Reduction of sewage sludge and N2O emissions by an Oxic Settling Anaerobic (OSA) process: The case study of Corleone (Italy) wastewater treatment plant

Accelerated dissemination of antibiotic resistant genes via conjugative transfer driven by deficient denitrification in biochar-based biofiltration systems

Biofiltration systems harbored and disseminated antibiotic resistance genes (ARGs), when confronting antibiotic-contained wastewater. Biochar, a widely used environmental remediation material, can mitigate antibiotic stress on adjoining microbes by lowering the availability of sorbed antibiotics, and enhance the attachment of denitrifiers. Herein, bench-scale biofiltration systems, packed with commercial biochars, were established to explore the pivotal drivers affecting ARG emergence. Results showed that biofiltration columns, achieving higher TN removal and denitrification capacity, showed a significant decrease in ARG accumulation (p < 0.05). The relative abundance of ARGs (0.014 ± 0.0008) in the attached biofilms decreased to 1/5-folds of that in the control group (0.065 ± 0.004). Functional analysis indicated ARGs' accumulation was less attributed to ARG activation or horizontal gene transfer (HGT) driven by sorbed antibiotics. Most denitrifiers, like Bradyrhizobium, Geothrix, etc., were found to be enriched and host ARGs. Nitrosative stress from deficient denitrification was demonstrated to be the dominant driver for affecting ARG accumulation and dissemination. Metagenomic and metaproteomic analysis revealed that nitrosative stress promoted the conjugative HGT of ARGs mainly via increasing the transmembrane permeability and enhancing the amino acid transport and metabolism, such as cysteine, methionine, and valine metabolism. Overall, this study highlighted the risks of deficient denitrification in promoting ARG transfer and transmission in biofiltration systems and natural ecosystems.

Carbon performance analysis of upgraded wastewater treatment plants in key control areas of the Ziya River Basin in China

Wastewater treatment plants (WWTPs) are one of the largest sources of greenhouse gas (GHG) emissions, and they are also one of the largest energy consumption industries in urban systems. With the progression of upgrading and standard-rising, WWTPs both directly and indirectly increase carbon emissions from the increased investments in facilities and usages in electricity as well as chemical agents. Here, we collected operational data from 15 WWTPs in the key control areas of the Ziya River Basin in North China and accounted for the changes in carbon performance at different technical upgrade methods. Results showed that the average carbon emission performance increased by 0.487 kg CO2/m3 after the upgrade. Carbon emissions from electricity consumption, chemical usage, biochemical process and sludge treatment accounted for 42%, 17%, 24%, and 17% of the total improvement in carbon emission performance, respectively. Reducing energy consumption, regulating chemical use and sludge comprehensive utilization are the key to carbon emission reduction. It further proposes that the development of wastewater treatment discharge standards should fully consider the comprehensive utilization of water quality classification. Regions with favorable natural conditions should make full use of their advantages by adopting economically feasible, low-energy-consuming technologies such as constructed wetlands, which offer carbon sequestration and landscaping benefits. This study provides guidance on the selection of technological pathways for pollution reduction and carbon mitigation in the wastewater treatment industry and on achieving sustainable water resource utilization.

The anaerobic exposure time (AET) as a novel process parameter in the anaerobic side-stream reactor (ASSR)-based process for excess sludge minimization

Minimization of excess sludge produced by wastewater treatment plants has become a topical theme nowadays. One of the most used approaches to achieve this aim is the anaerobic side-stream reactor (ASSR) process. This is considered affected by the hydraulic retention time (HRT) of the anaerobic reactor, the anaerobic sludge loading rate (ASLR) and the sludge interchange ratio (SIR), although, studies available in the literature did not reflect a clear relationship with the sludge minimization yields. To overcome this, a novel parameter namely anaerobic exposure time (AET) was defined and related to reduction of the observed yield coefficient (Yobs) in a lab-scale plant implementing the ASSR process. Furthermore, the AET was validated by performing a detailed and thorough review of previous literature. Excess sludge production was successfully reduced (10-60 %) with the increase of the AET (7.9-13 h/d), although maintaining the same HRT in the ASSR and a constant sludge interchange ratio (SIR) (100 %). A strong correlation (Pearson = 0.763) was found between the AET, and the Yobs reduction reported in previous studies, also indicating a linear relationship (R2 = 0.92) between these parameters. Contrarily, the correlation between the Yobs with the ASLR and the ASSR-HRT resulted moderate (Pearson = 0.186) or weak (Pearson=-0.346), respectively. Overall, while operating at low AET (< 6 h), maintenance and uncoupling metabolism were found the main sludge reduction mechanisms. Increasing the AET (>8 h) favoured the occurrence of extracellular polymeric substances (EPS) hydrolysis and endogenous decay mechanisms, which improved excess sludge reduction. To conclude, the AET could be considered a reliable parameter to be used for design or control purposes for the ASSR-based process.

Reducing biosolids from a membrane bioreactor system: Assessing the effects on carbon and nutrient removal, membrane fouling and greenhouse gas emissions

This study presents the effects on carbon and nutrient removal, membrane fouling and greenhouse gas (GHG) emissions of an Oxic-Settling-Anaerobic (OSA) - Membrane Bioreactor (MBR) pilot plant fed with real wastewater. The influence of three sludge return internal ratios (IR) was investigated by testing 45, 75 and 100%. The results showed that with the increase of IR, the biological sludge production substantially decreased by 85.8% due to the combination of cell lysis and endogenous metabolism. However, a worsening of ammonia removal efficiencies occurred (from 94.5 % to 84.7 with an IR value of 45 and 100%, respectively) mostly due to the ammonia release caused by cell lysis under anaerobic conditions. The N2O emission factor increased with the rise of IR (namely, from 2.17% to 2.54% of the total influent nitrogen). In addition, a variation of carbon footprint (CF) (0.78, 0.62 and 0.75 kgCO2eq m-3 with 45, 75 and 100% IR, respectively) occurred with IR mainly due to the different energy consumption and carbon oxidation during the three periods. The study's relevance is to address the optimal operating conditions in view of reducing sludge production. In this light, the need to identify a trade-off between the advantages of reducing sludge production and the disadvantages of increasing membrane fouling and GHG emissions must be identified in the future.