Patent Application
20160251245
The environmental management of landfill sites represents a serious environmental challenge. Indeed, these sites are a major source of contaminants and require strict environmental monitoring as regards two main problems.
Firstly, landfill waste is responsible for about 5% of Greenhouse Gas (GHG) in Quebec (Canada) with 4.1 Mt CO2eq in 2010. Specifically, landfill sites represent a major source of methane (CH4), a compound recognized by the Kyoto Protocol as being one of the six main GHGs. Its Global Warming Potential (GWP) is around 21 to 25 times higher than that of carbon dioxide (CO2). In some cases, methane (biogas) can be recovered or burned with the aid of flares when concentrations exceed 25%. But for lower concentrations, a make-up gas (propane, natural gas) must be added to enable its combustion. This approach is very costly, however, and runs counter to the aims of reducing GHG.
Subsequently, landfill sites generate leachates, namely water from precipitation that becomes heavily contaminated after percolating through layers of buried waste. This constitutes a potential source of groundwater contamination and requires special treatment before it can be discharged into the natural environment.
The main aim of the invention is to propose a biofiltration system and process for the combined and simultaneous treatment of methane (CH4) and leachate from sources such as landfill sites. Using one biofiltration system according to the invention avoids the need for two treatment chains specific to methane and leachate, reducing installation costs and simplifying the system's design. An embodiment of the system and process according to the invention aims to convert the CH4 contained in the landfill gas (LFG) to a concentration below around 20%, 12%, 5% or 3%, or between 0.5% and 1%, at conversion efficiency levels above around 50% or 0%.
Another embodiment of the system and process according to the invention relates to the treatment of leachates from landfill sites enabling one or more of the following purification performance levels to be achieved: BOD5<65 mg/l, Suspended Solids (SS)<35 mg/l, NH4<10 mg/L, fecal coliforms <100 CFU/100 ml, and pH between 6 and 9.5, with minimal by-products from the transformation of nitrogen in the form of nitrates at output.
Another embodiment of the system and process according to the invention is aimed at treating a leachate at a surface flow rate of up to 200 L/m2.d, or at an applicable load limit of 0.2 kgBODs/m2.d, 0.05 kg/SS/m2.d or 0.055 kgN/m2.d.
According to an embodiment of the system and process according to the invention, the microorganisms (e.g. heterotrophs, nitrificants, methanotrophs) participating in the combined treatment of the CH4 contained in landfill gases (LFG) and leachates from a landfill site cohabit on the basis of a profile within one and the same percolating biofiltration medium.
According to an embodiment of the system and process according to the invention, the nitrates (NO3) from the transformation of the NH4 in the leachate serve as nutrients for the methanotrophic microorganisms, a synergy that helps to reduce the nitrates in the leachates.
According to an embodiment of the system and process according to the invention, the transformation of NH4 nitrogen can be stabilized on an annual basis by recycling the exothermic process associated with the biological oxidation of NH4, enabling nitrification to continue during the winter.
Implementation of one of the embodiments of the system and process according to the invention has enabled the impact of the leachate load on the rates of conversion of CH4 to be determined.
The treatment system according to an embodiment of the invention uses a slow-percolation biofilter on a fixed-culture organic carrier. The passage of gaseous and liquid effluents therefore occurs through the filtering medium acting as a purification medium, which may consist of readily available natural elements, such as peat and woodchips with added calcite, on which the biomass responsible for the treatment of contaminated effluents develops.
Various purification mechanisms operate in the filtering medium, thus enabling treatment of the effluents. On the one hand, the physical-chemical mechanisms allow the pollutants to be retained either by filtration, adsorption or absorption. On the other hand, biological mechanisms using specific microorganisms act on the pollutants and cause them to biodegrade. The efficiency of the biofilters lies in maintaining an active biomass that is sufficiently large to ensure a satisfactory level of treatment.
As regards the nutritional needs of the biomass, in order to ensure optimum development of microorganisms known as methanotrophs, i.e. capable of converting CH4 into CO2, nitrogenous nutrients in the form of nitrates (NO3) must be available and for that reason an intake of nitrogen must be introduced into the process. The aim is for the nitrifying microorganisms present in the leachate to be treated to colonize the filtering medium sufficiently to generate a quantity of NO3 enabling the biofilter to be self-sufficient in nitrogenous nutrients. For this reason, the inoculum used to introduce methanotrophic microorganisms on start-up may contain manure treated by biofiltration, rich in nitrifying microorganisms.
