Recovery of Clean Water and Ammonia from Domestic Wastewater: Impacts on Embodied Energy and Greenhouse Gas Emissions

Anaerobic microbial core for municipal wastewater treatment - the sustainable platform for resource recovery

The requirement for carbon neutrality and bioresource recovery has shifted our views on water treatment from health and pollution avoidance to one of sustainability with water and nutrient circularity. Despite progress, the current process of wastewater treatment is linear, based on core aerobic microbiology, which is unlikely to be carbon neutral due to its large use of energy and production of waste sludge. Here, we outline a shift from aerobic to anaerobic microbiology at the core of wastewater treatment and resource recovery, illustrating the state-of-the-art technologies available for this paradigm shift. Anaerobic metabolism primarily offers the benefit of minimal energy input (up to 50% reduction) and minimal biomass production, resulting in up to 95% less waste sludge compared with aerobic treatment, which is increasingly attractive, given dialogue surrounding emerging contaminants in biosolids. Recent innovative research solutions have made ambient (mainstream) anaerobic treatment a ready substitute for the aerobic processes for municipal wastewater in temperate regions. Moreover, utilising anaerobic treatment as the core carbon removal step allows for more biological downstream resource recovery with several opportunities to couple the process with (anaerobic) nitrogen and phosphorus recovery, namely, potential mainstream anaerobic ammonium oxidation (anammox) and methane oxidation (N-DAMO). Furthermore, these technologies can be mixed and matched with membranes and ion-exchange systems, high-value biochemical production, and/or water reuse installations. As such, we propose the reconfiguration of the wastewater treatment plant of the futurewith anaerobic microbiology. Mainstream anaerobic treatment at the core of a truly sustainable platform for modern municipal wastewater treatment, facilitating circular economy and net-zero carbon goals.

Silver-π Interaction: A Diverse Approach to Hybrid Material and Its Efficacy in Electrocatalytic Reduction of Nitrate to Ammonia

Inspired by the intriguing nature of the metal-π interaction in organometallic chemistry, a novel 1D hybrid material has been designed. Herein, a functionalized tellurium allyl macrocycle (TAM) acts as a molecular building block and is knit together via silver-π interaction to obtain Ag-TAM. Ag is coordinated to two allyl groups and a phenyl ring in η2 mode. Instead of the conventional polymerization strategy, a metal-π interaction is employed to interlink macrocycles. TAM and Ag-TAM showed electrocatalytic capability for the conversion of nitrate to ammonia. Ag-TAM showed an NH3 yield rate 2-fold greater than TAM with a high faradaic efficiency of 94.6% with good durability, proving that interlinking of macrocycles via metal-π interaction improves the catalytic activity. Detailed periodic density functional theory (DFT) calculations unveil novel mechanistic insights, suggesting cooperative catalysis between neighboring Ag sites and contributing to the enhanced efficiency.

Electrochemical sulfate production from sulfide-containing wastewaters and integration with electrochemical nitrogen recovery

Electrochemical methods can help manage sulfide in wastewater, which poses environmental and health concerns due to its toxicity, malodor, and corrosiveness. In addition, sulfur could be recovered as fertilizer and commodity chemicals from sulfide-containing wastewaters. Wastewater characteristics vary widely among wastewaters; however, it remains unclear how these characteristics affect electrochemical sulfate production. In this study, we evaluated how four characteristics of influent wastewaters (electrolyte pH, composition, sulfide concentration, and buffer strength) affect sulfide removal (sulfide removal rate, sulfide removal efficiency) and sulfate production metrics (sulfate production rate, sulfate production selectivity). We identified that electrolyte pH (3 × difference, i.e., 25.1 to 84.9 μM h-1 in average removal rate within the studied pH range) and sulfide concentration (16 × difference, i.e., 82.1 to 1347.2 μM h-1 in average removal rate) were the most influential factors for electrochemical sulfide removal. Sulfate production was most sensitive to buffer strength (6 × difference, i.e., 4.4 to 27.4 μM h-1 in average production rate) and insensitive to electrolyte composition. Together, these results provide recommendations for the design of wastewater treatment trains and the feasibility of applying electrochemical methods to varying sulfide-containing wastewaters. In addition, we investigated a simultaneous multi-nutrient (sulfur and nitrogen) process that leverages electrochemical stripping to further enhance the versatility and compatibility of electrochemical nutrient recovery.

Anaerobic Wastewater Treatment and Potable Reuse: Energy and Life Cycle Considerations

Anaerobic secondary treatment has the potential to facilitate energy-positive operations at wastewater treatment plants, but post-treatment of the anaerobic effluent is needed to recover dissolved methane and nutrients and remove sulfide. In this study, a life cycle assessment was conducted to compare hypothetical full-scale wastewater treatment trains and direct potable reuse trains that combine the staged anaerobic fluidized membrane bioreactor (SAF-MBR) with appropriate post-treatment. We found that anaerobic wastewater treatment trains typically consumed less energy than conventional aerobic treatment, but overall global warming potentials were not significantly different. Generally, recovery of dissolved methane for energy production resulted in lower life cycle impacts than microbial transformation of methane, and microbial oxidation of sulfide resulted in lower environmental impacts than chemical precipitation. Use of reverse osmosis to produce potable water was also found to be a sustainable method for nutrient removal because direct potable reuse trains with the SAF-MBR consumed less energy and had lower life cycle impacts than activated sludge. Moving forward, dissolved methane recovery, reduced chemical usage, and investments that enable direct potable reuse have been flagged as key research areas for further investigation of anaerobic secondary treatment options.

