ADVERTISEMENTS:
Here is a term paper on ‘Biogas Plant’. Find paragraphs, long and short term papers on ‘Biogas Plant’ especially written for school and college students.
Term Paper on Biogas Plant
Term Paper Contents:
- Term Paper on the Historical Background of Biomethanation
- Term Paper on the Overview of Biomethanation in India
- Term Paper on the Basics of Anaerobic Fermentation
- Term Paper on Movable Drum Versus Fixed Dome Type Plant
- Term Paper on the Factors Affecting Biogas Yield
- Term Paper on the Commonly Used Feed Materials for Biomethanation
- Term Paper on the Potential of Biogas Plant Sludge as Enriched Fertiliser
- Term Paper on Biomethanation as an Aid for Environment Improvement
- Term Paper on Community Versus Family Size Plants
- Term Paper on the Integrated Farming System
ADVERTISEMENTS:
Term Paper # 1. Historical Background of Biomethanation:
ADVERTISEMENTS:
Perhaps the first person to observe the phenomenon of biomethanation was Alessandro Volta of Italy. Way back in 1776 he wrote to a friend that ‘combustible air’ was being produced continuously in lakes and ponds in the vicinity of Como in northern Italy. Volta observed that whenever he disturbed the sediments of the lake, bubbles of gas rose to the surface. He also noticed that if the sediment contained more plant material, more bubbles came up.
In 1806 William Henery showed that Volta’s gas was identical with methane gas. Humphrey Davy in the early 1800’s observed that methane was present in farmyard manure piles, Davy conducted the first laboratory experiment to produce methane by anaerobic fermentation of wastes. In earlier period anaerobic fermentation was carried out mainly as a municipal waste treatment process and energy recovery was not the primary concern.
In 1895, biogas from a waste treatment plant in Exter in England was collected and used to light nearby streets. Interest in biogas as a fuel received fillip during Second World War French scientists took particular interest in advancing biogas technology in forties and installed large number of plants in French colonies in Africa.
During this period fuel-starved French and Germans used biogas as a fuel for vehicles and farm tractors. Following the war, several nations such as England, USA, Canada, Russia, Japan, China, Kenya, Uganda, South Africa, New Zealand and India showed interest in biomethanation but later it waned following cheap availability of fossil fuels for over next three decades.
ADVERTISEMENTS:
However, series of energy shocks which rocked the world from 1973 onwards along with concern for environment protection revived interest in biomethanation. In view of its vast potential, large number of community and family-size plants have been set up in recent years in countries like China, India, Philippines and Nepal.
Term Paper # 2. An Overview of Biomethanation in India:
A number of estimates have been made from time to time about potential of biogas generation in India. With livestock population of over 300 million, India can set up over 30 million individual biogas plants and 560,000 community plants at a simple norm of minimum one in each village of the country. With a modest norm of 10 biogas plants per village, it is feasible to set up some 5.5 million plants in country’s 0.56 million villages.
ADVERTISEMENTS:
If this becomes reality, it would provide at 1981-82 price level fuel equivalent to 5341 million litres of kerosene valued at Rs. 5484 million and conserve organic manure to the extent of 89 million tonnes valued at Rs. 4464 million otherwise burnt as dung cake. Effective utilisation of organic wastes can yield gas equivalent of 285.5 million tonnes of coal, almost equal to the amount of energy consumed in the country during 1978-79.
A small biogas plant of 4 cubic meter gas per day capacity set up at a cost of Rs. 6000 including Rs. 2200 availed as Central Government subsidy, operating for 300 days a years can save 4.16 tonnes of fuel wood equivalent to 744 litres of kerosene, and produces manure worth Rs. 1656 per year at the then subsidised market price of fertilisers.
As per some broad estimates, over 30 per cent of 980 million tonnes of cattle dung generated annually on renewable basis is directly burnt as fuel without harnessing its full potential as gas and rich fertiliser following fermentation. Assessed in quantitative terms, it is equivalent to a third of India’s chemical fertiliser used.
