2.1 Landfill Degradation and Behaviour

CHAPTER 2

LITERATURE REVIEW

2.1 Landfill Degradation and Behaviour 2.2 Process-Based Landfill Enhancement Techniques 2.3 Development in Leachate Recirculation 2.4 Landfill Hydrology 2.5 Research Needs

This chapter provides a comprehensive review of the current knowledge and developments relating to process-based landfilling. The term process-based landfilling is used here to represent any active landfill management that aims to stabilise waste in contrast to the conventional permanent storage containment landfills. The chapter starts with some background information on landfill degradation and behaviour, followed by an overview of process-based landfill enhancement techniques. The review then focuses on the development of leachate recirculation and the associated landfill hydrology. Finally it identifies the research still required for a successful bioreactor landfill application.

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2.1 LANDFILL DEGRADATION AND BEHAVIOUR

2.1.1 Decomposition of Municipal Solid Waste

(i) Aerobic Degradation

In this degradation process, aerobic bacteria convert organic matter mainly into carbon dioxide, water, energy (heat), and biomass product.

Often a landfill is capped with a low permeability cover and the amount of oxygen that can penetrate into the waste is limited. Also the waste in most modern landfills is highly compacted with a minimum void. Hence the initial aerobic biodegradation generally lasts only a short time and the bacteria activities decline upon the depletion of oxygen.

After this short initial aerobic stage, the only part of the landfill body that may still involve aerobic metabolism will be the uppermost layer where oxygen may exist by means of diffusion and rainwater infiltration.

(ii) Anaerobic Degradation

Subsequent to the initial short aerobic degradation, the landfill will be predominated by an anaerobic condition. A consortium of anaerobic bacteria will start biodegrading the organic matter, eventually converting it mainly into carbon dioxide and methane. The microbial conversion processes are complex and have been described by various researchers (e.g. Christensen and Kjeldsen, 1989; Aragno, 1988). In brief, anaerobic degradation is mediated by a variety of microorganisms operating in series, i.e. product of one bio- reaction is used as substrate in the next bio-reaction. The flow chart in Figure 2.1 shows a simplified chain of anaerobic degradation. The chart also indicates the substrates and immediate products involved in each degradation step.

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Complex dissolved Solid organic matter organic matter

Fermentative bacteria HYDROLYSIS

Dissolved organic matter

ACID FERMENTATION Fermentative bacteria

Volatile fatty acids (except acetate) + Alcohols

Acetogenic bacteria ACETOGENESIS

Acetate Acetate Hydrogen Carbon dioxide

METHANOGENESIS Methanogenic bacteria Methanogenic bacteria (Acetophilic) (Hydrogenophilic)

Methane

Figure 2.1 – Simplified Anaerobic Degradation Processes Involving Various Bacteria Groups in a Landfill Ecosystem

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There are three main groups of bacteria involved in the anaerobic landfill ecosystem:

 Fermentative bacteria - These bacteria perform hydrolysis and organic acid fermentation. They are a large heterogeneous group of strictly-anaerobic and facultative-anaerobic bacteria (the latter prefer anaerobic conditions but can make use of oxygen on a temporary basis).

 Acetogenic bacteria - They are also heterogeneous bacteria that convert the products derived from the above fermentation into acetic acid.

 Methane producing bacteria - These bacteria are strictly-anaerobic and are very sensitive to the presence of oxygen and pH of the environment. They use only specific substrates.

The four important anaerobic degradation steps as indicated in Figure 2.1 are:

 Hydrolysis - This is an important process through which the solid and complex dissolved organic matters are broken down into smaller, soluble components required for subsequent microbial conversions (e.g. carbohydrates into simple sugars, proteins into amino acids and lipids into glycerol and long chain fatty acids). Hydrolysis is promoted by the extra-cellular enzymes produced by the fermentative bacteria (Christensen and Kjeldsen, 1989; Aragno, 1988).

 Acid Fermentation - The dissolved organic matter from hydrolysis is fermented by the fermentative bacteria primarily into volatile fatty acids (VFAs), alcohols, hydrogen and carbon dioxide. The acid-fermentation process thus results in a high concentration of volatile fatty acids.

 Acetogenesis - The acetogenic group of bacteria converts the longer chain volatile fatty acids (propionate, butyrate, isobutyrate, valerate, isovalerate and caproate) and alcohols into acetate (the shortest chain fatty acid), hydrogen, and carbon dioxide.

 Methanogenesis - Subsequently methane is produced by the methanogenic bacteria. The conversion is carried out either by the acetophilic group converting acetic acid to methane and carbon dioxide, or by the hydrogenophilic group converting hydrogen and carbon dioxide to methane. Both groups are strictly-anaerobic and require a condition of low redox potential. They are also sensitive to pH; the range of pH tolerated by the methanogenic bacteria is limited (at a range between 6 and 8).

Gas Composition (% by volume) 100%

N2 CO2 N2

O O2 H2 CH4 2 0 %

Leachate Concentrations COD

+ NH 4

1 2 3 4 5 Phase

Figure 2.2 - Typical Landfill Evolution Sequence in Terms of Gas and Leachate Composition (after Christensen and Kjeldsen, 1989; Farquhar and Rovers, 1973)

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2.1.2 Evolution Sequence

With knowledge of the decomposition processes, it is not difficult to understand that most landfills receiving MSW proceed through a series of rather predictable events. Such a sequence has been described by Farquhar and Rovers (1973), Ehrig (1983), Chian et al., 1985) and Christensen and Kjeldsen (1989). The sequence can be separated into five distinct phases in terms of gas composition and leachate concentrations as illustrated in Figure 2.2.

 Phase 1 is the short initial aerobic decomposition where oxygen is still present within the landfill mass. The amount of leachate generated in the aerobic process is generally not substantial.

 Phase 2 covers the immediate anaerobic degradation processes of acid-fermentation and acetogenesis. The two processes together are generally referred to as the acid production phase. The concentration of volatile fatty acids rises to a peak and pH of the leachate reaches its lowest. There is a concurrent increase in inorganic ions which is due to the leaching of easily soluble material in the more acidic environment. Thus, the leachate generated at this stage is of high strength. The content of nitrogen in the landfill gas diminishes as it is displaced by hydrogen and carbon dioxide generated in the fermentative and acetogenesis processes.

 Phase 3 is a transition from the above acid phase to the next methanogenic phase with a steady growth of methanogenic bacteria. As the growth of methanogenic bacteria is initially suppressed by the acidic environment, it usually takes some time for them to develop and dominate the system. The methane concentration increases slowly with a decrease in hydrogen and carbon dioxide. With the gradual development of the methanogenic microbial population, more acetic acid is converted into methane resulting in an increase in pH. With the redox potential dropping to its lowest value, nitrates and sulphates are reduced to ammonia and sulphides respectively.

 Phase 4 is the methanogenic phase at which the methanogenic bacteria have overcome the acidic environment and established themselves well in the system. It is

distinguished by a steady methane production. The methane concentration for a typical landfill would be around 50 to 60% by volume with the rest being mostly carbon dioxide. When the degradation reaches this stage, the composition of leachate is characterised by a close-to-neutral pH value, a low concentration of volatile acids, and a reduced amount of total dissolved solids. Thus, the high strength leachate generated from the preceding acid production is weakened by this methanogenesis process.

 Phase 5 is the final post-methanogenic stage. There is a lack of long term scientific data related to this maturation stage. It is generally expected that the landfill mass will eventually evolve towards an aerobic condition as the rate of oxygen diffusion into the waste exceeds the oxygen consumption rate. Other degradation, which requires an aerobic environment, will then take place. It is believed that the rate at which such a final evolution may progress, or whether it will occur at all, depends on specific landfill conditions such as moisture content and final cover.

This idealised evolution sequence, strictly speaking, applies to a homogeneous landfill mass. One should recognise that as most landfills are constructed of sub-cells or pockets of waste of different ages and conditions, it is common that more than one of the above phases may take place concurrently in a full-scale landfill. Therefore, the evolution of a real landfill situation could be more complicated. Nevertheless, the overall landfill gas and leachate development patterns as described in Figure 2.2 can be useful stabilisation indicators.

The time scale of the evolution sequence in Figure 2.2 is omitted intentionally as the duration of each phase may vary considerably depending on many influencing factors which are discussed below.

2.1.3 Influencing Factors

The fundamental factors that affect the efficiency of degradation in a landfill system are summarised in Table 2.1. They are discussed below.

