Browsing by Author "Steytler, Jan George"
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- ItemWater treatment residual: potential amendment to a sandy soil(Stellenbosch : Stellenbosch University, 2021-03) Steytler, Jan George; Clarke, Catherine E.; Hardie-Pieters, Ailsa G.; Stone, Wendy; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.ENGLISH ABSTRACT: Water treatment residual (WTR) is a waste product generated during the water treatment process. This study assesses the potential of using WTR as a potential amendment or co- amendment with commercial compost, to the sandy soils common in the Cape Town metropolitan area. The study set out to: i) characterise the Alum-WTR collected in 2019 and Ferric-WTR collected in 2018 and 2019, a sandy topsoil and compost through mineralogical and chemical characterization, ii) determine the soil-water interactions of Ferric-WTR (2018) single amendment and co-amendment with compost at application rates of 10% w/w (225 tons/ha) and 20% w/w (450 tons/ha), iii) conduct a greenhouse experiment on Swiss Chard (Beta vulgaris cicla.) growth in a sandy topsoil with 10% and 20% single amendments of compost and Ferric-WTR (2018) and a WTR-compost co-application (20%) under water and nutrient- induced stress, and iv) investigate the effect of the Ferric-WTR (2018) amended sandy topsoil on trace element plant availability and phytotoxicity. The properties of the Alum-WTR (2019), Ferric-WTR (2018 and 2019) samples was characterised in terms of morphology, mineralogy, specific surface area, EC, pH, aqua regia extractable elements, exchangeable cations and acidity and ammonium oxalate extractable Al, Mn and Fe. The adsorption of phosphate to the WTR was fitted to Langmuir and Freundlich isotherm models. The analysed WTRs exhibit irregular surface morphology and a large specific surface area (84.80 – 144.13 m²/g). The X-ray diffraction results showed that the crystalline phases from the raw water are quartz, muscovite, kaolinite and feldspar, while the Al and Fe derived from the treatment process is mainly held in poorly crystalline phases. The WTR from both water treatment plants (WTP) contained Al, Fe, Cr, Mn, Co, Ni, Cu, Zn, As, and Pb. The EC of the WTRs ranged between 0.24 -0.8 dS/m while the pH (KCl) ranged from 4.7 to 6.6 and the CEC of Ferric-WTR (2018) was the highest at 90 cmolc/kg, followed by 25 cmolc/kg for the Ferric WTR (2019) and 6 cmolc/kg for Alum-WTR (2019). The maximum P sorption was 12.62 mg/g and 12.01 mg/g for Ferric-WTR (2019) and Alum-WTR (2019), respectively. The soil water repellency (SWR) of the Ferric-WTR (2018), compost and WTR-compost co- application was investigated trough a Water Drop Penetration Time test (WDPT). The water retention characteristics of the amendment mixtures were determined with pressure plates. The SWR of the sandy topsoil was significantly reduced from 1661 seconds to 67 seconds by the 20% Ferric-WTR (2018) amendment. The Ferric-WTR (2018) amendment mixtures (10% and 20%) resulted in an upward shift in the water retention curve, without altering the retention curve shape. The 20% Ferric-WTR (2018) amendment resulted in a 12.5% increase in the total available-water holding capacity (TAWC). The increased water content at all matric potentials is related to water held in the micropores of the WTR. A 6 x 2 x 2 factorial ANOVA (6 amendment mixtures, 2 water levels and 2 fertiliser levels) on the water use efficiency (WUE) of Swiss Chard was conducted. The unfertilized treatment combinations were analysed colourimetrically for plant available P and N with a Mehlich-3 extract and a 2 M KCl solution and ammonium and nitrate test kits. The factorial ANOVA on WUE resulted in significant interaction. The treatment combinations were subsequently analysed by a single factor ANOVA. The WUE of the unfertilised water-unstressed WTR- compost co-application resulted in a 16-fold increase in WUE relative to the control. The WTR addition to the sandy soil reduced the plant availability of P. The WTR-compost co-application improves Swiss Chard biomass production and WUE under water and nutrient-limited conditions, and it outperformed both the control and the WTR single amendment. The plant-available trace elements of the unfertilized amendment mixtures was extracted in a Mehlich-3 extract. Both the unfertilised 20% Ferric-WTR (2018) single amendment and the WTR-compost co-amendment, WTR reduced the plant availability of As, Pb, Mo, Cu and Zn, while the Ferric-WTR is a source of Al, Cr, Mn, Fe, Co and Ni in comparison to the sandy topsoil. No treatment combination resulted in Pb, Ni, Mn, Cu and Zn exceeding the Mehlich-3 toxicity threshold. Land application of a WTR-compost co-application to a poor sandy soil is a viable conduit for diverting waste from landfill into agricultural potential.