157
Qualitative study of solid poultry digestate from an industrial digester installed in Tunisia
Y. M’Sadak*, I. Ghariani, A. Ben M’Barek, L. Tayachi
Department of the Genius of the Horticultural Systems and the Natural Environment, Higher Agronomic Institute of Chott-Mariem, University of Sousse, Tunisia
*Corresponding author. E-mail: msadak.youssef@yahoo.fr
Received 09 Dec 2014, Revised 16 Dec 2014, Accepted 19 Dec 2014
Abstract
: The Agricultural employment aboveground digestates or residues from anaerobic digestion can be used as an alternative to peat. The solid digestate poultry (called Poultry Methacompost: PMC) can be used as a partial substitute in the manufacture of culture substrates. The humified Organic Matter (OM) of mature substrates can be adopted only with the presence of necessary conditions for the development of the plant especially: respect of pH, percentage favorable of aeration, good sanitation, balanced contents of minerals. The present work aims to evaluate three different PMC from an industrial biomethaniser; one is obtained from the digester and the other two from the decanter (during two different periods). It appears from the physico-chemical characterization undertaken the following key results: In general, the PMC has a high bulk density, insufficient porosity of aeration, while the porosity of retention largely meets the standard.The phosphor and potassium are also eligible. The warranty of quality of PMC ensures greater acceptability and facilitates the incorporation with peat for soilless horticultural plants.
Keywords: Poultry manure, Industrial biomethanation, Poultry Methacompost , Physico-chemical characterization, Partial substitute, Peat, Aboveground culture.
1. Introduction
The current use of energy resources has led to environmental problems by causing the pollution of air, water and land. These adverse effects have increased the interest in developing new technologies for clean energy, mainly through the use of renewable sources [1]. Thus, major common energy resources such as coal, oil and natural gas are being depleted, and it is estimated that the destruction of oil resources could be in fifty years [2]. Wood and other biomass are among the main renewable energy resources [3, 4]. Biomass is the only source of fuels liquid, solid and gaseous [5]. Among renewable energy, biomass energy is the only comes from organic materials [6]. The recovery of organic waste, agricultural and animal wastes mainly by anaerobic digestion or biomethanation is considered as an attractive solution for many plans; environmental (reduction of pollution load), energetic (Production of biogas) and agricultural (farm employment direct and / or indirect of residues). In Tunisia, there are mainly two major sectors to produce biogas depending on the type of waste being used: the sector of valorization of wet and dry waste generated by the agrofood sector and the sector of valorization of liquid wastes from sewage treatment plants [7]. Anaerobic Digestion (AD)
158 is a method of biological treatment of organic solid and liquid residues [8], which leads to the formation of one major product called biogas mainly composed of carbon dioxide and biomethane and to a more or less liquid, called digestate [9]. Digestate (secondary co-product) can be valorised in two ways: either integrated as raw material in a string of composting, or spread directly on agricultural land as amendment. Post- processing of digestate usually leads to the production of a methacompost (solid digestate) and a juice process or leachate (liquid digestate). Peat, generally used in the production of horticultural substrates, is a non-renewable resource [10, 11]. It would be interesting to develop the research of alternative products and to encourage the partial use of substitute products, which help to retain the benefits of peat and allow limiting the quantities used, and therefore reducing imports and hemorrhage of devices. In this context, the work undertaken consist to determine some criterias for physico-chemical quality of three Poultry Methacomposts (PMC) from industrial biomethanation applied to biomass generated by intensive rearing of laying hens, enjoying the possibilities of their incorporation with peat as a substrate of growing market garden plants.
2. Materials and methods
2.1 Experimental equipment
The PMC, byproducts solid objects to this investigation are from the AD of poultry droppings. The digester producing these residues is part of a pilot project for industrial biomethanation located in Hammam Sousse, Governorate Sousse, Tunisia. This pilot project was conducted in 2000 as part of the new strategy adopted by the National Renewable Energy Agency (NREA) focusing on the development of industrial biogas systems. This is a digester with a capacity of 300 m3, with an uninterrupted alimentation daily by 10 m3 of substrate made up from approximately 1/3 of poultry droppings and 2/3 water [12].