The most favorable temperatures for the bioconversion of methane are between 25 and 35° C. Given that the methane degradation reaction releases heat (exothermic reaction), it is possible to assess the intensity of bacterial activity indirectly by measuring the temperature in the biofilter.
For optimum bioconversion of methane, the pH of the medium must ideally be close to neutral. In fact, methanotrophic microorganisms develop well in a medium where the pH is between 5.5 and 8.5.
As regards the introduction of methanotrophic microorganisms, it is advantageous to inoculate the filtering medium in order to reduce the start-up time for treatment of CH4. In fact, using an inoculum enables the specific introduction of methanotrophic microorganisms capable of treating CH4 and thus, of quickly starting its bioconversion. For example, an appropriate inoculum can be obtained by collecting some purified output liquid from a biofiltration system operating to treat a liquid effluent in the presence of CH4, such as pig's manure or other similar organic material, said output liquid being likely to contain methanotrophic microorganisms. The liquid collected can be used directly as an inoculum or undergo a procedure aimed at activating the proliferation of methanotrophic microorganisms prior to inoculation.
As regards the intake of microorganisms required to treat the leachate, the development of the biomass for this purpose can usually be assured without an external intake since the leachate is rich in microorganisms that are acclimatized. Thus, no inoculation is usually required to start the treatment of the leachate since the bacterial flora already present in the leachate can develop by itself within the filtering medium. There will then be a consortium of microorganisms capable of degrading the various compounds present in the leachate to be treated.
In the example of the system shown, each column uses an identical filtering medium composed of five layers, namely, from the bottom up: 5 cm of woodchips, 100 cm of an organic mixture (peat and woodchips) including a carbonate source (crushed oyster shells or calcite), 20 cm of woodchips (serving as a possible gas outlet in the event of overpressure), 30 cm of an organic mixture and 5 cm of woodchips. More precisely, the composition of the filtering medium can contain a mixture of peat (20% v/v), woodchips (80% v/v) and an additional volume of crushed oyster shells (15% v/v of the total mixture of woodchips and peat). The addition of crushed oyster shells in the filtering medium enables heavy metals that the leachate may contain to be captured. Moreover, the shells act as a buffer allowing the pH to be stabilized in the event of acidification of the liquids, caused by the nitrification process. The key design data of the biofiltration columns described above with regard to the example shown in
The following section presents the methodology and results obtained from a first test period of combined treatment of methane and leachate taking place over approximately 70 days and using the system described above with reference to
This first test period involved performing, inside one biofilter, a simultaneous biological treatment of a TLS leachate and a gas containing 1% v/v of methane. The circulation of the elements to be treated involved an upward gaseous flow, circulating in the opposite direction to a percolating flow of a leachate to be treated. Since this was a biological treatment, it was necessary to introduce the microorganisms essential for the treatment of the above-mentioned effluents. As a first step, during the start-up phase, the methanotrophic microorganisms capable of treating the methane were introduced by inoculation. To achieve this, an inoculation liquid, rich in methanotrophic microorganisms, was continually recirculated through the biofilters until a steady EE of at least 80% was reached. Achieving this level of efficiency in fact suggested that a methanotrophic biomass capable of efficiently converting methane into carbonic gas was established in the filtering medium of the biofilters. The inoculation of methanotrophic microorganisms considerably reduces the start-up time of the process by biofiltration. In fact, inoculation enables a biomass to be obtained that is capable of starting the treatment of methane after only 10 to 20 days rather than several weeks or even months. Using an inoculum liquid from the liquid outlet of a biofilter treating gaseous effluents containing methane is a simple and efficient way to achieve this. As a second step, during the combined treatment phase, the biomass capable of treating the leachate was established. To do this, leachate was fed in over several days to enable the development of microorganisms capable of reacting with the various compounds present in the leachate so as to biodegrade them. The aim was therefore to obtain a consortium of microorganisms capable of treating the methane and leachate circulating in the test columns at the same time. In other words, the aim of the test was to check that the two treatment processes were capable of coexisting in the same biofilter, more particularly that the methanotrophic microorganisms were able to tolerate the presence of the toxic elements present in the TLS leachates and the leachates were able to provide the nutrients required by the methane bioconversion mechanisms.
