Intensification of Constructed Wetlands for Nutrient Removal From Agricultural Runoff

ABSTRACT

Inspired by nature‐based solutions, constructed wetlands are increasingly acknowledged as effective wastewater treatment tools, with diverse structural designs suited to various contexts. In the present work, two typologies of these systems, a full‐scale farm wetland and three pilot scale systems, were tested in terms of nitrogen and phosphorus removal from agricultural runoff, as well as their potential to maintain treatment performance under intensified conditions and reduced space requirements. The farm wetland, a full‐scale surface flow wetland operating for more than 20 years on an agricultural farm in Italy, was monitored for different years. It showed high water retention rates (> 60% in most monitored years and up to 100% in some years) and high nutrient retention efficiencies, with TN retention generally exceeding 65% and TP retention exceeding 46%. However, the size of this system might be an obstacle to its wider application. With that goal, three pilot systems were tested, namely surface flow (SFCW) and horizontal sub‐surface flow (HFCW) constructed wetlands. The HFCWs were incorporated with two distinct compositions: one packed with gravel (HFCW‐G) and another containing 10% v/v biochar in addition to gravel (HFCW‐B). All CWs were monitored for 1 year, during which applied flow and nutrients concentration were studied as variables influencing their performance. The results showed that, overall, the SFCW performed the best, granting average TN reduction up to 78%, significantly higher in comparison to HFCW‐B and HFCW‐G (33% and 34%, respectively). In terms of TP removal, on average, the HFCW systems presented similarly high performances (43%) compared to SFCW system (33%). Seasonal fluctuations in nitrogen removal reflected a temperature and seasonal influence on the performance of the systems, particularly in the system containing biochar (HFCW‐B). This study therefore confirms that comparable or higher areal nutrient removal rates can be achieved under intensified operating conditions (e.g., increased hydraulic and nutrient loading rates), suggesting that farm wetland design might consider reduced footprint while maintaining treatment efficiency.