The poultry biogas undergoing a purification step (desulfurization with hematite iron) before use, the rate of biomethane (CH4) has increased from 60 to 75% following the applied purification (a removal efficiency of 25%). After purification, there is also a significant improvement in the Lower Calorific Value (LCV) which evolved from 5110 to 6389 kcal/Nm3 (or also, a relative increase of 25%) [13]. These poultry droppings produced through three different basins: A first tank for a short residence time, before staying in the settling tank or clarifier. From the latter, the sludge settled droppings are dried and transformed to PMC. As to the supernatant water the pond of settling, it passes into the third tank for serve to scraping of fresh poultry droppings which feeding the digester. The three MCA placed for this study, are reported below.
PMCdig: Refined Poultry Methacompost (Digester output).
PMC1dec: Refined Poultry Methacompost (Decanter output).
PMC2dec: Refined Poultry Methacompost (Decanter output).
The physico-chemical analysis were made during the same year for the PMCdig and PMC1dec, while those for the PMC2dec were performed in another campaign to highlight any differences due to the state of evolution.
2.2 Physical Characterization 2.2.1 Bulk density
The Bulk Density (BD) is the mass of the unit volume in the dry state. The measurement of BD was performed only for the MCA2dec. It consists to put the samples in stove at a temperature of 105°C for 24 hours to determine the dry mass [14]. It is expressed by the following formula:
159 BD (g/cm3) = (Ms – Mc) / V
Ms: dry weight of sample (g), Mc: mass of the empty capsule (g), V: volume of the capsule (cm3).
2.2.2 Porosity and hydraulic features
Porosity or poral space corresponds to the evaluation of voids spaces in relation to the overall dimensions of a substrate [15]. For PMCdig and PMC1dec, evaluating the porosity was accomplished by the standard porosity test. The equipment used for this test was formed especially by: containers for 15 cavities, graduated test tube, drilling nail, tumblers and stopwatch. The measured variables are:
Total volume (Vt) in m l= volume of container relative to 3 cavities of container.
Volume of poured water (Va) in ml = volume of pores: air and water.
Volume of collected water (Vr) in ml = volume of pores: air.
The studying parameters are:
Total porosity: Pt (%) = (Va / Vt) × 100 Porosity of aeration: Pa (%) = (Vr / Vt) × 100 Porosity of retention: Pr (%) = Pt – Pa
For the other methacompost (PMC2dec), estimating the porosity was accomplished from the hydraulic characterization. Determining the properties of water and air flows for substrates was extricated from pF curves. The total porosity (Pt) is expressed by a ratio between the volume of voids spaces (Vv) to the total volume or apparent volume (Va). The apparent volume is the sum of volume of the solid phase and the void volume. It is difficult to measure directly. There are, in fact, several formulations which are slightly different from each other [15], among which, we used that utilized by Gras [16] and it is given by the formula below.
Pt (%) = 95.83 – 32.43 BD
The energy with which the water is held by the porous medium will depend on the quality of the substrate, but also to its moisture content: we obtain a characteristic curve [17, 18] fluently called pF curve (potential of Free energy). The establishment of pF curves for each type of substrate allows comparing and characterizing them according to their hydraulic properties. In the field of plant growth, the forces of suction exerted through the roots correspond to a height of between 0 and 20 cm of water. Therefore, we use the logarithm of this suction [15, 17, 19].
Log 10 = pF
The samples of the substrates have been contacted with a water column by means of the layer of water saturated sand (suction table). It works by drying a product that has previously been brought to full saturation and the sample is then subjected to a suction corresponding to the chosen value. The hydric potential of the sample is equilibrated with the hydrostatic pressure of the water column.
