Sewer Systems of the Future: Developing a stochastic sewer model to support design of sustainable wastewater systems

Abstract

Global population growth, urbanisation and the threat of climate change are urging us to live more sustainably. This drive for sustainability extends to the urban water cycle. Wastewater systems were originally designed to protect human health but, in the transition to a more sustainable way of life, they could serve a greater purpose into the future. The water cycle is facing changes into the future that conserve water, improve efficiency and maximise the recovery and reuse of natural resources. This thesis sort to explore the effects of water conservation on the sewer system with a view to improve the sustainability of the wastewater system.
Initially, a hydraulic sewer model was developed to incorporate stochastic discharge patterns into a sewer network model. The stochastic sewer model integrated the stochastic water demand model SIMDEUM® with the InfoWorks® ICM (Sewer Edition) hydraulic model and software. The sewer model was tested and validated using sewer network data from real catchments in the Wessex Water area of the UK. This stochastic model, in which every household discharges a unique flow into the sewer, was compared to the continuous, deterministic model and deemed to be superior in representing the real system. The validated model performed well to predict sewer flow in the study catchment and was therefore used to study the impact of certain levels of water conservation. This early study showed how the diurnal wastewater pattern would be effected by up to 75% reduction in water use. It was found that overnight and daytime flow was reduced by up to 80% whereas evening flows remained largely similar. Extended stagnation times were observed in the street scale pipes (150 mm) in the low water use scenario.
The development of the model progressed by including wastewater pollutant loadings that were linked to specific household appliances, using the wastewater extension of SIMDEUM®, SIMDEUM WW®. By incorporating appliance-specific pollutographs into the stochastic model it was possible to simulate the effects of various water saving scenarios on wastewater concentration. The increasing concentration of wastewater is important for resource recovery and thus five future scenarios, developed by Artesia Consulting (on OFWAT’s behalf), were tested for their effect on flow, wastewater temperature and concentration of COD, TPH and TKN. These scenarios outlined how commercial and political factors may change water use in future. The scenario testing showed that a 15-60% reduction in domestic water use resulted in a 1-48% drop in the morning peak flow. The water use reductions increased wastewater concentrations of COD, TKN and TPH by 55-180%, 19-116% and 30-206% respectively. As such, this model had proved it could produce some useful outcomes to address future water use but the wastewater quality aspects were based purely on literature and lacked the necessary sewer measurements to ensure the model prediction was robust.
To investigate the model prediction further, the flow and quality model was applied to a sewer network in Amsterdam, The Netherlands. A week-long wastewater monitoring campaign was conducted in order to gain sufficient data on wastewater quality to validate the model’s outputs. Wastewater concentrations of TSS, COD, TKN, TPH were sampled on an hourly basis and wastewater temperature was recorded every 3-5 minutes. The results obtained from this campaign showed that the model predicted the mass flow of pollutants well but, due to the current lack of a time-varying solids transport model within InfoWorks® ICM, the prediction for wastewater concentration parameters was less good. Aside from this, the model was deemed capable of analysing the effects of three different water conservation strategies (greywater reuse, rainwater harvesting and installation of water-saving appliances) on flow, nutrient concentrations, and temperature in sewer networks. Resulting from this final scenario analysis, through a 62% reduction in sewer flow, an increase in concentration was achieved of COD, TKN and TPH by up to 111%, 84% and 75% respectively, offering more favourable conditions for nutrient recovery.
Finally, the concept of integrating this knowledge into the water industry was discussed. The strengths and weaknesses of this model were addressed and analysed as to how the water industry could best utilise the outputs of the model to improve sustainability in the water cycle.

Date of Award	16 Sept 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Jan Hofman (Supervisor) & Tom Arnot (Supervisor)