**Abstract**

Improper Solid Waste Management leads to the generation of landfill leachate at the landfills. To reduce the negative impacts of highly toxic and recalcitrant leachate on the environment, several techniques have been used. A lot of research is conducted to find suitable methods for the treatment of landfill leachate such as biological processes, chemical oxidation processes, coagulation, flocculation, chemical precipitation, and membrane procedures. The biological process is still being used widely for the treatment of leachate. The current system of leachate treatment consists of various unit processes which require larger area, energy and cost. In addition, the current aerobic treatment is not able to treat entirely the pollutants which require further treatment of the leachate. Anaerobic wastewater treatment has gained considerable attention among researchers and sanitary engineers primarily due to its economic advantages over conventional aerobic methods. The major advantages of anaerobic wastewater treatment in comparison to aerobic methods are: (a) the lack of aeration, which decreases costs and energy requirements; and (b) simple maintenance and control, which eliminates the need for skilled operators and manufacturers. Several anaerobic processes have been used for leachate treatment such as up-flow anaerobic sludge blanket (UASB) reactor, anaerobic filter, hybrid bed reactor, anaerobic sequencing batch reactor and Anaerobic baffled reactor. The following chapter provides an insight to the solid waste management at the landfills, generation of leachate and details of some of the highly efficient anaerobic treatment systems that are used for the overall treatment of landfill leachate.

**Keywords:** landfills, leachate, anaerobic reactors, biological, removal

#### **1. Introduction**

Currently, Municipal Solid Waste (MSW) generation is increasing day by day with the rapid growth of population, industrial developments to match the changing life standards of the people followed by uncontrolled urbanization are triggering the generation of municipal solid waste. It is estimated that currently about 2 billion tonnes per year of MSW is generated globally, which accounts to an average of about 0.74 kg/cap/day. It is predicted to reach a value of 3.4 billion tonnes in the year 2050.

The tragic situation even worsens when from the waste which is collected by municipalities (~67% of the total waste) about 70% is disposed in landfills and dumpsites, 19% gets recycled, about 11% goes for energy recovery [1]. Since most of the underdeveloped and developing countries are still far behind the efficient solid waste management system, therefore the study reveals that about 46% of the world population is unable to avail basic waste management facilities [2]. Researchers are suggesting the concept of circular economy where the preference of solid waste management is modified to the order of reduce, reuse, recycle, recovery (4R) and disposal of waste [3]. When the waste is disposed and carried forward to anaerobic digestion then biogas and the digestate is produced, this digestate is very rich in nutrients therefore it can be used as fertilizers creating the possibility of a fifth R that is rejuvenate.

The practice of landfilling is the organized disposal of MSW at a designated site called as landfill. But in terms of by-products landfill is extremely threatening to environment. Sanitary landfill is the most common MSW disposal method due to the simple disposal procedure, low cost, and landscape-restoring effect on holes from mineral workings. The primary objective of the landfill site design is to provide effective control measures to prevent negative effects on surface water, groundwater, soil and air [4]. Nevertheless, inappropriate management of the landfills and especially landfill leachate as it is declared as a hazardous substance leads to ecological and social problems, such as air, soil, surface water and groundwater pollution, flooding, noise from the garbage collection vehicles, and scavenging activities next to the landfills [5, 6]. Landfills can broadly be classified as open dumping landfills, semi-controlled landfills and sanitary landfills [7]. The details are clearly shown in **Figure 1** [8]. Open dump landfilling is mostly practiced in almost all the developing countries where the solid waste is dumped arbitrarily in open and low-lying areas causing serious environmental and health hazards. Semi controlled landfills are having basic facilities like sorting, segregation, shredding and compaction of solid waste followed by soil covering. While sanitary landfills are engineered and technologically advanced landfills. In addition to all the facilities of semi controlled landfills they have proper leachate collection and recirculation system, appropriate lining system and gas collection system [9].