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(i) Moisture

Moisture is essential for the activities of all microorganisms including the bacteria consortium in the landfill ecosystem. Many investigations have shown that the methane production rate of a landfill rises with an increase in moisture content of the waste (e.g. Eliasen, 1942; DeWalle and Chian, 1978; Rees, 1980 and Pohland, 1986). Hartz and Ham (1983) reported that methane production would reduce as moisture level in the waste decreases and would cease completely below the 10% moisture level (by wet mass).

Rees (1980) provided a summary of research findings which suggests that the landfill gas production rate rises exponentially with increase in moisture content up to 60% (by wet mass). A higher moisture content does not seem to enhance nor decrease the gas production rate (Pohland, 1986).

Furthermore, moisture movement in a landfill facilitates the following: (1) the exchange of substrates, nutrient and buffer, (2) the dilution of inhibitors, and (3) the distribution of bacteria within the landfill environment. A laboratory study (Hartz and Ham, 1983) showed that the rate of methane production with free moisture movement increased ten- fold as compared with a quiescent condition.

However, if moisture is added to the waste too rapidly, it may produce a negative effect by over-cooling the system (Rovers and Farquhar, 1973). This is because the anaerobic decomposition is also influenced by temperature (as discussed in 2.1.3 (v)).

Table 2.1 - Summary of Influencing Factors on Landfill Degradation

Influencing Criteria / Comments References factors Moisture Optimum moisture content : 60% and above (by wet mass) Pohland (1986) and Rees (1980) Oxygen Optimum redox potential for methanogenesis: -200mV Farquhar and Rovers (1973) -300mV Christensen and Kjelden (1989) below -100mV Pohland (1980) pH Optimum pH for methanogenesis: 6 to 8 Ehrig(1983)/ 6.4 to 7.2 Farquhar and Rovers(1973) Alkalinity Optimum alkalinity for methanogenesis : 2000mg/l Farquhar and Rovers (1973) Maximum organic acids concentration for methanogenesis : 3000mg/l Farquhar and Rovers (1973) Maximum acetic acid/alkalinity ratio for methanogenesis : 0.8 Ehrig (1983) Temperature Optimum temperature for methanogenesis : 40o Rees (1980) 41o Hartz et al. (1982) 34-38oC Mata-Alvarez et al. (1986) Hydrogen Partial hydrogen pressure for acetogenesis: Barlaz et al. (1987) Below 10-6 atm. Nutrients Generally adequate in most landfill except local Christensen and Kjelden (1989) systems due to heterogeneity Sulphate Increase in sulphate decreases methanogenesis Christensen and Kjelden (1989) Inhibitors Cation concentrations producing moderate inhibition McCarty and McKinney (1961) (mg/ l) : Sodium 3500-5500 Potassium 2500-4500 Calcium 2500-4500 Magnesium 1000-1500 Ammonium(total) 1500-3000

Heavy metals : No significant influence Ehrig(1983)

Organic compounds : Inhibitory only in significant amount Christensen and Kjelden(1989)

(ii) Oxygen

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The presence of oxygen inhibits the activities of anaerobic bacteria in a landfill. As discussed earlier, the methanogenic bacteria are strictly-anaerobic and are very sensitive to the presence of oxygen. It has been reported that they prefer an optimum redox potential between -200mV (Farquhar and Rovers, 1973) and -300mV (Christensen and Kjeldsen, 1989). However, lysimeter scale studies also showed that methanogenic phase could develop at a redox potential of about -100mV (Pohland, 1980 and Meta-Alvarez and Martinez-Viturtia, 1986).

Although atmospheric oxygen may penetrate into the landfill by means of air diffusion and rain water infiltration, the presence of aerobic bacteria in the uppermost layer of the waste would consume the oxygen and if there is a well-sealed capping, it would limit the presence of oxygen to the top 1m or less of the waste mass.

(iii) pH

The pH in a landfill system can have a significant influence on the methane conversion. While the fermentative and acetogenic bacteria can tolerate a wider pH environment, the methanogenic microbes are only active within a narrow pH range between 6 and 8 (Ehrig, 1983; Farquhar and Rovers, 1973).

It is not uncommon to observe a suppressed on-set of methanogenesis due to an over- stimulated acid production, where the system is said to become “sour”. Similarly, in a well established methanogenic system, if due to any reason (e.g. an ingress of oxygen) the activity of methanogenic bacteria in the landfill ecosystem is suppressed and the conversion of hydrogen, carbon dioxide and acetic acid to methane would not proceed, the pH of the system will drop as a result of the accumulation of volatile fatty acids. Any decrease in the pH will further slow down the growth of the methanogenic bacteria. In an extreme case, this may shut down the whole methane conversion.

(iv) Alkalinity

Alkalinity is generally expressed as a concentration of calcium carbonate. It acts as an effective pH buffer , which may significantly improve the efficiency of the degradation by maintaining a close to neutral pH range in the landfill ecosystem. Its major source would generally come from soil and demolition waste. It has been reported that an acetic acid to alkalinity ratio less than 0.8 is essential to start methane production (Ehrig, 1983). Farquhar and Rovers (1973) suggested that an alkalinity in excess of 2000mg per litre and a concentration of volatile acids less than 3000mg per litre are required for good methane production.

(v) Temperature

Farquhar and Rovers (1973) reported that there are three groups of methanogenic bacteria operating at different temperature ranges, namely psychrophilic (<20oC), thermophilic (>44oC), and mesophilic (20 to 44oC). It is the mesophilic group that is relevant to landfill methanogenesis.

Laboratory studies have reported that the production of methane increased significantly (up to 100 times) with temperature raised from 20 to 40oC (Christensen and Kjeldsen, 1989). Hartz et al. (1982) demonstrated by laboratory tests that the optimum temperature for methane production was at 41oC, while Meta-Alvarez and Martinez-Viturtia (1986) showed that the optimum temperature range was between 34 to 38oC in laboratory test cells with leachate recirculation.

The amount of heat energy generated by anaerobic decomposition processes is small compared to aerobic degradation. However, because landfill wastes and earth capping are good insulation materials, the heat loss to the external environment is generally minimal. Thus, the heat generated by the anaerobic processes is often enough to maintain an elevated temperature within the landfill mass. In a temperate climate, landfill temperature between 30 and 45 oC has been reported (Rees, 1980).

(vi) Hydrogen

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Since hydrogen acts as both a substrate and a product in the various anaerobic conversions, it plays an important role in the balance of the microbial ecosystem.

For the fermentation, at low hydrogen pressure the process yields hydrogen, carbon dioxide and acetate. At high hydrogen pressure, the process produces volatile fatty acids (except acetate) and alcohols (refer to Figure 2.1).

For the subsequent acetogenesis, if the hydrogen pressure in the landfill system is too high, longer chain volatile fatty acids generated in the above fermentation process will not be converted into acetate (along with hydrogen and carbon dioxide) but will accumulate. Barlaz et al. (1987) suggested that this conversion requires a hydrogen pressure below 10- 6 atmospheres.

(vii) Nutrient

Landfill micro-organisms require nutrient for their anaerobic activities. In this case, nutrient usually refers to nitrogen, phosphorus and other micro-nutrient including sulphur, calcium, magnesium, potassium, iron, zinc, copper and cobalt. Anaerobic assimilation requires much less nutrient than aerobic conversion processes. In most landfills, there are generally adequate supplies of these nutrients. However, heterogeneity of a landfill may create local nutrient-deficient pockets.

(viii) Sulphate

Christensen and Kjeldsen (1989) suggested that landfill methane production would reduce if sulphate is present. The suppression of methane formation by sulphate is not related to any toxic effects of sulphate on the methanogenic bacteria, but rather solely due to substrate competition as the sulphate-reducing bacteria also consume hydrogen and acetic acid during sulphate reduction.

(ix) Inhibitors

While oxygen, hydrogen, pH, and sulphate all have inhibitory effects on the methanogenic bacteria as discussed above, the inhibitors here referred to are cation concentrations, heavy metals and organic compounds.

Effects of cations (sodium, potassium, calcium, magnesium and ammonium) on methane production have been studied by McCarty and McKinney (1961). These cations, in low concentrations, are essential as micro-nutrient. But in high concentrations, they significantly inhibit methane production. The study reported a range of concentration levels that would provide different levels of effects (as listed in Table 2.1). However, in landfill environments, the concentrations of the above five cations are usually below the moderate inhibitory levels suggested in Table 2.1.

