Optimization of the treatment of an anaerobic pretreated landfill leachate by a coagulation–flocculation process using experimental design methodology

Abstract

Nowadays, landfill leachates are one of the most critical environmental issues faced by countries around the world. They result from the percolation of rain water through layers of solid waste, to which is added water from biochemical processes in waste’s cells and water that comes from wastes themselves. They are therefore highly charged with organic and mineral matter, which requires treatment before discharge to the receiving environment. In recent years, many techniques have been developed for leachate treatment. In this study, landfill leachate selected comes from Rabat city’s landfill. Landfill leachate is treated by anaerobic combined with coagulation flocculation (CF) process, using ferric chloride as coagulant and a cationic polymer as flocculant. The leachate is characterized by high COD and BOD₅. The BOD₅/COD ratio is equal to 0.58 ± 0.01, revealing that it is a young leachate with a very important biodegradability. The leachate was also loaded with phenolic compounds whose concentration reached 341.6 ± 21.3 mg/L. The optimization of the doses of coagulant and flocculant was performed using factorial design of experiments. Optimal dosages obtained were: 4.4 g/L of coagulant and 9.9 mL/L of flocculant. Removal efficiencies reached: 89 ± 6, 69 ± 4.8, 94 ± 1.3, 80 ± 8.7 and 89 ± 1.2% for phenol, turbidity, color, COD and absorbance at 254 nm (Abs 254 nm) respectively.

Introduction

In developing countries, landfilling is the most widely used technique for the treatment of solid waste. However, this technique causes the formation of another aspect of pollution: the landfill leachate. These liquids are loaded with organic and inorganic materials, biodegradable but also refractory to biodegradation. Leachate composition strongly depends on their age and the degree of stabilization of solid waste (Renou et al., 2008). Young leachates are characterized by an acidic pH, high organic load, and are generally biodegradable because of the values of BOD₅/COD ratio that are greater than 0,3, while old or stabilized leachate, are characterized by a low organic load and a basic pH.

Several methods have been used for the treatment of leachate, namely: biological treatment, physico-chemical and membrane processes (Renou et al., 2008, Souabi et al., 2011). The coupling between these processes brings together the advantages of each method, and thus leads to a more efficient treatment. Nevertheless, the choice of a technique in spite of another depends on the composition of the leachate and cost (Umar et al., 2010).

Among these techniques, CF has been widely used as pre or post processing for the treatment of leachate, because of its efficiency, simplicity to implement and reduced costs (Liu et al., 2012). The efficiency of this process depends on many parameters such as pH, nature and doses of coagulant and flocculant, the order of addition of reagents, stirring speed…. (Liu et al., 2012). Several researchers have studied the treatment of leachate by CF using different coagulants which, and showed that ferric chloride is more effective for the removal of organic pollution (Amokrane et al., 1997, Tatsi et al., 2003, Ntampou et al., 2006, Bakraouy et al., 2016).

The study of the CF process requires optimization of all parameters affecting it, in order to achieve high efficiency. Several studies have reported the advantages of experimental design with response surface methodology (RSM) (Martin et al., 2011; Liu et al., 2012). These designs, unlike conventional experimental designs, consider the influence of individual factors and their interactive influences. They thus lead to an optimization of many responses, allowing the simultaneous variation of several factors. RSM is used to establish a mathematical correlation between the factors and the studied responses.

The aim of this study is to treat the leachate from the landfill of Rabat city (pretreated anaerobically) by the CF process, using ferric chloride as coagulant and a cationic polymer as flocculant. The factors considered by the RSM are the doses of coagulant and flocculant, and selected responses are the removal efficiencies of phenol, turbidity, COD, color and absorbance at 254 nm.