Chapter 18 — Wastewater Story

What is wastewater? Wastewater is water that has been used and contains unwanted substances (solids, dissolved materials, microbes or chemicals). It comes from homes, farms, industries and cities and can pollute rivers, lakes and groundwater if not treated.

Main sources of wastewater

• Domestic (household) wastewater: water from toilets, kitchens, bathrooms, washing machines and sinks. It contains organic matter, food particles, grease, soaps and microbes.
• Industrial wastewater: water used in factories (textiles, tanneries, paper mills, chemical plants). It often contains harmful chemicals, dyes, heavy metals and oil.
• Agricultural runoff: water from fields that carries soil, fertilizers, pesticides and animal waste into nearby water bodies after rain or irrigation.
• Stormwater/urban runoff: rainwater that flows over roads, roofs and pavements collecting oil, dust, trash and chemicals.
• Commercial and institutional wastewater: from schools, hospitals, hotels and shops — may include food waste, disinfectants and medical waste.
• Leachate from landfills: water that percolates through garbage and dissolves harmful compounds.

Types of wastewater (by composition and effect)

• Biodegradable (organic) wastewater: contains substances (food waste, human waste) that microbes can break down. These increase Biological Oxygen Demand (BOD) in water bodies.
• Non-biodegradable (inorganic/toxic) wastewater: contains heavy metals, some chemicals and plastics that do not break down easily and are harmful.
• Pathogen-laden wastewater: contains disease-causing microbes (from toilets, hospitals) and causes health risks.
• Suspended-solids rich wastewater: has visible particles (soil, sand, grit) that increase turbidity.
• Nutrient-rich wastewater: contains excess nitrogen and phosphorus (from fertilizers, detergents) and can cause eutrophication (excessive plant growth) in lakes and rivers.

Why it matters: Untreated wastewater causes water pollution, spreads disease, kills aquatic life (due to low oxygen), and makes water unsafe for drinking and farming. Knowing sources and types helps design proper treatment methods.

Short pathway: Source (house/industry/field) → collection/drainage → (possible mixing with stormwater) → treatment (primary/secondary/tertiary) → discharge/reuse.

What is wastewater? Wastewater is water that has been used in homes, schools, farms or industries and contains solids, chemicals and microbes. It must be treated before being returned to nature.

Composition (what is in wastewater)

• Mostly water: Domestic wastewater is more than 99% water by weight. The remaining part contains different pollutants.
• Suspended solids: Small particles such as silt, food scraps, hair and fibres that make the water cloudy (turbidity).
• Dissolved solids and salts: Minerals, salts, dissolved household chemicals and detergents.
• Organic matter: Biodegradable material from food, human waste and plant matter. Microorganisms feed on this.
• Pathogens: Bacteria, viruses and parasites from human and animal waste.
• Oils and grease: From kitchens, vehicle runoff and some industries.
• Nutrients: Nitrogen and phosphorus from food waste, detergents and fertilizers (can cause algal growth in water bodies).
• Other chemicals: House-cleaners, medicines, paints or industrial chemicals (may be toxic).

Characteristics (how we describe wastewater)

• Physical: Colour, turbidity (cloudiness), temperature and odour. Turbidity is measured in NTU (nephelometric turbidity units).
• Chemical:
  • pH: Acidity or alkalinity on a scale 0–14 (7 is neutral).
  • Dissolved Oxygen (DO): Amount of oxygen dissolved in water. Healthy surface water has DO > 5 mg/L; polluted water often has low DO.
  • BOD (Biochemical Oxygen Demand): Oxygen required by microbes to decompose organic matter. High BOD means lots of organic pollution.
  • COD (Chemical Oxygen Demand): Oxygen required to chemically oxidize organic and some inorganic matter; usually higher than BOD.
  • TDS (Total Dissolved Solids): Total amount of dissolved salts and minerals (mg/L).
• Biological: Type and number of microorganisms present (e.g., coliform bacteria indicate faecal contamination).

Why these matter: High BOD/COD and low DO indicate the water will harm fish and plants. Pathogens can cause disease. Nutrients can lead to excessive algal growth (eutrophication). Suspended solids reduce sunlight and harm aquatic life.

