Combined clogging of suspended particle and algae during the recharge with the Yellow River water in the Yufuhe River channel of North China Plain

Nevertheless, aquifer clogging caused by suspended particles, chemical precipitation, biomass, and gas is the most serious problem during the recharge. According to the clogging causes, aquifer clogging can be divided into physical, chemical, and biological clogging, among which the physical plugging caused by suspended particle deposition is the most harmful clogging type in the MAR projects. The main form of suspended particles clogging is that the filter cake or silting layer formed by suspended particles deposition on the medium surface cuts off the hydraulic connection between surface runoff and pore runoff. Wang studied the rain-flood to recharge groundwater, the research conclusion found that the leakage channel of surface water flow was cut by the filter cake caused by suspended particles deposition on the sand sample surface (Wang et al. 2012). Zaidi found that physical clogging frequently appears on the surface of the medium (Zaidi et al. 2020). In the sand tank experiment simulating the piedmont river recharge, Zhao found that the surface sand sample of the sand tank formed a thin clay layer caused by the suspended particle deposition, which cut off the hydraulic connection between surface runoff and underground runoff (Zhao et al. 2022). In addition to physical plugging, bioclogging can also have a significant impact on the permeability of the aquifer (Wang et al. 2020; Cui et al. 2021). Bioclogging is a process in which microorganisms grow on the surface and inside of the medium by using the nutrient source in the recharge water, continuously accumulate biomass, and occupy the pore volume until the pores of the medium are all blocked. Xian studied the plugging process of the disjointed riverbed, the result found that the growth of microorganisms formed a layer of biofilm on the riverbed surface and blocked the infiltration process of the river (Xian et al. 2019). At present, the research on bioclogging mainly focuses on bacterial clogging, and the main concern is the microbial accumulation effect on the porous media permeability. Thuv and Cui simulated the bacterial plugging process through the soil column experiment, and believed that the soil column permeability decreased the fastest in the early stage of the seepage process, and there has been a rapid decline in the surface sand sample permeability compared with middle and bottom sand samples (Cui et al. 2021; Thuy et al. 2022). Chemical blockage is often accompanied by bioclogging. Cui studied the effect of iron on bioclogging through sand column experiments, and believed that the presence of iron would limit the microorganisms migration process in the medium, and the biofilm formed by biological growth would also limit the iron migration behavior in the medium (Cui et al. 2023). In practical MAR projects, a single type of aquifer plugging is almost non-existent. On the contrary, the combined clogging of aquifers is more reflective of the actual situation. The study on combined plugging mechanism formed by bacteria and suspended particle is a hot topic. Cui and Wang found that the combined clogging degree of bacteria and suspended particle on the medium pore in the SS-bacteria group was significantly higher than that of SS-only group and bacteria-only group through the batch sand column model experiment (Wang et al. 2020; Cui et al. 2021). The interaction of microorganisms, extracellular polymeric substance (EPS) secreted by microorganisms, and suspended particles in the medium pores is the primary reason of the medium combined plugging effect during the recharge. Algae are also microorganisms, so algae clogging and algae-related combined clogging should also be paid attention. The life activities of bacteria are rarely limited by light, whereas most algae depend on their own photosynthesis for growth, and the biomass accumulation is inseparable from light, which is completely different from bacterial growth. In addition, more importantly, a single algae cell particle size is in the micron scale, while the bacteria particle size is mostly at the nanometer level (Gong et al. 2019; Liao 2024). The difference of growth mode and particle size between algae and bacteria will directly affect their deposition and migration in the pore medium.

Zheng et al. (2019) and Zhao et al. (2021) have studied the influence of suspended particle in the Yellow River on the gravel riverbed permeability during the recharge of the Yufuhe River Basin through sand column and sand tank experiments, respectively. Li et al. (2019) studied the conversion efficiency of Surface Water and Groundwater during the Yufuhe recharge process by numerical simulation. The study of aquifer clogging during the recharge of Yufuhe River in the past mainly focuses on physical clogging. The runoff characteristics of the Yellow River vary greatly during the year. Therefore, in order to ensure the stable operation of the recharge project, the Yellow River water needs to be regulated and stored by reservoirs before the recharge. Especially in summer, the changes in the hydrodynamic conditions of the Yellow River water in the reservoir accelerate the algae growth. Thus, during the recharge process of the Yellow River in summer, the riverbed medium is faced with the risk of SS-only clogging, algae-only clogging, and SS-Algae combined clogging. Since suspended particles are not affected by season and temperature, recharge at any time of the year will cause physical blockage dominated by suspended particles. Algae are affected by temperature, light, season, and other factors. In summer, the algae concentration in the water will increase rapidly. Therefore, algae plugging occurs only in summer during the recharge process of the Yufuhe River. In this research, a sand column model was designed to simulate the recharge and infiltration process of the Yellow River in summer.

