32 Landfill bioreactor

Dr. Yogalakshmi K. N

epgp books

 

Objectives:

 

1.      To understand the concept of landfill bioreactor

 

2.      To understand the components of landfill bioreactor

 

3.      To know the different types of landfill bioreactor

 

4.      To know the various factors influencing the waste degradation in landfill bioreactor

 

5.      To realize the advantages and disadvantages of landfill bioreactor

 

6.      To study about the monitoring parameters of landfill bioreactor

 

 

1.0 Introduction

 

Disposal is the final step in the management of solid waste. All solid waste ultimately ends in disposal site either directly or after some processing and treatment. Most of the developing and under developed countries, dispose or dump the solid waste in an open land without any protection or environmental control. Such dumping is not safe and is called as open dumps or open dumping. Solid waste is also disposed off in water bodies e. g. Sea, ocean, lake, river etc. Sometimes they are also burned in open areas, the phenomenon is termed as open burning. Developed countries are practicing a system of safe or controlled dumping called the landfilling. Landfill is an engineered structure where the waste is safely disposed and contained to avoid environmental and health effects. They are considered as the largest route of disposal. Landfills are designed and operated according to acceptable standards. They are not homogeneous and are usually made up of cells in which a discrete volume of waste is kept isolated from adjacent waste cells by a suitable barrier. A liner system at the bottom and sides and cover at the top acts like a geological barrier. The requirement and type of the barrier system depends on the nature of waste being loaded in the landfill. Both natural and synthetic materials are used as barriers. The landfills are composed of three main components. They are

 

•      A liner system – used at the bottom to protect the soil and ground water.

 

•      Waste emplacement cells – pattern by which waste is loaded into the landfill

 

•      Cover or capping – It is the top most protection layer that prevents the entry of water or other animals into the waste pile

 

Figure 1 Landfill structure

 

Overall, the landfills are designed for the purpose of safe disposal and environmental protection. Though the system is provided with good leachate and gas collection system, still they fail in environmental protection due to certain problems. Moreover, the waste within the landfill takes a long period of time (i.e) many decades for degradation. The gas generation is also very slow. Thus, landfill bioreactors (LFBR) were developed with the purpose of accelerating the rate of waste degradation with simultaneous environmental protection.

 

2.0 Landfill bioreactor

 

A landfill bioreactor is an engineered landfill developed to speed up the process of waste degradation and promote environmental protection through proper gas and leachate management systems. The system promotes enhanced microbiological activity through moisture maintenance and leachate recirculation. The waste stabilization occurs within 5 to 10 years of implementation. Moisture is considered as single most component to fasten up the rate of degradation. A minimum of 35 – 40% moisture is required for increased rate of waste degradation. At certain times, leachate is not sufficient enough to maintain the required moisture. In those cases, other liquids such as water, storm water runoff and wastewater treatment sludges are used. The efficiency of the landfill bioreactor is also increased through other factors such as shredding of the incoming waste, nutrient addition, pH adjustment, waste pre-disposal and post-disposal conditioning and temperature control.

 

3.0 Types of landfill bioreactor: Landfill bioreactors are of four types. They are classified based on their working mechanism as aerobic, anaerobic, hybrid and facultative landfill bioreactors.

 

3.1 Aerobic landfill bioreactor: Waste in this landfill undergoes aerobic degradation. Air is supplied into the system through vertical and horizontal tubes or wells to facilitate aerobic degradation. The leachate generated during the decomposition of waste is collected and recirculated with other liquids. This helps in maintaining the moisture level, improves the degradation efficiency, maintains the microbial inoculum and stabilizes the waste. Some amount of air also enters through leachate circulation. The process of waste degradation is very fast in aerobic bioreactor landfills.

 

Source: https://archive.epa.gov/epawaste/nonhaz/municipal/web/html/aerobic.html

Figure 2 Aerobic bioreactor landfill

 

3.2 Anaerobic landfill bioreactor: The waste in the anaerobic landfill bioreactors undergoes anaerobic decomposition and results in the production of gases such as methane (CH4) and carbon dioxide (CO2), commonly referred as the landfill gases (LFG). In anaerobic landfill bioreactor, the gases are recovered and converted into energy. Greenhouse emissions are also minimized during the operation of anaerobic LFBR. The leachate generated from the anaerobic LFBR is collected and recirculated to maintain the moisture level within the system. Around 35-40% moisture I required for efficient degradation of waste. The leachate generated during the process is not sufficient to maintain the moisture within the LFBR, hence other sources such as sewage sludge, non-hazardous liquids, storm water is used for maintaining the moisture. Appropriate moisture increases the efficiency of waste degradation. It also densifies the waste. The gas generation is twice that of normal landfill. The rate of gas production is very fast. The anaerobic landfill bioreactors can also handle peak waste flow due to accelerated gas production in short period of time.

