Why is it said that artificial wetlands have a strong ability to remove organic matter from papermaking wastewater?

Ecological treatment refers to a method of purifying wastewater under natural conditions through the metabolic processes of environmental organisms. It has now become a hot topic in research and application, among which oxidation ponds and constructed wetlands have seen the most research and application. Their common features are low energy consumption, simple management, low operating costs, the ability to combine multiple ecosystems, and support for the comprehensive utilization of wastewater. Oxidation ponds, also known as biological ponds or stabilization ponds, are systems in which the organic matter in wastewater is primarily removed through symbiotic interactions between bacteria and algae. Heterotrophic microorganisms, namely aerobic bacteria and fungi, oxidize and degrade organic matter to generate energy and synthesize new cells ； Algae fix carbon dioxide through photosynthesis, and absorb nutrients such as nitrogen and phosphorus as well as organic substances to synthesize new cells while releasing oxygen. Under normal conditions, microorganisms and algae complement each other. The oxygen released by algae is used by aerobic and facultative bacteria to oxidize organic matter, producing carbon dioxide and water ； The carbon dioxide among them can be used by algae for photosynthesis. Some algae can carry out metabolism not only through photosynthesis but also through heterotrophy. Based on their biological characteristics, oxidation ponds can be divided into aerobic ponds, anaerobic ponds, and facultative ponds. The dissolved oxygen level in aerobic ponds should be maintained at above 2 ppm at all times. Too little dissolved oxygen creates facultatively anaerobic conditions. Theoretically, the depth of an aerobic pond should be limited to a depth that allows light to penetrate (no more than 0.5 meters), in order to ensure photosynthesis in algae. However, in such deep ponds, algae concentrations can become too high, preventing sunlight from reaching the bottom of the pond. In practical applications, to save land area, facultative ponds with a depth of 1–2 meters are often used. Excessively high concentrations of organic matter and suspended solids in the inlet water of the oxidation pond can affect the treatment efficiency. The use of oxidation ponds for the advanced treatment of pulp and paper wastewater not only removes organic pollutants but also reduces the content of inorganic salts in the wastewater, facilitating its reuse. For example, the inorganic salt concentration in the recycled water of Haisheng Paper Industry, which has oxidation ponds, is 2–3 g/L, while it is 7–8 g/L in the recycled water of Huapeng Paper Industry, which does not have such ponds. The accumulation of inorganic salts accelerates scaling and corrosion in equipment, resulting in an increased amount of sizing needed during production. Constructed wetland treatment technology is a wetland that is artificially designed and constructed to mimic the structure and functions of natural wetlands, as needed. Artificial wetlands have a strong capacity to remove organic matter from papermaking wastewater. On the one hand, insoluble organic matter is quickly trapped through physical deposition processes such as sedimentation and filtration in the filler bed of wetlands, and can be utilized by certain facultative or anaerobic microorganisms ； On the other hand, the soluble organic matter in wastewater is degraded and removed through adsorption, absorption, and biological metabolism by plant roots and the biofilms on the surface of the packing material. Ultimately, most of the organic matter in papermaking wastewater is converted by heterotrophic microorganisms into microbial biomass as well as CO2 and H2O; the newly formed organisms are eventually removed from the wetland system through sediment clearance and plant harvesting. In the advanced treatment of pulp and paper wastewater, artificial reed wetlands are widely used. With this type of wetland treatment, wastewater is thoroughly purified; at the same time, the grown reeds can be used as raw materials for pulp production, thereby enabling a circular economy. However, in practical engineering applications, it has been found that when using the constructed wetland method for the advanced treatment of papermaking wastewater, there is a problem of salt accumulation in the wetland soil; therefore, other water resources should be utilized to enhance soil desalination and leaching, thereby reducing salt accumulation in the soil. At the same time, in the northern regions of our country, due to significant temperature fluctuations associated with seasonal changes, the application of artificial wetland systems is greatly affected by these seasonal shifts. This is also a major constraint on the use of artificial wetland methods in those areas. In practical engineering applications, it is possible to address the issue of wetlands surviving the winter by increasing the capacity of the wetlands, thereby preventing drainage during winter. Ecological treatment is an effective method for the advanced treatment of pulp and paper wastewater, and it is particularly suitable for the advanced treatment of wastewater from small and medium-sized pulp and paper enterprises. This method not only achieves advanced treatment of wastewater but also helps to improve the surrounding ecological environment and enable the reuse of wastewater as a resource; therefore, it holds broad application prospects. However, ecological treatment systems require a large amount of space, are prone to breeding mosquitoes and insects, and there is a lack of experience in their design, operation, and management – these are also issues that need to be addressed in future work.

Artificial wetlands have a strong capacity to remove organic matter from papermaking wastewater, which is achieved primarily through physical interception and sedimentation as well as biological absorption and degradation. On the one hand, the insoluble organic matter in papermaking wastewater is quickly trapped through physical deposition processes such as sedimentation and filtration in wetlands, and can be utilized by certain facultative or anaerobic microorganisms; on the other hand, the soluble pollutants in the wastewater are degraded and removed through adsorption, absorption, and biological metabolism by plant roots and surface biofilms. Most of the organic matter in papermaking wastewater is ultimately converted by heterotrophic microorganisms into microbial biomass as well as CO2 and H2O, thereby achieving a high removal efficiency.

Is it related to the soil quality in some way?