Complex waste streams, such as sludge reject water from anaerobic digestion, often contain a multitude of cations at varying concentrations, with the ammonium ion (NH₄⁺) typically being the most abundant. Recently, electrodialysis (ED) has been developed as a technology for the removal and recovery of NH₄⁺ from reject water. However, the further development of ED for targeting NH₄⁺ recovery faces lack of reliable performance prediction and standardized operating strategies due to cation competition. It is postulated that selective rejection of divalent cations can be achieved through the use of monovalent-selective cation-exchange membranes (mCEMs). Due to their higher electrical resistance, questions remain regarding the cationic compositions under which mCEMs provide a substantial performance benefit. In the present study, three feed solutions simulating different molar cation compositions of reject water were subjected to ED using both mCEMs and conventional cation-exchange membranes (CEMs). The feed solutions were categorized based on the molar fraction of NH₄⁺ relative to the total cation content. Results showed that the perm-selectivity of NH₄⁺ over Mg²⁺ and Ca²⁺ was enhanced when using mCEMs. Moreover, mCEMs resulted in a higher overall NH₄⁺ removal efficiency compared to standard CEMs. At a relatively low NH₄⁺ molar fraction (0.32), mCEMs outperformed CEMs in terms of current efficiency. Notably, at an intermediate molar fraction (0.58), energy consumption was lower for mCEMs compared to CEMs, but only up to a critical degree of desalination (CDD). At a high molar fraction (0.89), the contribution of mCEMs was insignificant compared to that of CEMs. The CDD was identified as a pragmatic operational parameter beyond which further desalination leads to disproportionate energy penalties. For on-site-process-control of NH₄⁺ removal with ED, the CDD demonstrated that membrane selection and operating thresholds are strongly dependent on reject water composition.

Biogas, generated from small scale digesters, is a traditional energy source for satisfying the thermal energy demand in off-grid communities. Recent developments in small scale solid oxide fuel cells (SOFCs) technology and progress in research and development of dry reforming, opens perspectives to couple small scale SOFCs with already existing digesters to meet both thermal and electrical energy demand, enabling power access to off-grid communities. However, one of the major challenges for SOFC integration to small scale digesters is the effect of biogas impurities, such as H₂S, on the performance of SOFCs. Previous work has shown that local operational practices could influence the biogas quality and particularly the H₂S content in the biogas. The here presented research expanded on the use of cow urine instead of water as solvent in manure digestion as a potential operational strategy that enables in-situ reduction of H₂S in the evolving biogas. This research investigated the following hypotheses: 1) urine addition results in a high pH that favours HS⁻ over H₂S, 2) given the presence of metal elements in the cow urine, insoluble metal sulphides are being formed, reducing the biogas H₂S content. The research was carried out by measuring cow urine composition of various samples, assessing the effects of different urine/water/manure mixtures on the evolving biogas-H₂S concentration, and verifying the experimental findings with phreeqC equilibrium speciation. Bio-kinetic modelling, using the anaerobic digestion model nr 1 (ADM1), was subsequently performed to explore the influence of different feed compositions on the H₂S content in the biogas. It was observed that addition of cow urine in all experiments resulted in an elevated pH of the reactor compared to water dilution, yet both experiments I and II-2 showed an increased maximum H₂S content when urine dilution was applied, compared to water dilution. Cow urine and cow dung characterisation in terms of metals and S, showed that experiment II-1 had the highest Fe:S ratio of 1:0.3–1:0.9. Equilibrium modelling confirmed that despite the positive urine-induced pH effect, the measured Fe:S ratios could indeed be decisive, as with an Fe;S ratio of 1:6 and 1:0.5, the H₂S production at equilibrium was 61 and 10 mL/ kg of solution, respectively. Furthermore, it was predicted through bio-kinetic modelling that inconsistency in feedstock composition may result in temporary H₂S peaks exceeding 400 ppm. Overall, results showed that if a cow urine/manure mixture is characterised by a total metal:S ratio exceeding 1:0.5 and total S content of less than 0.5 mM, then hydrolysed cow urine addition presents an interesting in-situ H₂S cleaning strategy for biogas-SOFC applications.

Nitrification/denitrification mitigates excess nitrogen in wastewater and reduces nutrient pollution in recipient surface waters but emits substantial amounts of nitrous oxide (N₂O). Complete ammonia-oxidizing (comammox) bacteria provide novel opportunities to mitigate N₂O emissions from wastewater treatment systems. In this study, a down-flow hanging sponge (DHS) reactor with low-strength ammonia-based synthetic wastewater was used to culture comammox bacteria, to study the microbial community structure, and to assess the nitrogen removal performance. The results showed a high NH₄⁺-N removal efficiency of 98 ± 4 % and complete nitrification during the entire experimental period. 16S rRNA gene sequencing and metagenomic analysis showed that comammox-like Nitrospira dominated the DHS-retained sludge, and that comammox-like Nitrospira and ammonia-oxidizing archaea may have coexisted symbiotically. The dissolved N₂O emissions per NH₄⁺-N removed from the DHS reactor were much lower than those from conventional activated sludge processes, indicating that the DHS reactor could be effective in reducing N₂O emissions during wastewater treatment.

