**3.3. Selection of the treatment process**

the produced landfill leachate can be estimated by using empirical data based on flow measurements, or by using water mass balance between precipitation, evapotranspiration, surface runoff, and capacity of moisture storage. Waterproof covers and different covering liners contribute a lot to the reduction of landfill leachate quality, but they cannot completely reduce it. Both parameters (leachate quality and quantity) affect the attempts of uniform design of leachate treatment systems. The optimal treatment solution may change over time because of the changeable quality and quantity of the leachate, and the development of new technol‐

**phase**

pH (/) 6.7 4.7–7.7 6.3–8.8 7.1–8.8 COD (mg L-1) 480–18,000 1500–71,000 500–10,000 <1000 BOD5/COD (/) >0.5 0.5–1.0 0.5–1.0 <0.1

acids

Ammonium nitrogen (mg L-1) >100 <1000 <500 <500 Heavy metals concentration (/) Low Low - Medium Low Low Conductivity (μS cm-1) 2,500–3,300 1,600–17,100 2,900–7,700 1,400–4,500

Biodegradability Important Important Medium Low

be monitored even further, as defined by particular legislative requirements.

**Table 2.** Typical concentrations of landfill leachate concentrations as a function of landfill stabilization [30, 31, 34].

The landfill treatment and disposal represents one of the major landfill operational costs. Its management is not accomplished by the closure of the landfill, because its characteristics must

Traditionally, biological treatment is the most widely-used treatment strategy for wastewaters, mostly because of its low operational costs and complete as well as rapid destruction of pollution [2]. However, biological treatment is not always effective enough for the toxic and recalcitrant leachates (e.g., methane formation and maturation phase), in which case physicochemical treatment can take place. Usually, the leachate treatment involves a combination of various biological and chemical methods. Conventional landfill leachate treatment can be

**1.** Recycling and combined treatment with domestic sewage. In the past, it was common to treat landfill leachate mixed with municipal wastewater [2, 38]. This option is not so favorable nowadays, due to the identified presence of hazardous persistent compounds in the leachate, which are not removed in the conventional municipal treatment plant. On

**Methane formation**

5%–30% of volatile fatty acids + fulvic and humic acids

**Maturation phase**

fulvic and humic acids

**phase**

ogies and the legislation [37].

122 Wastewater Treatment Engineering

**3.2. Landfill leachate treatment**

classified into three major groups:

**Parameter Aerobic phase Acid formation**

Organic compounds (/) Various 80% of volatile fatty

It is well recognized that the selection of appropriate treatment method is strictly dependent upon the leachate characteristics and composition. For effective biological treatment, its biotreatability must be evaluated. In the case of a low-treatment efficiency of the biological plant, other treatment methods should be investigated.

Any determination of biological treatability of the wastewater must include data on toxicity and biodegradability. Toxicity could be assessed using one of the tests with mixed culture of microorganisms (activated sludge), which plays an essential role in a biological wastewater treatment plant. The test with measurement of inhibition of oxygen consumption and the test where growth inhibition of activated sludge is measured are widely applied [45]. If the impact of the leachate to anaerobic microorganisms is assessed, then the reduction of biogas produc‐ tion is often measured [46].


