Only 14 pages are availabe for public view
The target of this research is designing, fabricating and testing a portable digester, as a method to produce safe and cheap energy, from agricultural wastes namely Water hyacinth (WH) and cow manure (CM).
By increasing the fossil fuels demand and its depletion, in addition to its price rising, so the world searches for another method to getting the energy, which must be safe and cheap. Renewable energy sources include biomass, geothermal energy, hydropower, solar energy, and wind energy. They are called renewable energy sources because they are replenished in a short time. The production and utilization of renewable energy sources are justified not only by energy, political, environmental and competitive aspects, but by rural development aspects as well.
On other hand there are millions of tons of biomass waste being produced every year for which disposal is a problem. These wastes have a great potential for biogas production owing to its organic composition.
Sources of wastes can be broadly classified into four types, Industrial, Commercial, Domestic, and Agricultural wastes. Commonly waste is classified into two types, Biodegradable and Non-biodegradable waste. In general, all types of biomass can be used as substrates as long as they contain carbohydrates, proteins, fats, cellulose and hemicellulose as main components for producing biogas. The choice of a substrate depends on availability, its cost, ease of handling, safety for operators, storage and requirements for feeding.
Anaerobic digestion is “a process of controlled decomposition of biodegradable materials under managed conditions where free oxygen is absent, at temperatures suitable for naturally occurring”. The process of anaerobic digestion (AD) employs specialized facultative bacteria to break down organic waste, converting it into biogas and into a stable biomass (digestate). Anaerobic digestion completed at stages, Hydrolysis, Acidogenesis, Acetogenesis and Methanogenesis. The products of fermentation process are biogas and digestate. Biogas is a mixture of carbon dioxide and methane and traces of other gases. There are many factors affecting rate of Digestion and Biogas production like pH, temperature, sublayer materials, C/N ratio, agitation, inhibitors and pretreatment. Carbon and nitrogen are the main nutrients for anaerobic bacteria. While carbon supplies energy, nitrogen is needed for building up the cell structure.
Anaerobic digestion (AD) is a highly promising technology for converting biomass waste into biogas, while the natural gas is depleting. There are different types of anaerobic digesters for experimental purposes, pilot plant investigations and actual field use. Biogas digester could be divided according to the mode of feeding. These modes of biogas digesters are Batch, Semi-continuous and Continuous operation. Batch process means that, during fermentation, no further materials are added to or subtracted from digester. Semi-continuous operation involves feeding of the digester on a more regular basis. The digested organic matter is also removed on the same time interval basis. At Continuous operation mode the feeding and removal of organic matter take place at all times.
One of the major problems in water bodies is Water hyacinth. Several methods have been developed to help in weed Water hyacinth management like, biological controls, manual and mechanical harvesters and using of Water hyacinth registered aquatic chemical herbicides. Recent statistics show that infestation in Nile River is 5000 ha recorded in 1996. Infestation levels increase northward toward the Nile Delta and the large lakes of Maraut, Edco, Manzala, and Brolos in the north.
Infestation of Water hyacinth is a cause of some environmental, economic, and social problems and it is an added constraint on development. Water hyacinth is considered as harbours of poisonous snakes and insects. It is a strong reason of proliferation of agents of several deadly diseases like malaria, bilharzia, amoebic dysentery and typhoid.
Some Lab-scale experimental works were done to get the optimum conditions of the fermentation process and hence maximum biogas yield. Some agricultural wastes, namely Water hyacinth and cow manure were used. Also some blends of them were tested.
A laboratory scale digester system was made up of a large aspirator bottle with anaerobic head space. The working volume was 2L. The bottle mouth was tightly closed with a rubber stopper carrying two glass tubes one serving as a feed inlet, the other its function was gas outlet. The gas outlet was connected to a T-shaped glass tube, one end of which was used for gas sampling and the other connected to displacement system for measuring produced gas volume. Gas volume was recorded every 12-24 hrs.
The bottle was put in a water bath at the required temperature. The digester’s contents were mixed manually by shaking the bottle for 2 minutes every 8 hours.
