A Case of Study About the Influence of Organic Matter in Municipal Solid Waste Settlement

Municipal solid waste (MSW) settlement can be understood as volume reduction of the waste mass disposed in a landfill. Such phenomenon is mainly the result of organic matter degradation. Settlement can also be generated by MSW distortions, particle rearrengements, and other factors. These factors can be accurately analyzed if studied in an MSW experimental cell once it simulates the landfill behavior in a known and controlled way. This study aims to analyze the behavior of an experimental cell filled with MSW and how the biodegradation can influence the settlement over time and depth. The methodology was composed of waste sampling, experimental cell construction and filling, volatile solids analysis, and settlement measurements. The MSW gravimetric and volumetric analyses were performed in order to quantify the amount of organic matter contained inside the experimental cell. The tests indicated that the amount of organic matter drastically decreased in all MSW layers, and settlement varied according to the depth of the experimental cell due to mechanical factors and organic matter consumption.


Introduction
Landfill settlement is mainly a result of organic matter loss from the Municipal Solid Waste (MSW) disposed.It can also be generated by MSW rearrangement, mass deformation and other factors.The settlement is mainly generated by the degradable organic matter (OM) present in the waste mass composition.The complex biological degradation processes with numerous metabolic pathways can create MSW settlement.However, the waste biodegradation depends on environmental factors inside the landfill, which directly influence the settlement velocity and magnitude.According to Wall & Zeiss (1995), MSW settlement may range from 25% to 50% of the initial waste height.However, the literature does not cover many topics involving the influence of organic matter in waste settlement.
Most of the MSW constituents disposed in landfills is organic matter (especially in developing countries), so it is important to determine how much the organic matter influences the occurrence of settlement.The speed of organic matter degradation and hence the speed of settlement are issues that have to be analyzed so that the mechanical behavior of landfills can be understood.
These issues can be better analyzed if studied in a controlled environment by using a MSW experimental cell which can simulate the behavior of a full-scale landfill.In Brazil and all around the world, there are studies being de-veloped with experimental cells filled with MSW taking into account the peculiarities of different regions.
This paper aims to study the behavior of organic matter inside an MSW experimental cell regarding to its biodegradation and how it influences the settlement over time and depth.

Studied Literature
The organic matter in the waste mass inside the landfill is degraded through two processes: aerobic and anaerobic digestion (Alcantara, 2007;Araujo et. al., 2009).The anaerobic process is the most significant in terms of biodegradation because the oxygen inside the landfill runs out quickly after the landfill impermeabilization (Moreda, 2000).Leite (1997) states that the process of anaerobic biodegradation of MSW organic matter is quite complex.The physical and chemical composition of MSW and the presence of significant cellulolytic material demand some time to complete the bio stabilization of organic matter.According to Modesto (2002), the decomposition rate of anaerobic processes is slow and it requires a much longer time for the organic matter stabilization than in aerobic digestion.In the anaerobic digestion, most of the biodegradable organic matter is converted into biogas, and only a small portion of organic matter is converted into microbial mass.Melo (2003) and Monteiro et. al. (2006) suggest that anaerobic digestion of MSW occurs because of the succession of different types of microorganisms, including the fungi, that can accelerate the conversion process of some compounds, such as cellulose, lignocellulose and less complex materials.

Settlement in municipal solid waste landfills
The deformability of the waste disposed in landfills is a very important aspect related to the landfill behavior.According to Nascimento (2007), settlement studies are important for the evaluation of landfill components, such as cover soil layer, leachate and biogas drainage systems (Kudrna, 2009;Sharma, 2007).
MSW landfills settlement is defined as a vertical displacement of the landfill surface caused by waste or cover layer weight, external loads and, mainly, the waste complex biological degradation processes that results in leachate and biogas production.Melo (2003) states that primary and secondary settlement can happen simultaneously and the waste mass rearrangement happens because of the successive gaps collapse.Melo (2003) and McDougall et al. (2004) brought some new Acknowledgments in the study of waste settlement.They affirm that initial settlement in landfills is very intense because of the initial amount of organic matter.The rate of settlement decreases with time as the number of gaps decreases too.Organic matter degradation turns solid particles into liquids and gases, so the gaps once filled with the solid phase are now occupied by the liquid and gas phases.Initially, there are successive gaps enlargements due to the waste degradation, followed by collapses of these gaps.After a period of intense settlement, there is a period of zero settlement, when there is no settlement occurrence.However, once the weight of the waste mass is large enough to create gaps collapse, a new phase of settlement occurs.
It is difficult to define if the settlement is primary (first 30 days) or secondary.Melo (2003) says that both (primary and secondary) settlement occur simultaneously because as soon as the waste is disposed in the landfill, the microorganisms colonization takes place, causing the mass loss.

