A bilayer coarse-fine infiltration system minimizes bioclogging: The relevance of depth-dynamics

Unveiling the power of COD/N on constructed wetlands in a short-term experiment: Exploring microbiota co-occurrence patterns and assembly dynamics

Constructed wetlands (CWs) are a cost-effective and environmentally friendly wastewater treatment technology. The influent chemical oxygen demand (COD)/nitrogen (N) ratio (CNR) plays a crucial role in microbial activity and purification performance. However, the effects of CNR changes on microbial diversity, interactions, and assembly processes in CWs are not well understood. In this study, we conducted comprehensive mechanistic experiments to investigate the response of CWs to changes in influent CNR, focusing on the effluent, rhizosphere, and substrate microbiota. Our goal is to provide new insights into CW management by integrating microbial ecology and environmental engineering perspectives. We constructed two groups of horizontal subsurface flow constructed wetlands (HFCWs) and set up three influent CNRs to analyse the microbial responses and nutrient removal. The results indicated that increasing influent CNR led to a decrease in microbial α-diversity and niche width. Genera involved in nitrogen removal and denitrification, such as Rhodobacter, Desulfovibrio, and Zoogloea, were enriched under medium/high CNR conditions, resulting in higher nitrate (NO3--N) removal (up to 99 %) than that under lower CNR conditions (<60 %). Environmental factors, including water temperature (WT), pH, and phosphorus (P), along with CNR-induced COD and NO3--N play important roles in microbial succession in HFCWs. The genus Nitrospira, which is involved in nitrification, exhibited a significant negative correlation (p < 0.05) with WT, COD, and P. Co-occurrence network analysis revealed that increasing influent CNR reduced the complexity of the network structure and increased microbial competition. Analysis using null models demonstrated that the microbial community assembly in HFCWs was primarily driven by stochastic processes under increasing influent CNR conditions. Furthermore, HFCWs with more stochastic microbial communities exhibited better denitrification performance (NO3--N removal). Overall, this study enhances our understanding of nutrient removal, microbial co-occurrence, and assembly mechanisms in CWs under varying influent CNRs.

Rhamnolipid-induced alleviation of bioclogging in Managed Aquifer Recharge (MAR): Interactions with bacteria and porous media

The prevention and treatment of bioclogging is of great significance to the application of Managed Aquifer Recharge (MAR). This study investigated the alleviating effect of biosurfactant rhamnolipid (RL) on bioclogging by laboratory-scale percolation experiments. The results show that the addition of RL greatly reduced bioclogging. Compared with the group without RL, the relative hydraulic conductivity (K') of the 100 mg/L RL group increased 5 times at the end of the experiment (23 h), while the bacterial cell amount and extracellular polymeric substances (EPS) content on the sand column surface (0-2 cm) decreased by 60.8% and 85.7%, respectively. In addition, the richness and diversity of the microbial communities within the clogging matter decreased after the addition of RL. A variety of bacterial phyla were found, among which Proteobacteria were predominant in all groups. At the genus level, RL reduced the relative abundance of Acinetobacter, Bacillus, Klebsiella, and Pseudomonas. These microbes are known as strong adhesion, large size, and easy to form biofilms, therefore playing a critical role during MAR bioclogging. Moreover, RL changed the surface properties of bacteria and porous media, which results in the increase of electrostatic repulsion and decrease of hydrophobic interaction between them. Therefore, RL mediated the bacteria-porous media interaction to reduce biomass in porous media, thereby alleviating bioclogging. This study implies that RL's addition is an environmentally friendly and effective method to alleviate the bioclogging in MAR.