If the entire quantity of 980 million tonnes of cattle dung is first used for biomethanation, it can meet domestic fuel needs of 437 million people, raise manure production by 350 million tonnes and substantially improve hygienic conditions in rural areas.
If the entire quantity of dung produced is available for biomethanation, it can produce about 6375 x 107 cubic metre of gas having 3825 x 107 cubic metre of methane with net heat value as 312 x 1012 kcal. In addition, resulting plant effluent can generate one million tonnes of nitrogen, one million tonnes of phosphate, and half million tonnes of potash that can be used as fertiliser.
Based on several designs initially developed by scientists like N.V. Joshi, S.V. Desai, and S.C. Biswas. Y.N. Kotwal and C.N. Acharya, the noted agricultural scientist J.J. Patel while at the Agricultural College in Pune developed a biogas plant which later on came to be known as the ‘Gramlakshmi’ Gas Plant. Distinctive features of this plant covered integration of a gasholder and digester in one unit, underground positioning of the gasholder, daily feeding of slurry, facility of automatic overflow of the digested slurry, and gasholder rotation to break scum formation.
Based on pioneering studies carried out by S.C. Das- gupta, Swami Vishwakarmanand and C.N. Acharya in early fifties, original design of Gramlakshmi Gas Plant underwent further changes and finally the year in 1954, a much modified version emerged.
This improved design had two chambers, one to act as primary and the other as secondary digester with facility for gasholder to have rotary as well as vertical movements. Its digester made of brickwork was kept underground and fitted with two pipes one for feeding and the other for removing digested slurry.
The gasholder was designed in such a way as to maintain uniformity of pressure at 3 to 4 inch of water column but without using counterweights as was used in earlier design. This modified design of the Gramlakshmi Gas Plant became the major landmark in the programme for biomethanation in the country. It was adopted by the Khadi and Village Industries Commission (KVIC) for its countrywide popularisation during sixties and seventies.
ADVERTISEMENTS:
Apart from the KVIC, a Lucknow-based agency called Planning Research and Action Division (PRAD) also adopted it for promotion. The main pioneer at the PRAD in promoting biomethanation was Mr Rambux Singh who has now become a legend in this field. The KVIC and PRAD also tried to promote plants of the IARI (The Indian Agricultural Research Institute) design.
However, plants made to the IARI design with pulley and counterweights created alignment problems often causing tilting of the gasholder sideways which resulted in uneven and interrupted gas supply. The KVIC, therefore, preferred to popularise plants which had no attached pulley and counterweights.
Gas pressure was instead sought to be built-up with the weight of gasholders. KVIC also initiated R&D activities during sixties and seventies. Based on Chinese design and field trials carried out at the Gobar Gas Research Station, Ajitmal, Etawah in U.P., PRAD developed a fixed dome type plant which has come to be known as the Janata Biogas Plant (JBP).
From 1962 onwards, the KVIC launched a well-coordinated biogas programme in the country under which over a lakh biogas plants were set up till the beginning of eighties when the programme was finally taken over by the DNES (Department of Non-conventional Energy Sources). During the Sixth Plan (1980-85), the government set up the National Biogas Development Board (NBDB) with a view to advance grants, subsidies and loans to organisations associated with the implementation of the biogas programme.
It launched the National Project for Biogas Development (NPBD) to give a renewed thrust to biomethanation the country. DNES under the Ministry of Energy was in September, 1982 assigned the task to pursue the NPBD. The DNES also initiated several measures for the success of NPBD which covered upward revision of the Central Government subsidy for plant construction, availability of more effective post-installation follow-up services, creation of large cadre of trained manpower in the field, and supply of raw materials.
During the Seventh Plan (1985-90), it was planned to set up 15 lakh biogas plants with their break-up for successive years as 2.5, 2.75, 3, 3.25 and 3.5 lakh respectively. Under the NPBD, 18.52 lakh family type and 1009 community, institutional and night-soil based biogas plants had been set up by Dec. 31, 1993.