When rainwater and the moisture is mixed and gets percolated with the waste it forms highly polluted, toxic, colored, and odorous liquid called as landfill leachate (LFL). LFL is highly concentrated liquid containing organic and inorganic chemicals, heavy metals, nitrogen, ammonia, humic acids, fulvic acids and xenobiotics [10, 11]. The characteristics and composition of landfill leachate is varying, depending upon its age (young, intermediate, and old) and this governs primarily the selection of the treatment technology (**Table 1**). Till date, most of the research on the treatment of landfill leachate is focused on using physical, chemical, and biological processes. Young landfill leachate contains significant amount of biodegradable organic fraction and therefore conventional biological techniques can be employed while intermediate and old landfill leachate contains high amount of recalcitrant compounds and low BOD/COD ratio thereby requiring combined or integrated technologies [12]. Leachate treatment include anaerobic biological treatment technologies i.e. anaerobic bioreactors; aerobic biological treatment methods i.e. aerobic ponds/lagoons, activated sludge; physico-chemical treatment including coagulation, flocculation, air stripping, chemical precipitation, filtration and adsorption [13].

The selection of the optimum treatment technology depends upon the characteristics of landfill leachate and its composition [14]. Landfill leachate treatment generally involves multistage or integrated technologies for better removal efficiency, as any single technology cannot obtain desired results for the effluent of LFL to be discharged into water bodies [15]. The previous studies suggest that biological

*Effectiveness of Anaerobic Technologies in the Treatment of Landfill Leachate DOI: http://dx.doi.org/10.5772/intechopen.94741*

#### **Figure 1.**

*Details of a sanitary landfill (a) processes (b) structural {adapted from [8]}.*


#### **Table 1.**

*The composition of leachate based on age [15, 19].*

treatment can be utilized to treat the biodegradable matter present in waste, ammonia is removed by ion exchange, coagulation/flocculation is used for colloids, adsorption is adopted for the metals and organics while advanced oxidation process for the organic compounds [16, 17]. Anaerobic digestion of municipal solid waste is very advantageous because we can obtain biogas which contributes to about 35% of the bioenergy obtained from different biomass sources [18].

### **2. Anaerobic treatment**

Anaerobic treatment technology is an attractive and demanding pathway because it serves the purposes of pollutant removal and energy recovery. Anaerobic treatment can be achieved efficiently for the complex industrial wastewater which may contain toxic substances [20]. Anaerobic treatment of landfill leachate can become a viable option as it has following advantages: (i) less space is required (ii) low energy requirement (no aeration is required)(iii) no or little sludge production (iv) Methane production and recovery thus helping to reduce the emission of green-house gas (CH4 potential is 25 times more than that of CO2, [21]. Anaerobic digestion of waste includes biological action of different types of microorganisms acting together to breakdown the biomass typically in the absence of oxygen [22]. Anaerobic digestion is a process carried out by microorganisms that can live in an oxygen-deprived environment. The disintegration of organic substance happens in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis are shown in detail in **Figure 2** [23, 24].

The first stage of anaerobic digestion is called as hydrolysis in which the anaerobic microorganisms convert the organic matter into basic organic substances like monomers, while, the proteins, carbohydrates and fats are converted to amino acids, monosaccharide and fatty acids, respectively.

Eq. (1) explains how a hydrolysis reaction converts organic waste into a simple sugar (glucose) [25].

$$\rm{C}\_6\rm{H}\_{10}\rm{O}\_4 + 2\rm{H}\_2\rm{O} \rightarrow \rm{C}\_6\rm{H}\_{12}\rm{O}\_6 + 2\rm{H}\_2\tag{1}$$

During the second stage of anaerobic digestion the acidogenic bacteria convert the products of the hydrolytic reaction into alcohols, short chain VA, ketones, hydrogen, and carbon dioxide. The products obtained in the acidogenesis stage are propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), acetic acid (CH3COOH), formic acid (HCOOH), lactic acid (C3H6O3), ethanol (C2H5OH) and methanol (CH3OH). From these products, the hydrogen, carbon dioxide and acetic acid will omit the acetogenesis stage and be utilized by the methanogenic bacteria in the methanogenesis stage (**Figure 2**). Eqs. (2)-(4) [25] represent three typical acidogenesis reactions where glucose is converted to ethanol, propionate and acetic acid, respectively.

