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Introduction

This technical note is the last one of the section on recycling of organic waste. In this section, it is also question of treating organic waste by biological mode but in addition to the production of compost, a bio fuel known as biogas will be generated by the process.

Indeed, the anaerobic treatment of organic waste, if it is carried out properly, allows not only the transformation of the waste into compost but also into a gas composed of methan (CH4), and carbon dioxide (CO2) and nitrogen and sulfur components.

Biogas as a by-product of the methanic digestion of organic substrate is a fuel gas, which uses air as combustive agent in the same way as a fossil fuel, and which generates heat through oxidation.

The other difference is linked to the humidity level of the metha compost as it is not a dry product. It is always in a humid and even semi liquid form when it results from the digestion of pig manure or other waste in solution such as sludge from wastewater treatment plant.

The recent awareness of the population on global warming associated to the greenhouse gases GHG (methane is a GHG 21 times more harmful than carbon dioxide), the increasing costs and scarcity of fossil fuels are resulting in the growing interest for biogas technologies even if it is a technique which has been used for more than thousand years in China.

Indeed, it not only allows the sustainable treatment of waste but it is also attractive for its production of organic fertilizer and particularly bio fuel.

However, as a promising and efficient technology, biodigestion is also more complex and costly to implement than aerobic composting. It is only since a few decades that biodigestion is the subject of research and high investment technological applications, namely in the industrial sector, but as we will see, its technicity limits its viability in a certain number of contexts.

Basic Principles

Biochemical principles of methanization

Biodigestion concerns organic waste which can be digested by anaerobic mode. In the technical notes, we are specifically interested by the digestible fraction of domestic waste and equivalent, thus limiting the applications of biodigestion.

In this context, ligneous green waste have low potential for digestion, as well as fish scales, feathers, furs and horns from micro cattle breeding.

However, greasy papers or soiled cardboards with organic liquid can be digestible in thermophilic conditions. Another example, the green parts of a pineapple are not digestible whereas the peel and the fruit itself are.

Vegetable peels, residual waste from meat and fish as well as from cereals and tubers, grease and oils are not only digestible but also extremely methanogens.

We must also take into consideration that outside domestic waste, some waste such as sludge from WWTP, animal manure, industrial solid waste or effluents (slaughter house, fish transformation or oil industry) present interesting digestibility and should therefore not be under estimated.

To summarize we can say that biodigestion is a perfectly viable process as long as we are assured of adapted incoming material. This again underlines the need for separate collection and voluntary deposit scheme for specific waste.

Fundamentals

The treatment of organic matter by biodigestion is first of all subject to biological constraints and the techniques implemented aim at creating and maintaining an ecosystem favorable to particular microorganisms.

To simplify we can say that a biodigestor host and entertain strictly aerobic microbial populations which are brought to grow and reproduce themselves on an organic substrate composed of waste, by developing a bio-oxidation activity but in the absence of air.

These microbial populations are complex and relatively diversified but we know quite well their biochemical characteristics and the main peculiarities of their ecology.

  • Hydrolytic and fermentative bacteria. The hydrolysis phase is realized by several groups of anaerobic obligate or facultative eubacterias the nature of which depends on the qualitative and quantitative composition of the feeding substrate. The main species belong to the following genus: Clostridium, Bacillus, Ruminococcus, Enterobacteria, Propionibacterium and Butivibrio.
  • Acetogenic bacterias. During this phase, the oxidation of the substrate (specially with propionic and butyric acid and ethanol), is coupled to the formation of hydrogen, carbon dioxyde and acetate. It represents the activity of three groups of bacteris: the homoacetogenes of genus such as Clostridium, Acetobacterium, Sporomusa, Acetogenium, Acetoanaerobicum,  Pelobacter Butyribacterium, Eubacterium..., syntrophes of genus such as Syntrophobacter, Syntrophomonas, Syntrophus... and the sulfato-reductors of genus such as Desulfovibrio, Desulfobacter, Desulfotomaculum, Desulfomonas...
  • Methanogen bacterias. Active bacterias of this last phase form part of a unique group: Archae. They possess specific characteristics in relation to eubacterias and eucaryots, namely as far as coenzymes are concerned. Archae constitute one of the three primary states of the primary, with eubacterias and eucaryots.

