BIOGAS PRODUCTION FROM SOME ORGANIC WASTES
In view of the energy crisis and environmental pollution, biogas technology has recently attracted worldwide attention.
A list of some of the urgent research tasks on biogas was suggested by Sathianathan. This list includes the search for new materials for biogas production. In the present study, a laboratory investigation was undertaken to assess the feasibility of using some organic wastes that were not before investigated as substrates for biogas production. The wastes examined include geranium flour, akalona, and watermelon residues. In Egypt these materials are presently considered useless by-products of geranium oil extraction, the wheat milling process, and watermelon-seed production respectively. Each of the investigated wastes was digested individually or with cow dung at different mixing rations. The cow dung was also digested individually and served as a reference substrate.
MATERIALS AND METHODS
Organic Wastes
Cow dung fresh, undiluted dairy cow dung, Brown Swiss (Breed), was collected from the animal Production Farm of Fayum Faculty of Agriculture. The animal were fed a ration that was composed of approximately 50 percent rice straw and 50 percent of: 65 percent cotton seed meal, 20 percent rice bran, nine percent bran, three percent molasses, two percent lime stone, and one percent NaCl. No antibiotics or other additives were incorporated in the animal ration.
Geranium Flour
An air-dried sample from the residue remaining from geranium plants, Pelargonium graveolens Ait, after extraction of the essential oil, was obtained from a geranium oil extraction plant located in Fayum Governerate.
Akalona
Akalona is the outer portion (epidermis) of the wheat grains which results from the scouring of the wheat grains during the milling process. A sample of "Akalona" was obtained from the Fayum Mill.
Watermelon Residues Citrullus Vulgaris
This material was prepared in the laboratory from a watermelon fruit. After separating the seeds, residues of the watermelon fruit (i.e., rind, pulp and juice) were homogenized in a blender prior to use.
Starter
The starter for anaerobic digestion was prepared from the effluent of an actively operating laboratory digester fed with cow dung. The effluent was passed through four layers of cheesecloth to remove the undergraded materials. The starter was added to all digesters at the rate of 10 percent by volume.
Digestion Apparatus
Batch-fed laboratory digesters were constructed as described by Gamal-El-Din. The digester setup consists of a 1.25 litre brown bottle, a gas-measuring cylinder equipped with a leveling bulb, gas sampling port, and a sidearm through which liquid could be withdrawn or added.
Analytical Procedures
Gas Volume: The biogas produced was collected and measured by liquid displacement in a calibrated gas cylinder filled with Orsat confining solution: 20 percent sodium sulphate and five percent sulphuric acid in water.
Methane Content of the Biogas: This was determined by bubbling a known volume of biogas through 20 percent (W/V) potassium hydroxide solution. The loss in volume of biogas was taken as equal to the carbon- dioxide. The balance was assumed to be methane.
Determinations of Total Solids: (TS), ash, volatile solids (VS), total volatile acids (TVA), pH, alkalinity, and total nitrogen (TN) were made according to the procedures in "Standard Methods" devised by APHA. Organic carbon (OC) was determined by the rapid titration method of Walkaly and Black.
EXPERIMENTAL
The digesters were fed with cow dung, geranium flour, akalona, watemelon residues, or a mixture of a particular waste and cow dung. The proportions of the cow dung in the mixtures, based on the concentration of the total solids in the feed (five percent TS), were 20 percent, 50 percent, and 80 percent.
The required quantities from each waste or from the waste and the cow dung, to give a total solids concentration of five percent, were mixed with 100 ml of the starter and sufficient amount of tap water to give one litre total volume. Each treatment was replicated three times and a set of digesters was prepared for chemical analysis. The digesters was incubated at 350C. Mixing of the digester contents was done manually at the time of gas volume measurement.
The volumes of the biogas produced were recorded daily and biogas samples were analyzed periodically. All gas volumes were corrected to 00C and one atmospheric pressure (STP). The experiment lasted 30 days. Samples from the digester contents were analyzed for TS, VS at the beginning and at the end of the experiment. Determination of pH, TVA, alkalinity was carried out every 10 days.
