Laboratory scale landfill simulation tests

Landfills are huge heterogeneous reactors hosting several chemical and biochemical processes inside which result is hardly controllable and hardly monitor-able with precision.

Their complexity is mainly due to their long lifespan (some decades), to the several simultaneous biochemical reactions happening and to the heterogeneity of the materials deposited inside. All these factors increase the variability in humidity content, hydraulic conductivities and density, causing the formation of several interconnected microenvironments in the waste body. Despite all these complications, landfills require a periodical analysis of emissions, a continuous monitoring of internal conditions for their management and some prevision tools for their initial design and for calibrating possible in situ interventions.

Emissions monitoring is the general way to control landfill running conditions. In some cases, probes are installed inside the landfill body to have a continuous check of the processes of interest. Intense constant monitoring is particularly useful having ongoing processes or treatments, like biogas production and extraction, leachate recirculation, air injection, etc., which requires to be periodically calibrated according with the reactor performances. In particular, oxygen, methane and carbon dioxide in biogas are frequently analyzed, being the faster and the most abundant emissions of a Municipal Solid Waste (MSW) landfill. The evaluation of this compounds reciprocal percentages in biogas can immediately clarify macroscopically the landfill biochemical status. Similarly, leachate analysis are useful for monitoring biochemical processes in landfill: their trend gives information regarding the efficiency of the treatments, the residual substrates and nutrients presence in the waste body. Uncontrolled emissions monitoring is made sampling groundwater from wells placed around the landfill, according with groundwater flows itself.

While landfill routine management needs a constant monitoring of the parameters of concern, its design and the in-situ intervention calibration requires more sophisticated tools.

Lab-scale simulators are able to run the whole landfill life in a highly controllable reactor and in a “laboratory” time. These reactors are commonly used tools in chemical engineering and can be efficiently used for landfill simulation: they have different dimensions, operation time and design according with the aspect they want to simulate. Only some of these tests have a

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standard routine or an associated international method to be follow; however, some macro categories can be distinguished.

 Leaching tests consists in several methodologies used for estimating the potential mobility of compounds in a solid sample. Each test is calibrated specifically on the phenomena to simulate or to the compound to search for. Leaching test are very fast analysis, useful for evaluating macroscopically the total potential liquid emissions of a solid material. Biochemical processes are not so fast to be appreciable with these methods. The most used in waste management is an international standard methodology (UNI EN 12457-2) where the sample is milled at 4 mm diameter, distilled water is added until L/S reached 10 L/kgTS, the produced mix is turned for 24 hours and then filtered at 0.45 µm.

 Batch biochemical tests are a huge category of laboratory scale tests studied for evaluating the biochemical characteristics of the sample in a short time. Among them, respiration tests are focused on estimating the biochemical potential activity, measuring the oxygen consumed by a sample in a fixed amount of time. Anaerobic fermentation tests are used for lab-scale simulating the methane and the hydrogen production, evaluating the total convertible fraction of a sample and the kinetics of the process (Table 1.3.1).

Table 1.3.1: Stability Indexes overview.

Biological Stability Index

Characteristics

BOD5 – Biological Oxygen Demand

BOD5 is a static biochemical index for liquid samples, worldwide used. It measures the oxygen consumed by the sample in 5 days at the standard temperature of 20°C (Cossu et al., 2012).

BOD5/COD ratio BOD5/COD is a stability index for liquid samples derived from the BOD5. The advantage is that the ratio with COD is not influenced by the dilution effect (Cossu et al., 2012). In a landfill leachate, a BOD5/COD < 0.1 means an advanced methanogenic phase with low residual biodegradable matter content (Van Vossen et al., 2013; Sekman et al., 2011) SRT – Static

Respiration Test

SRT are respiration indexes commonly used for checking the residual biodegradability of compost or other solid samples. SRT measures the oxygen consumed by the sample without replacing it once consumed (STATIC) (Komilis and Kletas, 2012).

RI4 – Respiration Index

IR4 measures the oxygen consumption of a sample in 4 days, replacing the oxygen once consumed for maintaining its concentration constant for the whole test (SEMI-DYNAMIC). It is used for both solids and liquids. In a waste sample, IR4 < 2.5 mgO2/kgTS means low biodegradability (Laner et al., 2012).