Typical ranges (domestic wastewater, approximate): BOD: 100–400 mg/L; COD: 250–1000 mg/L; TDS: 200–2000 mg/L; DO: low in raw wastewater (<2 mg/L). These are classroom-level numbers to show the order of magnitude.

Simple treatment idea: Settling removes suspended solids, biological treatment reduces BOD by letting microbes eat organics, and disinfection (chlorine or sunlight) reduces pathogens.

Overview: Wastewater is water that has been used in homes, schools, industries and stormwater from roads. To protect health and the environment, wastewater must be collected and moved to a treatment facility or an appropriate disposal point.

Sources and how it is collected: Household wastewater comes from toilets, sinks, showers, washing machines and kitchen drains. Streets collect rainwater in gutters and storm drains. Wastewater from each building is connected by small pipes (house drains) to larger underground pipes (sewers) or open drains. Manholes give access for inspection and cleaning.

How wastewater moves: The main driving force for movement in most systems is gravity: pipes and drains are laid with a slight slope so water flows downhill to a sewer or treatment plant. Where gravity alone cannot move the water (flat ground, deep valleys, or tall buildings), pump stations lift the wastewater and push it along. There are two common systems: separate sewers (one for sewage, one for stormwater) and combined sewers (both flows in one pipe).

Key components: House connections (side drains), sewer lines (increasing sizes), manholes, pumping stations, stormwater drains, septic tanks (rural/isolated homes) and sewage treatment plants. Septic tanks collect solids and allow effluent to percolate into the ground; municipal sewers carry wastewater to a central treatment plant.

Problems and controls: Blockages (from fat, rags), illegal connections, overflowing in heavy rain, and contamination of water bodies are common problems. Regular cleaning, correct slope and cover design, use of grease traps, separate storm and sewage systems and pumping where needed reduce issues. Proper maintenance and treatment before discharge protect public health and ecosystems.

Simple science principle: The rate of movement (flow) depends on the pipe size, slope and roughness. Larger pipes or steeper slopes increase speed; rough or clogged pipes slow flow. Collecting wastewater in a planned network keeps streets clean and moves waste safely to treatment.

What is sewage? Sewage (wastewater) is the used water from homes, schools, industries and hospitals that contains organic matter, suspended solids, soaps, chemicals and microbes. Untreated sewage pollutes rivers, lakes and soil and is harmful to health.

What is an STP? A Sewage Treatment Plant (STP) is a facility that cleans sewage so that the treated water (effluent) can be safely returned to the environment or reused. The main goal is to remove solids, reduce harmful chemicals and kill disease-causing organisms.

Why do we need STPs?

• To protect water bodies and aquatic life.
• To prevent spread of water-borne diseases.
• To enable reuse of water for gardening, flushing or industry.

Main stages of treatment (simple overview)

• Preliminary treatment: Screens and grit chambers remove large objects (plastics, rags) and heavy sand/grit.
• Primary treatment (physical): Sedimentation tanks let suspended solids settle as sludge. Lighter oil/grease floats and is removed.
• Secondary treatment (biological): Microbes (bacteria) break down dissolved organic matter. Common methods include aeration tanks (activated sludge) and trickling filters. After biological action, the mixture goes to a secondary settling tank to separate the microbial biomass (sludge).
• Tertiary treatment (advanced/optional): Filtration, nutrient removal (nitrogen and phosphorus) and disinfection (chlorination or UV) to make water very safe for reuse or discharge.
• Sludge treatment: Sludge from primary and secondary treatment is thickened, digested (often anaerobically) to reduce pathogens and smell, then dried and safely disposed or used as manure after treatment.

How it works together (simple flow): Influent (sewage) → Screening → Grit removal → Primary sedimentation → Biological treatment (aeration/filters) → Secondary settling → Disinfection → Effluent out. Sludge from tanks → sludge treatment → disposal or use.

Safety & environment: Properly treated effluent reduces BOD (Biochemical Oxygen Demand), removes solids and pathogens so rivers do not become oxygen-depleted and aquatic life and human users remain safe.

Simple performance ideas: Effectiveness is often shown by how much BOD, suspended solids and bacteria are reduced. Small community STPs and septic tanks are examples for local use; large municipal STPs serve towns and cities.

