Automatic control and optimal operation for greenhouse gas mitigation in sustainable wastewater treatment plants: A review

Mechanistic investigation of ciprofloxacin degradation using NiFe2O4/CA-cellulose acetate composite films in a novel dielectric barrier discharge plasma system

Conventional wastewater treatments often exhibit limited efficiency in removing antimicrobial residues, thus requiring innovative methods to tackle antimicrobial contamination in the environment. This study employed a dielectric barrier discharge (DBD) plasma reactor with NiFe2O4-cellulose acetate (CA) composite films for ciprofloxacin (CIP) degradation in water. The catalytic efficiency of NiFe2O4/CA films was tested across the degradation rate of CIP in synthesized wastewater, reaction kinetics, energy utilization, and reductions in total organic carbon (TOC) and chemical oxygen demand (COD), both with and without the films in the DBD system. Optimal degradation conditions of 10 mg/L CIP concentration, 195 V, 6.5 Hz, 9% catalyst loading, and 4.32 L/min flow rate achieved 89.63% CIP removal within 60 min, with alkaline pH further enhancing degradation. UV-Vis analysis confirmed that extending DBD treatment time improved degradation rates. Variations in solution conductivity, pH, and concentrations of H2O2 and O3 were tracked to verify the catalytic role of NiFe2O4/CA films. Moreover, radical scavengers such as tert-butanol (TBA), benzoquinone (BQ), and triethylenediamine (TEDA) were introduced to the system which identified that •OH, ·O2-, and 1O2 were the key reactive oxygen species responsible for CIP degradation. Liquid chromatography-mass spectrometry (LC-MS) was used to determine the intermediate and by-products of the CIP degradation and four potential degradation pathways were proposed. Pathway III was considered the prominent route involving hydroxylation and piperazine ring cleavage, producing fewer toxic intermediates supported by density functional theory (DFT) calculations. Toxicity assessment showed most intermediates had reduced developmental toxicity and bioaccumulation potential compared to CIP. This highlights the environmental safety of the DBD plasma and NiFe2O4/CA system, as a promising, eco-friendly alternative to traditional methods, with reduced toxicity, minimal bioaccumulation, and potential for sustainable, large-scale application.

Utilization of trace metals for mitigating nitrous oxide generation in anaerobic digestion

In this study, trace metals were used to enhance the efficiency of anaerobic digestion and promote complete denitrification, thereby reducing nitrous oxide (N2O) production. The potential for greenhouse gas production during anaerobic digestion was evaluated. The effects of supplementation of Cu, Al, Mo, Fe, KI, Co, and Ni on greenhouse gas emissions were investigated through the anaerobic digestion of food waste and sewage sludge mixtures. The addition of trace metals increased CH4 production and decreased N2O production. The N2O mitigation rate ranged from 7.8% to 40.2% in all experiments with trace metals. The N2O production was caused by incomplete denitrification due to high concentrations of hydrogen sulfide and organic matter. Trace metals contribute to enhancing the efficiency of anaerobic digestion and mitigating N2O production.

Unraveling chained optimization strategies for carbon emission control in WWTPs and effective water utilization within complicated water systems

China faces severe water scarcity and significant greenhouse gas (GHG) emission pressures. As significant sources of GHG emission within urban system, wastewater treatment plants (WWTPs) exhibit a clear coupling relationship between wastewater treatment and urban water usage. However, the lack of trace of the water‑carbon nexus has created an obstacle to coordinating water utilization strategies and low-carbon wastewater treatment. In this context, the carbon emission intensity of the WWTPs and water use efficiency within the region were appropriately assessed focusing on the Yellow River Basin. A novel approach was introduced to analyze the chain of synergies between carbon neutrality and water conservation goals in urban water system. Specific chained optimization strategies tailored to each region were provided with dimensions linking different aspects of urban water system development, providing fresh perspectives and guidance for GHG mitigation, water resource utilization, and sustainable human development.

Assessment and optimization of wastewater treatment plant in terms of effluent quality, energy footprint, and greenhouse gas emissions: An integrated modeling approach

