Struvite recovery from aqueous waste streams: Feedstock modelling and environmental evaluation of commercial technologies

Saeede Soltani

Recovering phosphorus from wastewater as struvite (NH4MgPO4.6H2O) which is a slow-release and effective fertilizer, is a promising approach to utilise the valuable nutrient and mitigate eutrophication impacts to water bodies. However, there are research gaps in the field of struvite precipitation. A standardized method for preparing synthetic nutrient-rich solutions is lacking, hindering accurate assessment of the process. Additionally, concerns about the environmental and sustainability impacts of struvite production remain, due to the limited research focusing on these aspects. Most previous life cycle assessments have concentrated on the overall system rather than the struvite product itself. Furthermore, comparative studies evaluating full-scale struvite recovery from wastewater treatment plants are scarce. Addressing these research gaps is crucial for a comprehensive understanding of the environmental implications of struvite production and recovery. This PhD attempts to addresses three fundamental questions in phosphorus recovery as struvite. First, focused on using human urine as a source of nutrients for fertilizer. Struvite, a phosphate-rich mineral, can be extracted from urine and used as a valuable fertilizer. Due to biohazard concerns, synthetic urine was often used in research. The study developed a model to create synthetic urine that accurately mimics the elemental composition of real urine. This model considered various factors like mass balance, chemical speciation, and solution thermodynamics. The model was used to calculate the necessary salt quantities, pH, ionic strength, and struvite saturation index in both fresh and stored urine. The results from this model were verified using another software, PHREEQC. The model's accuracy was further validated by comparing the simulated urine composition to known recipes. Second, a comparative life cycle assessment of struvite recovered from five systems conducted based on commercial technologies, including Systems A (Phosnix), B (Ostara’s Pearl), C (Multiform), D (Phospaq), and E (NuReSys). The focus is on the impacts of feedstock P concentration and grid electricity mix on the energy and carbon footprints of the recovered struvite. The base-case results demonstrate that the energy and carbon footprints of struvite are in the ranges of 3.6 − 14.4 MJ/kg struvite and 0.46 − 1.88 kg CO2-e/kg struvite, respective, when using medium-P industrial wastewater as feedstock and the current grid electricity in Western Australia to meet energy demands. Struvite recovered from systems that use MgCl2 as Mg source exhibits greater energy and carbon footprints, as compared with those employ Mg(OH)2 and MgO. The struvite recovered from all five systems is energetically favourable as compared with conventional fertilizers. The impact of feedstock P concentration on the energy and carbon footprints of struvite depends on the recovery system. For Systems A, C, D and E, switching from low-P municipal wastewater to medium-P industrial wastewater significantly reduces energy and carbon footprints. Further switching to high-P farm wastewater results in additional reductions in these footprints, but to a lesser extent. System B shows increased energy and carbon footprints when switching to high-P farm wastewater due to higher electricity demand for product drying. Increasing the share of renewable electricity significantly reduces both the energy and carbon footprints of the struvite recovered with the five systems across all the feedstocks. Systems with higher electricity demands exhibit steeper reductions in both energy and carbon footprints when shifting to renewable electricity. These findings confirm the benefits of recovering struvite from wastewater, evaluate the environmental performance of five recovery systems based on commercial technologies, and highlight the significance of both feedstock P concentration and grid electricity mix on the energy and carbon footprints of the recovered struvite, contributing to sustainable wastewater treatment and a circular economy. In the third and final phase of our research, we conducted a detailed comparison between a traditional wastewater treatment plant (WWTP) and an integrated WWTP that incorporates struvite recovery. Our analysis focused on the environmental footprints of these two systems within the Australian context. To ensure a comprehensive assessment, we considered the environmental credits associated with biosolids in the conventional WWTP and struvite in the integrated WWTP, taking into account the quantity of treated water. This approach allowed us to gain a more accurate understanding of the overall environmental impact of each system. Our findings reveal that while struvite recovery offers several environmental advantages, such as reducing abiotic depletion, photochemical oxidation, acidification, eutrophication, particulate matter, and human toxicity (cancer), it also introduces new challenges. These challenges include increased chemical use, global warming, abiotic depletion (fossil fuels), ozone layer depletion, human toxicity (non-cancer), and freshwater ecotoxicity. Furthermore, our state-specific analysis highlights the variations in environmental impacts across different regions in Australia. For instance, the Northern Territory demonstrates the lowest overall abiotic depletion, while Tasmania exhibits significant reductions in photochemical oxidation, acidification, and eutrophication. However, it's important to note that struvite recovery also leads to increased global warming, abiotic depletion (fossil fuels), and ozone layer depletion in all states. In conclusion, our study underscores the trade-offs associated with struvite recovery. While it offers environmental benefits, it also introduces new challenges. Therefore, a comprehensive evaluation of wastewater treatment strategies is necessary to weigh the benefits and drawbacks of struvite recovery. A holistic approach that considers the long-term environmental and social impacts is crucial for making informed decisions about sustainable wastewater management.

Struvite recovery from aqueous waste streams: Feedstock modelling and environmental evaluation of commercial technologies

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