Our main research objectives include: (1) to compare the effects of SS-only clogging, algae-only clogging, and SS-Algae combined clogging on medium permeability; (2) to explore the migration characteristics of SS and algae within the medium between different clogging types; (3) to reveal the deposition law of SS and algae on the surface and inside of the medium. The research reveals the mechanism of algae clogging in Yufuhe River during the recharge improves the technical system of riverbed clogging study and provides a basis for the formulation of algae clogging prevention scheme, prolonging the life of the recharge project in the Yufuhe River. At the same time, it also enriches the theories about algae plugging in the bioclogging plugging system, and provides references for subsequent scholars.

Before the experiment began, the porosity and hydraulic conductivity of the riverbed surface sand gravel located in the study area were measured to be 0.28 and 50 m/day, respectively. In this paper, based on the principle of equal hydraulic conductivity coefficient, 0.25–0.5 mm quartz sand is used to replace the sand gravel layer covering the study area riverbed surface. The quartz sand used in infiltration experiments is acid, alkali, and high-temperature resistant, with extremely stable chemical properties. The clay minerals content in the Yellow River water sediments is very high, and the mass fraction of nano-scale montmorillonite is the largest (43%), consequently, the suspended particles were simulated by montmorillonite in this study. The water quality of Yuqinghu Reservoir and Wohushan Reservoir in summer was investigated in the laboratory, and the results show that the green algae represented by chlorella proteinucleus were the main plankton in the water. Therefore, chlorella was selected to configure the recharge water source to simulate the algae clogging process.

In entire experiment groups in this research, the infiltration water head of the sand column remains constant, and the water head is achieved by controlling the water depth, which is always maintained at 1 cm. Before the experiment begins, turn on the agitators in the water supply bucket to keep the suspended particles or algae in a suspended state. Turning on the peristaltic pump marks the beginning of the experiment. When the relative hydraulic conductivity coefficient of whole sand column drops below 0.01 or remains stable for a long time, the experiment is terminated. During the infiltration, the pressure water head, flow rate, suspended particles, and algae outflow concentration were measured every 30 min. All experiments in this study are divided into three types: SS-only group, Algae-only group, and SS-Algae group, and the SS concentration was set to 100, 300, and 500 mg/L, while the algae concentration was set to 10 and 15 nephelometric turbidity units (NTU). The detailed experimental scheme is presented in Table 1.

represents the particles concentration in the outflow at any time ⁠, which is used to describe the particles migration characteristics in the sand column. The SS concentration in water was measured by ultraviolet spectrophotometer at 600 nm wavelength and the algae concentration at 680 nm wavelength.

The particles distribution in the medium can reveal the permeability change of the medium, thus, the mass of SS and algae deposited in the medium needs to be measured immediately after the experiment. The measurement steps are as follows: the sand sample containing SS and algae was dried in a constant temperature drying oven at 120 °C to constant weight, and its mass was measured as m₁ by a balance. Then the dried sand sample was placed into the horse boiler and burnt for 2 h at a high temperature of 600 °C, then its mass was measured as m₂. The sand sample after burning at high temperature was repeatedly washed with water to remove the SS deposited in the sand sample. The mass of the washed sand sample measured after drying is m₃. The parameters m₁–m₂ and m₂–m₃ are the deposition mass of algae and SS in the sand sample, respectively.

In this research, the sand column medium is divided into surface layer, shallow layer, middle layer and bottom layer from top to bottom, among the clogging of surface layer is named as surface clogging, and the clogging of shallow layer, middle layer and bottom layer is named as internal clogging. As shown in Figure 5(b), by the end of the experiment, the K_t/K₀ of surface sand samples in the SS-only group, algae-only group, and SS-algae group were reduced by 0.98, 0.96, and 0.93, respectively. Compared with the SS-only group and algae-only group, the surface sand forms a more serious clogging issue in the SS-algae group. By contrast, the internal plugging degree of SS-algae group is the least of the three clogging types (Figure 3(c)–3(e)). Hence, one can see that the surface and internal clogging characteristics of the SS-only group, algae-only group, and SS-algae group are significantly different.

The algae continuous growth in summer will affect the algae concentration in the Yellow River water. Therefore, the effect of algae concentration changes on medium combined plugging should not be ignored. Algae concentration has a significant impact on the combined clogging process in the Figure 5(a)–5(e), after the clogging, the whole sand column K_t/K₀ value decreased to 0.04 and 0.03, respectively, with the algae concentration increase. The clogging characteristics on the surface and inside of the medium are the same as the whole. It follows that the algae concentration increase in recharge water has a negative effect on the whole, surface, and internal permeability of the medium. The time to reduce the entire sand column K_t/K₀ value by 50% is 270 min, when algae concentration is 10 and 15 NTU, respectively (Figure 5(a)). However, after 600 min, the experimental group with the high algae concentration was more clogged (Figure 5(a)). Consequently, the increase of algae concentration in the recharge water will advance the clogging process in the SS-algae group.