For heavy metals, Ehrig (1983) suggested that their concentrations commonly present in landfill wastes are not high enough to influence significantly the sensitive methanogenic bacteria.

The toxic effects caused by various organic compounds have been studied by several researchers and are summarised by Christensen and Kjeldsen (1989). They concluded that fairly high concentrations of these toxic organic compounds are required to impose a significant inhibitory effect on a methanogenic system. In MSW landfills, their concentrations would generally be too low to have any inhibitory effect.

2.2 PROCESS-BASED LANDFILL ENHANCEMENT TECHNIQUES

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Various enhancement techniques have been developed from the process-based landfill concept. They all share a common aim, that is, to control the influencing factors positively to enhance degradation and stabilisation. For discussion purposes, these techniques are grouped under the following eight headings.

2.2.1 Control and Selection of Waste

Results of many investigations (e.g. Farquhar, 1988 and Christensen et al., 1992) indicate that wastes of different composition can produce various effects on the degradation processes due to : (1) the availability of moisture and substrate, (2) the presence of potential inhibitors, and (3) the isolation of local pockets of different environments. For example, wet rapid decomposable organic matter from kitchen and garden wastes may delay the methane production due to an early intensive acid phase generated by the readily degradable organic. Synthetic organic in the form of plastics may be biologically degradable in the very long term, but would degrade at an extremely slow rate that is generally considered to contribute very little in the production of methane. The presence of large amounts of high sulphate content waste (e.g. demolition waste and incinerator waste) may also reduce methane formation (refer Section 2.13 (viii)).

Thus the control or selection of waste prior to landfilling can affect the degradation processes. Krol et al. (1994) suggested that an enhanced MSW treatment can be achieved by:  using landfills as bioreactors to treat raw MSW,  pre-treating different wastes prior to landfill, and/or  treating source-separated biodegradable organics to produce marketable solid products and landfilling only the biologically stable (inert) residuals.

2.2.2 Shredding of Waste

Although shredding of MSW prior to landfilling is often carried out primarily to provide more landfill space, it has also been promoted with an aim to enhance degradation. The

2-14 Literature Review arguments are that shredding may help to increase the homogeneity of the waste by size reduction and mixing, increase the specific surface area of the waste for biodegradation, remove moisture barriers caused by impermeable materials, and improve the water content distribution in the waste.

However, limited data from some investigations (e.g. Christensen et al., 1992) have suggested that shredding of waste may actually induce a negative effect on degradation. A logical explanation is that shredding promotes hydrolysis and acid formation, and this intensified acid development will produce a low pH condition that prevents the onset of a methanogenic environment. However, if this negative effect can be controlled (e.g. by pH buffering), shredding may prove to be beneficial when hydrolysis and acid production are the limiting steps in the conversion processes.

2.2.3 Waste Compaction

Modern landfills often aim to achieve the highest possible waste compaction in order to maximise the use of space for economical reasons. There is another benefit with high compaction, which is often overlooked. Minimising void to limit the amount of free oxygen within the waste would shorten the duration of the initial aerobic decomposition. This would enable the subsequent anaerobic conversion to take place at the earliest possible time.

Results from limited studies (e.g. Dewalle and Chian, 1978) show that compaction also affects anaerobic decomposition directly. If a waste is relatively dry, increasing the compaction (or the dry density) may significantly speed up the degradation processes. This positive effect can be explained by the higher moisture content (by volume) available in the more compacted solids which may help to enhance the distribution of nutrient and the contact between substrates and bacteria. However, for a waste with a high water content, an increase in dry density may actually slow down methane production. This is due to the development of an undesirable early intensive acid phase over-stimulated by high moisture.

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2.2.4 Buffer Addition

In an unbalanced landfill ecosystem, a low pH environment caused by a vigorous acid production would inhibit the growth of methanogenic bacteria. This understanding has led to attempts to introduce buffer artificially into landfill systems. Results of small scale experiments (e.g. Christensen et al., 1992) suggest that the addition of buffer generally has a positive effect on the degradation. If a landfill fails to generate methane due to a low pH, buffer addition is an obvious measure to help the establishment of a methanogenic condition.

2.2.5 Sewage Sludge Addition

Co-disposal of sewage sludge with MSW in a landfill may promote degradation by increasing the availability of moisture, nutrient and anaerobic micro-organism in the waste. However, in situations where methanogenic conditions are already established or the landfill environment is favourable to methanogenic development, addition of sewage sludge may not have any beneficial effect. Studies (e.g. Leckie et al., 1979, Leuschner, 1989) have reported that the addition of sewage sludge in some cases have actually induced negative effects. In these cases, the sludge added to the landfill system was low in pH (septic tank sludge), and the natural buffer capacity of the landfill environment was exceeded.

2.2.6 Pre-composting Part of Landfill Waste

Researchers in Germany (e.g. Stegmann, 1983, Stegmann and Spendlin, 1986, Stegmann and Spendlin, 1989, Doedens and Cord-Landwehr, 1989) have conducted both laboratory and full-scale tests which showed that the pre-composting of part of the waste in a landfill can provide a positive effect on leachate strength reduction. Its background theory is to allow the more readily degradable organic material in the waste to be first degraded by

2-16 Literature Review aerobic process via composting, thereby moderating the development of an intensive acid phase in the later anaerobic degradation.

The part of landfill waste that has been pre-composted also provides an acid "diluting" effect in the system. Thus, a balance between the acid production and methanogenic phase in the landfill ecosystem can be achieved much sooner.

Based on the same principle, an alternative technique called "thin-layer" operation has been developed. In practice, layers of pre-composted waste are first constructed in the bottom of a landfill cell by placing the waste in thin layers with low compaction (in order to allow adequate aeration for composting). These pre-composted layers will later act as an effective "anaerobic filter" for the high strength acid generated from the landfill waste above. Both laboratory and full-scale tests conducted on the pre-composted "thin-layer" operation have shown positive results (Stegmann and Spendlin, 1989, Doedens and Cord- Landwehr, 1989).

2.2.7 Enzymes Addition

As hydrolysis is promoted by enzymes produced by fermentative bacteria (refer Section 2.1.1(ii)), research has been conducted to study the possibility of controlling and enhancing the hydrolysis process by manipulating the natural enzyme activity. Lagerkvist and Chen (1993) used laboratory scale simulators to investigate the effect of adding industrial celluloytic enzymes into MSW during both acidogenic and methanogenic states. The laboratory results suggested that it is viable to intensify both acidogenic and methanogenic conditions by enzyme addition.

2.2.8 Leachate Recirculation

Among all the enhancement techniques, leachate recirculation is by far the most investigated process-based management option. It is commonly performed in containment

2-17 Chapter 2 landfills that have effective leachate collection systems so that all leachate produced and recycled can be accounted for.

Its main drive is being generated from the arguments that the recirculated leachate helps : (1) to promote an active microbial degradation by providing an optimum moisture, (2) to induce a water flux to provide a mechanism for the effective transfer of microbes, substrates, and nutrient throughout the refuse mass, and (3) to dilute local high concentration of inhibitors.

Also there are potential operational benefits claimed, including:  the temporary storage of leachate and the partial in-situ treatment of leachate,  the improvement of landfill gas production rate and total yield,  the accelerated waste compression/ settlement to create additional space for disposal and to allow earlier use of the land asset, and  the reduction in time and cost of post-closure monitoring.

However, there are also controversies regarding leachate recirculation (e.g. Lee et al., 1986 and Maloney, 1986). The main criticism comes from the concern that feeding back the highly polluted leachate into the landfill may concentrate the pollutants and overload the hydraulic capacity of the containment system. Others are sceptical regarding its ability to treat leachate and reduce leachate volume. Operational difficulties together with environmental constraints (such as problems associated with odour) also hinder its development.

The following section aims to provide an up-to-date summary of research developments and current findings in relation to leachate recirculation.

2.3 DEVELOPMENTS IN LEACHATE RECIRCULATION

Researchers started exploring the potential benefits of recirculating leachate in landfills as early as the 1970's, although the early emphasis was more on the in-situ treatment of leachate and gas enhancement rather than waste stabilisation. Up to now, most of the

2-18 Literature Review studies have been conducted at laboratory or lysimeter-scale. This section aims to provide a review of developments from the earlier pilot laboratory tests to some of the recent full- scale studies.