Visual analysis of greenhouse gas emissions from sewage treatment plants based on CiteSpace: from the perspective of bibliometrics

With the global reduction actions of greenhouse gas (GHG) emissions, environmental facilities, including sewage treatment plants (STPs), need to reduce pollutants while minimizing GHG emissions. Therefore, more and more publications revealed the formation mechanism of GHGs in STPs and committed to finding better reduction schemes. From the perspective of bibliometrics, this study used CiteSpace to conduct quantitative and visual analysis based on 1,543 publications retrieved from Web of Science between 2000 and 2021 around the world. We have systematically evaluated the structure, development trend, hot spots, and research frontier in the field of GHG emissions from STPs and compared with the contents of top journals to verify the scientificity of the analysis. The results show that the number of publications has increased year by year, and the networks of authors and institutions show a strong correlation. Among them, the clusters of nitrous oxide, anaerobic digestion, and life cycle assessment (LCA) started earlier and received extensive attention, which derived other clusters in the research process. With the development of the field, researchers have gradually changed from single water treatment facilities to multi-carriers that can realize energy regeneration and utilization simultaneously. Accordingly, the GHG reduction of STPs through energy regeneration and resource recovery has become a hot point and frontier direction, which also challenges the breakthroughs in relevant technologies. Furthermore, it provides scientific support for the formulation of relevant incentive policies and economic subsidy systems, so as to alleviate the pressure of global warming and realize the sustainable development of STPs concurrently.

Ionic resource recovery for carbon neutral papermaking wastewater reclamation by a chemical self-sufficiency zero liquid discharge system

Papermaking industry discharges large quantities of wastewater and waste gas, whose treatment is limited by extra chemicals requirements, insufficient resource recovery and high energy consumption. Herein, a chemical self-sufficiency zero liquid discharge (ZLD) system, which integrates nanofiltration, bipolar membrane electrodialysis and membrane contactor (NF-BMED-MC), is designed for the resource recovery from wastewater and waste gas. The key features of this system include: 1) recovery of NaCl from pretreated papermaking wastewater by NF, 2) HCl/NaOH generation and fresh water recovery by BMED, and 3) CO₂ capture and NaOH/Na₂CO₃ generation by MC. This integrated system shows great synergy. By precipitating hardness ions in papermaking wastewater and NF concentrate with NaOH/Na₂CO₃, the inorganic scaling on NF membrane is mitigated. Moreover, the NF-BMED-MC system with high stability can simultaneously achieve efficient CO₂ removal and sustainable recovery of fresh water and high-purity resources (NaCl, Na₂SO₄, NaOH and HCl) from wastewater and waste gas without introducing any extra chemicals. The environmental evaluation indicates the carbon-neutral papermaking wastewater reclamation can be achieved through the application of NF-BMED-MC system. This study establishes the promising of NF-BMED-MC as a sustainable alternative to current membrane methods for ZLD of papermaking industry discharges treatment.

Dissolved Methane Recovery and Trace Contaminant Fate Following Mainstream Anaerobic Treatment of Municipal Wastewater

Anaerobic treatment of municipal wastewater with the staged anaerobic fluidized bed membrane bioreactor (SAF-MBR) shows promise to transform secondary wastewater treatment into an energy-positive process. However, the dissolved methane in SAF-MBR effluent needs to be recovered to reach net energy positive. To recover this methane for energy generation, an air stripping system was constructed downstream of a pilot-scale SAF-MBR facility and operated for over 80 days. The process removed 98% of effluent dissolved methane, and with the addition of intermittent disinfection recovered an average of 90% of the dissolved methane. The exit gas from air-stripping comprised 1.5-2.5% methane and could be utilized by blending with biogas produced from primary solids digestion and the SAF-MBR in an on-site combustion process. The direct energy costs for air stripping methane are <1% of the energy recoverable from the dissolved methane, not accounting for siloxane or sulfide scrubbing. Only siloxanes were observed at levels impacting combustion in this study, with 1.6 mg Si/m³ present in the blended biogas and air stripping mixture. The fate of a subset of trace organic contaminants was examined across the air stripping unit to check for aerobic degradation by methanotrophs or other opportunistic aerobes. Only 1,4-dioxane and benzotriazole showed statistically significant removal among 17 compounds screened, with 0.53 ± 0.13 and 0.34 ± 0.15 fraction removal, respectively. Our results indicate that air stripping is an energy efficient and robust technology for dissolved methane removal and onsite utilization for heat and electricity generation from anaerobic treatment of municipal wastewater.