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 17^th 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 1. Biogas was reported as mixture of gases including methane, carbon dioxide, hydrogen and others 234. Whereas kitchen waste is any substance raw or cooked which is discharged or remains after 5 cattle dung is black-greenish material that passes out of the rumen of herbivores after feeding on grass and other materials 6. It was reported that digestion of kitchen waste as single substrate yielded 27% of gas 7 yet cattle dung as single substrate yielded 17.9%8 . There are variations in quantities of gas formed when single or mixed substrates are digested 9.

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 ¹⁰11. Anaerobiosis involves a series of processes in which microbes biodegrade organic materials 112. Synthesis of biogas can be coupled to waste management because it produces bio residue which serves a s manure and a gas suitable to replace fossil fuels 1314. The process of bio digestion starts with bacteria hydrolyzing organic matter placed in a digester to transform insoluble organic polymers like cellulose to soluble products, on which different protozoa act 13. Acetogenic bacteria convert amino acids and sugars to methane, hydrogen, carbon dioxide and ammonia 15.

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 Figure 2. Each experiment was repeated thrice and the average volume recorded over the 28 days’ period was used to plot Figure 2 below;

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 Figure 2 for which the coefficients of linearity were and the equations were respectively.

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 40.

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 40, and the microbes needed acclimatization period 41. The steady increase in biogas was explained by the fact that the microbes’ population was fully established in large enough numbers and were therefore progressively acting on more and more substrate as their numbers increased 40.

By the time the peak production was attained, the anaerobes were acting on maximum quantity of organic matter suspended in the slurry 28.

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 41. The volume of gas formed from different generating substrates varied with cattle dung yielding least yet cogeneration mixture made of 50% cattle dung produced highest volume. It was observed that after day 9, the volume gas formed from all digestion mixtures kept increasing steadily.

The rate of change in volume with time in Figure 3 revealed that the higher the rate of evolution of gas was attained by the 15^th day for single substrate and mixed substrate digestions. However, smaller rates of evolution of gas occurred for single substrate than for mixed substrates due to positive synergism brought about by balance of the carbon to nitrogen ratio getting closer to 30:1 41. After day 15, the rate of degradation decreased due to depletion suspended ingredients. It would therefore be recommendable that if biogas is generated for commercial needs, one needs six to seven digesters arranged in series such that thy are started one after the other after a day and left to run up to the 15^th day the restarted on the 18^th day by feeding the first in the same series as was done at the beginning.

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 40. The microbes survive better in media containing more nitrogen than those containing less because nitrogen is an essential element for their life. The anaerobes metabolize organic matter with aid of enzymes reducing carbohydrates, proteins and fats to methane. There is dependency of quantity of gas formed on the carbon to nitrogen (C/N) ratio of the slurry digested 43. The optimum ratio of C/N is 30:1 was reported. The anaerobes consume carbon 30 times faster than nitrogen 41 to convert organic waste to a renewable energy source, biogas. The chemical composition and structure of lignocellulosic materials hinders the rate of bio digestion of slurry as hydrolysis of complex matter to soluble compounds must be the are determining or limiting step for the decomposition of the slurry with high solid content like cattle dung slurry 2844.

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 Figure 3.

As shown on Figure 3, the average rate of evolution biogas from aqueous slurry of cd and kwf because the combination brought together the positive characteristic of feed stocks and potentially bringing better digestion performance as well as more rapid growth of microbial population in the mixed substrate than in the single substrate 4045.

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 46. The maximum rate of volume increase biogas formed was for mixture made of 25% cd on the 15^th day. This was interpreted as showing that the slurry contained the best C/N ratio of all the samples tested 41. So this mixed substrate slurry containing cd and kwf approached the optimum C/N ratio of 30:1 47.

The average rate of decomposition expresses volume of gas formed/g of substrate digested is shown in Figure 4 below for cd, kwf and mixtures cd and kwfs Figure 4 above illustrates that when equal total masses of substrates were fed in the digesters at ambient temperatures differing mean rates of decomposition resulted because the volumes of biogas registered were different. The cogeneration slurries showed higher average rates of decomposition than single substrate slurries of cd or kwf. The average rate of decomposition for cd was1.42+ 0.29ml/g that for kwf was 1.58+ 0.21ml/g that made of 75% cd was 1.78+ 0.35mL/g; for 50% cd was 1.80+ 0.37mL/g and for 25% cd was 1.92+ 0.40mL/g.

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 4045 and this brought about by synergistic tendencies whereby carbohydrates, fats and proteins simultaneously contribute to formation of biogas.

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 15^th 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 37^oC, 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.