This is aimed at removing dissolved materials which have survived the primary and secondarily treatment. The most important solutes are compounds of phosphorous and nitrogen.
Phosphorous is more important than nitrogen in causing eutrophication, which is the process which results in excessive and undesirable growth of plant life in lakes and harbours.
Other toxic substances may also be present and special processes are used in each case. The end result should be water which is suitable for drinking.
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Nitrogen is more difficult to remove than phosphorus, but, where it is in the form of NH3, it can be tripped by air at pH 11.0. This means that effluent, from the lime treatment, is particularly suitable for ammonia removal.
Apart from nitrogen and phosphorus, there are a variety of substances which contribute to the appearance and even the taste of the water. These can be removed by filtration through certain media, including activated charcoal. The charcoal can be reactivated and reused.
Many of the substances, which require removal by tertiary treatment, can theoretically be removed by biological processes, particularly in conjunction with the activated-sludge- step of the secondary treatment.
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Care must be taken, however, not to upset the chemistry to such an extent that the beneficial organisms are harmed. In conditions of adequate sunlight, plants such as algae can consume phosphate and nitrates.
For this to be effective, the plants have to be removed at regular intervals. If successful, this method will have the added advantage when aquatic plants can accumulate, and, thus, remove toxic metals.