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Stellenbosch University (2020)

THE EFFECT OF DOMESTIC GREYWATER ON SOIL QUALITY OF URBAN SOILS FROM THE CAPE TOWN AND STELLENBOSCH AREAS

Madubela, Ncumisa

Titre : THE EFFECT OF DOMESTIC GREYWATER ON SOIL QUALITY OF URBAN SOILS FROM THE CAPE TOWN AND STELLENBOSCH AREAS

Auteur : Madubela, Ncumisa

Université de soutenance : Stellenbosch University

Grade : Master of Agricultural Sciences (Soil Science) 2020

Résumé partiel
During a recent drought and water scarcity in the Western Cape, the reuse of greywater for garden irrigation was encouraged. Greywater, although considered less polluted than same other wastewaters, can be environmental hazardous due to the pathogens, salts, alkalinity and micropollutants it contains. Some greywater streams are easier to capture and reuse than others, and types of detergent can have a significant effect on greywater quality. In previous research the role of soil properties in soil susceptibility to greywater degradation has received little or no attention. Therefore, this study investigated the effect of irrigation with different domestic greywater streams on soil quality of a variety of representative urban soils from the Greater Cape Town area. Six domestic greywater streams were characterised in terms of water quality parameters. Two of better (shower and liquid laundry detergent) and two of poorer quality greywater streams (dishwasher and powdered laundry detergent) were selected for use in subsequent soil application experiments. Twenty soil samples, representing the five major soil groups from the Cape Town and Stellenbosch areas, were collected and characterised. These groups consisted of aeolian coastal sands (avg. 5% clay), alluvial soils (avg. 10% clay), granite-derived soils (avg. 11% clay), shale-derived soils (avg. 20% clay) and Fe-rich chromic soils (avg. 23% clay). In the first experiment, a laboratory soil column infiltration experiment was used to investigate the vulnerability of the five soil groups to degradation (pore sealing and dissolved organic carbon removal) by liquid laundry detergent (LLD) and powdered laundry detergent (PLD) greywaters in comparison to tap water (TW). Application of 200 mm PLD greywater had significantly more detrimental effects on soil permeability, clay dispersion and dissolved organic carbon (DOC) removal compared to 200 mm LLD or TW. This was attributed to PLD’s high pH (ca. 9.95) and SAR (ca. 147). The saturated hydraulic conductivity (Ksat) of the LLD greywater was 1.3 - 2.3 times lower than that of TW, while PLD Ksat was 2.2 - 8.4 times lower. Granite and shale soils were more inclined to Ksat reduction (ca. -81% and -82%, respectively) while the chromic soils were the least susceptible (ca. -47%). PLD greywater resulted in greatest extent of DOC removal, with aeolian sands being most susceptible to DOC stripping (ca. 7.5% C lost) while the chromic soils were the least susceptible (ca. 1.5% C lost). In the second leaching column experiment, the effect of the shower (SH) and dishwasher (DW) greywaters on soil degradation was compared to that of the laundry greywaters and TW on a smaller selection of (11) soils. Application of 200 mm of SH and DW reduced soil infiltration by ca. 50% compared to TW, although it was not statistically significant. Shower and dishwasher greywaters did not significantly remove DOC from the soils as compared to TW. In the third experiment, a column experiment was conducted to simulate the effect of repeated summer greywater irrigation, followed by winter rainfall, on soil properties.

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