Some of young Water hyacinth was harvested from Nile River. Wet Cow manure (CM) was collected from cow breeding farm. To start experiments whole WH plants were washed thoroughly from grit and mud with tap water. To ensure lowest level of heavy metals contamination aerial parts (stems and leaves) of WH were used.
Pretreatment means two steps. First, is removing of undesired particles (impurities) like, soil, sand, stones, glass, mineral materials and wood. Secondly, Size reduction of bulky substrate. Several pretreatment strategies are available for the fractionation, solubilisation, hydrolysis and separation of cellulose, hemicellulose and lignin. This includes physical methods, chemical methods and hybrid strategies.
Two methods of pretreatments to WH were followed. The first method of pretreatment was collecting and chopping WH into 0.5-1.5 cm pieces manually. Then the WH was air-dried. The second pretreatment method was chopping the stems and leaves by electric chopper to get WH in small pieces with diameter of 5 - 10 mm. The effect of these two pretreatment methods on the composition of WH was found have no noticeable difference. As a result of proximity of the composition of the two pretreatments of WH the output results of pretreatment of chopped WH were only concerned.
Chemical analyses of the different parts of WH were got by using Walkley and Black methods and Kjeldal method to get nitrogen ratio. The analyses showed that roots of WH had higher ash and lower volatile matter contents than other parts of the plant, so stems and leaves only were used at the experiments. So by fermentation of stems and leaves more biogas yield could be obtained than the fermentation of the whole plant.
Also Walkley and Black methods and Kjeldal method were used to get the chemical composition analysis of CM. Chemical composition analysis of Cow manure is explained that CM has low C/N ratio about 10 while the C/N ratio for WH was 25-30.
By using a feed stock containing 30 g TS/L for 30 days at constant temperatures namely 25°C, 37°C and 45°C were fixed. The temperature inside the Lab. digester was kept constant by controlling the temperature of water bath. It was found that the performance at 37°C was more than the other temperatures (25°C, 45°C). The biogas production yields at 25°C, 37°C and 45°C were 200 ml, 390 ml and 270 ml respectively.So 37°C was the best one for fermentation.
Two Lab. digesters were loaded with 2 litters of WH slurry containing 20, 30 and 50 g TS/L. Fermentation was continued for 30 days at temperature 37°C. The slurries of concentration of 20 TS/L, 30 TS/L and 50 g TS/L gave 530 ml/kg TS, 590 ml/kg TS 650 ml/kg TS respectively. So gas yield per unit weight of solids is directly proportional to the solids concentration.
A Lab. Scale digester was filled by WH slurry of 50 g TS/1with CM as inoculum with rate of 10% (v/v). And another digester served as an uninoculated control (CO). Experiments were run for 30 days at temperature 37°C. Regarding biogas yield per kg TS it was found the value which was giving by fermentation of (CO) was 230 l/kg TS while the biogas yield per kg TS for the inoculated blend was 420 l/kg TS. Three blends of Water hyacinth and Cow manure with Carbon/ Nitrogen ratios (C/N) namely, CN20, CN25, and CN30. The experiments were conducted in the period of 30 days at temperature 37°C. The results showed that the yield of Biogas in descending order for CN30, CN25 and CN20 where it were 350 ml, 330 ml and 300 ml respectively. Regarding the average pH values produced among the treatments, showed that the pH value ranged from 6.9 to 7.1 with no significant differences among the treatments. This range is suitable for biogas digester operation.
After repeating the above experiments Tri- replication, the pilot digester was designed and fabricated.
The digester vessel was manufactured from stainless steel material to give long life time. Digester was opaque to prevent the effect of light on bacteria, incase that bacteria are sensitive to light. The digester was double walled; between its two walls there is a sheet of thermal insulating material to prevent any affect of the outside surrounding temperature on internal temperature of the digester. The upper vessel lid is in dome - shape to bear maximum internal pressure and to collect the produced gasses. Gaskets were applied to the contact surfaces of the two lids and the digester vessel to prevent any leakage of produced gas. To achieve complete mixing of substrates inside the fermenter, an agitator was used. The agitator was driven by electrical motor through gear box to control its rotation speed. The agitation could be done by drawing the substrates from an orifice at the bottom of the digester and repumping it through the feeding inlet. The fabricated digester could be used as batch digester or as semi continuous digester. This digester requires little maintenance where it has small number of moving items.
