The term biopolymers may be used to describe macromolecules that are obtained from a wide variety of biological sources. In the wake of growing concerns surrounding the use of non-biodegradable synthetic polymers, they serve as ideal replacements for a wide range of engineering applications. However, given the versatility of (charged or uncharged) chemical species that form biopolymers, there are considerably large uncertainties in determining their exact chemical structure. The presence of charged species in biopolymers further complicates matters, as they remain sensitive to parameters such as acidity (pH) and ionic strength (conductivity), thereby challenging the traditional physical models that describe polymer dissolved within specific solvents.

The work presented in this thesis tries to overcome these limitations through an abstract and idealised description of their structure, using both, specific knowledge about the type of charges present within the biopolymer, as well as parameters such as pH and conductivity. Further, intentional changes are made to the pH and conductivity by introducing specific (counter) ions that are known to influence the thermodynamic stability of the biopolymer in solution. These include both, changes to the conformation of the biopolymers coils due to electrostatic interactions, as well as intermolecular interactions between neighbouring biopolymer coils due to the formation of (weak→ strong) physical bonds. Naturally, the accompanying changes to the structural conformations has an predictable influence on the properties of the biopolymer solution, which in this case is captured using rheology, i.e. the deformation of (solid, liquid and viscoelastic) materials due to the application of a load.

Specifically, rheology is used to track changes to the following parameters, the intrinsic viscosity: which is a measure for the size of the dissolved macromolecules, the Herschel-Bulkley consistency index: which is a measure for the viscous dissipation of concentrated biopolymer solutions, and the storage modulus and the yield stress. These latter two parameters collectively provide quantitative insight about the packing of weakly bonded biopolymer gel particles. In turn this information, i.e. changes to the rheological properties of a wide variety of representative biopolymer systems, is used to derive relevant structure-property relationships that may be exploited for engineering applications.

Finally, the collective insight gained by establishing these structure-property relationships is condensed to provide a simple rheology experiment. This simple experiment is practically relevant to ensure the on site quality of novel biopolymer formulations, as well as to aid suitable modifications to the existing extraction protocol.

Our current resource consumption practices are unsustainable due to our linear approach, which rapidly depletes resources as populations grow and demands increase. To address this issue, we are transitioning towards circular practices aimed at prolonging the use of products, ma-terials, and resources, thereby minimizing waste. This shift is critical for ensuring a sustainable and secure future for the next generations. Consider wastewater as an example: it's not merely dirty water that needs disposal; rather, it represents a concentrated source of valuable resources such as energy, reusable water, and essential nutrients like nitrogen and phosphorus. By embracing these concentrated streams, we have the opportunity to transform wastewater treatment plants into re-source recovery facilities. Here, we can efficiently extract and reuse these precious materials, thus maximizing their value and minimizing environmental impact...

This study explores the potential use of extracellular polymeric substances (EPS), commercially known as Kaumera Nereda Gum®, extracted from aerobic granular sludge (AGS), for foliar fertilization in agriculture. Koppert, a company specializing in sustainable agricultural products, is interested in using Kaumera Nereda Gum® for its biostimulating effects, water-absorbing capacity, biofilm formation, and adhesive properties. They are currently spraying leaves with a solution mixed with Kaumera Nereda Gum®, water, and other fertilizers.

Nonetheless, the utility of using Kaumera Nereda Gum® for foliar fertilization can be affected by several factors, such as seawater intrusion in AGS projects like Faro-Olhão in Portugal and the type of acid utilized for the precipitation of the polymer. In certain places like Utrecht, hydrochloric acid is used, whereas in Faro, sulfuric acid is employed. The latter acid is more favorable when considering agricultural applications.

Thus, the chemical composition of Kaumera samples from Faro and Utrecht was compared, and their suitability for foliar fertilization assessed. Parameters such as total solids, volatile solids, carbohydrate content, and protein content were quantified, and FTIR-ATR analysis was conducted to gain a better understanding of each polymer. Conductivity and pH levels were also measured and compared to expert recommendations against the ideal values for foliar fertilization. Additionally, nutrient levels were quantified and compared to regulatory guidelines and nutritional recommendations. An adhesion protocol was also designed to compare the adhesive properties between Kaumera Utrecht, Kaumera Faro, and Kaumera Zutphen (established benchmark).

Results indicated that Kaumera Faro has 0.114 ± 0.005 grams of carbohydrate per gram VS, whereas Kaumera Utrecht has 0.168 ± 0.002 grams of carbohydrate per gram VS (47% more). On the other hand, Kaumera Utrecht has 0.323 ± 0.009 grams of protein per gram VS, while Kaumera Faro has 0.456 ± 0.004 grams of protein per gram VS (41% more). High carbohydrate content could be positively correlated with strong adhesion properties, whereas a high protein content might demonstrate enhanced biostimulating effects. In addition, the pH and salinity adjustments required for applying the Kaumera solution to leaves are determined by its mixing ratios. When preparing the solution, it is essential to consider the low pH of Kaumera Nereda Gum® and evaluate whether washing the polymer to reduce salinity is necessary. Heavy metal concentrations in samples of Kaumera Utrecht and Kaumera Faro remained within permissible limits according to regulatory thresholds when considering the applied dosage of 35 liters per hectare per year (as suggested by Koppert). However, with such dosage, nutrient levels in the Kaumera samples were lower than recommended, confirming that additional fertilizers are to be mixed with Kaumera Nereda Gum®. Furthermore, the adhesion protocol proved to be a valuable screening tool, showing that Kaumera Utrecht has superior adhesive properties compared to the benchmark. In summary, this study confirms that Kaumera Utrecht and Kaumera Faro exhibit advantageous properties in foliar fertilization, but their effective use requires pH adjustment, salinity reduction, and nutrient supplementation.