Application of response surface methodology (RSM) to optimize coagulation–flocculation treatment of leachate using poly-aluminum chloride (PAC) and alum

Abstract

Coagulation–flocculation is a relatively simple physical–chemical technique in treatment of old and stabilized leachate which has been practiced using a variety of conventional coagulants. Polymeric forms of metal coagulants which are increasingly applied in water treatment are not well documented in leachate treatment. In this research, capability of poly-aluminum chloride (PAC) in the treatment of stabilized leachate from Pulau Burung Landfill Site (PBLS), Penang, Malaysia was studied. The removal efficiencies for chemical oxygen demand (COD), turbidity, color and total suspended solid (TSS) obtained using PAC were compared with those obtained using alum as a conventional coagulant. Central composite design (CCD) and response surface method (RSM) were applied to optimize the operating variables viz. coagulant dosage and pH. Quadratic models developed for the four responses (COD, turbidity, color and TSS) studied indicated the optimum conditions to be PAC dosage of 2 g/L at pH 7.5 and alum dosage of 9.5 g/L at pH 7. The experimental data and model predictions agreed well. COD, turbidity, color and TSS removal efficiencies of 43.1, 94.0, 90.7, and 92.2% for PAC, and 62.8, 88.4, 86.4, and 90.1% for alum were demonstrated.

Introduction

Sanitary landfill leachate, a highly polluted industrial wastewater, has been a cause for significant concern with landfilling being the most common technique in solid waste disposal [1]. The implementation of the most suitable technique for the treatment of leachate is directly governed by the characteristics of the leachate. Leachates from different landfills vary considerably in their chemical compositions due to factors such as the type of solid wastes deposited, hydrogeology of the landfill site, specific climate conditions, moisture routing through the landfill, landfill age as well as design and operation of the landfill [2], [3], [4], [5]. Biological treatment processes are effective for young or freshly produced leachate, but are ineffective for leachate from older landfills (>10 years old). In contrast, physical–chemical methods which are not favored for young leachate treatment, are advised for older leachate [6].

Coagulation–flocculation is a relatively simple physical–chemical technique commonly used for water and wastewater treatment. The removal mechanism of this process mainly consists of charge neutralization of negatively charged colloids by cationic hydrolysis products, followed by incorporation of impurities in an amorphous hydroxide precipitate through flocculation [7]. This technique has been employed successfully for the treatment of old landfill leachates [8]. The method is mainly proposed for pretreatment of fresh leachates, or for post-treatment of partially stabilized leachates with low biodegradability, i.e. low BOD₅/COD ratio [4].

Inorganic metal salts such as aluminum (alum) sulfate, ferrous sulfate, ferric chloride and ferric chloro-sulfate are generally used in coagulation–flocculation. Among these inorganic coagulants, iron salts are often more efficient than aluminum ones [8]. In recent years, there has been a rise in the use of polymerized forms of metal coagulants such as poly-aluminum chloride (PAC) for water treatment in Europe, Japan and North America due to their reduced cost and wider availability [9], [10]. Such products are claimed to be more advantageous over conventional coagulants because of their higher removal of particulate and/or organic matters as well as natural advantages of lower alkalinity consumption and lesser sludge production [10]. Amokrane et al. [8] reported that conventional coagulants generally remove 10–25% COD from young leachates and 50–65% COD from stabilized leachates or biologically pretreated leachates. However, application of polymerized forms of metal coagulants in leachate treatment is not well documented.

The appropriate implementation of this method depends upon how precisely coagulant dosage and pH are chosen. Therefore, trial and error has been conventionally practiced to optimize these variables. These studies were conducted using “changing one factor at a time” method, i.e. a single factor is varied while all other factors are kept unchanged for a particular set of experiments. Likewise, other variables would be individually optimized through the single-dimensional searches which are time consuming and incapable of reaching the true optimum as interaction among variables is not taken into consideration [11]. As a solution, the statistical method of response surface methodology (RSM) has been proposed to include the influences of individual factors as well as their interactive influences. RSM which is a technique for designing experiment helps researchers to build models, evaluate the effects of several factors and achieve the optimum conditions for desirable responses in addition to reducing the number of experiments [12]. Analysis of variance (ANOVA) provides the statistical results and diagnostic checking tests which enables researchers to evaluate adequacy of the models.

The present study investigates the comparative suitability of PAC and alum as coagulants for leachate treatment. Central composite design (CCD) and RSM was used to design the experiments, build models and determine the optimum conditions. Removal of COD, turbidity, color and TSS were monitored throughout the experiments. Thus, the statistical design was based on two factors (coagulant dosage and pH) and four responses (COD, turbidity, color and TSS).