[en] Biochar as a substrate has great potential to promote pollutant removal efficiency in subsurface flow constructed wetlands (SSFCWs). However, information about the effect of different influent C/N ratios on treatment efficiency in SSFCWs with biochar is still scarce. In this study, SSFCW microcosms added with and without biochar were performed to investigate comparatively nitrogen removal in response to the domestic wastewater with varying C/N ratios. The results demonstrated that nitrogen removal increased at influent C/N ratios from1 to 3, and then decreased with the rising influent C/N ratios from 3 to 15 in different SSFCWs. Much higher removal efficiencies for COD (92%), NH₄⁺-N (50%), and TN (50%) were obtained in the biochar-added SSFCW especially at the C/N ratio of 3 when comparing to CW without biochar. The higher pollutants removal ability of biochar-added SSFCWs was mainly attributed to the stronger adsorption ability in the porous biochar. However, poor nitrification was observed at various influent C/N ratios in SSFCWs with and without biochar due to lack of enough oxygen. This study supported that the use of biochar could increase the treatment performance in SSFCWs under various influent C/N ratios.

[en] Highlights: • Impact of organic substances in strength water to struvite was measured. • Presence of organic substances reduces nutrient recovery only at low pH (8.0–9.0). • Particulate and colloidal organic substances slowed down reaction speed. • pH variation influenced organic substances relocation and metals accumulation. The high content of organic substances in strength agro-industrial wastewater has been documented to be among the major barriers hampering nutrient recovery efficiency of struvite precipitation. However, our results in this study show that the previously reported negative impacts of organic substances in high-strength agricultural wastewater on struvite precipitation might be overestimated. This study is the first to test the influence of three forms of organic substances from real high-strength wastewater that contains a complex of particulate, colloidal and soluble organic substances, on nutrient recovery efficiency and product quality through struvite precipitation at varying pH conditions. Our results demonstrated that the inhibition of organic substances on struvite formation only happens at the pH levels of 3− (5–15%) and NH⁺ (6–13%). The inhibitory effect of the organic substances at the optimal pH range (9.5–10) reported from the literature review is only ≤5%. Moreover, the transformation in the contents of humic- and protein-like substances with an increment in pH was characterized and may contribute to mitigate the inhibition of nutrient recovery. Even though the particulate and colloidal organic substances slowed the precipitation reaction, they substantially increased the particle size (i.e., 70% and 40%, respectively) of the formed struvite. The presence of organic substances in all tested forms does not significantly influence the purity and crystalline structure of struvite which can still be used as a slow-releasing fertilizer. Regarding the relocation process of organic substances during struvite precipitation under varying pH conditions, understanding the interaction between organics and heavy metals which in turn affect the dynamics of heavy metals in solution and precipitates remains limited; thus, additional research is needed.

[en] Highlights: • Nutrients recovery performance was optimized by using poultry slurry. • The count of total coliform and E. coli reduced during struvite precipitation. • Struvite precipitate was free of pathogen and with traces of heavy metals. • Struvite precipitation recovered nutrients along with pathogens reduction. The aim of this study was to assess the potential of struvite precipitation to recover nutrients from anaerobically-processed poultry slurry and struvite's interactions with heavy metals (Zn, Cu, Pb, Cr, and Ni) and pathogens (total coliforms and Escherichia coli). The impacts of pH, Mg, N, and P molar proportion, reaction time, and mixing rate and duration were explored to determine the optimal conditions for nutrient recovery through struvite precipitation. A pH range of 9.5 to 10.5, was ideal for P and N removal and recovery, with a molar ratio of 1:1:1 for Mg:N:P. A mixing rate of 150 rpm for 10 min could allow nutrient recovery with little loss (3.32%) of NH through volatilization, and also achieve an optimal struvite crystal size (50–60 μm). The results of X-ray diffractometry and scanning electron microscopy confirmed that the precipitates generated at pH 9 and 10 were orthorhombic struvite. Moreover, along with the recovery of nutrients, 40, 45, 66, 30, and 20% of Zn, Cu, Pb, Cr, and Ni, respectively, and 70% total coliforms and E. coli were removed by struvite precipitation from poultry slurry. This was observed despite that the levels of contaminants (heavy metals) detected in struvite were well below the permissible limits and free of pathogens. Consequently, it was inferred that the struvite quality was reasonable by virtue of its heavy metal and pathogen content, and therefore appropriate for application in the field. Similarly, struvite precipitation has multiple benefits as it can effectively recover nutrients as well as reducing pathogenic populations.

