Modelling Wastewater Quantity and Quality in Mexico -- using an agent-based model
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Wastewater is a key element in regional and global water circles, and the discharge of a large quantity of untreated wastewater is posing serious threats to the environment and public health in Mexico. To have a thorough understanding of the mechanism of how wastewater is produced and flows in the city is a crucial step to address the problem. The thesis built a network-based ABM (agent-based model) to simulate the wastewater-related socio-ecological system, taking Huehuetla, Mexico as the study area. It aims to represent the change of wastewater quantity and quality over time and space that is caused by human behaviour, at the hourly temporal resolution. For this purpose, how local people decide on their activities and how they interact with each other and with the environment are modelled. People’s activities defined here refer to the human water-use practices that could generate wastewater, including individual water-use activities (i.e., the use of bath, wash basin, toilet, and shower) and collectives’ behaviour (i.e., the use of kitchen sink and washing machine, industrial processes, and socio-economic activities). The exact activity adopted by an individual at a certain tick is determined by its own decision-making rule that considers personal characteristics (i.e., age, educational age, water-saving consciousness, and employment status) and exogenous factors, such as weather (i.e., temperature and precipitation) and intervention actions taken by the government (i.e., organise a water-saving campaign). Weather has a direct impact on individual decision-making rules, while the water-saving campaign comes into play by raising individual water-saving awareness that is also influenced by others via the social network. On the other hand, wastewater quality is calculated based on the quantity and concentrations of TSS, BOD5, and COD in wastewater from various sources. Meanwhile, wastewater flow along the sewerage network in the city towards the wastewater treatment plant is simulated mathematically. The thesis provides an overall picture of the current situation of wastewater production in the study area by plotting the time series of wastewater inflow and quality to the treatment plant, and analyses the main source of wastewater pollutants – industrial processes, and peak hours of household wastewater production – hours near waking-up time and bedtime. Besides, it also shows the role the government plays in controlling wastewater quantity and quality to improve the current wastewater situation. Scenario experiments are conducted in the study area to test the optimal place and timing of a water-saving campaign, with the conclusion being that the campaign held at school in March can lead to the maximum effect on reducing household wastewater production.