These three communities must constitute a balanced ecosystem in order to allow that most of the reducing agents (carbon atoms and hydrogen) produced as waste during bacterial anabolism (hydrolysis then acidophilic and acetogenesis) are finally found in the methane (methanogenesis CH4).

In a summarized way we can present the three phases of the biodigestion process as in the figure and the carbon flows associated to each phase are given in %.

The management of the artificial ecosystem constituted by the anaerobic bioreactor requires an intervention to ensure that certain essential physico chemical conditions prevail: pH, temperature and oxido reduction potential as well as the nutritional needs.

The pH

  • The optimum pH for anaerobic digestion is around neutral. It is the result of the optimal pH for each bacterial population :
  • The optimum pH for acidifying bacterias is between 5.5 and 6, acetogens prefer a pH close to neutral whereas methanogens have their maximal activity with a pH between 6 and 8.
  • However, methanisation can occur in lightly acidic or alkalines environments.

Temperature

  • The activity of the methanogen consortium is closely linked to the temperature. Two ranges of optimal temperatures can be defined: the mesophilic zone (around 35°C) and the thermophilic zone (between 55 and 60°C) with a decrease of the activity beyond these two low and high values.
  • The majority of bacterial species has been isolated in mesophilic environments, but all the trophic groups of the different stages of the aerobic digestion possess thermophilic species which use the same metabolic channels as the mesophilic bacterias with similar performances. It remains possible to work at different temperatures optimums with lower performances.


Oxidoreduction potential

  • This parameter represents the reducing state of the system, it affects the activity of methanogen bacterias.
  • These bacterias indeed require in addition to the absence of oxygen, an oxido reduction potential lower than 330mV to initiate their growth.


Nutritional and metabolic needs

  • As all micro-organisms, each bacteria forming part of the methanogen flora require a sufficient level of macro elements (C, N, P, S) and trace elements for its growth.
  • The needs in macro elements can be roughly evaluated from the gross formula describing the composition of a cell (C5H9O3N). For methanogen bacterias, the culture environments must have carbon contents (expressed in COD), in nitrogen and in phosphorous at minima in proportions equal to 400/7/1.
  • Ammonia is the main nitrogen source. Some species fix molecular nitrogen whereas others need amino acids. The needs in nitrogen represent 11% of the dry volatile matter of the biomass and the needs in phosphorous 1/5 of those of nitrogen.
  • Methanogen bacterias possess high content in proteins Fe-S which play an important role in the electron transportation system and in the synthesis of coenzymes. Thus the optimal concentration of sulfur varies from 1 to 2 mM in the cell. This flora generally uses the reduced forms such as hydrogen sulfur. The methanogens assimilate phosphorous under mineral form.
  • Some trace elements are necessary for the growth of methanogens, more particularly for nickel, iron or cobalt. Indeed these are components of coenzymes and proteins involved in their metabolism. Magnesium is essential as it plays a role in the final synthesis reaction of methane as well as sodium which appears in the chemical osmotic synthesis process of ATP.
  • Some growth stimulating factors for the activity of some methanogens exist: fatty acids, vitamins as well as complex mixtures such as yeast or the trypticase peptone.

In conclusion, if nowadays, we master correctly the « macro-model » which simulates a biodigestion process, to a point that we can forecast the methane productions and the composition of the metha compost, the processes remain difficult to implement.

Indeed if we want to treat the volatile organic fraction of domestic waste and equivalent (including from industrial or agricultural sources), the process must be designed for each case in order to reach the best possible return on investment, as each substrate has its own particular microbiologic ecosystem and the yield ranges are not very wide.

