Retention of nutrients in surface-flow constructed wetlands treating domestic wastewater

Abstract

The duration of filter operation had an impact on the removal efficiency of total P in the first filter. During the first year of filter operation Typha latifolia was planted and vegetation density was low. During that period the same or even greater P concentration was observed in effluent water when compared to influent. The following year the plants proliferated intensively and became dense. However, the efficiency of phosphorus removal increased. The average load of the first filter according to total P was 0.13 g m"2 d"1 and its removal reached 0.05 g m"2 d"1, i.e. estimated removal efficiency was 38.5%. Initial contamination of wastewater in the second filter was higher, therefore, the dependence of phosphorus removal on vegetation density was not observed. The average load of the second filter according to total P was 0.47±0.26 g m"2 d"1 its removal reached 0.13±0.07 g rrf2 d"1, i.e. estimated removal efficiency was 27.6%. As concerns low efficiency of N and P removal, the use of surface - flow constructed wetlands is advisable for additional wastewater treatment from nutrients. Constructed wetlands are considered to be one of the wastewater treatment methods. By construction, they may be designed as surface-flow wetlands. These are wastewater treatment systems in which the processes of pollutants removal occur while wastewater goes through the roots of plants in shallow ponds. The research was carried out in two wastewater treatment facilities of Lithuanian countryside. With regard to climatic conditions, Lithuania is considered as a country of cold climate. The amount of precipitation is 670 mm, average air temperature is 6.2 °C. The coldest month is January (-5.1 °C). Average temperatures are: in winter -4 °C, in spring 5.5 °C, in summer 16 °C, in autumn 7 °C. In winter period snow cover is 10-20 cm deep, frozen ground can reach 50-70 cm deep. Wastewater enters the first surface-flow filter after initial treatment in biological ponds, the second filter after treatment in septic tanks. For statistical data analysis 60 samples in different periods of 2003-2010 were taken. The load with total N in the first surface - flow filter reached 1.3±1.0, in the second one - 13.0±5.0 g rrf2 d"1. The results indicate that in the filter with lower load the average removal of total N reached 0.116 in winter and 0.20 g m"2 d"1 in summer time, while statistically significant difference in the higher load filter was not observed with removal of total N reaching 2.4-2.9 g rrf2 d'1. After treatment in filters, different forms and amounts of N is observed in water effluent. Increasing concentration of total N in water effluent determines higher amount of NH4-N. Filter in which total N concentration in effluent reached 24.6 mg I"1, contained 66.3% of ammonia nitrogen, while the second filter with the concentration of 62.0 mg I"1 contained 76.8%. This demonstrates that anaerobic conditions dominate in the filters.