Plant-wide modelling of a full-scale industrial waste treatment plant

Decoupling progress and destruction of the environment is the engineering paradigm nowadays. Since the first industrial revolution, the rate at which natural resources are consumed has been increasing exponentially. Given that humans will not fall back in their former living standards, the only way to preserve the environment is to find new ways to sustain our society. In the industrial context, this can be achieved in two ways: By increasing the efficiency of the current production processes, or by developing new technologies that do not consume scarce resources. Waste treatment is particularly entangled with environmental protection, because it can be regarded as the gate connecting the human society with the natural environment.

In this thesis, the focus is set on the industrial waste treatment system (iWTS) that processes both solid and liquid waste streams from two Danish biotech companies, Novozymes and Novo Nordisk. Both companies use, as their main manufacturing tool, fermentation technology, a process that generates by-products such as the product-synthetizing biomass itself, and an array of liquid streams rich in organic material. Although the plant does not have a large incoming volumetric flow rate, the high concentration of pollutants in those incoming streams translates into an organic treatment capacity equivalent to a 2.5 million people municipal wastewater treatment plant. This makes it the biggest iWTS in northern Europe, in terms of organic load. Therefore, optimization of the waste management process would have a great impact on the sustainability of the overall process, and on the economic aspect. In this project, the objective is to obtain a deeper understanding of the behavior of the iWTS, both holistically and of its individual parts. Due to the complexity and the high degree of interconnection of the system, mathematical modelling is used to facilitate the understanding of the main phenomena taking place.

The first steps of the project were taken on studying one subsystem of the iWTS: the solids line. For that purpose, a detailed measuring campaign was conducted, from which the most relevant components found in the waste streams were analyzed. A dedicated model was developed and steady state as well as dynamic simulations provided a clear picture of the origin, transformation, and fate of the principal pollutants. Moreover, some optimization scenarios affecting fundamentally the solids line were tested in silico.

With the successful completion of modelling the solids line, the ground was ready for developing a truly plant-wide model. In this case, the solids line subsystem was integrated in a major framework, where the anaerobic water line and the aerobic water line were also present. The developed models were designed in a way that they could be easily connected, to one another. A new, more comprehensive measuring campaign was conducted. This time advanced data reconciliation methods were used to create a robust and consistent set of process data that could serve to calibrate the plant-wide model.

Additionally, the plant-wide model was calibrated in order to accurately describe the OPEX (Operating Expenses) of the plant based on assumed raw material costs, with the aim of performing techno-economic evaluations. Several alternative scenarios were designed, introducing modifications in different parts/aspects of the process. Their technical feasibility, effect on OPEX and on liberation of total treatment capacity was assessed, showing the full potential of mechanistic mathematical modelling. Combined stream refluxing, operation modifications, and new technology implementation had a synergistic effect over the individual changes in the process, predicting significant improvements in process economy and sustainability.