Numerical and experimental investigation of chlorine demand and decay in water distribution systems.
Maphanga, Donald
Maphanga, Donald
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Abstract
It is crucial to control the chlorine residual within the portable water distribution network/system to ensure the delivery of safe water to end-users as per regulatory requirements. In this study COMSOL Multiphysics and EPANET softwares were employed concurrently to respectively develop solute transport and water quality regression-based mathematical and computational models for first order and second order chlorine decay to simulate the impact of reaction mechanisms, chlorine initial dose, bulk decay coefficient, pH, chlorine demand, breakpoint chlorination, and chlorine concentration profiles at different distances within the water distribution system supplied by the Vaalkop Water Treatment Plant (WTP). The developed computational models were validated by laboratory experimental results and literature reviews as part of quality assurance. Experimental results were simulated using a pipe length of 1 meter for solute transport and 100 kilometers for the water quality model. The chlorine dosages ranged from 2.5 mg/L to 20 mg/L, with a reaction time spanning from 0 to 4419 minutes. The chlorine demand for lower dosages, ranging from 2.5 mg/L to 5 mg/L, was high, with 98% of the chlorine dose being consumed within 15 minutes of reaction time. Elevated chlorine dosages, between 10 mg/L and 20 mg/L, exhibited a chlorine demand of less than 60% of the initial chlorine dose within 15 minutes, and caused formation of Trihalomethanes disinfection byproducts after 15 minutes of reaction time. The bulk decay rate for the mathematical models displayed exponential decay, ranging from 0.3 (mg/L.min) to 0.0001 (mg/L.min), for chlorine dosages ranging from 2.5 mg/L to 20 mg/L. The regression-based second order models achieved a determination of coefficient (R2) of 98% with respect to chlorine dose, while the first order models achieved 86%. The bulk decay rate for the simulated models ranged from 0.035 (mg/L.hr.m) to 0.000021 (mg/L.hr.m) for dosages ranging from 2.5 mg/L to 20 mg/L in the solute transport models, and 0.3 (kg/hr) to 0.00021 (kg/hr) for the water quality models. Rapid chlorine decay, with 98% of the chlorine dose being observed within 10 kilometers for the water quality models and 0.1 meters for the solute transport models, was observed for lower chlorine dosages below 5 mg/L. Chlorine dosages exceeding 10 mg/L satisfied the chlorine demand of the system and maintained chlorine residual levels above 1.3 mg/L at the exit points for both solute transport (1 meter) and water quality models (100 kilometers).
The optimal chlorine dosage for the system was determined to be 6.30 mg/L, which would maintain chlorine residual within ±0.5 mg/L at the point of use without the formation of disinfection byproducts. However, the upper limit for chlorine supply within the system was set at 5 mg/L that would require the installation of chlorine boosters within a 5-kilometer radius of the system. First order chlorine decay models are contingent upon the chlorine reaction time, whereas second-order chlorine decay models are reliant on the initial chlorine dose. The parameters of the second order chlorine decay model are linearly associated with the chlorine dose. The reaction of chlorine in aqueous solutions is pH-dependent, and chlorine decay does not affect water pH. The results of this study demonstrated the applicability of the developed solute transport and water quality models for determining accurate chlorine dosing requirements to reduce costs and ensure compliance with regulatory frameworks.
Description
Submitted in partial fulfilment of the requirements for the degree Magister of Engineering: Chemical Engineering In the Department of Chemical, Metallurgical, and Materials Engineering Faculty of Engineering and the Built Environment. at the Tshwane University of Technology.
Date
2023-01-01
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Tshwane University of Technology
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Keywords
Chlorine, Decay, Water distribution system