Some soil chemical and fertility aspects of the land disposal of a water treatment residue on selected soils of KwaZulu-Natal, South Africa.

Abstract

The environmental and agricultural viability of land disposal of a water treatment residue (WTR) from the Midmar Water Treatment Works of Umgeni Water was investigated by determining answers to four broad questions: 1. What effects would the application of the WTR have on plants growing on the treated soils? 2. What effects would application of the WTR have on soil chemical properties? 3. What effects would the WTR have on the soil solution composition (and by implication the quality of the groundwater)? 4. Could this material be used to reduce solubility of potential pollutants? To answer these questions, the following experiments were set up, and their respective results are reported. 1. Effects of the water treatment residue on plant growth This was investigated in a pot experiment and two field experiments. In the pot experiment five soils, two Huttons (Hu-M and Hu-T), an Inanda (la-C), a Namib (Nb-F) and a Shortlands (Sd) were used to grow perennial ryegrass ((Lolium perellne). All samples were fertilized with a basal dressing of N, P, K, Mg and S. Two lime levels were added to the Ia-C and Nb-F soils, the higher calculated to reduce acid saturation to 1%, and the lower being half of that. The WTR was applied at rates of 0, 40, 80 and 120 Mg ha-1. All treatments were in triplicate. Eight cuts in all were made of the perennial ryegrass. The dry matter (DM) yield of perennial ryegrass grown in the pot experiment increased with the WTR applied in all five soils although the highest increase was with the acidic Ia-C and Nb-F soils. The fact that the highest yields were on the strongly acid soils suggests that the liming effect of the WTR could have contributed, more so considering that lime also increased yields in these soils. It was, however, clear that no one factor was responsible for the increase in yield as the timing effect could not explain the results of the other three soils. At the two field experiments perennial ryegrass was grown at Brookdale Farm from 1998 to 2001, after which the site was re-seeded with tall fescue (Festuca arundinaceae). At Ukulinga Farm tall fescue was grown from the outset in 2000. In the two field experiments with both perennial ryegrass and tall fescue, no significant increase in yield was apparent. Importantly, however, from an environmental point of view there was no decrease in yield whether the WTR was incorporated or applied as a mulch. This was observed even at the highest rates of application, namely 1280 Mg ha-1. The growth on the mulched plots was often observed to be better than any of the other treatments, including the control. Analysis of the plant material from both pot and field experiments indicated that the WTR neither pollution of the groundwater by nitrates. However, analysis of saturated pastes from soils at both field experiments showed that the levels of nitrate were increased by application of the WTR in only the fallow plots. 4. The water treatment residue as a possible pollutant-reducing agent The effect of the water treatment residue on the sorption of P and heavy metals (Cd, Ni and Zn) was studied in the laboratory. Soils treated with WTR were equilibrated for 6 hours in 0.005 M calcium cWoride solution containing a known concentration of each element. For the coarse-textured soils, initial P concentrations ranged from 0 to 1000 mg kg-1 as opposed to 0 to 1800 mg kg-1 for the clay soils. Treatments of WTR used were 0, 80, 320 and 1280 Mg ha-1, both incubated and non-incubated. At high initial P solution concentrations, the WTR increased the extent of sorption in the coarser textured soils (Hu-T, Nb-A, Nb-F, Va and We), and decreased it in highly sorbing Av, Hu-M, la-C and la-W soils. In general though, the WTR greatly reduced soluble P. For Cd, Ni and Zn only one concentration, 50 mg kg-1, was studied using the incubated soil samples as affected by WTR rates from 0 to 1280 Mg ha-1. For all three metals, the amount sorbed increased with increase in amount of WTR for the nine soils studied, namely the Av, Hu-F, Hu-M, Hu-T, la-C, la-W, Nb-F, Va and We. In many cases the sorption was so high that more than 40 mg kg-1 of the initial concentration was removed from solution. Even for those soils with high sorption capacity e.g. the Va and We, the WTR still increased sorption by up to an average of more than 25% for Cd and more than 40% for Ni and Zn. Because for the Av and la-W soils liming also increased sorption, it could be assumed that the accompanying increase in pH as a result of the addition of WTR promoted precipitation of metals, and/or the resultant increase in negative charge increased their adsorption. These results show that where excess concentrations of soluble heavy metals may occur (especially in coarse-textured soils), and where there is concern about run-off with high P concentrations then this WTR could be considered to immobilize these elements and render them less harmful to the environment. General comments and management guidelines. Based on the results reported above, it is apparent that the WTR can be safely disposed of onto land. It has been demonstrated in the current investigation that rates of application can be as high as 1280 Mg ha-1. Rates of application to land higher than 1280 Mp; ha-1 could probably be acceptable - this was the highest rate tested in this investigation - where the residue is produced in large amounts at the plant, and land for disposal is somewhat limited.