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# The calculations behind circular breakthroughs

Published on
December 17, 2020

Fish–plant systems in which the urine and faeces of the farmed fish serve as food for the plants. Or high-quality, circular eco-industrial parks on the outskirts of the city. Karel Keesman, professor of Systems Theory for Sustainability, develops mathematical models for calculation of the feasibility of sustainable food systems. These are all systems in which multiple cycles come together.

Keesman, who is convinced we are increasingly moving towards combined food production systems in the world, explains, ‘The future belongs to systems in which the nutrient, water and energy cycles are closed. The end product is healthy food produced sustainably. These systems do not just happen; they require a great deal of calculation using mathematical models, and not just to design the system but also to predict its daily operation. That is what I work on within Biometris, the mathematical and statistical methods group at Wageningen University & Research.’

## Food for fish and plants

For example, Keesman calculates fish–plant systems, in which fish are farmed in large tanks. ‘The urine and faeces excreted by the fish contain a lot of fertilisers, such as phosphate and nitrogen. We send these fertilisers to the plants, which use them for growth. Particles of faecal matter and uneaten fish food sink to the bottom, and these particles end up in a bioreactor for the production of biogas. The fertilisers contained in the nutrient-rich water within the reactor are also sent to the plants. Therefore, in this system we use fish food to grow both fish and plants. This involves considerable calculations: how much feed and water do you need? What is the optimum moment to send the water from the reactor to the plant? These are the kinds of questions we want to be able to answer with mathematics.’

## Urban agriculture

Keesman is also involved in a major international project to develop these kinds of aquaponic systems on the outskirts of large cities, under the name Cityfood. Aquaponics is a portmanteau of aquaculture and hydroponics: the cultivation of crops in the gutters of greenhouses. ‘These kinds of urban farming systems contribute directly to the food supply of cities – and especially high quality food: fish and fresh vegetables. This solves various problems: it is a solution to the lack of agricultural land in urban areas, and it reduces the need for transport and storage. In the city itself, even parking garages or cellars beneath buildings could be used for this form of food production. This would involve leaving the necessary LED lighting on, powered by the solar panels on the roof. As we work out the calculations we come up with all kinds of new cycles to add to the mix.’

## Eco-industrial parks near the city

At the time of writing, Keesman is working on an EU proposal that combines all these different ideas. He envisions a future with eco-industrial parks near urban areas, with a mix of agro-activities, all with closed cycles. Take Westland, the epicentre of greenhouse cultivation, for example. Keesman sketches the situation, ‘In the immediate vicinity of this area there is heavy industry, such as refineries, which produces large volumes of CO2 emissions. That CO2 is important for plant growth. At present, the practice among growers is still to burn natural gas to raise the CO2 level in the greenhouse. But that use of gas is coming to an end, so the CO2 has to come from somewhere else. Then it quickly becomes clear how the puzzle pieces fit together. So, the first step is to start making the necessary calculations.’

## Organic desulphurisation

He is also enthusiastic about another development: organic gas desulphurisation. ‘Natural gas contains sulphur, and biogas does too. In the past this caused acid rain, but these days it is removed from the gas. This is done on a large scale, using chemical methods. At Wageningen we have already been developing organic desulphurisation methods for thirty years. Bacteria are used to convert the sulphur compounds from the gas into particles that can be collected. There is now a project under way to use these sulphur particles for agriculture, where there is an increasing shortage of this substance.’

For Keesman and his colleagues this means taking a deep dive into the fundamental science: what happens in the bacteria during the conversion to sulphur particles? ‘Extremely complex, yet immensely interesting subject matter for calculations. Particularly to predict how the start-up process should proceed: under what conditions do the bacteria work optimally? By answering questions like these we are developing good circular solutions to the great challenges of our time.’