| Livestock Research for Rural Development 30 (4) 2018 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
This work aimed to determine the optimum amount of inoculum for anaerobic digestion of cassava waste in three digesters. These digesters are supplied with different inoculums as follows: effluent + urine + 5 kg of cow dung (B5); effluent + urine + 10 kg of cow dung (B10); effluent + urine + 15 kg of cow dung (B15).
The results of this work show that the three digesters operated in mesophilic conditions with a pH of about 7. Also, the purification yields all digesters ranged between 95.7 to 96.5%. A conservation of nitrogen was observed with values ranging from 41.12 to 50.35%. The proper functioning of these digesters enabled the production of biogas. However, the combustion time differs from a digester to another. Indeed, the biogas produced (1.47 m3) with the digester B5 burns after 36 days of operation. As for the digester B10, with 2.98 m3 of biogas the combustion begins after 6 days. Concerning the digester B15, with a production of 3.87 m3 of biogas, the inflammability was observed after 9 days. Given these results, we must remember that the co-digestion from 10 kg of cow dung is the best fit for a good anaerobic digestion of cassava waste with human urine as a co-substrate.
Keywords: anaerobic digestion, biogas, cassava effluent, cow dung, human urine
In Ivory Coast, the main dietary form of cassava tuberous roots is " attiéké" (steamed manioc) (Kakou 2000). The production of this popular food within the majority of Ivorian population generates significant waste. Indeed, the process produces 0.74 m3 of waste water per tons of processed cassava. According to Aboua et al (1990), using 40000 to 50000 tons of cassava fresh tubers produces 28000 to 34000 tons of "attiéké". This corresponds to a production of 20720 to 25160 m 3 of effluent. However, these effluents are firstly highly loaded with organic matter with loads of COD and BOD, respectively from 6 to 50 g/L and 1.5 to 35 g/L (Ubalua 2007; Kpata-Konan et al 2011; Kpata-Konan et al 2013) and secondly with toxic cyanide contents up to 500 ppm (Goualo et al 2007).
In the case of the 93 Ebrié villages, engulfed by the Autonomous District of Abidjan, where many women are principally engaged in the manufacture of "attiéké", the waste water from this activity, untreated, are released into the natural environment including the lagoon Ebrié. These effluents degrade the quality of life, generate odors, or involve the spread of pathogens and cause risks to human and animal health (Marache 2001).
To remedy the pollution of the received environment, anaerobic digestion, a natural process of transformation of organic matter into gas under the action of microorganisms without oxygen (Kalloum et al 2001; Osak et al 2015), appears as a credible alternative for these effluents treatment. This process is widely used for the treatment of food waste heavily loaded with organic matter (Ubalua 2007; Triet et al 2017; Yen et al 2017).
Like other food waste, cassava effluent is rich in organic matter, but deficient in nitrogen (0.6 to 0.8 g/L) and acidic, with pH values below 3 (Kpata-Konan et al 2011; Kpata-Konan et al 2013). These properties make it bio-recalcitrant. It would then be necessary to neutralize and to inoculate these effluents to allow good anaerobic digestion.
The work aims to determine the optimal amount of inoculum for anaerobic digestion through monitoring of pH, temperature, COD, TKN and biogas production.
The digestion substrate consists of effluent from "attiéké" mill of Azito village (pressed cassava juice and peeled cassava wash water), human urine and cow dung.
The experimental reactor (Figure 1) consists of two parts. The first part consists of a large metal drum which capacity was 186 L with a diameter of 57 cm and a height of 90 cm.
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| Figure 1. Schema describing the experimental digester |
The second part is made of smaller metal drum, with a capacity of 100 L, whose open side is immersed in the biggest barrel until touching its bottom. It has a diameter of 45.5 cm and a height of 83 cm. This small barrel serves as gasometer in which the produced gas is stored. On this small drum, a mixer and a valve are mounted. The mixer is used to homogenize the mixture in the large barrel so as to avoid settling. The valve prevents leakage of the gas produced during the fermentation and allows to collect it for the different analyzes.
Feeding digesters was to vary the amount of cow dung to determine the amount needed for optimal anaerobic digestion. The reactions of these digesters were fertilized with human urine and buffered to pH 7. Quantities of cow dung used to inoculate the three digesters are 5 kg, 10 kg and 15 kg. The composition of the different digesters is follows: B5 (Effluent+urine+5 kg cow dung), B10 (Effluents+urine+10 Kg cow dung) and B15 (Effluents+urine+15 Kg cow dung).
All digesters operated staple ways and stirred manually on a daily basis for 10 minutes before all levies and all measures. Temperature, pH, COD and TKN were determined by AFNOR (1994).
The volume of biogas produced is determined from the expression:
V = π R2 H
H = height of the uprising of the gasometer.
R = the radius of the gas holder (piston).
Table 1 shows COD removal in the three digesters. The treatment efficiency t are 95.72% in the digester B5, 95.85% in the digester B10 and 96.55% in the digester B15.
Concerning the nitrogen pollution, the treatment performance of the digesters are 48.81 % for B5, 50.35 % for B10 and 41.12 forB15.
For DCO / NTK ratios, the values of the treatment performance of the digesters ranged between 0.51 and 6.12 between 0.38 and 4.55 and between 0.34 and 5.90 respectively for reactors B5, B10 and B15.
