Assessing phosphorus recovery from anaerobic digestion effluent of tapioca starch processing in a pilot – scale fluidized – bed homogeneous crystallizer: Effects of operation modes

Abstract

This present study aimed to investigate the effects of two crystallization operation modes, batchwise operation (BO) and continuous operation (CO), on P recovery efficiency and characteristics of precipitates in a pilot – scale fluidized – bed homogenous crystallizer, using real anaerobic digestion (AD) effluent derived from tapioca starch processing as influent. Characterization techniques, including SEM – EDX, powder XRD, profile – matching, FT – IR, and crystal size distribution (CSD) analysis, were employed to identify phase compositions. Additionally, the purity and %P content of the target product were quantified using principles of hierarchical equilibrium thermodynamics and elemental analysis. As a result, the highest P recovery efficiency was 72.5 % for BO and 83.2 % for CO at a pH of 9 and a mixing rate of 400 rpm. Phase identification results revealed that struvite formed with Ca – rich solids, such as hydroxyapatite (HAP), notably exhibited higher impurity levels originating from CO modes. Precipitates from BO modes predominantly held 65 – 75 % struvite, with %P content ranging from 7.4 to 13.1 % — surpassing that of single superphosphate fertilizer (SSP, 18 – 22 % as P2O5), resulting in a suitable process in this study. Moreover, results obtained from the cost – benefit analysis (CBA) highlighted that BO scenario had economic viability (NP +0.05 to +1.53) with an estimate production cost of 3.47 USD/kg Premoved. Hence, the findings not only indicated that real AD effluent is suitable for P recovery, yielding high – quality struvite products, but also contribute to understanding the nuanced impact of crystallization modes on recovery yields and characteristics. This insight is invaluable for optimizing the process, leading to environmental benefits through reduced P and nitrogen discharge.