Performance evaluation of a low-cost loess-based filler for bioretention cells

The sand and gravel fillers used in traditional bioretention cells are expensive and becoming increasingly scarce, and their performance is unstable. It is important to find a stable, reliable, and low-cost alternative filler for bioretention facilities. Using cement as a modified loess filler for bioretention cells is a low-cost and easily obtainable alternative. The loss rate and anti-scouring index of the cement-modified loess (CM) were analyzed under different curing times, cement addition amount, and compactness control conditions. This study found that the stability and strength of the cement-modified loess in water with a density of not less than 1.3 g/cm³, a curing time, of not less than 28 d and a cement addition amount not less than 10% meets the use requirements of the bioretention cell filler. X-ray diffraction and Fourier transform infrared spectroscopy of cement-modified materials with a 10% cement addition and a curing time of 28 days (CM28) and 56 days (CM56). Cement-modified materials with 2% straw and a curing time of 56 days (CS56) showed that the three kinds of modified loess all contain calcium carbonate and that the surface contains hydroxyl and amino functional groups that can effectively remove phosphorus. The specific surface areas of the CM56, CM28, and CS56 samples were 12.53 m2/g, 24.731 m2/g, and 26.252 m2/g, respectively, which are significantly higher than that of sand (0.791 m2/g). At the same time, the adsorption capacity of the ammonia nitrogen and the phosphate that was present in the three modified materials is better than that of sand. CM56, like sand, has rich microbial communities, which can entirely remove nitrate nitrogen in water under anaerobic conditions, indicating that CM56 can be used as an alternative filler for bioretention cells. The production of cement-modified loess is simple and cost-effective, and using modified loess as a filler can reduce the use of stone resources or other on-site materials. Current methods for improving the filler of bioretention cells are mainly based on sand. This experiment used loess to improve the filler. The performance of loess is better than sand, and can completely replace sand as the filler in bioretention cells.