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The inhibitory effects of oxytetracycline (OTC) on the anaerobic ammonium oxidation (anammox) performance were relieved by employing bio-augmentation (BA) tactics. However, the recovery mechanism was vague. The response of specific anammox activity (SAA), heme c, functional genes, extracellular polymeric substance (EPS) and antibiotics resistance genes (ARGs) to OTC inhibition and BA aid were traced in the present study. The results indicated that response of SAA, heme c content and functional genes, such as nir S, hzs A and hdh to OTC inhibition were not synchronous. The presence of the tet C, tet G, tet X, and int I1 genes enhanced the resistance of anammox sludge to OTC, thus accelerating the performance recovery when aided by BA. A significant correlation existed between number of anammox 16S rRNA gene copies and protein level in the soluble microbial products (SMP), between tet G gene relative abundance and polysaccharose in SMP and between tet G gene relative abundance and protein in bound EPS (EPSs). In nutshell, the current findings provide the first description of a recovery mechanism regarding OTC-inhibited anammox performance aided by BA based on functional genes and highlights the contribution of ARGs and the self-resistance ability of EPS. Unlabelled Image • The recovery mechanism for OTC-inhibited anammox was proposed for the first time. • Bioactivity and functional genes do not always coincide with each other. • The presence of ARGs improved the resistance of anammox sludge to OTC. • A significant correlation was observed between anammox, EPS composition and ARGs. [ABSTRACT FROM AUTHOR]

• Raw piggery digestate was used for Desmodesmus sp. CHX1 cultivation. • Inoculation with nitrifying bacteria showed better performance. • Microalgae aggregating bacteria dominated in the optimized systems. The microalgae-based system has been applied in anaerobic digestate treatment for nutrient removal and biomass production. To optimize its performance in treating piggery digestate, here, commercial bacterial agents, including organic degrading bacteria (Cb) and nitrifying bacteria (Nb), were inoculated into the microalgae-based system dominated by Desmodesmus sp. CHX1 (D). Reactor DN (inoculated with D and Nb) and DCN (inoculated with D, and Cb to Nb at a ratio of 1:2) have better performance on NH 4 +-N removal, with a final efficiency at 40.26% and 39.87%, respectively, and no NO 3 −-N or NO 2 −-N accumulations. The final total chlorophyll concentration, an indicator of microalgal growth, reached 4.74 and 5.47 mg/L in DN and DCN, respectively, three times more than that in D. These results suggested that high NH 4 +-N removal was achieved by the assimilation into high microalgal biomass after the inoculation with functional bacteria. High-throughput sequencing showed that the richness of microbial community decreased but the evenness increased by inoculating functional microorganisms. Microalgae aggregating bacteria were Cellvibrio , Sphingobacterium , Flavobacterium , Comamonas , Microbacterium , Dyadobacter , and Paenibacillus. This study revealed that the inoculation with functional bacteria reconstructed the microbial community which benefited for the microalgal growth and nutrient removal, providing a promising strategy for treating highly-concentrated digestate. [ABSTRACT FROM AUTHOR]