[en] Highlights: • Limited hydrolysis under high loading rate resulted in process failure. • Recirculation improves methane production under high loading rate. • Short HRT under high recirculation rate caused instability of methanogenic reactor. • Increased recirculation and retention time further improved overall performance. - Abstract: Continuous laboratory-scale experiments were conducted on two-stage anaerobic systems treating vegetable waste (VW) to improve CH_4 production. The acidogenic reactors were employed with a serial methanogenic reactor configuration with volume distribution ratios of 3 L/4 L and 3 L/7 L (acidogenic reactor/methanogenic reactor), as well as recirculation rates (RRs) of 0.8 and 1.6. Results showed that recirculation improved the performance of VW anaerobic digestion under an organic loading rate (OLR) of 2.6 g VS/L/d. The OLR increased from 2.6 g VS/L/d to 3.0 g VS/L/d to compare the stability of the processes and to study the behavior response of serial systems. System control without recirculation was susceptible to overloading and volatile fatty acids (VFAs) utilization was inhibited in the methanogenic reactor, which was implemented as a fixed-bed biofilm reactor with active carbon fiber textiles. These findings indicated overall process failure. The ratio of total volatile fatty acid (TVFA) and alkalinity gives a good indication of the process stability of anaerobic digestion. The TVFA/alkalinity ratio of the methanogenic reactor in the 3 L/4 L configuration, with RR of 1.6, increased to approximately 0.5, which indicates potential imminent failure of the methanogenic process. However, the methanogenic reactorin the 3 L/7 L configuration helped in utilizing the VFA produced by the overloading in the acidogenic reactor, which improved the CH_4 production and conversion efficiency of the system

[en] Highlights: • Increasing concern regarding negative effect of ENPs on bioprocesses formed this review. • Exposure of low level ENP shows no effect on heterotrophics for organic removal. • Impacts of ENPs on microbial nitrogen transformations have been well reported. • No negative effect on P removal is reported after short-term nor long-term exposure. • Inhibition effects are overestimated in cultivated toxicity test compared to real wastewater. -- Abstract: Concern regarding the potential negative impacts of released engineered nanoparticles (ENPs) on pollutant removal performance of wastewater treatment systems has received booming attention in recent years. However, the conclusions drawn from different studies often lead to fragmented overall knowledge, some of which are even contradictory. This scenario shows the necessity for a comprehensive review of the interactions of ENPs in wastewater treatment systems, particularly on the impacts of ENPs on microbial processes of carbon (C), nitrogen (N), and phosphorus (P) removal in water treatment systems. This review introduced the impact of 6 often reported ENPs in 5 types of treatment systems. We found that exposure to most of the investigated ENPs at low concentrations doesn't adversely influence the growth of the heterotrophic microbes, which are responsible for organic matter removal. The impacts of ENPs on various microbial nitrogen transformation processes have been investigated. Dosing of ENPs often causes acute microbial nitrogen removal inhibition at various concentrations, but does not influence long-term operation due to microbial adaption. No significant negative effects on biological phosphorus removal in different wastewater treatment processes have been reported after both short-term and long-term exposure (except copper nanoparticles). Environmentally relevant concentrations of ENPs have been reported to enhance the photosynthetic capacity of wetland plants, whereas chronic inhibition to photosynthesis was found in exposure to high concentrations of ENPs. Inhibition effects are often overestimated in pure cultivated toxicity test assays compared to testing with artificially prepared wastewater containing various ingredients or with real wastewater. Potential ligands in real wastewater can bind with ENPs and lower their dissolution. Some challenges exist regarding detection and quantification techniques of ENPs at environmental concentrations, modeling of engineered nanomaterial release on a worldwide scale, and inhibitory mechanisms to microbial transformations.