2.3.1 Small-Scale Studies

Table 2.2 lists all laboratory/ lysimeter-scale studies identified in the literature. The table includes the scale of testing, number of test cells, enhancement techniques investigated and assessment criteria of each study. The highlights of each study are presented below.

(i) Pohland (1975) [U.S.]

Pohland (1975) contributed the first major research effort to examine the effects of leachate recirculation on the degradation of MSW landfills. His first study conducted at the Georgia Institute of Technology aimed to investigate the potential of accelerating the stabilisation of leachate by : (a) recirculation alone; (b) recirculation with pH control; and (c) recirculation with pH control and sewage sludge addition.

The laboratory tests demonstrated that recirculating leachate in the simulated landfill cell managed to develop a more active anaerobic biological system as reflected by the increase in the rate of degradation of the readily available organic. The effect of recirculation with pH control appeared to promote an earlier onset of methanogenesis. The addition of sludge seemed to create an environment most beneficial to the rapid formation of volatile acids but unfavourable to the methane forming bacteria.

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Table 2.2

(ii) Leckie et al. (1979) [U.S.]

The objectives were to investigate the quality and quantity of leachate stimulated by leachate recirculation, accelerated moisture addition (both initial addition and flow-through effects), and microbial seeding by sludge addition.

The study showed that bringing the MSW to field capacity immediately after placement accelerated the stabilisation processes to an observable degree as reflected by an early methane production.

A second cell, with a continual flow-through of water, accelerated the degradation, the flushing out of soluble materials, and the rate of settlement. It also provided an early production of methane. The soluble constituents in the leachate were significantly reduced by the process. However, it was suggested that this mode of operation was not feasible for full-scale landfills due to the substantial amount of leachate generated.

Septic tank sludge was used as microbial seeding in one cell. It seemed that the accelerated acid fermentation suppressed the buffer capacity of the system and inhibited the subsequent methanogenic phase development. This negative effect was caused by the low pH inherent with septic tank sludges.

(iii) Pohland (1980) [U.S.]

Two field lysimeter cells were employed. One was left open and the other sealed. The open and the sealed cells were both allowed to reach field capacity with incident rainfall and equivalent water addition respectively. As soon as sufficient leachate was generated, recirculation was carried out in both cells, first weekly and later daily.

Leachate composition was monitored. Both cells experienced a high volatile acid production in the early phase of degradation. However, subsequent methane production was observed to take place earlier in the sealed cell than in the open cell. This was attributed to the anaerobic environment in the sealed cell preferred by the methanogenic bacteria.

(iv) Tittlebaum (1982) [U.S.]

This study aimed to investigate the effects due to leachate recirculation with pH control, waste shredding, and nutrient addition.

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It demonstrated that recirculation with pH neutralisation established an efficient anaerobic degradation system - the concentrations of volatile acids rose to a peak value and diminished rapidly following the establishment of an effective methanogenic environment.

The recirculation combing either waste shredding or nutrient addition did not seem to provide any further significant enhancement.

(v) Robinson et al. (1982) [U.K.]

The objective was to determine effects of leachate recirculation on leachate production and quality. Leachate recirculation alone was performed in one cell. In the second cell, leachate recirculation was also conducted but the leachate collected was first aerated and dosed with nutrient (phosphate) prior to recycle.

During the recirculation, spraying raw leachate collected from the first cell resulted in the death of grass in the soil cover. The aerated leachate from the second cell had no adverse effect on the grass growth due mainly to the reduction in leachate strength following aeration.

Regarding leachate production, the overall water balance showed that although evaporation reduced a certain volume of the leachate, the amount of leachate collected and recirculated increased with time. The total amount of leachate in the system would eventually become excessive and require external disposal.

The analysis of leachate composition showed that in the case of leachate recirculation alone, the initially high concentrations of COD, ammonia and chloride stabilised to a reasonably constant level in 12 months. In the case of aerated leachate with nutrient addition, the leachate concentrations were generally lower but fluctuated over a longer period.

(vi) Hartz and Ham (1983) [U.S.]

The main objective was to investigate the change in the rate of methane production due to a variation in moisture content.

Extrapolation of the test data suggested that methane production would cease entirely below the 10% moisture level (wet mass basis). The field capacity was found to be at about 40%. Below this, no free moisture was available for recirculation.

The study also concluded that, keeping the same moisture content, the test with moisture flow produced a methane production rate about 10 times higher than the one under a quiescent condition.

As the study was carried out primarily to look at methane generation, no data on leachate composition were reported.

(vii) Mata-Alvarez and Martinez-Viturtia (1986) [Spain]

The study aimed to investigate the effect of temperature on landfill decomposition. Five tests were conducted to cover the temperature range between 30 to 46 oC. Shredded MSW waste was used and each test was enhanced by leachate recirculation with pH control and digested pig manure addition. Leachate composition and methane production rate were used as reaction indicators.

The test data suggested that the optimum operating temperature for a landfill anaerobic degradation was between 34 to 38oC.

A good correlation between leachate composition pattern and gas production pattern was observed. Maximum gas productions were achieved at pH values of about 7.5. Initially, concentrations of volatile fatty acids, total solids, and volatile solids in the leachate rose to high values, each up to a peak coinciding with the commencement of high gas production. After that, the concentrations of these components dropped rapidly. Redox potential was at its lowest of about -100mV during the period of maximum gas production.

The study concluded that buffer addition was very important in the starting up of methanogenesis. By extrapolation the study also suggested that biodegradable matter in a real landfill, operating under same conditions simulated in the study, should be stabilised in about two years.

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(viii) Walsh et al. (1986) and Kinman et al. (1987) [U.S.]

Sixteen cells were used to evaluate the effectiveness of gas enhancement promoted by moisture flow, elevated moisture content, leachate recirculation, buffer addition, nutrient addition, anaerobic digested sludge addition, and elevated temperature.

One conclusive finding from the study was that, by leachate recirculation alone, the operation intensified the early phase of acid production, which produced a negative effect on the subsequent methanogenesis phase.

By recirculation with buffer addition, the operation provided positive results in methane production by maintaining a steady close-to-neutral pH. It also gave a rapid reduction in leachate strength.

(ix) Barlaz et al. (1987) [U.S.]

The study investigated effects of leachate recirculation with pH control, addition of old anaerobically degraded refuse, addition of anaerobically digested sludge, use of sterile cover soil, and addition of acetate.

It demonstrated that recirculation with pH control stimulated a good methane production.

The addition of old anaerobically degraded refuse provided positive results. It appeared that the old waste provided an effective seeding of anaerobic bacteria.

The addition of sludge alone did not seem to provide any beneficial effect on methane production.

The use of sterile soil cover did not prohibit methanogenic activities. It ruled out the belief that soil could be the only source of methanogenic bacteria for anaerobic degradation.

The addition of acetate in the system did not stimulate any positive effect on methane production. This suggested that, under the test condition, hydrolysis or acid fermentation was not the critical step in terms of limiting the rate of production.

(x) Stegmann and Spendlin (1986 and, 1989) [Germany]

A series of laboratory and lysimeter experiments were reported. Apart from leachate recirculation, the study also investigated other enhancement techniques including leachate recirculation with pH control, mixing MSW with pre-composted waste prior to landfilling, and sewage sludge addition.

The study demonstrated that mixing MSW with pre-composted waste has a positive result on leachate strength reduction. The background theory of this has been discussed previously in Section 2.2.6. The findings of other tests were not conclusive.

(xi) Leuschner (1989) [U.S.]

Six laboratory reactors were employed to cover the following combinations :

Reactor (1) - control cell (without enhancement), Reactor (2) - recirculation only, Reactor (3) - recirculation with pH buffer, Reactor (4) - recirculation with pH buffer and nutrient, Reactor (5) - recirculation with pH buffer, nutrient and anaerobically digested sludge, and Reactor (6) - recirculation with pH buffer, nutrient, and septic tank sludge.

The most significant feature of this study was to quantify various enhancement effects with a mathematical model. By fitting the experimental data in the model, the times required to degrade 80% of the biodegradable volatile solids in various reactors were calculated.

Comparing Reactor (2) with Reactor (1) (control), the increase in moisture level and movement due to recirculation alone did have a beneficial effect in stimulating the hydrolysis and acid fermentation. However, it did not stimulate methane production

2-25 Chapter 2 because the natural buffer capacity of the MSW was unable to mediate the drop in pH. The 80% degradation time for Reactor (2) was 10.2 years, and for the control cell 19.3 years.

