Algal-bacterial biofilm reactors for petrochemical wastewater: Matrix-dependent DOM transformation and stage-specific...

Petrochemical wastewater (PCW) treatment remains challenging because residual dissolved organic matter (DOM) after conventional treatment still contains complex, refractory, and heteroatom-rich molecular components. Algal-bacterial biofilm (AB) systems have potential for both secondary and advanced treatment; however, their stage-dependent applicability within PCW treatment trains remains unclear. In this study, three AB reactors were fed with dissolved air flotation effluent (DAFE), secondary biological treatment effluent (SBTE), and effluent from a high-activity sludge carbon-capture reactor (ASCE). The three wastewater matrices resulted in distinct patterns of treatment performance, DOM transformation, and biofilm response. After PCW feeding began, all reactors showed short-term inhibition followed by gradual recovery, with the SBTE-fed reactor showing comparatively higher total nitrogen and total phosphorus removal after recovery. Excitation-emission matrix fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry further showed that the SBTE-fed reactor achieved greater attenuation of fluorescent DOM, a decrease in molecular diversity, reduced DOM unsaturation, and removal of nitrogen- and sulfur-containing molecular components. In contrast, low-unsaturation formulas were preferentially removed in the DAFE-fed reactor, whereas the ASCE-fed AB reactor produced more extracellular polymeric substances and soluble microbial products and exhibited stronger antioxidant responses but showed limited DOM attenuation. Microbial community and metatranscriptomic analyses indicated that the SBTE-fed reactor was characterized by functional assemblages and expression patterns related to organic matter transformation, aromatic degradation, energy metabolism, and nitrogen/sulfur transformation. These results suggest that AB reactors may be more suitable as an advanced treatment step following conventional biological treatment in PCW treatment trains.