Sustainable Phosphate Recovery from Wastewater using Biowaste-based Material

Abstract

Phosphorus plays a crucial role in plant growth and biological processes. Recovering phosphorus from waste streams is important for sustainable food production and environmental stewardship. Bio-based materials can be used for effective phosphate recovery through adsorption. This sustainable and cost-effective methodology reduces environmental pollution and eutrophication and contributes to the circular economy. The recovered phosphate solid can be used as a soil conditioner or fertilizer, promoting efficient nutrient management and sustainable agriculture. Thus, following this approach, this work aimed to explore low-cost biowaste materials for effective phosphate recovery from real wastewater. Various biowastes, including orange peels, spent coffee residues, fish scales, seagrass residues of P. oceanica, biochar produced from olive kernels, and biochar generated from vineyard prunings, were tested for their phosphate adsorption capacity. Thermally treated seagrass (SG-TT) and eggshell (EGSL-TT) residues exhibited the highest capacity from the biowastes examined. The optimum pre-treatment temperature and exposure time were determined as 500ºC for 1 h and 900ºC for 30 min, respectively. Moreover, chemical leaching experiments of phosphate from dewatered anaerobic sludge (DWAS) were conducted, evaluating sonication and inorganic acids (sulfuric acid (SA), thermal-sulfuric acid (TSA), and nitric acid (NA)) as extraction methods to determine the most suitable acid medium to leach out phosphate. SA and TSA processes with 84.9 and 93.2% extraction efficiency, respectively. Adsorption batch experiments with real wastes (anaerobic effluent wastewater and leached solution from DWAS), demonstrated that SG-TT and EGSL-TT have high adsorption efficiency and selectivity towards phosphate (>78.4% for both materials). After phosphate adsorption, the solid residues were mixed with compost in different ratios and then tested as fertilizer substitutes on plant growth. The SG solid residue after adsorption produced from anaerobic effluent or synthetic solutions imposed a positive effect on plant growth with germination index (GI) values 96.7 – 111.1%, for all types of seeds tested (Solanum lycopersicum, Lepidium sativum, and Sinapis alba), while the solid residue after adsorption produced from DWAS leached solution negatively affected the germination of seeds, probably due to potentially refractory compounds contained in DWAS. Similar behaviour was observed in EGSL solid residue remaining after adsorption from DWAS leachate, while a positively effect was distinguished on plant growth for Sinapis alba and Lepidium sativum seeds. EGSL-TT were additionally tested in AD systems as a new approach to counteract excessive acidification and alleviate low pH in anaerobic digestion and showed substantially higher methane generation than the control. This new proof of concept contributes to the circular economy; the EGSL-TT is integrated with anaerobic digestion both in-situ for buffering acidification and ex-situ for phosphorous removal and potential use as a soil conditioner. Finally, a process for the valorisation of different type of wastewaters was developed, which was including SAnMBR system in combination with adsorption method. The first system involved the combination of a submerged anaerobic membrane bioreactor (SAnMBR) and two biowaste adsorption columns (SG-TT and EGSL-TT) to assess the recovery of phosphates from low-strength wastewater relatively high in phosphate ion (reject wastewater from anaerobic sludge dewatering process). The results showed a chemical oxygen demand (COD) removal of 50%. However, the phosphate removal efficiency in the reactor was low and as such, the effluent from SAnMBR was then passed through two columns containing SG-TT and EGSL-TT residues for further removal (resulting in over 95% recovery). In addition, the fractionation analysis of phosphorus showed that inorganic phosphorus was the substantial phosphorus fraction in eggshell and seagrass end product, accounting for 92.6 and 95.7% of TP, respectively. Eggshell end product had the highest proportion of apatite phosphorus fraction (87.4%), which require further time for the dissolution process, leading to low concentrations of bioavailable phosphorus fractions and consequently poor germination of seeds. Overall, this study shows that the seagrass and eggshell could be effectively reclaimed as selective adsorbents towards phosphates in advanced wastewater treatment processes when combined with a SAnMBR. The second system used a SAnMBR to treat synthetic and domestic wastewater. The COD was effectively treated, with average removal percentages of 82.3 and 87.8% for synthetic and domestic wastewater, respectively. Then, the SAnMBR effluent was independently exposed to EGSL-TT and SG-TT, and the phosphate ions were recovered with percentages up to 71.8 - 99.9% and 60.5 - 78.0%, respectively, for all the flow rates tested. The effluent from EGSL and SG was exposed to powder-activated carbon (PAC), and the COD was further reduced to a concentration of 20.2 ± 5.2 and 57.0 ± 13.3 mg L-1. The respective final effluent was evaluated in phytotoxicity trials demonstrating that the SAnMBR effluent after the PAC treatment was significantly better for Lepidium sativum and Sinapis alba seeds tested than the untreated domestic wastewater. Finally, the determination of volatile organic compounds (VOCs) depicted a significant decrease from the very first steps of the process, with a reduction of common wastewater contaminants such as dimethyl disulfide, dimethyl trisulfide, phenol, p-cresol, nonanal and decanal. The tested technology might be considered a promising treatment system since can effectively treat domestic wastewater, reduce the VOC, generates biogas for energy, produces solid products high in inorganically bound phosphate and the effluent can be used for irrigation.