Gravimetric and volumetric composition of municipal solid waste
The gravimetric and volumetric characterization of MSW is an important tool in waste management and it indicates the amount of weight and volume that each type of waste occupies in the landfill cells (Pereira et al., 2010).According to Pereira et al. (2010), knowing some aspects (weight and volume) of each MSW constituent individually is important because it shows the amount of existing biodegradable compounds within the waste mass, which contributes to the correct sizing of drainage systems and leachate treatment systems and to the evaluation of biogas generation potential.
The gravimetric composition of MSW is important to understand the waste mass behavior, and it expresses the presence of each component related to the total weight of the waste sample (Jucá & Melo, 2000).Both gravimetric and volumetric compositions are linked to the mechanic behavior of waste, and both are affected by the specific weight and compression of that waste.El-Fadel & Khoury (2000) states that high initial specific weight values associated with a high magnitude of waste compression reduces both primary and secondary settlement rates.

Development of the research
An experimental cell was built on Universidade Federal de Campina Grande (Federal University of Campina Grande by English translation) campus, in the city of Campina Grande, state of Paraíba, Brazil.The experimental cell or lysismeter filled with MSW was projected as an attempt to simulate and analyze a landfill behavior and how it loses mass due to waste decomposition.The experimental cell allows controlling the acceleration of the waste decomposition.

The experimental cell construction
The experimental cell was built with handmade bricks, and it had a diameter of 2 m, height of 3 m and volume capacity of 9 m 3 .The instrumentation was made with surface settlement gauges (plates A and B), thermocouples, piezometers (to measure the liquid level) and biogas drains (Fig. 1).For cover and bottom soil layers, a low permeability soil was chosen (velocity: 10 -8 m/s).

Waste collection, sampling and lysimeter filling
The procedure used in order to perform the waste collection and sampling was recommended by the ABNT: NBR 10007 (2004).The waste was collected and its homogenization was performed based on LIPOR's (2000) methodology -adapted by Leite (2008) and Pereira et.al. (2010).The determination of the MSW gravimetric and volumetric composition was based on the same method previously listed.
The MSW was collected from three different neighborhoods in Campina Grande.The criteria used to choose neighborhoods were the social class.This way, neighborhoods representing the high, middle and low classes were chosen (one neighborhood for each class).After the collection, the MSW was homogenized and quartered (according to ABNT NBR 10007, 2004) so that a final sample was obtained to fill the experimental cell.

Field parameters monitoring and laboratory trials
After filling, MSW samples from the lysimeter were collected in order to determine their mechanical, physical, chemical and microbiological parameters.Table 1 shows the physicochemical parameters of the MSW in the lisymeter monitored over time.

