Denitrifying bioreactor surface subsidence varies with age and cover

Surface subsidence at denitrifying woodchip bioreactors treating subsurface drainage has been anecdotally noted but has not been consistently documented and is thus poorly understood. Subsidence is of concern due to safety and potential exacerbation of ponding within the bioreactor but could also indicate flow restrictions within the woodchip bed. This study used 3D light detection and ranging (LiDAR) surveying on handheld devices (iPhone 12 Pro, iPad Pro) to provide minimum estimates of surface subsidence at 17 full-size bioreactors across a range of ages (0.1 to 14 years). Bioreactors with woodchips extending to the surface subsided faster than bioreactors with soil covers with median subsidence rates over the entire surface of 7.3 and 1.0 cm/y, respectively. Maximum subsidence averaged 40 ± 14 cm across all sites and tended to occur near the inflow manifold where subsidence could disproportionately impact hydraulic performance. Although these findings are limited to the bioreactors in the present evaluation and other sites may show different trends, it may be that subsidence is not reducible to aerobicity alone. Subsidence is not necessarily, on its own, the best individual indicator of the end of design life. In practice, checking for a consistent reduction in the amount of outflow over time is the best way to assess the need for a bioreactor woodchip recharge. Nevertheless, in this study, subsidence at full-size bioreactors was successfully approximated using a hand-held LiDAR device, and use of this method at additional sites is suggested, especially following bioreactor construction.