Bioleaching of toxic metals from anaerobically digested sludge without external chemical addition

Superior mainstream partial nitritation in an acidic membrane-aerated biofilm reactor

Shortcut nitrogen removal holds significant economic appeal for mainstream wastewater treatment. Nevertheless, it is too difficult to achieve the stable suppression of nitrite-oxidizing bacteria (NOB), and simultaneously maintain the activity of ammonia-oxidizing bacteria (AOB). This study proposes to overcome this challenge by employing the novel acid-tolerant AOB, namely "Candidatus Nitrosoglobus", in a membrane-aerated biofilm reactor (MABR). Superior partial nitritation was demonstrated in low-strength wastewater from two aspects. First, the long-term operation (256 days) under the acidic pH range of 5.0 to 5.2 showed the successful NOB washout by the in situ free nitrous acid (FNA) of approximately 1 mg N/L. This was evidenced by the stable nitrite accumulation ratio (NAR) close to 100 % and the disappearance of NOB shown by 16S rRNA gene amplicon sequencing and fluorescence in situ hybridization. Second, oxygen was sufficiently supplied in the MABR, leading to an unprecedentedly high ammonia oxidation rate (AOR) at 2.4 ± 0.1 kg N/(m3 d) at a short hydraulic retention time (HRT) of a mere 30 min. Due to the counter diffusion of substrates, the present acidic MABR displayed a significantly higher apparent oxygen affinity (0.36 ± 0.03 mg O2/L), a marginally lower apparent ammonia affinity (14.9 ± 1.9 mg N/L), and a heightened sensitivity to FNA and pH variations, compared with counterparts determined by flocculant acid-tolerant AOB. Beyond supporting the potential application of shortcut nitrogen removal in mainstream wastewater, this study also offers the attractive prospect of intensifying wastewater treatment by markedly reducing the HRT of the aerobic unit.

Ammonium-based bioleaching of toxic metals from sewage sludge in a continuous bioreactor

The broader reuse of sewage sludge as a soil fertilizer or conditioner is impeded by the presence of toxic metals. Bioleaching, a process that leverages microbial metabolisms and metabolites for metal extraction, is viewed as an economically and environmentally feasible approach for metal removal. This study presents an innovative bioleaching process based on microbial oxidation of ammonia released from sludge hydrolysis, mediated by a novel acid tolerant ammonia-oxidizing bacteria (AOB), Ca. Nitrosoglobus. Over a span of 1024 days, a laboratory-scale bioleaching reactor processing anaerobically digested (AD) sludge achieved an in-situ pH of 2.5 ± 0.3. This acidic environment facilitated efficient leaching of toxic metals from AD sludge, upgrading its quality from Grade C to Grade A (qualified for unrestricted use), according to both stabilization and contaminants criteria. The improved quality of AD sludge could potentially reduce sludge disposal expenses and enable a broader reuse of biosolids. Furthermore, this study revealed a pH-dependent total ammonia affinity of Ca. Nitrosoglobus, with a higher affinity constant at pH 3.5 (67.3 ± 20.7 mg N/L) compared to pH 4.5-7.5 (7.6 - 9.6 mg N/L). This finding indicates that by optimizing ammonium concentrations, the efficiency of this novel ammonium-based bioleaching process could be significantly increased.

Biomass conversion and radiocaesium (Rad-Cs) leaching behaviors of radioactive grass in anaerobic wet fermentation systems: Effects of pre-treatments

Persistent concerns regarding environmental hazards arise from the difficulty in disposing of radioactive plant-based wastes originating from the nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FNPP) in Japan in 2011. In this study, three anaerobic digestion (AD) strategies were proposed: Sole anaerobic wet fermentation, and wet fermentations with either alkaline-heat or ultrasonic pre-treatment, which were employed for long-term anaerobic treatment of a genuine radioactive grass stemming from the FNPP accident. The objectives of this work are to investigate the effects of pre-treatments on biomass conversion efficiency and to gain insight into the leaching behavior of radiocaesium (Rad-Cs) within AD processes. Experimental results indicate that by introducing alkaline-heat and ultrasonic pre-treatments to AD systems, the removal efficiencies of total solids (TS) from the raw grass increased by 60.8 % and 42.5 %, respectively, compared to sole wet fermentation. Pre-treatments have been shown to enhance the stability of AD systems, both in terms of enhancing methane production and mitigating pH fluctuations triggered by the accumulation of organic acids. Remarkably, even though the Rad-Cs leaching rate was highest when the AD system was fed with the alkaline-heat pre-treated grass, it remained unsatisfactory at only 5.77 %. We inadvertently isolated a soil-like component from the raw grass, and analyzed both its proportion in the raw grass and the radioactivity intensity. The results indicate that although the soil constituted only 9.51 % TS of the raw grass, it accounted for a significant 81.35 % of the total radioactivity. The soil, which has a pronounced affinity for ionic Cs, being mixed into the raw grass, was identified as the primary factor limiting the leaching efficiency of Rad-Cs throughout both the pre-treatment and wet fermentation phases.

