Colmatação em sistemas alagados construídos de escoamento horizontal subsuperficial: principais fatores e métodos de identificação em unidades plantadas e não plantadas

• Ano de defesa: 2015
• Autor(a) principal: Mateus Pimentel de Matos
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Meio Ambiente e Recursos Hídricos; Saneamento
• Link de acesso: http://hdl.handle.net/1843/BUBD-A3JGV8

Clogging in constructed wetlands (CWs) has received attention in several studies, but the understanding of this phenomenon still requires substantial clarifications, especially in relation to its genesis. Moreover, there is no available method that allows a precise characterization of the bed conditions. Thus, this study aims to evaluate the main influencing factors of clogging on horizontal subsurface flow constructed wetlands (HSSF-CWs) units in operation for eight years, as well as to use methods to characterize the obstruction degree of the bed media porous. The units evaluated were two HSSF-CWs, with 25.0 x 3.0 meters, filled with blast furnace slag (d = 19.1 mm) until the height of 0.40 m, proividing porosity of 0.40 m3 m-3 in the bed. The systems are continuously fed with sewage previously treated, in Phases I and III by UASB reactors, and in Phase II, by UASB and open trickling filter (OTF), with a flow of 7.5 m3 d-1 horizontally flowing at 10 cm below the surface. To observe if the plants presence attenuates or accelerates clogging, one of the units was cultivated with cattail (Typha latifolia) (P-CW), while the other was maintained without plants (C-CW). In order to investigate the possible loss of efficiency due to clogging, the three phases were divided into sub-phases, based on the cycle periods between cuttings. Thus, the removal efficiencies of BOD, COD, solids, ammonia, Total Kjeldahl Nitrogen (TKN), nitrates and nitrites, sulfates and phosphates of each sub-phase were compared statistically by non-parametric tests. The methods evaluated to characterize the degree of clogging were: Falling Head Method (FHM) to obtain the hydraulic conductivity in saturated porous medium; direct method, with extraction of solids from the bed; tracer tests using NaCl; and probing with GPR, using antennas of different frequencies: 1.6 GHz, to obtain images of the cross sections in order to estimate the porosity, and 200 MHz to investigate potential points of infiltration/percolation. All the results derived were compared to those obtained by the visual field analysis, measuring the extent of the surface runoff and the water level on both units. As the two units are being operated for a long period, the pattern of their generated images was compared with another treatment unit recently installed, which is a Coarse Rock Filter, with an expected less solids concentration. This unplanted rock filter of 25.0 x 5.25 m, receives, since July 2014, the effluent from two maturation ponds in series, operating similarly to a not planted CW with the peculiarity to have three different particle sizes (gravel # 3, 2 and 1, in that order). Samples of clogging solids were collected at CW for determination of total (TS), volatile (VS) and fixed solids (FS), specific gravity (), neutralizing power (NP), and the chemical and mineralogical characterization of the material. These results were compared to those obtained on the substrate to enable an evaluation of the origin of the clogging solids. With the same objective, slag samples collected in the CWs had its particle size composition, porosity and specific surface area (SSA) analyzed. The results indicated that C-CW is currently more clogged, with a higher extension of surface flow, despite having better hydrodynamic condition in the first meters of the unit, where the wastewater height is less than that observed in P-CW. The expansion in the pore space P-CW, provided by the roots, caused better recovery of bed hydrodynamic conditions, both spatially and temporally, since five years ago, P-CW was in a worse condition. The test with saline tracer confirmed these observations, indicating a higher volumetric efficiency in the P-CW. The characterization of CWs from the VS/TS ratio and from images generated by the GPR also corroborate what was examined visually. The probe with the highest frequency antenna provided the identification of the most critical region in terms of solids concentration and total porosity in the beds, which were coincident with field observations, allowing the estimation of the porosity of each section with the use of equations developed for evaluation of the color standard. Differently, hydraulic conductivity, measured using the FHM, did not allow an adequate description of current conditions in terms of degree of clogging in the CWs. Added to this, the difficulty of conducting the test and measurement values of ks, especially as the state of these treatment units become more critical. Thus, changes in the methodology are necessary, to enable making measurements in situ or by assessing undisturbed samples taken from the beds. The analysis of made from clogging solids indicated that there is a great predominance of inorganic components, and with the wear of the slag it tends to increase. However, in spite of being in low concentration, the organic material is that most influences the pore clogging, due to its lower specific gravity and hydrophilic characteristics, with a matching of their larger proportion and the most critical points in terms of clogging in CW. Prospecting with the antenna for greater penetration depth allowed the identification of a possible point of failure of the clay lining in P-CW, with risk of infiltration/percolation of the liquid being treated.