MODELLING
WASTEWATER
PROCESS
MODELLING
South Africa has a long and prestigious history of wastewater treatment process modelling that is internationally recognised. Sadly, there is a slow, minimal uptake of this research and work by our own municipalities.
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arge, better-resourced municipalities and water services authorities – like the City of Cape Town, eThekwini and the Ekurhuleni Water Care Company – have some capability to implement process modelling,” says Barbara Brouckaert, research fellow at the University of KwaZulu-Natal (UKZN) WASH R&D Centre. However, academics and consultants take up most of the work. “In theory, some municipalities have some very capable people to do modelling, but these people also have a host of other responsibilities and there are time constraints. The time and data needed to set up a working model are often underestimated. Effective modellers have a specific mindset and need space to work intensely for long periods without being distracted by other responsibilities. It is very difficult to persuade municipalities to invest in that resource,” explains Chris Brouckaert, senior research fellow at the WASH R&D Centre.
Advantages
This is a pity, as wastewater process modelling can provide a host of benefits in terms of the management and planning of wastewater treatment works (WWTWs), such as: • creating a better understanding on how to improve the design and operation of WWTWs • evaluating the benefits, impacts, and feasibility of planned upgrades
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• accurately estimating potential chemical/energy savings • risk analysis • predicting performance under various operating conditions and control strategies, including future changes in the quantity and quality of water/wastewater • analysing system dynamics, e.g. the impact that a sewer system may have on the WWTW • prioritising interventions that will have the greatest system-wide impact.
Local context
“Instead of trying to pursue ultimate sophistication, we have adapted process modelling to suit local conditions in the wastewater sector, where there is poor
data and information availability, and unreliable infrastructure,” says Chris. Barbara elaborates that the process modelling of WWTWs requires a lot of data, and much of that data is not routinely available (like biodegradability). “Furthermore, the available data is frequently incomplete and inconsistent, because most of it is seldom actually reviewed or used for anything. Collecting data and getting it into a usable form is usually the biggest part of the modelling effort.” Conducting measurement campaigns for additional data is expensive and time consuming, and it is difficult to get representative values due to the highly variable nature of domestic wastewater.
MODELLING: BASIC TERMS AND DEFINITIONS Process modelling: Process models are equations or sets of equations that describe the behaviour of processes or systems under various conditions, often coded in modelling software packages. Unit process modelling: This would represent a specific par t/equipment of WWTWs, like an activated sludge reactor, anaerobic digester, settler or dewatering unit. Plant-wide modelling: Where all the unit processes are linked together in a model of an interconnected system. Integrated system modelling: Combining models of systems that interact with each other, like a WWTW with a catchment and sewer system. Steady-state model: A model that provides time-invariant results typically based on average conditions. Dynamic model: This is a model in which the inputs, operating conditions and model outputs all change with time. For example, it would be used to simulate how plant per formance changes as raw water and operating conditions change, or when an item of equipment fails.
