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30BIOTECHNOLOGY OF LEATHER PRODUCTION FAT-CONTAINING WASTE RECYCLING
USING CO-FERMENTATION
Golub N.
Doctor of Technical Sciences Professor of Department of ecobiotechnology and bioenergy, Igor Sikorsky Kyiv Polytechnic Institute, Shinkarchuk M.
PhD student of Department of ecobiotechnology and bioenergy Igor Sikorsky Kyiv Polytechnic Institute, Kyiv, Ukraine,
Kozlovets A.
PE "Kievbudproekt", technological engineer, PhD
Abstract
Anaerobic fermentation of fat-containing wastes is a complex process that requires the use of additional elements to ensure a complete process and increase in biogas yield. The process of fatty raw materials co-fermentation with corn, apples and potato wastes is investigated. It is shown that the ratio of fat/co-substrate 1:1 is rational. Under these conditions, when co-fermenting with corn waste, the biogas yield reaches 76.2 ± 3.8 cm3/g DOM with a methane content of 63.5%, with apple wastes - 54.1 ± 2.7 cm3/g DOM with a methane content of 71%. With an increase in the fat content, the biogas output reduces by 10%, but the methane content in biogas is increased by 8.5%.
Keywords: co-fermentation, anaerobic digestion, biogas, methane, fat, co-substrate, corn waste
INTRODUCTION
The waste from the leather production, which is formed during the preparation, processing and treatment processes of raw materials at leather factories, can be divided into realizable (can serve as secondary raw material for various products manufacture) and those that are stored without further processing at the containing areas (Rubanka et al., 2015). The most common of such waste is fat-containing raw material with chemical impurities (chloride, sulfide and sodium carbonate, calcium hydroxide, etc.), which are introduced in treatment process and limit the possibility of waste usage (Golub et al., 2017). Proceeding from the above, the task of developing technology for such raw materials recycling appears.
The addition of other wastes that can be used as a co-substrate in anaerobic fermentation of fat-containing raw materials is a rational solution for such a task, since it enables the following to be carried out:
- reduce the impact on the environment, as in the process of methane fermentation following occurs:
A) death of the most of pathogenic microorganisms species dangerous for humans and animals;
B) elimination of unpleasant smells of fat-containing raw materials during storage;
C) reduction of waste storage sites area due to the higher ability to release moisture compared with undigested waste (Golub et al., 2018);
D) elimination of surface water contamination risks.
- obtain useful products, namely:
A) biogas, during combustion can produce heat and electricity that can compensate the costs of use and implementation of such technology;
B) bio-fertilizer, which is formed in the raw materials fermentation process (Golub et al., 2014), which makes it possible to increase the profitability of fat-containing waste processing.
- Co-fermentation of fat-containing waste positively affects:
- anaerobic digestion process, because it provides the necessary amount of nitrogen and trace elements that are absent in only fat-containing raw materials;
- regulation of the working medium pH value inside methane tank;
- reduction of salt concentrations by diluting of fat-containing raw materials by co-substrate (Golub et al., 2018).
The use of leather production solid waste co-fermentation at Khazaribagh, Dhaka (Hazaribagh, Dhaka) together with domestic sewage and cow manure allowed to solve the problem of low C / N (2.64) ratio and high pH (10.99) value in wastes (Basak et al., 2014). Co-fermentation were performed at different ratios of co-substrates with a total dry substances content of 6% in mesophilic mode. As a result of research, it was found that the sample containing 75% of leather production wastes with domestic wastewater (dry matter ratio (on dry matter) 1: 1) and 25% of cow manure, showed the optimal result, in which the specific biogas yield increases up two times compared to control sample, and reaches 0.476 dm3/g DOM (dry organic matter) with 73% methane content.