Such a technique is physically limited to -100 mbar (pF2). This is a sandbox used for determining the pF from 0 to 2. The hydraulic potentials are obtained by creating a series of depressions and overpressure. The weighing of each sample after adjustment renders the content of water for each tension. The tested substrate is represented by two capsules each of which represents one repetition. According to Gras [17], the air content is determined after establishing the curve pF. It is complementary to the content of water, since these two fluids share the poral space. Therefore we have the following relation given by Morard [15].
Pt (% vol) = Water content (% vol) + Air content (% vol) With:
Air content at pF1 (% vol) = Pt (% vol) – MG at pF1 (% vol) Air content at pF1 = Porosity of aeration (Pa)
160 Water content at pF1 = Porosity of retention (Pr)
Moisture Gravity (MG) to pF1 was determined by using the following formula:
𝑀𝐺 𝑎𝑡 𝑝𝐹1 % 𝑣𝑜𝑙 =𝑀𝑎𝑠𝑠 𝑎𝑡 𝑝𝐹1 – 𝑇𝑜𝑡𝑎𝑙 𝑑𝑟𝑦 𝑚𝑎𝑠𝑠
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑐𝑎𝑝𝑠𝑢𝑙𝑒 2.3 Chemical Characterization
2.3.1 pH
The pH is measured after put in solution of 20 g of the sample in 100 ml of distilled water. The method consists of preparing a suspension of diluted substrate dried in 5 times its volume of water (1: 5), let it in agitation for 5 minutes, and then let it rest for minimum two hours. The pH reading is made by a pH meter.
2.3.2 Electrical Conductivity
The Electrical Conductivity (EC) allows measuring of the concentration of soluble ions of a substrate by the mean of the property of an aqueous solution to lead electricity relatively to its ion concentration [20]. It is measured by a conductivity meter, and it is commonly expressed in mS/cm or mmhos/cm3. A sample substrate is extracted with water at 20 ± 1 °C (extraction ratio of 1: 5 to dissolve the electrolyte).
2.3.3 Organic Matter
The determination of Organic Matter (OM) and ashes is carried out according to the Tunisian Standard (TS) relative to dosage of OM manure. To assess the rate of OM at each substrate, we must weigh 20 g that is placed in the stove for 24 hours at 70°C, then the calcination is made of 3 g predried in the stove for 2 hours at 900°C for at least 6 hours in a muffle furnace and we can determine the Dry Residue (DR).
The content of OM is determined according to the following formula:
OM (%) = [(M1 – M2) / M1] × 100
With: M1: Mass before calcination (mg) and M2: Mass after calcination (mg). From the OM, a deduction of the carbon content is possible by applying the following equation:
OM (%) = 1.4 × Corg (%) + 1.5 (TS) C (%) = (OM – 1.5) / 1.4 (TS)
2.3.4 Determination of nitrogen
The nitrogen content is determined by the Kjeldahl method [21], whose principle is based on the attack of the extract by concentrated sulfuric acid (H2SO4).
2.3.5 Determination of some mineral elements
The dosage of the chemical elements has also affected the phosphorus (P) and the potassium (K). It requires the mineralization and the preparation of calibration solutions. Mineralization is a common step, only the nature and the solution of calibration differs from a chemical parameter to another. The percentage of each element is calculated as follows.
P, K (%) = (n × V × D) / (D × 104)
n: value found (ug / ml), D: dilution of the solution to photometer (D = 1) ,V: solution volume (V = 100 ml) p: test sample (g)
161 The mineral forms are oxidized forms of major elements. They are obtained from the calcination of the DM.
The % of P2O5 and K2O were determined by using the following expressions:
P2O5 (%) = 2.3 × P (%) K2O (%) = 1.2 × K (%)
For technical reasons, P and P2O5 were evaluated only for MCAdig and MCA1déc.
3 Results and discussion
3.1 Evaluation of physical quality of substrates 3.1.1 Global Physical Results
This assessment has affected the physical parameters related to the BD such as those relative to various porosities. Table 1 summarizes the various physical parameters measured for each substrate considered.