 

3.3 Hybrid landfill bioreactor: Hybrid landfill bioreactors are designed to combine the advantages of the aerobic and anaerobic landfill bioreactors. The operational simplicity of the anaerobic process and efficient degradation of aerobic system is combined in hybrid LFBRs. It is also known as Aerobic-Anaerobic bioreactor landfills. The upper portion of the bioreactor is operated under aerobic condition where air is injected into the waste. Aerobic conditions facilitate faster decomposition. The bottom layer of the bioreactor is maintained in the absence of oxygen. The purpose is to attain faster degradation with simultaneous recovery of energy at the lower portion of the bioreactor. Faster degradation reduces the production of organic acids, thereby favoring faster onset of methanogenic phase.

 

3.4 Facultative landfill bioreactors: Facultative bioreactors are developed to control high ammonia within the landfill bioreactors. High ammonia concentration develops due to the leachate recirculation within the landfill bioreactor. The leachate containing ammonia is treated by biological nitrification process. The nitrified effluent is then introduced into the anaerobic LFBR. Under anaerobic condition, the facultative anaerobes, use nitrates as oxygen source and reduce them to molecular nitrogen by a process of denitrification. Molecular nitrogen is safe and do not cause any harm to the environment. Like other landfill bioreactors, the moisture of facultative bioreactor should also be maintained to achieve increased degradation.

Figure 3 Hybrid landfill bioreactor

Source: https://www.epa.gov/sites/production/files/2016-03/facultative_new.jpg

 

Figure 4 Facultative landfill bioreactor

 

4.0 Components of landfill bioreactor

 

The landfill bioreactor consists of seven important components. They include

  1. Liner system at the base and sides of the landfill: Liners are natural or synthetic polymers used to protect the soil and groundwater underneath. The liners should essentially be strong, impermeable in nature, flexible, high tensile strength, tolerant to temperature fluctuation and UV resistant. The liners should be easy to install, economical and resistance to scratch, and chemical degradation by leachate. The common liners used in leachate management are geomembranes, geosynthetic clay liners, geotextiles, geogrids, geonets, and geocomposites.
  2. Leachate collection system: The leachate drainage system is used to collect and transport the leachate which is collected inside the liner. Leachate collection system is designed in such a way to collect maximum amount of leachate. The pipes of the drainage system are located at the bottom of the cell. The pipeline network can be flexible or rigid but the joints connecting the pipes should be flexible to withhold enormous amount of weight and pressure. The size of the pipes is larger than the conventional landfills. The pipes get clogged due to deposition of suspended materials. The collection pipe network drains, collects, and transports leachate throughout the drainage layer to a collection sump where it is separated for the treatment or disposal. After recirculation, the excess water must me removed from the bioreactor to maintain the 30 cm or more hydraulic head. If not the leachate leaks out of the system. The leachate distribution system is of five types (Table 1). Factors such as environmental impacts, climatic factors, worker exposure, evaporation loss, reliability, uniformity and aesthetics is considered in selection of distribution system.

 

Table 1 Leachate distribution System

 

 

3. Gas collection system: An ideal collection system consists of a sequence of gas collection wells which are positioned throughout the landfill. The gas collection wells are designed based on the volume and density of waste, depth, and area of landfill. Collection wells in the landfill offer pathways for the passage of gas. The gas collection system is very important in bioreactor landfill as the gas generation occurs from the early phase. Moreover, when compared to conventional landfill, the landfill bioreactor generates high amount of gas.

 

4. Daily and final cover at the top of the landfill: Cover protects the entry of water into the waste during precipitation. The cover layer is composed of synthetic and natural materials. Daily cover is the 150 mm of soil placed over each layer of waste. The cover of LFBR prevents the leachate from escaping through the sides of the landfill; provides safety against diseases, vectors, fires, odors, blowing litters, and scavenging and contaminate the environment. Agricultural waste, sludge, worn out geotextiles and composts are alternatives that can be used as covering material. They also degrade with the waste material.