Physical characterization of the MSW deposited in the experimental cell
The physical characterization of the MSW was composed of gravimetric and volumetric composition tests and MSW density determination.For the gravimetric composition, Table 2 shows that the MSW has about 66% of organic matter, indicating a high percentage of waste mass deformation that results in large settlement values since the biodegradation increases with the amount of putrescible material in the MSW composition (Dixon & Jones, 2005).
Table 2 also shows that the amount of plastics reached a value of 11% of total waste weight.Such percentage of plastics reflects their low density that usually contributes to a lower value on the total weight compared to organic matter.The volumetric composition for loose and compressed MSW shows a higher percentage of plastics than the one found in gravimetric composition.Grisolia et al. (1995) states that compressed materials such as metals and plastics can release retained liquids or gases, causing an apparent decrease in their volume.The organic matter releases its liquid, so the volume variation is even higher.Such observation is directly related to settlement creation and water content within the waste mass.
The volumetric composition of organic matter for compacted and loose waste was 37% and 38%, respectively (Table 2) .This data could initially lead to the conclusion that settlement due to the degradation of organic matter would also be around that value.However, McDougal et al. (2004) andFirmo et al. (2010) suggest that not all the organic matter is biodegradable and Melo (2003) affirms that inhibitors products intrinsic to the degradation of organic matter can exist in the waste mass disposed in landfills.Those elements can decrease the metabolic activity of microorganisms and, consequently, the waste decomposition.
In regards to plastics, Fucale (2002) reports that these materials can work as a reinforcement matrix of the landfill structure.Plastic retards the degradation of organic matter  The volumetric composition of loose and compacted waste for paper and cardboard, composite, sanitary, textiles, glass, metals and others was less than 40% of the total waste volume.Nevertheless, these materials use to have very high values, according to studies conducted by Catapreta & Simoes (2008).
Table 2 also shows that loose and compacted waste densities for the organic matter was 270 kg/m 3 and 400 kg/m 3 , respectively.The density values of the compressed waste is greater that the non-compressed waste ones because after compression, the volume is reduced, so the density increases.
Loose and compacted waste density results were, respectively, 400 kg/m 3 and 700 kg/m 3 , which is in accordance with the technical literature (Leite, 2008;Landva & Clark 1990;Kaimoto & Cepollina, 1996).These authors report that a compressed density of 700 kg/m 3 is excellent for mechanical operation in landfills.

MSW settlement over depth
The plates to measure the MSW settlement were arranged according to the specified depths showed in Table 3.The largest MSW deformations occurred with the plate 1 (Fig. 2).Plate 1 is located in the center of the experimental cell, in the top layer, right before plate 2 (from top to the bottom).The settlement values measured by plates 1 and 2 were 0.37 m and 0.40 m, respectively, by the end of the monitoring time.
Surface plates 'A' (on the left side of plate 1) and 'B' (on the right side of plate 1) showed settlement values of 0.35 m and 0.33 m, respectively.Such results were predictable since these plates were 2.00 m far from the bottom layer.
Plates 1 and 2 had higher rates of settlement than surface plates because the central region of the cell has less influence of the friction caused by the lysimeter walls.The walls interact with the stuck waste by impeding its consoli-dation.Melo (2003) and Alcantara (2007) found the same results when studying small-size experimental cells and full-scale cells as reported in their studies.
The settlement values observed in the experimental cell represented a typical behavior of a full-scale cell once the settlement magnitude decreased over time as it is usually observed in landfills (Fig. 2).Mariano (1999), Melo (2003) and Monteiro (2003) found out that the settlement decreases as the depth increases in full scale MSW cells.Such behavior occurs since the waste in the deepest layers are more influenced by the compression due to the weight of the top layers, so as the plates on the surface have more available space to move downwards.The same behavior was observed in this study, as it is seen in Table 4.The plate 6 did not move because it is located in the bottom layer already, so the high waste compaction retarded the settlement in that layer.
The largest settlement values occurred in the early months after waste disposal.The highest settlement speed rates also occurred in these days (first 90 days after closing the experimental cell).During the first 30 days, the settlement occurred because of the loads imposed by the waste mass and compacted cover soil weights (Fig. 2).According to Valozer (1989), in the early days, the microbiota of the waste mass is still adapting itself to the environment in the experimental cell, so the settlement created is mostly due to  mechanical factors.After this phase, settlement was generated by the degradation of organic matter from the MSW.

Settlement due to MSW biodegradation
In regards to the settlement fraction related to MSW biodegradation (Table 5), the surface plates 'A' and 'B' showed a waste mass deformation of 71% and 73%, respectively.For plate 1, 77% of settlement was originated by degradation of organic matter, and for plate 2, 70% of settlement was created due to biodegradation.The waste layers in which plates 3, 4 and 5 are located had the largest amount of vertical deformation caused by biodegradation.
As the depth increased, a higher biodegradation settlement percentage was found.Immediate settlement oc-curred during the experimental cell filling process when the lower waste layers were constantly compacted by the upper layers.The settlement during the filling process was not recorded.Only when the experimental cell was closed, the settlement measurements started, so the results taken from the first day of measurement does not represent the magnitude of the immediate settlement in the lower waste layers.This way, the biodegradation settlement (non-immediate settlement) will always be proportionally greater in deeper waste layers because immediate settlement practically ended by the time that settlement measurements started.
The studied literature only makes observations about the general behavior of MSW settlement.However, the settlement of the organic fraction of the waste is not thoroughly analyzed.In Brazilian cities, the organic fraction corresponds to the highest percentage of MSW composition (by terms of weight and volume).In order to predict the lifespan of a landfill, it is necessary to take into account the waste degradation time for each layer considering also the immediate settlement.