Wastewater treatment plants (WWTPs) can benefit from utilizing digital technologies to reduce greenhouse gas (GHG) emissions and to comply with effluent quality standards. In this study, the GHG emissions and electricity consumption of a WWTP were evaluated via computer simulation by varying the dissolved oxygen (DO), mixed liquor recirculation (MLR), and return activated sludge (RAS) parameters. Three different measures, namely, effluent water quality, GHG emissions, and energy consumption, were combined as water-energy-carbon coupling index (WECCI) to compare the effects of the parameters on WWTPs, and the optimal operating condition was determined. The initial conditions of the A2O process were set to 4.0 mg/L of DO, 100 % MLR, and 90.7 % RAS. Eighty scenarios with various DO, MLR, and RAS were simulated under steady-state condition to optimize the biological treatment process. The optimal operating conditions were found to be 1.5 mg/L of DO, 190 % MLR, and 90.9 % RAS, which had the highest WECCI of 2.40 when compared to the WECCI of the initial condition (1.07). This optimal condition simultaneously reduced GHG emissions by 1348 kg CO2-eq/d and energy consumption by 11.64 MWh/d. This implies that controlling DO, MLR, and RAS through sensors, valves, and pumps offers a promising approach to operating WWTPs with reduced electricity consumption and GHG emissions while attaining effluent quality standards. Additionally, the nitrous oxide stripping rate exhibited linear relationships with the effluent total ammonia and nitrite concentrations in the aerobic reactor, suggesting that monitoring dissolved nitrogen compounds in the effluent and reactor could be a viable strategy to control MLR and DO in the biological reactor. The digital-based assessment and optimization tools developed in this study are expected to hold promise for application in broader environmental management efforts.

The effect of aeration mode (intermittent vs. continuous) on nutrient removal and greenhouse gas emissions in the wastewater treatment plant of Corleone (Italy)

The paper reports the results of an experimental study aimed at comparing two configurations of a full-scale wastewater treatment plant (WWTP): conventional activated sludge (CAS) and oxic-settling-anaerobic process (OSA) with intermittent aeration (IA). A comprehensive monitoring campaign was carried out to assess multiple parameters for comparing the two configurations: carbon and nutrient removal, greenhouse gas emissions, respirometric analysis, and sludge production. A holistic approach has been adopted in the study with the novelty of including the carbon footprint (CF) contribution (as direct, indirect and derivative emissions) in comparing the two configurations. Results showed that the OSA-IA configuration performed better in total chemical oxygen demand (TCOD) and ortho-phosphate (PO4-P) removal. CAS performed better for Total Suspended Solids (TSS) removal showing a worsening of settling properties for OSA-IA. The heterotrophic yield coefficient and maximum growth rate decreased, suggesting a shift to sludge reduction metabolism in the OSA-IA configuration. Autotrophic biomass showed a reduced yield coefficient and maximum growth yield due to the negative effects of the sludge holding tank in the OSA-IA configuration on nitrification. The OSA-IA configuration had higher indirect emissions (30.5 % vs 21.3 % in CAS) from additional energy consumption due to additional mixers and sludge recirculation pumps. The CF value was lower for OSA-IA than for CAS configuration (0.36 kgCO2/m3 vs 0.39 kgCO2/m3 in CAS).

Decoupling the simultaneous effects of NO2-, pH and free nitrous acid on N2O and NO production from enriched nitrifying activated sludge

In the pursuit of energy and carbon neutrality, nitrogen removal technologies have been developed featuring nitrite (NO2-) accumulation. However, high NO2- accumulations are often associated with stimulated greenhouse gas (i.e., nitrous oxide, N2O) emissions. Furthermore, the coexistence of free nitrous acid (FNA) formed by NO2- and proton (pH) makes the consequence of NO2- accumulation on N2O emissions complicated. The concurrent three factors, NO2-, pH and FNA may play different roles on N2O and nitric oxide (NO) emissions simultaneously, which has not been systematically studied. This study aims to decouple the effects of NO2- (0-200 mg N/L), pH (6.5-8) and FNA (0-0.15 mg N/L) on the N2O and NO production rates and the production pathways by ammonia oxidizing bacteria (AOB), with the use of a series of precisely executed batch tests and isotope site-preference analysis. Results suggested the dominant factors affecting the N2O production rate were NO2- and FNA concentrations, while pH alone played a relatively insignificant role. The most influential factor shifted from NO2- to FNA as FNA concentrations increased from 0 to 0.15 mg N/L. At concentrations below 0.0045 mg HNO2-N/L, nitrite rather than FNA played a significant role stimulating N2O production at elevated nitrite concentrations. The inhibition effect of FNA emerged with further increase of FNA between 0.0045-0.015 mg HNO2-N/L, weakening the promoting effect of increased nitrite. While at concentrations above 0.015 mg HNO2-N/L, FNA inhibited N2O production especially from nitrifier denitrification pathway with the level of inhibition linearly correlated with the FNA concentration. pH and the nitrite concentration regulated the production pathways, with elevated pH promoting the nitrifier nitrification pathway, while elevated NO2- concentrations promoting the nitrifier denitrification pathway. In contrast to N2O, NO emission was less susceptible to FNA at concentrations up to 0.015 mg N/L but was stimulated by increasing NO2- concentrations. This study, for the first time, distinguished the effects of pH, NO2- and FNA on N2O and NO production, thereby providing support to the design and operation of novel nitrogen removal systems with NO2- accumulation.