Studies on the treatment of urban landfill leachates by Fenton-like processes with zero-valent iron

Abstract

A deposição de resíduos sólidos urbanos em aterro dá origem a vários impactes negativos, de entre os quais se destaca a produção de lixiviados, que têm associada uma carga poluente muito elevada. Esta dissertação surge no âmbito do tratamento de lixiviados de aterro, tendo como objectivo principal a proposta de um novo processo de tratamento que possa ser aplicado à escala industrial e que garanta o cumprimento dos limites de descarga do efluente para o colector municipal local.

Os métodos de tratamento seleccionados para o tratamento do lixiviado tiveram por base a utilização de ferro de valência zero (ZVI), o processo de Fenton-like (Fe3+ + H2O2) e o processo de Fenton-like com ZVI. Este último tratamento foi estudado, numa primeira fase com ferro finamente dividido e, seguidamente, com limalhas de ferro. Estes tratamentos foram aplicados a lixiviados recolhidos em três fases distintas do processo de tratamento: antes do tratamento (ERaw), após o tratamento biológico (EPost-Bio) e à saída da estação de tratamento (ETreat).

Os resultados experimentais a dois dos três lixiviados testados (ERaw e ETreat) revelam que as condições óptimas são: gama de pH entre 2 e 4, 13.40 g H2O2/L, 62.5 g Fe0/L (ERaw) e 25 g Fe0/L (ETreat), em 60 min. As limalhas de ferro foram testadas nas condições óptimas, obtendo uma remoção de CQO de 38 % em 5 min (ETreat). Contudo, para o EPost-Bio, após os 7 min ocorreu uma remoção de 49 % de CQO, o que permitiu cumprir o limite legal para descarga num colector municipal local. A biodegradabilidade (CBO5/CQO) foi melhorada de 0.01 para 0.11, no caso do ETreat, e de 0.04 para 0.27, no caso do EPost-Bio, em 30 min. O reuso das limalhas de ferro foi realizado em 10 testes cíclicos, de 1 h cada, onde houve uma remoção de CQO máxima de 65.3 % no segundo reuso. É importante salientar que as limalhas de ferro continuaram a apresentar eficiências elevadas, após este máximo.

ABSTRACT

Urban solid waste disposal in landfills originates several negative impacts, among them stands out leachates generation, which have associated a high pollutant load. This thesis appears in the scope of landfill leachate’s treatment and the principal objective is to propose a new treatment process that can be applied at industrial scale and to ensure compliance with the effluent discharge limits for the local municipal sewage.

The selected treatment methods for the treatment of the leachate were based on the use of zero-valent iron (ZVI), the Fenton-like process (Fe3+ + H2O2) and Fenton-like process with ZVI. This last treatment was studied in a first phase with iron powder and then with iron shavings. These treatments were applied to leachate collected in three distinct phases of the treatment process: before treatment (ERaw), after the biological treatment (EPost-Bio) and at the exit of the treatment plant (ETreat).

The experimental results on two of three tested leachates (ERaw and ETreat) show that the optimal conditions are: a pH range between 2 and 4, 13.40 g H2O2/L, 62.5 g Fe0/L (ERaw) and 25 g Fe0/L (ETreat), in 60 min. Iron shavings were tested in these optimal conditions, achieving a COD removal of 38 % in 5 min (ETreat). However, for the EPost-Bio after 7 min with 49 % removal of COD was attained meeting the legal limit for discharge to the local municipal sewage. The biodegradability (BOD5/COD) was improved from 0.01 to 0.11, in the case of ETreat, and 0.04 to 0.27, in the case of EPost-Bio in 30 min. The reuse of iron shavings was conducted in 10 test cycles of 1 h each, where there was a maximum COD removal of 65.3 % in the second reuse. It is noteworthy that iron shavings continued to show high efficiencies after this maximum.

Within these results, the main conclusion is that the treatment of leachate by Fenton-like process with iron shavings brings environmental and economic benefits due to the potential use of an industrial by-product, which can be acquired at low cost. It was shown that this system seems more appropriate to be integrated into the existing process after the biological reactor and prior to discharge the treated leachate to the local municipal sewage, replacing the physico-chemical used in the landfill leachate treatment plant studied.