What is sludge?
Sludge is the thick, semi-solid material that settles out from wastewater during treatment. It contains organic matter, tiny solids, microbes and sometimes harmful substances. Sludge is produced in both primary settling (primary sludge) and biological treatment (secondary or activated sludge).

Why must sludge be handled?
Fresh sludge contains water, smells bad, can carry disease-causing organisms (pathogens) and takes up space. Proper handling reduces smell and pathogens, recovers useful resources (like compost or biogas), and makes final disposal safe for the environment.

Main steps in sludge handling

• Thickening: Reduces water content so sludge becomes more concentrated. Methods: gravity thickeners or flotation.
• Stabilization: Reduces organic matter and kills many pathogens. Common methods: anaerobic digestion (in absence of oxygen) and aerobic digestion (with oxygen). Anaerobic digestion also produces biogas (methane).
• Dewatering: Removes more water so sludge becomes cake-like. Methods: drying beds, belt filter presses, centrifuges.
• Final disposal or reuse: Options include land application as soil conditioner or fertilizer (only after safe treatment), composting with organic wastes, incineration, or secure landfill. Choice depends on contamination level and local rules.

Safety and environmental points
Untreated sludge can spread disease and pollute water. Before using sludge on farmland, it must be stabilized and tested for pathogens and heavy metals. Workers must use protective equipment when handling sludge.

Practical benefits
Proper sludge handling turns waste into resources: stabilized sludge can improve soil (as compost) and anaerobic digestion produces biogas that can be used for cooking or electricity.

Simple classroom summary
Imagine the solids that settle in a glass of muddy water—that settled material is like sludge. In treatment plants, we concentrate and treat that material so it is safer and sometimes useful.

What are on-site sanitation systems?

On-site sanitation systems are arrangements that collect, treat and dispose of human excreta and household wastewater at or near the source (household, cluster of houses, or community) rather than transporting it through a sewer network to a distant treatment plant. Common on-site systems include pit latrines, twin-pit pour-flush latrines, septic tanks with soak pits, composting toilets and small biodigesters.

How they work (basic principles)

• Containment: Wastewater and fecal matter are directed into a closed pit or tank to prevent direct discharge into the environment.
• Separation and settling: In systems like septic tanks, solids settle to the bottom (sludge) and lighter materials (oils, grease, scum) float to the top. The middle layer (clearer liquid) is the effluent.
• Biological treatment: Anaerobic bacteria inside tanks or pits break down organic matter, reducing solids and odour.
• Disposal/infiltration: Treated liquid effluent is often released to ground through a soak pit or leach field where further natural treatment occurs in soil. In some systems (composting toilets) solids are decomposed and converted into compost.

Main types — short description

• Pit latrine: A simple covered pit dug in the ground. Waste collects in the pit and decomposes over time. Needs emptying or pit relocation when full.
• Twin-pit pour-flush latrine: Two pits used alternately. One fills while the other decomposes; after about 12–18 months the filled pit's contents are safe to remove as compost.
• Septic tank + soak pit: A watertight tank where solids settle and anaerobic digestion occurs; effluent flows to a soak pit or dispersal trench to percolate into soil.
• Composting toilet: Dry system where fecal matter is mixed with bulking material (e.g., sawdust) and composts aerobically to safe material for soil amendment.

Key design and operational points (class 7 level)

• Septic tanks are typically designed to provide a certain hydraulic retention time (HRT) — the time wastewater remains in the tank — to allow solids to settle and biological treatment to occur.
• Location matters: tanks and pits should be placed at safe distances from wells, rivers and groundwater (local regulations apply) to avoid contamination.
• Regular maintenance is essential: desludging of septic tanks, emptying or alternating pits, and keeping soak pits free of clogging materials.
• Soil type controls infiltration: sandy soils percolate well; clayey soils permit less infiltration and may require different designs.

Advantages

• Can be installed where sewer networks do not exist.
• Lower initial cost and simple technology for rural and peri-urban areas.
• Twin-pit systems can safely convert human waste into compost for non-food uses.

Limitations and environmental concerns

• Poorly designed or maintained systems can contaminate groundwater with pathogens, nitrates and other pollutants.
• Odour and fly breeding if pits/tanks are not sealed or vented properly.
• Septic systems do not remove all pathogens or nutrients; effluent disposal must be managed carefully.

