Nitrogen removal from industrial wastewater by hybrid constructed wetland systems

The efficiency in nitrogen removal from wastewater generated at CSBP Ltd fertiliser and chemical manufacturer by two hybrid constructed wetland mesocosms was compared. The mesocosms were set up in order to help clarify the reasons why the two year old constructed wetland located at the industrial facility, a 1.2 ha free water surface/vertical flow conjugated system, has performed under the 50% removal expectation (16% for NH4-N, 39% for NO3-N and 20% for total nitrogen). It is believed that the short retention time and insufficient oxygen transfer may be limiting nitrification. A free water surface/vertical flow wetland, simulating the design and operation mode of the industry’s wetland, and a vertical/horizontal flow, an alternative system, were tested. All systems were set in duplicates and planted with Schoenoplectus validus. On average, removals of NH4-N, NO3-N and total nitrogen (TN) for the free water surface/vertical flow mesocosm were 79%, 78% and 79% respectively, hydraulic retention time (HRT) was 10.4 days. The vertical/horizontal flow system performed better for ammonium removal (97%) but it failed to remove nitrate properly (-10%), TN removal was 66%, HRT = 8.5 days. The vertical flow stage of the vertical/horizontal flow mesocosm alone removed 95% of the NH4-N and almost doubled the concentration of nitrate in its outflow in 2.4 days, indicating the nitrification capacity of vertical flow systems. The experimental free water surface/vertical flow system which simulates the CSBP wetland outperformed the full scale one and this may be attributed to the lower hydraulic loading rate received by the mesocosm.