Investigation of cockleshell waste as a natural biocarrier and its performance efficiency for blackwater treatment in a free-floating plant-constructed wetland

Abstract

This study investigated the potential of using waste cockleshell (CS) as a natural biocarrier for blackwater treatment. The CS were analysed for surface porosity, area, volume, elements, and crystal structure. Then, the pilot-scale study confirmed biofilm growth on the CS surface after 29 days, using a crystal violet assay and scanning electron microscopy (SEM) analysis. The free-floating plant-constructed wetland (FFP-CW) was optimally designed with a redox gradient: an aerobic-aquatic zone for nitrification and an anoxic gravel-filled zone for denitrification. The effect of integrating CS biocarriers and active aeration in FFP-CW (aquatic zone) treatment efficiency was investigated. The FFP-CW treatment efficiency for nitrate, chemical oxygen demand (COD), and total ammonia nitrogen (TAN) increased by 9.62%, 6.8%, and 4.83%, respectively, after integrating CS biocarriers of 0.55% filling ratio. Furthermore, the optimal dosage of 192.51 mol of oxygen in FFP-CW increased in TAN and COD removal by 31.11% and 16.54%, respectively. The X-ray diffractometry of CS unravelled aragonite and calcite crystalline forms at 2θ range from 10 to 70°C. The stability of CS biocarriers during 427 days of treatment was monitored using SEM-energy-dispersive X-ray spectroscopy to assess the physical and chemical changes. Thus, CS holds the potential to be a sustainable biocarrier.