Demonstration study of bypass multipond wetland system to enhance river water quality Open Access

Over the past few decades, China has suffered from severe water quality degradation due to industrial economy-oriented development (Wang & Xiong 2022). In the process, a large amount of domestic and industrial wastewater is directly discharged into rivers, resulting in the serious degradation of river water quality (Hsu et al. 2019). The destruction of urban rivers not only has a major impact on the urban landscape but also seriously impacts the physical and mental health of nearby residents. Therefore, urban river management is an urgent matter. Among river water quality management techniques, artificial wetlands and stabilization ponds have had some of the best effects on improving water quality (Bai et al. 2020; Xu et al. 2022).

As a green ecological water purification process, artificial wetlands are widely used in wastewater treatment because of their low cost, ease of operation, eco-friendliness, and aesthetics (Wang et al. 2022). Artificial wetlands consist of substrates, aquatic plants, and microorganisms that purify wastewater through the synergistic biological, chemical, and physical effects of microorganisms, plants, and substrates (Wei et al. 2020; Xu et al. 2021). Microorganisms play a key role in removing pollutants from artificial wetlands and can degrade pollutants into insoluble or harmless substances (Kumar & Dutta 2019; Zhu et al. 2021). Aquatic plants are an important part of artificial wetlands and can directly remove pollutants from wastewater through absorption and sorption (Zhou et al. 2018b; Gu et al. 2021). Artificial wetland substrates play an important role in pollutant removal, not only by providing a place for the growth and reproduction of microorganisms but also by pollutant sorption (Lu et al. 2021). Artificial wetlands have been widely used in China and other regions of the world over the past two decades as surface water discharge standards have increased (Yu et al. 2022).

Stabilization ponds have the advantages of simplicity, low construction and operating costs, low energy consumption, and sustainability (Abd-Elmaksoud et al. 2021). They provide a completely natural purification process and are increasingly used in wastewater treatment (Ho et al. 2017). Stabilized ponds can be divided into aerobic, parthenogenic, and anaerobic ponds, according to their dissolved oxygen (DO) content (Butler et al. 2015). Different types of stabilization ponds provide better purification; among these, partition ponds stabilize organic matter and remove biochemical oxygen demand from wastewater (Schwarz et al. 2019). Although stabilization ponds are widely used, this low-cost technology is now facing the dilemma of either upgrades or replacement because of more stringent effluent discharge standards (Ho & Goethals 2020). For this reason, artificial wetlands in combination with stabilization ponds were used in the current study, along with the addition of bio-bacteria and artificial plants to enhance the water quality of the river.

In this study, the water quality of a river with a bypass multipond wetland system in China was evaluated. The bypass multipond wetland system was selected to enhance the river water quality after studying and comparing domestic and foreign river treatment technologies and the actual river conditions. Using actual topography, the ponds around the river were converted into surface-flow artificial wetlands, improved parthenogenic ponds, aeration reoxygenation ponds, and ecological ponds. Using artificial plants instead of wetland substrates in the surface-flow artificial wetlands simplified the wetland modification and reduced the modification cost. In the parthenogenic pond, deep water and shallow water areas were set up to not only improve the treatment efficiency of the parthenogenic ponds, but also have landscape effects. In the aeration reoxygenation pond, microbial bacteria were injected into the ecological pond with the water flow. Many artificial plants were planted in the ecological ponds to provide a large amount of habitat for microorganisms. The main aims of the study were: (1) to renovate existing ponds and use the renovation process to enhance river water quality; (2) to achieve the following water quality objectives: DO > 5 mg/L, ammonia as nitrogen (NH₃-N) <1.0 mg/L, total phosphorus (TP) <0.2 mg/L, chemical oxygen demand (COD) < 20 mg/L, and water transparency >60 cm.

At the beginning of the study, the river water was relatively turbid with low transparency, and the main pollution sources were domestic sewage and agricultural water discharge, as shown in Table 1. Using the existing topography, the fishponds and lotus ponds around the river were transformed into surface-flow artificial wetlands, improved parthenogenic ponds, aeration reoxygenation ponds, and ecological ponds to form a bypass multipond wetland system to improve river water quality. The system had a design influent of 15,000 m³/d, covered an area of approximately 103,490 m², and had a water residence time of approximately 21.42 d.

Five sampling points were set up: at the inlet of the bypass multipond wetland system, the outlet of each process, and the outlet of the system; these were denoted as P1, P2, P3, P4, and P5, respectively. The sampling points were set up as shown in Figure 1.

The water quality testing methods in this study are shown in Table 2.

In this study, the pollution sources are mainly domestic sewage and agricultural wastewater discharge. Surface flow artificial wetlands effectively remove suspended matter and nutrients from water due to their plants, substrates, and microbial communities (Xu et al. 2021). These effectively treat domestic and agricultural wastewater (Yang et al. 2018; Li et al. 2020). Surface flow artificial wetlands are widely used because of their advantages, such as low construction, operation, and maintenance costs (Ma et al. 2020). In this study, after considering pond renovation costs, treatment effects, and landscape effects, surface-flow artificial wetlands were selected and used as the treatment process at the front end of the multipond wetland system.

