C34: Waste Water Treatment
How waste water and sewage are treated to make them safe before release into the environment, including the key stages of sewage treatment and handling industrial waste water.
What is Waste Water?
Waste water is water that has been used and contaminated. It includes domestic sewage, industrial effluent and agricultural run-off. Waste water must be treated before it can be released into the environment.
Waste water comes from several sources:
- Domestic sewage — from homes (toilets, sinks, showers, washing machines)
- Industrial waste water — from factories and manufacturing processes
- Agricultural run-off — containing fertilisers, pesticides and animal waste
- Surface run-off — rainwater carrying pollutants from roads and urban areas
Untreated waste water contains:
- Organic matter (human waste, food waste)
- Harmful bacteria and pathogens
- Dissolved chemicals and heavy metals
- Solid debris and litter
Sewage Treatment Stages
Sewage treatment involves several stages to progressively remove contaminants from waste water.
Stage 1: Screening
Raw sewage passes through large mesh screens. These remove large solid objects such as sticks, rags, plastic and other debris. The screened material is removed and disposed of separately. Grit and gravel also settle out at this stage in grit channels.
Stage 2: Primary Treatment (Sedimentation)
The screened sewage flows into large settlement tanks called primary sedimentation tanks. Here, the flow rate is slowed down so that heavier solid particles settle to the bottom as sludge. The lighter material floats to the surface and is scraped off. The liquid that remains is called effluent and flows on to the next stage.
Stage 3: Secondary Treatment (Biological/Aerobic Treatment)
The effluent is treated with aerobic microorganisms (bacteria) that break down the remaining organic matter. This can be done in two ways:
- Trickling filter beds — effluent is sprayed over beds of stones coated with aerobic bacteria. Air circulates through the bed, providing oxygen for the bacteria.
- Activated sludge tanks — effluent is mixed with activated sludge (containing aerobic bacteria) and air is bubbled through the tank to supply oxygen.
The bacteria break down organic waste through aerobic respiration, producing carbon dioxide and water.
Stage 4: Anaerobic Digestion of Sludge
The sludge collected from the primary sedimentation tanks is broken down by anaerobic bacteria in large heated tanks called anaerobic digesters. This process:
- Produces methane gas — which can be burned to generate electricity or heat the digester
- Reduces the volume of sludge
- Produces a remaining solid that can be used as fertiliser or sent to landfill
Stage 5: Final Treatment and Effluent Release
After biological treatment, the effluent passes into final settlement tanks where any remaining bacteria settle out. The treated effluent may then be disinfected (using UV light or chlorine) before being released into rivers or the sea. The water should now be clean enough not to harm the environment.
The overall process: Screening → Primary sedimentation → Aerobic biological treatment → Anaerobic digestion of sludge → Effluent release
Summary of Sewage Treatment Stages
| Stage | Process | What is Removed | Method |
|---|---|---|---|
| Screening | Mechanical | Large solids and grit | Mesh screens and grit channels |
| Primary sedimentation | Physical | Suspended solids (sludge) | Settlement tanks — gravity |
| Aerobic treatment | Biological | Dissolved organic matter | Aerobic bacteria with oxygen |
| Anaerobic digestion | Biological | Organic matter in sludge | Anaerobic bacteria without oxygen |
| Effluent release | Physical/Chemical | Remaining bacteria | Settlement and disinfection |
Anaerobic vs Aerobic Digestion
| Feature | Aerobic Treatment | Anaerobic Digestion |
|---|---|---|
| Oxygen required | Yes | No |
| Treats | Liquid effluent | Sludge |
| Products | CO₂ and water | Methane and remaining solid |
| Conditions | Oxygen-rich, ambient temperature | Oxygen-free, heated tanks |
| Energy value | None — consumes energy for aeration | Produces methane for energy |
| Speed | Relatively fast | Slower (weeks) |
Remember: aerobic treatment uses oxygen to break down organic matter in the effluent. Anaerobic digestion breaks down sludge without oxygen and produces useful methane gas.
Industrial Waste Water
Industrial waste water often contains different pollutants compared to domestic sewage and may require specialised treatment.
Industrial waste water may contain toxic chemicals, heavy metals, acids or alkalis that require specific treatment processes before release.
Types of industrial waste water and their treatment:
- Heavy metals (e.g. lead, mercury, cadmium) — removed by chemical precipitation, where a reagent is added to form an insoluble metal compound that can be filtered out
- Acidic or alkaline waste — neutralised before release (acids neutralised with alkalis, alkalis neutralised with acids)
- Organic solvents — removed by distillation or activated carbon filtration
- Oil and grease — separated using oil-water separators or dissolved air flotation
Neutralising Industrial Waste Water
An industrial waste water sample has a pH of 2 (strongly acidic). Before release, it must be neutralised. An alkali such as sodium hydroxide or calcium hydroxide (lime) is added to bring the pH close to 7. The neutralised water can then be safely released or sent for further treatment.
Industrial waste water often needs additional treatment steps compared to domestic sewage because it can contain hazardous chemicals not found in normal sewage.
Environmental Impact of Untreated Waste Water
If waste water is not properly treated before release, it can cause serious environmental problems:
- Eutrophication — excess nutrients (nitrates and phosphates) cause algal blooms, which deplete oxygen in water and kill aquatic organisms
- Disease — pathogens in untreated sewage can spread waterborne diseases such as cholera and typhoid
- Toxicity — heavy metals and chemicals can poison aquatic life and accumulate in food chains
- Oxygen depletion — organic waste is decomposed by aerobic bacteria, which use up dissolved oxygen, suffocating fish and other organisms
Biological Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen needed by aerobic bacteria to break down organic matter in water. High BOD indicates high levels of pollution.
