 |
| Photo 1. The micro and macro biodigesters installed in the same area |
| Livestock Research for Rural Development 29 (2) 2017 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
Cassava factories that produce starch from cassava roots discharge large quantities of waste-water to the environment. The waste-water is rich in fermentable substrate (about 3000 mg TSS/liter) but with low pH (4.2-4.4). The present study aimed to investigate the addition of a buffer (sodium bicarbonate), and the inoculation with effluent from a working biodigester, on gas and methane production from cassava waste-water in a plug-flow biodigester batch system. The experiment was carried out in Toan Xuan Hung cassava factory in Dong Nai Province, Vietnam, from March to June 2016. The treatments in micro (4 liter) and macro (480 liters) plug-flow biodigesters were addition of sodium biocarbonate (5 g/liter liquid volume of biodigester) and increasing proportions (fresh basis) of cassava waste-water (CW) relative to biodigester effluent (EF) (75:25, 50:50 and 25:75).
Addition of bicarbonate raised the pH to above 6.0, the degree of increase being greater with increasing concentration of cassava waste-water in the input material. Gas production with biocarbonate was 6.4, 18.2 and 15.5 liters/g suspended solids for CW:EF ratios of 25:75, 50:50 and 75:25. In the absence of bicarbonate, there was no gas production for CW:EF input ratios of 75:25 and 50:50 and only 6 liters/g suspended solids for the CW:ED ratio of 25:75. Concentration of methane was low on all treatments.
It is concluded that addition of sodium bicarbonate to biodigesters charged with cassava waste-water and biodigester effluent will increase the initial pH to > 6.0 and maintain it through a 21 day batch fermentation, provided that a minimum of 25% of the digester volume is added as effluent from a working biodigester as a source of inoculum.
Key words: buffer, effluent, gas production, methane, pH
Cassava, also known as tapioca, is a starch-containing root crop of worldwide importance as food, feed and non-food products. More than 70% of this production is produced by small-scale farmers in the subtropical and tropical regions between 30oN and 30oS of Africa, Latin America and Asia (Jansson et al 2009). Cassava is a major source of food for more than 700 million people in tropical developing countries and it is cultivated in a total global area of 18.6 million ha with a total production of 238 million tonnes (Patil and Fauquet 2009).
In Vietnam, at the present time, cassava is mainly used to extract starch for export in large scale factories. Large quantities of waste-waters are generated from the starch extraction process, which involves cleaning of the roots, starch extraction, separation and drying. The process generates 20–60 m3 of waste-water per tonneof roots processed. The waste-water has a low pH, high chemical oxygen demand (COD), high biochemical oxygen demand (BOD) and high levels of suspended solids (SS) (Annachhatre and Amatya 2000; Annachhatre and Amornkaew 2001). Although starch processing plants may dilute the waste-water, it is a source of pollution and causes environmental problems, such as high toxicity to tropical duckweed (Bengtsson and Triet 1994). Thus, effective technologies are necessary for the treatment of cassava starch waste-water.
A range of anaerobic biological systems, such as up-flow anaerobic sludge blanket (UASB) reactor for sewage and starch waste-water treatment (Mahmoud 2008), horizontal flow filter with bamboo and anaerobic ponds (Rajbhandari and Annachhatre 2004), has been used in the treatment of waste-water. However, in practical applications, all the anaerobic treatments suffer from the low growth rate of the microorganisms, a low settling rate, process instability, high hydraulic retention time (HRT) and the need for post -treatment (Chan et al 2009; Ndon and Dague 1997).
Another option is the co-digestion of cassava waste-water with other organic wastes to produce biogas. Apart from increasing biogas production, co-digestion offers several other benefits including: increased loading of readily biodegradable organics, improved balance of nutrients and C:N ratio, dilution of toxic substances, a better quality of the digested product, and reduced costs. Several studies have shown that mixtures of agricultural, municipal and industrial wastes can be digested successfully and efficiently together. A stimulatory effect on synthesis of methane gas has been observed when industrial sludge was co-digested with municipal solid waste (Agdag and Sponza 2007). The co-digestion of municipal solid waste with an industrial sludge ratio of 1:2 yielded the highest amount of methane gas, compared to municipal solid waste alone. Similarly, in a two-phase anaerobic digestion system, Fezzani and Cheikh (2010) recorded the highest methane production when a mixture of olive mill waste-water and olive mill solid waste was co-digested. The process has also been useful in obtaining a valuable sludge which can eventually be used as a soil amendment after minor treatments (Gomez et al 2006).
The present study aimed to investigate the effects of sodium bicarbonate (NaHCO3) as a buffer in the co-digestion of cassava waste-water (CW) and effluent from a working digester (EF).
The experiment was carried out at Toan Xuan Hung cassava factory, 5 Ward, Xuan Hung commune, Xuan Loc district, Dong Nai province, from March 2016 to June 2016.
Two factors were studied: (i) ratio of CW to EF; and (ii) with and without NaHCO3 (NaB) as buffer. A minimum proportion of biodigester effluent (25% liquid volume basis) was used as initial observations indicated that with 100% cassava waste-water there was no gas production even after 2 months.