The recent experimental and analytical approach from which biodigestion has progressed has enabled the establishment of a certain number of functional laws which are applied to distinct biodigestion technologies, each one having a privileged adaptation to the treatment of a certain type of waste, in variable quantities and with different quality levels of metha compost and biogas.

Schematic representation of a biodigestor with fixed biomass

Schematic representation of a biodigestor with free bacterial strain

Batch biodigesters (sequential loading)

Schematic representation of a biodigestor with continuous flow

The different types of process

A bioreactor is therefore an “artifact” which tries to optimize the living conditions of a given colony of microorganisms, at a certain point in time or in a given location in a view to favor the production of methane resulting from the digestion of substrate put in solution.

To simplify we can say that a biodigestor is composed of 4 major elements:

• A waterproof, airtight and often insulated cell
• A mixing and moving system
• A heating device
• Devices for incoming and outgoing flows (substrate, digested matter and biogas)
Depending on the processes implemented, we can differentiate two main types of ecosystems:

  • Biodigestor with fixed biomass


In this type of biodigestor, the container not only allows the separation of the substrate from air but its interior walls, often structured as “honey combs” are used to fix bacterial strains.

The advantage of this process consists in maintaining the availability of bacterial strains despite the permanent or sequential transfer of flows of treated substrate. Indeed the objective is to avoid having to start over the seeding of bacterias or having to specialize the bacterial flora according to the chemical variations.

Several types of fixing processes exist, some for example granulate the substrate or part of the substrate entering the biodigestor in a view to seed it and to circulate it within the biodigestor.

We can however note that this process operates properly only when the waste and substrate are put in solution.

  • Biodigestors with free bacterial strains

This type of biodigestor is based on processes which use reinforced active biomass which consist in the heating and circulation of the liquid part and eventually in addition of trace elements and pH adjustors.

The process is adaptative and relies on the spontaneous capacity of the bacterial flora to specialize according to the constraints of the given environment, namely when nutrients are present in important quantities.

The adaptability of the biomass, which can “freely” leave the cell with sequential or continuous discharge flow and evolve according to the constraints of the ecosystem, is reinforced by external actions:

Thermal: temperature maintained at 36°C for the mesophiles or 55°C for the thermophiles
Chemical : neutralization of acidic or alkaline pH and mechanical (fluidization or mixing)

This type of biodigestor requires a good follow-up which often lies on the careful observation of signals transmitted by captors.

Beyond the differentiation between fixed biomass and free population, the types of flow dynamics are also differentiated and two main ones prevail.

  • Sequential loading (batch)


The processes with sequential loading present the major characteristic of establishing the succession of three major phases of methanic digestion in the same containment and for a single dose of substrate.

In other words, we can say that bacterial populations evolve on the same substrate from the beginning to the end of the cycle and do not therefore have to use energy to adapt to changes regarding their ecosystem. They transform the ecosystem and not the other way round.

Hence once the content of the digestor is loaded (it can take one day or more), the optimal conditions for the start of the hydrolysis phase are brought (temperature, pH, nutrients and seeding).

It is then the turn of the acidogenesis phase which is regulated to allow the activation of the acetogenesis and then the methanogenesis.

From a general point of view, this process present the advantage of having an hydraulic retention time (HRT) shorter than for protocols with continuous flow and of being easier to manage.

It is namely the case when a cell does not function properly as we can continue to use the others. It is also a process where the cells are smaller and accept substrate with higher density of dry matter.

Nevertheless, sequential loading means that the cells and associated devices (loading hoppers, valves and pumps) have to be multiplied.

  • Continuous (feeding) flow


Continuous feeding strictly differs from sequential loading on several aspects. First, it is because the ecosystem and more particularly the bacterial flora are brought to be polyvalent or more precisely to have bacterias and their co-enzymes to cohabitate in the same cell and at the same time throughout the four phases of the cycle.