Recall that the porosity results were obtained using different methods, which explains the observed difference in standards adopted.
Table 1. Results of the physical characterization of studied poultry methacomposts
3.1.2 Findings of the bulk density
The PMC2Dec presents a BD very high (0.78 g/cm3), which exceeds widely the usual values for substrates of crop: 0.10 <BD (g/cm3) <0.30, according to Morel and al [19]. This material is considered heavy substrate. The findings reported by Lamhamedi and al [22] ensure that the BD has a direct consequences on growth and morphology of roots. The PMC2Dec can not in any way be used in pure form, as it can cause major constraints during its partial incorporation with peat. In this regard, we must look for the adequate mixing ratio.
3.1.3 Findings of the porosity
The porosity of the PMC can be influenced, inter alia, by the dimensions of particles and the nature of the substrate (retentive and / or aerator). The quality and the composition of the culture substrate play an important role for obtaining of good seedlings. A good substrate of crop has an order of magnitude of the total porosity of 80 to 90% as the brown peat [23], but it can reach 95% [19, 24].
PMC Parameters Average Results Norm Reference
PMCdig
Pt (%) 67.30 Pt ≥50
[24, 25,26]
Pa (%) 11.20 Pa ≥ 20
Pr (%) 56.10 Pr ≥ 30
PMC1dec
Pt (%) 70.60 Pt ≥ 50
Pa (%) 12.10 Pa ≥ 20
Pr (%) 58.50 Pr ≥ 30
PMC2dec
BD (g/cm3) 0.78 0.10 < BD <
0.30
[19]
Pt (%) 70.54 80 < Pt <95
[19, 23, 24]
Pa (%) 16.19 20 à 30
Pr (%) 54.35 55 à 70
162 The three PMC have insufficient values of Pt (Table 1) compared to the standard reported by various authors [19, 23, 24]: 80 <Pt (%) <95. This may be due to the heterogeneous dimensions of particles (irregular ripening). They have a Pr complies with the standard and a low Pa, regardless of the standard applied. The fine particles of such substrates can store more water in the micropores and therefore, having a high water retention capacity (Pr), so, they can be considered as retentive substrates.
Based on the standards set by various authors [24, 25, 26], the both PMC put in porosity test (according to the standard test) show in this case a suitable porosity (Pt and Pr), but the Pa is still non-congruent. It should be noted that the Tunisian conditions require the following values of porosity: Pt ≥ 50%, Pa ≥ 20% and Pr ≥ 30%. These rules were inspired from the Canadian standards [24] by promoting retention on ventilation, due to the dry climate of Tunisia [25, 26].
A good substrate must meet certain criteria including a good ability to retain water, to drain and to rewet easily. [27] Peat is widely used in nurseries as a major component of the culture substrate through to its large holding capacity of water [28]. The PMC have a holding capacity of water close to that of the peat, while holding capacity of air is far from satisfactory.
According to Morard [15], presently we looked to use a substrate with good Pt, Pa and Pr. Hence, the three PMC studied, in pure state, should be avoided in order to not affect the air, by reducing the lacunar space and promoting the impasto of the substrate. Up to now, there is no real alternative material to the peat in terms of physical quality. In addition, the PMC, in the three states under consideration, requires an aerator substrate to correct the Pa and get a physical balance. It is necessary in this case to seek the best mix ratio for better physical optimization.
3.2 Assessment of the chemical quality of the substrates 3.2.1 Assessment of pH and electrical conductivity
Table 2 summarizes the pH and the EC for each substrate studied. The pH can be an indicator for the full maturity of a substrate. The pH of a ripe compost is normally between 7 and 8 [29] or between 7 and 9 [30].
The PMC tested can be considered like ripe substrates, since they have pH values according to the standard (Table 2).
