Chemicals are difficult to eliminate during wastewater treatment. In this article I discuss two harmful chemicals found in wastewater that any treatment system must first seek to eliminate:

1. Nitrate: nitrates are widely distributed in soil, water and plants and are found in most foods and drinking waters. The excess nitrate in wastewater may come from agricultural sources, because nitrates form the basis of many fertilizers. Nitrate as a chemical in wastewater can also come from domestic and industrial effluents, and decaying animal and vegetable matter. Symptoms of acute poisoning include headache, vomiting, flushing of the skin, hypotension, collapse, convulsions and coma.

In infants, methaemoglobinaemia can develop after exposure to high levels of nitrate. Methaemoglobinaemia occurs when the ferrous ion of haemoglobin is oxidized to the ferric form. This form of haemoglobin is less effective at carrying oxygen. The earliest clinical feature is cyanosis, but after 30 – 40% of haemoglobin has been converted to methaemoglobin, there is weakness and excertional dyspnoea. After about 60% has been converted, respiratory depression and stupor develop. Death may follow soon after. Diagnosis of acute nitrate poisoning is aided by the demonstration of methaemoglobin in blood samples. Treatment is by giving oxygen and possibly the antidote, which is methylene blue or ascorbic acid.

2. Sodium: the relative importance of drinking-water sodium depends on the sodium concentration in water and the total sodium intake from all sources. In adults acute poisoning from sodium chloride is virtually unheard of due to the efficient way that the kidneys handle sodium. Salt has been used therapeutically as an emetic, and large oral intakes will usually induce vomiting, with the ejection of the salt. However, hypernatraemia may develop in the very young and in those with impaired renal function. Clinical features of hypernatraemia may include cerebral and pulmonary oedema with convulsions, muscle twitching and breathlessness.

For most healthy adults, drinking-water sodium contributes only marginally to daily sodium intake. Nevertheless, it is important to eliminate this chemical in wastewater before discharge into water bodies. For the very young and those on severely reduced sodium intakes for medical reasons, water-sodium levels can cause problems when moderately elevated. Sodium in wastewater can contaminate drinking water sources leading to the above mentioned problems.



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