The experiments in Chapter 2 were carried out by using six identical lab-scale columnar reactors (Figure 1.3.1). The equipment body is a Plexiglass® (polymethyl methacrylate) transparent pipe height 106 cm, with a diameter of 24 cm and a thickness of 0.5 cm; the total internal volume amounts to 48 L. The bottom of each reactor is pasted to a HDPE trolley support and the top is closed with two sealing rings fixed to a HDPE cap, screwed to the main Plexiglas body (Figure 1.3.2). The reactor loading can be done from the upper side, manually compacting the waste.
Figure 1.3.1: Picture of columnar reactor equipment with thermo-regulation suits (B) and without (A).
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Figure 1.3.2: Technical sketch of columnar reactor equipment with graphical description of the regulation systems.
A 10 cm thick gravel layer (Ø 20-30 mm) is placed at the bottom of each reactor as a drainage layer to facilitate the extraction of leachate. Another 8 cm gravel layer is placed at the top of each column to increase the distilled water and recirculated leachate distribution (Figure 1.3.2). Leaving 5 cm of headspace between the cap and the top gravel layer, the height available for waste sample is 83 cm and the maximum waste volume is reduced to 38 Litres.
For guaranteeing structural integrity of the Plexiglas pipe, the sample compaction cannot exceed 0.8 kg/L; as consequence, this kind of reactor can contain at maximum 30 kg of wet waste.
The reactors allows a high control level of the process utilizing a heating system for temperature control, a leachate system for moisture management, a gas extraction system and an air injection one. All these tools can be facultative used in line with test design and process necessities. The upper cap of the reactor body is equipped with three valves providing for the introduction of air, sampling and extraction of gas as well as the introduction of liquids, while under the bottom another valve is placed for leachate extraction, by gravity.
48 Heating system
The exothermic biological processes happening inside a landfill generate sufficient heat to maintain mesophilic conditions (35-45 °C) during the anaerobic phases and to reach even 60-70 °C during the aeration of the waste body. In lab scale, even if biological process are faster than in real sites, the generated temperature is dispersed into the environment because the specific heat exchange surface is too high.
Figure 1.3.3: Picture of columnar reactor equipment with Tedlar bags for biogas collection and leachate recirculation system.
A constant temperature in laboratory test is also preferred for excluding its variation effect in the monitored processes. For these reasons, temperature is maintained constant at 38 - 40°C by means of a thermo-regulated insulation system covering all the reactor lateral surfaces (Figure 1.3.1, 1.3.2 and 1.3.3). Another used system comprises a spiral circuit of silicon pipes placed around the columns with circulating hot water at 40°C and a bubble-wrap cover as insulation. The internal temperature was monitored with Thermo Systems TS100 temperature probes or PT 100 (Endress+Hauser) probes installed inside the core of the reactor. The reactors were placed in a room in which the temperature is maintained constantly around 20 °C.
Leachate system
The reactors can be equipped for not only leachate extraction and distilled water injection;
a temporized leachate recirculation can be adopted for better control the humidity inside and
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for guaranteeing a homogeneous distribution of nutrients, microorganisms and substrates.
The leachate collection port is located at the bottom of each column and the liquid extracted falls directly into a collection tank of 5 L capacity (Figure 1.3.4, B). The recirculation of leachate is carried out using programmable peristaltic pumps (Heidolph PD 5001), which automatically pump up leachate form the tank to the introduction of liquids valve, placed in the top of the reactor (Figure 1.3.2). Flowing into the valve, leachate is uniformly distributed in the entire sample surface by means of a shower, placed into the internal part of the cap (Figure 1.3.4, A). The leachate collection as well as the recirculation system are built with the purpose to avoiding any leakage of leachate and any loss of gas in the circuit.
Figure 1.3.4: Particular of the leachate distribution shower placed in the internal top part of the reactor (A) and of the leachate collection tanks placed under the reactors, with leachate recirculation pumps (B).
Gas extraction system
The gas generated from each column can freely exit form a valve placed in the top cap and flow into a Tedlar® sampling bag (Figure 1.3.3). The gas volume is calculated measuring the time necessary to empty the bag at the constant flow of 200 L/h, regulated by means of a volumetric flow meter. This methodology reliability has been certified by comparing it with a direct volumetric measurement. If the volume exceed the Tedlar bag capacity during aeration, an emergency system will discharge away the gas to not generate excessive pressure in the reactors. A portable analyser (Eco-Control LFG20) measures oxygen, carbon dioxide and methane concentrations. Stripped N-NH3(g) can be caught through an acid scrubber, placed immediately after the off-gas valve of each reactor (Figure 1.3.2). Boric acid 0.5 M was used as scrubber and the gaseous ammonia emission results were periodically compared with a portable gas analyser (Analitica Strumenti LFG 2000).
Air injection system
To channel air into the waste body, a vertical PVC pipe with side perforations was installed at the centre of the reactor (Figure 1.3.2). This system was designed to guarantee the uniform distribution of air throughout the reactor and to simulate a vertical well injection. The
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injection of air from the bottom of the reactor is possible, but not compatible with continuous leachate extraction. A Prodac Air Professional pump 360 provides for airflow generation, while a Sho-Rate GT1135 flow meter regulates the flow quantity (Figure 1.3.5). The air injection is possible only if gas extraction system avoids the increase of pressure in the column. Fundamental for air distribution efficiency is considering the effects of waste compaction due to the biochemical processes happening in the waste mass. In particular, compaction must be made avoiding voids in waste mass and holes in the air distribution pipes has to remain under the minimum height reached by the waste volume reduction.
Figure 1.3.5: Particular of the air injection pumps and flow meters. The temperature monitoring equipment is visible on the bottom left side of the picture.