Purpose of Review: Liquid crystal monomers (LCMs), used extensively in liquid crystal displays (LCDs), have emerged as persistent, bioaccumulative, and toxic organic pollutants. A network analysis of SCOPUS data revealed significant knowledge gaps, especially concerning the fate of LCMs in WWTPs. The available literature highlights that influent LCM concentrations vary widely, with elevated levels linked to industrial and e-waste recycling activities. This review examines the occurrence, fate, and treatment of LCMs, particularly fluorinated LCMs (F-LCMs), in wastewater treatment plants (WWTPs). Recent Findings: Conventional WWTP processes achieve moderate removal efficiencies (~ 84%) for LCMs, but F-LCMs often persist. Advanced treatment techniques such as UV/peroxydisulfate (UV/PDS) showed removal rates of 77–84% for LCMs with biphenyl and ethoxy groups. These groups alter electron distribution, making the molecules more susceptible to oxidative attack by reactive species such as hydroxyl and sulfate radicals. Degradation pathways include cleavage of biphenyl, ethoxy, and C-F bonds, producing less toxic by-products such as oxalic acid and cyclohexane. However, some degradation intermediates formed are toxic, necessitating further research of the treatment processes. Summary: This review underscores the need for systematic monitoring of LCMs in wastewater and their transformation products in treated wastewater and sludge, alongside advancements in treatment technologies to mitigate environmental and health risks. This review highlights the urgency of improving wastewater management strategies for LCMs and the need for future research to address the critical knowledge gaps.

The anaerobic degradation of phenolic compounds presents substantial challenges due to their toxicity to methanogenic biomass and inherently low conversion rates. Recent studies indicate that nano-magnetite can stimulate direct interspecies electron transfer (DIET), potentially enhancing phenol conversion and methane production. This study employed two anaerobic membrane bioreactors (AnMBRs) to investigate phenol and p-cresol degradation under stepwise increasing loading rates, with complete retention of biomass in both reactors. While AnMBR-C served as a control, nano-magnetite was additionally supplemented to reactor AnMBR-M at a concentration of 40 mmol/L in Phase 1 and 20 mmol/L in Phase 2. Results demonstrated that AnMBR-M supplemented with 20 mmol/L nano-magnetite tolerated higher phenolic loading rates compared to AnMBR-C. In Phase 2, a higher total Fe concentration was observed in AnMBR-M, suggesting an enhanced electron transfer mechanism via dissimilatory iron reduction-oxidation cycle. Follow-up batch experiments revealed that magnetite-adapted biomass had more tolerance to phenol inhibition. A 16S-rRNA sequencing was conducted to characterize microbial communities within both reactors. Results suggested that DIET was stimulated in Phase 1, as shown by the enrichment of the electrogenic Pseudomonas and Methanolinea in AnMBR-M. However, the possibly stimulated DIET in Phase 1 could not alleviate the inhibition caused by excessive 40 mmol/L nano-magnetite dosage. Notably, there was no significant difference between the genera of AnMBR-C and AnMBR-M by the end of Phase 2. However, short-chain fatty acid degrader Mesotoga was more enriched in AnMBR-M. Moreover, species-level analysis showed that AnMBR-M had a sixfold higher relative abundance of Methanosaeta harundinacea compared to AnMBR-C.

