Algal growth in human urine pretreated with ion exchange process: Effect of different coagulants on harvesting efficiency

Abstract

Recently, the global population has reached 8.2 billion and is expected to increase, eventually reaching 10.3 billion by the 2080s. This population growth has led to extensive consumption of natural resources such as water, food, and energy. By 2030, the predicted increase in demand for food, water, and energy is expected to rise by 35%, 40%, and 50%, respectively. Meeting these growing demands will require innovative and sustainable approaches to resource management. Nitrogen and phosphorus are essential elements for life, and their consumption will increase proportionally with population growth. However, nitrogen, the most abundant element in the air, must be fixed through the energy-intensive Haber-Bosch process, while phosphorus is obtained via mining, with limited and unevenly distributed reserves worldwide. These elements are crucial for agriculture; however, the reliance on non-renewable energy sources for ammonia synthesis via the Haber-Bosch process, combined with the scarcity and unequal distribution of phosphate rocks, poses a risk to future fertilizer production. Therefore, shifting to alternative nutrient recovery methods is not only essential for the sustainability of agricultural production, but also a step toward reducing greenhouse gas emissions associated with fertilizer production. Wastewater, generated by human activities, contains high levels of valuable nutrients. However, current wastewater management systems follow a "treat and discharge" approach, which not only leads to the loss of these nutrients but also contributes to eutrophication in natural water bodies. Sustainable management practices need to be developed to reduce energy consumption and preserve limited resources. Ecological sanitation (ECOSAN) systems offer a sustainable alternative to conventional wastewater treatment. This approach separates wastewater streams at the source, enabling the recovery and reuse of valuable nutrients. Depending on the intended reuse or recovery purpose, wastewater can be separated into two or three streams. The two-stream system divides wastewater into black water and grey water, whereas the three-stream system separates it into yellow water, grey water, and brown water. Among these streams, yellow water (urine) constitutes only 1% of domestic wastewater but contains 80% of nitrogen, over 50% of phosphorus, and more than 50% of potassium. Separating yellow water from domestic wastewater could effectively close the nutrient loop in wastewater management. Microalgae cultivation using wastewater as a nutrient source has attracted significant attention due to its economic and environmental benefits. Nutrient removal, primarily nitrogen and phosphorus, can be achieved cost-effectively, while simultaneously producing microalgae biomass rich in proteins, lipids, carbohydrates, and essential vitamins. This biomass can serve as a sustainable resource for biofertilizers and biofuels. While nutrient removal from source-separated human urine (SSHU) and the cultivation of microalgae species using diluted or concentrated urine have been widely studied, research specifically on the cultivation of Spirulina platensis using pretreated urine remains limited. This study aimed to evaluate the potential of SSHU as a nutrient medium for microalgae cultivation within the ECOSAN framework. Ion exchange pretreatment was applied to reduce the high ammonium concentration in hydrolyzed urine and create a non-toxic growth medium suitable for Spirulina platensis. The study also investigated the effectiveness of alum and chitosan as chemical coagulants for biomass harvesting. For this purpose, human urine was collected from two urine-diverting toilets and a urinal located at the Department of Environmental Engineering, Istanbul Technical University. The collected urine was stored to convert urea into ammonium through hydrolysis. Ammonium was removed using clinoptilolite, a natural zeolite, as an adsorbent. Pretreated human urine after ion exchange was used as a growth medium to cultivate acclimated Spirulina platensis microalgae in a photobioreactor. Biomass growth was monitored daily by measuring the optical density (OD) at 680 nm, 690 nm, 720 nm, and 750 nm, and through dry weight analysis. After reaching maximum growth, the microalgae were harvested. Ammonium concentration was 1747 ± 26 mg/L, increasing to 4400 ± 100 mg/L by the end of the collection period. During the hydrolysis process, ammonium concentration reached 5010 ± 10 mg/L, while pH and electrical conductivity (EC) were measured as 9.03 ± 0.005 and 26650 ± 50 µS/cm, respectively. Based on previous studies, an ammonium loading rate of 10 mg NH₄⁺/g clinoptilolite was selected as the optimal initial loading rate. The initial pretreatment experiment was conducted over five days. the majority of the removal occurred on the first day of operation and no significant changes in concentration were observed after three days. Through ion exchange with an initial clinoptilolite loading of 10 mg NH₄⁺/g, 74.3% of ammonium, 94.4% of total phosphorus (TP), 72.3% of total nitrogen (TN), and 52.2% of chemical oxygen demand (COD) were removed from hydrolyzed urine. The Spirulina platensis was successfully cultivated in the photobioreactor using pretreated human urine as a growth medium. Spirulina growth was achieved under a free ammonia concentration of 927 mg/L, with a growth rate of 0.185 d-1. After a nine-day cultivation period, the biomass concentration reached 1.62 g/L, while the OD at 680nm (OD680) reached 2.21. Throughout the entire cultivation period, pH level was maintained at 10 ± 0.02. The removal efficiencies for TN, NH4+-N, TP, and COD parameters were 64%, 48%, 80%, and 48%, respectively. A jar test was carried out to determine the coagulant type and optimum dosage for chitosan and alum. Three different experiments were performed for chitosan and alum coagulants. The chitosan coagulant achieved a maximum harvesting efficiency of 38.9 ± 0.5% at a dosage of 3000 mg/L, whereas the alum coagulant achieved a maximum harvesting efficiency of 97.3 ± 0.1% at a dosage of 1250 mg/L. The results demonstrated that alum, at a concentration of 1250 mg/L, was the most effective for harvesting. Harvesting was conducted using 1250 mg/L alum dosage, achieving 97.3% efficiency at OD680. This research highlights the potential of cultivating Spirulina platensis using pretreated, source-separated urine (SSU) as a sustainable approach to nutrient recovery via microalgae uptake. It demonstrates a promising, resource-conserving, and energy-conserving method that supports sustainable agriculture.