**2. Methodology**

### *2.1. Collection of the Samples and Landfill Characteristics*

Samples of leachate were sampled from the Ampar Tenang Closed Landfill Site (ATCL). ATCL is situated nearly 40 km southwest of Kuala Lumpur, Malaysia at a latitude of 02◦48.9250 north (N) and a longitude of 101◦4.9330 east (E) [23]. ATCL is mostly surrounded by oil palm plantations. Labu River flows along the landfill 300 m away. The ATCL area is characterized as tropical. The average temperature is 27.2 ◦C, and the average rainfall is 2287 mm/year [24]. The landfill site is located on the eastern sector of the confined alluvial aquifer of the Langat Basin, composed primarily of silt (50%–70%), clay (<25%), and sand (<25%) [25–27]. This site is more clayey close to the surface of the ground, but changes to sandy in lower layers [25,26]. ATCL operated as of 1994 for a total of 15 years. About 100 tons of waste are dumped into this site every day during the operation of the landfill. This results in the on-site disposal of a total of half a million tons of solid waste. The site was completely closed in 2010. Before it was closed, it was converted to sanitary classification (Level 1) from a disposal site (Level 0) [8,27,28]. In 2018, the samples were manually collected and placed in 1000-mL containers manufactured from polyethylene. The samples were collected at 4 ◦C and immediately transformed to prevent significant biological degradation and chemical reactions. The samples were transported in a preservation refrigerator from the site to the laboratory before being stored in the laboratory refrigerator until the next day where the experiment was prepared; no acids were used for preservation.

### *2.2. Leachate and POME Characterizations*

The physico-chemical characterization of the leachate is presented in Table 1.


**Table 1.** Physico-chemical characteristics and heavy metals of leachate.

\* [29], \*\* [30]. Abbreviation. EC: electrical conductivity; TDS: total dissolved solids; TSS: total suspended solids; COD: chemical oxygen demand; BOD5: biochemical oxygen demand; NH3-N: ammoniacal nitrogen; DO: dissolved oxygen; USEPA: U.S. Environmental Protection Agency; DOE: Department of Environment.

The physico-chemical characterization of the POME is presented in Table 2.

**Table 2.** Physico-chemical characteristics and heavy metals of palm oil mill e ffluent (POME).



**Table 2.** *Cont*.

DOE: Department of Environment, Malaysia; NA—not available [30].

### *2.3. Experimental Procedure*

\*

A bubble column bioreactor was used for the aeration process. It is characterized by its simple construction, higher efficiency in removal, and efficient control of the liquid residence time [31]. The aeration process was conducted at room temperature (25 ◦C). In the aeration process, the air pump is connected to four bottles of one liter each by a tube of 1 mm in diameter; these bottles contain different ratios of leachate/POME, and the process continues for 21 days.

The experiment was conducted in two steps; the first step was a preliminary experiment performed utilizing one factor at a time to determine the area of concern for each influential variable of the leachate/POME ratio and the aeration time to determine the optimal levels. The selected levels for the leachate/POME ratio and the aeration time were utilized to conduct the second step utilizing response surface methodology (RSM). RSM consists of a group of experimental methods devoted to estimating the relationship between a group of experimental variables (factors) and the (targeted) measured responses. To build a more practical model, the process variables under investigation need to be understood. Central composite face-centered (CCF), a type of central composite design (CCD), was used for two independent variables to estimate the effect value of POME dosages and aeration time on four response variables: COD, TSS, color, and NH3-N.

### *2.4. E*ff*ect of the Leachate*/*POME Ratio*

For the first step, POME was used to improve the biodegradation of leachate. In 1000 mL of leachate samples, different leachate/POME ratios (1:0, 0.9:0.1, 0.7:0.3, and 0.5:0.50) were used. The initial pH for the leachate sample (8.4) was left unadjusted. The liquid was aerated using an aeration bump (HAILEA) model V-20 with output 20 L/min and pressure >0.02 MPa for 24 days. The treatment efficiency was evaluated based on COD, TSS, color, and NH3-N removal efficiency.

The efficiency for COD removal was estimated using Equation (2).

$$\text{COD Removal} \left( \% \right) = \left[ (\text{C}\_{\text{r}} - \text{C}\_{\text{k}}) / \text{C}\_{\text{i}} \right] \times 100 \,, \tag{2}$$

where Cr is the initial COD concentration, and Ck is the final COD concentration.

### *2.5. Optimization of Treatment E*ffi*ciencies of Targeted Parameters*

Using Design-Expert software (version 6.0.7), a central composite design (CCD) for the leachate/POME ratio was developed to examine whether COD, TSS, color, and NH3-N affected the leachate/POME ratio and aeration time. Depending on the preliminary experiments stated in Section 2.3, the amounts and rates of each factor were chosen. Thirteen experiments were conducted to include all possible combinations of the leachate/POME ratio and aeration time.

Data from di fferent CCD experiments were utilized to appropriate a polynomial model and a second-order model (Equation (3)).

$$\mathcal{Y} = \beta \mathbf{0} + \sum\_{j=1}^{k} \beta\_j \mathbf{X}\_j + \sum\_{j=1}^{k} \beta\_{j\bar{j}} \mathbf{x}^2 \boldsymbol{\beta} + \sum\_{i} \sum\_{$$

where *Y* is the response, *Xi* and *Xj* are the variables, β is the regression coe fficient, *k* is the number of variables tested and optimized in this experiment, and e is the random error. A *p*-value less than 0.05 was reported as significant.