According to the annual MNES sources, 23.05 lakh plants had been installed till Feb. 29, 1996 which are estimated to generate gas equivalent of over 72 lakh tonnes of fuel wood approximately valued at Rs. 360 crores per annum besides producing 345 lakh tonnes of enriched fertiliser.
Over two lakh biogas plants were installed during 1993-94 against a target of 1.75 lakh plants. Matching with the average cattle holding size of rural families, emphasis in recent years has shifted towards installing small-sized plants of 2m3 capacity.
Considering that 23.05 lakh biogas plants constructed upto 1995-96 represent just over 1 per cent of the total rural households in the country, there is vast untapped potential for installing many times the existing number of plants in the country. With an assumed norm that adult bovine generates 10 kg of animal wastes per day, 75 per cent of which is collected, 262 million adult bovines can generate nearly 2 million tonnes of animal wastes per day in the country.
With the established norm that 25 kg of animal wastes can produce 1 m of biogas, it is possible to feed some 40 million biogas plants of 2m3 capacity daily which are 25 times the plants (16 lakhs) installed upto 1991-92. The biogas produced by these plants can meet most of the fuel needs of 120 million rural households in the country. By the end of Eighth plan (1992-97), biogas plants tally is expected to cross 26 lakh mark in the country. MNES is planning to draw ambitious targets for the Ninth Plan (1997-2002).
Term Paper # 3. Basics of Anaerobic Fermentation
:
Anaerobic fermentation process can be broadly divided into two phases, acid forming and methanogenesis. In the first phase, acid forming bacteria breakdown and liquefy volatile solids of feed into fatty acids. In the second phase, the methane-forming bacteria convert these volatile acids into methane and carbon dioxide which are the main constituents of biogas.
These bacteria are sensitive to changes in their environment. Rapid digestion and efficient biogas generation occur within narrow ranges of temperature and are influenced by the composition of raw material, Consistent temperature, pH and use of fresh organic matter as feed help to maximise the gas yield.
Temperature is usually maintained at approximately 95°F (35°C) but different temperatures can also be used provided no variation is allowed. For every 20°F (11°C) decrease, gas production is nearly halved. Hence a constant temperature is essential for higher yield. Temperature variations of as little as 5°F (2.7°C) can inhibit methane forming bacteria causing acid accumulation and possible digester failure.
Biogas plant requires no special skill or scientific knowledge to operate. However, acidity of digester contents needs to be occasionally checked. This can be accomplished by dipping a strip of litmus paper in the effluent and noting its colour. Increasing acidity indicates an imbalance in various bacteria which can be corrected by adding burnt lime until the slurry becomes neutral.
Generally maximum biogas production requires daily loading of the digester. When the loading is done once a week or even less, use of warm water for slurry preparation can compensate provided biogas demand is low. When the demand rises, output can be quickly increased by resorting to daily loading.
A simple apparatus or plant is enough to produce biogas. Desired capacity and life of a plant affects cost and complexity of a biogas system. A simple batch loaded digester requires an oxygen free container, relatively constant temperature, a means for collecting gas and some mixing device. A comparison of different types of commonly used digester systems. As biogas is explosive gas, adequate safety precautions need to be observed while handling it.
Size of biogas plant digester is controlled by a number of factors such as number, size and type of animals providing feed, slurry dilution and retention time. The most easily controllable factor (that can be changed with tank size) is retention time. Ten days is minimum retention time but a longer period can be used. The longer the retention time, the larger is the size of digester.
Loading rate gives an indication of weight of volatile solids fed to a digester per day. Volatile solids are that portion of total solids which are organic in content. Biological organisms utilise a portion of this material as a substrate to produce gas. The degree of utilisation of the material affects gas production rate. Not all volatile solids present in feed are biodegradable and normally around 50 per cent of volatile solids are destroyed depending upon feed composition.
Volatile solids concentration in the digester determines the rate of gas production. For instance, a digester loaded with four units by weight of volatile solids produces twice the volume gas as the same digester loaded with only two units. Gas production rate is quite often expressed in terms of volume of the gas produced per unit weight of the volatile solids added or destroyed.
