduces open discharge, avoids unnecessary long sewers, and allows phased expansion. Its success depends on tariff collection, operator training, routine sludge removal, community communication, inspection of commercial pretreatment, and enforcement against illegal stormwater connections.

Learning points

1. A sanitation system must be selected from site and institutional conditions, not from population size alone.
2. Flow estimates are preliminary until local measurements and future conditions are assessed.
3. Containment, treatment, conveyance, and sludge management are one linked system.
4. Drawings must show maintenance and public-health safeguards, not only pipes and tanks.

8. Challenges, Regulation, and Contemporary Practice

Sanitation planning faces uncertain population growth, water scarcity, climate-related flooding, energy costs, ageing infrastructure, informal development, and limited municipal capacity. Climate resilience may require flood-resistant manholes, protected electrical equipment, standby power, emergency storage, and treatment processes that tolerate variable loads.

Regulatory requirements normally specify effluent quality, discharge permits, groundwater protection, worker safety, sludge use or disposal, and reuse standards. The designer must consult the current national and local regulations because numerical limits differ by receiving environment and intended reuse. Public participation is important where land acquisition, tariffs, odour, reuse, or sludge transport may affect communities.

Digital tools can improve flow mapping, geographic information systems, hydraulic modelling, asset registers, and inspection records. Sensors and automated alarms can identify pump failure, abnormal levels, or wet-weather inflow. AI may assist with anomaly detection or maintenance scheduling, but it cannot replace sampling, engineering judgement, regulatory approval, or responsibility for public safety. Data quality, cybersecurity, privacy, and transparent decision-making must be considered.

Best-Practice Checklist

• Characterize each significant wastewater source before design.
• Separate sanitary flow, stormwater, and industrial risk wherever practicable.
• Use average, peak, minimum, wet-weather, and future design conditions.
• Express pollutant loads using compatible flow and concentration periods.
• Provide access for inspection, cleaning, sampling, and sludge removal.
• Protect groundwater and surface water through suitable siting and containment.
• Design for reliable operation under local energy and management conditions.
• Show emergency routes, bypass controls, safety barriers, and hazard warnings.
• Verify all dimensions, levels, gradients, materials, and standards before issue.
• Treat sanitation as a complete service chain from toilet to final reuse or disposal.

Chapter Summary

Wastewater may be domestic, commercial, institutional, industrial, stormwater, or a mixture of these categories. Its physical, chemical, and biological characteristics determine collection, treatment, worker protection, and environmental controls. BOD₅ measures biodegradable oxygen demand over a standardized test period, COD measures chemically oxidizable material, and TSS measures filterable suspended material. Concentration must be combined with flow to determine pollutant mass loading.

Wastewater flow varies with population, water use, commercial activity, time of day, season, rainfall, infiltration, and inflow. Basic estimates use average flow and a locally selected peak factor, but final design requires standards, surveys, monitoring, and allowance for future conditions. Centralized systems provide large-scale collection and professional treatment, whereas decentralized systems provide local or phased service. Neither is universally superior. The correct choice depends on density, land, soil, groundwater, topography, finance, energy, management, and regulation.

For civil draftsmen, competent sanitation work requires more than pipe representation. Drawings must communicate hydraulic function, treatment processes, access, safety, maintenance, environmental protection, and future operation.

Key Terms

Wastewater: Water altered by domestic, commercial, industrial, or other human use.

Domestic wastewater: Wastewater generated by households.

Blackwater: Toilet wastewater containing urine and faeces.

Greywater: Wastewater from bathing, washing, kitchens, and basins.

Commercial wastewater: Wastewater from businesses such as restaurants, markets, and hotels.

Infiltration: Groundwater entering a sewer through defects or porous components.

Inflow: Surface water entering a sewer through direct or illegal connections.

BOD₅: Five-day biochemical oxygen demand measured under standardized conditions.

COD: Chemical oxygen demand for chemically oxidizable substances.

TSS: Total suspended solids retained by a specified filtration method.

Pollutant load: Mass of a pollutant entering a system per unit time.

Peak flow: Short-duration maximum wastewater flow used for hydraulic design.

Centralized sanitation: Collection and treatment serving a large connected area.

Decentralized sanitation: Local or on-site collection and treatment.

Pretreatment: Removal or control of specific contaminants before wastewater enters a main system.