The material inside any digester should be agitated through mechanical or gas mixers or by hydraulic method that keep the solids in suspension. Agitation release biogas bubbles trapped in the substrate. The agitation makes the temperature gradient uniform inside the digester. Stirring prevents and breaks down the scum.
The pilot digester was charged by different blends of WH and CM. Referring to the results of Bench scale experiments main parameters were adjusted to get maximum gas production. Temperature was adjusted to be 37°C by using heated water which was controlled by a thermostate. Water was added to the substrates inside the digester to keep TS concentration as 50g/ L. The ratio of CM to WH in the mixture was based on the organic dry matter (ODM) at these substrates. Five different blends with different mixing ratios were prepared from CM and WH. 100% CM + 0% WH (control treatment), 75% CM + 25% WH, 50% CM + 50% WH, 25% CM + 75% WH and 0% CM + 100% WH. pH was noticed to be kept at level 7. The Retention time was noticed till the gas production tends to be decreased which an indication that degradation of the substrates inside the digester is completed.
By performing the experiments using the prototype digester some results were got. It was found that high produced biogas volume was produced with higher percentages of WH in the mixture. Total gas production was (122 L, 148 L, 176 L, 153 L, and 171 L respectively) in the treatment of 100% CM; 75% CM + 25% WH; 50% CM + 50% WH; 25% CM + 75% WH; and 100% WH, respectively.
Improvement in biogas production caused by co-digestion varied with digestion time and generally exhibited the largest values early in the digestion process (10–15 days after process initiation) compared to mono-digestion.
In addition to the measurement of biogas volume produced in each treatment, a certain amount of gas was extracted to know the main gas components. The main gas component in biogas is CH4. The average value was varied during the interval of the experiment. The average percentage of methane in biogas were 60%, 58%, 57%, 55% and 52% for the mixture of CM and WH of ratios100% CM; 75% CM + 25% WH; 50% CM + 50% WH; 25% CM + 75% WH; and 100% WH, respectively.
- Biogas production rate increased steadily at first, and then increased sharply until it reached its peak on the 18th day. Gas production remained stable around the period of peak production until it started declining gradually.
- Biogas volume reached maximum value in the 2nd week; and between the 2nd and the 5th biogas yield was week, this observation showed that the material decomposed most in the first 2 weeks. During the period between set-up and initial gas production microorganism responsible for the process were completely inactive. - Methane production increased with the increase in substrate concentration which is in agreement with the works carried out with other substrates. The reason of this increase is due to the high substrate concentrations resulted in high VFA concentrations favoring to methane production.
During co-digestion most of the CH4 potential in the biomass is exhausted within the first 10-15 days of the experiments after which the CH4 production becomes relatively small (cumulative CH4 production reach a plateau and becomes constant). During mono-digestion it generally takes about 20–25 days to reach the plateau in cumulative CH4 production. Thus, co-digestion not only has a generally positive impact on CH4 potential but it also seems to results in a more rapid release of the CH4 in comparison to mono-digestion.
-The produced biogas commenced flammable gas production from the 8th day. At start of fermentation produced biogas contains too little methane, so the first gas produced did not burn. The gas formed in the first 4-5 days must be discharged unused. This is due to the methane forming bacteria were not fully active yet.
For small scale digesters it is perfable to use the produced biogas as it is, for cooking and heating purposes.
For a family which consists of eight persons, it needs one butane bottle for cocking and heating which is about 12 kg of C4H10/ month. where the heating value for Biogas is half of butane gas= 9.5 kWh/Kg biogas (34200 kJ /kg) hence this family needs 24 kg of biogas /month.
The search recommends by the following:
- More experiments could be carried by using digester as a semi continuous batch digester, where the experiments of this work were done using digester as batch one.
- Other different materials could be used as substrates. The results of produced biogas should be recorded for different blends. It allows using secondary products obtained from agriculture, food industry waste, household waste, and animal manure from livestock.