This first test period of combined treatment was performed in biofiltration column L1 while biofiltration column L2 acted as a control. During the test, the operating conditions were identical for both columns. This means that the liquid and gaseous flow rates as well as their methane content in the synthetic gas were the same for L1 and L2. The distinction between the two columns focused on the nature of the liquids used and their mode of circulation. Thus, during the combined treatment phase, L1 received, without recirculation, TLS leachate while L2 received, with recirculation, inoculation liquid containing nitrogenous nutrients (nitrates). These differences allowed a distinction to be drawn between the hydraulic effects of the flows and the physico-chemical effects caused by the different nature of the liquids used. Moreover, using a control in these conditions ensured that an inoculation liquid rich in methanotrophic microorganisms, possibly required for the inoculation of new columns, was readily available. Moreover, it should be noted that, for the entire duration of the first test period, no nitrogenous nutrients were added to the leachate to be treated, received by L1.
A first start-up phase was performed, having as its aim the inoculation of the filtering medium by the methanotrophic microorganisms. To do this, the two columns L1 and L2 were inoculated with an inoculation liquid of which half comprised an inoculation liquid rich in methanotrophic microorganisms from an operating biofiltration system, as explained above. This inoculation liquid was recirculated in the columns throughout the start-up period in order to promote the development of biomass in the filtering medium. Also, in order to inoculate the filtering medium of nitrifying microorganisms, half of the recirculated liquid contained effluent from a biofilter having treated pig's manure rich in nitrifying microorganisms. The aim of using this mixture is the simultaneous inoculation of methanotrophic microorganisms for the treatment of methane and of nitrifying microorganisms for the nitrate requirements of the methanotrophs, thus reducing the time required to establish the biomass needed for treatment. Using an inoculum in the form of inoculation liquid during the start-up of a methane biofiltration process, in recirculation mode, enables a steady state to be reached more quickly, that is, in only about ten days. The effective operating parameters during the start-up phase of the first test period are set out in Table 2, the start-up configuration being represented schematically in
After completing the first start-up phase, when the methanotrophic microorganisms were sufficiently active to perform a stable and effective treatment of the methane in terms of elimination efficiency (EE ≧80% for L1 and the control L2), a second phase involving combined treatment was begun, by changing the nature of the inoculation liquid of column L1, which was replaced by TLS leachate without recirculation. Moreover, from that moment, there was no more external intake of inoculum and liquid from a biofilter treating pig's manure for the liquid in control column L2. The effective operating parameters during the combined treatment phase of the first test period are shown in Table 3, the configuration of combined treatment being schematically shown in
Various analysis parameters were constantly monitored throughout the first test period. Tables 4A and 4B show the nature of the analytical parameters considered as well as their respective frequency of monitoring. The temperature measurement positions correspond to those of the thermometers shown in
Note that for control column L2, considering the recirculation of the inoculation liquid, the concentrations at inlet must be the same as those at outlet (except for potassium because KNO3 is added). Consequently, the verifying analyses in the laboratory only related to the liquid at the outlet of control column L2.
As mentioned above, the leachate to be treated was taken from a TLS, the sampling point having been chosen upstream of a treatment system in order to obtain raw untreated leachate. The leachate taken from the TLS was stored at a temperature of 4° C. in a 200-liter tank, and each time the liquid tank of column L1 was refilled, the leachate contained in the tank was mixed in order to homogenize it, so as to provide an unsettled leachate representative of the leachate taken from the TLS.
The elimination efficiency (EE) parameter was used during the first test period to measure the performance of the methane treatment process. By constantly monitoring the evolution of this parameter, it was possible to check the level of activity of the methanotrophic biomass. The graphs in
It should be noted from the graph in
pH
As the bioconversion of methane by methanotrophs is an exothermic reaction, it was possible to follow the methane treatment activity by measuring the temperatures inside columns L1 and L2. As shown in
The graphs in
By using an infrared thermometer, it was possible to follow the evolution of the temperature along the walls of column L1 and control column L2. This enabled the hottest zone to be located and so the most active treatment zone to be identified. The use of said thermometer therefore enabled temperature readings to be obtained along the entire length of the columns and not be limited merely to the three reading points offered by the rod thermometers. The graph in
The graph in
The following section sets out the results obtained concerning the analytical monitoring parameters considered during the first test period, relating to the liquid phase of columns L1 and L2.
