Constructed wetlands, as a low-tech and low-cost water treatment technology, have been used worldwide in the wastewater treatment or stormwater disposal or reclaim (Kong et al. 2021; Chai et al. 2019). However, clogging is one of the most crucial operation problems that reduce the removal efficiency and life span of the constructed wetland, especially bioclogging, which was due to the accumulation of biomass in the porous media and considered to be the primary reason for the clogging of the infiltration system (Zhou et al. 2018). Bioclogging will occur inevitably in the substrate layer (Liu et al. 2019a), and thus, its mitigation or elimination is an urgent issue in the operation of the constructed wetlands (CWs).
Prevention and restoration are the two general strategies to control clogging in CWs management. Preventative strategies aim to delay or alleviate the negative effects of clogging, such as optimizing operational conditions (Pedescoll et al. 2011), size selection and filling strategies of substrate (Suliman et al.2007, Zhong et al. 2022), and applying resting operation (Hua et al. 2014). Physical and chemical pre-treatment is also an effective preventative strategy, such as adding flocculants, biological pretreatment (De la Varga et al. 2013), which can postpone clogging by removing organic and suspended solids loads (Srivastava et al. 2021).
Restorative strategies are objective to solve the clogging-related hydraulic problems or poor treatment efficiency of CWs (Nivala et al. 2012). The restoration of bioclogging CWs is traditionally through renovation, such as substrate replacement and backwashing. On the other hand, in situ restorations is also becoming widely used, such as adding wetland animals (e.g., earthworms and loaches) (Chiarawatchai et al.2007, Ye et al. 2018), or dosing chemicals (e.g., H2O2, HCl, NaOH and NaClO). Recently, several innovative dosing strategies were presented, such as addition of B. subtilis (Ping et al. 2021), enzymes (Tang et al. 2018), biosurfactants (Du et al. 2016) and rhamnolipids-citric acid compound (Cao et al. 2021), which have good solubilizing effect and environmental friendliness. Although there have been many technologies for prevention and recovery of the bioclogging for CWs, they will increase investment, operating costs, or the land occupation in practical applications. Therefore, it is still imperative to explore the in situ, non-intrusive, environmental friendly and cost saving method to alleviate bioclogging.
Bioclogging is induced by the biofilm accumulation in the porous media. Recent research found that the interactions between the biofilm growth and liquid flow hydrodynamics affected the biofilm accumulation in the porous media (Zhou et al. 2021). An increase in fluid flow velocity facilitates cells and nutrients transport but also triggers biofilm detachment events (Krsmanovic et al. 2021). Thus, it is hypothesized that effectiveness of bioclogging control in CWs may be enhanced by optimizing hydraulic loading. In this paper, we investigated the biofilm distribution and hydrodynamic characteristics in the porous media under the different hydraulic loadings through two-dimensional (2D) flow cell. Besides, the effectiveness of hydraulic loading on bioclogging alleviation was also evaluated through column testing. The results will provide a safe and effective method for solving bioclogging issue in CWs.