Effect of effluent recirculation on the performance and microbial structures of a downflow hanging sponge (DHS) reactor for treating high-strength organic wastewater

A downflow hanging sponge (DHS) reactor’s performances were evaluated under varying organic loading rates (OLRs) and hydraulic retention times (HRTs) with/without effluent recirculation, using high-strength industrial wastewater. The experiment was conducted for 389 days at 30°C in four main phases. In phase 1, the OLR was increased from 1.5 to 9.7 kg· chemical oxygen demand (COD) m-³·day −1 while HRT decreased from 14.4 to 2.9 h. In Phase 2, half of the sponges were removed, doubling the volumetric OLR. In Phase 3, four sponges were reintroduced (OLR = 6.86 kg· COD m-³·day −1 , HRT = 2.1 h), and in Phase 4, OLR was reduced to 3.77 kg·COD m⁻³·day⁻¹with an HRT of 4.3 h. The organic matter removal was inversely related to the OLR. In the recirculation reactor (R2), the total biochemical oxygen demand (tBOD) and tCOD removals decreased from 87-75% (Phase 1) to 50% (Phase 2). The tCOD removal increased in Phase 4 and reached 74%. NH 4 + -N removal followed a similar pattern, declining from 97% (Phase 1) to 38% (Phase 2) and then recovering to 77% (Phase 4). The R2 demonstrated a higher removal rate, with 85% tBOD and 58% total nitrogen (TN) removal, compared to the R1 (without recirculation) results of 80% tBOD and 52% TN removal. Comamonadaceae and Rhodocyclaceae were prominent in the sponges, though the nitrite-oxidizing bacteria were low. Effluent recirculation significantly enhances nitrogen removal (~20%), while organic matter removal improvements were condition-specific. Recirculation improved oxygen transfer and created favorable conditions for microbes, making the DHS system effective for high-strength organic wastewater treatment. • Organic removal declined at extreme OLRs but recovered at moderate loading. • Recirculation achieved 85% tBOD and 58% TN removal, outperforming the control. • Denitrifying bacteria dominated; low NOB limited complete nitrification. • Recirculation significantly enhanced ammonia removal, but low impact on organic removal