Moreover, in order to obtain a sufficient HRT, the cell must be designed for very important volumes which results in important energy costs to maintain an adapted temperature (rarely thermophilic except for small units) and to have a continuous mixing in a view to avoid the formation of a crust at the surface and of too dense deposits at the bottom of the cell.

We can however note that this process, which is very old as domestic biodigestors or those found at farms in China are mostly fed with a continuous flow and are quite well adapted to small quantities.

Indeed with small dimensions (less than 100 m3), homogeneous waste and with stable quality, they are easy to maintain if we do not try to evacuate the deposits in real time but rather the loaded flows which can be used for spreading in agriculture.

After several operation cycles, these small units must be stopped so that the deposits can be removed as they not only reduce the available space in the cell but also disturb the development of the bacterial flora.

Only some industrial processes can produce in addition to biogas, discharge flows which are sufficiently loaded to allow the obtention, after settlement and spinning, of metha-compost, which can be used as an organic fertilizer.

The advantage of this process, whether domestic or industrial, resides essentially in its capacity to accept a continuous flow of waste or effluent with low organic load with medium biogas productions but with possible valorization of extracted effluent, more rarely metha-compost.

Single Phase and differentiated phases:

Based on what has been developed above, we can say that two main types of process remain in competition: biodigestors with a single phase and biodigestors with differentiated phases.

In the first case, for both sequential or continuous flow biodigestors, and either the biomass is fixed or free, all the phases occur in the same cell. In the second case, hydrolysis, acidogenesis and acetogenesis are confined in a first cell and methanogenesis is carrie out separately in a second cell.

The objective for these multi-phase processes is to better manage the different phases taken separately by controlling the micro conditions optimizing these different ecosystems.

More complex and costly, the processes with differentiated phases do nevertheless have a better yield in terms of biodegradability for certain types of waste which necessitate enzymatic or chemical additions.

However, for a flow of waste which is homogeneous in time and with a composition presenting no particular risks (namely at the acetogenesis stage), this process does not bring enough added value to legitimate its complexity and the investment. We can note that fermentation with separate phases is presently applied only for fermentation with high load of digestible organic matter.

Finally we must consider three types of biodigestors based on the concentration of Total Solids in Suspension (TSS) in the flow, i.e. the proportion of dry matter (DM) put into aqueous phase in the biodigestor.

  • Low concentration of dry matter with less than 10 % of TSS: Low solids systems (LS)


Some applications have industrial or domestic effluents as main input material as is the case for waste water treatment plants and biodigestors which treat these wastewater flows have a particular configuration.

The principle is in a certain way to use the biodigestor as a settlement tank where the TSS remain whereas the purified water is discharged.

More clearly said, the retention time for TSS is more important than for the incoming flow as the biodigestor include a settlement system (passive or active) and a retention/ anaerobic degradation of digestible dry matter process.

As such these biodigestors are not adapted to the treatment of the digestible solid fraction of domestic waste except if they are shredded and mixed with the effluent flow which shall constitute the largest part of the material to be treated.

Under these protocols, the production of biogas and of methacompost (in the form of sludge) is relatively low but the capacity for primary treatment of an effluent is very good and the energy use is balanced with the production of biogas. Eventually the liquid methacompost can be sold as organic fertilizers.

Maximal volumic loads applicable are of the order of 2 to 5 kg of COD/m3/day.

  • Medium concentration of dry matter with 15% to 20% of TSS: Medium solids (MS)


It is the most common type of biodigestor, where the digestible substrate is put in aqueous phase in 2 to 3 times its weight if water. It corresponds to a search of equilibrium between the quantity of digestible matter and its viscosity within the cell.

Indeed, in order to enable the bacterial activity to operate in the best conditions, it is necessary that the substrate does not get compacted as long as it can be mobilized during the different phases of the biodigestion.

This process can therefore adapt to the treatment of the digestible fraction of solid organic waste provided a good sorting is carried out upstream in order to evacuate the non desired matter and a relatively fine shredding is undertaken in order to authorize the hydraulic transfer of the digestible mass.