In Reactor (3), the buffering of recycled leachate helped to establish a neutral pH. Once a proper pH was reached, rapid methane production began. The 80% degradation time was 4.2 years.

In Reactor (4), the addition of both nutrient and buffer significantly shortened the time to commence methane production. This suggested that there could be a nutrient deficiency during the initial phase. However, the continued addition of nutrient after methane production did not seem to improve the rate of production. The 80% degradation time was 7.8 years.

Reactor (5) exhibited a methanogenesis commencement time similar to that of Reactor (4) but produced methane at a substantially higher rate than any other reactors. This was most likely due to the microbial inoculum from the sludge. The 80% degradation time was 3.2 years.

The poor result from Reactor (6) led to the conclusion that the septic tank sludge was not a suitable source of microbial inoculum due to its low pH nature.

(xii) Doedens and Cord-Landwehr (1989) [Germany]

One laboratory test and one lysimeter experiment were reported. The laboratory test was conducted to look into the effects of moisture level and moisture movement induced by : (1) leachate recirculation, (2) initial saturation with old stabilised leachate, and (3) moisture due to rainfall infiltration. Based on leachate composition and landfill gas production, the effectiveness of enhancement was assessed. However, no conclusive results were obtained in this laboratory study.

The field experiment comprising three large lysimeters aimed to compare the effects due to leachate recirculation and "thin-layer" operation (refer Section 2.2.6). The study reported that the "thin-layer" operation proved to be more effective than the leachate recirculation in terms of leachate strength reduction.

(xiii) Scrudato and Pagano (1991) [U.S.]

The laboratory experiment involved the feeding of a high strength leachate collected from an active landfill into a flow-through anaerobic digester. By monitoring the composition of the influent and effluent, it was demonstrated that the leachate experienced a significant reduction in trace metal concentrations, BOD and VFAs, together with a concomitant increase in pH. However, in the residual sludge, the concentration of trace metals increased. This reflected that the trace metals had precipitated in the sludge as the leachate pH increased. The results thus suggested that recirculation would help to reduce trace metals concentration in leachate.

(xiv) Woelders et al. (1993) [Netherlands]

Three column cells were used to evaluate the effects of water infiltration and leachate recirculation on a mechanically separated organic residue (derived from a MSW pre- treatment). Column 1 was drained with water at a rate equivalent to natural infiltration. Column 2 was recirculated with methanogenic leachate at twice the natural infiltration rate. Column 3 was similar to Column 2 but with a recirculation rate 5 times the natural infiltration. The accelerated degradation effects were assessed based on organic matter balance, gas yield, methane concentration, pH, and leachate COD.

One important feature was the use of methanogenic leachate to initiate the production of leachate in Columns 2 and 3. This resulted in an early onset of methanogenesis as reflected by the leachate composition and gas production patterns. The two systems were also characterised by a relatively high pH value throughout the test. Column 1, without the buffering and methanogenic bacteria seeding advantages, managed to start up the gas production only after the addition of buffer at a later stage.

The fact that Column 3 exhibited a better enhancement result than Column 2 suggested that the higher recirculation rate promoted a more efficient system.

(xv) Otientio (1994) [Kenya]

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The study involved four column cells to simulate effects due to leachate recirculation and due to initial saturation with leachate (without recirculation) on raw, shredded or aged domestic refuse, with no buffer or other additions. The tests were assessed based on leachate concentrations. The results did not provide any conclusive findings.

(xvi) Chugh (1996) [Australia]

The study investigated the “Sequential Batch Anaerobic Reactor” (SBAR) based on the earlier work by Chynoweth et al. (1992). The process involves an exchange of leachate between a fresh waste cell and an anaerobically stabilised cell. First, the methanogenic leachate obtained from the stabilised cell is recirculated in the fresh cell aiming to provide inoculum to initiate degradation and to flush out the inhibitory products (mainly organic acids). The leachate collected from the fresh cell is then recirculated back to the stabilised cell, where the established microbial population can convert the organic acids to methane. This sequencing of leachate recirculation continues until the cell is stabilised.

The study, conducted in laboratory test cells, demonstrated that the process successfully converted approximately 80% of the degradable organic portion of unsorted MSW to methane in 60 days.

The study also investigated the effects of leachate recirculation volume in the SBAR process. It concluded that an increase in recirculation volume (up to 30% waste volume) provided an earlier onset of methanogenesis and increased the rate of degradation. However, this management option may not be easily put into practice in real landfills due to the high volume of leachate involved. On the contrary, the results also demonstrated that small recirculation leachate volume (at 2% waste volume) could also start up the degradation of a fresh waste cell successfully. Hence for practical operational reasons, a lower leachate recirculation volume may be more desirable in bioreactor applications.

2.3.2 Full-Scale Studies

Compared with small-scale studies, there is an apparent limitation generally associated with full-scale studies - in most cases only limited qualitative data were reported. Table 2.3 attempts to list all full-scale studies available in the literature. The table includes the scale of the test cells, location of the landfill, leachate recirculation method, effects of enhancement and the assessment criteria of each study. The highlight of each study is described below.

(i) Barber and Maris (1984 and 1992)

This full-scale demonstration project, which commenced in 1980, was conducted at the Seamer Carr Landfill in North Yorkshire, England. One unusual feature of this trial was the use of shredded waste in the full-scale landfill cell. The principal aim was to look at the production and in-situ treatment of leachate through recirculation. The developments in landfill gas, waste temperature and settlement were not reported.

The test cell, with a plan area of 2 hectares, was divided into two halves. Leachate recirculation was performed in one half and the remaining half was used as control. However, the two sections were not hydraulically isolated but shared a common leachate collection system. This introduced the difficulty of differentiating the leachate in terms of both quantity and quality.

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Table 2.3

Another difficulty encountered was the presence of a layer of highly impermeable intermediate soil cover, which created a perched saturated zone hydraulically separated from the rest of the 4m deep waste leading to an uneven moisture distribution. Depths of saturation were measured by vertical penetration wells. However, the unsaturated moisture distribution pattern was not measured.

Spray irrigation was chosen as the recirculation method. Prolonged irrigation led to the formation of a hard pan at the surface and resulted in a substantial reduction in infiltration rate. The surface had to be broken up at regular intervals in order to improve infiltration and reduce run-off.

The results of a water balance of the cell suggested that leachate recirculation in a wet climate could not provide a solution to the complete elimination of leachate. Although a substantial amount of leachate could be reduced through evaporation, it would eventually require external disposal.

The results of leachate composition monitoring showed that in-situ leachate treatment achievable in smaller scale studies could also be achieved in the full-scale landfill, but required a longer period of recirculation (in excess of two to three years). Also the concentrations of residual COD, ammonia and chloride in the leachate were still high and it would need dilution or further treatment prior to direct disposal. The greatest reduction in leachate strength was achieved where the waste was fully saturated (i.e. at local saturation zones).

(ii) Croft (1991); Campbell et al. (1995); Knox (1997)

This full-scale investigation commenced in the late 1980s at the Brogborough Landfill in Bedfordshire, England (Croft, 1991). The objective was to examine various enhancement techniques in the improvement of landfill gas quality and production. In addition to a control cell, five test cells were employed to investigate the following variables:

(a) low density tipping, (b) mixing with 50% industrial/ commercial waste, (c) addition of sewage sludge during infilling, (d) retrospective water addition, and (e) retrospective air injection.

Leachate production, leachate composition, waste temperature and settlement were measured. As it was primarily an energy recovery project, emphasis was given to the monitoring of landfill gas. Knox (1997) reviewed the monitoring results and summarised that:

(a) Low density tipping made little difference.

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(b) The inclusion of a high proportion of non-hazardous industrial/ commercial waste accelerated the initiation of landfill gas production. The natural pH buffer offered by the less degradable industrial/ commercial waste might have contributed to the early methanogenesis.

(c) The addition of sewerage sludge promoted an earlier gas generation and a faster rate of production. This was attributed to the significant amount of moisture and the readily degradable organic matter associated with the sewerage sludge.

(d) The retrospective water addition led to an increase in gas production.

(e) The retrospective air injection also led to an increase in gas production. It was suggested that its effectiveness was mainly caused by the injected air forcing some leachate movement within the waste, which improved the moisture distribution. This contradicted the original intention to increase waste temperature by allowing a brief aerobic composting activity by the injected air. This appeared to be unsuccessful as no apparent rise in temperature was detected.

