The application of nitrification–denitrification over nitrite (nitritation–denitritation) with municipal (i.e. diluted and cold (or low-temperature)) wastewater can substantially improve the energy balance of municipal wastewater treatment plants. For the accumulation of nitrite, it is crucial to inhibit nitrite-oxidizing bacteria (NOB) with simultaneous proliferation of ammonium-oxidizing bacteria (AOB). The present study describes the effect of the influent total ammonium nitrogen (TAN) concentration on AOB and NOB activity in two moving bed biofilm reactors operated as sequencing batch reactors (SBR) at 15 °C (SBR I) and 21 °C (SBR II). The reactors were fed with diluted reject water containing 600, 300, 150 and 75 mg TAN L−1. The only factor limiting NOB activity in these reactors was the high concentrations of free ammonia and/or free nitrous acid (FNA) during the SBR cycles. Nitrite accumulation was observed with influents containing 600, 300 and 150 mg TAN L−1 in SBR I and 600 and 300 in SBR II. Once nitrate production established in the reactors, the increase of influent TAN concentration up to the original 600 mg TAN L−1 did not limit NOB activity. This was due to the massive development of NOB clusters throughout the biofilm that were able to cope with faster formation of FNA. The results of the fluorescence in situ hybridization analysis preliminarily showed the stratification of bacteria in the biofilm.

Sewage sludge (SS) contains potential toxic elements (PTEs) that are harmful to the environment, and their bioaccumulation in the food chain is a major environmental health concern. Vermicomposting has been shown to reduce PTEs during composting of sewage sludge. However, the extent of PTE’s assimilation into the earthworm tissues during composting is largely unknown. The objectives of this study were to evaluate the potential of vermicomposting to decrease PTEs (As, Cd, Cr, Cu, Pb, and Zn) during composting of SS and whether the bioaccumulation of PTEs in earthworm tissue depends on feed quality. The initial SS was mixed in triplicate with varying proportions of pelletized wheat straw (PWS) (0%, 25%, 50%, and 75% (w/w)) along with a control (100% SS, no earthworms), and the variants were named VC1, VC2, VC3, VC4, and C0 (control), respectively. The experiment was conducted for 120 days using Eisenia andrei. In comparison to the control, mixing SS with PWS reduced Arsenic content by 14–67%, Cadmium content by 4–39%, Chromium contents by 24–77%, Copper content by 20–68%, Lead content by 39–75%, and Zinc content by 16–65%. The bioaccumulation factor’s (BCF) ranges were 20–80% for Arsenic, 20–60% for Cadmium, 6–16% for Chromium, 32–80% for Copper, and 37–115% for Zinc, demonstrating that the accumulation of PTEs in the earthworm tissues explains the low content of PTEs in the vermicompost. In terms of removal rate, the sludge mixtures with bulking agent can be arranged in the following order: VC4 > VC3 > VC2 > VC1. The total carbon loss showed a significant relationship with BCF_As (r = 0.989, p < 0.011), BCF_Cd (r = 0.996, p < 0.004), BCF_Cr (r = 0.977, p < 0.023), BCF_Cu (r = 0.999, p < 0.000), and BCF_Zn (r = 0.994, p < 0.006). The variant containing 75% PWS (VC4) appeared to be a suitable SS mixture to reduce PTEs. Hence, it is suggested that vermicomposting reduces the content of PTEs in SS.