The EC can inquire about the availability of mineral nutrients in the culture medium. The high content of salt affects the growth of plants that can cause burns in roots and in leaves. We must keep in mind that roots of plants rooted better in a substrate containing few nutrients [31].
Indeed, a high value represents a large amount of ions in solution, which makes more difficult the absorption of water and nutrients by the plant and may even burn the roots. Salinity can also develop from the nitrogen mineralization and production of organic acids [20]. The biomethanation, on the contrary, further reduces the salinity of the starting material (poultry droppings), due that animal droppings are known by their high salt content. According to Fuchs and al [32], the EC of the compost should not exceed 4 mS/cm. Excessive salinity of compost may be the cause of its phytotoxicity [20].
Referring to the constatations reported by CPVQ [24] and Comtois and Legaré [31], deficiencies in (Fe, Cu, Mn, B) may occur and possible excess in (Ca, N) may occur at the substrate. Soumaré and al. [33] assert that the substrates of crop should have a low EC less than 3 mS/cm. Beyond this value, negative impacts could occur on the germination and the emergence of seed. According to Morel and al [19], the EC is acceptable between 0.5 and 2.0 mS/cm. Based on this last standard, PMC2dec has a congruent value (1.30 mS/cm), while the other two were high (Table 2).
163 Table 2. pH and EC of tested poultry methacomposts
pH EC (mS/cm)
Norm 7 < pH < 9 0.5 < CE < 2.00
Reference [29, 30] [19]
PMCdig 8.20 2.90
PMC1dec 8.70 2.60
PMC2dec 7.43 1.30
3.2.2 Evaluation of the organic fraction
Table 3 summarizes the OM, nitrogen and C/N ratio observed for each substrate considered. OM content is of fundamental importance for the fertility of the substrate, because of its physical, chemical and biological effects. Peat is widely used in nurseries as a major component of substrates of crop due to its high content of OM [27]. Droppings of laying hens, according to Levasseur and Aubert [34], have a degree of OM around of 58%. AD is the source of a significant reduction in OM. The significant degradation of OM of poultry droppings during the AD is explained by the results found for the three studied methacomposts which are respectively in the range of 32.50%, 36.50% and 37.33% for PMCdig, and PMC1dec PMC2dec (Table 3). A too high degree of degradation of poultry droppings, resulting in a longer time of digestion, riking to decreas the value of organic value of PMC produced. This result is consistent with that reached by ADEME [35]
which states that the digestate produced is low in OM.
Table 3. % OM, % N and C/N of poultry methacomposts
OM (%) N (%) C/N
Norm 35 < MO < 45 0.92 < N < 2.76 8 < C/N < 15
Reference [27] [32] [17]
PMCdig 32.50 2.40 7.50
PMC1dec 36.50 2.60 7.80
PMC2dec 37.33 2.28 11.25
According to the instructions reported by Fuchs and al [32], ripe compost should have content less than 50%
OM (% DM). For quality compost, in viewpoint maturity and stability, the value of OM should be between 35 and 45% (% DM). Under this test, the PMC are considered ripe and stable products.
3.2.3 Evaluation of C/N
The AD is responsible for a significant reduction in the rate of OM. Despite this sharp decline, the levels of OM for PMC are still high to allow the production of a high-quality substrate. There is a significant mineralization of nitrogen, during the AD, proportional to the rate of biodegradation of carbon [36]. The nitrogen contents of the PMC (Table 3) are adequate in accordance with the standards described by Vanai [27]. According to the latter, compost for use in horticulture, must have nitrogen content greater than 1% of DM. The minimum value required by the Directive of the European Community is about 0.6% while the maximum value imposed by the French Standard FS U 44-051 is about 2% for the substrates of crop.
164 Beyond a value of 2%, the compost should be considered as an organic fertilizer [27]. The content found depends on the degree of maturity that is often enjoyed by the C/N ratio. According to Fuchs and al [32], the nitrogen content must be between 0.92 and 2.76%. The high nitrogen content of the PMC is due to the wealth at nitrogen of animals waste in general, and of poultry droppings in particular.
