Urban water reuse in the Bengal delta: Prospects, challenges and socio-technological solutions

Water is indispensable for sustaining life on Earth and the uneven distribution of freshwater resources is impacting water availability. Additionally, climate change and rapid urbanization restrict water availability, causing global water scarcity. The Bengal delta is the largest delta in the world and is suffering from economic water scarcity. Sea level rise, salinity intrusion, discharge of untreated wastewater degrades surface water quality and threaten the availability and quality of drinking and irrigation water. Urban water defined as a combination of greywater and sealed surface runoff often has been widely used in agriculture as an alternative source of irrigation. Reuse of urban water offers a cost-effective solution by enhancing the socio-environmental sustainability of water resources. However, adequate planning and in-depth understanding of the socio-economic, health and technological aspects are necessary for successfully implementing reuse projects.

Considering the growing demand for quality irrigation water in the Bengal delta, this research explored the possibilities and barriers related to safe urban water reuse in agriculture. Khulna, the coastal city in Bangladesh, the most vulnerable to climate change, has been taken as a case study. Several quantitative and qualitative research methods were employed to gather data from the study area.

Chapter 1 starts with the global overview on water availability and narrows down to the negative impact of climate change and rapid urbanization on growing water scarcity in different regions. Literature was consulted to illustrate the history, major trends and achievements in urban water reuse worldwide.

In Chapter 2, the urban water reuse potential in peri-urban agriculture was quantitatively assessed. Firstly, the irrigation water requirement of Boro rice during the dry season was assessed using the FAO AquaCrop model. Then the greywater and sealed surface runoff generation were calculated. The analysis indicated that the urban water could positively contribute to the irrigation water demand of peri-urban agriculture while the net irrigation requirement of Boro rice has declined over the last decades (1984-2017). The water requirement is highest during February and March and the lack of rainfall in these months fails to satisfy the total irrigation demand. However, with the introduction of storage systems, urban water could supply the required amount throughout the year. The results provided a solid quantitative assessment of the matching potential of urban water to sustain agricultural activities.

In Chapter 3, the spatio-temporal variability in macro-chemical surface water quality was analyzed and the subsequent usability in agriculture was mapped. Statistical analysis to correlate the water quality with urban land uses and mapping using ArcGIS were carried out. The negative impact of direct discharge of urban wastewater and solid waste on water quality is reflected by elevated values of the related parameters such as solids (TSS), organic matter (BOD5, COD) and low DO concentrations. The adjacent salt-carrying tributary rivers impact the water quality which is evident in elevated saltwater influenced parameters such as TDS, Na+ and Cl−. Results showed that the current surface water quality does not meet FAO guideline. The influence of surrounding land use was evident which can be used to improve surface water quality and future planning strategies. The method of integrating water quality information at a spatial scale provided valuable insights on the variability of water quality and usefulness, restrictions and treatment requirements for further use in agriculture.

In Chapter 4, the microbial and heavy metal contamination in the surface water was assessed through laboratory analysis and subsequent risk was assessed. A screening level QMRA was performed for assessing the health risk of farmers considering E. coli concentrations in water. Results show that the mean concentrations of microbial indicators exceeded the thresholds of the WHO and local guidelines for safe irrigation. However, no such significant thresholds were observed for heavy metals. The health risk assessment suggested that the existing surface water quality poses a health risk for farmers as they are in direct contact with the polluted surface water and do not use any protective equipment. However, farmers do not prioritize their health due to their longstanding practices and lack of alternative irrigation sources. A multi-barrier approach containing reduction of pathogen concentrations and awareness among farmers was discussed to lower the risk. The results further reiterated the need for the necessary treatment of wastewater for ensuring safe use.

In Chapter 5, the governance aspects of urban water reuse were investigated employing several participatory methods such as questionnaire survey, Key Informant Interview and Focus Group Discussion. Results indicated a high level of awareness among urban citizens (80%) about the negative impacts of wastewater discharge. There is a positive attitude towards urban water reuse as an alternative to combat irrigation water scarcity. Citizens are willing to pay for the treatment and the willingness of citizens is influenced by their socio-economic conditions. Several governmental agencies are parallelly involved in urban water-related issues; however, there is no clear strategy to work together on an interdisciplinary issue like urban water reuse. Adjustments in existing rules and regulations are necessary for the organization to collaborate and work together cohesively.

Finally, in Chapter 6, the findings of this thesis were synthesized and four socio-technological scenarios for treating and reusing urban water in agriculture were illustrated. The potential and drawbacks of urban water reuse were discussed to highlight the need for proper treatment infrastructures. An extensive literature review was carried out to list the crucial factors that need to be considered for selecting suitable treatment technologies and defining a proper technical collection and treatment strategy including storage and redistribution to agriculture. The chapter also identifies several limitations of current research and provided suggestions for future research.

Considering the future uncertainties around the provision of freshwater supply, planned urban water reuse presents a viable alternative to meet the growing water demand in agriculture. Improvement of existing water quality with adequate treatment and infrastructures for collection, storage, and supply would ensure safe urban water reuse. Supportive governance arrangement and stakeholder participation in the decision-making process could ensure the implementation of urban water reuse projects for sustaining agricultural activities in the Bengal delta.