The domestic wastewater in this study contained more pollutants and the partition ponds can degrade a large amount of organic matter in domestic wastewater (Sunarsih et al. 2020); therefore, partition ponds were installed after the surface-flow artificial wetlands. Parthenogenic ponds are large open ponds prone to the vertical stratification of heat, pH, and DO (Buchanan et al. 2018). This is because parthenogenic ponds can provide a suitable living environment for aerobic, anaerobic, and parthenogenic microorganisms. The overall performance of the parthenogenic ponds is based on the symbiotic relationship between aerobic and anaerobic microorganisms and algae.

Maintaining high DO concentrations is essential for the survival of higher life forms in aquatic ecosystems (Mayo & Abbas 2014). Therefore, in this study, an aeration reoxygenation pond was set up after the improvement of the parthenogenic pond. Selection of an appropriate aeration method was also crucial. One study illustrated that there was no significant difference in the COD removal between continuous aeration, intermittent aeration, and non-aerated systems, but there was a significant effect on nitrogen removal from wastewater (Uggetti et al. 2016). Intermittent aeration helped achieve aerobic and anoxic conditions to achieve higher nitrogen removal concentrations (Liu et al. 2019). According to previous research, intermittent aeration can reduce the photosynthetic process of plants, but 4 h of intermittent aeration per day has very little effect on plants (Fan et al. 2013). Intermittent aeration of 4–6 h per day not only reduces aeration costs, but also improves nitrogen and organic removal efficiency (Shukla et al. 2021).

To improve the nitrogen removal rate in the water column, intermittent aeration was used to restore the DO in the water column. Microbial agents, mainly ammonia nitrogen agents, were added to the aeration reoxygenation pond. The synergistic effect of artificial aeration and microbial agents can effectively enhance the anti-pollution ability of the ecosystem and the self-purification ability of the water body, thereby removing pollutants from the water body and improving the water quality of the river (Li et al. 2021). The injected microbial agents flowed into the subsequent treatment facility under water flow. The dosing frequency was twice per week, and the dosing volume was 20 L each time. The aeration reoxygenation pond covered an area of 3,298.74 m², with a design water depth of 1.5 m, a hydraulic retention time of a 0.2 d, and an aeration time of 6 h per day. In the aeration reoxygenation pond, four groups of fountain aerators (model ZL-CFA1500) were set up, with a power of 1.5 KW and an oxygenation capacity of 0.9–1.1 kgO₂/h.

Two ponds were transformed into ecological ponds, mainly with aquatic plants and animals. Aquatic plants are the main producers in aquatic ecosystems and are critical for the management of the water environment. Plant remediation is an economical and aesthetic solution to many global environmental problems, and to the use of plants to remove and accumulate pollutants in the environment, which involves using plants to mitigate, transfer, stabilize, or degrade pollutants in soil, sediment, and water (Lu et al. 2018; Nash et al. 2020). Aquatic plants not only provide habitats for various aquatic animals but also enhance the biodiversity and stability of aquatic ecosystems (Zhou et al. 2018a). Aquatic animals can reduce phytoplankton biomass and debris in the water column through their filtering properties, thereby improving the water quality and clarity (Gao et al. 2017).

Aquatic plants, submerged plants, and artificial plants were planted in the ecological ponds, into which aquatic animals were then introduced. The plants used were mainly submerged species, such as M. elatinoides, H. verticillata, and V. natans, and aquatic plants such as P. australis, T. dealbata, and C. indica. Aquatic animals were fish (such as Ctenopharyngodon idella and Lateolabrax maculatus), snails, and shrimp. The ecological ponds covered an area of 25,461.43 m², with a design water depth of 0.8 m, a hydraulic retention time of approximately 8.56 d, and a hydraulic load of approximately 0.09 m³/(m²·d).

Technologies that can achieve the goal of water quality treatment are mainly divided into two types: in-situ treatment and bypass treatment. To choose the better process based on cost, this study focused on comparing the cost of in-situ treatment technology and bypass treatment technology. A comparison of the technology costs is presented in Table 3.

The study was conducted over a period of 4 months, with samples taken from 9:00 to 10:00 am daily. Samples were taken every ten days for a total of 12 times. The measurement indices mainly included water transparency, DO, COD, NH₃-N, and TP.

In this study, by reasonably modifying the ponds around the river, the river water passed through a bypass multipond wetland system consisting of surface-flow artificial wetlands, improved parthenogenic ponds, aeration reoxygenation ponds, and ecological ponds. After 4 months of treatment by this system, the water quality of the river reached the treatment targets of DO >5 mg/L, NH₃-N <1.0 mg/L, TP <0.2 mg/L, COD <20 mg/L, and transparency >60 cm. After monitoring the results of each process, it was found that the processes with best transparency enhancement were the surface-flow artificial wetlands and ecological ponds. The aeration reoxygenation pond had the best effect on the DO enhancement. The processes that most effectively removed NH₃-N and TP were the surface-flow artificial wetlands and ecological ponds, while the process with the best effect on COD removal was the improved parthenogenic pond. After treatment, the water quality was enhanced and the landscape was improved, which demonstrates the application value of this wetland system. These results can provide a reference for similar river water enhancements.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.