Nitrogen balance of duckweed covered sewage stabilization ponds

Nitrogen removal is nowadays one of the most important effluent treatment objectives because of the serious pollution problems it causes to the environment. The conventional processes for wastewater treatment, including chemical and biological removal of nutrients are costly in terms of investment and operation. Duckweed covered sewage stabilization ponds offer a low cost solution to both TSS, COD, pathogen and nutrient removal and includes the possibility of nutrient recovery and re-use. It is important to study how nitrogen is transformed and removed in duckweed ponds. There are not many results available in literature that could be useful to optimize pond design for nitrogen removal and recovery. How much nutrients should be removed may differ depending on the end use of the final effluent. In case of effluent irrigation, residual N could be useful, while in case of discharge levels should be as low as possible. The experiment was performed in a pilot plant, which consisted of two lines of seven duckweed ponds in series. One line received de-gritted domestic wastewater. The other line received effluent of a real scale Up-flow Anaerobic Sludge Blanket (UASB) Reactor. Both !irres were operated at a hydraulic retention time of 21 days, pond depth of 0.70 m, and harvesting interval of 4 days. Nitrogen balances were established for every pond and for every line. Ammonia volatilization is not an important removal mechanism in duckweed ponds (less than 1 %). Rem oval by sedimentation was also low at 2.1 % and 4.7% for the systems with and without anaerobic pre-treatment respectively. Instead, denitrification was found to be the most important removal mechanism (42% and 48 %), followed by duckweed biomass up-take (15.6% and 15.1 %. Average nitrogen biomass up-take rates were 199 mg N m -2 d -1 and 193 mg N m -2 d -1 for the system with and without pre-treatment, respectively. Nitrification rates were in the range of 112 - 1190 mg N m -2 d -1 and 58-1123 mg N m -2 d -1 for the system with and without anaerobic pretreatment respectively. Denitrification rates were in the range of 112 - 937 mg N m -2 d -1 and 59 - 1039 mg N m -2 d -1 for the system with and without pre-treatment respectively. The configuration of the system, in particular the down and up flow pattern seems to have an important positive effect on denitrification rates.