Overall, the digesters functioned mesophilically. The B5 digester had average temperatures of 28.98 °C. As for the digesters B10 and B15, the average temperatures are respectively 29.29 °C and 29.31 °C.
At the pH level, the average values were 7.80 for the digester B5, 7.73 for the digester B10 and 7.84 for the digester B15.
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Table 1. Summary of physico-chemical parameters of the effluent at the outlet of the digesters |
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Parameters |
B5 |
B10 |
B15 |
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COD (g/L) |
23.3 - 0.99 |
20.5 - 0.85 |
22.1 - 0.76 |
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Carbon treatment efficiency (%) |
95.7 |
95.8 |
96.5 |
|
TKN (g/L) |
3.80 - 1.94 |
4.50 - 2.23 |
3.74 - 2.20 |
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Nitrogen treatment efficiency (%) |
48.8 |
50.4 |
41.1 |
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COD/TKN |
6.12 - 0.51 |
4.55 - 0.38 |
5.90 – 0.34 |
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average T°C |
29.0 |
29.3 |
29.3 |
|
average pH |
7.80 |
7.73 |
7.84 |
Table 2 presents a maximum gas observed in digesters B10 (2.98 m 3) and B15 (3.07 m3). Over the entire experimental period, the digester B5 produced a total volume of 1.47 m3 after 106 days. The B10 digester provided cumulative gas volumes of 2.98 m 3 in 106 days. For the B15 digester, it produced a total of 3.07 m3 during 106 days.
The biogas flammability test in the digesters B10 and B15 took place respectively from the 6th and 9th day of experimentation.
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Table 2. Summary of biogas produced |
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B5 |
B10 |
B15 |
|
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Biogas volume (m3) |
1.47 |
2.98 |
3.07 |
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Combustion start |
from the 36th day |
From the 6th day |
From the 9th day |
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Anaerobic digestion time |
106 |
106 |
106 |
Analysis of the results shows the concentrations of nitrogen which slightly decrease indicates, retention of nitrogen in the reaction area as also observed by Lansing et al (2010). According to Liu and Sung (2002), nitrogen plays an important role in the proliferation of bacteria in anaerobic digesters and serves as a catalyst in the production of biogas by Peu et al (2011).
The ration COD/TKN, which is a nutrient intake, is 18.97/3.7. According to Giroux and Audesse (2004), plant effluents have ratios of COD/TKN varied and are between 10 and 100 g/L. Anaerobic intakes of nutrients for good microbial growth must respect the C/N ratio (COD/TKN) equal to values lying between 15/1 and 20/1 for Giroux and Audesse (2004) and between 20/1 and 30/1 for Moletta (2002). These results show that the reaction area is not deficient in carbon and nitrogen.
Overall, the results show that the three types of digesters (B5, B10, B15) have performed as mesophilic systems. In this system, the temperature ranges from 24 to 35°C, and it is fundamentally important for effective purification. Indeed, the high temperatures in the mesophilic systems accelerate bacterial activity (Liu et al 2001) for good bioconversion (Wen et al 2010). According to Wen et al (2010), the activity of methanogenic consortium is optimal for digesters whose temperature is inserted into the mesophilic range. For Vedrenne (2007), the temperature is as a primary factor in anaerobic digestion because it affects the speed of enzymatic reactions and influences the balance between the different microflora present.
Overall, the changing in quantities of cow dung in the different reactors had no significant effect on the digestive process. Indeed, digesters operated in similar temperature ranges. At pH values fall in the three types of reactors during the first 7 days of anaerobic digestion. This pH decrease could be related to the production of organic acids by Corynebacterium manihoti and Geotricum candida as indicated by Bougrier (2005).
Furthermore, according to Vidal et al (2000) and Kayode and Jude (2015) this reduction does not appear to affect the phase of the hydrolysis and acidogenesis, even at lower values. However, the pH range allowing the expression of the acetogenic and methanogenic metabolisms is more limited and usually strictly greater than 6 (Prescott et al 2000). The gradual increase in the pH in the digesters and stable values above 7 during the experiment could be explained by desorption of proteins or volatilization of acid compounds or CO2 as noted by Murillo (2004). These comments were reported by Raposo et al (2008) in their studies on the treatment of sewage.
All three reactors generated biogas. The combustion of this biogas indicates that it contains methane. According to Bougrier (2005), a fuel gas is composed of at least 40% of methane. However, analysis of biogas profiles shows that there are significant differences in the types of reaction area. These observed differences are the amount of biogas produced as well as the time taken by the gas before flammability.
Maximum biogas production was observed in the digesters B10 and B15 respectively containing 10 kg and 15 kg of cow dung. This higher biogas production in these two types of reaction area could be explained by the higher growth of microorganisms. Indeed, the gradual increase in cow dung (5, 10 and 15 kg) contributes to bacterial methanogenic activity more intense. According to Vedrenne (2007) and Djaâfri et al (2009), the inoculation of a substrate significantly aids in digestion and accelerates the development of methanogenic bacteria, and therefore, the faster trigger the methanogenic phase. In addition, the time taken for the reactors to produce combustible biogas is 36 days for the digester B5. It is 6 days for the digester B10 and 9 days for the digester B15. These observations show the implementation of the methanogenic phase in a very short time in the digesters containing 10 and 15 kg of cow dung.
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Received 14 October 2017; Accepted 26 February 2018; Published 1 April 2018