Defluoridation of water by the Homa* method, a co-precipitation technique using wood ash leachate and alum

Abstract

High fluoride level in drinking water is an endemic public health concern in East Africa. Unlike in Kenya where it is absent, the Nalgonda technique, a defluoridation method that uses two chemicals, alum, and CaO, has seen mixed results in its application and adoption in Ethiopia and Tanzania. This has been due to the low capacity of communities to manage the process and the breakdown in the supply chain of chemicals used in the technique. In the present study, we attempted to bridge the gap in the chemical deficit by investigating the possible substitution of CaO with leachate from wood ash, a by-product of wood combustion commonly found in Kenya. The leachate was prepared from one part of wood ash mixed with two parts of distilled water and stirred for 24 hours followed by decantation. The new technique, the Homa method, using alum and wood ash leachate was then tested on H2O samples from three areas in Kenya with high F- concentrations ranging from 5.1 mg L-1, 9.1 mg L-1 to 91.0 mg L-1. The determination of F- concentration by SPADNS Spectrophotometry was applied throughout the experiment. Four levels of alum i.e. 1%, 2%, 3%, and 4% were dosed on five volumes of water i.e. 100, 200, 300, 400, and 500 ml raw water at 5.1 and 9.1 mg L-1 F-. For water samples at 91.0 mg L-1 F-, the same volumes were treated with 5 higher alum levels i.e. 5%, 6%, 7%, 8%, and 9%. The final pH was then adjusted to 7 with ash leachate for defluoridation. The set-up was a factorial design experiment where the final F- concentration was the dependent variable and the volume of raw water, the percentages, and volume of alum and wood ash leachate constituted the different factors. A fitted multivariate regression model of the general form; where Y = Residual fluoride, X = wood Leachate volume, W = alum Concentration, X*W = Interaction α, β, γ were regression coefficients, ε = error term, showed that only in the Baringo area did we have an interaction between wood ash leachate and alum concentration significant (p < 0.05). Defluoridation occurred (p < 0.05) at as low as 10% and as high as 99%, depending on the initial F- content. Total coliform decreased from 310, 290 and 270 count/l respectively to zero. Unfortunately, high chemical and TDS (from 558 mg L-1 to more than 9,000 mg L-1) enrichment were recorded in addition to the mixed data on turbidity. The overall results show that wood ash can substitute CaO in the Nalgonda process. Further investigation is however required to make it applicable for potable water production.