Influence of double-layer filling structure on nitrogen removal and internal microbial distribution in bioretention cells

The nitrogen removal effect of traditional bioretention cells on runoff rainwater is not stable. The nitrogen removal effect of bioretention cells can be improved by setting up a layered filling structure, but the effect of changes in filling structure on the nitrogen removal process and microbial community characteristics is still unclear. Two types of porosity fillers were set up in the experiment, and a homogeneous bioretention cell and three bioretention cells with layered fillers were constructed by changing the depth range of the upper and lower layers to analyze the influence of the pore variation of different depth fillers on the nitrogen removal process and microbial community characteristics. The experimental results showed that, compared with the homogeneous filing structure, the layered filling structure can strengthen the adsorption of NH₄⁺-N and the conversion of NO₃^--N, so as to increase the removal rates of NH₄⁺-N and NO₃^--N by 20.71-81.56% and 9.25%-78.19%, respectively. Although the low porosity filler structure will reduce the nitrification activity and urease activity by 48.63%-66.68% and 8.00%-20.64% respectively, it can increase the denitrification activity by 19.14%-31.92%, thus significantly reducing the nitrate content in the filler. The low porosity filler structure can affect the growth and reproduction of various phylum bacteria such as Proteobacteria, Chloroflexi, Acidobacteria, and genus bacteria such as Nitrospira, Ellin6067, Rhizobacter, Pseudomonas, which can improve the diversity and richness of microorganisms.

Cultivation-dependent and cultivation-independent investigation of O-methylated pollutant-producing bacteria in three drinking water treatment plants

O-methylated pollutants (OMPs) are emerging contaminants in drinking water and mainly produced through bacterial O-methylation. However, the information of OMP-producing bacteria (OMPPB) in drinking water treatment plant (DWTP) is largely unknown so far. In this study, the OMPPB in water samples from three DWTPs (XL, JX and NX) were investigated by using cultivation-dependent and cultivation-independent technologies. Four OMPs were detected and their odor and toxicity risks were assessed. Formation potentials (FPs) of 2,4,6-trichloanisole, 2,3,6-trichloanisole, 2,4,6-tribromoanisole, pentachloroanisole and diclofenac methyl ester were determined in water samples and their values shifted significantly among DWTPs. A most probable number (MPN) method was established to quantify OMPPB numbers and the relationships between total haloanisole FPs (HAFPs) (y) and OMPPB numbers (x) in three DWTPs could be described by the following functions: y = 0.496×^0.373 (XL), y = 0.041×^0.465 (JX) and y = 0.218×^0.237 (NX). Several genera like Bacillus, Ralstonia, Brevundimonas, etc. were newly found OMPPB among the cultivable bacteria, and their OMP products were evaluated in terms of quantity and environment risks (odor, toxicity and bioaccumulation). High-throughput sequencing revealed treatment process was the main driving factor to shape the OMPPB community structures and Mantel test showed HAFP profile was significantly influenced by Mycobacterium and Pelomonas. PICURSt2 analysis discovered four phenolic O-methyltransferases (OMTs) and four carboxylic OMTs which might be responsible for OMP formation. Several strategies were recommended to assess risk and control contamination brought by OMPPB in DWTPs.

Influence of pH on bioclogging in porous media during managed aquifer recharge (MAR): Effectiveness and mechanism

To investigate the effect of pH on bioclogging process during managed aquifer recharge, three laboratory-scale column experiments were conducted and the relative hydraulic conductivity, bacterial cell number and the concentrations of polysaccharide, protein and EPS were measured under pH 5, 7, and 9, respectively. High-throughput sequencing was also used to determine the characteristics of bacterial community under different pH conditions. The development of bioclogging was rather different for the case of pH 5. 7, and 9; i.e., the growth process and number of bacteria differed with pH. The shortest growth period and lowest number of bacteria were observed at pH 5. In addition, the difference in bacterial EPS concentration was mainly associated with the polysaccharides. The variation in pH led to different bacterial community composition and diversity. The acid-resistant Elizabethkingia and Bacillus were abundant at pH 5, while Chryseobacterium and Klebsiella had relatively high abundances at pH 7. In contrast, the basophilic Exiguobacterium accounted for >80% of the total bacterial abundance at pH 9. This work is of great significance to explore bioclogging mechanism during MAR process, and provides insights and guidances for field-based managed aquifer recharge.