Being more appropriate to processes based on continuous loading rather than those with sequential ones, the principle of the medium concentration in DM is particularly beneficial to systems with fixed biomass as the incoming substrate has a sufficient flow to impoverish the resident floras.

Generally, the volumic load to be applied can reach 15 to 20 kg COD/m3/day. HRT vary between 4 to 5 weeks.

Under this configuration, biogas yield are good and the production of metha-compost as a settled fibred material is correct but necessitates at least a decantation if not a centrifugation process.

  • High concentration of dry matter with 22% to 40% of TSS: High solids (HS)


Some categories of organic waste, among which the domestic waste, are constituted of an important solid fraction with low digestibility.

In other words, the weight of DM is important but the proportion of VOM on the DM is not.

Being given that we can not valuably concentrate the VOM of these wastes, it is opportune to have a technology which allows the treatment by anaerobic mode and some biodigestors are designed for this type of application.

The specificity of these applications resides in the advancing and mixing modes for the substrate and in the fact that they are nearly exclusively bioreactors with sequential load and with free biomass but with seeding.

We can note that beyond a certain level, the risk of overloading in VOM exists which can result in the inhibition of the methanogenesis which is especially true for waste rich in animal proteins (carcass and fats).

Generally, the volumic load to apply can reach 40 kg COD/m3 /day. The retention time vary from 2 to 3 weeks.

We can also take into account the fact that beyond 3g/l, ammonia (NH4+) inhibits the methanogenesis process. It is also known that this limit must not be exceeded for waste with a C/N ratio less or equal to 20 with a rate of VOM of the order of 60% of the organic matter.

The most commonly used technique to maintain organic substrate below this threshold consists in mixing carcass and other meat waste with carbonated substrate.

Another alternative consists in reducing the rate of VOM of the waste (mainly the proportion of ammonia) by making them go through an intense aerobic thermophilic fermentation but this requires in any way that the meat waste are mixed with carbonated substrates.

Critical analysis of the potential for treatment by methanization of DWE

Critical values for the methanization of solid and liquid waste

Representative values of the biodigestible fraction of DWE

Main modes of methanic digestion for DWE

Parameters for the design of a biodigestor

Simplified configuration of a biodigestor for DWE (modalities)

Simplified configuration of a biodigestor for DWE(characteristics)

Schematic representation of a biodigestor adapted to the treatment of DWE for ReCoMaP targeted countries/regions

Which process to choose for the digestible fraction of domestic waste and equivalent ?

Before deciding on methanic digestion for the treatment of the organic choose for the fraction of domestic waste, we must validate certain points as illustrated in the figure:

To answer to the question regarding technical feasibility, we must in priority validate the main parameters for acceptability of two major types of process, liquid or solid phase, which can be expressed in terms of:

  • DM of waste to be treated ;
  • Volumic load (VL) which is expressed in kg COD / L / H, but this calculation require that the average composition of the waste is perfectly known ;
  • HRT which applies to the liquid fraction and BRT (Biomass Retention Time) which applied to the solid fraction of the treated waste.

Notes:

  • In order to envisage these two alternatives, we must first validate the possibility for an efficient sorting, waste shredding and the availability of water or effluent in sufficient quantity. It is understood that in aqueous phase, both shredding/mixing and the availability in water are more demanding than in solid phases.
  • Generally, the fraction of DM in domestic organic waste is of the order of 47.5% (varying from 40 to 75%), and it varies according to the composition of meals (for example, the more fresh vegetables, the less DM). The more waste are put into solution, the lower the DM of the substrate will be.
  • The VL of the organic fraction of the domestic waste is a data which is difficult to obtain as its calculation depends directly on the nature of the envisaged process. However we can take the COD as a replacement value and for domestic organic waste in aqueous phase in the digestor, the initial average COD is 3.3kg/l (varying from 1.6 to 7).
  • The HRT depends on the envisaged process and it is easily calculated for a sequential digestor, which normally has a BRT equal to the HRT, more difficult to evaluate for the digestors with continuous flow, and even more for multi-phase digestors with continuous flow.


