ROLES Project – Second Year 2017
Denitrification capacity of old MSW landfill under anoxic conditions
Introduction and aim of the experiment
The present research aimed at studying whether MSW landfill bioreactor is suitable for denitrification of a nitrified landfill leachate characterized by very low COD/NO3⁻-N mass ratios, ranging from approximately 0.2 g/g to 3.0 g/g.

Materials and methods
The waste used for this study comes from a previous lab-scale experiment, during which it was aerated for testing in situ aeration technique. In the origin, the waste was taken from a mechanical biological treatment (MBT) plant in Legnago (Verona) landfill site, northern Italy and was the residual fraction from the separate collection of the municipal solid waste.
A composition analysis was then carried out and the results (wet weight – % w/w) are reported in Figure 1.

Figure 1. Composition analysis (% w/w) of municipal solid waste (MSW) employed for this study

The physical and chemical properties of the solid waste material at the beginning of the present experiment were detected and they are listed in Table 1.

Table 1. Physical and chemical characterization of the solid waste material at the beginning of the present experiment (VS = Volatile Solids; DM = Dry Matter; RI = Respiration Index)

Density (t/m3)
VS (% DM) Moisture content (%) RI4 (mg O2/g DM)TKN (g N/kg DM) TOC (g C/kg DM)
0.815 13.2 55.8 4.4 10.725 174

The present study was performed using six Plexiglass® (polymethyl methacrylate) reactors with a height of 106 cm and a diameter of 24 cm (Figure 2).

Figure 2. Lab equipment configuration that simulates the landfill processes.

The top lid had three valves and a hole, used to insert a Thermo Systems TS100 temperature probe connected to an Endress-Hauser monitor.
One valve was used for the leachate recirculation, one connected to a 25 L Restek® bag filled with N2 in order to avoid negative pressure inside the column and one for the biogas analysis, performed with a portable Eco-Control analyzer, model LFG 20.
At the top and at the bottom of the waste body, two gravel layers (Ø < 2 mm), both with thickness of 10 cm were placed to distribute leachate and prevent clogging.
Leachate was sampled by means of a valve set at the bottom of the column and its recirculation was accomplished through PVC pipes and the hydraulic head was ensured by a Heidolph PD 5001® peristaltic pump. A slotted PVC pipe allowed the homogeneous distribution of the leachate inside the column.
Approximately 18 kg of waste were loaded into the columns and compacted at an average wet density of 0.82 t/m3. About 2 L of leachate were added in each bioreactor;
During the whole experiment, all reactors were kept at the constant temperature of 30 ± 1.5 °C, using thermo-regulated insulation system covering the lateral surface, and under anoxic conditions. Nitrogen gas (N2) was flushed within all reactors to ensure anoxic conditions before the start of the operations.
The experiment was performed in three runs. At the beginning of the first and second run, COD/NO3⁻-N ratio of the leachate was adjusted in all reactors by dilution with deionized water and by addition of calcium nitrate tetrahydrate (Ca(NO3)2·4H2O). Leachate dilution was performed only at the beginning of the first run while calcium nitrate tetrahydrate (Ca(NO3)2 was added at the beginning of all runs in all reactors. Leachate was recirculated in all reactors every day.
At the beginning of the third run, sucrose was added as COD in three reactors (C4-3, C5-3, C6-3) in order to get the same high COD/NO3⁻-N ratio of the first run in C1-1 and C2-1 and compare the nitrate removal rate having a different quality of COD.
The desired COD/NO3⁻-N mass ratios (g O2 /g NO3⁻-N) in each run are reported in Table 3.

Table 3. Desired COD/NO3⁻-N mass ratios (g O2/g NO3⁻-N) in leachate in each run.

C1 C2 C3 C4 C5 C6
First run
1.0 1.0 2.02.03.0 3.0
Second run0.20.2 0.4 0.4 0.6 0.6
Third run0.20.20.23.0*3.0* 3.0*

* Sucrose addition

In the third run, leachate recirculation was stopped in two reactors (C3-3 without sucrose addition and C6-3 with sucrose addition) in order to better understand its influence on denitrification process.
During the experiment, leachate characterization was carried out on pH, nitrate (NO3⁻-N), nitrite (NO2⁻-N), ammonia (NH4+-N), TKN, BOD5, COD, TOC and sulfate (SO42-). Leachate in each column was sampled and analyzed twice a week.
Nitrate removal rate (NRR) was calculated using a central difference method of analysis (Berge et al., 2006; Fu et al., 2009):

where NRR (mg NO3⁻-N/L/d) is the nitrate removal rate at time t (d) and C is the nitrate concentration in the leachate (mg NO3⁻-N/L).
NRR can be also expressed as mg NO3⁻-N/kg DM/d in order to make it comparable with other similar studies; it is obtained multiplying the mg NO3⁻-N/L/d for the initial total liquid volume inside each reactor (moisture content plus the process water) and dividing for the initial dry matter (DM) of solid waste material.

Results
The evolution of nitrate (NO3⁻-N), COD and sulfate concentration during the experiment is reported in Figure 3 and Figure 4.

Figure 3. Evolution of nitrate (NO3⁻-N) and COD concentrations in C1 (●), C2 (○), C3 (▲), C4 (Δ), C5 (■), C6 (□) during the first (a, d), second (b, e) and third (c, f) run of the experiment.

Figure 4. Evolution of sulfate concentrations in C1 (●), C2 (○), C3 (▲), C4 (Δ), C5 (■), C6 (□) during the second (a) and third (b) run of the experiment.

During the whole experiment, the nitrate removal efficiency was always higher than 96 %, except in C3-3 (only 55.5 %), where leachate recirculation was not performed and COD was not added.
Nitrate removal efficiency was calculated as in the following:

Nitrate\ removal\ efficiency=\ \frac{C_f-C_i}{C_i}·100

Where C_f is the final nitrate concentration in the leachate and C_i is the initial nitrate concentration in the leachate.
During the third run, nitrate reduction in C1-3, C2-3 and C3-3, where COD was not added, was complete, but it occurred more slowly than in the second run, suggesting a reduction in the denitrification capacity of those reactors.

Conclusions
Denitrification occurred in all bioreactors during the whole experiment, even at low biodegradable COD, except in the one where leachate recirculation was not performed.
In particular, during the first and second run of the experiment, characterized by a low COD concentration, the COD/NO3⁻-N ratio did not affect the average nitrate removal rate (ANRR) which was highly influenced by the initial nitrate concentration: higher initial nitrate concentration favored higher ANRR.
When biodegradable COD was added at the beginning of the third run, the ANRR increased and COD/NO3⁻-N ratio highly affected the denitrification process.

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