Research Progress and Prospects of Constructed Wetland Treatment Systems for Maricultural Wastewater and Its Nitrogen Removal Process

Abstract

In the process of mariculture, a large number of toxic and harmful substances such as organic matter, ammonia, and nitrite are produced during the metabolism of cultured organisms and the decomposition of feed residuals. If such maricultural wastewater is discharged without purification treatment, it will aggravate the occurrence of eutrophication in the receiving sea area. Constructed wetlands (CW) have received widespread attention due to their low operating costs, simple maintenance, and management advantages. Using CW to treat maricultural wastewater has great prospects. Nitrogen removal is one of the main tasks of constructed wetlands. The characteristics of high salinity and low C/N of maricultural wastewater result in the unique treatment environment and operating mechanism of CW. The substrate can adsorb nitrogen in the constructed wetlands, and nitrogen-cycling microorganisms such as nitrifying bacteria and denitrifying bacteria can attach to the surface to form biofilms. The selection of suitable substrate materials, in addition to zeolite, cinder, sand, and other commonly used water purification materials, can strengthen water purification. Given the low C/N characteristics of maricultural wastewater, materials with slow-release carbon sources can be selected as the filling substrate of constructed marine wetlands. For example, biological carbon sources such as corncob and wood chips, and polymer materials such as PCL and PLC, have recently been used as substrates to fill constructed wetlands and release carbon sources. Meanwhile, substrates that can drive the autotrophic denitrification process of microorganisms such as sulfur autotrophic, hydrogen autotrophic, and iron autotrophic have also been used as a solution. Plants are an important component of constructed marine wetlands, supporting nitrogen removal in four aspects: Nitrogen absorption, oxygen transport, carbon source secretion, and root enrichment of microorganisms. The high salinity environment determines that the wetland plants should be salt-tolerant, and the screening of salt-tolerant plants is a key step in constructed marine wetlands. Currently, Spartina alterniflora, Suaeda salsa, Salicornia bigelovii, Kandelia candel, and similar plants are chosen as candidate plants for constructed marine wetlands. The selection of plants should also consider local conditions, choosing salt-tolerant plants suitable for growing in the local environment. The nitrogen cycle of microorganisms is the main path of biological nitrogen removal in CWs. Various nitrogen-metabolizing bacteria cooperate and restrict each other in CWs, including autotrophic and heterotrophic bacteria, as well as aerobic and anaerobic bacteria. In the process of nitrogen removal in constructed wetlands, dissolved oxygen (DO) is an important environmental factor affecting the distribution and functioning of nitrogen-removing microorganisms. The relatively high DO in the upper layer of the constructed wetland favors the growth and reproduction of aerobic microorganisms, promoting the traditional nitrification process dominated by AOA, AOB, and NOB. The relatively low DO in the bottom layer is more conducive to the growth and colonization of anoxic and anaerobic microorganisms, favoring anaerobic denitrification, Anammox, and DNRA. The occurrence of Comammox can be driven under low nutrient and low oxygen conditions. These bacteria with nitrogen metabolism functions are distributed in different areas, cooperating and restricting each other, forming a complex nitrogen cycle network. Clarifying the basic path of the nitrogen cycle in seawater constructed wetlands is the fundamental basis for regulating the operating parameters of constructed wetlands. The low C/N of mariculture wastewater is not favorable for denitrification by microorganisms. Carbon sources can be supplemented with additional liquid carbon sources, solid carbon sources, and plant litter. DO is the key control index of constructed marine wetlands. The dissolved oxygen content in constructed wetlands is significantly correlated with the community composition of denitrification microorganisms. Therefore, oxygen supply regulation modes, such as continuous aeration, intermittent aeration, and tidal flow, may be effective measures for mariculture wastewater constructed wetlands to improve the overall nitrogen removal performance of wetlands. Accurate regulation of the oxygen supply mode and oxygen supply in constructed wetlands and optimization of dissolved oxygen distribution in different times and spaces within the system are the development trends of nitrogen removal technology in constructed wetlands in the future. The hydraulic operation conditions of CW play an important role in its nitrogen removal effect. Too high or too low indices will affect the efficiency of nitrogen removal in wetlands. Therefore, the optimal control values of hydraulic retention time (HRT), hydraulic loading rate (HLR), and other hydraulic parameters of constructed wetlands also need to be studied. The hydraulic conditions of constructed wetlands also have a significant impact on plant growth, affecting the purification efficiency of plants. In this paper, recent research progress and perspectives on constructed wetlands for the purification of maricultural wastewater and its biological nitrogen removal process were reviewed from four aspects: Selection of substrate, screening of salt-tolerant plants, nitrogen cycling microorganisms, and operation regulation. It is expected to provide a theoretical basis and support for regulating the actual operation of maricultural wastewater constructed wetlands and improving the technical level of maricultural wastewater treatment.

Keywords

• maricultural wastewater
• constructed wetlands
• biological nitrogen removal
• salt-tolerant plants
• nitrogen cycling microorganism