Despite growing scientific interest in the past decade, bipolar membrane electrodialysis has seen limited advancement in controlled operation of water dissociation via the bipolar membrane (BPM). For nutrient recovery applications, such as ammonia (NH₃) extraction from anaerobic digestion reject water, implementing in-situ pH control in the base solution could enhance energy efficiency. By controlling the electric current, pH is regulated through OH⁻ generation from the bipolar membrane (BPM). Once the targeted pH is reached, the electric current is applied in pulses and pauses with the purpose to sustain and to not overpass the pH setpoint. A selective electrodialysis reversal (SEDR) combined with a two-compartment bipolar membrane electrodialysis (BPMC) and vacuum membrane stripping (VMS) enabled the recovery and conversion of ammonium ions (NH₄⁺) into volatile ammonia (NH₃). Operating the BPMC with the developed pH control method lowered energy consumption (E_NH4⁺) and improved current efficiency for NH₄⁺ removal compared to constant current (CC) operation. Under pH control, the BPMC maintained the target pH throughout the whole operation, with an E_NH4⁺ between 12.5 and 35.3 MJ·kgN⁻¹, compared to 12.1 and 78.6 MJ·kgN⁻¹ under CC. The current efficiency was maintained across setpoints with pH control, ranging between 25 % and 29 %. With CC, the current efficiency declined from 27 % to 12 % at higher current densities. Furthermore, pH control applying a pulsed electric current reduced the occurrence of scaling by minimising the transport of divalent cations across the cation exchange membrane and CO₂ formation in the acid compartment. Similar removal efficiencies were attained, applying pH controlled operation and CC; however, both methods performed a declining removal efficiency during 30 h operation. The developed pH control method can provide distinct improvement in scale-up applications, where energy reduction by preventing excessive water dissociation by the BPM is of interest. In addition, external caustic dosing can be substituted by pH control with a BPMC layout of the stack, reducing the residual impurities of the chemical dosing.

Abstract: Presence of carbohydrates hampers protein degradation in anaerobic digesters. To understand this phenomenon, we used proteogenomics to identify the active protein-degraders in the presence of low and high carbohydrates concentrations. Active metabolic pathways of the identified protein-degraders were investigated using proteomics with ¹³C-protein substrates (protein stable isotope probing). Results showed that 1) Acinetobacter was the active protein-degraders under both protein-fed and protein-glucose mixture-fed conditions, 2) the relative abundance of Acinetobacter was not affected by the presence of carbohydrates, 3) the incorporation of the ¹³C-labelled protein substrate was predominantly observed in outer membrane-bound proteins and porin proteins, which are associated with proteinases or the transportation of amino acids across the cell wall. The Acinetobacter metabolic model and the incubation conditions suggested that glucose and proteins were degraded through anaerobic respiration. The negative impact of carbohydrates on protein biodegradation was attributed to Acinetobacter's preference for carbohydrates. This work highlights that efficient degradation of protein and carbohydrate mixtures in anaerobic digesters requires a staged or time-phased approach and enrichment of active protein-degraders, offering a new direction for process optimization in anaerobic digestion systems.

Oil palm empty fruit bunch (OPEFB) is an abundant organic waste in Malaysia that is often disposed of through field burning. A previous study has shown that solar-driven steam gasification of OPEFB can produce hydrogen-rich syngas with an energy upgrade factor of 1.2 and a carbon conversion efficiency of 95.1 %. Beyond its potential as a biofuel, OPEFB can also act as a carbon sink, capturing photosynthetically stored carbon. This study explores the potential of amplifying OPEFB's negative carbon emissions through solar-driven gasification, using CO₂ as the gasifying agent. In this work, a Central Composite Design (CCD) approach was employed to assess the influence of temperature (1100–1300 °C) and CO₂/OPEFB molar ratio (1.6–3.0) on H₂/CO molar ratio and energy upgrade factor, with a constant OPEFB flow rate of 1.8 g/min. The results demonstrated that at an energy upgrade factor of 1.4, 94.9 % of the total carbon was converted into syngas with a H₂/CO molar ratio of 0.3. The maximum observed net carbon capture yield of 0.4 g C/g OPEFB was achieved at 1300 °C and a CO₂/OPEFB molar ratio of 3.0. The remaining carbon (94.4–95.7 wt %) was converted into biochar with low heavy metal content, which has potential as a soil enhancer.

Recent research showed that the recovery of ammonia from simulated ammonium citrate scrubber effluent via bipolar membrane electrodialysis (BPMED) is less energy-intensive than from ammonium sulfate solutions. Nonetheless, the application of citric acid as scrubbing agent is limited by its high costs. This study aimed to improve BPMED performance for ammonium recovery using ammonium salts mixtures (ammonium sulfate and ammonium citrate) as feed solutions. Unlike previous studies that focused mainly on single-salt systems, it investigated how this combination affects ammonium recovery efficiency, current efficiency, energy consumption, ammonia diffusion, H⁺ and OH⁻ leakage to the diluate compartment, and anions transport across anion exchange membranes (AEMs) during BPMED. The ammonium recovery efficiency was higher for pure ammonium citrate (45.2 %) and mixture solutions (32.0–45.9 %) than for pure ammonium sulfate (26.8 %). Higher efficiency resulted from reduced competition between protons and ammonium across the cation exchange membrane (CEM). Feed with a higher ammonium citrate proportion increased buffer capacity, preventing protons leakage from the acid to the diluate compartment. This resulted in higher ammonium current efficiency for pure ammonium citrate (34.8 %) and mixture solutions (24.9–35.7 %) than for pure ammonium sulfate (20.4 %). The energy consumption was lower for pure ammonium citrate (14.1 kWh/kg-N recovered) and mixture solutions (13.0–17.4 kWh/kg-N recovered), than for pure ammonium sulfate (22.3 kWh/kg-N recovered). Ammonia diffusion from the base to the acid compartment reduced current efficiency by 19–23 % and accounted for 30–40 % of the total ammonium transported from the feed. This study demonstrated the effective use of ammonium citrate as one of the salts in the mixture to achieve high ammonium recovery efficiency with reduced energy consumption.