Individual treatment were:
The experiment was done at both micro and macro level, corresponding to the liquid volume of the biodigesters (4 and 480 liters). Both types of biodigester were of the “plug-flow” configuration (Triet et al 2016) and were installed in the same area (Photo 1) to ensure a uniform microclimate condition and overall environment (ambient temperature 31- 33o C).
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| Photo 1. The micro and macro biodigesters installed in the same area |
The procedure used in the micro biodigester was similar to that described by Triet et al (2016) (Photo 2). The biodigesters were made from recycled 5 liter “pep” plastic water bottles. Each system was fitted with an inlet port for the substrate, an outlet port for the effluent and a gas outlet leading to a 1 liter “pep” water bottle calibrated at 50ml intervals, with the bottom cut-away, and suspended in a second 5 liter water bottle with the top removed. At the beginning, the “gas” bottle was filled with water so that gas production could be measured by water displacement. Each “system” was replicated 3 times according to a completely randomized block design. The liquid volume of the bio-digester was 4 liters and the retention time was 21 days.
 |
| Photo 2. Micro biodigester in plug-flow configuration with gas collection bottle suspended in the water reservoir bottle |
The macro biodigesters (Photo 3) were made from tubular polyethylene film (internal diameter 0.6m; 2m length) according to the design developed by San Thy et al (2003). The total biodigester volume was 565 liters, of which 85% corresponded to the liquid volume (480 liters). The gas outlet from the biodigesters was connected to an inverted plastic bag suspended in a 50 liter oil drum filled with water.
 |
| Photo 3. Macro biodigester made from tubular polyethylene |
The two input materials were taken from the waste-water treatment plant in the Toan Xuan Hung cassava factory (Diagram 1). Cassava waste-water was collected directly from the mixed stream of waste-waters coming from the root washing and the starch extraction process. The biodigester effluent was collected from the outlet of the large scale biodigester that received the two streams of waste-water. Sodium bicarbonate was purchased from the “Techlab Company” in Ho Chi Minh City.
The CW. EF and NaB were mixed and introduced into the biodigesters on day 1 (Table 1).
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Table 1. Quantities of CW, EF and NaB introduced into the biodigesters |
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|
CW |
EF |
NaHCO3 |
|
|
Micro experiment |
|||
|
75CW25EF |
3 |
1 |
0 |
|
50CW50EF |
2 |
2 |
0 |
|
25CW75EF |
1 |
3 |
0 |
|
75CW25EF0.5NaB |
3 |
1 |
20 |
|
50CW50EF0.5NaB |
2 |
2 |
20 |
|
25CW75EF0.5NaB |
1 |
3 |
20 |
|
Macro experiment |
|||
|
75CW25EF |
360 |
120 |
0 |
|
50CW50EF |
240 |
240 |
0 |
|
25CW75EF |
120 |
360 |
0 |
|
75CW25EF0.5NaB |
360 |
120 |
2400 |
|
50CW50EF0.5NaB |
240 |
240 |
2400 |
|
25CW75EF0.5NaB |
120 |
360 |
2400 |
 |
| Diagram 1. The two input materials were taken from the waste-water treatment plant in the Toan Xuan Hung cassava factory |
The gas volume and the percentage of methane were recorded every 3 days over the trial period of 21 days. Methane in the gas was measured with a Crowcon infra-red analyzer (Crowcon Instruments Ltd, UK). A sample of the digester contents was taken every 3 days (using a probe inserted through the outlet tube of the biodigester) and the pH recorded with an electronic meter.
The data were analyzed by the General Liner Model (GLM) option in the ANOVA program of the Minitab 16 software (Minitab 2015). Source of the variation in the model were: NaB, CW-EF ratios, interaction NaB*CW-EF and error.
The range of pH was much lower in the waste-water than in the effluent (Table 2). All indicators of biological contamination were much higher in the waste-water than in the effluent.
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Table 2. The characteristics of the cassava waste-water, biodigester effluent and the mixtures put into the experimental biodigesters |
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Parameters |
CW |
EF |
75CW25EF |
50CW50EF |
25CW75EF |
|
pH |
4.2 – 4.4 |
6.6 – 6.8 |
4.7 |
5.7 |
6.1 |
|
COD (mgO2/L) |
12218 |
1207 |
10076 |
6283 |
4020 |
|
TSS (mg/L) |
2920 |
570 |
2479 |
1685 |
1186 |
|
TN (mg/L) |
457 |
252 |
429 |
369 |
316 |
|
TP (mg/L) |
66.2 |
40.7 |
63.1 |
56 |
49.1 |
| L = liters | |||||
The pH inside the biodigesters was raised by addition of sodium bicarbonate, the degree of increase being proportional to the concentration of cassava waste-water in the input material (Table 3; Figures 1 and 2). In the absence of bicarbonate, there was no gas production for CW:EF input ratios of 75:25 and 50:50. Gas was produced when the CW:EF input ratio was 25:75, but the quantities were small and with no benefit from the bicarbonate. Concentrations of methane were low on all treatments.