 

5. Surface water drainage system: These systems are established to avoid water accumulation on the surface of the landfill. It serves as a channel to collect the surface run off caused during precipitation.

 

6.  Environmental monitoring system: This component helps in assessing leachate or gas leakage if any so that environmental contaminations can be avoided.

 

7. Closure and post-closure monitoring

 

5.0 Waste degradation in landfill bioreactor

 

The waste inside the landfill bioreactor undergoes physical, chemical and biological changes. Due to these changes the waste gets compacted and its volume gets reduced. The various physiochemical process that takes place inside the landfill bioreactor is chemical precipitation, adsorption, and many more. The biological process of waste degradation inside the landfill occurs in phases which is elaborated below. Three broad biological groups namely carbohydrates, proteins and lipids are present in organic portion of the waste. Waste degradation occurs in five stages. Figure depicts the stages of waste degradation in landfill bioreactor.

 

Stage I – Hydrolysis/ aerobic degradation

 

This is the first stage of waste degradation and it occurs in the presence of oxygen conditions. The oxygen in the void spaces and those locked in the waste are used in this stage for degradation. It occurs immediately after waste is placed in the landfill and lasts for few days or weeks depending on the availability of oxygen for the process. The aerobic micro-organisms metabolize the available oxygen and degrade the organic fraction of the waste into hydrocarbons, carbon dioxide, water and heat. The heat increases the temperature of the waste to up to 70-90 °C. The water and carbon dioxide react to form carbonic acid thereby increasing the acidity of the leachate.

 

Stage II – Hydrolysis and Fermentation

 

This stage begins as soon as the oxygen in the waste pile gets depleted. During this stage the facultative anaerobes establishes themselves in the waste pile due to reduced oxygen conditions. Carbohydrates, proteins and lipids are hydrolysed to sugars which are then further decomposed to carbon-dioxide, hydrogen, ammonia and organic acids. Likewise, proteins are decomposed via deaminisation to form ammonia, carboxylic acids and carbon dioxide. Organic acids namely acetic acid, but also propionic, butyric, lactic and formic acids are formed. The leachate produced at this phase contains ammonical nitrogen in high concentration. The temperature at this phase will be around 30 and 50 °C. The gas concentrations in the waste in this stage was 80% carbon dioxide and 20% hydrogen.

 

Stage III – Acetogenesis

 

The organic acids produced in stage II are converted by acetogen micro-organisms to acetic acid, carbon dioxide and hydrogen. After few days the hydrogen and carbon dioxide levels continue to decrease in the landfill. It is at this point the methane-generating micro-organisms, the methanogens grow and results in the production of methane and carbon dioxide. The pH at this phase is acidic (pH 4) which leads to increase in the solubility of metal ions which can be analyzed in the leachate. Ions such as chloride, ammonium and phosphate are also found in high concentration in the leachate.

Figure 5 Stages of waste degradation within the landfill

 

Stage IV – Methanogenesis

 

Stage IV is the main stage for landfill gas generation (i.e) 60% methane and 40% carbon dioxide is produced. This phase is very slow and takes many years for completion. The waste is devoid of oxygen and anaerobic condition exits in this phase. There are two classes of micro-organisms which are active in the methanogenic stage, the mesophilic bacteria which are active in the temperature range 30-35 °C and thermophilic bacteria active in the range 45-65 °C. When temperatures drops below 15 °C in cold weather the rate of biological degradation drops. In addition to temperature, pH also plays an important role in this phase. The ideal conditions for the methanogenic micro-organisms are a pH range from 6.8 to 7.5. Moisture also determines the level of waste degradation in this phase.

 

Stage V – Oxidation

 

The last and final stage of waste degradation is oxidation. During this phase new aerobic micro-organisms slowly replace the anaerobic forms and re-establish aerobic conditions. Oxygen and other oxidized species will establish themselves in the bioreactor. A dramatic decrease in gas production was observed. The residual methane is converted to carbon dioxide and water. All the different stages may be progressing simultaneously until all the waste reaches stage five and stabilization occurs.

 

Figure 6 depicts the gas generation during the five stages of waste degradation in the landfill.

