Remoção de fósforo em sistemas alagados construídos de escoamento horizontal subsuperficial com substrato de lodo de estação de tratamento de água

Abstract

The excess of phosphorus (P) in aquatic environments can unleash the eutrophication, so it is necessary the use of effluent treatment technologies of lower cost and satisfactory pollutant removal to prevent the pollution of watercourses. The constructed wetland system (CWS) is a low cost technology with satisfactory performance in the removal of pollutants; however,the maximization of P removal is associated with the use of substrates with a higher adsorption capacity for this element, such as water treatment plant sludge (WTPS), a sanitation residue with lower cost. The purpose of this work was to produce a substrate using WTPS, cement and additives and to evaluate its performance as P adsorbent in horizontal subsurface flow constructed wetlands systems (HSSF-CWs) in the treatment of sanitary effluent. The substrate was characterized by ATR-FTIR, SEM, EDS and compressive strength. Studies of pH and ionic strength effect, kinetics and adsorption isotherm in synthetic aqueous solution and phosphate adsorption in real sample of sanitary effluent were performed. The substrate was also evaluated in an experimental unit, in pilot scale, with three HSSF-CWs: 1) planted with Tulbaghia violacea and without substrate; 2) planted with T. violacea and with substrate; and 3) without vegetation and with substrate. The monitoring of the units was divided: stage 1, without addition of WTS sludge substrate; and stage 2, with addition of WTS sludge substrate in HSSF-CWs 2 and 3. Single inflow and effluent samplings were performed to each HSSF-CWs, to determine pH, electrical conductivity (EC), total solids (TS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), total phosphorus (TP) and phosphate (Pfosf), with the data submitted to Kruskal-Wallis test, at 5% significance level. At the end of monitoring, water content, productivity and nutrient extraction capacity for the aerial and root parts of T.violacea were evaluated. The results indicated that the WTS sludge substrate can be applied over a wide pH range and with little interference from the addition of chloride ions. The pseudo-second order model showed good fit to the experimental kinetics data. The Freundlich, Langmuir and Sips models described well the P adsorption process in the WTP sludge substrate, with maximum adsorption capacity of 7.34 mg g-1, obtained by the Langmuir fit. The complexity of the wastewater effluent reduced the phosphate removal efficiency compared to the synthetic solution. In the field, in stage 2, there was substrate defragmentation in the immediate contact with the sanitary effluent, with an increase in pH, EC, TS and COD at the outlet of HSSFs-CWS 2 and 3, but with a tendency to reestablish the conditions observed in stage 1. The substrate application reduced the concentrations of TKN, TP and Pfosf, with average removal efficiencies of 35.42% of TP and 60.52% of Pfosf, in HSSF-CWs 2, higher than those found in HSSF-CWs 1, of 13.73% of TP and 15.32% of Pfosf. The productivity and nutrient extraction capacity of T.violacea is in the same range as the values of species of consolidated use in HSSF-CWs. The maximum adsorption capacity of P by the WTS sludge substrate is within the range of values reported in studies using other wastes as adsorbents. The species T.violacea can be recommended as an ornamental plant for cultivation in HSSF-CWs in the treatment of sanitary effluent.