To conclude, we can retain that in general rule, the organic fraction of domestic waste is treated by solid mode in order to take into account their DM rate but specially for the digestibility of the OM.

Moreover, if properly shredded and eventually amended by animal manure and with a significant fraction of shredded green waste (25% of the DM), the substrate to be digested will not be difficult to mix. The digestibility and characteristics of the organic fraction of domestic waste are important parameters needed for decision making regarding process modalities. The tables give the main values and proposes a summarized analysis of modalities of biodigestion applicable to the organic fraction of domestic waste.
 
In Europe, most of the biodigestors operating with high loads and which are presently on the market, have been designed for the treatment of municipal solid waste.

Outside the choice of this mode of operation, the preference (arguable) for a process with sequential flow and the option for a single phase process, the question regarding the choice of a mesophilic or a thermophilic digestor remains.

However, having opted for this mode and knowing the type of waste which is to be treated, the sizing of a biodigestor will be made on the digestion temperature which can be mesophilic or thermophilic.

The thermophilic mode has an influence on the HRT/BRT, the quality of the metha compost and on the productivity in biogas (expressed per T of waste being digested).

Indeed we can say to simplify that in a thermophilic biodigestor (55 C°) with high load in digestible organic matter, microbial activity during methanogenesis is more intense and more complete than in mesophilic conditions (35°C) with two major consequences. We reach the peak of production in a shorter period and the metha-compost is nearly hygienized.

However, to maintain the temperature constant at 55°C +/- 2°C requires an additional installation ensuring thermal exchanges and a reliable source of heat.

However this additional investment, required for the thermal exchange which recovers the calories lost by the electric generator, is compensated by the gain obtained through the biogas production and the savings associated to the reduction of the size of the tanks of the biodigestor.

Moreover, in hot climates, it is unlikely that thermal energy of the generator is recovered for domestic uses (heating devices) and the thermal losses on the tanks are low when compared to the average ambient temperature, which reduces the need in thermal insulation.

For all these reasons and notwithstanding the additional required technology, the thermophilic process must be given due attention.

It is understood that this approach is very general and it is strongly recommended to obtain the advice of experts who are aware of site constraints and who will be able to deliver a rigorous evaluation. In any case, we can retain that the evaluation of a biodigestor concept will be based on the detailed parameters shown in the figure.

Particular attention must be brought to the necessity of a prior rigorous study as the investment required for a biodigestion unit is much higher than for a composting station. The environmental risk is also high as inflammable and explosive gas (with some toxic components e.g. H2S) is stored and burnt.

Moreover, a high production of biogas of good quality, i.e. with a high content in methane (CH4) and correctly purified is not sufficient to guarantee the valorization of the electrical productions as a cost effective centralized electrical generation unit and a reliable market with revenues are necessary to ensure the return on investment.

To conclude we can say that the profile of a medium-size biodigestor capable of treating the digestible organic fraction of domestic waste in priority regions of ReCoMaP can be described as in the figure.

Energy recovery and agronomic uses

Estimates for the production of biogas with a typical installation

Estimates of the energetic potential for biogas on a typical installation

Typical composition of biogas

Modelization

Estimation of the production of biogas is a relatively difficult operation if we want to consider all the characteristics and modalities regarding the configuration of the biodigestor.

Models presently available are extrapolations of numerous and detailed empirical approaches which not only result from measures taken on existing installations but also on laboratory prototypes easier to equip and to observe.

Manufacturers have always the possibility to test the biogas production in laboratories from samples of representative waste and on the basis of the configuration they propose. But analysis of the results from a laboratory prototype is a minimal precaution which should be taken before deciding on any investment.

Estimation

If we retain the configuration proposed above, and on the basis of the relatively rich technical documentation relative to biomethanisation of domestic waste, it is possible propose, for a waste flow of 10T/day.