MSW settlement velocity
The settlement velocity (Fig. 3) shows how considerable were the MSW settlement occurrence on the first 30 days after the experimental cell closing.The settlement rate was up to 10 times higher in some of the plates during the first 30 days if compared to the other days.However, the settlement speed decreased after 90 days.Plate 5 (Fig. 3) showed zero settlement speed after 330 days of monitoring which indicates that either the organic matter is unavailable to the microorganisms by excessive compression, or there is an accumulation of liquids at that region, retarding the settlement.Leite et al. (2007) affirm that excessive compression may inhibit the degradation of organic compounds.For the other plates, the settlement is decreasing, but it will still take place for a while because there is still organic matter to be degraded inside the experimental cell.

Settlement vs. organic matter
The value of 66% of the initial organic matter in the MSW composition indicates that the experimental cell is filled with approximately 2,900 kg of organic compounds.However, not all the organic material is biodegradable.The degradation of organic material results in settlement, so it must be measured since it will determine the landfill lifespan (Melo, 2003;Monteiro, et al., 2006).
The rate putrescible organic matter within the waste mass per volume of the experimental cell is 400 kg/m 3 , which indicates a total of 1,700 kg of highly degradable organic compounds.The results obtained by many researchers around the world show that even the putrescible waste fraction might take a considerable amount of time to be completely degraded.Within the waste mass, there are materials that are aggressive to the microorganisms due to the release of toxic substances, the waste heterogeneity and the different temperatures and pH ranges (Mcdougall et al., 2004;Melo, 2003).Such fact explains why many landfills produce biogas for decades after their closure.Firmo et al. (2010) states that, despite the volatile solids (VS) amount of 87% for plastics, 64% for rubber and leather, 98% for textiles and 81% for paper/cardboard, these materials are considered moderately or slowly biodegradable once they also contain a high fraction of non-biodegradable substances.
Regarding to the total content of organic matter in the studied experimental cell, the settlement account would be around 1.33 m, which corresponds to 66% of total reduction of waste height.That did not occur because not all the organic matter is biodegradable, and the waste disposal time (427 days) is still too short if compared to full-scale landfill cells (20-30 years).According to the results found in the monitored experimental cell, the settlement was much smaller than the one suggested by theoretical methods when considering the easily degradable compounds.
Most studies about settlement do not take into account that waste deformation is divided into: i) immediate settlement (30 days after MSW disposal); ii) settlement created by degradation of putrescible organic matter; iii) settlement due to hardly degradable organic matter; iv) settlement originated from the rearrangements of inert waste particles.All these types of settlement occur in different periods and they generate a high volume reduction.
The Table 6 shows the quantity of initial organic matter for each waste layer of the experimental cell.
According to Table 6, most of the settlement should happen in the bottom layer, but this is contradictory with the results showed before.The settlement in the bottom layer was the lowest one even when there was more organic matter available in that layer.

MSW layer
Total quantity of OM (kg)