Proper mixing of digester contents provides intimate contact between micro-organisms and the substrate and maintains uniform temperature, distribution of bacteria and volatile solids throughout the slurry. It also minimises sludge formation and prevents a crust or scum from being formed on the surface of the slurry which interferes with release of biogas. Mixing also, prevents accumulation of undigested feed in digester.
Scum is a collection of light weight, inert material that collects on liquid surface in digester. This accumulation, unless broken up or removed, tends to markedly reduce the effective volume of digester and obstruct release of gas from digester contents. Methods of mixing or agitation include slurry recirculation pumps and mechanical paddles.
However mechanical components exposed to slurry are susceptible to corrosion which makes there difficult to repair without disrupting digestion by opening the digester. Mixing can also be accomplished by recycling gas through diffusers at the bottom of digester tank. An electrically driven vacuum pump draws biogas from storage under roof and injects it at the bottom of both stages of digester. Recirculation of biogas helps in agitation of slurry.
Amount of water required in slurry preparation is an important factor in digestion process. Considering that optimum total solids (TS) concentration in an anaerobic digester is normally 7 to 9 per cent, and since animal faeces may contain 10 to 25 per cent solids in general, considerable dilution is quite often required. Furthermore, as far as possible, total solids in digester content should be maintained fairly uniform, allowing only gradual changes whenever it becomes necessary.
Very little reduction of feedstock occurs in a plant by volume. In fact, there occurs an increase in the volume of effluent on account of slurry preparation with water. Digested slurry after anaerobic fermentation has far less odour. As it still contains most of the original nitrogen, phosphorus and potassium and is still highly polluted, affluent cannot enter a sewer stream after it leaves the digester.
Lagoons are commonly used to hold the waste in most European countries until it can be disposed of by either hauling or pumping onto the agricultural land. As a result of fermentation, there occurs no increase in the amount of nitrogen, phosphorus or potassium although it ensures their larger availability through concentration.
Biogas plants can be fed with slurry continuously or in batches. A batch loaded digester is filled to capacity sealed until it has produced all it can, emptied and filled again. Gas production is uneven because bacterial digestion starts slowly, peaks and then tapers off with growing consumption of volatile solids. This difficulty can be overcome by connecting the batch loaded digesters in series loaded at different times so that adequate biogas is available daily.
This method uses feed efficiently but is less efficient in terms of digester space. In a continuous type biogas plant fresh slurry is added daily. Gas production in this method is consistent because the bacteria always have a fresh supply of volatile solids to digest. The continuously loaded digester uses expensive digester space efficiently although it may not produce quite as much gas per kg of feed as a batch type may do.
Incoming slurry displaces an equal amount of processed slurry from digester each time it is fed since maximum allowable volume of slurry in a digester is fixed for a particular plant. In view of digester volume being constant, fraction of digester’s liquid volume replaced each day determines retention period. For example, if slurry equal to one-tenth of digester’s liquid volume is added daily, digester slurry has an average retention time of 10 days.
Considering that retention period depends on digester’s volume, its selection to suit a particular feedstock and digester determines size or capacity of a digester. Other factors that have bearing on plant capacity include number, size and type of animals providing feed and optimum amount of dilution-water required for slurry preparation.
A very brief retention time does not allow bacteria enough time to digest and hence results in low gas production. Long retention time allows complete decomposition of wastes and hence increased gas production rate follows. However, if retention time is made very long, it does not furnish enough fresh slurry to promote bacterial growth, and hence again a low gas production rate results. In most biogas plants, fifteen days is usually chosen retention period.
Considering that peaks of gas generation and usage of biogas do not always match, some gas storage either within gasholder or outside is necessary. Low pressure storage systems usually suffice but if the gas is to be used for such application as vehicle fuel, high pressure storage is required. At low pressures of 100 to 150 mm of water column, polystyrene or butyl rubber bags can be used for storage.
