Nitrate removal from a drinking water supply with large free-surface constructed wetlands prior to groundwater recharge

Abstract

A constructed wetland was used to remove nitrate from the municipal drinking water supply of two million people in Orange County, southern California, USA. Nitrate removal is needed to protect human health and to reduce eutrophication and algal clogging in deep groundwater recharge ponds. The source water was the effluent-dominated Santa Ana River and up to 1.5 m 3 s −1 (33×10 6 gallon day −1) was treated prior to groundwater recharge. The influent was mostly highly treated, nitrified municipal wastewater containing 3.1–10.9 mg l −1 NO 3–N and was applied to 170 ha (425 acres) of shallow, open water and vegetated marsh in the Prado Basin. Nitrate removal rates as high as 1,000 mg NO 3–N m −2 d −1 were observed in some portions of the marsh and exiting nitrate concentrations sometimes fell to as low as 0.1 mg l −1 NO 3–N with residence times of less than 10 days. High nitrate removal rates were observed at loading rates higher than comparable systems; hydraulic surface areas ranged from 0.04 to 0.55 ha per m 3 s −1 (2–33 acres per 10 6 gal d −1) and hydraulic detention times ranged from 0.3 to 9.6 days. Average nitrate removal was 522 mg NO 3–N m −2 d −1 (range 4–1071), average efficiency for the entire wetland was 79% (range 14–100). Nitrate loading rates averaged 1458 mg NO 3–N m −2 d −1. Calculated nitrate removal by plant growth or groundwater seepage was too small to account for the measured losses, so we conclude that bacterial denitrification was the primary nitrate loss mechanism. Denitrification was probably regulated by available carbon since nitrate removal efficiencies dropped substantially when floods removed most of the vegetation and detritus. This study demonstrates the effectiveness and reliability of constructed wetlands for large-scale treatment of low BOD, high nitrate, low ammonia, effluent-dominated streams in urbanized, semi-arid regions like southern and central California and similar regions throughout the world. Unlike many wetlands, the Prado system was able to remove large amounts of nitrate in a short period, reducing the amount of land needed and making large-scale nitrate removal from rivers a feasible proposition for nitrate-polluted rivers worldwide.