Top layer 640
Intermediate layer 1,000 Figures 4 to 6 show the existent relation between settlement and volatile solids decreasing by the action of microorganisms over time.In the top layer (0 to -1.57m), vertical deformations distributed and verified by the settlement plates have a very similar displacement pattern (Fig. 4).As it was already expected, large vertical displacements occurred when the amount of organic matter was high.However, settlement still happened when almost all the organic matter was already degraded (after 210 days), and this is probably because of the settlement produced by the inferior layers.
Concerning the intermediate layer (Fig. 5) of the experimental cell (-1.57m to -0.89 m), settlement data had a better correlation with the organic matter (VS) decreasement over time.There was a high occurrence of settlement on the first 60 days of measurements while the volatile solids content was between 50-70%.After this period (between 60 to 120 days), there was organic matter degradation due to the increasing of gaps, but such fact does not create considerable vertical displacements.The collapse of these gaps will only happen when they cannot support the upper loads.
There is still occurrence of settlement after 120 days, but it is less intense because of the continued consummation of organic matter and the association with settlement of the bottom layer.After 120 days, there was a gradual decreasing of organic matter from 50% to 40%.On the first 60 days, there was a vertical displacement of 0.115 m, which is much accentuated if compared with the later settlement occurred in the following 307 days (day 120 to 427) of monitoring.
Figure 6 shows the settlement that happened in the bottom layer (-0.89 m to 0 m).The bottom layer has the plates 4, 5 and 6, but only plates 4 and 5 indicated settlement occurrence and, as the organic matter was degraded, the settlement occurred.The initial settlement (like in the superior layers) were also greater during the first days of  monitoring when there was a higher concentration of volatile solids.
For all the bottom plates, there was a period of zero settlement between 90 and 120 days.After this period, there was another settlement phase between 120 and 300 days, followed by another zero displacement phase.After 300 days, there was no settlement occurrence on plate 5, which is probably due to the excessive compression since there was still organic matter left in this plate.
During the volatile solids monitoring process, it was observed that, in the bottom layer, the VS values randomly increased instead of decreasing.Such result is due to the influence of the accumulated leachate, which is composed of organic matter and other constituents.On the first 150 days, the volatile solids were related to the settlement once the deformation patterns were inversely proportional to the volatile solids decreasing.
As it is seen in Table 7, the MSW top layer had 600 kg of organic matter transformed into liquids, biogas and energy, which corresponds to 93% of organic matter volume reduction.The intermediate layer presented a reduction of 40%, which is around of 400 kg of transformed organic matter.For the bottom layer, an organic matter volume reduction of 38% occurred.Table 7 also shows that the initial amount of organic matter inside the experimental cell was 2,900 kg and the final amount was 1,440 kg, which means a total organic matter reduction of approximately 49%.
Therefore, there is a height reduction of about 0.66 m, which is 33% of the initial height.
Overall, the results of the organic matter decomposition in this study were greater than the ones for full-scale cells taken into account the same monitoring time (Melo, 2003;Monteiro et al., 2006).It indicates that cells with smaller dimensions might accelerate the organic matter decomposition process once they have a bigger surface area that facilitates the exchange of energy and matter with the environment.
Table 8 shows that most of the settlement happens in the intermediate layer, although this is not the thickest layer.The intermediate layer had greater vertical deformation due to the loads imposed by the top layer and the loss of liquids to the bottom layer.Even with the intense settlement in the intermediate layer, most of the settlement created by organic waste degradation happened in the top layer due to the influence of the external environment.

Conclusions and Suggestions
The results obtained through physical characterization showed that organic matter represents most of the MSW (66%) from the city of Campina Grande.This large amount of organic matter is probably the reason for the high MSW settlement values found in the experimental cell studied in this research.
The amount of organic matter decreased in all MSW layers of the experimental cell.The MSW settlement oc-  curred due to organic matter degradation and mechanical factors.The top layer had the best results regarding the settlement and degradation of organic matter, as it is more influenced by the local environmental conditions.
Figure 1 -Top view and vertical cut of MSW experimental cell.
Figure 2 -MSW settlement values in different lysimeter depths over time.

Figure 3 -
Figure 3 -Settlement velocity of the waste layers.

Table 1 -
Physicochemical parameters monitored over time.

Table 2 -
Values obtained by the physical characterization of MSW.

Table 3 -
Position of settlement plate inside the experimental cell.

Table 4 -
Final MSW settlement values for each plate located in the experimental cell.

Table 5 -
Settlement percentage due to MSW weight and organic matter degradation.
the lowest settlement rate over time if compared to the other plates.

Table 6 -
Quantity of organic matter (OM) by MSW extract.

Table 7 -
Organic matter (OM) initial quantity and transformation into other constituents.

Table 8 -
Height and MSW amount variation over time and depth.