During the recharge, the SS concentration variation in the Yellow River water can also affect the migration process of algae and SS itself in the medium. Figure 6(b) and 6(e), respectively, show the migration characteristics of algae and SS with the different SS concentration in the recharge water. With the SS concentration increase, after the experiment is stable, the C_t (SS) was 6.43, 13, and 20 mg/L, and the C_t (algae) was 6.94, 3.4, and 2.22 mg/L, respectively. In the SS-algae group (Figure 6(b) and 6(e)) increasing the SS concentration in recharge water can promote the SS migration, but can hinder the algae migration process in the medium.

Figure 6(c) and 6(f) show that the increase of algae concentration in the river water will expand the algae migration range in the medium, and also limit the SS migration process in the recharge process. After the experiment, the C_t (algae) concentrations were 3.4 and 4.4 mg/L, and the C_t (SS) were 13 and 6.9 mg/L, respectively, when the algae concentration is 10 and 15 NTU (Figure 6(c) and 6(f)). Thus, the effect of algae concentration change on the recharge process should be considered in the actual recharge process.

Table 2 shows the SS deposition mass in sand sample. Compared with the SS-only group, the SS mass deposited in the medium pores in the SS-algae group decreased at least 8% (Table 2). And in the SS-algae group, the SS deposited in sand sample increases with the SS concentration increase (Table 2). On the contrary, the increase in algae concentration in recharge water will reduce the SS deposition mass. In addition, the SS mass deposited in surface sand samples is the largest regardless of the SS-only group or the SS-algae group.

The algae deposition characteristic in the sand is similar to that of SS, and Table 3 presents the algae deposition mass. In the SS-algae group, except the surface medium, the algae mass deposited in the sand sample pore was reduced at 3% than that in the algae-only group (Table 3). Meanwhile, increasing the algae concentration in the recharge water will promote the algae deposition process in the medium, but increasing the SS concentration will have a significant bad effect on the algae deposition (Table 3).

At present, the research of bioclogging and combined clogging related to microorganisms is centered on bacteria (Cui et al. 2021; Xia et al. 2022; Liu et al. 2023a, 2023b). Most researchers believe that bacteria cells and EPS secreted by bacteria are the main substances that clog aquifer pores (Xia et al. 2016; Li et al. 2023; Wang et al. 2023), and EPS contribute a lot to the combined clogging formed by bacteria and SS in the medium pore during the recharge (Wang et al. 2020; Cui et al. 2021). However, the studies on algae plugging are extremely scarce. Moreover, the aquifer medium pores are more likely to be blocked by the algae cells with large particle sizes compared to bacteria in the recharging.

Notwithstanding the fact that this research has completed plenty of work on investigating the algae and SS combined clogging mechanism, there are still several problems that need to be further explored. The sand column model is the main means to study SS-microorganism combined clogging in the recharge process, and the numerical model of combined plugging should be supplemented and developed (Wang et al. 2020; Cui et al. 2021). The microorganisms’ cell particle size, EPS, pore structure, suspended particle size, and so on will affect the clogging process of the medium. In this study, the combined clogging mechanism was analyzed only from the perspective of algae and particle size. In the infiltration process, algae are also in a growing state, and the influence of algae growth process on algae clogging and SS-Algae combined clogging should be considered. Therefore, the next step in this work is to investigate the combined plugging numerical model in detail, and analyze the influence of the algae growth process on the SS-Algae combined plugging process.

In this research, the single and combined clogging mechanisms related to suspended particle and algae are studied by the sand column model. The main conclusions are as follows:

• (1) The filter cake and algae mat are the signs of suspended particle clogging and algae clogging, respectively, and the algae mat structure formed by the suspended solids and algae jointly is denser, which has a more negative impact on the medium's permeability. In the SS-Algae group, the clogging degree of the whole sand column and the surface layer is at least 2% higher than the SS-only group and Algae-only group.

• (2) The filter cake and algae mat in the recharge process are mainly deposited on the riverbed surface. Replacing the surface medium of the riverbed regularly is one of the effective measures to ensure the long-term operation of the recharge project.

• (3) The research method in this paper is mainly based on the sand column model. However, the experimental data that can be obtained through physical experiments is very limited. Therefore, establishing a numerical model that can reveal the deposition and migration processes of the suspended particles and algae in the riverbed medium pores can effectively predict the evolution mechanism of riverbed clogging, and is more conducive to formulating a fine recharge plan to prevent clogging.

This paper was supported by the Natural Science Foundation of Shandong Province (No. ZR2021ME069), the National Natural Scientific Foundation of China (NSFC) (grant numbers U21A2004), and the Major Science and Technology Projects of the Ministry of Water Resources (SKS-2022041).

W. Z. investigated the data, performed data analysis, wrote the original manuscripts, and conceptualized the whole article. W. W. supervised the work, helped with funding acquisition, and arranged the resources. S. Q. is a part of experiment, J. Z. and Z. W. helped revising the paper.

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

The authors declare there is no conflict.