DRI – Dynamic Respiration Index

DRI test measures the oxygen consumption of a solid sample continuously injecting new air for the whole test (DYNAMIC) (Cossu et al., 2001). This system has the advantage to be able to use up to some kilos of sample for decreasing the heterogeneity effect.

GB21 – Gas Generation Sum

GB21 index measures the anaerobic biogas production of a mud sample in 21 days at the standard condition of 40 °C. The index results can be correlated with the respiration test ones (Cossu and Raga, 2008).

BMP – Bio Methane Potential

BMP tests measures the total methane production of a sample, until reaching the endo of biochemical process (Esposito et al., 2012). This test is longer than GB21 but can supply data useful for the evaluation of the reaction kinetics and the total methane expectable.

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 Columnar reactors (Figure 1.3.1) are useful for simulating the whole landfill life as well as a single part of it or an in situ treatment. According with the initial project, a test can be run in some months (Cossu et al., 2003) or require many years to be completed (Hrad et al., 2013; Ritzkowski et al., 2006; Cossu et al., 2016). Reactor body is a sealed cylinder generally made of steel, glass or plastic, which can contain 10-50 kg of waste depending on compaction. The laboratory dimensions allow to place them directly inside a building and to achieve a high control level on processes and emissions. In particular, water and air injection or extraction can be efficiently managed, as well as the temperature. This tool is described in Chapter 2.

 Large lysimeters (Figure 1.3.6) are reactors conceptually similar to columnar ones but working with more than 500 kg of waste. This increased dimension is useful to simulate the phenomena influenced by waste heterogeneity inside waste body: water flow preferential paths, impermeable lens, effects of deepness as well as differences in compaction, interstitial gas diffusion, oxidative conditions and internal concentration of compounds. These systems have the disadvantage to increase the run time to even to a decade. This tool is described in Chapter 3.

Leaching and biodegradability test outputs generally are “maximum potential values”

useful for calibrating mathematical models and comparable directly to other test results or real landfills monitoring data. Columnar reactors and lysimeter tests are complete simulations of landfill processes so their timescale must be readapted to the real application one to have a correct interpretation of the results obtained. This operation is complicated by the order of magnitude of difference (in both dimensions and time) between the real landfill and the laboratory equipment. The main control variables of any landfill process are time and leaching, which are only partially correlated while acting simultaneously on the same substrates (Hjelmar O. and Van der Sloot H., 2011). As consequence, one factor cannot be freely substituted by the other, without consistent uncertainty: leachate emissions remove substrates otherwise available to microorganisms while biological processes cannot overlook liquid movements to work properly (Ritzkowski et al., 2013). The lag-factor between lab and field performance depends also on the differences between calculated and real parameters, systems efficiency, temperature, moisture and the higher heterogeneity of landfill respect to a laboratory equipment (Hrad et al., 2013).

The easiest linking method consists in progressively calibrate the reactor timescale with the real landfill time. However, generally, better comparison results are obtained working with the liquid variable instead of the time variable.

The probably most used methodology is the Liquid-Solid (L/S) ratio (L/kgTS) where: “L is the cumulated amount of water that at any given time has percolated through the system and S is the dry weight of waste that has been percolated” (Hjelmar O. and Van der Sloot H., 2011). L/S scale is build “confusing” the time with the corresponding L/S and using it for compare different reactors and/or real landfills too. This tool is also used backwords for laboratory scale tests design (Cossu et al., 2016; Morello et al., 2016). In mathematical

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models, more complicate equations are used, correcting directly the time with a factor influenced by L/S (Hjelmar O. and Van der Sloot H., 2011).

A tracer in leachate (Chloride as instance) can be used to normalise liquid emissions both in real landfill and in lab-scale equipment. In this case, timescale is change with the correspondent tracer emission scale (Fellner et al., 2009). Similarly, rationalise a generic compounds concentration with the tracer one is a procedure used for comparing leaching status of the landfill (Ritzkowski et al., 2016).

Whatever method is chosen, the direct comparison between lab-tests and field data must be always managed with caution and experience. In addition, data obtained from these tests can be elaborated for closing a mass balance, testing the efficiency of a new concept and used for evaluating kinetics to design pilot scale applications.