Mitigation of soil and ground water pollution caused by on-land disposal of olive mill wastewater
Thesis (PhD)--Stellenbosch University, 2016.
ENGLISH ABSTRACT: Olive mill wastewater (OMW) is generated in large quantities, particularly in the regions with a Mediterranean climate where olive oil is produced on a commercial scale. Some producers collect the effluent and dispose of it as hazardous waste at significant expense, while others dispose of it directly on land, claiming the potential benefits to productivity from the plant nutrients present in the OMW. It was shown that the OMW also contains some phytotoxic phenols, which may have both immediate and cumulative negative effects on plant growth. The long-term effects on the soil and crop growth have been shown to be detrimental. Sandy soils are of particular concern due to the possibility of phenol penetration into deeper soil layers and potential ground water contamination. The study explores in-situ (soil amendment with biochar prior to the OMW disposal) and ex-situ (OMW filtration through a biochar bed) options to mitigate the negative effects of the OMW on-land disposal. A laboratory batch sorption experiment was set up using 0.2 g pinewood biochar to explore the possibilities of removing the phenols from 50 mL of the OMW or gallic acid (GA) solutions at different concentrations. The results showed that the sorption process was rapid and stabilized within one hour. The kinetic process followed a pseudo-second-order model and was described by the Freundlich multi-layer isotherm. The pinewood biochar had a sorption capacity of 30 mg·g-1 and 100 % removal was obtained with 300 g·l-1 of the OMW load. It was found that pinewood biochar could be used to remove the phenols contained in the effluent. A column experiment was set up to determine the effectiveness of biochar and biochar-soil mixtures in removal of phenol and Chemical Oxygen Demand (COD) from the OMW compared to sand filtration. The breakthrough curves for phenol and COD were determined, while the pH and EC of the filtrates were monitored. Ten PVC columns of 30 cm height and 5 cm diameter were filled with five different materials: sand, sand + biochar, Hutton clay loam soil, Hutton clay loam soil + biochar and biochar alone. Two different treatments were given to the columns; five of the columns were prewashed with 2 liters of deionized water and the other five were not washed before the OMW filtration. The performance of the columns was determined in respect of the phenol and COD removal capacities, hydraulic conductivities and porosity changes. The results showed that washing enhanced the phenol sorption but not the COD sorption. The addition of the biochar at 2%wt load significantly improved the effectiveness of the filtration. The best performance was achieved in terms of COD removal in pure biochar columns, but in terms of the phenol, the best performance was on a pre-washed Hutton clay loam soil with 2%wt biochar addition. Both the washing and biochar addition affected the porosity and reduced the hydraulic conductivity of the columns. The greenhouse experiments were conducted to confirm the above statement using pot trials laid out in a 4 x 4 factorial Randomized Complete Design (CRD) to determine the effect of effluent and biochar on wheat and green beans on alkaline sand. Results showed that the increasing effluent rate up to 200 m3·ha-1 gave significantly negative results on wheat growth, even with fertilizer application. But the effect was different for beans where low effluent loads gave positive results though not significant while with fertilizer (N and P) 50 m3·ha-1 performed better. With the addition of biochar there was no significant effect on wheat, but it significantly affected beans at the application rate of 2.5 and 5%wt. The interaction of biochar and effluent showed that the best performance was at 5% biochar application and effluent loads of 50 and 100 m3·ha-1, but increased effluent rate decreased production even with a 5% wt biochar application rate. It was suggested that a leguminous crop should tolerate OMW application better compared to wheat even in the adverse conditions of the alkaline sand. A second greenhouse experiment was conducted with another legume, an indigenous African crop, the bambara groundnut, on an acidic Hutton clay-loam soil (Oxisol) sourced locally. The experiment was laid out in a 2 x 6 CRD factorial design to determine the effect of the biochar and effluent combination on the yield and growth parameters of bambara as well as the effect on soil conditions and nutrient availability. The result showed that biochar addition improved seed germination, which was retarded by effluent loading. The effluent rate of 200 m3·ha-1 and biochar 2% gave the best yield performance. The biochar addition increased the pH and hence affected the release of P and N whereas Na and K availability were reduced. We conclude that biochar may be used for both ex-situ filtration to treat the OMW, and as a soil amendment to allow safe on-land disposal of the OMW. The estimations of safe disposal loads and the required application rates of the biochar should be made individually for a specific soil type. Pinewood biochar was proven to be a cheaper source of activated carbon for the treatment of olive mill wastewater organic contaminants in South Africa.
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