However, for storing several days’ gas output, volume needs become excessive. For example, to store biogas equivalent of 200 litres of petrol, a low pressure storage system requires a volume of about 300 cubic metres which is broadly equivalent to the volume of average size house. High pressure systems use a three or four stage compressor to compress biogas to pressures of around 3500 psi which can be stored only in a high pressure cylinder. Even at such high pressures which are expensive to attain, biogas has only one-sixth of energy density of liquid fuels.
Methane which is the principal constituent of biogas cannot be liquefied by any pressure at ambient temperature. The critical pressure of methane is 4710 kPA at – 82.3°C. In contrast, propane has a critical pressure of 1290.3 kPA at 38°C and can thus be liquefied at ordinary temperatures (Critical temperature of a gas is that temperature above which it is not possible to liquefy it under any pressure. Critical pressure is the pressure needed to liquefy a gas at its critical temperature). Liquefaction poses further problems following presence of large quantities of carbon dioxide and other impurities. In case of natural gas, it is possible to store it by adsorption on propane or adsorption on activated charcoal.
If this method is tried on biogas, it may pose several difficulties such as high initial cost of adsorbent, large storage needs, and also release of gas from adsorbent. Propane tanks, have been successfully used for biogas storage at pressures upto 1400 kPA (200 psi) on farm digester installations in Canada. The Biomass Energy Institute at Manitoba (Canada) demonstrated that this pressure is easily achievable by using a conventional cold storage compressor.
Term Paper # 4. Movable Drum Versus Fixed Dome Type Plant
:
The conventional movable drum type model comprises a masonry digester with an inlet on one side for feeding slurry and an outlet on the other side for removing digested slurry. The gas collects in a steel gasholder which is inverted over the slurry and moves up and down depending upon accumulation and discharge of gas guided by a central guide pipe (Fig. 1.1).
The main drawback of this design is that metal construction cost are very high. The movable gasholder made of mild steel alone accounts for about 40 per cent of the total cost of plant. The maintenance costs of this type of plant are also high considering the fact that vital components like gasholder are corrosion-prone which need to be given anti-corrosive painting at least once a year.
Moreover, several construction materials are not readily and locally available at plant sites. Janata Biogas Plant (JBP) which is a fixed dome plant dispenses with movable steel drum type gasholder, and its gasholder is built with bricks and cement as immovable and fixed dome cover of the well-like digester itself (Fig. 1.2).
When biogas is formed inside the digester, it rises towards the top of the dome and pushes slurry down. Displaced level of slurry provides the necessary pressure for release of the gas up to the point of its use. The first pilot drumless JBP was developed by the Gobar Gas Research Station, Ajitmal in early part of 1977. The depth and diameter of digester were 2m and 3m, respectively with the digester so designed that it could hold about 50 days feed.
The JBPs have no moving parts and hence the wear and tear and consequent maintenance problems are relatively few. Accordingly these plants have longer working life. Service life of a biogas plant can be expected to range anywhere between 15 to 25 years. In a movable drum type plant, digester is generally expected to last for 40 years, gasholder for 10 years and pipes and accessories for about 20 years. Gas yield of JBPs is considered to be higher (0.04 m3) per kg of cattle dung at 24-27°C, than those of other plants.
However, gas yield per unit volume of the digester is more for a movable drum type than for a fixed dome type plant. Operating costs per cubic meter of biogas is generally more for a movable drum type than a fixed dome type biogas plant. Effluent obtained from JBPs has the same carbon and nitrogen contents and it is richer in phosphorus and potassium contents by 2.5 to 6.5 per cent and 7.2 to 18.5 per cent, respectively as compared to those from other plants.
On account of JBPs being underground, rate of gas yield from them shows relatively uniform pattern during days and nights due to their being less susceptible to temperature variations. One side advantage of JBPs is that the space available above the plant can be gainfully used for a variety of applications such as providing a cattle shed. A comparison between fixed dome type (JBP) and movable drum type biogas plant is summarised in Table 1.2.
