An investigation into untreated greywater as supplementary household water source to augment potable municipal supply with consideration of associated risks.

Nel, Nicole (2022-04)

Thesis (PhD)--Stellenbosch University, 2022.


ENGLISH ABSTRACT: Despite the available body of research regarding supplementary household water sources and in particular, greywater use, there is a critical gap when it comes to understanding the uptake of untreated greywater in suburban areas and the trade-off between the risks and potential water savings. This dissertation focuses on untreated greywater use in residential, fully serviced houses equipped with regular water use appliances and with conventional waterborne sewers. The main objective is to gain an improved understanding of the uptake of untreated greywater and the potential for use and application in suburban areas by exploring the trade-off between expected water savings (associated with quantity) and potential risks (associated with quality) as related to untreated greywater use. This study starts with addressing on-site supplementary household water sources with a focus on groundwater abstraction, rainwater harvesting, and greywater use as available non-potable supplementary water sources to residential consumers. The legal position in South Africa and an end use model to assess the theoretical impact of these sources on water demand in formal residential areas, is presented. The model provides valuable strategic direction and indicates a significant theoretical reduction in potable municipal water demand of between 55% and 69% for relatively large properties with irrigated gardens when supplementary household sources are maximally utilised (when compared to exclusive municipal use as a baseline). This load reduction on piped reticulation systems could be an advantage through augmenting municipal supply. However, water service planning and demand management are complicated by the introduction, and possible future decommissioning, of any household water source. The trade off between the advantages and disadvantages of this load reduction defines whether there is a nett positive benefit linked to the use of the household water sources. Groundwater is the household water source considered to have the most notable penetration and intensity to impact potable water demand in residential areas and is coupled to a relatively low risk in terms of water quality relative to other uses such as greywater use. Groundwater, however, has the biggest barrier to entry and requires the highest capital investment of the three supplementary household water sources. The distinct trade off between the advantages and disadvantages of untreated greywater, particularly in comparison to the other supplementary household water sources, provides justification towards it being the focus of this study. Untreated greywater use at household level is an accessible water source to supplement non-potable water requirements in times of emergency water curtailments, but poses various risks to the consumer, the wider community, infrastructure and the environment. Little is known about unregulated, untreated greywater use practices in suburban communities where consumers have become accustomed to reliable potable water supplied via a pressurised, piped distribution system. There is a lack of knowledge regarding the sources of greywater used, collection methods, -storage and -distribution, the application points, the level of treatment (if any) and the perceived risks related to the greywater use. The City of Cape Town was selected as a case study site for research into greywater use under the threat of “Day Zero” and stringent water restrictions, implemented during the 2017/2018 summer season. A consumer survey and analysis of relevant online forums was conducted in order to obtain the necessary information. Greywater use practices from a sample group of 351 consumers were identified and classified. Untreated greywater use was found to be common, mainly for garden irrigation and toilet flushing. The results point to high-risk activities in the study group. By using these reported ad hoc greywater use practices identified through the Cape Town case study, the volume of untreated greywater used by households in formal residential settings was evaluated by means of a stochastic end use model. Untreated greywater use practices (e.g. bucketing) were found to reduce water consumption in a single person suburban household by less than 10%, which is lower than values reported in literature. This relatively low volume weighed up against the high risk of using untreated greywater may result in a negative nett benefit, providing decision making insights for both water service providers and consumers. This quantification of the volumes associated with untreated ad hoc greywater use is the first step in understanding the trade-off between expected water savings (quantity) and potential risks (quality) of untreated greywater use. The second component of the water saving-risk trade off involved an investigation of untreated greywater quality and related risks, through a statistical analysis of greywater quality results, as sourced from South African studies. Greywater sources included were the bathroom, kitchen, laundry, mixed and general residential sources. Variability in terms of each of the reported physical, chemical and microbiological constituents by source and between result sets was noted. Statistically significant differences were evident between the pH, conductivity and phosphorous values of certain sources. A risk assessment undertaken for each of the constituents revealed further variability. The constituent with the highest number of high-risk samples was total dissolved solids, although further research into specific constituent elements that are of real danger to humans is warranted here. The finding that water savings due to untreated greywater through manual collection methods is <10% is markedly less than the water savings through the use of multiple household water sources (up to 69% for large properties). This coupled with the relatively high risk and high consequences in greywater practices in terms of public health, the environment, and infrastructure, given its variability, provide insight into the quality-quantity space. There is a need for a more nuanced view of the potential potable savings associated with greywater use and a need for improved risk management. Risk management and drivers of consumer decision making in the water use space were therefore explored further. As a result, a decision-making matrix was designed as an interim conceptual tool to assist consumers when faced with water use decisions during emergency drought conditions. This research is unique in that while the use of greywater with purpose-built infrastructure and treatment systems has been studied for a number of locations and configurations, the practices used by individuals in the absence of such infrastructure was not well understood. This study has shed light on the reported volume of untreated greywater used by households in formal residential settings, based on reported ad hoc greywater use practices and on the extent of these potentially risky practices. A novel holistic picture of the risks and trade-offs associated with untreated greywater use was developed, allowing for advancement of knowledge in the field.

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