New Insights into Changes in DOM Fractions in a Crab Farming Park and Key Factors in the Removal Process Using Fluorescence Spectra with MW-2DCOS and SEM

With the explosion of crab farming in China, the urgent need to treat crab wastewater can never be overemphasized. Hence, in this study, excitation–emission matrix (EEM) fluorescence spectroscopy with parallel factor analysis (PARAFAC), moving window two-dimensional correlation spectroscopy (MW-2DCOS) and structural equation modeling (SEM) were employed to identify changes in the dissolved organic matter (DOM) fractions in a crab farming park and reveal latent factors associated with removal processes. Seven components (C1–C7) were extracted from DOMs by EEM-PARAFAC as follows: C1: microbial byproduct-like substances, C2: visible-tryptophan-like substances, C3: fulvic-like substances, C4: phenolic-like substances, C5: ultraviolet tyrosine-like substances, C6: D-tryptophan-like substances and C7: L-tryptophan-like substances. Interestingly, C7 (39.20%), a representative component of DOM in the crab farming pond, was deeply degraded in the aeration pond by aerobic microbes, whereas C6 was absent in the crab pond. According to 2DCOS, the changing order of the components was C7 → C4 → C6 → C5 → C2 → C1 → C3, and the changing order of the functional groups was carboxylic → phenolic → aromatic. As assessed by MW-2DCOS, the Fmax of the components, especially components C2, C5 and C6 (and with the exception of C4 and C7) exponentially increased in the aeration pond, where an accumulative effect occurred. C2, C5 and C7 were removed by 24.26%, 39.42% and 98.25% in the crab farming system, and were deeply degraded in the paddy-field, purification pond and aeration pond, respectively. As assessed by SEM, the latent factors of organic matter removal were C1, C2, C4, C5, SUVA254, CODMn and DO. This study could be conducive to comprehensively characterizing the removal of components and functional groups of DOMs in crab farming parks.