Thermal hydrolysis process (THP) is a widely used pre-treatment method in the anaerobic digestion (AD) of waste municipal sewage sludge. A post AD dewatering step of the digestate produces a liquid stream called reject water. THP increases the concentration of humic substances (HSs) and nutrients in the produced reject water. Struvite precipitation is a widely used technique to remove and (potentially) recover PO₄³⁻ -P and the corresponding amount of total ammoniacal nitrogen from reject water. The chemical characteristics of the THP-produced HSs influence reaction yields and morphology of struvite. In our current study, struvite batch precipitation experiments were conducted at different pHs, and different concentrations of HSs, consisting of either melanoidins or humic acids. Our results showed that at pH 6.5 struvite precipitation was severely retarded. However, increased concentrations of melanoidins at pH 6.5 enhanced struvite precipitation. Batch experiments conducted at pH 7.25 and 8 with increased melanoidins concentrations showed PO₄³⁻-P precipitation yields over 86 %. Humic acids negatively impacted struvite precipitation at all analysed pH values, presumably because of Mg²⁺ complexation. Morphological analysis showed that the presence of both HSs affected Feret diameters, aspect ratio, and cleavage pattern of struvite. Also, HSs rendered coloured crystals. Overall, our results showed that struvite precipitation is affected by HSs intrinsic characteristics, affecting yield, morphology, and colour of the formed precipitates.

The study investigated the effects of long-chain fatty acids (LCFA) on anaerobic sludge treating lipid-rich wastewater. It involved batch experiments with three sludge samples: two acclimated to lipids and one non-acclimated. The experiments aimed to observe the degradation of LCFA, specifically oleate and palmitate, by dosing them at concentrations ranging from 50 to 600 mg/L. Measurements of the cumulative methane production and the LCFA concentration, quantified as fat, oil, and grease (FOG) were performed. To ensure the sludge was free from other biodegradable substrates, part of the samples was pre-incubated without feed. The tests were conducted with both pre-incubated and non-incubated inoculum sludge. The findings revealed that oleate was degraded more efficiently than palmitate across all sludge samples, with a greater conversion rate to methane. Sludge samples acclimated to lipids showed a superior capacity to degrade LCFA compared to non-acclimated ones. It was noted that at concentrations above 400 mg/L, the conversion of LCFAs to intermediate compounds was inhibited, although this did not affect the subsequent methane production. The study concludes with a recommendation for sludge adaptation strategies to boost the efficiency of anaerobic wastewater treatment systems dealing with lipid-rich waste. The presence of LCFA-degrading bacteria families like Kosmotogaceae, Petrotogaceae, and Synergistaceae in the acclimated sludge samples underscores the adaptation and potential for improved degradation performance.

A biokinetic model based on BioWin's Activated Sludge Digestion Model (ASDM) coupled with a nitrous oxide (N₂O) model was setup and calibrated for a full-scale wastewater treatment plant (WWTP) Amsterdam West, in the Netherlands. The model was calibrated using one year of continuous data to predict the seasonal variations of N₂O emissions in the gaseous phase. This, according to our best knowledge, is the most complete full-scale data set used to date for this purpose. The results obtained suggest that the currently available biokinetic model predicted the winter, summer, and autumn N₂O emissions well but failed to satisfactorily simulate the spring peak. During the calibration process, it was found that the nitrifier denitrification pathway could explain the observed emissions during all seasons while a combination of the nitrifier denitrification and incomplete heterotrophic denitrification pathways seemed to be dominant during the emissions peak observed during the spring season. Specifically, kinetic parameters related to free nitrous acid (FNA) displayed significant sensitivity leading to increased N₂O production. The obtained values of two kinetic parameters, i.e., the FNA half-saturation during ammonia oxidising bacteria (AOB) denitrification and the FNA inhibition concentration related to heterotrophic denitrification, suggested a strong influence of the FNA bulk concentration on the N₂O emissions and the observed seasonal variations. Based on the suboptimal performance and limitations of the biokinetic model, further research is needed to better understand the biochemical processes behind the seasonal peak and the influence of FNA.