(iii) Stegmann and Spendlin (1986 and 1989)

This full-scale investigation was conducted at the Sanitary Landfill of Lingen in Germany to examine the in-situ leachate treatment by a combination of the "thin-layer" operation (refer 2.26) and leachate recirculation.

Two full-scale cells (each about one hectare) were used - one constructed by the "thin- layer" operation and the other as control (without the pre-composted bottom layer). Leachate recirculation was performed in both cells. The comparison of leachate composition over a four-year monitoring period showed that the combined operation was much more effective in terms of leachate strength (BOD and COD) reduction. However, the enhancement effects on landfill gas and other stabilisation aspects were not addressed.

(iv) Doedens and Cord-Landwehr (1989)

Doedens and Cord-Landwehr (1989) reported a survey conducted in Germany on 13 operating landfills, which had practised leachate recirculation since the late 1970's. BOD and COD concentrations against ages of landfills were studied and compared with data obtained from landfills without recirculation. A trend could be observed which showed that higher reduction of BOD and COD was taking place in those recirculation sites.

Two full-scale studies were also reported. The first one was conducted in the Bornhausen Landfill to assess results on leachate treatment by a two-stage leachate recirculation scheme. High strength leachate collected from a young landfill cell was recycled and treated in-situ in an old landfill cell. "Thin-layer" pre-composting construction was also incorporated in the reactor cell. Data obtained from BOD and COD monitoring indicated that the two-stage scheme was very effective in terms of leachate strength reduction.

A second full-scale study was conducted in the Bornum Landfill where leachate was recirculated in a cell constructed with a pre-composted bottom layer. The results of a three year monitoring indicated that the COD and BOD of the recycled leachate dropped rapidly. The study showed that the combined technique was effective for in-situ leachate treatment.

However, neither study investigated the enhancement in terms of landfill gas production and waste stabilisation.

(v) Pacey (1989)

This study commenced in 1981 at the Mountain View Landfill in California, which appears to be the first full-scale demonstration project reported in the United States. Six cells, each containing approximately 6000 tons of refuse, were employed to evaluate the variables of initial high moisture content, leachate recirculation, and sludge with buffer additions. Methane production rate, gas yield, waste temperature and settlement were monitored as reaction indicators. As the main objective of the project was to look at landfill gas enhancement, no leachate analysis was reported.

The test results did not seem to follow the trends commonly demonstrated in laboratory simulations. For example, the cell tested with leachate recirculation together with buffer

2-33 Chapter 2 and sludge addition gave a lower measured methane production than the control cell (although the former recorded a higher waste temperature and a higher rate of settlement). The study suggested a possible reason for this unexpected outcome - incorrect gas measurement caused by a possible leakage in the containment system.

The monitoring records also indicated that unexpected surface water and ground water had entered all cells including the control cell. This fault invalidated the interpretation of the results regarding moisture enhancement. Nevertheless, the waste temperature variation, settlement patterns, and the chemical analysis of volatile solids content of the waste all indicated that the cells with enhancement treatments experienced a more active degradation than the control cell.

(vi) Watson (1987) and Vasuki (1988)

This case study reported three landfill cells in the Central and Southern Solid Waste Facilities Landfills in Delaware, United States, which had been practising recirculation since the early 1980s.

The results suggested that a combination of spray irrigation, surface infiltration and deep injection wells was required to achieve an efficient recirculation rate. However, the moisture distribution that could be achieved by the combined method was not discussed.

The analysis of leachate quality indicated that there was a general trend of enhanced biodegradation in terms of leachate strength reduction. However, no gas production, waste temperature or settlement data were reported.

(vii) Kilmer (1991)

This case study described the experience gained in a leachate recirculation landfill located at Worcester County in Maryland, United States. The landfill employed a recirculation system comprising spray irrigation, surface infiltration and recharge wells. No detailed performance of the recirculation system was reported apart from quoting that almost four million litres of leachate were recycled in a cell within a 16 month operation.

The study showed that the leachate BOD level had risen to some high values as expected. This reflected that the recycled leachate did accelerate the initial waste decomposition in a relatively short time. However, the development of the subsequent methanogenic phase was not studied. No gas production, waste temperature or settlement data were reported.

(viii) Scrudato and Pagano (1991)

This full-scale leachate recirculation trial was conducted at the Seneca Meadows Sanitary Landfill in New York, United States. The 45-hectare site consists of an older unlined section filled from the late 1970s to 1983. It was subsequently bordered by nine fully lined cells constructed later during the period between 1984 and 1990.

Recirculation was performed using leachate collected from the older cells into the new cells. The recycled leachate drained out from the new cells was then monitored for a 12- month period covering BOD, VFAs, pH and selected trace metals.

The study reported that within the 12-month monitoring period, the recycled leachate, initially of high strength, experienced a significant reduction in BOD and VFAs concentration and a concurrent increase in pH. It also reported a substantial decrease in trace metals concurrent with the rise in pH. The study then concluded that in-situ treatment was a viable option in reducing the more biodegradable organic and trace metals concentration in leachate. No data on gas generation were reported.

(ix) Miller et al. (1991)

The investigation commenced in 1990 at the Alachua County Southwest Landfill in Florida, United States. The objective was to evaluate leachate recirculation effects on waste stabilisation, landfill gas production, and leachate quality and quantity.

The experiment was conducted in a lined active landfill cell that had an advancing operation face. Two percolation ponds were used for recirculation, which covered only part of the cell. The rest of the area, which was not subjected to recirculation, was treated as a control zone. There was no physical isolation between the recirculation area and the control area as far as leachate and gas collection systems were concerned. The leachate

2-35 Chapter 2 collected in the drainage system accounted for both the recirculated area and the rest of the cell.

Water balance was conducted to assess the leachate percolation rate and leachate production. However, the study did not address the efficiency of the recirculation system in terms of moisture distribution.

Only 10 months of leachate quality monitoring records were available then. The limited monitoring results did not show any distinct trend in leachate composition. The pH was observed to remain close to neutral throughout the monitoring period even without buffer addition. This suggested that there was a good natural buffering capacity in the landfill system.

Chemical analysis of the waste was performed on samples collected from both the recirculated area and control area with an attempt to compare the degree of biodegradation. However, again due to the limited period of recirculation, the result was not conclusive.

Temperature of the waste was not monitored in-situ but based on the temperature measured in augered samples. The data showed that the landfill waste possessed a slightly lower temperature in the leachate recirculation area than in the control area, which was explained by a cooling effect caused by the recirculated leachate.

As the recirculation area was not isolated from the rest of the cell, the increase in gas production in a particular vent could not indicate conclusively that it was a result of leachate recirculation. But the percentage of methane in the landfill gas remained relatively constant from all vents.

As a whole, data from the study could not provide any conclusive results. This was due partly to the relatively short period of monitoring and partly to limitations in cell design and instrumentation. Also, without a properly designed control cell isolated from the recirculation zone, a quantitative comparison of the enhancement effects was not possible.

(x) Reynolds and Blakey (1992); Knox (1997)

This “Landfill 2000” project commenced in 1990/91 at the Lower Spen Valley Landfill in West Yorkshire, England. The principal purpose was to investigate the practicality of accelerating the stabilisation of domestic waste with an aim to look at the possibility of re- mining the waste and re-using the engineered landfill cells. In this case, in-situ leachate treatment and gas enhancement were not the primary objectives.

Reynolds and Blakey (1992) described the design and construction of the test cells. Two cells each containing 1000 tonnes of domestic waste were employed. Each measures 36m long and 23m wide. The cells were very shallow with an average depth of 1.4m and a maximum depth of 5m. Sewerage sludge (12% by wet weight) was added to both cells during infilling. Sewerage effluent (10% by volume) was added to one cell and the leachate produced was subsequently recirculated. The second cell was taken to be the control with no recirculation. The monitoring results reported by Knox (1997) can be summarised below:

(a) Methanogenesis became established within one year in both cells. This was largely attributed to the moisture and bacteria seeding associated with the sewerage sludge addition during infilling. In spite of low ambient temperatures (range of 7 to 17oC), high gas production rates were achieved in both cells.

(b) The recirculation cell produced landfill gas at a substantially higher rate (17 m3 per tonne per year) than the non-recirculation cell (8 m3 per tonne per year).

(c) The original intention of waste stabilisation was not achieved in three years, as revealed by the biochemical methane potential (BMP) measurements of waste samples. BMP of 76 was measured in the recirculation cell and BMP of 161 in the non-recirculation cell.