It is estimated that a production of approximately 220 m3/day, with a HRT of 20 days, will be obtained from this substrate, which is a relatively low production when compared to the one which will have been obtained from waste without green waste, but the mixing will be easier.

To synthesize, on this flow of waste which we can consider as typical of the waste produced by a coastal village of 25,000 inh. the yield of this biodigestor can be estimated at 170m3/T of FM, which is relatively good specially with a methane rate which can be estimated at 65% based on the nature of the digestible OM and on the process configured.

The energetic value of biogas is essentially supplied by the methane (CH4) it contains. The Low Calorific Value (LCV) of a fuel is the quantity of heat produced by the complete combustion of a unit of this fuel. In other words, it is the quantity of energy supplied when part of its total energy is used to vaporize the water or to oxidize its non combustible components.

  • The LCV of methane is 36,5 MJ/m3or 9,94 KWh/m3 and it is on this component of biogas that energetic yield should be estimated as the LCV of biogas is proportional to its content in CH4.
  • Thus we have : LCV (biogas) = Q * 36,5 MJ/m³ = Q * 9,94 kWh/m³ where Q is the volumic content (%) of CH4 in biogas.
  • The average content of CH4 in biogas can be approximated at 60%, we can therefore estimate the average LCV of biogas at 21, 9 MJ/m³ or 5,96 kWh/m3.

The evaluation of the energetic potential and the electrical production of our example can be summarized as in the figure.

We note that the energetic yield of the system motor/generator is not very high and that a significant part of the potential energy of the biogas is wasted in heat losses.

However, in a thermophilic process we can recover up to 85% of these calories through a heat exchanger which can supply the heat required to maintain the temperature in biodigestors.

Optimizations

The use of biogas directly from the biodigestor is not recommended as biogas is composed of several gases which are not all combustible. Moreover some of them can harm certain parts of the engine or turbine of the generator.

Taking into consideration the nature of the substrate, the composition of biogas supplied by the installation taken in our example above, can be estimated as in the figure.

It is therefore judicious to undertake a purification which can be carried out as follows:

  • Extraction of CO2 in alkaline water (solubilization)
  • Use of a biofilter (compost seeded with a sulfato reducing flora) to extract the H2S (chemical processes also exist or use of membrane but are more delicate to manage)
  • Dehumidification by condensation

Non purified biogas is less calorific (18 to 25 MJ/m3) depending on the proportion of CO2) than purified CH4(36.5 MJ/m3). The extraction of CO2 also allows the reduction of the storage volume of gas.

In our case, the purifying treatment of biogas would allow the optimization by 30% of the energetic production of the system, i.e. the equivalent of 70 kWh.

Production of metha-compost and agronomic uses

Biodigestion protocols by solid mode generate metha compost in addition to biogas, which unlike the one produced by liquid mode, is quite easy to spin and mature.

This by product of the methanic digestion generally presents the following characteristics:

  • Dry matter: 50 % to 75 % Organic matter : > 20 %
  • COD (mgO2/g VDM) : < 100 (stabilized product)
  • Nitrogen, phosphorus, potassium : <3 %
  • C/N ratio : between 10 and 20
  • Color: black.

The agronomic value of metha compost depends first on the type of waste and material treated but also of the process used.

Generally, metha composts are richer in total nitrogen and ammonia than compost made from aerobic processes.

However, they are often less rich in fibers and they are more rapidly washed out and part of the nitrogen not readily available to plants is lost and can eventually contribute to soil pollution.

Thermophilic processes present the advantage of a high hygienization of the metha compost but where nevertheless some viral strains and some bacteria or bacillus remain alive.

Depending on whether the substrate initially contains risky products (animal manure, human faeces, waste from slaughter house), it can be opportune to apply a thermophilic aerobic composting phase which will in addition accelerate the final maturation of the metha compost.

To conclude, we can say that a biodigestion unit designed for the treatment of the organic fraction of domestic waste and equivalent is a complete system but a complex one. Therefore a detailed study undertaken by specialist prior to decision making is strongly recommended.

