Effects and mechanism of using Nanobubble to inhibit biofouling and scaling in biogas slurry drip irrigation emitters

Abstract: Biogas slurry irrigation can effectively alleviate the shortage of agricultural water and fertilizer resources as well as the environmental pollution caused by improper discharge, and the nutrient elements in biogas slurry can improve the yield and quality of crops. However, biogas slurry contains a large number of microorganisms, salt ions, particulate matter and other scaling substances which are easy to cause the irrigation system of drip irrigation clogging, which has become a key obstacle to the application and promotion of biogas slurry drip irrigation technology. Nanobubbles (NBs) with an average diameter of less than 200 nm are characterized by large specific surface area, negative surface charge and high adsorption performance and are expected to be a reliable method for synergistic mitigation of compound fouling in biogas slurry irrigation system. The purpose of this study was to explore the mitigation effect of Nanobubbles on the composite clogging of the emitter of the biogas drip irrigation system, and to clarify the mitigation effect of NBs on the epiphytic fouling, chemical precipitation and particle fouling of the emitter. Different volume ratios of NBs-free water and saturated NBs water were prepared, and three kinds of irrigation water with NBs concentrations were configured: control group (non-NBs water), saturated NBs water (all NBs water), and half NBs water (1:1). The clogging mitigation test was carried out for three kinds of inner insert type emitter device respectively. During the experiment, Relative Average discharge (Dra) and Uniformity coefficient (CU) of the emitter are calculated. At 112.5, 225, 337.5, 450, 562.5, 675, 787.5 and 900 h, the dry weight in the emitter was recorded. The Extracellular Polymeric Substances (EPS) was calculated at 225, 450, 675 and 900 h. Polycrystalline diffraction patterns and mineral composition of the clogging material were determined. The results showed that, compared with the control group, NBs effectively relieved the clogging of the emitter, and the dry mass of the clogging substances in the three types of emitters at the end of the system operation decreased by 22%-49%, 18%-41% and 25%-47%, and increased the Dra and CU of the drip irrigation system by 15%-44% and 26%-48%, respectively. Meanwhile, NBs significantly decreased the biofouling, chemical precipitation and particulate fouling adhering to the emitter. Among them, the content of EPS in the dirt was reduced by 29%-53%, the content of precipitate such as carbonate and phosphate was reduced by 52%-75% respectively, while the total amount of particulate fouling was reduced by 35%-68%. In contrast, the dry weight of foulants in the 0:1 treatment was lower than that in the 1:1 treatment. It is recommended to use saturated NBs in the biogas drip irrigation project to alleviate the clogging of the irrigator. This study proves that NBs is an effective irrigation clogging mitigation technology, which is helpful for the further promotion of biogas drip irrigation technology.