(d) Methanogenic activity developed very early in the basal drainage layers, which was believed to have been promoted by the biologically inert environment of the gravel drainage media, where there is no production but only consumption of organic acids.

(e) High methane production rates were recorded even though acetogenic leachate was still detected in the unsaturated waste.

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Knox (1997) also identified some weaknesses of the study. The two test cells were too shallow to develop optimum temperatures. There was uncertainty over the recirculation efficiency of the system employed. The reliability of the passive gas venting system without allowing excessive uncontrolled gas egress was in doubt. Similarly there was uncertainty over control of water ingress in terms of the integrity of the cell containment system.

(xi) Nilsson et al. (1995 a and b)

Six test cells were employed in the Spillepeng Landfill, Malmö, Sweden to investigate the optimisation of methane production. Each cell had a base area measuring 35m x 35m with the depth of waste varying from 2 to 9m. While the modification of waste composition was the primary factor investigated, leachate recirculation was also examined in one of the cells as listed below:

 Cell 1 - 30% domestic and 70% non-hazardous industrial/ commercial waste,  Cell 2 - as Cell 1 but with 5% grease trap sludge,  Cell 3 - high organic, moist waste from MSW sorting plants and restaurants,  Cell 4 - 100% domestic waste,  Cell 5 - 95% domestic waste and 5% grease trap sludge, and  Cell 6 - 100% domestic waste with leachate recirculation.

All six cells began producing gas almost immediately upon completion of filling, which took one year. The methane concentration of all cells reached 50% or more within the first six months and peaked at between 55% to 60% in just over two years.

The two cells containing a large proportion of non-hazardous industrial/ commercial waste (Cells 1 and 2) produced the highest total gas quantities. The other four cells (Cells 4 to 6) produced less gas and behaved very similarly to each other. There was no strong evidence to substantiate why the two cells with the non-hazardous industrial/ commercial waste produced methane at higher rates. There is also uncertainty regarding why the recirculation conducted in Cell 6 had no apparent enhancement compared with the control

(Cell 4). No details were presented regarding the performance of the recirculation in terms of leachate volume and moisture distribution.

The temperature monitoring data indicated that there was no apparent correlation between waste temperature and gas flow rate.

2.3.3 Summary of Developments in Leachate Recirculation

While the current literature provides a reasonable understanding of landfill degradation and behaviour, the preceding review illustrates the complexity of waste degradation enhancement. Performing leachate recirculation to enhance landfill stabilisation, particularly in a real-scale situation, does demand a comprehensive knowledge of the whole stimulation process. The complexity grows when other supplementary enhancement techniques are combined with leachate recirculation.

Summarising the literature, some general findings regarding leachate recirculation enhancement can be observed:

(i) Leachate Recirculation Alone - This generally only accelerates early hydrolysis and acid production, which results in a high volatile acids concentration in the leachate. If the natural buffering capacity of the system is insufficient, the acidic environment will inhibit the growth of methanogens and delay methane production (e.g. Walsh et al., 1986; Kinman et al., 1987). However, limited full-scale data tend to suggest that MSW landfills generally provide a good natural buffering capacity (e.g. Barber and Morris, 1984; Millers et al., 1991).

(ii) Leachate Recirculation with pH Neutralisation – As buffer addition helps to mediate the acidic environment caused by any vigorous acid production, it thus enables early onset of methanogenesis (e.g. Pohland, 1975; Tittlebaum, 1982;

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Leuschner, 1989). This is by far the most important supplementary operation if the natural buffering capacity is inadequate.

(iii) Recirculation with Methanogenic Leachate - Both small and large-scale studies have shown that there are benefits to be gained in the recycling of old methanogenic leachate in young landfills (e.g. Woelders et al., 1993; Scrudato and Pagano, 1991; Chugh 1996). Such benefits include rapid reduction in leachate strength and early methane production, which are attributed to the high alkalinity and the seeding of methanogens from the methanogenic leachate.

(iv) Leachate Recirculation with Sludge Addition – Co-disposing with anaerobically digested sewerage sludge generally serves the purpose of moisture enhancement, nutrient addition and microbial seeding. Both small and large-scale studies (e.g. Leuschner, 1989; Knox 1997) have produced positive results which suggest that it promotes early methanogenesis as well as higher gas production rates. However, one has to be cautious regarding the characteristics of the sludge as it has been demonstrated that, for instance, septic tank sludge exhibits a detrimental effect due to its low pH nature (Leuschner, 1989).

(v) Leachate Recirculation with Waste Shredding - Generally no conclusive findings have been reported to suggest that leachate recirculation combining waste shredding would provide a better enhancement effect than without shredding (e.g. Tittlebaum, 1982).

(vi) Leachate Recirculation with Nutrient Addition - Combining nutrient addition with recirculation does not seem to provide any further enhancement as nutrient deficit is generally not a limiting factor (e.g. Tittlebaum, 1982).

(vii) Leachate Recirculation with Temperature Control - Laboratory studies have indicated that the optimum temperature range for anaerobic degradation lies between 34 and 38 oC, with or without leachate recirculation (Mata-Alvarez et al., 1986). In terms of full-scale studies, there are insufficient data available. The original intention of one of the Brogborough test cells (Croft, 1991) was to induce higher waste temperatures by stimulating a brief period of aerobic composting

activity. However it was not successful, as there was no apparent temperature rise detected in the waste. Knox (1997) explained that the enhancement obtained from air injection was primarily caused by an improved moisture distribution due to leachate movement forced by the injected air. The full-scale experience reported by Nilsson et al. (1995a and b) suggested that there was no direct correlation between waste temperature and landfill gas generation rate.

(viii) Leachate Recirculation with Waste Modification – This covers the mixing of old anaerobically degraded refuse (e.g. Barlaz et al., 1987) or the use of a pre- composted bottom layer/ "thin-layer" operation (e.g. Stegmann and Spendlin, 1986 and 1989). Both have demonstrated positive effects on leachate strength reduction. The co-disposal of a high proportion of non-hazardous commercial/ industrial waste with domestic waste has also proved to be effective in promoting early methanogenesis (e.g. Nilsson et al., 1995a and b), which appears to benefit from the natural pH-buffer offered by the less readily degradable commercial/ industrial waste.

(ix) Leachate Recirculation at Different Rates – Laboratory research generally supports the view that a higher rate of recirculation provides a better anaerobic degradation (Hartz and Ham, 1983; Chugh, 1996). However, any secondary effects as a result of a high recirculation rate should also be considered. For example, the drop in waste temperature due to a large turn-over of cold leachate in the system may lead to negative results (Rovers and Farquhar, 1973). Generally in full-scale landfills, there is an uncertainty regarding the effectiveness of the recirculation system (e.g. Knox, 1997). Furthermore, because of the difficulty in managing a large volume of leachate in practice, no full-scale experiment has yet demonstrated an effective and high recirculation rate comparable to laboratory tests.

(x) Aeration of Leachate prior to Recirculation - Aeration may be used to pre-treat the leachate to reduce its high organic load prior to recycling. This is particularly beneficial if the leachate is to be recycled by direct spray irrigation onto landfill surface with vegetation cover. The pre-treated leachate would sustain vegetation growth by providing nutrient (Robinson et al., 1982). However, direct injection of aerated leachate into the waste has not been investigated. The effect can be negative

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as the increased oxygen content carried by the aerated leachate may upset the sensitive methanogenic bacteria.

There are other important observations particularly relevant to full-scale tests:

 When analysing the results of individual studies, one ought to consider all possible secondary variables, which the studies often did not describe in detail. For example, waste composition, natural buffering capacity of the waste, ambient temperature, nature of added sludge, and rate/ efficiency of recirculation all are important secondary variables that could vary substantially from test to test. This explains to a large extent why some studies have generated results that appear to be inconclusive or even contradictory to the results of other studies.

 It is important to employ a well-designed and adequately instrumented test cell. This point appears to be obvious but turned out to be a major limitation in many of the full- scale studies (e.g. Barber and Maris 1984 and Millet et al., 1991). The problems generally came from a lack of previous experience, inadequate planning, site-specific restrictions, and to a large extent, constraints associated with the limited resources committed to the test. These problems are less significant with small-scale tests.

 It is important to use a control cell to gauge against the test cells in order to quantify the net effects of the variable being investigated. This appears to have been overlooked by many of the studies (e.g. Miller et al., 1991).