Use of pilot-scale geomedia-amended biofiltration system for removal of polar trace organic and inorganic contaminants from stormwater runoff

Stormwater runoff capture and groundwater recharge can provide a sustainable means of augmenting the local water resources in water-stressed cities while simultaneously mitigating flood risk, provided that these processes do not compromise groundwater quality. We developed and tested for one year an innovative pilot-scale stormwater treatment train that employs cost-effective engineered geomedia in a continuous-flow unit-process system to remove contaminants from urban runoff during aquifer recharge. The system consisted of an iron-enhanced sand filter for phosphate removal, a woodchip bioreactor for nitrate removal coupled to an aeration step, and columns packed with different configurations of biochar- and manganese oxide-containing sand to remove trace metals and persistent, mobile, and toxic trace organic contaminants. During conditioning with authentic stormwater runoff over an extended period (8 months), the woodchip bioreactor removed 98% of the influent nitrate (9 g-N m^-3 d^-1), while phosphate broke through the iron-enhanced sand filter. During the challenge test (4 months), geomedia removed more than 80% of the mass of metals and trace organic compounds. Column hydraulic performance was stable during the entire study, and the weathered biochar and manganese oxide were effective at removing trace organic contaminants and metals, respectively. Under conditions likely encountered in the field, sustained nutrient removal is probable, but polar organic compounds such as 2,4-D could breakthrough after about a decade for conditions at the study site.

Physicochemical and microbiological characteristics of waste foundry sand used in landfills

Biodegradation of solid waste is a process that depends on the characteristics of waste and soil, and these characteristics may create a waterproof barrier in the landfill, changing its performance. Some residues, such as waste foundry sand (WFS), whose final destination is the sanitary landfill, can create waterproofing and alter the performance of the landfill. This study was carried out to test this hypothesis by evaluating two prototypes composed of layers of organic residues, one covered by a mixture of 30% clay soil + 70% WFS and the other covered only with clay soil, monitored for 24 months. The generated leachate was analyzed regarding the parameters of chemical oxygen demand (COD), pH and electrical conductivity. In addition, after the monitoring period, semi-undeformed samples were collected for quantification of microorganisms and physical-chemical analysis (pH, electrical conductivity, moisture content and images with scanning electron microscopy). In the soil prototype, there was the formation of a waterproofing barrier in its deepest layer (soil₃). Factors such as the clay-mineral portion, moisture content (33,18%) and amount of microorganisms influenced this formation (650.000 and 15.000 CFU/g bacterial and fungal biomass, respectively), showing that WFS avoids the formation of this waterproofing barrier, as indicated for covering organic waste in landfills.

Power assisted MFC-based biosensor for continuous assessment of microbial activity and biomass in freshwater ecosystems

Microbial activity and biomass are important factors that determine nutrient and carbon fluxes in freshwater ecosystems and, therefore are also related to both water quality and climate change induced stressors. This study aimed at assessing the feasibility of a power assisted Microbial Fuel Cell (MFC)-based biosensors for the continuous monitoring of microbial activity and biomass concentrations in saturated freshwater ecosystems. For this purpose, four lab-scale reactors were constructed and operated for 30 weeks. Reactors were fed with four different organic matter concentrations to promote a suite of microbial activity and biomass conditions. The reactors consisted of 3.8 L PVC vessels filled with 23 extractable gravel- sockets, used for microbial activity and biomass assessment, and 1 MFC granular-graphite socket, for biosensing assessment. Microbial activity was determined by the ATP content and the hydrolytic enzymatic activity, and the biomass content was assessed as the volatile solids attached to the gravel. Very significant linear relationships could be established between the parameters studied and the current density produced by the MFC with a very short detection time: 10 min for the ATP content (R² = 0.88) and 1 h for the enzymatic activity (R² = 0.78) and biomass (R² = 0.74). Moreover, the power assisted MFC-based biosensing tool demonstrated to be functional after a long operation time and under a wide range of organic loading conditions. Overall, the results highlight the feasibility to develop a power assisted MFC-based biosensor for on-line monitoring of the microbial activity and biomass of a given ecosystem (either natural or artificial) even in remote locations.

