Wastewater process modelling

WASTEWATER MODELLINGPROCESS

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.

Large, 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 • 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 part/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 performance changes as raw water and operating conditions change, or when an item of equipment fails.

The UKZN modelling group and Professor David Ikumi’s Water Research Group at the University of Cape Town (UCT) make up the South African Development Centre for WEST, the Danish Hydraulic Institute’s software for advanced wastewater systems modelling. However, both groups also focus on developing models and modelling tools that are adapted for local conditions. For example, UCT has developed several steady-state modelling tools in Excel, including a tool for estimating the capacity of an existing WWTW.

“The design capacity – the number of megalitres per day of wastewater a plant can treat – is always a focus point for local government. However, it has become important to determine how many megalitres per day a plant can treat with ageing infrastructure and failing equipment. We need to calculate the actual functional capacity of WWTWs,” explains Chris.

The term ‘functional capacity’ was coined by a process engineer in eThekwini Municipality’s Water and Sanitation Department (EWS). It refers to the megalitres per day a WWTW can process with its existing equipment and operational arrangements. There are frequent queries regarding the capacity of WWTWs to process wastewater from prospective housing developments or businesses. While the design capacity may indicate that a WWTW could accommodate a new development, the functional capacity often presents a different picture.

“This is where modelling WWTWs can provide value – it can scientifically demonstrate the functional capacity of a WWTW and provide an analysis of what needs to be done (in terms of upgrades, replacement of equipment, operational changes, additional resources) at the WWTW in order to accommodate more wastewater,” states Chris.

In addition to improving the generation and collection of accurate data, as well as employing full-time modellers, Barbara adds that there needs to be dialogue between the model, theory, concept and the reality in the WWTW with its operational staff. “Typically, the operational staff generate the data and they have to understand why that data is important. Additionally, clear modelling objectives that align with the priorities of the municipality or WWTW management will determine the level of detail

Chris Brouckaert, senior research fellow at the UKZN WASH R&D Centre Barbara Brouckaert, research fellow at the UKZN WASH R&D Centre

“The design capacity – the number of megalitres per day of wastewater a plant can treat – is always a focus point for local government. However, it has become important to determine how many megalitres per day a plant can treat with ageing infrastructure and failing equipment. We need to calculate the actual functional capacity of WWTWs.”

Modelling schematic

required, and where the modelling efforts need to be focused.”

Work done with eThekwini Municipality

UKZN works very closely with eThekwini’s Process Engineering Services group, which works as an internal consultant to EWS.

They have established a process modelling group, with support from UKZN, with the ambitious long-term aim of developing an integrated model of the metro’s entire wastewater system, eventually including sewers, treatment works and rivers. It is intended to be a decision support system for the municipality to plan and manage all aspects related to wastewater.

EWS has also collaborated with UKZN on several Water Research Commission-sponsored projects, including: • The potential for including modelling in the Green Drop programme: This project evaluated UCT’s capacity estimation tool, as well as the use of dynamic modelling for risk assessment. For example: the design capacity of a number of older plants is not known accurately, due to lost design plans; the importance of storing necessary equipment spares in case something fails; and the impact on a WWTW if equipment has failed or is not running optimally. • Using modelling to investigate the impact of low-flush toilets on sewers and treatment works: Low-flush toilets and other water-saving devices reduce the minimum flows in local reticulation, which can increase the risk of sewer clogging due the deposition of solid material. They also increase the concentration of the sewage under dry weather conditions, which can potentially lead to corrosion problems. It was furthermore found that low-flush toilets have little impact on peak flows, which are rather determined by stormwater ingress, and they do not increase the capacity of the system in terms of the population it can serve.

Based on the modelling results, it was concluded that sewer design and maintenance guidelines need to be updated to account for increases in the efficiency of water use. UKZN is currently working with EWS on another Water Research Commission-funded project to provide modelling support to South African municipalities. One of the outputs of this project has been the launch of the WISA Modelling and Data (MAD) Division, which aims to facilitate the sharing of data and modelling expertise in the Southern African water sector.

UKZN is also collaborating with EWS, UCT and Danish Technical University on a Danish International Development Aid-funded project called ‘Evaluation of Resource Recovery Alternatives in South African Water Treatment Systems’. This project includes a modelling case study of eThekwini’s KwaMashu WWTW in Durban.

Future trends

With experience in the process modelling of wastewater treatment plants since 2005, Chris notes that while modelling software and knowledge in applying models have evolved over the years, the fundamentals of modelling have remained the same.

According to Barbara, there is a move towards integrated urban water system modelling, where the entire water system is modelled and water quality issues can be assessed in a holistic manner. “There is only a limited amount of improvement that can be made by solely focusing on WWTWs.”

Another international trend is digital twins, which provide a virtual representation of a real-world system, running in parallel to the actual system, thereby offering continuous operational guidance. Digital twins of WWTWs are running in China and some European countries.

Real-time control (RTC) is when there is a direct link between the model and the WWTW, and a computer will make changes to the plant the moment a problem is detected.

“Digital twins and RTC are quite a few years away in South Africa; the immediate priority is to restore and improve the efficiency of our existing WWTWs around the country, and process modelling can definitely assist with that,” concludes Chris.