Application of constructed wetlands in treating rural sewage from source separation with high-influent nitrogen load: a review

Recent progress, challenges, and future prospects in constructed wetlands employing biochar as a substrate: a comprehensive review

Constructed wetlands (CWs) are a cost-effective, efficient, and long-term wastewater treatment solution in various countries. The efficacy and performance of constructed wetlands are greatly influenced by the substrate. Recently, biochar as a substrate, along with sand and gravel in constructed wetlands, has gained importance due to its various physical, chemical, and biological properties. This review presents a detailed study of biochar as a substrate in CWs and the mechanism involved in efficiency enhancement in pollutant removal. Different methods for producing biochar using various types of biomasses are also addressed. The effect of biochar in removing pollutants like biological oxygen demand (BOD), chemical oxygen demand (COD), nitrogen, heavy metals, and non-conventional pollutants (microcystin, phenanthrene, antibiotics, etc.) are also discussed. Furthermore, post-harvest utilization of constructed wetland macrophytic biomass via bioenergy production, biochar formation, and biosorbent formation is explained. Various challenges and future prospects in biochar-amended constructed wetlands are also discussed. Biochar proved to be an effective substrate in the removal of pollutants and proved to be a promising technique for wastewater treatment, especially for developing countries where the cost of treatment is a constraint. Biochar is an effective substrate; further modification in biochar with the right plant combination for different wastewater needs to be explored in the future. Future researchers in the field of constructed wetlands will benefit from this review during the utilization of biochar in constructed wetlands and optimization of biochar characteristics, viz., quantity, size, preparation method, and other biochar modifications.

A critical review of characteristics of domestic wastewater and key treatment techniques in Chinese villages

As of 2022, China's rural sewage treatment rate is only approximately 31 %. Rapid rural development has led to higher demand. However, China's rural areas are complex and face many problems, such as uneven economic development, population distribution, and water availability. Long-lasting and low-cost wastewater treatment measures are needed for application in rural areas. The quantity and quality of rural domestic wastewater in China were characterized first. Next, the hot topic of domestic wastewater in Chinese villages was confirmed via bibliometric analysis using CiteSpace, and the treatment technologies for rural domestic wastewater were compared. Specifically, the technical status and challenges of the most common technology in rural domestic wastewater treatment, constructed wetlands, were summarized.

Performance and microbial community analysis on nitrate removal in a bioelectrochemical reactor

In this experiment, we took reflux sludge, sludge from an aeration tank, and soil from roots as microbial inoculating sources for an electrochemical device for denitrification with high-throughput sequencing on cathodic biofilms. The efficiency of nitrate nitrogen removal using different microbial inoculates varied among voltages. The optimal voltages for denitrification of reflux sludge, aeration tank sludge, and root soil were 0.7V, 0.5V, and 0.5V, respectively. Further analysis revealed that the respective voltages had a significant effect upon microbial growth from the respective inoculates. Proteobacteria and Firmicutes were the main denitrifying microbes. With the addition of low current (produced by the applied voltage), the Chao1, Shannon and Simpson indexes of the diversity of microorganisms in soil inoculation sources increased, indicating that low current can increase the diversity and richness of the microorganisms, while the reflux sludge and aeration tank sludge showed different changes. Low-current stimulation decreased microbial diversity to a certain extent. Pseudomonas showed a trend of decline with increasing applied voltage, in which the MEC (microbial electrolysis cell) of rhizosphere soil as inoculates decreased most significantly from 77.05% to 12.58%, while the MEC of Fusibacter showed a significant increase, and the sludge of reflux sludge, aeration tank and rhizosphere soil increased by 31.12%, 18.7% and 34.6%, respectively. The applied voltage also significantly increased the abundance of Azoarcus in communities from the respective inoculates.

Removal Efficiency of Heavy Metals Such as Lead and Cadmium by Different Substrates in Constructed Wetlands

In order to find an efficient and economical wetland substrate to treat mine wastewater containing various heavy metals, and effectively realize the resource utilization of water treatment residuals, in this paper, the treatment efficiency of mine wastewater containing various heavy metals was investigated using unburned ceramsite prepared from water treatment residuals (UCWTR) and clay ceramsite. The continuous dynamic test results showed that the removal rate of Pb, Cd, Cu, Zn, and Fe can reach more than 98.5% after the UCWTR-based CWs runs for 56 days, and its concentration was 30.05%, 24.85%, 20.82%, 14.63%, and 7.91% lower than that of the clay ceramsite-based CWs, respectively. SEM, XPS, and FT-IR showed that the characteristic peaks of two ceramsites were basically similar. The ceramsite undergoes ion exchange, coordination complexation, and chelation reaction with Pb, Cd, Cu, Zn, and Fe under the action of the gel of internal groups -OH, C=O, Al-OH, Si-Fe-O and C-S-H. Compared with clay ceramsite, the ion exchange reaction and chelation reaction of -OH effect and the coordination reaction of C=O effect of carboxyl group in UCWTR were enhanced. In conclusion, using UCWTR as a CWs substrate can effectively enhance the adsorption capacity of heavy metals, providing a scientific basis for the application of UCWTR-based CWs in mine wastewater treatment.

Systematically assess the advancing and limiting factors of using the multi-soil-layering system for treating rural sewage in China: From the economic, social, and environmental perspectives

Solving the problem of rural sewage is considered an essential task in China's rural revitalization strategy. Based on the yearbook data of sewage treatment in rural areas between 2014 and 2019, although the rate of sewage treatment in rural areas of China showed an upward trend, it was still below 35%, mainly due to the lack of suitable sewage treatment technologies. Here, we discuss the multi-soil-layering (MSL) system, which is an emerging technology suitable for rural sewage treatment. It was deemed to overcome the shortcomings of current biological and ecological treatment technologies, such as complex operation, large area, and high operating costs. We used system dynamics to evaluate the advancing and limiting factors of MSL application for rural sewage treatment from the social, environmental, and economic dimensions. The results illustrated a complete causal loop diagram in which essential variables and relationships were concentrated in the technology, operation and maintenance, and satisfaction of farmers. The efficiency of MSL is the key variable affecting the final decision of the MSL application. Overall, using MSL to treat rural sewage could be an option to improve the rural environment in China. However, the scientific technological model for MSL should be further explored. This review provides guidance on how to promote MSL systems in rural areas.