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examBoard: Cambridge
examType: IGCSE
lessonTitle: Domestic Waste in Urban Areas
Urban areas are home to large concentrations of people, with over 56% of the world's population now living in cities. This concentration creates enormous amounts of domestic waste that can pollute water sources if not properly managed. In this lesson, we'll explore how everyday activities in our homes and cities contribute to water pollution.
Key Definitions:
Every day, the average person produces about 150 litres of wastewater. This includes water from toilets, showers, sinks and washing machines. In cities with proper infrastructure, this wastewater travels through sewers to treatment plants. However, in many urban areas worldwide, sewage systems are inadequate or non-existent, leading to untreated sewage entering rivers, lakes and coastal waters.
Rain that falls on roofs, roads and pavements washes pollutants into storm drains. This urban runoff can contain oil, petrol, litter, pet waste and garden chemicals. Unlike sewage, urban runoff often flows directly into water bodies without treatment, making it a significant source of water pollution in cities.
Untreated sewage is one of the most harmful forms of water pollution from urban areas. It contains:
When sewage enters water bodies, it increases the Biochemical Oxygen Demand (BOD), reducing available oxygen for fish and other aquatic organisms. High BOD levels can create "dead zones" where few organisms can survive.
Contains high levels of nitrogen and phosphorus, contributing to eutrophication. Also contains pathogens that can spread disease.
Cleaning products, medicines and personal care items contain chemicals that can harm aquatic life, disrupt hormones and persist in the environment.
Tiny plastic particles from clothing, cosmetics and breakdown of larger plastics enter waterways through washing machines and urban runoff.
When domestic waste enters water bodies, it can have several serious consequences:
In the 1950s, the River Thames was so polluted with sewage and industrial waste that it was declared biologically dead. No fish could survive in its waters. The main cause was untreated sewage from London's growing population.
Following the implementation of sewage treatment works and stricter regulations, the river has made a remarkable recovery. Today, the Thames supports over 125 fish species and is one of the cleanest metropolitan rivers in the world. However, the system still faces challenges during heavy rainfall when combined sewer overflows release untreated sewage into the river.
The Thames Tideway Tunnel, a major infrastructure project due for completion in 2025, aims to capture and store sewage that currently overflows into the river during rainstorms, further improving water quality.
Modern households use numerous chemical products that can end up in water systems:
Phosphates in laundry and dishwashing detergents act as fertilisers in water bodies, promoting algal growth. Many countries have now banned or limited phosphates in detergents, but they remain a problem in some regions. Even "biodegradable" detergents can cause problems if they reach water bodies before breaking down.
Products like paint, solvents, batteries and electronic waste contain toxic chemicals that can leach into groundwater from landfills or enter water systems if poured down drains. Even small amounts of these chemicals can be harmful to aquatic life and potentially contaminate drinking water sources.
Unlike sewage, which typically comes from identifiable pipes (point sources), much of urban water pollution comes from diffuse sources across the urban landscape:
Effective management of domestic waste is essential to prevent water pollution in urban areas. Key strategies include:
Using fewer harmful chemicals, proper disposal of hazardous waste and reducing water usage can all help prevent pollution at the source.
Modern sewage treatment plants, separate stormwater systems and sustainable urban drainage systems (SUDS) can reduce pollution from urban areas.
Laws governing sewage treatment, product ingredients and waste disposal help ensure domestic waste is managed properly.
Modern sewage treatment typically involves three stages:
Singapore, a densely populated island city-state with limited freshwater resources, has developed an innovative solution to its water challenges. NEWater is high-grade reclaimed water produced from treated sewage.
The process involves three stages: microfiltration to filter out solids, reverse osmosis to remove bacteria and viruses and ultraviolet disinfection as a final safety measure. The resulting water is cleaner than most tap water and meets World Health Organisation drinking water standards.
While most NEWater is used for industrial purposes and air conditioning cooling, a small percentage is added to reservoirs for indirect potable use. This approach has helped Singapore reduce its dependence on imported water and provides a model for sustainable urban water management.
As cities continue to grow, sustainable approaches to managing domestic waste are becoming increasingly important:
Understanding the sources and impacts of domestic waste is the first step toward protecting our water resources. As urban populations continue to grow, managing this waste sustainably will become increasingly important for environmental and public health.
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