Extracellular Polymeric Substances Facilitate the Adsorption and Migration of Cu2+ and Cd2+ in Saturated Porous Media

Heavy metal contamination in groundwater is a serious environmental problem. Many microorganisms that survive in subsurface porous media also produce extracellular polymeric substances (EPS), but little is known about the effect of these EPS on the fate and transport of heavy metals in aquifers. In this study, EPS extracted from soil with a steam method were used to study the adsorption behaviors of Cu2+ and Cd2+, employing quartz sand as a subsurface porous medium. The results showed that EPS had a good adsorption capacity for Cu2+ (13.5 mg/g) and Cd2+ (14.1 mg/g) that can be viewed using the Temkin and Freundlich models, respectively. At a pH value of 6.5 ± 0.1 and a temperature of 20 °C, EPS showed a greater affinity for Cu2+ than for Cd2+. The binding force between EPS and quartz sand was weak. The prior saturation of the sand media with EPS solution can significantly promote the migration of the Cu2+ and Cd2+ in sand columns by 8.8% and 32.1%, respectively. When treating both metals simultaneously, the migration of Cd2+ was found to be greater than that of Cu2+. This also demonstrated that EPS can promote the co-migration of Cu2+ and Cd2+ in saturated porous media.

Impact of compost reactive layer on hydraulic transport and C & N cycles: Biogeochemical modeling of infiltration column experiments

Managed Aquifer Recharge (MAR) is a key strategy to increase freshwater resources in many regions facing water scarcity. MAR issues are related to both quantity and quality of the infiltrating water. In most countries, very high quality of the infiltrating water is required, to limit the impact on the aquifer geochemistry. In this paper, the possibility of injecting water of lower quality in the aquifer and letting the biogeochemical reactions take place in order to enhance its quality is explored. Here, we present the fate of nutrients (C, N) in the biogeochemical system of a reactive barrier formed by mixture of different proportions of sand and compost, supplied with treated wastewater to mimic MAR. An integrated conceptual model involving the nutrient cycles and biomass dynamics (auto- and heterotrophic) was developed, and then tested with a number of solute transport experiments in columns with different compost fraction in the column filling. The model incorporated both saturation and inhibition processes (regarding the nutrients and their byproducts) to provide a comprehensive picture of the nutrient dynamics within the column. The model developed (three if considering the 3 column setups) allowed to discriminate the processes that govern the fate of nutrients in relation with the compost enhancing long-term nutrient degradation, yet hindering hydraulic parameters that affect infiltration rates.

Laboratory investigation on Bacillus subtilis addition to alleviate bio-clogging for constructed wetlands

Bio-clogging is a major problem in the operation of constructed wetlands (CWs) and is caused by accumulation of biofilm and extracellular polymeric substances (EPS) in the substrate. B. subtilis can successfully produce α-amylase and endoglucanase, which can degrade polysaccharides and, consequently, disperse the EPS. Therefore, the addition of B. subtilis was used to decrease the bio-clogging of lab-scale vertical-flow constructed wetlands (VFCW) in this study, and the feasibility and performance of VFCWs were assessed. The results indicate that the addition of B. subtilis can degrade the polysaccharides in the clogging matter and thereby increase the porosity of the substrate. The hydraulic conductivity of Column 1 (with addition) increased by six times, which was 57 times that of control (Column 2). Meanwhile, the chemical oxygen demand (COD) removal rate also increased after the addition of B. subtilis. The microbial communities show that the richness and diversity within the substrate increased after addition. The relative abundance of functional groups of chemoheterotrophy, aerobic chemoheterotrophy, as well as that connected to N cycles also increased, which implied the improvement of the pollution removal efficiency. Meanwhile, the copy number of α-amylase and endoglucanase increased significantly in Column 1 with the addition of B. subtilis, which offers further support for a hydrolase-induced reduction of polysaccharides and the efficiency of B. subtilis on bio-clogging alleviation. The results showed that B. subtilis addition is an effective and safe solution to control the bio-clogging for CWs. However, further research about long-term effect assessment and dosing strategy optimization should be conducted.

