VMBO Year 3 · Sustainable living · Section 7.6

Water

What happens to organic material in water?

Surface water occurs in rivers, lakes, ponds and seas. Groundwater fills spaces in soil and rock below the water table. Dead plant and animal material enters water and is broken down by microorganisms, including bacteria and fungi.

This contributes to water's self-purification capacity:

• Decomposers break down organic material.
• Aerobic decomposition uses dissolved oxygen.
• Carbon dioxide and mineral nutrients are released.
• Producers can take up these nutrients again.

This capacity is limited. Natural decomposition does not remove every pollutant or make water automatically safe to drink.

Nutrient enrichment and algal blooms

Eutrophication occurs when excessive nutrients, particularly nitrogen and phosphorus compounds, enter water. Sources include agricultural runoff, wastewater and some industrial discharges. Nutrients can trigger an algal bloom.

• Dense algae make the water cloudy and block light.
• Submerged plants receive less light and may die.
• Dead algae and plants increase the amount of material to decompose.
• Decomposers use large amounts of oxygen.
• Oxygen shortage can kill fish and other aquatic animals.

Severely oxygen-depleted water may develop unpleasant smells. Some blooms also include organisms producing toxins.

Other forms of water pollution

Water may contain harmful chemicals, including some metals and cleaning agents, or pathogens making it unsafe to drink. Litter such as plastic also causes damage. Large amounts of marine plastic pollution are sometimes called plastic soup. This consists of scattered material of many sizes, not a continuous solid island. Animals may ingest plastic or become entangled, and some particles can pass through food webs.

Wastewater treatment

A wastewater treatment plant removes many pollutants in stages.

• Screens trap large pieces of waste.
• Primary settling tanks allow settleable solids to sink.
• Aeration tanks supply oxygen so microorganisms break down organic substances.
• Secondary settling tanks separate microbial sludge from treated water.

Some plants include additional steps to remove nutrients or other pollutants. Treated wastewater can be discharged under regulated conditions; it is not automatically drinking water.