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 \leftrightarrow 2\text{CH}\_3\text{CH}\_2\text{OH} + 2\text{CO}\_2 \tag{2}$$

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 + 2\text{H}\_2 \leftrightarrow 2\text{CH}\_3\text{CH}\_2\text{COOH} + 2\text{H}\_2\text{O} \tag{3}$$

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 \rightarrow \text{3CH}\_3\text{COOH} \tag{4}$$

Acetogenesis is the stage in which all the acidogenesis products (butyric acid propionic acid and alcohols) are converted into carbon dioxide, hydrogen and acetic acid with the help of acetogenic bacteria (**Figure 2**). Eq. (5) shows the conversion of propionate to acetate. Glucose and ethanol are also converted to acetate during the third stage of anaerobic fermentation (Eqs. (6) and (7)) [25].

$$\mathrm{CH\_3CH\_2COO^- + 3H\_2O} \leftrightarrow \mathrm{CH\_3COO^- + H^+ + HCO\_3^- + 3H\_2.} \tag{5}$$

*Effectiveness of Anaerobic Technologies in the Treatment of Landfill Leachate DOI: http://dx.doi.org/10.5772/intechopen.94741*

**Figure 2.** *Degradation steps of anaerobic digestion process [23, 24].*

$$\rm{C}\_6\rm{H}\_{12}\rm{O}\_6 + 2\rm{H}\_2\rm{O} \leftrightarrow 2\rm{CH}\_3\rm{COOH} + 2\rm{CO}\_2 + 4\rm{H}\_2\tag{6}$$

$$\text{CH}\_3\text{CH}\_2\text{OH} + 2\text{H}\_2\text{O} \leftrightarrow \text{CH}\_3\text{COO}^- + 2\text{H}\_2 + \text{H}^+\tag{7}$$

The last accomplishing stage of the anaerobic digestion is termed as methanogenesis. During methanogenesis the microbes convert the acetic acid and hydrogen to methane gas and carbon dioxide [25]. The anaerobic microorganisms that help to perform this conversion are called as methanogens. Waste is considered completely reduced in anaerobic treatment when methane gas and carbon dioxide are produced.

$$\text{CH}\_2 + 4\text{H}\_2 \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \tag{8}$$

$$2\text{C}\_2\text{H}\_5\text{OH} + \text{CO}\_2 \rightarrow \text{CH}\_4 + 2\text{CH}\_3\text{COOH} \tag{9}$$

$$\text{CH}\_3\text{COOH} \rightarrow \text{CH}\_4 + \text{CO}\_2 \tag{10}$$

#### **2.1 Factors Affecting the Anaerobic treatment of landfill leachate**

Anaerobic digestion of the pollutants present in landfill leachate depends on several factors such as temperature, pH, OLR and HRT, they are discussed below:


$$\text{OLR} = \frac{Q \times COD}{V}, \text{ upon simplifying } \text{OLR} = \frac{COD}{HRT} \text{ where } \text{OLR is organic}$$

loading rate (kg COD/m3 ⋅d), is flow rate (m3 /d), COD is chemical oxygen demand (kg COD/m3 ), and is reactor volume (m3 ). OLR does not directly influence the performance of an ABR but has an impact on the removal efficiencies. ABR treating a complex wastewater was operated at different OLRs ranging from 0.6 to 2 kg COD/m3 /day, for about 600 days without wasting sludge at temperatures of 20 to 38°C.The average COD removal decreased with a decrease in OLR. At max OLR i.e. at minimum HRT, the COD removal exceeded 88% [30]. It can be concluded that the OLR is an indicator of the nutritional condition of microorganisms.

Therefore, when low-concentration wastewater is being treated, lower HRT and higher OLR are preferred to ensure the availability of nutrients to the microorganisms. When high-concentration wastewater is being treated, lower OLR is suggested to enable complete biodegradation of the substrate and prevent sludge floating caused by higher yields of biogas [26].

iv.Hydraulic Retention Time (HRT): Hydraulic retention time is the volume of the aeration tank divided by the influent flow rate can be shown by the expression HRT [d] = [ ] [ ] Volume of reactor m3 Influent flow rate m3 / d <sup>=</sup> *<sup>V</sup> Q* where HRT is in days

*Effectiveness of Anaerobic Technologies in the Treatment of Landfill Leachate DOI: http://dx.doi.org/10.5772/intechopen.94741*

> or hours, V is volume of reactor in m3 and Q is influent discharge in m3 /d [31]. HRT is the macro-conceptual time of the stay of organic material in the reactor the inverse of which is called as dilution rate and if the dilution rate is greater than the growth rate of microorganisms the microbes will be washed out. Otherwise the accumulation of microbes will take place [24]. The decrease in efficiencies at very lower HRTs could be because the bacteria did not get enough time to consume the substrate. Hydraulic shock loads can also result in process souring and failure due to the accumulation of VFAs, as they could not be degraded effectively by the heterotrophic bacteria and methanogens. HRTs also can influence the dead space volume, at lower HRTs, hydraulic dead space increases, and at higher HRT, biological dead space increases [26].
