Effects of a water treatment residue, lime, gypsum, and polyacrylamide on the water retention and hydraulic conductivity of two contrasting soils under field conditions in KwaZulu-Natal, South Africa

Abstract

Water treatment residue (WTR), a waste by-product of the 'drinking' water treatment industry, consists mainly of clay and fine silt flocculated out of suspension by chemicals such as polyelectrolytes and aluminium sulfate. This residue was disposed of almost exclusively in landfill, but land treatment is increasingly being seen as a possible alternative for this form of waste. A key concern, however, is that should the WTR decompose to its constituent fractions, this could cause blockage of pores and decrease the hydraulic properties of soil. To test this hypothesis, 2 field experiments were established on physically contrasting soils in KwaZulu-Natal, South Africa. Application rates from 0 to 1280 Mg/ha of WTR were compared with amendments of lime, gypsum, and polyacrylamide to determine their effects on water retention and hydraulic conductivity. Although after 3 years at one experiment and 2 years at the second differences between the WTR treatments were measurable, only the 1280 Mg/ha application rate was significantly different from the control. The chemical amendments that were applied purely as comparative treatments had no significant influence on either water retention or hydraulic conductivity. Neither soil showed any difference in readily available water content. The experiments showed that, in both soil types, the WTR increased water retention and hydraulic conductivity but that improvement in both of these properties was only significant at the 1280 Mg/ha application rate. Thus, very high amounts of WTR must be added to affect these physical properties of soils.