The effect of domestic greywater on soil quality of urban soils from the Cape Town and Stellenbosch areas

Madubela, Ncumisa (2020-03)

Thesis (MScAgric)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: 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. The effect of repeated application (370 mm applied over 10 weeks) of the four greywater streams on soil quality of a representative dispersive (granite – SP1) and stable (chromic – BD1) soil types was determined. This was followed by repeated application of 370 mm of rainwater to see whether the soils could be rehabilitated. As expected, the PLD and DW had the most harmful effects on soil quality, resulting in the formation of alkaline and saline-sodic soils. Powdered laundry detergent greywater and DW also significantly increased plant available P. All the treatments lowered soil bacterial diversity, while no significant change was observed on the fungal community. Subsequent application of rainwater showed that no water was able to infiltrate into the dispersive granite soil after treatment with PLD or DW. This indicated that it would be very difficult to remediate this soil type after irrigation with these types of greywaters. Application of all four greywaters significantly decreased rainwater infiltration in the chromic (ca. -42% to -93%) and granitic (ca. -25% to -100%) soils. Application of rainwater was, however, able to decrease the exchangeable sodium percentage of the DW and PLD irrigated soils to around ca. 13%, but the pH values remained high. Total C content of the PLD treated chromic soil was significantly decreased (ca. -22% of total C) due to DOC stripping. The results of this study demonstrate that soils vary in their susceptibility to degradation due to greywater application, depending mainly on texture and clay mineralogy. It is concluded that PLD and DW greywater should not be used for soil irrigation, whereas LLD and SH greywater should be used cautiously, especially on dispersive granite and shale-derived soils. The results of this study should be incorporated into the establishment of greywater irrigation guidelines.