RESULTS AND DISCUSSION
Feasibility of biogas production from geranium flour (GF), akalona (AK), watermelon residues (WR), and from mixtures of each waste with cow dung (CD) in different proportions was investigated and compared with biogas production from cow dung, which served as a reference substrate.
Prior to the anaerobic digestion, sample from the investigated wastes were analyzed for TS, ash, OC, TN, and pH. The Vs and the C/N ratio of each waste were calculated. The data obtained are summarized in Table 1.
Table 1 Characteristics of the Investigated wastes
| Waste |
TS% |
VS% |
VS/TS% |
OC% |
TN% |
C/N |
pH in 5% TS Slurry |
| Cow Dung |
17.63 |
13.65 |
77.42 |
44.01 |
1.37 |
32.10 |
7.10 |
| Geranium Flour |
92.41 |
74.32 |
80.42 |
40.61 |
1.35 |
30.08 |
4.80 |
| Akalona |
91.85 |
88.32 |
96.20 |
51.59 |
1.03 |
50.09 |
6.70 |
| Watermelon Residues |
6.88 |
6.12 |
88.95 |
49.42 |
0.36 |
137.28 |
5.50 |
Biogas Production from Geranium Flour (GF)
Geranium oil is one of the major essential oils for export in Egypt. The geranium plant was first grown in Egypt in 1930. Since then, its area has progressively increased to about 14,000 feddans. The geranium cultivated area at fayum Governerate is about 7,000 feddans.
At present, geranium flour, the residue remaining from geranium plant leaves after extraction of the essential oil is either left where it is produced or may be used as a source of energy by direct burning.
The results obtained in the present study showed that the geranium flour and its mixture with cow dung gave lower biogas volumes than those produced from cow dung. The 100 percent "GF" digesters produced the lowest biogas volumes with low methane content. This result could be explained by the low initial pH value (5.0) of the "GF" digester contents. McCarty reported that methane production proceeds quite well as long as the pH is maintained between 6.6 and 7.6, with an optimum range between 7.0 and 7.2. At pH values below 6.2, acute toxicity occurs.
Table 2 Changes in pH, Alkalinity and Total Volatile Acids (TVA) concentration during the anaerobic digestion of Geranium Flour, Cow dung and different mixtures of the two Wastes at 350C.
Figure 1 Cumulative Biogas volume and biogas production rate (STP) from batch digestion of geranium flour cow-dung and mixtures ofthe two wates at 35°C (geranium flour 100%__80%__50%__20%__cow-dung 100%__ ). *Average CH4 percentage.
The analysis of the effluent after the 30 days digestion period (data not presented) indicated that the approximate percentage VS destruction incase of geranium flour was 3.9 percent as compared to 36.1 percent for the cow dung. This build-up of solids in the "GF" digesters without a significant increase in TVA concentration suggests that "GF" is resistant to attack by the bacterial population.
In addition to the effect of the low pH, the geranium flour may contain an antimicrobial substance(s). Drabkin studied the phytocidal substances of pelargonium and found that the sap of fresh, crushed, leaves and that of autoclaved leaves has an anti-mircrobial activity which was particularly high in the case of the autoclaved leaves.
The results also showed that the biogas production rate dropped to practically zero in the case of the 80 percent GF and the 50 percent GF digesters before the end of the experimental period. According to Kroeker, digester failure appeared to occur approximately at pH 6.5 when the concentration of TVA was 1650 mg/L as acetic acid.
From the results obtained, it may be concluded that under the conditions of the present study, geranium flour is not a convenient substrate for bio-gas production, since it is not easily degradable and should be pretreated to be more suitable for bacterial attack. In addition, adjustment of the pH of the waste slurry is needed. However, it seems doubuful whether the cost of pretreatment of the waste and/or chemical addition can be offset by the increase in gas production.
Biogas Production from Akalona (AK)
Akalona represents about 0.5 percent of weight of the wheat grains. At present, it is considered as useless residue because it is not usable as animal fed, nor is it profitable for further processing.
The results of the anaerobic digestion of akalona showed that "AK" digesters produced lower biogas volumes than those produced from the "CD" digesters. However, such a result was expected because of the relatively high C/N ratio of the former waste (about 50).
