Remediação de metais pesados e HPAs em lodo de esgoto por meio do cultivo de Pennisetum purpureum, aeração e compostagem

Abstract

Sewage sludge (SS) has been used in agriculture as a fertilizer due to its chemical characteristics and the presence of organic matter, allowing savings in the use of mineral fertilizers and the proper disposal of waste in the environment. However, the presence of toxic contaminants requires the adoption of remediation practices to make it fit. The studies aimed to evaluate practices for the remediation of heavy metals and polycyclic aromatic hydrocarbons (PAHs), through the direct cultivation of Pennisetum purpureum (PP), forced aeration and composting. Studies I and II were carried out in a randomized block design with four replications, in a 2 x 5 factorial scheme, corresponding to the presence or not of cultivation with PP in SS in the planting density of 50 plants per m2 combined with five periods of intermittent aeration for 30 minutes: 0; 14; 28; 42 and 60 days, with an air flow of 1,059 dm3 h-1 kg-1 of volatile solids. In the first, heavy metals and sludge decomposition were characterized for use as organic fertilizer and in the second, the levels of PAHs were evaluated. In the third study, in the same statistical design, the composting of the residues from the previous studies was carried out with a duration of 72 days. In study I, there was an increase in the total levels of N, Ca, S, Na, EC and soluble mineral residues (SMR) with the cultivation of PP, indicating a higher rate of decomposition of the SS; on the other hand, cultivation decreased the total levels of Fe and Pb, providing a reduction in the risk of toxicity of these elements. The injection of air into the SS did not influence the decomposition of organic matter. In study II, the following HPAs were found in SS: naphthalene (NAF), phenanthrene (FEN), fluoranthene (FLUO), pyrene (PIR), benzo(a)anthracene (BaA), chrysene (CRI) and indene(1,2,3-cd)pyrene (IND). In the absence of forced aeration in the SS, the cultivation of PP provided a reduction in the levels of PAHs, in comparison to the non-cultivated SS. In the presence of aeration, uncultivated SS was more effective in reducing of PAHs levels with the aeration time varying from 15 to 26 days. In study III, the composting process of organic material from the cultivation and mixing of PP biomass in SS with an aeration time of 42 days reduced the levels of NAF, FEN, PIR, BaA, IND and the sum of the PAHs, in comparison to composting of material derived only from SS. The cultivation of PP without aeration and the aeration of the sludge without the cultivation showed satisfactory results in improving the chemical characteristics of the SS, and indicate the use of these techniques separately to adapt it as organic fertilizer, as they promote the mineralization of macronutrients and the reduction of contaminants. With the cultivation of grass the advantage of using plant residues for composting with the SS itself to achieve a better quality of the material.