Indeed, the production of metha compost and biogas vary according to processes and protocols selected and the quality of treatment of the waste vary accordingly.

Synthesis

The conception and the implementation of the organic waste treatment system by anaerobic mode can be analyzed on three major parameters:

Efficiency of the treatment of waste

The efficiency of the system depends particularly on the adequation of the process with the nature of the flow of waste to be treated.

The digestible organic fraction of domestic waste exists and is particularly important in priority regions of ReCoMap but it can only be considered if sorting at source, voluntary deposit schemes or separate collection systems have been implemented.

The digestibility and productivity of the substrate are values which are linked. They should be considered together with the frequency and regularity of supply to the station to determine the type of process which should be privileged. For example if the flow of waste contains an important part of semi solid waste (animal manure, fats and oils, organic sediments and WWTP sludge), the addition of material rich in fibres (green waste, saw dust and rice husks) to remain in the configuration of a solid treatment and to respect a balanced C/N ratio.

It is only with such an approach that the treatment will be able to operate effectively without big variations and costly or complex requirements in terms of “raw material” and energy.

Indeed the measure of efficiency of the treatment of waste through a biodigestor is undertaken by first observing the regularity of the biological process occurring in the biodigestor (conditions of start, duration of primary phases, productivity of the methanogenesis) and this regularity, if it depends on a good initial configuration, remains strongly dependant on the capacity of the system to absorb the quantitative and qualitative variations of the incoming flow. In this regard, a biodigestor is much more sensitive than a composting station. To summarize, we can say that if we envisage to set up an anaerobic digestion unit for a given load of waste and if we aim at achieving the proper treatment of the waste, we must first have a perfect control on the upstream waste management.

Feasibility of the technology transfer

It is clear that the implementation and operation of a biodigestion unit for the organic fraction of domestic waste is a complex project which require, from the design stage to the operation and maintenance phases, that the beneficiary possess the minimum knowledge to be able to evaluate risks and to take the responsibility of its management.

This difficulty can be overcome by good training and transfer of competencies and the recruitment of specialists to cover the technology needs. However, the level of technicity of this type of installation also results in high investment costs and therefore brings the question of economic feasibility.

Economic feasibility

At this stage of reflection, we must not only consider the costs of the technology and its maintenance but also the income that are expected from the sales of the products of the biodigestor. They can be differentiated in three categories:

  • Public service for solid waste management
  • Energy recovery (biogas)
  • Agronomic use of by product

It is the waste manager of the local, regional or national authority who will be in charge of establishing the balance and ensure that the return on investment will match with the budget available.

We can however note that if it is correctly designed, a methanic digestion unit can beneficiate from funding under the Cleaner Development Mechanism (CDM) and that this source of financing can be significant.

Introduction to Recycling of : Plastics, Paper & cardboard, Non-ferrous metal and Glass

This group is the last one of the series of technical notes proposed for the ReCoMap project within the sustainable coastal waste management theme. They deal about the recycling of non organic waste for the four major group of waste:

  1. Recycling of plastics
  2. Recycling of paper and cardboards
  3. Recycling of non-ferrous metals
  4. Recycling of glass

The choice of recycling as a major issue for waste management in the ReCoMap countries is based on the fact that a global international market for recycling of non organic waste exists but it often encourages waste managers in developing countries to collect and export the sorted waste whereas simple techniques are available for the setting up of complete recycling options at national, regional or even urban level.

In this context, the present TNs will not aim at listing the industrial or commercial exporting options for sorted recyclable waste, this global industry being very active in terms of commercial prospection and communication. We will rather opt for the presentation of a few recycling techniques which appear to be the most adapted to the ReCoMap targeted countries.

The objectives of these TNs therefore consist in describing to the waste managers the techniques which are adapted to their countries and regions in a view to recycle locally a large part of their non organic waste by supporting the implementation of financially sustainable and environmentally positive activities.