Fertilising water bodies

Cities grew steadily during the 20th century and water consumption and wastewater loads increased. In addition to being affected by increasing loads, water quality also suffered as a result of the increased use of chemicals in everyday life. Since the beginning of the century nitrogen loading has increased fourfold and phosphorous eightfold. As a result pollution and eutrophication problems have become commonplace in all cities.

Today most cities treat wastewater, but even after treatment the water contains organic matter, nutrients and bacteria. This load is commonly directed into the open sea or outside city limits - out of sight. As a result, the formerly polluted urban water bodies have been recovering during recent decades. Meanwhile, the open sea is showing the signs of severe eutrophication: blue-green algal blooms are becoming common. Many of these algae species are poisonous and are unwelcome in shore waters.

The impacts of nutrients

When we dump our waste and wastewater into a water body, we add nutrients to it. These include phosphates from soaps, ammonia from urea, and organic matter that breaks down into nutrients. Nutrients are needed for plant growth, but over-fertilisation of a water body occurs easily. If the nutrient load is limited, the receiving water body may succeed in coping with it. But if the nutrient load is pronounced, the increase of plant production is high and its consequences are notable: increased shore vegetation, algal blooms, slime production, turbidity, etc.

Algal growth has several consequences. Increased plant production means that more decaying matter is produced. In normal conditions, aerobic bacteria decompose and need oxygen to break down the dead biomass that eventually settles to the bottom. Increased oxygen consumption leads to depletation and finally to total anoxia at the bottom. As anaerobic bacteria continue decomposition of the organic matter, hydrogen sulphide is produced and released to the surface, causing bad odours. In anoxic conditions nutrients stored in the sediments are also released. Under these conditions the water body fertilises itself. Enhanced phytoplankton production will decrease transparency, which in turn gradually limits production and increases decomposition. Continuous nutrient loading eventually results in the eutrophication of all water bodies with limited water exchange and volume.