The ratio of co-substrates in the fermentation process affects the biogas production. As a result of joint fermentation of slaughter waste (DOM 18.96%, DM 91.97%) (Adrar, Algeria) with sludge from water treatment facilities (DOM 8.21%, DM 52.51%) used as inoculum, during 47 days at a temperature of 35°C in a reactor of 1 liter volume at a ratio of inoculum / substrate 1, 0,5 and 0,3. Biogas output was 504, 856 and 864 cm3, respectively, for the entire period of fermentation (Slimane et al., 2014).
Also, the raw materials concentration affects the biogas yield. So, with the digestion of 1:1 mixture by weight of the myast, which was formed during stages before and after leather irritation (DOM 55.6%), and
primary sediment of wastewater treatment plant from leather production (DOM 35.24%) at a dry organic matter content in reactors of 17.2 , 21.2 and 26.7 g/dm3. Specific biogas yield was 0.419 dm3/g DOM (content of methane 71-76%), 0.635 dm3/g DOM (methane content 75-77%) and 0.535 dm3/g DOM (methane content 72-77%) respectively (Thangamani et al., 2009).
The use of co-substrates allows to increase the load on the fat-containing waste reactor. At the same time, the biogas output is increasing, but the specific yield per g DOM decreases in the case of tannery waste with the primary sediment from leather production wastewater treatment plant in Leon, Mexico (Polizzi et al., 2018).
It was shown in (Nayono et al., 2010) that with the co-fermentation of artificially created organic fraction of solid household waste with 28% of pork fat (food waste), the content of methane in biogas increased from 58 to 61%, compared to organic fraction fermentation of solid household wastes without fat. However, with the addition of animal fats as a co-substrate, the digestion process is initially accompanied by a decrease in biogas yield, which is due to long chain fatty acids accumulation. The addition of biodiesel production waste - raw glycerine - to solid household waste increases the content of methane in biogas by almost 50% (Fountoulakis et al., 2009).
The addition of enzymes to increase the hydrolysis rate of fat-containing raw materials leads to a reduction in fermentation duration and an increase in the biogas output. With co-fermentation of the myast (DOM 14.0 ± 0.17 mg/g of wet mass) with primary (DOM 37.8 ± 0.49 g/dm3) and secondary (DOM 19.95 ± 0.51 g/dm3) sewage treatment plants sludge from leather plant (Chennai, India) at a ratio of 5:9:1 DOM, the biogas output reached 379 cm3/dm3 DOM. The use of lipase mount - steapsin (0.75 g/7.5 g DOM) in the process of co-fermentation increases the biogas output by 15% and reaches a specific yield of 440 cm3/g DOM). The addition of this enzyme reduces the digestion period by 30% (Sri Bala Kameswari et al., 2011).
It is determined that the most rational on component compositions of co-substrate are poultry droppings (Golub et al., 2018). The addition of droppings as a co-substrate to fat-containing waste
from the leather production at different ratios "fatty waste: chicken litter" (DOM in a reactor is 7.5%) allows biogas to be obtained, with a methane concentration reaching 64 ± 1.8% - 66 ± 2, 64% during the 23-day digestion period. By the availability of application in biogas production droppings can be equated to plant waste, which is why this study was taken as the basis for the search for an optimal ratio of plant co-substrates.
Most researches on waste recycling of leather industry relate to the co-substrate digestion of fat-containing waste with sludge from wastewater treatment facilities. At the same time, there are no studies on the digestion of fat-containing waste with plant waste. Therefore, the purpose of the proposed work is to research the process of co-fermentation of fat-containing waste with plant wastes of various composition. To achieve this goal, it is necessary to investigate the dynamics of biogas output changes during co-fermentation of fat with various co-substrates and to determine the rational fat:co-substrate ratio for each type of co-substrate, at which it is possible to obtain the maximum energy yield.
MATERIALS AND METHODS As a model substrate for fermentation, pure porcine fat with addition of salt (3% NaCl by weight DOM) were used to simulate the composition of raw material similar to fat-containing leather production waste. As a co-substrate, corn waste (leaves, wrappers), potatoes peels, apple pulp. The ratio of fat-containing raw material (F): co-substrate (C) was calculated by DOM. As the control points of the experiment, the ratio of F:C - 1:1, 3:1, 4:1, 9:1 was selected as the experimental samples.
