The authors have declared that no competing interests exist.
Biogas is anaerobic degradation product formed from aqueous slurry of organic waste in a digester. It can be produced from cattle dung,(cd)chicken droppings, decaying leaves, kitchen waste foods(kwf), sewage sludge, slaughter house, goat, pig or sheep manure, Aqueous slurry of 200g/L of mixed or single substrate of cattle dung or/and kitchen waste evolved up to 400mL of biogas at ambient temperatures. The rate of gas evolution reached 5mL/day on the 15th day using 25% cd mixed slurry. The rates of degradation attained in the mixtures were 1.42ml/g for cd; 1.58mL/g for kwf; 1.78mL/g for 75% cd mixed substrate; 1.78mL/g for 50% cd mixed substrate; 1.92mL/g for 25% cd mixed substrate slurries. The comparative rate of biogas formation ranged from1.25 to 1.35 which was in agreement with the range published in literature of 0.8 to 5.5. Biogas can be synthesized efficiently at ambient temperature in Kampala as was done at mesophilic temperatures elsewhere. However, it may be necessary to attempt producing biogas at different pH and temperatures as well as using other substrates and inoculums.
Biogas was defined as gas formed by biological decomposition of organic matter in absence of oxygen and it originates from biogenic materials so it is a biofuel. Interest in synthesis of gas formed by decomposing organic matter was first reported in the 17th century. Later it was found that gas produced from cattle manure and kitchen waste can be used for lighting and cooking in much the same as natural gas is used. It is now known that it contains up to 50% methane, a renewable source of energy that can be used for heating, generating electricity and other operations based on internal combustion engines
The processes yielding biogas involve anaerobiosis whereby archaea bacteria, algae, fungi, protozoa and or viruses degrade organic matter. Bio digestion has been employed to treat organic wastes to recover renewable 10
The two key processes in bio digestion were reported to be mesophilic and thermophilic in nature. Further, it was shown that digesters that generate biogas from kitchen waste involve thermophilic microbes
Cogeneration or co-digestion of biogas is simultaneous decomposition of homogeneous slurry of two or more substrates 10. Cogeneration of biogas from mixed slurry of solid from slaughter house, manure, fruits and vegetables was reported to have increase the yield of methane by 44% as compared to single substrate digestion of cattle dung or kitchen waste
Fats and oils were degraded in high percentages in cogeneration with simulated organic fractions of municipal solid waste with result indicating anaerobic digestion of lipids
The yield of biogas was shown to be affected by type and composition of the substrate, microbial composition, temperature, moisture, bioreactor design and pH
Low temperatures were reported to decrease microbial growth, rates, substrate utilization and rate of biogas formation
From a kraal (zero-grazing facility), wet cattle dung (10kg) was collected. From the same kraal, cattle urine (10L) was collected. From the garbage dumping site, cooked waste food materials (10kg) from nearby restaurant at Wandegeya market was collected.
Wet cattle dung (50g) was put in a can and cattle urine (200ml) and sludge inoculum (50mL) was added. The mixture was stirred to form slurry.
Waste food materials (50g) were put in a can and cattle urine (200ml) and sludge inoculum (50mL) was added. The mixture was stirred to form slurry. The slurry was fed in the digester.
Wet cattle dung (25g) and food waste (25g) were put in a can and cattle urine (200mL) and sludge inoculum (50mL) was added. The mixture was stirred to form slurry. The slurry was fed in the digester and carbon dioxide bubbled through slurry to eliminate oxygen.
Wet cattle dung (13g) and waste food (37g) were put in a can and cattle urine (200mL) and sludge inoculum (50mL) was added. The mixture was stirred to form a slurry. The slurry was fed in the digester and carbon dioxide bubbled through slurry to eliminate oxygen.
Wet cattle dung (37g) and waste food (13g) were put in a can and cattle urine (200mL) and sludge inoculum (50mL) was added. The mixture was stirred to form a slurry. The slurry was fed in the digester and carbon dioxide bubbled through slurry to eliminate oxygen.
