Primary pollution Historically speaking, it was not so very long ago that lakes, rivers and coastal waters were clean and supported a balanced aquatic plant and animal life. This process is known as primary pollution. The more organic material there is in sewage, the greater the amount of oxygen needed to support the decomposition of these pollutants and, consequently, the greater the primary pollution. These fertilized plants, in turn, die and decompose, further robbing the water of its naturally dissolved oxygen. The principal nutrients causing secondary pollution are nitrogen, phosphorous and potassium.
Secondary pollution is measured by how much fertilizer is added to water.
To understand their growth potential in water, it is necessary to know which nutrients in it are in short supply in the water. Some lakes are growth-restricted by the lack of phosphorous, others by lack of nitrogen and others, yet again, by the lack of potassium. Generally speaking, combined sewage is rich in all three nutrients and contributes greatly to unbalanced plant growth in water, be it in a lake, a stream, or an estuary. The diagram below gives a rough idea of primary and secondary pollution derived from flush-toilets and various greywater sources. None of these sources carries water which is likely to contain disease organisms of anywhere the same magnitude as those in toilet wastes.