**Table 3.** Commonly used methods in Toxicity Identification Evaluation (TIE) procedures [12, 49, 57].

Biodegradability of the wastewaters is usually determined by using various non-standardized laboratory or pilot-scale long-term tests with activated sludge as a source of active microor‐ ganisms [5, 6]. At the same time, some of the standardized test methods, developed for biodegradability assessment of pure chemicals, could be applied [7]. Biodegradability assessment usually starts with the determination of ready biodegradation in common envi‐ ronmental conditions and it is upgraded with the assessment of biodegradation potential in the inherent biodegradability assessment test under optimal conditions [5]. All of the above mentioned tests are based on the measurement of summary parameters, such as COD or DOC removal, O2 consumption, etc. [47, 48]. A combination of different measurement techniques to follow biodegradation is recommended to distinguish between the biodegradation and the complete mineralization of the sample [5]. Inherent biodegradability assessment tests provide the data on adsorption potential of the sample to the activated sludge and allow us to estimate its impact on biological wastewater treatment plants [49]. Preliminary estimation should then be verified in an actual laboratory or a pilot-scale aerobic treatment plant, to determine the impact of the wastewater on activated sludge processes. Another possibility to assess bio‐ treatability is also the stabilization study, which represents a link between toxicity and biodegradability, to correlate the changes of toxicity versus the extent and rate of the biode‐ gradation. The initial and final biodegradability testing of the test mixtures allows us to confirm the measured degradation or the persistency of final residue [10]. If the wastewater consists of mainly degradable components, resulting in the toxicity elimination, the biological treat‐ ment is a good alternative, while in the case of poorly biodegradable wastewater with negligible decrease in toxicity, other treatment methods should be considered [8-10].

biotreatability must be evaluated. In the case of a low-treatment efficiency of the biological

Any determination of biological treatability of the wastewater must include data on toxicity and biodegradability. Toxicity could be assessed using one of the tests with mixed culture of microorganisms (activated sludge), which plays an essential role in a biological wastewater treatment plant. The test with measurement of inhibition of oxygen consumption and the test where growth inhibition of activated sludge is measured are widely applied [45]. If the impact of the leachate to anaerobic microorganisms is assessed, then the reduction of biogas produc‐

Filtration at different pHs Suspended solids ⋅ Toxicity is related to soluble or insoluble material

Biodegradability testing Biodegradable fraction ⋅ Decrease in toxicity due to the biological treatment

Oxidant reduction Oxidants ⋅ Toxicity is related to oxidants. Metal chelation (EDTA) Cationic metals (no Hg) ⋅ Toxicity is related to metals.

Adsorption Adsorbable organics ⋅ Toxicity is related to adsorbable organics

Oxidation Oxidizable organics ⋅ Toxicity is related to oxidizable organics

**Table 3.** Commonly used methods in Toxicity Identification Evaluation (TIE) procedures [12, 49, 57].

Ammonia Volatile organics **Data obtained**

compounds

not purged out

investigated condition

are removed during filtration

⋅ Metals form insoluble complexes at higher pHs and

⋅ Toxicity is related to inorganic compounds or ions

⋅ Toxicity is related to recalcitrant or biodegradable

⋅ Possible sorption of pollutants to microorganisms. ⋅ Biodegradability or mineralization potential

⋅ At low pH (pH = 3) small molecular weight organic acids will be effectively removed, while at higher pH (pH = 11) they may be dissociated or form salts and are

⋅ Ammonia could be stripped out at higher pH.

⋅ Results very dependent upon adsorbent used and

⋅ Results very dependent upon oxidant used and

⋅ Presence of compounds causing color

⋅ The extent of biodegradation of wastewater at

⋅ Toxicity is related to volatile organics

⋅ Toxicity is related to ammonia

experimental conditions

experimental conditions

plant, other treatment methods should be investigated.

**removed**

ions

tion is often measured [46].