The slurry was fed in the digester shown below through the reservoir while tap leading to out the effluent was open to allow air out of the reactor. Once the reservoir was nearly full, addition of slurry was stopped, carbon dioxide was bubbled through the slurry for five minutes and tap leading to the effluent was closed. The stirring started and slurry left to decompose while the electric stirrer was running. The gas formed was collected in graduated glass tube over sodium hydroxide solution to absorb carbon dioxide. (
The volume of biogas formed by degradation of cattle dung, kitchen waste and admixtures of these two were measured and recorded using the apparatus shown in
Production of gas from 50g of cattle dung, 50g of kitchen food waste and their admixtures in 50%, 75% and 25% cattle dung in slurry with cattle urine (200mL) and sludge inoculums yielded graphs in
The production of biogas started by day 3 of experiment showing that the time between set up of experiment and initial formation of gas was not utilized by the anaerobes to act on the slurry. This has been explained as time used by the anaerobes to use up oxygen present in the slurry; and after oxygen is depleted, acid forming anaerobes became active so gas production started
Production of biogas increased steadily at first and the sharply after day 9 until it attained its peak on day 18. When gas production had just begun, the microbes in the slurry had just become active and began increasing their population
By the time the peak production was attained, the anaerobes were acting on maximum quantity of organic matter suspended in the slurry
The drop in volume of gas formed beyond the peak may have resulted from decrease in quantity of substrate available to the microbes to act on or even shift in the balance of carbon to nitrogen ration available to the anaerobes to use
The rate of change in volume with time in
By comparison, the volume of biogas formed from slurry of cattle dung was less than that formed from kitchen food waste in cattle urine probably due to kitchen food waste providing a better nutrient balance for carbon to nitrogen than cattle dung which was largely lignified cellulose
Cogeneration is simultaneous generation of biogas using homogeneous slurry of two or more substrates, each of which can produce the gas if digested singly. The results on cogeneration of biogas using slurries containing cattle dung cd and kitchen food wastes kwf in a laboratory scale digester at ambient temperatures is shown in
As shown on
It can be observed that very significant evolution of gas started after day 3. Evolution of biogas was slower for cd than kwf because cd slurry had higher content of lignified cellulose than kwf. Cellulose requires more time and adverse conditions to hydrolyze than ordinary carbohydrates in present kwfs. Additionally, cooking could have weakened bonds in kwfs. So the retention time for cd was higher than for kwf
The average rate of decomposition expresses volume of gas formed/g of substrate digested is shown in
The mean rate of biogas formation for the cogeneration slurries containing cd and kwfs was obtained to be higher than for cd by the respective factors of 1.254 for 75% cd; 1.268 for 75% cd and 1.352 for 25% cd and these values lie within the range of values of enhancement that were reported to lie between 0.8 to 5.5 as compared to single substrate digestion slurries alone
The average optimum amount of biogas was produced by anaerobic digestion of cd and kwf in a period of 18 days, the slurry of cd 50g/200mL yielded 275+ 2.03mL/L; kwf 50g/200mL yielded 329.2+ 5.77mL/L; 50% cd yielded329.2+ 3.10mL/L; 75% cd yielded 422.0+ 3.56 mL/L; 25% yielded 431.5+ 4.65 mL/L.
The average rate of gas evolution reached 5mL/day on the 15th day using 25% cd mixed slurry. The overall rates of degradation attained in the mixtures were 1.42 + 0.26ml/g for cd; 1.58+0.33mL/g for kwf; 1.78+ 0.38mL/g for 75% cd mixed substrate; 1.78+ 0.29 mL/g for 50% cd mixed substrate; 1.92+ 0.21 mL/g for 25% cd mixed substrate slurries in the 200g/L load. The comparative rate of biogas formation ranged from1.25 to 1.35 which was in agreement with the range published in literature of 0.8 to 5.5.
Biogas can be synthesized efficiently at ambient temperatures in Kampala as was done at mesophilic temperatures elsewhere.
Cd and kwf can produce significant quantities of biogas if digested anaerobically.
The digestion of slurry of single cd, kwf and mixed substrates of cd, kwf should be tested for evolution of gas at 37oC, the reported optimum temperatures.
Attempts to test on the effect of pH on yield of biogas need be determined.
Studies on C/N ratios for cd and kwf should be documented to assert the nutrient balance levels.
However, it may be necessary to attempt producing biogas at different pH and temperatures as well as using other substrates and inoculums.
We are indebted to Prof G.W. Nyakairu and Mr. Moses Mutenyo for the design and technical advice on the design of the bio-digester.