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.

Bipolar membrane electrodialysis (BPMED) is a promising technology for recovering dissolved ammonia (NH₃) from concentrated wastewater streams, such as scrubber effluents. To date, most research remains laboratory-scale, while existing pilot-scale studies operate at effluent concentrations of 2 g-N/L, membrane areas below 3.5 m², and current densities under 150 A/m². In this study, a pilot-scale BPMED system with an active membrane area of 21.3 m² was operated in a sequential batch mode. Real scrubber effluents (18–22 g-N/L) were treated to simultaneously recover dissolved NH₃ and regenerate citric acid. For the first time, the influence of non-ideal phenomena including proton (H⁺) competition, NH₃ back diffusion, electro-osmotic and osmotic water fluxes on NH₃ recovery, was systematically investigated at current densities beyond typical laboratory values. At feed pH below 3, decreasing current efficiency and a sharply increasing energy consumption established an operating window for the pilot-scale BP-C system, irrespective of the applied current density. NH₃ concentrations in the base compartment increased with increasing current density, reaching 48 g-N/L at 500 A/m² and corresponding to treatment capacities of up to 1.8 kg-N/m²d. At 150 A/m², NH₃ recovery was limited by cyclic NH₄⁺ transport associated with NH₃ back diffusion, coinciding with increased electro-osmotic water flux. At higher current densities, NH₃ recovery was instead limited by electro-osmotic and osmotic water flux driven by current density and osmotic pressure gradients across the membrane. Despite comparable energy consumption (7–9 kWh/kg-N recovered) to previous pilot-scale studies, this work achieved higher NH₃ concentrations and treatment capacities while advancing mechanistic understanding of transport limitations in BPMED systems.

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.

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.

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.

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.

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.

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.

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 removal of ammonium and ammonia, represented as total ammoniacal nitrogen (TAN), from reject water through electro-dialysis (ED) and bipolar membrane electrodialysis (BPMED) encounters challenges such as organic fouling, NH₃ back-diffusion, and high energy consumption. The efficacy of electrodialysis reversal (EDR) combined with bipolar membrane electrodialysis using cation-exchange membranes (BPC) was assessed as a more practical configuration (EDR + BPC). Additionally, a novel configuration involving monovalent selective cation-exchange membranes (MSCEMs) in an EDR + BPC setup (SEDR + BPC) was investigated. Comparisons were made among BPMED, EDR + BPC, and SEDR + BPC under three load ratios (L_N) of 0.8, 1, and 1.3 during continuous operation. The innovative SEDR + BPC configuration, with an L_N of 0.8, exhibited the lowest energy consumption for transported TAN (E_TAN) at 4.4 MJ·kgN⁻¹ removal and achieved the highest TAN removal efficiency of 78 % with an L_N of 1.3. In contrast to conventional BPMED, SEDR + BPC allowed for the recovery of potentially back-diffused NH₃ into the acid chamber, minimizing transport losses. Furthermore, scaling in the base chamber was reduced due to the contribution of MSCEMs when applying an L_N of 0.8. The MSCEMs increased the molar ratio of TAN over (Mg²⁺ + Ca²⁺) in the concentrate and decreased it in the diluate. EDR + BPC and SEDR + BPC configurations exhibited stable and lower cell resistance throughout the operation compared to BPMED, attributed to their ability to generate higher concentration gradients. The results clearly demonstrated the feasibility of low-energy TAN removal from real reject water from sludge anaerobic digestion using the SEDR + BPC setup.

Energy-efficient wastewater treatment plants (WWTPs) utilize systems like high-rate activated sludge (A-stage) system to redirect organics from wastewater are redirected into energy-rich sludge (A-sludge). Anaerobic membrane bioreactors (AnMBRs) offer lower footprint and higher effluent quality compared to conventional digesters. In this study, the biological treatment and the filtration performances of AnMBRs for A-sludge digestion under mesophilic and thermophilic conditions were comparatively evaluated through lab-scale experiments, mass balancing and dynamic modeling. Under thermophilic conditions, a higher COD fraction of the influent sludge was converted into methane gas than under mesophilic conditions (65% versus 57%). The energy balance indicated that the surplus energy recovery under thermophilic conditions was less than the additional energy required for heating the AnMBR, resulting in a more than three-fold higher net energy recovery under mesophilic conditions. Therefore, operating an AnMBR for sludge digestion under mesophilic conditions has a higher potential to improve the energy balance in WWTPs.