Dyllon Garth Randall

This study presents a novel, modular 3D printing system for the scalable production of microbially induced calcium carbonate (CaCO ₃ ) precipitation (MICP) produced bio-materials. Through modular design principles and operation automation, the technique offers a biochemical route to cementitious materials with the potential for lower energy and carbon footprints compared to traditional kiln-fired ceramics. The bio-tiles produced using this technique exceed international standards of breaking strength and modulus of rupture. By leveraging freeze-dried urease-active bio-slurry powder, the process ensures uniform bacterial distribution within the sand matrix, enhancing reproducibility and scalability. To establish a technical baseline for this automated platform, the effect of CaCO ₃ seed supplementation, magnesium-enriched cementation solution, and manual daily nutrient broth treatments was evaluated as a function of ureolytic activity and mineralization performance. Results revealed that magnesium addition paired with nutrient broth supplementation was a notable factor associated with the observed strength enhancement, achieving an average of 622 ± 84 N. While individual seed loadings showed variability consistent with gravity-driven infiltration, the aggregate performance of magnesium-supplemented tiles established a clear general technical trend, achieving strengths 3.3 times higher than bio-tiles treated without magnesium and only nutrient broth. However, nutrient broth treatments were found to be redundant as ureolytic requirements were likely satisfied by the bio-slurry itself. Additional CaCO ₃ seeds minimally influenced strength but improved edge solidification. Despite challenges in achieving uniform solidification without specific additives, the automated system successfully integrated submersion and pumping methodologies. The bio-tiles exceeded strength standards for tiles with a water absorption of greater than 10%, underscoring their potential for sustainable construction applications. This proof-of-concept highlights the mechanical automation as a key enabler for scalable MICP-based manufacturing, reducing labour intensity and optimising bio-cementation for sustainable, high-performance materials. Future research should focus on enhancing solidification consistency and advancing MICP-based bio-materials for structural applications. The integration of automation, modularity, and optimised MICP conditions marks a significant advancement toward the commercial-scale production of materials with improved environmental profiles.

Human urine is rich in valuable nitrogen which can easily be lost due to biological urea hydrolysis and subsequent ammonia volatilization. While this enzymatic reaction can be prevented by alkalizing the urine, recent studies suggest that chemical urea hydrolysis can result in substantial nitrogen losses when drying alkalinized urine at high temperatures. Furthermore, it was previously suggested that CO₂ dissolution from the air used to evaporate water from alkalinized urine could result in a pH decrease to values which allows for biological urea hydrolysis and subsequent ammonia losses. This study aimed to determine the kinetics of chemical urea hydrolysis in alkalinized human urine and confirm the effect of CO₂ dissolution with controlled laboratory experiments. We measured the change in urea concentration at different temperatures and pH values for real human urine and determined the corresponding rate constants for chemical urea hydrolysis. We showed that the rate constant increases as a function of temperature and that pH has a negligible effect on the rate of chemical urea hydrolysis in the high pH range of alkalized urine (>11). The rate constants for chemical urea hydrolysis in a saturated calcium hydroxide solution were found to be 0.00147 d⁻¹, 0.00595 d⁻¹, 0.0204 d⁻¹ and 0.0848 d⁻¹ for temperatures of 25°C, 40°C, 55°C and 70°C, respectively. The effect of CO₂ dissolution on urea hydrolysis was determined by aerating human urine alkalinized with calcium hydroxide (Ca(OH)₂). In order to represent biological urea hydrolysis, urease was added to the solution. The computer simulations of the experimental results showed that CO₂ dissolution and the subsequent dissociation of carbonic acid to carbonate ions, bicarbonate ions and protons is the main cause of the pH decrease, but CaCO₃ precipitation, and NH₃ volatilization foster the pH decrease. However, biological urea hydrolysis prevents the pH from decreasing below 9. Residual undissolved Ca(OH)₂ was shown to substantially delay the pH decrease. Overall, this work provides valuable insights into the mechanisms of urea hydrolysis in alkalinized urine during dehydration, which can be used to design more efficient decentralized sanitation systems and minimize nitrogen losses.