As a control for biogas yield comparison at different ratios of plant co-substrates, samples with the same ratios of fat-containing raw material: droppings were used. Poultry droppings is selected as a control, because in contrast to the plant material, the droppings contain proteins and fats, it is rich in all the trace elements that are necessary for anaerobic microorganisms association development. The chicken droppings from the non-laced technology of poultry farming was used (from a farm at Zhytomyr region, Ukraine). Parameters of raw materials used are given in table. 1
Table 1.
Raw material parameters for reactor feed
Raw material Humidity, % Ash, % DOM, % Note
Pure pig fat 1,9-0,095 0,13z0,007 97,97-4,90 3 % NaCl by dry organic mass (DOM)
Corn waste 68,143z3,407 2,48-0,099 29,377z 1,469 Particles 2-3 mm, nitrogen content 2,8 %, Р2О5 1,8 % by dry matter1
Potato peels 73,703z3,68 0,323z0,012 25,974-1,03 Particles 2-4 mm, nitrogen content 0,8 %, Р2О5 0,2 % by dry matter1
Apple pulp 79,511=2,38 0,116=0,004 20,37^0,815 Imitated, nitrogen content 1,1 %, Р2О5 1,4 % by dry matter1
Chicken droppings 61,1±3,05 28,9±1,445 10±0,5 Grinded, nitrogen content 18,4%, Р2О5 14,3 % by dry matter1
1 Literature data (Biogas manual et al., 1988).
Determination of raw material main characteristics for fermentation (humidity, ash), on which the reactors loading was calculated, were carried out using the analytical weight Scout PRO model SPE-123 according to standard methods (GOST 26712-94, 1986; GOST 26713-85, 1986; GOST 26714-85, 1986). The rational content of dry organic matter in the
reactor was 7.5%. Determination of working environment pH value was carried out using the ionmeter MI-150 (Russia). The qualitative and quantitative composition of the biogas components determined using the gas chromatograph «CRYSTALUX 4000M» (RF) according to the standard method (Leybnyts et al., 1988). The
calculation of the ratio C:N:P (table 2) was calculated according to the methodology given in the manual (Biogas manual et al., 1988).
Table 2
_Calculation of C: N: P ratio for fat (X) and co-substrate (Y)_
Co-substrate ratio X:Y
Co-substrate
Equasion
C:N:P ratio
Corn
1:1
Potato
Apple
Droppings
70:2:1
620:4:1
85:1:1
8:1:1
Corn
3:1
Potato
Apple
Droppings
179:2:1
1600:4:1
224:1:1
21:1:1
Corn
4:1
Potato
Apple
Droppings
218:2:1
1960:1:1
280:1:1
27:1:1
Corn
9:1
Potato
Apple
Droppings
506:2:1
4538:4:1
644:1:1
62:1:1
As inoculum of anaerobic microorganisms association, sludge from fat-containing raw materials anaerobic fermentation reactor was used (Department of Environmental Biotechnology and Bioenergy of the National Technical University of Ukraine "Igor Sikorsky Kiev Polytechnic Institute").
Co-fermentation was carried out in anaerobic conditions at a temperature of 37 ± 1 °C in a dry-air thermostat TC-80M with stirring 1 time per day (when measuring parameters). Reactors with a volume of 1 dm3 with a filling factor of 0.7 were used. Inoculum
load was 3.5 g by DOM for all samples. The period of co-fermentation ranged from 14 (for all samples) up to 23 days (for samples that showed the largest biogas output).