Accumulation of TVA was observed in the akalona digester contents (3282 mg/L as acetic acid/30 days). This caused a drop in pH from 6.6 to 4.6. The combined effect of pH depression and TVA concentration increase may include toxic conditions. It seems that the low alkalinity found in the akalona digester (584 mg/L as CaCo3), at the end of the experiment cannot protect the system. McCarty indicates that a bicarbonate alkalinity in the range of 2500 to 5000mg/L as CaCo3 provides a safe buffering capacity for anaerobic treatment of wastes.
The performance of the digesters fed with different mixtures of "AK" and "CD" was highly variable, and this makes it impossible to draw general conclusions. However, at about the 20th day of digestion, the cumulative volumes and the methane content of the biogas produced from both the "20 percent AK" and "CD" digesters were approximately equal. This means that akalona, without any pretreatment, can replace about 20 percent of the cow dung solids for biogas production.
Figure 2 Cumulative Biogas volume and biogas production rate (STP) from batch digestion of akalona, cow-dung and mixtures ofthe two wates at 35°C (akalona 100%__80%__50%__20%__cow-dung 100%__ ). *Average CH4 percentage.
Table 3 Changes in pH, Alkalinity and Total Volatile Acids (TVA) concentration during the anaerobic digestion of Akalona, Cow dung and different mixtures of the two wastes at 350C.
In conclusion, the biogas production from akalona can be improved by nitrogen and/or alkali addition to overcome low pH caused by the rapid formation of the volatile acids. This can be achieved by mixing the akalona with a nitrogen-rich waste such as poultry excreta.
Biogas Production from Watermelon Residue (WR)
In Egypt, more than 3000 feddans of watemelon per year are cultivated for the production of seeds. The watermelons from this area produce about 15,000 tons of juice as a by-product. In addition to the juice, other residues are produced, i.e., rind, pulp, and fibers. Khattak found that charliston watermelon fruits contain about 33.6 percent juice, 43.4 percent rind, and 24 percent seeds, pulp and fibers. They also found the total carbohydrate content of the watemelon juice (reducing and non-reducing sugars) ranged from 6.26 to 7.28 gm/100 ml juice.
Presently, the watermelon residue are considered a useless by-product of the seed-production industry. Because that watermelon juice is rich in sugars, it can represent a serious pollution problem. Some proposals have been made to recover useful products from the watermelon juice. However, none of the systems so far suggested have yielded a satisfactory result.
Regarding biogas production from watermelon residues, the data obtained in the present study showed that the "WR" digesters produced low bio-gas volumes of a relatively low methane content; and generally the bio-gas production and methane content increased by increasing the percentage of cow dung in the mixture. Such a result is not surprising since the C/N ratio (about 137) and the pH value (about 5.5) of the watermelon residues are not favorable for anaerobic digestion.
Table 4 Changes in pH, Alkalinity and Total Volatile Acids (TVA) concentration during the Anaerobic digestion of watermelon residues, Cow dung and different mixtures of the two Wastes at 350C.
The high C/N ratio and the low pH caused as increase in TVA concentration and a decrease in pH. Both affected the activity of the methane-forming becteria. Moreover, the marked drop in the pH value of the "WR" digester contents may inhibit the acid-producing bacteria. It was found that the optimum pH for the separate acidogenesis of soluble carbohydrate containing wastewaters is in the range of 5.7-6.0.
Figure 3 Cumulative Biogas volume and Biogas production rate (STP) from batch digestion of watermelon resifues, cow-dung and mixtures of the two wates at 350C (watermelon reaiduea 100%_80%_50%_20%_cow-dung 100%_).Average CH4 percentage.
In conclusion, the watermelon residues should be pretreated before they can be successfully treated with the anaerobic digestion process. Pretreatment could include pH adjustment to 7.0 and nutrient addition, particularly nitrogen. Reducing the loading rate may also improve the biogas production. Also two-phase anaerobic digestion could be suitable for treating these residues. Because this type of waste would cause serious pollution, additional work is required to identify the suitable pretreatment.