As regards the pH, the graph at the top of
As regards nitrogenous compounds, the reduction of ammonium (NH4) and the variation of the nitrates (NO3) at input and output depending on the duration of operation are shown in the graphs in
The graph at the top of
As regards COD and BOD5, the reduction of these parameters as a function of the duration of operation is shown in the graphs of
With reference to the graph in
In order to assess the pressure loss of columns L1 and L2 caused by their clogging, as a function of the duration of operation during the first test period, the pressure of the gas entering the columns was measured. Since the gaseous output from the columns was at atmospheric pressure, the measurement of the pressure at inlet corresponded directly to the difference in relative pressure between the inlet and outlet, i.e. the pressure loss. The graph in
The above results of the first test period confirm the possibility of performing in one biofilter a combined treatment of methane and leachate from a TLS. For 75 days of combined treatment, it was possible to achieve significant performance levels, both for CH4 (>90% reduction of a gas at 1% CH4) and for the main elements present in the leachate at an application rate of 77 Lm2.d (load of 0.039 kgN-NH4/m2.d=efficiency >90%—av. 11 mg/l at output; max. load 0.04 kg BOD5/m2.d=efficiency of 75%—av. 85 mg/l at output, load of 0.004 kgSM/m2.d=13 mg/l at output). In fact, the EE levels were on average in the order of 95%, i.e. well above the acceptable level of 75%. The methanotrophic bacteria therefore withstood contact with the leachate well during this period, despite the presence of toxic elements in the leachate. The results obtained for ammonia reached the target of 10 mg/L. In fact, the NH3 content dropped on average from 507 mg/L at the inlet of L1 to 11 mg/L at its outlet. The treatment of the leachate as regards this parameter therefore performed very well. Furthermore, it is interesting to note that despite its inhibiting effect, the ammonium present in the leachate did not affect the methanotrophs. For suspended solids (SS), the maximum target value of 35 mg/L at output was easily met, since output values in the order of 13 mg/L were achieved. The results showed that the combined treatment process withstood the imposition of a flow rate of leachate to be treated of 2.4 L/d, which represents a hydraulic load of 0.077 m3/m2/d, but does not necessarily represent the operating limit of the system tested. The first test period also allowed the efficiency of an inoculation and a method for starting the methane treatment process to be checked. This first test period in fact showed that, just a few days after starting, EE levels of around 95% could be obtained by inoculation. This result confirms the considerable time saving that can be achieved when an inoculation rich in methanotrophs is available at start-up.
The following section sets out the methodology and results obtained for the entire methane and leachate combined treatment test period to enable prolonged monitoring extending over almost 700 days from start-up after the acclimatization phase, and using the system described above with reference to
As indicated in the graph in
As indicated in the graph in
The following section presents an analysis of the test results over the entire period of monitoring biofiltration columns L1 and L2. Tables 6 and 7 show, for columns L1 and L2 respectively, the statistical data (minimums, maximums, averages, standard deviations) of the main parameters that were measured mainly in the laboratory and formed the subject of the analysis of the results. Some data were obtained using a colorimeter when the number of laboratory analyses was deemed insufficient.
It will be noted that in Table 6, the maximum NO3 input value was obtained during the external addition of nitrates, and that the maximum and average [CH4] input values were 1.2, 1.1 and 0.07 respectively after eliminating identified aberrant data.
It will be noted in Table 7 that the maximum NO3 input value was obtained during the external addition of nitrates.