[en] The primary cause of anaerobic digester failure includes accumulation of inhibitory substances and intermediate products such as volatile fatty acids (VFAs), free ammonia (NH₃⁺), and ammonium (NH₄⁺). They (except VFAs) are however required as essential nutrients for bacteria growth. The current study specifically investigated the effect of oil content on the biogas production and the stability of anaerobic digestion of food waste. Two lab scale reactors were designed with different organic loading rates and feeding adjustment of used oil addition to testing the effects of lipids on biodegradation and biogas production. The results indicate that, at 2.0 g VS L⁻¹ d⁻¹, the addition of oil (5% v/v), caused the reactor failure, whereas, at 4.0 g VS L⁻¹ d⁻¹, the reactor remained stable for 10 days before the accumulation of VFAs, which resulted in low pH, and thus reduced the biogas and methane production. The addition of NaOH to reactivate the reactors can only improve pH, alkalinity and negatively increased viscosity, but there was no significant effect on biogas production and VFAs concentration. An effective solution to reactivate the reactors was achieved by recirculating 50% of both reactor’s effluent back to the reactors. This resulted in biogas recovery and stable process performance of the reactors. Surprisingly, NH₄⁺–N remained stable (1400 mg L⁻¹) throughout the period, far less than the critical concentration of 3000 mg L⁻¹. On the contrary, the low NH₄⁺–N couldn’t contribute to buffering the reactor’s high VFA concentration during the unstable period, thereby raising new questions on its roles in anaerobic digestion process.

[en] The application of constructed wetlands (CWs) has significantly expanded to treatment of various industrial effluents, but knowledge in this field is still insufficiently summarized. This review is accordingly necessary to better understand this state-of-the-art technology for further design development and new ideas. Full-scale cases of CWs for treating various industrial effluents are summarized, and challenges including high organic loading, salinity, extreme pH, and low biodegradability and color are evaluated. Even horizontal flow CWs are widely used because of their passive operation, tolerance to high organic loading, and decolorization capacity, free water surface flow CWs are effective for treating oil field/refinery and milking parlor/cheese making wastewater for settlement of total suspended solids, oil, and grease. Proper pretreatment, inflow dilutions through re-circulated effluent, pH adjustment, plant selection and intensifications in the wetland bed, such as aeration and bioaugmentation, are recommended according to the specific characteristics of industrial effluents. - Highlights: • Knowledge on use of wetlands treating various industrial effluents is insufficient. • Updated concept and functional mechanisms in wetlands were summarized. • Benefits and limitations of wetlands treating industrial effluents were evaluated. • Effluent recirculation and intensifications in the wetlands are recommended. - A comprehensive knowledge on application of CWs for treatment of various industrial effluents is reviewed in aspects of challenges, operational strategies and overall performance

[en] Highlights: • Intensified CWs effectively treated alkaline ammonia-stripped digestate effluent. • Biological N transformation consumed the majority of alkalinity and decreased pH. • NH₃ volatilization accounts for less than 5% of the total NH₄⁺-N removal. • Biodegradation was the main N removal pathway indicated by N-isotopic assessment. • Anammox showed high potential for N removal when treating highly alkaline effluent. This study investigated the treatment performance and nitrogen removal mechanism of highly alkaline ammonia-stripped digestate effluent in horizontal subsurface flow constructed wetlands (CWs). A promising nitrogen removal performance (up to 91%) was observed in CWs coupled with intensified configurations, i.e., aeration and effluent recirculation. The results clearly supported that the higher aeration ratio and presence of effluent recirculation are important to improve the alkalinity and pollutant removal in CWs. The influent pH (>10) was significantly decreased to 8.2–8.8 under the volumetric hydraulic loading rates of 0.105 and 0.21 d⁻¹ in the CWs. Simultaneously, up to 91% of NH₄⁺-N removal was achieved under the operation of a higher aeration ratio and effluent recirculation. Biological nitrogen transformations accounted for 94% of the consumption of alkalinity in the CWs. The significant enrichment of δ¹⁵N-NH₄⁺ in the effluent (47–58‰) strongly supports the occurrence of microbial transformations for NH₄⁺-N removal. However, relatively lower enrichment factors of δ¹⁵N-NH₄⁺ (−1.8‰ to −11.6‰) compared to the values reported in previous studies reflected the inhibition effect of the high pH alkaline environment on nitrifiers in these CWs.