Research on the experiment of the enhancement removal of fine sand by hydrocyclone in sewage treatment plant

The content of fine sand (< 200 μm) in primary sludge is relatively high in Chongqing sewage treatment plant owing to the part of rainwater will be mixed with sand and discharged into the municipal pipe network. Due to the insufficient separation of the sand, different obstacles to subsequent treatment processes may increase equipment wear, reduce effective volume of the tank, or shorten the cleaning cycle. There is a common use of grit chamber for the separation. Nevertheless, the use of hydrocyclone shows an outstanding performance in cost effectiveness and ease operation. The primary sludge in a sewage plant in Chongqing was monitored, and the average concentration of total suspend solids (TSS), total sand content, and volatile suspended solid (VSS) were 40.25 g/L, 17.51 g/L, and 13.41 g/L, respectively. The size of sand in the sludge was small, and the sand below 30 μm accounted for about 70% of the total sand. It formed flocs with organic matter and was removed in subsequent process units. While the size between 30-200 μm, called fine sand, was the main separation object, accounted for about 28.5%. According to XRF and XRD analysis, the sludge composition was mainly composed of quartz (SiO₂), plagioclase (Na(AlSi₃O₈)), and calcite (CaCO₃), which were similar to the main mineral composition of surface sediments and mountain rocks in the main urban area of Chongqing. A single-factor experiment on two types (FX100 and FX50) of hydrocylones was conducted to determine their abilities concerning the separation of fine sand and enrichment of organic matters from primary sludge. FX100 and FX50 showed best performance in the case of P = 0.17 Mpa, underflow diameter (Du) = 18 mm and P = 0.20 Mpa, Du = 6 mm, respectively. The removal efficiency of fine sand by hydrocyclone FX50 was 71.39%. While, it had poor performance on organic matter enrichment and the removal efficiency of which was 17.38%. By contrast, the removal rate by FX100 reached 61.89% for fine sand and only 6.89% for organic matters detached. The superimposition effect did not appear in the serial experiments on hydrocylone FX100 and FX50, but the power is 3.5 times of that of single-stage hydrocylone FX100. Comprehensive consideration of the processing capacity per unit time and operating power, the hydrocylone FX100 was more suitable for actual operation.

The influence mechanism of bioclogging on pollution removal efficiency of vertical flow constructed wetland

The effect of change of hydraulic characteristic and microbial community on pollution removal efficiency of the infiltration systems in the bioclogging development process remain poorly understood. In this study, therefore, the pollutant removal as a response to hydraulic conductivity reduction and the change of diversity and structure of microbial communities in vertical flow constructed wetlands (VFCWs) was investigated. The results indicated that the richness and diversity of the bacterial communities in the columns at different depths were decreased, and the microbial communities of the genus level were changed in the process of bioclogging. However, the variation of microbial communities has a low impact on the purification performance of VFCWs because the abundance of function groups, respiratory activity, and degradation potentiality of microorganisms remain steady or even get improved in the columns after bioclogging. On the contrary, the hydraulic efficiency of VFCWs decreased greatly by 16.9%, 9.9%, and 57.1% for VFCWs filled with zeolite (Column I), gravel (Column II), and ceramsite (Column III), respectively. The existence of short-circuiting and dead zones in the filter media cause the poor pollution removal efficiency of VFCWs due to the short contact time and decrease of oxygenation renewal, as well as low activity in the dead zone.