Prospects for Using Oyster Shells (Crassostrea gigas) and Plastic Waste (Polyethylene) in Lab-Scale Vertical...

Replacing conventional filter media with recycled filters can enhance treatment efficiency and reduce the operational cost of vertical subsurface flow constructed wetlands (VSF CWs). This study evaluated the feasibility of using oyster shells (Crassostrea gigas) and polyethylene plastic waste as substitutes for traditional gravel media in VSF CWs treating swine wastewater. Seven media configurations were designed: CP1 (sand, small gravel [1 × 2 cm], and large gravel [3 × 5 cm]); CP2 and CP3 replaced small gravel with oyster shells and plastic waste, respectively; CP4, CP5, and CP6 used mixtures of oyster shells and plastics in varying volume ratios (1:3, 1:1, and 3:1, respectively); CP7 was a control system with no media. Four-month experiments were conducted at a laboratory scale, monitoring different hydraulic retention times (HRTs) (1, 2, 3, and 4 days) under two conditions: planted with Cyperus alternifolius and unplanted. Importantly, replacing conventional filter media with oyster shells and plastic waste did not adversely affect the growth of C. alternifolius in any of the experimental systems. The results showed that planted systems achieved 0.8%-28.7% higher removal efficiencies than unplanted ones (p < 0.05), with an optimal HRT of 2-3 days to meet QCVN 62-MT:2016/BTNMT Column B. CP1 consistently demonstrated the highest treatment efficiency. Meanwhile, CP2 and CP6 exhibited slightly lower removal efficiencies than CP1 but still achieved outstanding TP removal rates of over 70%. CP3 showed the lowest treatment performance. However, its effectiveness could be improved by blending with oyster shells at appropriate ratios. Among all experiments using recycled filter media, CP6 exhibited the most promising performance, with removal efficiencies for TSS, COD, TN, NH₄ ⁺, and TP ranging from 62.6% to 87.6%, 36.1% to 77.2%, 30.8% to 74.0%, 33.6% to 85.3%, and 29.6% to 69.0%, respectively. Substituting small gravel with recycled plastic waste and oyster shells reduces material costs by three to four times, mitigates environmental pollution, and promotes solid waste recycling toward a circular economy.