Avaliação do desenvolvimento e desempenho do capim-vetiver após submetido a diferentes lâminas de esgoto no período de aclimatação em sistemas alagados construídos

Abstract

The stimulation of root growth of plants cultivated in constructed wetlands systems of horizontal subsurface flow (CWs-HSSF) can result in a greater removal of pollutants, besides allowing the use of deeper units and with less area demand, reducing one of the main disadvantages of the use of these treatment units. Thus, the present work aimed to evaluate the performance of pollutant removal efficiency in three CWs-HSSF and the phytotechnical aspects of vetiver grass (vegetal species used and of extensive root system), after submitted to different sewage level heights in the reactors during the acclimatization phase. The pilot-scale units were made of 8 mm thick tempered glass with dimensions of 0.60 m length x 0.40 m height x 0.25 m width, being fed with a flow rate of 6 mL min-1 (HRT, hydraulic retention time of 2 d) with sanitary sewage from the chlorination tank of the wastewater (sewage) treatment plant of the Federal University of Lavras (UFLA WWTP), presenting a surface organic load application around 5.18 kg ha-1d-1 of BOD. The three CWs were submitted to two stages of acclimatization, where the effluent slurry was lowered at each biweekly cycle. In the first stage, all units had the same effluent slurry (35, 33 and 30 centimetres) so that the seedlings of vetiver grass could develop in equal conditions. Later, in the second stage, the units varied as to the effluent level, being kept at 30 cm in CW 1, while CW 2 and 3, respectively, had the level gradually reduced to 20 cm and 10 cm (after 15 days) with the objective of visualizing root development under stress. During the monitoring phase, the three CWs had the water depth adjusted to 30 cm, to provide a HRT of 2.0 d. Based on the results, although there was no significant difference in the reduction of the sewage sludge during the acclimatization period in the efficiency of pollutant removal between the units, CW 2 presented the best vegetative development and performance, possibly due to the root stimulation. On the other hand, the lowering to 10 cm and/or the insufficient time for adaptation to this condition did not provide greater productivity and growth in CW 3. Regarding root length, all plants grown in the three units presented root growth to the bottom of the reservoir. However, the more accentuated lowering of the sewage level resulted in an increase in root volume in the corresponding unit (CW 3), practice that also caused a more intense stress in the unit in question, leading to a higher root density. However, these procedure result in a lower growth of the aerial part and a lower extracting capacity in comparison with CW 2, that had an intermediate lowering (effluent depth kept at 20 cm). Finally, it was observed that the lowering of the sewage level had no influence on the nutrient content both in the aboveground and in the roots of the plants (in acclimation phase).