Sewage sludge as an acidity filter for groundwater-fed lakes

Abstract

The acidification of lakes by acid rain has increased awareness of the problems associated with acid waters¹, and has stimulated work on self-neutralization mechanisms² and liming strategies³. However, some very acid lakes, particularly those which have been newly formed by the widespread extraction of coal, mineral ores and sand and gravel, owe their acidity to another source. If pyrite is present, as it often is, its exposure to air induces microbially mediated oxidation, resulting in the production of sulphuric acid⁴. The waters of the lakes then resemble dilute acid, typically with pH <3, with an impoverished flora and fauna. Neutralizing such waters with lime ameliorates the situation, but only temporarily because further supply of acidic groundwater inexorably lowers the pH. Here I report the use of a combined treatment of lime and sewage sludge, which has produced a self-regulating system, capable of maintaining a stable pH. The organic material which is spread over the bottom of the lake acts as a chemical filter, removing acidic sulphate as it enters, and converting it to neutral sulphide.