4. DESARROLLO DE NUEVOS Y MEJORES MÉTODOS DE
4.3 Desarrollo de derivados enzimáticos rodeados de microambientes
Co-digestion is a process whereby homogenous mixtures of two or more of energy-rich organic substrates are added to a digester with the aim of enhancing the methane production [69]. Traditionally, AD uses a single substrate for single purpose treatment. With more work on the biogas technology, it has been noted that the process become more stable when a variety of substrates are applied into the system simultaneously [69]. This improves nutritional supplements for microorganisms, dilute such as sulphur-containing compounds
which can potentially be toxic thus affecting the microbial environment and it increase the performance of the system, thus more biogas [70].
Supportive studies by Weiland [71] reported 40 – 80% increase in biogas from co-digestion of organic wastes and by-products. Results by Alvarez and Liden [72] showed of 80% methane production from co-digestion of slaughterhouse waste, manure and fruit and vegetable waste. This was supported by Ukpai and Nnabuhi [73] with 76% methane yield when co-digesting cow dung with cow pea and cassava pealing. A similar study was done by Ward [74] on optimization of anaerobic digestion of agricultural resources showed 82% conversion of volatile solids, showing an increase in methane production.
In an attempt to optimize biogas production of various industrial sludges, Mahanty et al. [70] treated waste sludges from waste treatment facilities and reported maximum methane recovery increasing from one batch to three batches shown in Figure 2.8. This was due to the positively interacting pairs in co-digestion process as described by Abdullah et al. [75]. Furthermore, they noted that methane yield was found to decrease in five batches based optimized co-digestion process. Thus, the digestibility of various industrial sludges is improved under various batches.
Figure 2.8: Methane yield from utilisation of sludges in different co-digestion process scenarios consisting of one to five independent co-digestion batches [70].
Xiao et al. [76] presented a pilot scale anaerobic co-digestion of municipal biomass waste. About 78% methane recovery was reported. Similarly, Grisel et al. [77] investigated the biogas production from of co-digestion of coffee pulp and cow-dung under solar radiation and discovered that 50% of biogas was recovered during the first month of gas production,
The gas further increased to 60% and remained constant for at least eight months of further digestion. Thong et al. [78] observed 98% biodegradability of the feedstock and 82% methane yield with the corresponding energy content of 36 MJ per m3. This was found to be feasible in the thermophillic acidogenic hydrolysis of lignocellulosic in the empty fruit bunches up to mixing ratio of 2:3:1.
Despite the well-known benefits of co-digestion, such as optimum humidity, buffering capacity and C/N ratio or inhibitory substances dilution [79], it is not clear whether some co- substrates have adverse impact when they are co-digested with another waste in particular if there is synergisms or antagonisms among the co-digested substrates and if several co- substrates of similar biochemical composition can be co-digested [79]. Therefore, it is critical to obtain an optimal mixture of the available co-substrates as well as the optimum operating conditions, which allow high biogas yields without compromising the stability of the process [79].
A recent review by Pastor et al. [80] on the effect of substrate composition on biogas production showed that an adequate mixture formulation is needed in order to ensure the correct functioning of the anaerobic digestion process. The following parameters have been taken into account in order to obtain an adequate mixture formulation for co-digestion: biogas production improvement, composition, nutrient balance and risk of inhibition by long chain fatty acids (LCFA). Additional co-digestion studies are presented in Table 2.9.
Table 2.9: Studies of co-digestion of animal manures with other wastes
Manure Co-digestion
substrates
Operation conditions
Reactor type Findings References
Chicken Fruits and vegetables T=35˚C, HRT= 21d, OLR=3.19-5.01kg VS/m3/d
18 litre CSTR Overall methane improved from 0.23- 0.45 CH4/g Vs added
[81]
Chicken Corn stover T=37˚C, working
volume of 0.5L
1 litre Co-digestion of CS
and CM significantly increased methane yield, with methane yield reaching as much as 218.8 mL/g
[82]
Cow Whey T=35˚C 128 litre Batch
reactors on a pilot scale
Cow and whey mixture enhanced biogas production [83] Cow OFMSW T=55˚C, HRT= 14- 18d, OLR=3.3-4. kg VS/m3/d
4.5 litres reactors 0.63–0.71 biogas /g VS added., again no sign of inhibition at free ammonia was observed
Cow grass silage, sugar beet tops and oat straw
T=35˚C,OLR=2 kg VS/m3/d
5 litre CSTR 16–65% improved methane in
comparison with that obtained from digestion of manure alone
[85]
Cow sugar beet leaves (by-
product from sugar production)
T=37 ˚C and 55˚C,
HRT= 20 d,
OLR=1.76-6.75 g VS/l/d
3 litre CSTR Co-digestion was only successful at high dilution with either manure or water
[86]
Diary sugar beet tops 1.5 higher methane
than mono digestion of diary manure was reported
[87]
Pig Sewage waste and
vegetable waste
T=35˚C, HRT= 36 d, OLR=2.6 kg VS/m3/d
3 litre glass reactor The process worked well with high methane yields of 0.56-0.618 CH4/g VS
[88]
Pig Herbal extraction
residues
T=35˚C, HRT= 30 d, OLR=2.9 kg VS/m3/d
7 litre CSTR Biogas production was enhanced with
the addition of VS
Pig vegetable waste,
(greenpeas, maize,carrots and leeks) T=37˚C, HRT= 25 and 15 d, OLR=0.4 and 0.6 kg VS/m3/d and ratio of 1:1 5 litre CSTR in an improvement of 3 and 1.4 folds in methane yields compared with mono- digestion at HRT of 25 and 15 days
[90]
Pig Potato tuber and its
industrial
T=35˚C, HRT= 60 d, OLR=2 kg VS/m3/d
5 litre CSTR 0.12-0.15 l CH4/g VS
added [91]
Pig Glycerine T=35˚C 500 ml Batch Highest CH4 of 0.215
l CH4/ g COD was obtained with the mixture of 80% PM
2.5.4.2 Pre-treatment of feedstocks
During anaerobic digestion, composition of the cell wall is a major limiting factor resulting in low biodegradability of feedstock [93]. Highly aliphatic molecules normally show high resistance against bacterial and chemical hydrolysis [94]. Therefore, pre-treatment of feedstock prior to anaerobic digestion process will increase the biodegradability during digestion [93]. The following techniques are currently used in treating feedstock prior to AD.