AFRIKAANSE OPSOMMING: Gedurende ‘n onlangse droogte en waterskaarste in die Wes-Kaap, was die hergebruik van gryswater vir tuinbesproeiing aangemoedig. In vergelyking met sommige ander afvalwaterbronne, kan gryswater kan as minder besoedelend beskou word, alhoewel dit steeds 'n omgewingsgevaar kan inhou as gevolg van potensiële patogene, soute, alkaliniteit en mikro-besoedelingstowwe wat dit bevat. Sommige gryswaterbronne is makliker herwinbaar as ander, en verskillende soorte skoonmaakmiddels kan 'n beduidende effek op die kwaliteit van die gryswater hê. In vorige navorsing was daar min klem gelê op die rol wat grondeienskappe op die vatbaarheid vir degeradering weens gryswater toediening het. Dus was die fokus van hierdie studie op die effek van besproeiing met huishoudelike gryswaterbronne op die grondkwaliteit van 'n verskeidenheid verteenwoordigende stedelike gronde uit die Groter Kaapstad area. Ses huishoudelike gryswaterstrome is gekaraktiseer in terme van waterkwaliteitparameters. Twee beter (stort- en vloeibare wasgoedmiddel) en twee slegter gehalte (skottelgoedwassermiddel en wasgoedpoeier) gryswaterbronne was gekies vir gebruik in opvolgende eksperimente vir grondtoediening. Twintig grondmonsters, wat die vyf belangrikste grondgroepe uit die Kaapstad en Stellenbosch gebiede verteenwoordig, is versamel en gekarakteriseer. Hierdie groepe het bestaan uit eoliese sand (gemiddeld 5% klei), alluviale grond (gemiddeld 10% klei), graniet afkomstige gronde (gemiddeld 11% klei), skalie-afkomstige gronde (gemiddeld 20% klei) en Fe-ryke chromiese gronde (gemiddeld 23% klei). In die eerste eksperiment, was 'n laboratorium grondkolominfiltrasie eksperiment gebruik om die kwesbaarheid van die vyf grondgroepe vir degredasie (porie-verseëling en verwydering van opgeloste organiese koolstof) deur vloeibare wasgoedmiddel (LLD) en wasgoedpoeier (PLD) te ondersoek, in vergelyking met kraan water (TW). Die toediening van 200 mm PLD gryswater het aansienlik meer nadelige uitwerking op gronddeurlaatbaarheid, klei deflokkulasie en verwydering van opgeloste organiese koolstof (DOC) gehad in vergelyking met 200 mm LLD of TW toegedien. Dit word toegeskryf aan PLD se hoë pH (ca. 9.95) en NAV (ca. 147). Die versadigde hidroliese geleidingsvermoë (Ksat) van die LLD-gryswater was 1.3 - 2.3 keer laer as dié van TW, terwyl PLD Ksat 2.2 - 8.4 keer laer. Graniet- en skaliegrond was meer geneig tot Ksat-vermindering (ca. -81% en -82%, onderskeidelik), terwyl die chromiese gronde die minste vatbaar was (ca. -47%). PLD-gryswater het die grootste mate van verwydering van DOC tot gevolg gehad, met eoliese sand was die mees vatbaarste vir stroping van DOC was (verlies van 7.5% C), terwyl die chromiese gronde die minste vatbaar was (verlies van ca. 1.5% C). In die tweede uitlogingskolom eksperiment is die effek van gryswater afkomstig van stort (SH) en skottelgoedwasser (DW) op die degredasie van grond vergelyk met wasmasjien gryswaters en TW op 'n kleiner verskeidenheid (11) gronde. Toediening van 200 mm SH en DW het die grondinfiltrasie met ca. 50% verminder in vergelyking met TW, hoewel dit nie statisties beduidend was nie. SH- en DW-gryswaters het nie meer opgeloste organiese koolstof van die grond as TW nie. In die derde eksperiment is 'n kolomeksperiment uitgevoer om die effek van herhaaldelike somer gryswater besproeiing, gevolg deur winter reënval, op grondeienskappe te simuleer. Die effek van die herhaaldelike toediening (370 mm toegedien oor 10 weke) van vier gryswaterbronne op die grondkwaliteit is op twee verteenwoordigende disperse (graniet - SP1) en stabiele (chromiese - BD1) grondsoorte bepaal. Dit was gevolg deur die herhaaldelike toediening van 370 mm reënwater om te bepaal of die grond gerehabiliteer kon word. Soos verwag, het die PLD en DW die mees nadeligste gevolge vir die grondkwaliteit gehad, wat gelei het tot die vorming van alkaliese en natriumbrak gronde. Waspoeier gryswater en DW het ook die plantbeskikbare P aansienlik verhoog. Al die behandelings het die bakteriële diversiteit van die grond verlaag, terwyl daar geen noemenswaardige verandering in die swamgemeenskap waargeneem is nie. Die daaropvolgende toediening van reënwater het getoon dat geen water na die behandeling met PLD of DW in die disperse granietiesegrond kon infiltreer nie. Dit het aangedui dat rehabilitasie van hierdie grond na die besproeiing van hierdie tipe gryswaters uiters moeilik sal wees. Toediening van al vier gryswaters het die reënwaterinfiltrasie in die BD1 (ca. -42% tot -93%) en SP1 (ca. -25% tot -100%) gronde beduidend verminder. Die toediening van reënwater kon die uitruilbare Natrium persentasie van die besproeide grond DW en PLD egter met ongeveer 13% verlaag, maar die pH-waardes was steeds hoog. Die totale C-inhoud van die PLD-behandelde chromiese grond het beduidend afgeneem (ca. -22% van die totale C) as gevolg van die strooping van opgeloste organiese koolstof. Die resultate van hierdie studie demonstreer dat gronde variëer in hul vatbaarheid vir degredasie a.g.v. gryswater toedining, hoofsaaklik as gevolg van tekstuur en kleimineralogie verskille. Daar kan tot die gevolgtrekking gekom word dat gryswater van PLD en DW nie vir besproeiing gebruik moet word nie, terwyl LLD en SH gryswater oordeelkundig gebruik moet word, veral op skalie- en graniet verweerde gronde. Die resultate van hierdie studie kan gebruik word in die opstel van riglyne vir gryswaterbesproeiing.

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