Figure 6 Gas generation during the five stages of waste degradation in the landfill

 

The optimum conditions that are to be maintained in the landfill bioreactor to achieve better degradation of waste is summarized in Table 2 given below

 

Table 2 Conditions to be maintained for better waste degradation in landfill bioreactor

Source: Yuen et al., (1994)

 

6.0 Monitoring parameters

 

Continuous monitoring of the landfill bioreactor process will minimize the operational failures and improve the performance of the process. The monitoring parameters is summarized below

 

Leachate Flux – The rate of leachate generation and recirculation should be continuously monitored. Likewise, the moisture levels within the landfill bioreactor must also be monitored regularly to identify the requirement of leachate for recirculation. Excess moisture might reduce the degradation efficiency and gas production rate. According to previous reports, a relationship exists between influx, in situ moisture levels and leachate removal rates

 

Temperature – Continuous temperature monitoring within the landfill bioreactor might indicate the stage of biological activity. Temperature monitoring is very essential especially for aerobic landfill bioreactors to avoid fires.

 

Moisture – As mentioned earlier, moisture determines the rate of degradation within the landfill. Hence waste moisture monitoring will give an idea about effectiveness of leachate recirculation systems and requirement of moisture at a particular location.

 

Cellulose and Lignin – Measurement of cellulose and lignin within the landfill bioreactor will facilitate in determining the phase of biological activity and degradation rate.

 

Leachate Yield and Quality – Leachate yield provides information about the rate of degradation and saturation levels of waste. Short circuits in the leachate system can also be determined. Likewise, characterization of leachate (i.e) leachate quality might indicate phase of degradation and biological activity within the bioreactor.

 

Waste Density and settlement – The rate of degradation and waste saturation is determined from waste density monitoring. Monitoring of settlement in the surfaceof landfill bioreactor might help in calculating the available space in each cell. This might facilitate more loading of waste.

 

Gas Flow and Quality – Again, analysis of quantity and quality of gas generation indicate the biological activity within the bioreactor. The progress and rate of degradation can also be identified. This information is also needed to plan expansion and modification of gas management systems.

 

7.0 Advantages of landfill bioreactor

  • The advantages of a landfill bioreactor over a normal landfill are as follows
  • The rate of degradation is fast when compared to normal landfill. The waste is degraded within 5-10 years when compared to dry tomb landfill which takes 20-30 years
  • The waste within the landfill bioreactor shows rapid settlement and stabilization Impact on ground and surface water is very minimum
  • LFBR generates high quantity of gas in a short span of time.
  • The gas can be collected efficiently and converted into electricity.
  • Huge amount of revenue can be generated from the landfill gas when converted into electricity or other forms of energy. Moreover, the gas generated in LFBR in controlled condition are of improved quality.
  • The greenhouse gas emissions are reduced.
  • The waste density is increased within the LFBR and henceforth facilitate loading of more waste. Leachate circulation minimizes the cost involved in leachate treatment. Recirculation avoids entry of pollutant into the environment during leachate discharge Improved leachate quality is not an environmental concern and gets stabilized within 3 years of closure of the landfill
  • The residue left after the degradation in LFBR do not need further landfilling. They can be used as a compost. Removal of inert materials periodically through landfill mining can improve the space availability in LFBR so that more waste can be loaded.
  • After closure of landfill bioreactors, the land can be reused for other purposes
  • Faster waste degradation in LFBR avoids the requirement of large area of land, thereby reducing the overall cost of the landfilling process.
  • Long term environmental risks are avoided due to minimum leachate migration, recovery of landfill gas and controlled waste settlement.

 

8.0 Drawbacks of landfill bioreactors:

 

Cost of construction is too high as it involves many engineering aspects. Numerous networks of pipes for leachate and gas collection needs to be established which further increases the cost of construction.

 

Skilled labours are required for operation of LFBR

 

Temperature control is very important especially in aerobic bioreactors Other drawbacks include

 

o   Geotechnical stability

 

o   Liner chemical compatibility

 

o   Odour control

 

Availability of other liquids such as storm water, sewage sludge and non-hazardous liquid for recirculation is little difficult

 

Summary

 

To summarize, in this module we have studied about

  • Landfill bioreactor and its components
  • Types of landfill bioreactor
  • Process of waste degradation inside the landfill bioreactor
  • Parameters that needs to be monitored for improved degradation
  • Advantage and drawbacks of landfill bioreactor

 

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References