Nitrification in acidic and alkaline environments

Aerobic nitrification is a key process in the global nitrogen cycle mediated by microorganisms. While nitrification has primarily been studied in near-neutral environments, this process occurs at a wide range of pH values, spanning ecosystems from acidic soils to soda lakes. Aerobic nitrification primarily occurs through the activities of ammonia-oxidising bacteria and archaea, nitrite-oxidising bacteria, and complete ammonia-oxidising (comammox) bacteria adapted to these environments. Here, we review the literature and identify knowledge gaps on the metabolic diversity, ecological distribution, and physiological adaptations of nitrifying microorganisms in acidic and alkaline environments. We emphasise that nitrifying microorganisms depend on a suite of physiological adaptations to maintain pH homeostasis, acquire energy and carbon sources, detoxify reactive nitrogen species, and generate a membrane potential at pH extremes. We also recognize the broader implications of their activities primarily in acidic environments, with a focus on agricultural productivity and nitrous oxide emissions, as well as promising applications in treating municipal wastewater.

Towards the carbon neutrality of sludge treatment and disposal in China: A nationwide analysis based on life cycle assessment and scenario discovery

Motivated by the carbon neutrality target, strategic planning for a low-carbon transition of sludge treatment and disposal in China is challenging due to the unpredictability of technical, regional, socioeconomic, and political factors affecting greenhouse gas (GHG) emissions. This study combines the use of a Life Cycle Assessment and the Patient Rule Induction Method, accounting for possibilities that could achieve net-zero carbon emissions by exploring multiple plausible future profiles of sludge treatment and disposal. Results show that reducing sludge landfill and increasing anaerobic digestion are effective methods to facilitate GHG reduction. Achieving carbon neutrality is closely linked to developing a cleaner electricity mix. Based on a cascaded scenario analysis considering regional differences for 31 Chinese provinces, results demonstrated a maximum cumulative reduction potential of 371 Mt CO₂ equivalents from 2020 to 2050, equal to 59.84% of the business-as-usual scenario. Together with GHG reductions, terrestrial acidification and ecotoxicity as well as freshwater ecotoxicity are synergistically reduced. However, the shifting environmental burden results in freshwater eutrophication, human toxicity, marine ecotoxicity, marine eutrophication, and photochemical oxidant formation. This study presents a novel method for systematically identifying possible future development paths toward carbon neutrality. The findings may support policy designs for achieving target carbon reduction effects for sludge disposal.

Determining Factors for Nitrite Accumulation in an Acidic Nitrifying System: Influent Ammonium Concentration, Operational pH, and Ammonia-Oxidizing Community

Acidic nitrification is attracting wide attention because it can enable robust suppression of nitrite-oxidizing bacteria (NOB) in wastewater treatment. This study reports a comprehensive assessment of the novel acidic nitrification process to identify the key factors that govern stable nitrite accumulation. A laboratory-scale moving-bed biofilm reactor receiving low-alkalinity wastewater was continuously operated under acidic conditions (pH < 6) for around two years, including nine stages varying influent and operational conditions. The results revealed that nitrite accumulation was related to three factors, i.e., influent ammonium concentration, operating pH, and ammonia-oxidizing microbial community. These three factors impact nitrite accumulation by altering the in situ concentration of free nitrous acid (FNA), which is a potent inhibitor of NOB. The critical FNA concentration is approximately one part per million (ppm, ∼1 mg HNO₂-N/L), above which nitrite accumulation is stably maintained in an acidic nitrifying system. The findings of this study suggest that stable nitrite accumulation via acidic ammonia oxidation can be maintained under a range of influent and operational conditions, as long as a ppm-level of FNA is established. Taking low-strength mainstream wastewater (40-50 mg NH₄⁺-N/L) with limited alkalinity as an example, stable nitrite accumulation was experimentally demonstrated at a pH of 4.35, under which an in situ FNA of 2.3 ± 0.6 mg HNO₂-N/L was attained. Under these conditions, Candidatus Nitrosoglobus became the only ammonia oxidizer detectable by 16S rRNA gene sequencing. The results of this study deepen our understanding of acidic nitrifying systems, informing further development of novel wastewater treatment technologies.

The combination of aerobic digestion and bioleaching for heavy metal removal from excess sludge

In this study, bioleaching is employed for removing heavy metals from excess sludge generated during municipal wastewater treatment. To avoid organic matter impact on bioleaching, aerobic digestion was performed as pretreatment of the bioleaching or accompanied with the bioleaching. The results showed that the leaching amounts of heavy metals from the process of aerobic digestion accompanied with bioleaching was 2.3 times more than that of the process of aerobic digestion followed by bioleaching. The stable-state proportions of Zn, Cu, Ni and Mn increased by 83%, 94%, 96% and 91%, respectively, in the process of aerobic digestion accompanied with bioleaching, and moreover, the reduction rate of MLSS increased by 22.7%. Although the content of ammonia nitrogen and total phosphorus in sludge decreased after bioleaching treatment, they were still much higher than the soil background value. It indicates that the treated sludge still has agricultural value. High throughput sequencing analysis showed that the relative abundance of acid-producing bacteria (Romboutsia, Clostridium, Tricibacter, and Intestinibacter) significantly increased from 0% to 28.6%, 6.9%, 3.9%, and 2.4%. The enrichment of these acidogenic bacteria was the main reason for the pH decrease, which was conducive to the removal of heavy metals from sludge.