Although some ofthe WTRs studied had low nutrient concentrations, in particular Nand P, they may be useful to improve conditions for plant growth in degraded or nutrient-poor soils.
The alkaline nature ofmost of the WTRs (in particular the Rand WTR) would enable them to increase the pH of acid soils. Fertiliser additions may help overcome some of the nutrient deficiencies, possibly with the exception of P. The moderately high P sorbing capacity of these WTRs indicates that large amounts of added P could be removed, but this study has not considered desorption of P from these residues. Desorption and plant growth studies may better show how these residues might affect P availability.A number of studies have reported on the P sorption capacity of WTRs and how they may reduce P uptake by plants grown in glasshouse investigations in either pure WTR or mixtures of soil or potting media and WTR (inter alia Elliott and Singer, 1988; Heil and Barbarick, 1989; Skene et al., 1995; Ahrned et al., 1997; Basta et al., 2000; Codling et al., 2002). However, field experiments have shown
that plant uptake is apparently not affected by additions of WTR (Grabarek and Krug, 1987;
Geertsemaet al., 1994).
In some instances, the high amounts ofMn may lead to toxicity problems (in particular the Faure and Midmar WTRs), especially if used under acid or reducing conditions.It is unlikely that any of the other metals would be problematic. Furthermore, consideration needs to be given to land application rates as well as existing background soil concentrations and characteristics and the intended purpose of the land treated with the WTR (Elliott et al., 1990a). These factors may influence the rate and frequency of application.
A number of studies have reported on improved soil physical properties as a result of WTR additions (Rengasamy et al., 1980; Skene et al., 1995;Ahmed et al., 1997; Moodley, 2001).
While not intensively investigated here, some WTRs had high water holding capacity. This may improve water retention of some soils, while improving infiltration of heavy textured soils, if the WTRs are applied to land. However, Moodley (2001) has indicated that if the WTR applied to a soil were to decompose to its constituent fractions (clay and silt in most instances) then clogging of soil pores may lead to reduced infiltration and water retention in affected soils. In an ongoing investigation using the field trials reported on by Moodley (2001), data suggest that, after five years, the physical properties of a Button soil treated with WTR at application rates as high as 1280 Mg ha'! are returning, after having shown increased water retention, to similar conditions as the control treatments (personal observation, see project website: http://www.wrc.org.za/interest/sludgeweb/index.htm).
As part of the present study, two WTRs were selected for further investigation of their potential for land application. As a larger study is currently in progress using the Midmar WTR (Moodley, 2001; Buyeye, unpublished data) it was decided that WTRs with different characteristics to the Midmar WTR should be selected and ideally should represent the other two major water suppliers in South Africa, namely the Cape Metropolitan Council and Rand Water. Rand Water supplies the greater Johannesburg region, while the Faure WTP is the Cape Metropolitan Council's largest plant supplying Khayelitsha (and Cape Town at times) in the Western Cape.
The Rand Water WTR was chosen for the following reasons:
• the very different chemical nature of the material (when compared to other WTRs);
• the availability of material from Rand Water'sPanfontein disposal ponds; and
• pre-existing communications between Rand Water and a nearby coal mine to use the residue on the mine as part of the mine's reclamation strategy.
The choice of the Faure 1 WTR was based on similar criteria to the Rand WTR i.e.:
• the high Fe and organic carbon content of the residue (obviously different from the Rand WTR);
• the large volume of residue produced and its availability; and
• the potential of the residue to be used on nutrient-poor sands (common to the local region) to improve fertility.
The Rand WTR was used to test the growth of selected grass species on mixtures of Rand WTR and material from the nearby mine (Chapter 3). The Faure 1 WTR was applied to a nutrient-poor sand to test whether the growth of a crop could be improved (Chapter 4).
Chapter 3
Land application of Rand Water's water treatment residue to selected mine material and coal combustion ash: laboratory and glasshouse investigations
3.1 Introduction
The use of industrial by-products to improve the conditions of mine wastes for plant growth and to simultaneously act as a potential disposal option for the by-product have been reported extensively in the literature. Sopper (1992) reviews a number of studies that consider the use of sewage sludge as an ameliorant on mine dumps to improve soil properties and plant growth. Other materials used on mine tailings have included fly ash (Taylor and Schuman, 1988;Welden et al., 1999; Bhumbla et al.,2000;Seoane and Leiros, 2001);sawdust (Roberts et al., 1988); and manures and papermill sludges (Haering et al.,2000). Revegetation of coal combustion ash dumps has been reported by Mulhern et al. (1989), Carlson and Adriano (1991), and van Rensburg et al.(1998).There are no studiesthatexamine the use of WTRs on mined land, although Dayton and Basta (2001) and Zupancic (1996) consider the use of WTR as a soil substitute, and the potential of WTR to aid in mined land reclamation. However, they did not specifically test the effects of applying WTR to mined land,thusmaking it difficult to predict how either material would be affected by the other.
As outlined in Section 2.4, the Rand Water WTR (RWTR) was selected for investigations into its use on a coal mine (New Vaal Colliery) near Vereeniging, South Africa. Potential uses of the RWTR included, amongst others, as fill material in voids, a liming material, neutralisation of acid hotspots and to improve plant growth. As the particular interest of the wider project (Introduction) was land application of the WTRs and the effects on soil properties and plant growth, it was decided that laboratory and pot experiments would be used to investigate the latter aspect. The mine management suggested materialsto be used in the investigations, and these included the sandy soil material removed from the land prior to mining (3 to 6 m in depth), the overburden material removed from above the coal seams consisting of shale interspersed with calcareous nodules and coal fragments, and a coal combustion residue (fly ash) from a nearby power plant (Lethabo Power Station).
The primary objectives of the study were:
• to characterise the spoil, soil and ash;
• to examine the effect ofRWTR on the properties of the spoil, soil and ash;
• to measure the yield response of three grass species grown in mixtures of RWTR and the selected materials; and
• to determine nutrient uptake by the grass species tested.
It is important to note that this was a preliminary investigation into the land disposal or application of RWTR at this mine. Land disposal offers a potentially viable option of discarding of one waste type onto another or using properties of a soil to assimilate the waste.
In this instance the disposal of RWTR onto the mine may present Rand Water with a suitable disposal option, reducing the need to lagoon and store large quantities of waste WTR. The benefit to the mine would only be realised if use of the RWTR on their materials proved successful and assisted reclamation efforts.