124 Wastewater Treatment Engineering

Air stripping at different

pHs

**Method Group of pollutants**

Ion exchange Inorganic compounds

However, the discussion on the selection of the treatment method is based on the knowledge on wastewater quantity and quality, as well as the required effluent quality. The costs and the availability of the land are also very important; a detailed cost analysis should therefore always be made prior to the final process selection and design. The main characteristics, which should be considered are [12]: i) soluble organics responsible for oxygen consumption; ii) suspended solids; iii) priority substances that have hazardous environmental impact due to their persis‐ tency, toxicity, bioaccumulation potential and they could pose endocrine disruptive effect; iv) heavy metals; v) substances and particles causing color and turbidity; vi) nitrogen and phosphorous content; vii) refractory substances; viii) floating oils and grease; ix) volatile compounds (organics and H2S), etc. For wastewaters containing nontoxic and biodegradable organics, the process design criteria can be obtained from the data from laboratory or pilot studies, while more defined screening procedures are often needed for more complex and changeable wastewaters, such as landfill leachates. To set up appropriate treatment technol‐ ogy, the toxicity identification (TIE) approach is sometimes feasible, especially when a biological treatment is considered [53]. The TIE is a wastewater-specific study to isolate, identify and confirm the causative agents of toxicity. It is based on procedures, developed by the United States Environmental Protection Agency (USEPA). The Toxicity Reduction Evaluation (TRE) procedure is used as a tool to identify toxic components that may be removed or reduced in an effluent to reduce toxicity problems.


\*...Depends upon the combination of treatment processes and landfill leachate sample characteristics.

TP = Total phosphorous TN = Total nitrogen SS = Suspended solids COD = Chemical Oxygen Demand DOC = Dissolved Organic Carbon OM = Organic Matter

DOM = Dissolved Organic Matter

WWTP = wastewater treatment plant

**Table 4.** Some of the recently investigated combinations for treatment of heavily polluted landfill leachates [61-67].

The TIE methodology uses the responses of the test organisms to detect the presence of toxic substances in the sample before and after the samples are subjected to a series of physical and chemical treatments. This combination of physical/chemical manipulations of toxic samples, followed by the toxicity testing, allows one to isolate and identify the problematic group of compounds [50, 54, 55]. The most often used procedures are listed in Table 3. Results could be efficiently utilized to set up the appropriate treatment procedure for the particular wastewater, because it can be clearly estimated which treatment method is efficient in the removal of the particular group of pollutants and where the toxicity of the wastewater comes from [56]. Usually these simple, cost-effective methods could reduce the need for a complex and detailed characterization of the wastewater before setting up treatment procedure. However, they have to be designed and performed with caution (blank sample) to avoid any impact of the applied method (pH manipulation, addition of chemicals, etc.) to the final characteristics of the sample.

**Combined processes Experimental**

⋅ Sequencing batch reactor (SBR) ⋅ Coagulation with polyferric sulphate + Fenton system ⋅ Upflow biological aerated filters

126 Wastewater Treatment Engineering

⋅ Sequencing Batch Biofilter

Granular Reactor ⋅ With/no ozone ⋅ Solar photo-Fenton

⋅ FeCl3 coagulation ⋅ Magnetic ion exchange ⋅ Reverse osmosis ⋅ Nanofiltration

⋅ Aerated lagoon ⋅ Solar photo-Fenton ⋅ Conventional biological WWTP (with nitrification/

denitrification)

⋅ Agitation/stripping ⋅ FeSO4 coagulation

⋅ TiO2/UV photolysis ⋅ Bioreactors with various inoculums (raw leachate/soil extract/activated sludge)

⋅ Coagulation/flocculation

⋅ Solar photo-Fenton

TP = Total phosphorous TN = Total nitrogen SS = Suspended solids

OM = Organic Matter

COD = Chemical Oxygen Demand DOC = Dissolved Organic Carbon

DOM = Dissolved Organic Matter WWTP = wastewater treatment plant

⋅ Fenton

⋅ SBR, mixed with sewage: anoxicaerobic-anoxic conditions ⋅ Sand and carbon filtration

**scale**

Full Mature/

Laboratory Mature/

Pilot Young/

Laboratory Mature/

Laboratory Mature/

Laboratory Pilot

**Type of the leachate**

> COD NH3-N TP SS

COD DOC

DOC

Salts DOM

DOC TN

COD TOC BOD<sup>5</sup> SS NH3-N

COD BOD<sup>5</sup> NH3-N

Toxicity: -respirometry COD

Biodegradability


UV245 adsorbing OM

Biodegradability

Stabilized

Laboratory Medium aged Toxicity:

Stabilized

After lagooning

Stabilized

Stabilized

Mature/ Stabilized

**Table 4.** Some of the recently investigated combinations for treatment of heavily polluted landfill leachates [61-67].

