Abstract:
Water scarcity has become a global problem due to diminishing water resource and
pollution of the remaining resources. The problems arising fromwater scarcity are exacerbated
rapid urbanisation and industrialisation. Water quality management systems are introduced.
Numerous water management methods exist some of which, if applied e ectively, can
remedy these problems. In South Africa, water management systems are urgently needed
to start addressing issues around the longterm sustainability of our limited water resource.
Water quality modelling is one of the tools employed to assist in validating decisions
made during the planning phase of a water quality management system. It also provides
a means of exploring viable options to be considered when these decisions are to be made.
A range of management options exist and implementing all of them may prove costly,
therefore optimisation techniques are utilised to narrow down options to the most e ective
and least costly among the available choices. Commonly, water quality models are used to
predict concentrations in the river from which constraint equations are generated. The
constraint equations are used in optimisation models to generate feasible solutions by
either maximising or minimising the objective function. In this case the objective function
is wastewater treatment cost. Constraints equations are based on the set in-stream water
quality standard at selected theoretical measuring stations (checkpoints) in the stream
and a feasible solution is one that suggests a treatment method that will ensure water
quality standards are met at the lowest regional treatment cost.
This study focused on the Upper Olifants river catchment near Witbank in Mpumalanga
province. This catchment is subjected to extensive wastewater e uents from various
mining operations and wastewater treatment plants. The aim here was to develop a
water quality model for predicting dissolved oxygen (DO) concentration in the river, and
to use a modelling approach to generate constraint equations for the system.
A Streeter-Phelps stream simulation model was employed to predict DO concentration in
the river. A mixed-integer programming technique was then used to evaluate the impact
of nine wastewater treatment facilities discharging e uent into the river. Treatment levels
were varied to test model reliability. The coupled stream simulation and optimisation model produced feasible solutions under
2 minutes, with each solution suggesting a range of treatment levels which ensured that
the critical DO concentration was above 5 mg/L and the most stringent DO concentration
the system could manage without violations anywhere else in the stream was obtained to
be 7mg/L.