Since the PMC have fairly high levels which exceed acceptable levels relating to the substrates of crop, these organic materials cannot be used only as growing media. They usually lead to a weakening of the mechanical resistance of the plant and a greater sensitivity to certain fungal diseases [37].
The C/N ratio is often used to evaluate the process of mineralization of OM [38] and as an indicator of compost maturity [29]. According to Vanai [27], this ratio varies widely depending by the origin of composted waste and is influenced directly by the nitrogen content on the substrate, while the determination of the C/N of compost is not sufficient to evaluate its maturity [39]. The low C/N of PMC can be explained by the fact that the mineralization of animal composts is faster than that of vegetable origin (hard polymer degradation). According to Lemaire and al. [40], the OM with C/N medium or low are not suitable for the production of substrates of crop, because they evolve over time through mineralization. It results from settlements, variations related to porosity and loss of DM and the clogging by fine or colloidal elements.
Competition for oxygen between microorganisms appears, especially as the porosity decreases.
The C/N ratio constantly decreases during composting to stabilize at 10 (8-15) in ripe compost [16].
However, this parameter has a major drawback, because of the lack of accurate reference values. Starting from the standard announced, it can be said that the C/N ratio is acceptable in all three cases (Table 3).
Jimenez and Garcia, cited by Larbi [29] suggest a ratio less than 12 that reflects the degree of maturity of the compost. Brewer and Sullivan, cited by Tambone and al [41] proposed a C/N ratio ideal ranging from 12 to 25. This latter standard penalizes the results found. In general, the standard for 8 to 15 is the most used.
3.2.4 Evaluation of the phosphorus and potassium
Table 4 summarizes the various values of mineral elements (P and K) analyzed for each substrate studied.
Generally, the P content in compost varies from 0.7 to 0.9 (% DM) whose 50-60% are assimilable. Measured values for PMC tested meet this range (Table 4). The PMC is a material rich in K. Average grades of K for the PMC produced by biomethanation are of the order of 1.73%, 1.86% and 1.83% for the three successively PMC (Table 4). The K2O values are highly above to the minimum value required by the Directive of the European Union (0.3% K2O), cited by M'Sadak and al [25] to allow the marketing of compost. Overall wealth in minerals for the PMC could be a good index to reduce the number of fertigations practiced in the nursery, from where the double advantage of PMC as a retentive substrates (physical role) and rich in minerals. On the other side, their aeration is inadequate.
Table 4. Mineral substances identified for poultry methacomposts
P (%) P2O5 (%) K (%) K2O (%)
Norm 0.7 à 0.9 (% MS) 1.14 à 2.06 (% MS) K0.25 K2O0. 3
Reference [25] [25] [25] [25]
MCAdig 0.75 1.72 1.73 2.08
MCA1dec 0.78 1.79 1.86 2.23
MCA2dec - - 1.83 0.30
165
4. Conclusion
The agricultural exploitation in above-ground of residues or solid digestate from AD applied to poultry droppings proves environmentally interesting, while demanding quality and maximum efficiency for better valorization.
The solid digestate or Poultry Methacomposts (PMC), object of this study, has a fine granulometry of particules, which may create barriers to gas exchange (poor ventilation and excess water retention). Taking into account this major physical constraint and the large bulk density, they can be used only as partial substitutes for peat in the making of culture substrates.Incorporation in an amount of from 10 to 20% might be possible and profitable.
On chemical quality, the PMC showed a pH and salinity relatively acceptable. So they can be considered ripe and correspondingly stable and can be used in above-ground culture with great caution. PMC studied have a degree insufficient of OM, but they are rich in nitrogen, while achieving levels of phosphorus and potassium more or less valid.
Overall, the results obtained in this study show significant effects of poultry methacomposts in multiple parameters physical, hydric and chemical of growing media made up. This constatation justifies their use only as a partial substitute for peat, thereby reducing some of the quantities imported.
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