 The integrity of the containment system is of paramount importance. The leachate within the test cell needs to be contained and there should be no unaccounted ingress/ egress of water and gas. The latter appears to have contributed to the uncertainty regarding misleading moisture budgeting and inaccurate landfill gas measurements reported in some studies (e.g. Pacey, 1989 and Knox, 1997).

 Generally there has been a lack of understanding regarding moisture movement and distribution within a recirculation landfill. Barber and Maris (1992) concluded in their study that “it is clear that any study of recirculation at other sites should consider

carefully the hydrology of landfill”. The research regarding landfill hydrology is discussed next.

2.4 LANDFILL HYDROLOGY

With the emerging “wet cell” approach, a better understanding of the hydrological aspects of landfill becomes more crucial, as the decomposition of waste, in-situ treatment of leachate, and production of gas are all closely related to landfill moisture level. However, compared with the attention that has been given to degradation enhancement, relatively little work has been conducted in the related hydrological research. These studies are discussed below.

2.4.1 Performance of Leachate Recirculation System

An ideal leachate recirculation system should aim to accomplish: an even and optimum moisture distribution; a sufficient feeding capacity to meet the required rate of recirculation; a minimum leachate hydraulic head on liner, and; minimal operational constraints including potential blockage and damage.

Various leachate recirculation methods have been reported in the literature. They generally cover surface irrigation (e.g. Barber and Maris, 1992), surface ponding (e.g. Miller et al., 1991), buried drip irrigation tubing or in-ground piping grid (e.g. Croft, 1991), sub-surface infiltration trench/field (e.g. Maier et al., 1995), and vertical recharge well (e.g. Morelli, 1992).

In general, surface application of leachate is not considered to be a favourable option due to environmental constraints associated with odour control and wind-blown misting.

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The slow percolation rate is also a limiting factor if a reasonable recirculation rate has to be achieved (Barber and Morris, 1992; Miller et al, 1991).

Sub-surface infiltration trench and deep injection well are the two most common techniques reported. Often different methods are combined with an aim to achieve the best system to suit site-specific factors such as climate, waste depth, compaction, final cover, and liner design (Vasuki, 1988; Harper, 1993; Maier et al., 1995; Reinhart, 1996).

Other more specific methods have also been proposed - for example, the "bio-trench" (Scrudato and Pagano, 1993) which is essentially a deep trench filled with drainage material serving as a leachate distribution header, a storage reservoir and an anaerobic bioreactor.

While many studies have reported the total recirculation volume achieved, there has been very little information regarding the effectiveness and performance of various recirculation techniques, particularly in terms of their influence zone, feeding capacity, spatial and temporal moisture distribution pattern, and induced hydraulic head on liner. Generally, field data are limited and design guidelines are unavailable.

2.4.2 Water Balance of Landfill Cells

The study of water balance of landfills can help to identify the significance of various water components. This is important for leachate management in terms of in-situ storage, treatment and disposal. It is particularly relevant to leachate recirculation volume control in wet cell operations.

Various water balance models have been reported in the literature although none was developed specifically for wet cell operations. Some examples are the WBM (Fenn et al., 1975), HSSWDS (Perrier and Gibson, 1981), LSM (Meeks et al., 1989), and the most popular HELP model (Schroeder et al., 1994; Peyton and Schroeder, 1988).

In these water balance models, the amount of natural infiltration into the refuse mass is generally obtained by subtracting the runoff, change in soil moisture content, and evapotranspiration from the total precipitation. However, in the computation of the timing of leachate formation, the actual process of moisture movement through the refuse is commonly not taken into consideration. For example, the WBM and HSSWDS models do not account for the period of time required for the refuse to be brought up to field capacity. Other models use a simplified approach to account for the transient unsaturated flow-through effect, e.g. the HELP model uses a quasi-unsaturated flow solution by simplifying the highly non-linear unsaturated hydraulic conductivity function to have a linear relationship with moisture content.

Despite the popularity of these models, there appears to be inadequate field data to calibrate and validate them, and particularly to test their suitability for a certain climate.

For wet cell water balance, a scheme has been proposed by Baetz and Onysko (1993) to specifically address the storage volume sizing for leachate recirculation management. The methodology has been based on water balance and some simplified assumptions, which neglect the lag time required for percolation, the antecedent moisture conditions, and the boundary condition effects (e.g. surface ponding and evapotranspiration). Again, the approach has not been tested with large-scale operations.

2.4.3 Hydraulic Properties of Municipal Solid Waste

The saturated hydraulic conductivity of in-situ MSW was investigated by Oweis et al. (1990) in a full-scale pumping test. A correlation between hydraulic conductivity and shredding/ compaction of waste was reported by Fungaroli et al. (1979) and Canziani and Cossu (1989). The unsaturated hydraulic conductivity/ suction/ moisture characteristic functions of MSW were studied by Korfiatis et al. (1984) and Ahmed et al. (1992). Field capacity of waste, which is an important feature describing moisture storage characteristics, has been discussed by various researchers (e.g. Canziani and Cossu, 1989).

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Blight et al. (1992) reported a full-scale study to investigate the moisture states of three landfills. Moisture measurement by direct sampling and gravimetric method was conducted by drilling 1.2 m diameter holes through the landfills using a pile-augering machine and lowering an operator wearing a full-face diving mask down the holes to hand pick waste samples.

In general, while some hydraulic parameters of MSW have been proposed in the literature, studies involving field validation of the parameters have been limited.

2.4.4 Saturated/Unsaturated Flow in Municipal Solid Waste Medium

Attempts have been made to develop saturated/unsaturated flow mathematical models to predict moisture movement in MSW landfills. Most of the studies were conducted to simulate infiltration using one-dimensional vertical flow model (Straub and Lynch, 1982; Korfiatis et al., 1984; Demetracopoulos et al., 1986; and Baldi et al., 1993). A two- dimensional model has also been reported by Ahmed et al. (1992). While still at its infant stage, the literature tends to suggest that modelling MSW waste as a porous medium provides a reasonable tool to predict its moisture movement. Again, very limited field data have been available to validate these mathematical models.

2.5 RESEARCH NEEDS

Despite all the potential benefits of the new landfilling concept, the literature indicates that the many uncertainties and practical constraints associated with full-scale operations still hinder its implementation. To overcome the impediments, the following two priority areas have been identified, which are addressed in the thesis by the four specific study tasks as defined in Section 1.2.

(i) Full-Scale Bioreactor Behaviour Studies

While smaller scale studies can allow the flexibility to study a large number of operational variables under controlled conditions, it is obvious that they cannot accurately simulate the

2-46 Literature Review natural degradation processes taking place in full-scale landfills due to scale effects. For example, almost all of the small-scale studies worked with shredded waste but very rarely was the same treatment given to the MSW in full-scale landfills. It also appears that the natural pH buffering capacity in a real landfill environment generally performs far better than that in a bench-scale simulator. This is possibly a result of the presence of soil covers and a more diversified source of waste material (Sections 2.33 (viii)). Also, the kind of recirculation rate and uniformity of moisture distribution that can be achieved in a laboratory test cannot be easily obtained in a full-scale landfill cell (Section 2.3.3 (ix)).

The above concerns together with the limited full-scale research work conducted so far reflect the need for more full-scale experiments. They should be implemented with an aim to obtain qualitative data to confirm the real benefits associated with bioreactor landfilling. The full-scale experiments should also be used to identify existing operational constraints. This is especially true for countries such as Australia where no full-scale experience is available and certain test variables (e.g. climate and waste composition) may vary significantly from cases reported in other countries.

(ii) Landfill Hydrological Studies

An optimum leachate recirculation strategy demands a good knowledge of the performance of the recirculation system in terms of its influence zone, feeding capacity, spatial and temporal moisture distribution pattern, and induced hydraulic head on liner. The same is required to understand and predict moisture flow in MSW. Unfortunately there is still not enough research in this area.

Water balance studies should be incorporated in all future full-scale recirculation experiments. This would help to identify the significance of various water components in the wet cell, thus enabling an efficient moisture budgeting.

Another area that certainly demands research attention is the methods of in-situ moisture measurement of MSW. While this is an essential tool required for most hydrological studies, surprisingly little information is available. The measurement of moisture content can be achieved by a direct gravimetric moisture determination of

2-47 Chapter 2 waste samples. However, the availability of a rapid, non-destructive, and easily repeatable indirect method would be highly desirable, particularly in a situation where a large spatial moisture measurement of a repetitive nature is required.