\*...Depends upon the combination of treatment processes and landfill leachate sample characteristics.

**Measured parameters Removal**

**efficiency**

97.3% > 99% < 1 mg/L-1 < 10 mg/L-1

High High High 95.3% 95%

45–71%\* 84–94%\* > 93%\* > 99%\*

90% 56%–90%\* Increased

97.4% 92.3% 94.4% 97.5% 99.2%

87% 90% 43%–79%\*

Remains low 89% Increased

**Reference**

[62]

[63]

[64]

[61]

[66]

[65]

[67]

According to the results of the methods described in Table 3, suitable treatment methodology could be set up. If, for example, the wastewater contains a significant fraction of a nonbiodegradable organic fraction (determined by biodegradability testing) and it contains a lot of oxidizable organics (proved by oxidation experiment), one of the advanced oxidation processes would seem to be to be the most viable treatment option. On the other hand, if it contains organics that are able to mineralize almost completely in the biodegradability test, and it contains a lot of ammonia, one of the biological treatments involving nitrification/ denitrification would seem to be to be the best choice. It can be clearly concluded that, in the case of municipal landfill leachates, a technically and economically viable methodology for the effective treatment has yet to be designed. The available options are similar to those used in the treatment of industrial wastewaters, involving a combination of physical, chemical, and biological processes. Primarily due to their low costs, the biological processes, in their various forms according to redox regime (aerobic, anaerobic, anoxic), a type of biomass (a mixed or a pure bacterial culture, fungi, etc.) and a biomass fixation (dispersed, attached), remain the most widely implemented type of treatment processes [2, 58, 59].

However, a combination of biological and physico-chemical processes is usually employed for heavily polluted leachates. Many examples of efficient treatment combinations could be found in literature. As presented in [60], an aerobic biological treatment, a chemical coagulation, an advanced oxidation process (AOP), and some combined treatment strategies were compared. Laboratory experiments were done with 200 mL samples in a glass vessel. The efficiency of these treatment procedures was evaluated by analyzing the COD and color removals. In the extended aeration process, the maximum COD and color removals were 36% and 20%, respectively. They could be achieved during the optimum retention time of 7 days. Chemical coagulation with an optimum aluminum sulphate dose of 15,000 mg/L at pH = 7.0, gave the maximum COD and color removals of 34% and 66%, respectively. Using Fenton oxidation process at optimum pH = 5.0 and optimum dosages of reagents, with H2O2/Fe2+ molar ratio of 1:3, the highest removals of COD and color were 68% and 87%, respectively. The combined treatment, the extended aeration followed by Fenton oxidation, was found to be the most suitable.

Some additional, recently investigated and proposed treatment designs are presented in Table 4.

A large-scale multistage treatment system was also designed for the treatment of a mature raw landfill leachate [61]. The system consisted of an activated sludge biological oxidation (ASBO) reactor for aerobic and anoxic conditions (volume 3.3 m3 ) and a solar compound parabolic collector (CPC) for photo-Fenton process (total collector surface 39.52 m2 and illuminated volume 482 L). The raw leachate was characterized by a high concentration of humic substan‐ ces, representing 39% of the DOC content and high nitrogen content, mostly in the form of ammonium nitrogen. In the first biological oxidation step, a 95% removal of total nitrogen and a 39% mineralization in terms of DOC were achieved. The following photo-Fenton reaction led to the depletion of humic substances > 80% of low-molecular-weight carboxylate anions > 70% and other organic micropollutants, thus resulting in a total biodegradability increase of > 70%. The neutralized photo-bio-treated leachate was finally treated with the second stage biological oxidation, where the rest of biodegradable organic carbon and nitrogen content were eliminated. This way, a high efficiency of the overall treatment process was achieved.
