More than half of the farmland in Denmark is artificially drained by subsurface tile drains or ditches. New ideas for remediatinng agricultural drainage losses of nutrients include small-scale constructed wetlands (CWs) and other innovative flow-through filter structures.
Small constructed wetlands are designed and optimized to enhance the filtering efficiency found in natural wetlands. The research within SupremeTech will consider two main types of constructed wetlands surface-flow constructed wetlands (SF-CW) and subsurface-flow constructed (SSF-CW) wetlands. Although CWs are widely used and studied for wastewater treatment, they are often described as “black boxes” because of complex interactions between hydrology, biogeochemistry and vegetation. Surface-flow constructed wetlands is known to retain and transform phosphorus and nitrogen in agricultural drainage water, however retention efficiencies are highly variable mainly due to episodic flow and highly variable nutrient concentrations in agricultural drainage water. In general, there is little information on CWs with unregulated event-driven inflows.
Subsurface-flow CWs (SSF-CW) have rarely been tested on agricultural drainage waters. Evidence from on-site domestic wastewater systems with high nutrient loads suggests that SSF-CW may be more efficient than surface flow CWs due to enhanced contact between the percolating water and the active porous media. To practically reduce comparatively low nutrient concentrations in drainage water to below environmental threshold values under transient flow, however, remains, a challenge. Also the system life-time and long-term efficiency depends on both the filter structural stability, e.g. avoid clogging or structural dissolution that reduces the hydraulic efficiency, as well as the sorption affinity and capacity of the filter substrate.
Recommended design of a surface-flow constructed wetland.
Specific demostratation site – a subsurface-flow CWs with matrices at Gjern, Denmark
Innovative flowthrough filter structures
Due to decades of surplus P additions to agricultural land, the source of today’s P losses is accumulated soil P, now posing a long-term risk.
Mitigating agricultural P losses is particularly challenging, as critical losses are only a small fraction of actual soil P contents. The SupremeTech research will consider drainage well filters containing both particle trapping filters and high-affinity P-retaining filter substrates as a new approach in mitigating the loss of P in agricultural drainage from high-risk areas.