RESULTS AND DISCUSSION
The beginning of gas formation was observed on 1-2 days of fermentation for all ratio of co-substrate. The total yield of biogas depending on added co-substrate for 14 days of fermentation is shown on the figure 1.
b
а
1. day t, dny
c d
Figure. 1. Relation of the total biogas yield (V) from time (t) with co-fermentation offat-containing raw materials with waste: a) corn, b) potatoes, c) apples, d) droppings (control) at different ratios of the substrate
F:C.
Since the biogas output in potato waste samples stopped after 5-9 day, depending on the co-substrate ratio, for further study co-substrates with corn and apples waste were used. Digestion was carried out during 23 days for the fat/co-substrate ratio of 1:1 and 9:1. On figure 2 the daily biogas output at co-fermentation with plant raw material and sewage is shown.
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Figure 2. Daily biogas yield (V) for period (t) with co-fermentation of fat-containing raw material with co-substrate at different ratios of F:C: a) 1: 1, b) 9: 1, where control is poultry droppings.
a
b
The total yield of biogas and methane for 23 days of fermentation is shown on the histogram (Figure. 3)
Figure 3. Total biogasyield (V) and methane content in it (V) during co-fermentation F:C.
Based on the obtained results, shown on Fig. 1, the least effective co-substrate for fat containing raw materials were potato wastes - starchy raw material. For all co-substrates ratios F:C, except for 3:1, biogas production was observed during the first 5 days of fermentation. Inhibition of the biogas generation process was due to medium pH value reduction down to 5. In this case, the generated biogas couldn't support combustion, and had a smell characteristic for propionic acid. That is, with the starch presence, its rapid hydrolysis occurs with the subsequent formation of organic acids, which are not suitable for further transformation into methane, which leads to a pH value decrease. In the case of fat to potato waste ratio 3:1, the process methanogenesis stops on 10 day, also becouse of pH value reduction down to 5.28. Increasing and decreasing the fat content relative to the ratio of F:C 3:1 leads to the accelerated formation of organic acids, which changes the fermentation process direction and inhibits the biogas formation. That is, for the process of raw materials anaerobic fermentation, which is characterized by low pH value, when combined with fat-containing raw materials, the ratio of carbon: nitrogen: phosphorus does not affect the process, as the reduction of pH value inhibits the co-fermentation process, regardless of the C:N:P ratio.
For corn waste, characterized by the cellulose content of 21-39 % and 12-17 % lignin, and apple wastes, which include 2 % fiber without lignin and hemicellulose, the process of co-fermentation with fat-containing raw materials lasts for 23 days (Figure 2). That is, the presence of hemicellulose and cellulose in raw materials allows it to be used as a co-substrate in the digestion of fat-containing waste, as the process goes without medium acidification.
For corn, the highest biogas output is characteristic for the ratio of F:C 1:1 and 3:1 (Fig. 1, a). To create the rational conditions of the digestion process, the optimum ratio C:N should be 30:1 (Biogas manual et al., 1988), which is provided at a ratio of F:C 1:1. With an increase in fat content, the C:N ratio increases and reaches the ratio C:N:P as 218:2:1 and 506:2:1 at a content of fat raw materials 4:1 and 9:1, respectively. Deficiency of nitrogen and phosphorus, as well as other trace elements, provided from plant material, reduces the biogas yield down to 54,3 ± 2,7 cm3/g DOM for the 1:1 ratio, to 38,0 ± 1,9 cm3/g DOM for 4: 1 and down to 41.2 ± 2.1 cm3/g DOM for the ratio of 9: 1.
