Integrated electrocoagulation-flotation of microalgae to produce Mg-laden microalgal biochar for seeding struvite crystallization

Abstract

Developing sustainable materials for recovering and recycling nutrients from wastewater is critically needed for nutrients such as phosphorus that have a diminishing supply. Struvite crystallization is emerging as a promising strategy for phosphorus recovery which can be enhanced with seeding through microalgal biochar. The main bottleneck of using microalgae is its high harvesting cost. In this study, an integrated electrocoagulation-flotation (ECF) process is used to recover and at the same time modify the algal surface with magnesium anode and inert carbon cathode. Harvesting efficiency of 98% was achieved with 40.78 mA cm−2, 0.5 cm inter-electrode distance and energy consumption of 4.03 kWh kg−1 in 15 min. The harvested microalgae were pyrolyzed to obtain a yield of 52.90% Mg-laden microalgal biochar. Simultaneously, surface impregnation of 28% magnesium was attained as confirmed by Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Phosphorus recovery and struvite yield of 93.70% and 2.66 g L−1, respectively, were obtained from dosing 1.50 g L−1 Mg-laden microalgal biochar. Comparison of physicochemical characteristics of residual supernatant after microalgal harvesting and struvite recovery showed that the combined use of both the residuals can serve as a sustainable growth medium for microalgae. The overall operating cost of the integrated process was found to be 2.48 USD kg−1 with a total energy consumption of 10.76 kWh kg−1, which was found to be lower than conventional harvesting unit processes such as centrifugation and filtration. This novel approach can help attaining a circular bioeconomy by encompassing nutrient recovery and waste management in an integrated process.

Biochar integrated reactive filtration of wastewater for P removal and recovery, micropollutant catalytic oxidation, and negative CO2e: Process operation and mechanism

“…With the biochar modification (i.e., Fe‐BC P in this study), the Fe treatment on the biochar creates nanoparticulate iron oxide (e.g., hydrous ferric oxide) that increases the adsorption affinity of the biochar for P (Bakshi et al, 2021; Zhang et al, 2020). This is supported by studies that show metal modifications on biochar facilitate the formation of metal‐biochar bridges between phosphate anions and negatively charged weak‐acid functional groups on the biochar (Garcia‐Perez et al, 2022; Kubar et al, 2021; Nageshwari et al, 2022).…”

Sustainable Fertilizers: Publication Landscape on Wastes as Nutrient Sources, Wastewater Treatment Processes for Nutrient Recovery, Biorefineries, and Green Ammonia Synthesis

“…Biochar sorbents are less expensive than struvite or direct sludge water applications and the phosphorus from the biochar is more bioavailable to plants. From these studies, we can see the promise of biochar for recycling waste feedstocks into useful adsorbent carriers of nutrients, as alternative crystallizing agents for struvite that could replace more expensive chemical precipitants, or as sorbents for the removal of pollutants from wastewaters …”

“…It was found that the applied current, the CaCO 3 particle size, and the feed rate can affect calcium phosphate precipitation . As an alternative seeding agent microalgae-derived biochar enriched with magnesium has also been used for seeding struvite crystallization …”

Sustainable Fertilizers: Current Publication Landscape and Challenges

“…From these studies and more we can see the promise of biochar for recycling waste feedstocks into useful adsorbent carriers of nutrients or other products, as alternative crystallizing agents for struvite potentially replacing more expensive chemical precipitants, or as sorbents for the removal of pollutants from wastewaters. 114 In the following study on the hydrothermal carbonization of digestate, an anaerobic digestate may be used to recover nutrients. Hydrothermal carbonization of anaerobically digested biomass is a way to make another biochar, referred to by the authors as "hydrochar" due to the process used to make it.…”

“…156 Microalgae-derived biochar enriched with magnesium has been used for seeding struvite crystallization. 114 In Ion-Exchange Membrane Electrodialysis (ED), extraction of nutrients from wastewater occurs via application of ion-exchange membranes. 157 Over 95-98% of the phosphate and nitrate nutrients were recovered from the wastewater from a membrane bioreactor using ion exchange.…”