Struvite recovery from anaerobically digested waste activated sludge
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Date
2024-09
Authors
Funani, Charmaine Kgomotso
Journal Title
Journal ISSN
Volume Title
Publisher
Vaal University of Technology
Abstract
The current state of nutrient recovery from wastewaters based on struvite crystallization is investigated widely, emphasizing the process, formation conditions in various wastewaters, and nutrient recovery effectiveness improving water quality and minimizing water pollution at the same time sustaining and conserving the environment. Anaerobically digested wasteactivated sludge (WAS) from municipal wastewater treatment plants is a promising substrate for phosphorus recovery and recycling given the high phosphate content. This study aimed to recover struvite from the supernatant of anaerobically digested WAS following a one factor a time (OFAT) experimental design, optimise the recovery process using Central Composite Design (CCD) and Response Surface Methodology (RSM), and characterise the recovered struvite.
The supernatant solution from anaerobically digested WAS was collected from the ERWATMidvaal wastewater treatment plant in Gauteng. Thereafter, using Sodium hydroxide (NaOH, 99%) the pH was adjusted to the desired alkaline conditions, the molar ratio of Mg:P was adjusted, and struvite precipitation was conducted using Magnesium Sulphate heptahydrate (MgSO4.7H2O,99%) and Ammonium Chloride (NH4Cl, 99.5%) salts. The precipitation followed batch experiments conducted at various pH levels and various Mg:P ratios at room temperature. Response Surface Methodology (RSM) was applied for the optimisation of the struvite recovery process and to determine the interactive effects of the key parameters (Mg:P and pH). The formed precipitates were separated from the liquor and oven-dried at 50 ◦C for 48 hours. The phase purity of the recovered struvite was validated using an X-ray diffraction (XRD) pattern and Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy. An elemental analyzer determined the proportion of phosphorus and nitrogen in the recovered product and inductively coupled plasma optical emission spectroscopy also determined the metal element composition.
Phosphate removal efficiency was highest at pH 10 with 75% P removal at Mg/P ratio of 1. For the optimisation studies, a second-order polynomial equation was determined to correlate the parameters. Analysis of variance (ANOVA) was applied and showed that p-values for all the investigated parameters were less than 0.05 showing that they had a statistically significant effect on the phosphorus recovery. Above a pH of 9 and Mg/P ratio of 1, the phosphorus recovery was high at over 80%. Energy dispersive x-ray spectroscopy (SEM-EDX) showed the formation of uneven structure crystals and the presence of Mg, P, and O indicating the formation of struvite. Low compositions of interfering ions were found (K and Ca), meaning that Ca-P and K-P compounds might have formed in insignificant quantities. XRD and FTIR spectroscopy confirmed struvite formation.
Past research has shown that recovered struvite offers a sustainable alternative to conventional fertilizers, reducing nutrient leaching and environmental impacts. Its slow-release properties improve soil health and crop growth, despite initial high costs. The long-term benefits, including reduced application frequency and lower remediation costs, make it economically viable.
The current study has potentially significant impacts on the environment, economy, health, and community. The findings were based on local parameters and are viable for performance improvement. The adoption of the proposed struvite recovery approach is significant in the economics of wastewater and environmental management and community development. Struvite recovery contributes to a sustainable sludge management approach, which creates a circular economy. In the circular economy, nutrients that are lost along the food value chain as organic and human excreta waste are recycled back into the soils, improving food security. This could be achieved through increased low-cost nutrient recycling as well as minimal use of expensive inorganic fertilizers. Future studies should be conducted to improve the quality and purity of the recovered struvite and determine cost implications before upscaling.
Description
M. Eng. (Department of Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology.
Keywords
Struvite crystallization, Wastewaters, Nutrient recovery, Wasteactivated sludge (WAS), Water pollution, Municipal wastewater treatment plants, Phosphorus recovery, Central Composite Design (CCD), Response Surface Methodology (RSM)