Septic system: On-site containment and partial treatment system, commonly including a septic tank and soil dispersal.

Pathogen: Disease-causing organism or biological agent.

Eutrophication: Nutrient-driven excessive plant or algal growth in water.

Sludge: Concentrated solids separated during wastewater treatment.

Reuse: Beneficial use of treated wastewater or treatment products under controlled conditions.

Review Questions

Short Answer Questions

1. Distinguish between blackwater and greywater.
2. What is the difference between infiltration and inflow?
3. Define BOD₅ and explain why it is useful.
4. Why is pollutant load more informative than concentration alone?
5. State two advantages and two limitations of decentralized sanitation.

Long Answer Questions

1. Explain the physical, chemical, and biological characteristics used to describe wastewater.
2. Discuss domestic, commercial, institutional, industrial, and stormwater sources and their design implications.
3. Explain average flow, peak flow, diurnal variation, infiltration, and inflow in sewer design.
4. Compare centralized and decentralized sanitation systems using technical, economic, environmental, and management criteria.
5. Describe the information that should appear on a professional sanitation layout and treatment-plant drawing.

Analytical / Application Questions

1. A settlement of 2,500 people produces an estimated 75 L/person·day of wastewater. Calculate its average flow in m³/day. If the approved preliminary peak factor is 3.0, calculate the planning peak flow.
2. A plant receives 900 m³/day at 280 mg/L BOD₅. Calculate the BOD₅ load in kg/day and explain why this value is important for treatment planning.
3. For a community with high groundwater, unreliable electricity, dispersed houses, and a dense commercial centre, recommend a sanitation arrangement and justify the roles of centralized, decentralized, or hybrid components.

Multiple Choice Questions with Answers

Question 1

Which wastewater stream normally contains urine and faeces?

A. Greywater

B. Blackwater

C. Stormwater

D. Infiltration water

Correct Answer: B. Blackwater

Question 2

A wastewater flow of 1,000 m³/day has a BOD₅ concentration of 200 mg/L. What is the approximate BOD₅ load?

A. 20 kg/day

B. 100 kg/day

C. 200 kg/day

D. 2,000 kg/day

Correct Answer: C. 200 kg/day

Question 3

What does COD generally measure?

A. Only settleable solids

B. Chemically oxidizable substances

C. Only disease-causing organisms

D. Groundwater entering a sewer

Correct Answer: B. Chemically oxidizable substances

Question 4

Which condition is most directly associated with infiltration?

A. An open roof drain connected to a sewer

B. A cracked sewer pipe below the groundwater table

C. Grease discharged from a restaurant

D. A high morning domestic peak

Correct Answer: B. A cracked sewer pipe below the groundwater table

Question 5

Why is TSS important in wastewater treatment?

A. It indicates suspended particulate material and affects settling and turbidity

B. It directly measures all pathogens

C. It is identical to pH

D. It measures only dissolved oxygen

Correct Answer: A. It indicates suspended particulate material and affects settling and turbidity

Question 6

Which is a typical advantage of a centralized sanitation system?

A. It requires no maintenance

B. It can provide professional treatment for a large connected service area

C. It is always cheaper for dispersed settlements

D. It eliminates pumping in all terrains

Correct Answer: B. It can provide professional treatment for a large connected service area

Question 7

A population of 2,000 produces 100 L/person·day of wastewater. What is the estimated average flow?

A. 20 m³/day

B. 100 m³/day

C. 200 m³/day

D. 2,000 m³/day

Correct Answer: C. 200 m³/day

Question 8

If the average flow is 300 m³/day and the approved peak factor is 2.5, what is the preliminary peak flow?

A. 120 m³/day

B. 302.5 m³/day

C. 750 m³/day

D. 7,500 m³/day

Correct Answer: C. 750 m³/day

Question 9

Which wastewater source most commonly requires grease control before entering a sewer?

A. Restaurant

B. Office

C. Roof runoff

D. Groundwater

Correct Answer: A. Restaurant

Question 10

Which factor is especially important when selecting on-site sanitation in a high-groundwater area?

A. Decorative pipe colour

B. Groundwater protection and watertight containment

C. Increasing stormwater inflow

D. Removing all access covers

Correct Answer: B. Groundwater protection and watertight containment