For apple waste, the optimal ratio of F:C is 1:1 (Fig. 1, c). Increasing the fat content in the ratio of components 9:1 reduces the biogas output by 11% compared to ratio of 1:1 (Fig. 3). One of the possible assumptions may be to change in C:N:P ratio from 85:1:1 for F:C 1:1 to 644:1:1. for F:C 9:1. With an increase in the proportion of fat-containing raw materials, the share of C increases, which affects biogas output, cousing reduction, due to lack of nitrogen and phosphorus. Despite a significant proportion of carbon for F:C 9:1, the process of methanogenesis continues, as there is no pH value decrease. It is possible that pH value changes do not occur due to a relative reduction in the amount of hemicellulose, which is hydrolyzes faster than fat and is a substrate for the low molecular organic acids formation. Under these conditions, the self-regulation of the system occurs becouse of carbonate buffer, which is formed during fermentation (Golub et al., 2014). The prolonged decomposition of fat-containing material provides microorganisms with a substrate for long periods. However, the biogas output at co-fermentation of fat-containing raw materials with apple waste is 29% less compared to the process of co-fermentation with corn waste, although biogas will have a higher methane content (71 ± 1%).
An important factor in the selection of a co-substrate is its availability and the ability to store it unchanged for a long time period. The apple waste is mainly seasonal raw material, which can be used to replace other co-substrate, for example, corn or droppings.
For all ratios of F:C (Figure 2), where corn and apple wastes are used as a co-susbstrate, there is a peak biogas output on 2-3 day, in contrast to control sample where droppings are used. In this case, the peak yield falls on the 9-th day, which can be explained by the slower decomposition of high molecular compounds contained in droppings. The maximum biogas yield during the co-fermentation of fat-containing waste with plant raw material is characteristic for corn as a co-substrate and reaches 4000 cm3, which is 18,86 ± 1% less compared with bird droppings. Droppings is not only a source of trace elements, vitamins and other biologically active substances for microorganisms association development (Golub et al., 2018), but also a source of additional methanogenic microorganisms that are capable to fermentate various types of organic wastes, and during adaptation process are included in the association renewing it causing increase in biogas
production. However, despite this, corn residues remain one of the best co-substrates for the fat-containing raw materials digestion, since the parts of leaves and stalks serve as a immobilization material for fat and microorganisms, which increases the concentration of raw material and inoculum in the reactor.
For the ratio of F:C 1:1, the biogas output reaches: for F:D - 93.9 ± 4.7 cm3/g DOM with a methane content of 64.3%, F:C - 76.2 ± 3.8 cm3/g DOM with a methane content of 63.5%, F:D - 54.1 ± 2.7 cm3/g DOM with a methane content of 71%. With an increase in the fat content in substrate, the biogas yield for plant raw material decreases compared to droppings. For a ratio of F:C 9:1, the biogas yield co-substrate from corn waste goes up to 68.7 ± 3.4 cm3/g DOM, which is 19 ± 1% lower than with droppings, for the apples waste up to 48.1 ± 2 , 4 cm3/g DOM, which is 43 ± 1% lower than with droppings. With increase in fat content in the substrate, the methane content in biogas also increases
Since the goal is the processing of fat-containing waste with high salt content, the rational ratio of fat/plant raw materials co-substrates is 9:1 for the technological solutions development for raw materials recycling with the simultaneous biogas production. The use of corn waste as a co-substrate reduces the biogas yield by 10 ± 1% compared to the ratio of F:C 1:1, but due to an increase in the methane content by 8.5% in biogas, the overall yield of energy carrier remains the same. This increases biogas energy capacity. With the use of apple wastes biogas output is reduced by 11% compared with the ratio of F:C 1:1. The output of methane also decreases by 11%. That is, the most rational co-substrate for fat-containing waste digestion is plant raw materials containing cellulose and lignin.
CONCLUSIONS
It has been shown that for the fat-containing waste processing in biogas it is possible to use plant wastes as co-substrate, containing cellulose and lignin and provide microorganisms association with the necessary nutritional elements. It is shown that the use of corn and apple wastes is possible for such process.
It has been established that co-substrate ratio by DOM of fat-containing raw materials/corn waste affects both the biogas yield and the methane content in it. At a 1:1 ratio, the biogas output reaches 76.2 ± 3.8 cm3/g DOM containing 63.5% methane, at a ratio of 9:1 up to 68.7 ± 3.4 cm3/g DOM with methane content of 69.5 %
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