[en] Highlights: • MFC based biosensor for acetate monitoring in AD process with high accuracy • Linear detection range of 0.5–20 mM established within 5 hour response • High selectivity to acetate and less interference from other VFAs • Digestate with temperature of 37 °C had no significant influence on biosensor -- Abstract: Volatile fatty acids (VFAs) especially acetate concentration have been proved to be a sensitive and reliable indicator for many anaerobic processes such as anaerobic digestion (AD). Microbial fuel cells (MFC) have been demonstrated as a promising VFAs sensor due to simple reactor design and operating conditions among microbial electrochemical biosensors. However, the conventional MFC biosensors may fail to distinguish between VFAs and other organics as real digestates containing complex organics and microbes are fed into anode directly. In the present study, an MFC based biosensor was developed and operated in a smart way for selective acetate detection. In the biosensor, acetate ions contained in the AD sample was first fed into the cathode, and then acetic ion transferred through the membrane from the cathode to anode chamber where it was further used as the sole substrate by pre-enriched electroactive biofilm for the current generation. A linear correlation between the current density and acetate concentrations (0.5–20 mM) at varied reaction time (1–5 h) was established. Then, the interference from propionate, butyrate, isobutyrate, and glucose on the performance of the biosensor was evaluated. Furthermore, the influence of sample temperatures (37 and 55 °C) was also studied. Finally, the VFAs content in real AD effluent with this biosensor was measured. The results corresponded well with gas chromatographic measurements. This simple, and reliable biosensor could serve as a promising alternative method for acetate detection in the AD process or any other acetate-rich fluids.

[en] Highlights: • Synergistic effect of NaOH treatment and Fe dosage to maize straw was investigated. • Combining NaOH treatment and Fe dosing resulted in 57% and 56% higher biogas and methane yield respectively. • Combined treatment shortened the technical digestion time from 48 days to 7 days. • Methane content did not differ significantly among the straw treatments. - Abstract: The synergistic effect of alkaline pretreatment and Fe dosing on anaerobic digestion of maize straw was investigated using mesophilic batch reactors. Three straw treatments were investigated as follows: NaOH (4% and 6%) pretreatment, Fe dosage (50, 200, 1000 and 2000 mg/L), and combined NaOH pretreatment and Fe dosage. Compared to the control, NaOH pretreatment alone increased methane yield by 3.5% (313.3 mL CH_4/gVS) and 22.5% (370.9 mL CH_4/gVS) and shortened the technical digestion time (TDT) from 48 days to 19 days and 10 days in 4% NaOH and 6% NaOH pretreatment respectively. Moreover, Fe dosing (200–1000 mg/L) alone gave a methane yield higher (9.4%) than that obtained from 4% NaOH and 7.5% less than the methane yield from 6% NaOH pretreatment; however, the TDT was 10 days longer. Combining NaOH pretreatment and Fe dosage (200–1000 mg/L) significantly increased the methane yield even further to 21.8% (368.8 mL CH_4/gVS) and 56.2% (472.9 mL CH_4/gVS), and shortened TDT from 48 days to 13 days and 7 days in 4% NaOH and 6% NaOH pretreatment respectively. This synergistic effect may be attributed to the fact that the alkaline treatment improved accessibility of the biodegradable fraction of the straw while Fe contributed to increased microbial enzyme activity.