Term Paper # 5. Factors Affecting Biogas Yield:
Regardless of specific feed used, biogas generation proceeds most efficiently when raw materials fed to the digester have a desired pH and carbon-to-nitrogen ratio (C: N). Bacteria thrive in a slurry with a pH value of around 7 and consequently If the input slurry has a pH close to 7, fermentation proceeds more smoothly. Under normal conditions, fermentation process balances excess acidity or alkalinity on its own.
Carbon is the major chemical element in organic wastes which bacteria digest releasing finally methane and carbon dioxide in the process. However, microorganisms require certain amount of nitrogen in the feed to perform their function. Ratio of carbon-to-nitrogen is thus crucial for anaerobic fermentation. A high C: N ratio means that the nitrogen will be exhausted before carbon is digested. Conversely, a low C : N ratio means too much nitrogen in relation to carbon.
It results in high ammonium concentrations which tend to become toxic to anaerobic bacteria. It is, possible to adjust the C : N ratio in a biogas plant by adding another material to supplement or counter material already in the plant. For instance, saw dust which has a high C : N ratio could be added to poultry droppings which have a low C : N ratio. Dairy cow manure has a C: N ratio just slightly below that required by bacteria.
Total biogas production rate depends on organic material being digested, digester loading rate and environmental conditions. Under ideal condition; 95°F or 35°C temperature and proper pH, it is possible to produce about 45 ft3 (1.4 m3) of biogas at atmospheric pressure from one day’s faeces of a 100- pound (54 kg) cow. Not all of the biogas produced is available for utilisation as part of it goes waste.
In very aggregate and general terms, 1m3 of slurry fed to biogas plant produces an average of 0.15 to 0.2 m3 of biogas daily; one cubic metre of biogas produces about 5500 kcal of heat; and each member of a family requires 1.5 to 2m3 of digester space to meet his mean requirement of lighting and cooking; and 32 kg of cattle dung per day, 20 kg of pig faeces per day, and 12 kg of poultry droppings per day produce 1 m (34 ft3) of biogas every day under normal conditions.
Term Paper # 6. Commonly Used Feed Materials for Biomethanation
:
Common feed materials include animal wastes, crop-residues and urban wastes including night-soil. These are broad categories which cover variety of materials. Some of the waste materials are directly used as fuel or fertiliser. Utilisation of these materials for biogas generation harnesses additional benefit while retaining their existing benefits which are anyway tapped.
The list of materials which fall under the above three broad categories can be further expanded to include dairy cow wastes, swine wastes, poultry litter, sheep and goat droppings, crop-residues of various plants, sugar-cane trash, bagasse, rice-husk, rice-bran, tobacco wastes and seeds, cotton dust from textile mills, tea waste, fruit and vegetable wastes, marine algae, seaweeds, water- hyacinth, press-mud from sugar plants, twigs, bark, tree branches, plant leaves, municipal wastes and night-soil etc.
Another development in the area of anaerobic digestion is application of industrial wastes which constitute an important category of feed-stocks. Wastes available from food and drink industry are particularly useful for this purpose. Vegetables and fruit-processing and canning units produce large quantities of carbohydrates-rich-wastes which are easily amenable to anaerobic fermentation.
Drink manufacturers use enormous quantities of biomass in the form of barley, apples, grapes etc., and residues from these plants can be anaerobically fermented. Methane gas production from vegetable wastes has in past been particularly popular in countries like Australia and South Africa.
Wastes from sugar distilleries which have high carbohydrate contents are also a valuable feedstock. Several sugar factories world over have established fermentation units. One manufacturer has successfully completed trials for a digestion plant utilising wastes from an olive-oil factory in Spain which besides generating usable gas also solved problems of pollution-control.
Term Paper # 7. Potential of Biogas Plant Sludge as Enriched Fertiliser
:
Effluent or sludge resulting from a biogas plant is a resource as it contains almost all of nitrogen present in raw feed material prior to digestion. The effluent is an excellent fertiliser because nitrogen in the effluent is more readily absorbed by plants than nitrogen in raw manure.