Simple safety and maintenance tips

• Never pour oil, grease, large solids or chemicals into the system; these cause clogging and kill helpful bacteria.
• Ensure periodic desludging of septic tanks by trained workers and transport sludge to an approved treatment facility.
• Keep trees a safe distance from tanks and soak pits to avoid roots damaging the structure.

For Class 7 students: think of on-site sanitation as a local mini-treatment plant under or near your house that uses natural settling and bacteria to make wastewater safer before it soaks into the ground or turns into compost.

What is untreated wastewater? Untreated wastewater is water that has been used in homes, industries or farms and released into the environment without proper cleaning. It contains organic matter, pathogens (bacteria, viruses, parasites), nutrients (nitrate, phosphate), and sometimes toxic chemicals or heavy metals.

Environmental impacts

• Pollution of rivers, lakes and groundwater: Pathogens and chemicals make water unsafe for drinking, bathing or irrigation.
• Oxygen depletion and fish kills: Organic matter in wastewater is broken down by bacteria that consume dissolved oxygen (DO). Low DO levels starve fish and other aquatic life.
• Eutrophication and algal blooms: Excess nutrients (nitrates and phosphates) cause rapid algae growth. When algae die and decompose they further reduce DO and create foul smell and dead zones.
• Soil and crop contamination: Using untreated wastewater for irrigation can add harmful salts, heavy metals and pathogens to soil and crops, reducing soil fertility and making food unsafe.
• Bioaccumulation: Toxic chemicals (like heavy metals) build up in aquatic organisms and move up the food chain, affecting birds, animals and humans who eat them.
• Loss of biodiversity: Polluted water bodies support fewer species; sensitive plants and animals disappear.

Health impacts

• Waterborne diseases: Drinking or coming in contact with untreated wastewater can cause cholera, typhoid, diarrhoea, dysentery, hepatitis A, and parasitic infections.
• Acute poisoning and chronic effects: Chemicals and heavy metals can cause headaches, stomach problems, nervous system damage and long-term diseases including cancer.
• Methemoglobinemia ("blue baby" syndrome): High nitrate levels in drinking water can reduce the ability of infants' blood to carry oxygen.
• Skin and eye infections: Bathing or washing in contaminated water can cause rashes, eye irritation and other infections.
• Respiratory problems: Gases and foul-smelling compounds from polluted water can cause irritation and breathing difficulties in nearby communities.

How it affects daily life and economy

• Unsafe water increases illness and healthcare costs and reduces school and work attendance.
• Fisheries and tourism decline when water bodies are polluted.
• Agricultural productivity falls if soil is damaged by salts, pathogens or toxic substances.

Simple prevention ideas: Install toilets and septic systems, treat wastewater in sewage treatment plants, avoid dumping industrial effluents untreated, practise safe irrigation, and raise community awareness about sanitation and hygiene.

Overview
Reuse, recycling and pollution prevention are three allied approaches to manage wastewater and other wastes so that we conserve resources, protect the environment and reduce the amount of pollutants reaching rivers, lakes and soils.

Definitions

• Reuse – Using a product or resource again for the same or a different purpose without major reprocessing (for example, using household greywater to water plants).
• Recycling – Converting a wasted material into raw material that can be processed into a new product (for example, melting glass bottles to make new glass).
• Pollution prevention – Actions taken at the source to reduce or eliminate the generation of pollutants (for example, using less detergent or changing industrial processes to produce less waste).

Why these matter
They reduce demand for fresh water and raw materials, lower energy use, decrease landfill loads and minimize harmful discharges (BOD, suspended solids, nutrients, toxic chemicals) into the environment.

How reuse works (examples and methods)

• Domestic greywater (bath/sink water) can be reused for flushing toilets and garden irrigation after simple screening or filtration.
• Industrial process water can be treated and recirculated within the plant (cooling towers, rinsing processes) to reduce freshwater intake.
• Rainwater harvesting collects roof runoff for non-potable uses (gardening, toilet flushing).

How recycling works
Recycling involves collection, sorting, cleaning and reprocessing. Common recycled streams: paper, glass, metals, plastics, and organic wastes (composting).