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Table 3. Effect on pH, gas production and methane content of the
gas, of adding sodium bicarbonate to |
|||||
|
75:25 |
50:50 |
25:75 |
SEM |
p |
|
|
pH |
|||||
|
0.5 NaB |
6.4 |
6.63 |
6.73 |
0.077 |
<0.001 |
|
No NaB |
4.52 |
5.13 |
6.32 |
||
|
Gas production, ml |
|||||
|
0.5NaB |
149 |
131 |
30.7 |
19.3 |
<0.001 |
|
NoNaB |
0 |
0 |
28.8 |
||
|
Methane, % |
|||||
|
0.5NaB |
9.73 |
10.5 |
3.16 |
2.24 |
0.037 |
|
NoNaB |
0 |
0 |
3.1 |
||
|
Methane, ml |
|||||
|
0.5NaB |
14.5 |
13.8 |
0.970 |
||
|
NoNaB |
0.00 |
0.00 |
0.893 |
||
|
Gas, g/liter SS |
|||||
|
O.5BaB |
15.5 |
18.2 |
6.40 |
||
|
NoNaB |
0 |
0 |
6 |
||
 |
 |
|
Figure 1.
Effect on pH of adding sodium bicarbonate to biodigesters charged with different proportions of cassava waste water and biodigester effluent (micro biodigesters) |
Figure 2.
Effect of sodium bicarbonate on the pH in the micro biodigesters charged with 75% cassava waste water and 25% of effluent |
 |
 |
|
Figure 3.
Effect on gas production of adding sodium bicarbonate to biodigesters charged with different proportions of cassava waste water and biodigester effluent |
Figure 4.
Effect on methane content of the gas of adding sodium bicarbonate to biodigestors charged with different proportions of cassava waste water and biodigester effluent |
The trends in the results from the macro biodigesters were almost identical with those from the micro biodigesters, differing only in magnitude for gas production and production of methane (Table 4; Figures 5, 6, 7 and 8).
|
Table 4.
Effect on pH of adding sodium bicarbonate to
bio-digesters charged with different |
|||||
|
25 |
50 |
75 |
SEM |
p |
|
|
pH |
|||||
|
0.5NaB |
6.44 |
6.62 |
6.73 |
0.081 |
<0.001 |
|
NoNaB |
4.55 |
5.15 |
6.32 |
||
|
Gas production, liters |
|||||
|
0.5NaB |
14.3 |
12.7 |
2.88 |
1.85 |
<0.001 |
|
NoNaB |
0 |
0 |
2.71 |
||
|
Methane, % |
|||||
|
0.5NaB |
9.87 |
10.4 |
3.10 |
2.24 |
0.037 |
|
NoNaB |
0 |
0 |
3.10 |
||
|
Methane, liters |
|||||
|
0.5NaB |
1.41 |
1.32 |
0.09 |
||
|
NoNaB |
0.00 |
0.00 |
0.08 |
||
|
Gas, g/liter SS |
|||||
|
0.5BaB |
12.41 |
14.70 |
5.00 |
||
|
NoNaB |
0.00 |
0.00 |
4.70 |
||
 |
 |
| Figure 5. Effect on pH of adding sodium bicarbonate to macro biodigesters charged with different proportions of cassava waste water and biodigester effluent |
Figure 6. Development of the pH in the macro biodigesters charged with 75% cassava waste water and 25% of effluent |
 |
 |
|
Figure 7.
Effect on gas production of adding sodium bicarbonate to macro-biodigesters charged with different proportions of cassava waste water and bio-digester effluent |
Figure 8.
Effect on methane content of the gas of adding sodium bicarbonate to macro biodigesters charged with different proportions of cassava waste water and bio-digester effluent |
The lack of gas production from the biodigesters charged with 75 and 50% of waste water, and no biocarbonate, would appear to be directly related to the low pH (<5.1). The critical role of pH in determining the efficient functioning of biodigesters charged with biological wastes is well understood. The culture pH affects the activities of specific acidogenic (Zhang et al 2012) and methanogenic bacteria (Ghosh et al 2000), and thereby the stability of the co-digestion process. Thus, pH is a pivotal factor that affects the methane production efficiency (Jiang et al 2010; Liu et al 2008). The optimal range of pH for methane production was 6.5 – 7.5 according to several researchers (Liu et al 2008; Eckcnfelder 1989; Cheremisinoff 1994). Only the treatments containing biocarbonate were within this range. However, gas production is also a function of the fermentability of the substrate and it is apparent that in the combinations of CW:EF 25:75 there was insufficient substrate to support gas production, even though the pH was within the acceptable range.
This research was done by the senior author as part of the requirements for the MSc degree in Animal Production "Specialized in Response to Climate Change and Depletion of Non-renewable Resources" of Cantho University, Vietnam. The authors acknowledge support for this research from the MEKARN II project financed by Sida. They also acknowledge the Research and Technology Transfer Center, Nong Lam University, Vietnam for providing infrastructure support
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Received 18 October 2016; Accepted 14 January 2017; Published 1 February 2017