a. Thermal pre-treatment
The temperature appears to be an important factor during thermal pre-treatment. Thermal pre- treatment of algal biomass from sewage treatment ponds for 8 h at 100 ˚C resulted in 33% increased methane production [95]. The biodegradability of Scenedesmus biomass was 22– 24% for raw and pre-treated microalgae at 70 ˚C and increased to 48% with 90 ˚C pretreatment [96]. Nevertheless, the degradability of microalgae for biogas production is strongly dependent on the species and on the pre-treatment [97]. Schwede et al. [98] found out that thermal pretreatment prior to anaerobic digestion significantly increased the methane yield from 0.2 to 0.57 m3 kg VS-1 under batch conditions and from 0.13 to 0.27 m3 kg VS-1 in semi-continuous digestion (Figure 2.9).
Figure 2.9: Biogas and methane yield of untreated (A) and thermal pre-treated (100 ˚C and 120 ˚C pressure compensated (PC), 2 h and 8 h) Nannochloropsis salina biomass after 21 and
b. Alkali pre-treatment
During alkali pre-treatment, the first reactions that occur are solvation and saphonication, which induce the swelling of solids [99]. As a result, the specific surface area is increased and the substrates are easily accessible to anaerobic microbes [100]. Then, COD solubilisation is increased through various simultaneous reactions such as saponification of uronic acids and acetyl esters as well as neutralization of various acids formed by the degradation of the particulates [103]. When substrates are pre-treated with alkali solutions, an important aspect is that the biomass itself consumes some of the alkali [100]; thus, higher alkali reagents might be required for obtaining the desired AD enhancement. The hydrolysis rate was increased with increasing NaOH dose, due to higher removal rates of lignin and hemicelluloses. However, the optimal NaOH dose was 6% (w/w) according to the specific methane production (SMP). Under this condition, the SMP and the technical digestion time of the NaOH-treated asparagus stem were 242.3 mL/g VS and 18 days, which were 38.4% higher and 51.4% shorter than those of the untreated sample, respectively [104].
Figure 2.10: Specific methane potential of the alkaline pre-treated a asparagus stem with different NaOH additions [104].
c. Acid pre-treatment
Acid pre-treatment is more desirable for lignocellulosic substrates, not only because it breaks down the lignin but also because the hydrolytic microbes are capable of acclimating to acidic conditions [105]. The main reaction that occurs during acid pre-treatment is the hydrolysis of hemicellulose into perspective monosaccharides, while the lignin condensates and precipitates [100]. Strong acidic pre-treatment may result in the production of inhibitory by-
products, such as furfural and hydroxyl methyl furfural (HMF) [101]. Hence, strong acidic pre-treatment is avoided and pre-treatment with dilute acids is coupled with thermal methods (see also Section 2.5). Other disadvantages associated with the acid pre-treatment include the loss of fermentable sugar due to the increased degradation of complex substrates, a high cost of acids and the additional cost for neutralizing the acidic conditions prior to the AD process [101].
Summary 2.6
This chapter presented a brief overview on the history of biogas technology. Previous research on AD process, key operational parameters of the process, feedstock criterion, alternative energy sources, current advancement of biogas and the technology of co-digestion have been reviewed. Improvements of biogas production using pre-treatment methods have also been discussed. The current review signify that further technological advancements in diverse concerns of biogas production is crucial for process enhancement that desires entire possibilities on various aspects bring together to decode this technology in to an outstanding option for sustainable development in future.
Based on the previous studies the technology has shown to have good potential in limiting environmental challenges. However, the present literature on anaerobic digestion is not sufficient to use it at industrial scale with complete control of all its parameters and factors. The influence of microbial community structure on process stability, important bacterial strains actively involved in the degradation, requires further efforts and must be analyzed in detail. Furthermore, a clear understanding of the effect of particle size on this process is crucial for optimization of their performance and attainment of the stable operation process.
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