As for the pH, the aim for this parameter was to stay between values of 6 and 9.5, which was achieved for column L1. The average pH at input was 8.1 and at output 8.6. During the biofiltration process, production or transformation of one or more substances responsible for the rise in pH therefore occurred. In fact, the nitrification process (biotransformation of ammoniacal nitrogen to nitrate) should have caused the pH to fall. In any event, the average pH at output was slightly above the optimal pH of methanotrophic bacteria (between 5.5 and 8.5), but within the optimal pH zone of nitrifying bacteria (between 8 and 9). By contrast, the pH having been measured at the output of the columns, and considering that the methanotrophic and nitrifying microorganisms do not colonize the lining in the same place, the measurement obtained was therefore a general indication of the pH within the biofiltration columns. In the graph in
In the graph in
As regards monitoring ammoniacal nitrogen (NH3) even though like some of the other substances contained in TLS leachates, its concentration can vary considerably, the discharge standard of a maximum of 10 mg of NH3/L should be found during the test. In the graph in
In the graph in
As regards monitoring the BOD5, which gives an indication of the amount of O2 used by the microorganisms over a period of 5 days (the higher its value, the greater the amount of biodegradable organic material), the graph in
As regards monitoring the BOD5 for column L2, the graph in
As regards monitoring suspended solids (SS) for column L1, the graph in
As regards monitoring SS for column L2, the graph in
As regards monitoring the nitrates (NO3), the graph in
As regards monitoring the nitrates (NO3) for column L2, the graph in
As regards monitoring methane (CH4), the graph in
As regards monitoring the methane (CH4) in column L2, the graph in
As regards the methane elimination capacity (EC), the graph in
As regards the methane elimination capacity (EC) for column L2, the graph in
As regards the active methane treatment zones, the graph in
As regards the active methane treatment zones for the test in column L2, the graph in
As regards monitoring the chemical oxygen demand (COD), considering that this parameter is not covered by the standard considered for the purposes of the test, it was monitored as an indicator. The graph in
As regards monitoring the COD for column L2, the graph in
As regards monitoring the fecal coliform level, the graph in
As regards monitoring the fecal coliform level for column L2, the graph in
The purpose of the successive stoppages and re-starts that occurred during the test was to observe the behavior of the biofiltration columns in response to this method of operation. Although it was observed that the elimination efficiency increased somewhat during certain stoppages of liquid feed, the elimination capacity fell to 56%, whereas it was nearly 90% before beginning the successive stoppages and restarts. This loss of performance could be explained by the fact that the production of nitrates fell almost to zero. Thus, the methanotrophic microorganisms may have lacked nutrients, reducing by this fact their capacity to oxidize the methane. Moreover, these microorganisms seem to be relatively seriously affected by changes in the liquid flow rate, which could explain the drop in performance levels observed earlier during the test.
In summary, the results of the test that extended over 631 days confirmed that all of the objectives set for biofiltration column L1, operating in methane and TLS leachate combined treatment mode, were met. It was possible to achieve an EE of CH4=83%, an average N_NH4 at output of 5.7 mg/l, an average BOD5 at output of 20 mg/L, average pH values at output of 8.6, an average SS at output of 27 mg/L and average fecal coliform levels at output of 3.9 CFU/100 ml. Just as all of the performance criteria were met, so too did the process tested in L1 prove to be efficient for the simultaneous combined treatment of gaseous effluents containing methane and TLS leachates.
As regards biofiltration column L2, it was possible to achieve an average CH4 EE of 83%, an average N_NH4 at output of 5.9 mg/L, an average BOD5 at output of 23.9 mg/L, average pH values of 8.7 and average fecal coliform levels of 10 CFU/100 ml. Only the average SS of 62.9 mg/L observed for column L2 failed to achieve the set objective.
It was observed that changes in the liquid feed flow rate affect the bioconversion of methane. As the concentrations at inlet are already very variable, the fact of drastically changing the feed flow rate, by doubling it for example, would involve a major adaptation of the biofiltration system on an operational level. A constant-flow method could therefore prove advantageous in this case. If changes in flow rate should prove necessary, they could be applied gradually over a longer period of time.
The phase of stoppages and restarts carried out during the test involved a succession of 4-day periods of feeding liquid at 1.2 L/d followed by 3-day stoppages, for nearly 60 days. At the start of this phase, the CH4 elimination efficiency was not affected, unlike the nitrification process. In fact, the production of nitrates was greatly influenced during this phase, dropping from 210 mg NO3/L to 1.9 mg NO3/l at output a few days after the start of this phase. As nitrates form an important substrate for methanotrophic microorganisms, their absence definitely has an adverse effect on methane elimination efficiency. This is borne out by the fact that the CH4 EE dropped from 90% to 56% in 17 days. The reduction in nutrients was by no means the only cause for this reduction. In fact, for both column L1 and L2, all of the reductions in efficiency occurred during changes in liquid flow rate. So, as this phase is characterized by frequent changes in flow rates, the methanotrophic microorganisms could have been negatively affected by these changes in operating conditions. The ideal situation would be for the level of nitrates at output to be low but without affecting the treatment of methane. Moreover, an operating method involving successive reductions and increases in the liquid flow rate could be applied with the aim of reducing the adaptation needs of the methanotrophic microorganisms.
In the graphs showing the active treatment zones, it is possible to determine the place where the methanotrophic microorganisms are more likely to be established, namely in the intermediate woodchip zone.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2014/051028 | 10/23/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61894668 | Oct 2013 | US |
