High-rate nitrogen removal in a two-stage subsurface vertical flow constructed wetland

A two-stage constructed wetland (CW) system consisting of two vertical flow (VF) beds with intermittent loading operated in series has been investigated. The first stage uses sand with a grain size of 2–3.2 mm for the main layer and has a drainage layer that is impounded; the second stage uses sand with a grain size of 0.06–4 mm and a conventional drainage layer. The two-stage system was operated with an organic load of 40 g COD mÀ2 dÀ1 for the whole system, that is with a specific surface area requirement of 2 m2 per person equivalent. The system was operated over a period of 2 years including two full winter seasons. No operational problems occurred, and the Austrian effluent standards for organic matter and ammonium nitrogen could be met during the whole investigation period. Additionally, average nitrogen removal efficiencies of 53% and average nitrogen elimination rates of 2.7 g NmÀ2 dÀ1 and 986 g NmÀ2 yrÀ1, respectively, could be achieved without recirculation, thus allowing construction with energy-free loading systems. It can be concluded that a better effluent quality can be reached with such two-stage systems as compared to a conventional single-stage VF CW system with intermittent loading, even if the two-stage system is loaded with the double organic load and therefore has only half of the specific surface area requirement.

A two-stage CW system (first stage – grain size 2–3.2 mm for the 50 cm main layer, impounded drainage layer; second stage – grain size 0.06– 4 mm for the 50 cm main layer, conventional drainage layer) was operated with an organic load of 40 g CODmÀ2 dÀ1 on the whole system (a design area requirement of 2 m2 per person), resulting in an effective load of 80 g CODmÀ2 dÀ1 on the first stage. The findings of the investigations can be summarized as follows: • Very low and stable effluent concentrations • • • • were measured for organic matter and ammonia nitrogen in the effluent of the two-stage CW system, as well as high removal rates for microbial parameters. The effluent concentrations of the two-stage + system for BOD5, COD and NH4 -N were observed to be lower than the ones of the single-stage VF bed designed and operated according to the Austrian design standard € ONORM B 2505 [11]. A whole year round operation of the two-stage system was maintained, without clogging tendencies even during the very cold winter of 2005/2006. It was shown that it is possible to operate the two-stage CW system without any problems even during long winters. The Austrian effluent standards [3] could be met during the whole investigation period. Additionally, by using the two-stage CW system it was possible to reach about 53% nitrogen elimination and a high nitrogen removal rate in average of 986 g NmÀ2 yrÀ1 (maximum more than 1’400 g NmÀ2 yrÀ1) without recirculation. The nitrogen removal rate is very high compared to reported literature values of 630 g NmÀ2 yrÀ1 for VF beds [8]. In conclusion it can be said that a higher effluent quality could be reached with the two-stage system as compared to the single-stage VF CW system designed with 4 m2 per person. The higher effluent quality could be reached although the two-stage CW system was operated with the double organic load resulting in half the specific surface area requirement (2 m2 per person).