Potable Water Production from Waste Water
In some regions, treated waste water undergoes additional processing to become potable water. This involves:
- Advanced filtration (including reverse osmosis)
- UV disinfection
- Adding chlorine for residual protection
- Adding minerals back to improve taste
This is more common in areas with limited fresh water supplies but is carefully monitored to ensure safety.
Practice Questions
1. Describe the stages involved in sewage treatment.
Screening removes large solids using mesh screens. Primary sedimentation allows suspended solids to settle as sludge in settlement tanks. Aerobic biological treatment uses aerobic bacteria to break down dissolved organic matter in the effluent. Anaerobic digestion of sludge by anaerobic bacteria produces methane gas. Final treatment involves settlement and disinfection before effluent release.
2. Explain the difference between aerobic and anaerobic treatment in sewage works.
Aerobic treatment uses bacteria with oxygen to break down organic matter in the liquid effluent, producing CO₂ and water. Anaerobic digestion uses bacteria without oxygen to break down the sludge, producing methane gas which can be used as fuel. Aerobic treatment treats the liquid; anaerobic treatment treats the solid sludge.
3. Why must industrial waste water often receive additional treatment compared to domestic sewage?
Industrial waste water can contain toxic chemicals, heavy metals, and strong acids or alkalis that are not found in domestic sewage. These require specific treatment processes such as chemical precipitation, neutralisation, or distillation before the water can be safely released into the environment.
4. What useful product is formed during anaerobic digestion of sludge and how can it be used?
Methane gas is produced during anaerobic digestion. It can be burned as a fuel to generate electricity or to heat the anaerobic digester tanks, making the process more energy-efficient and sustainable.
5. Explain how acidic industrial waste water can be made safe for release.
Acidic waste water is neutralised by adding an alkali such as sodium hydroxide or calcium hydroxide (lime). The alkali reacts with the acid to form a neutral salt and water, bringing the pH close to 7 so it is safe to release into the environment.
Common Misconceptions
Misconceptions About Waste Water Treatment
Treated sewage is clean drinking water.
Treated sewage meets environmental standards for safe release into rivers or the sea, but it does NOT meet the standards for drinking water. Treated effluent still contains some dissolved substances and may have low levels of bacteria. It would need further extensive treatment before it could be safe to drink.
Bacteria in sewage treatment are harmful and should be killed.
The bacteria used in sewage treatment are essential — they are aerobic and anaerobic microorganisms that break down organic matter in the waste water. Without these bacteria, the treatment process could not work. They are carefully managed and are different from the harmful pathogenic bacteria that the treatment aims to remove.
Screening removes all harmful substances from sewage.
Screening only removes large solid objects like sticks, rags and plastic. It is just the first stage. Harmful dissolved substances, pathogens and organic matter are removed by later stages including sedimentation, biological treatment and chemical treatment.
6-Mark Question
Describe the stages of sewage treatment and explain the role of bacteria.
Sewage treatment involves several stages. First, screening removes large solid objects like sticks and rags using mesh screens. In primary treatment, the sewage flows into large settlement tanks where heavier solids sink to the bottom as sludge and lighter materials float to the surface as scum — both are removed. In secondary (biological) treatment, the liquid effluent is broken down by aerobic bacteria. This can happen in filter beds where the effluent trickles over stones coated with microorganisms, or in activated sludge tanks where air is bubbled through the effluent to supply oxygen for the bacteria. The aerobic bacteria respire and break down the organic matter in the effluent into harmless products like CO2 and water. The treated effluent then passes to final settlement tanks where any remaining bacteria settle out. The sludge from the primary stage is treated separately by anaerobic digestion, where bacteria break it down in the absence of oxygen, producing methane gas which can be burned as fuel and a harmless solid residue that can be used as fertiliser. Finally, the treated effluent is discharged into rivers or the sea.
AO3: Analysis and Evaluation
Comparing Waste Water and Potable Water Treatment
Both waste water treatment and potable water treatment aim to produce safe water, but the processes and standards are very different.
Compare and evaluate the two processes, explaining why different standards are applied.
Answer: Potable water treatment starts with relatively clean fresh water and removes suspended particles and microbes through filtration, sedimentation and chlorination. The standards are very strict because the water must have very low levels of bacteria and dissolved substances to be safe for human consumption. Waste water treatment starts with highly contaminated sewage and progressively removes solids, organic matter and harmful microorganisms. The treated effluent must meet environmental standards for release into waterways, but these standards are less strict than drinking water standards because the water will not be directly consumed — it will be further diluted and naturally purified in rivers. Applying drinking water standards to sewage effluent would be prohibitively expensive and unnecessary. The two processes differ in complexity, cost and energy because they serve different purposes.
Maths Skills
Interpreting Waste Water Treatment Data
Waste water treatment data often includes measurements of Biochemical Oxygen Demand (BOD), suspended solids, and concentrations of pollutants before and after treatment.
BOD measures the amount of oxygen consumed by microorganisms breaking down organic matter in the water. High BOD indicates high levels of organic pollution.
Example: Untreated sewage has a BOD of 300 mg/dm3. After biological treatment, the BOD is 20 mg/dm3. Calculate the percentage reduction:
Percentage reduction = ((300 - 20) / 300) x 100 = 93.3%
Comparing treatment efficiency: If Treatment Plant A reduces suspended solids from 250 mg/dm3 to 15 mg/dm3 and Plant B reduces from 250 mg/dm3 to 30 mg/dm3, Plant A achieves (250-15)/250 x 100 = 94% removal while Plant B achieves 88% removal. Plant A is more efficient.
Flow rate calculations: If a treatment plant processes 500,000 m3 of waste water per day and the population is 1,000,000, the water use per person per day is 500,000 / 1,000,000 = 0.5 m3 = 500 litres per person per day.
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