The sludge that collects at the bottom of a digester should be regularly removed because accumulated sludge tends to reduce active digester space. The end result of applying anaerobically digested sludge to soils has the same beneficial effect as that obtained by applying any other organic matter.
The humus materials formed in sludge tend to improve physical properties of soil; for example, aeration, moisture-holding and water-infiltration capacity of soils are improved. Furthermore, sludge serves as a source of energy and nutrient for development of microbial populations which directly or indirectly improve the solubility and thus availability of essential chemical nutrients contained in soil minerals.
The organic fraction of sludges from an anaerobic digester operating on plant and animal waste may contain 30 to 40 per cent lignin, undigested cellulose and lipid materials on dry weight basis depending upon the type of raw material used. The remainder consists of substances originally present in the raw material but protected from bacterial decomposition by lignin, newly synthesized bacterial cellular substances and relatively small amount of volatile fatty acids.
The amount of bacterial cell mass is small as 10 – 20 per cent of substrate is converted to cells. Thus there is less risk of creating odour and insect-breeding problems when anaerobically digested sludges are stored or spread on land than when untreated or partially treated organic waste materials are similarly handled, indiscriminately disposed off or stored. The sludge is an almost odourless, homogenised liquid which does not attract rodents or flies.
Term Paper # 8. Biomethanation as an Aid for Environment Improvement:
Biogas plants help to bring improvements in ecology and environment. Biomethanation provides a non-traditional way for disposal of sewage, animal and human faeces in urban and rural areas. In biogas plants as these wastes are no longer exposed to open air, menace of mosquitoes, flies and hookworms is sufficiently controlled.
Following check on hookworms, Biomethanation also helps to control diseases like Schistosomiasis, hepatitis and intestinal infections. It improves household hygiene as it prevents occurrence of polluting smoke that emanates for traditional fuel wood stoves (chulhas). Faeces contains countless parasitic ova and pathogenic organisms which often cause ailments like typhoid, dysentery, hepatitis and poliomyelitis.
Chances of health hazards posed, basically depend upon three factors, namely, proportion of viable pathogenic organisms in faeces, survival rate of these organisms in sludge, and length of time the sludge is stored prior to use.
Survival rate of pathogenic micro-organisms varies from situation to situation and there are no fixed rates available for this purpose. Most of these bacteria do not survive beyond a period of 14 days. Anaerobic fermentation process is considered to be one of the most effective ways of killing enteric micro-organisms which is highly beneficial to public health.
The survival of enteric micro-organisms has been the subject of many investigations, the outcome which depends on numerous factors such as the type of organism, ambient temperature, type of soil, moisture and pH of digester contents, and as a result survival is found to vary from few hours to several months. Free ammonia which is released during anaerobic fermentation is another factor that influences survival and growth of micro-organisms.
The usual proportion of ammonia in digester contents is of the order of 0.07 per cent. If free ammonia level is around 0.2 per cent, the ova of Schistosoma can hardly survive beyond six days and hookworm and Ascaris only marginally longer. Free ammonia reduces the survival period of micro-organisms as it tends to permeate egg-shell and cell membrane and in the process kills ova and bacteria.
Addition of certain plant materials like algae to sludge checks the growth of some bacteria. For instance, algae when added to effluent retards the activity of Anopheles larvae following change in pH level and in the process makes the bacteria ineffective.
In spite of the fact that certain pathogens and parasites do survive even after anaerobic fermentation, and some remain active much longer, no cases of outbreak of diseases have been reported from any quarter. This can be explained by the fact that most of the rural-folk are anyway vulnerable to higher health risks, and biomethanation reduces the health hazards significantly, if not completely.
Term Paper # 9. Community Versus Family Size Plants
:
Biogas plants can be constructed either as individual family units or community plants. Whereas individual plants are appropriate for families, community plants are set up to meet fuel and fertiliser needs of groups, institutions and village as a whole. Community plants are particularly appropriate where individuals do not have adequate manure or funds to set up family units.