Pollution prevention practices

• Source reduction: use less packaging, buy concentrated products, avoid single-use items.
• Substitution: choose biodegradable/less-toxic chemicals (phosphate-free detergents).
• Process changes: modify industrial methods to generate fewer wastes (closed-loop systems).
• Proper operation and maintenance: fix leaks, install oil traps, maintain sewage systems.
• Education and segregation: separate wet (compostable) and dry wastes at home/school to make recycling and composting effective.

Connection with wastewater treatment
Wastewater treatment reduces pollutants so water can be safely reused or released. Typical stages are:

• Primary treatment – screening and sedimentation to remove large solids and settleable matter.
• Secondary treatment – biological processes (activated sludge, biofilters) to reduce BOD and suspended solids.
• Tertiary treatment – advanced removal of nutrients (nitrogen, phosphorus), pathogens and fine particulates to make water suitable for reuse.

Environmental and economic benefits

• Conserves freshwater and reduces pressure on rivers and groundwater.
• Saves energy and raw materials (recycling metals saves large amounts of energy compared to extracting ores).
• Reduces contamination of ecosystems and human health risks.
• Can reduce municipal costs for water supply and waste disposal.

Practical tips for students and households

• Reuse jars and containers; repair items instead of discarding them.
• Collect greywater (from baths and sinks) in a bucket and use it for plants—avoid using water contaminated with strong chemicals.
• Separate organic waste for composting to recycle nutrients and reduce landfill methane.
• Use a dustbin with separate compartments and follow local recycling rules.
• Save water: turn off taps while brushing, fix leaking taps and cisterns.

Overview
The chapter 'Wastewater Story' introduces the water that has already been used (wastewater), its types, what pollutants it carries, and how it is treated before being returned to the environment or reused. Understanding key terms helps explain why treatment is needed and how different processes remove different kinds of pollution.

• Wastewater (Sewage) — Water that has been used in homes, schools, industries or that runs off streets and fields. It contains organic matter, chemicals, solids and microbes.
• Domestic wastewater — From households: bathing, washing, toilets, kitchen. Often contains food waste, detergents and pathogens.
• Industrial wastewater — From factories and plants; may contain hazardous chemicals, dyes, heavy metals and high temperature water.
• Stormwater / Runoff — Rainwater that flows over roads and fields picking up oil, pesticides, and trash.
• Pollutant load (mass load) — Amount of a pollutant delivered per unit time (e.g., kg/day). It equals concentration × flow.
• BOD (Biochemical Oxygen Demand) — A measure of the amount of oxygen required by microbes to break down biodegradable organic matter in water. High BOD indicates more organic pollution and greater stress on aquatic life.
• COD (Chemical Oxygen Demand) — Amount of oxygen needed to chemically oxidize organic and inorganic substances in water; often higher than BOD.
• Suspended solids (SS) — Small solid particles suspended in water; they can make water cloudy and settle to form sludge.
• Pathogens — Disease-causing microorganisms (bacteria, viruses, protozoa) present in untreated sewage.
• Primary treatment — Physical processes (screening, grit removal, sedimentation) to remove large solids and settleable particles.
• Secondary treatment — Biological processes (aeration tanks, activated sludge, biofilms) where microbes decompose organic matter, reducing BOD.
• Tertiary treatment — Advanced physical/chemical/biological processes (filtration, nutrient removal, disinfection) to remove remaining contaminants and pathogens.
• Sludge — Solid material that settles out during treatment. It is treated separately (thickening, digestion, dewatering) before safe disposal or use.
• Effluent — Treated water released from a treatment plant. It should meet standards before discharge or reuse.
• Disinfection — Process (chlorination, UV) to kill or inactivate pathogens before water is discharged or reused.
• Reuse / Recycle — Using treated wastewater for irrigation, industrial cooling, or groundwater recharge to conserve freshwater.

Why these concepts matter
Untreated wastewater can reduce oxygen in rivers (through high BOD), spread diseases, and harm ecosystems. Treatment stages progressively remove solids, organic matter and pathogens so water can be safely returned or reused.

Simple process summary
Screening → Grit removal → Primary sedimentation (settling) → Secondary (biological) treatment → Secondary sedimentation → Tertiary treatment/disinfection → Discharge or reuse. Sludge is collected and treated separately.