Community plants can be operated by a group of families, cooperatives or members of a locality. Utility of a community plant is further enhanced if some small scale industry is made to run on biogas as a fuel which may also create some employment opportunities for rural youth. In case of community plants, common utilisation of gas in the direct neighbourhood of the plant, such as for running a community kitchen is generally desirable.
Many energy planners argue that individual plants should be constructed only when idea of setting up community plant found is infeasible. It is often seen that a farmer who is reluctant to invest a large sum of money for his own private plant may not find it difficult to make a small deposit and pay monthly charges as his share in the running of the plant.
Community plants provide economies of scale which arise from savings in initial capital cost, land utilised and labour charges needed in running such plants. Community plants make large-scale composting of organic waste feasible by mixing them with plant sludge.
Community biogas plants are socially more egalitarian. Out of some 100 odd million households in India, only about 20 million own three to four heads of cattle needed to feed a family-size biogas plant. According to the report of the Planning Commission’s Working Group on Energy Policy (1979), even if all those households which can set-up family-size plants were to do so, still enough cattle would be available to feed 560,000 community plants at a simple norm of one in each village of the country.
In view of their vast potential, community plants received major thrust from the Sixth Plan (1980-85) onwards when an ambitious target of 100 plants was fixed. Plant economics can be further improved if night-soil from community latrines is also used as valuable feedstock. Animal wastes often lying on village roads and grazing lands which otherwise go waste can also be gainfully utilised in the community plants.
Community plants ensure higher gas yield per kg of feed on account of economic viability of providing facilities such as warming up of slurry during winter season, which is not cost-effective in family units. Higher biogas supply from community plants besides meeting fuel requirement of households, can be used for generating power, fuelling rural industrial units, operating irrigation pump sets, and lighting village streets.
Community plants are considered to be more cost-effective. The capital cost needed to set up a 500-ft3/day (CFD) capacity plant is nearly half of that required to install 50 plants of 10 CFD each. However, running of these plants is not without problems. For instance, due to countrywide social and economic disparities task of dung collection and gas distribution poses practical difficulties. Similarly, evolving a pricing policy for the gas and fertiliser produced by a plant which meets the requirement of both rich and poor farmers alike is difficult to work out.
Term Paper # 10. Integrated Farming System
:
Integrated farming system may comprise a biogas plant, energy farming, wind-mills; solar heating and photovoltaic systems; food, fodder and algal cultivation and pisciculture. Effluent from the biogas plant provides a nutritious protein-rich feed for animals, particularly fish. Solids are recovered from sludge by allowing it to settle in tanks or by draining and drying. These solids along with other additives after being meshed and detoxified can be used as animal feed.
After separation of solids from sludge, remaining liquid can be utilised as a nutrient for algal growth. Chlorella, a single-celled high protein, can be harvested with the liquid separated from sludge in a shallow pond. The pond is usually lined with concrete, metal or plastic material which makes harvesting easier and minimises contamination. Chlorella can be used in proportions upto 10 per cent of animal feed to replace soyabean used as a meal for protein supplementation.
With the availability of fish meal from sludge, it is advisable to have a fish-pond as part of the integrated farming system. Algae produced can be utilised in two ways. Part of it can be used as feedstock for producing biogas and part as fish meal along with other additives. A large portion of biogas plant sludge is used as fertiliser for growing food, fodder and energy crops.
Biogas plant sludge contains a variety of products including nitrogen which is mostly lost during aerobic composting. Nitrogen is one of the most important nutrients for plant growth. Nitrogen content can be increased by adding animal urine to influent slurry of a biogas plant. For maximising fertilising effect, slurry should be ploughed into the soil about one week before sowing is planned.
In integrated farming system, sludge is stored till the time of its spread. In a farm, it can be stored in two ways, as a liquid fertiliser in a covered pit and as solid manure in a compost pit. As per rough norm, one cubic metre of slurry can fertilise about 100 sq. m of land annually.