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In this article we will discuss about the potential of biogas plant effluents.
Introduction to Biogas Plant Effluents:
Effluent from a biogas plant provides rich fertiliser. Utilisation of animal wastes in a biogas plant whereas on one hand it generates a clean fuel, on the other it provides a high quality fertiliser. Not only it provides improved fertiliser but also it generates more in quantity. Higher quantity results from addition of certain crop-residues and night-soil in feedstock which otherwise remain unutilised.
Digested slurry has a higher fertiliser potential due to its having larger nitrogen content which is an important ingredient for plant growth. In case of air-drying of animal wastes, 30 to 35 per cent of nitrogen is lost in air, whereas through anaerobic fermentation route it is limited to 10 to 15 per cent. If animal urine is also used in slurry preparation, fertiliser properties of plant sludge improve further following increased availability of nitrogen and potassium.
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Like compost and farmyard manure, plant effluent improves soil-fertility, soil-porosity and water-holding capacity. It supports bacterial growth in soil that facilitates release of vital nutrients which contribute to increased crop productivity. Digested slurry can be used effectively for any type of soil and crop. Effluent can be applied at a norm of 10 tonnes per hectare in irrigated areas and 5 tonnes per hectare in dry land areas. Effluent can be either directly used in liquid form or after composting in a pit.
Composting of effluent with other organic wastes is a common practice. However, it requires adequate land available near the biogas plant. Effluent can also be used in semi-dry form which can be done by spreading effluent in shallow pits and allowing partial sun-drying. It is then scraped and stored in piles and covered with plastic sheets or mats. Effluent can also be used as supplementary fish meal in fish ponds.
Plant effluent also helps to improve village hygiene. Despite the fact that certain micro-organisms continue to exist in effluent which are injurious to health, effluent does not cause any harmful effect. In the absence of oxygen and presence of ammonia, pathogenic organisms and parasitic ova cannot survive, develop and multiply for long which contributes to improved hygiene though resulting health benefits are not ultimate hygienic standards.
Organic fraction of effluent may contain 30 to 40 per cent of lignin, undigested cellulose and lipid materials on dry-weight basis depending upon type of raw material used. Remaining portion consists of substances originally present in raw material but protected from bacterial decomposition and small amounts of volatile fatty acids.
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 disposed off or stored.
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One of the direct benefits of anaerobic fermentation process is that nutrient elements in plant residues and animal wastes such as nitrogen all of which are conserved. The only materials that are removed from the system are gases comprising methane, carbon dioxide and hydrogen sulphide. Biogas plant digested slurry retains about 99 per cent of nitrogen present in input slurry.
State-of-Art of Effluent Utilisation as Enriched Fertiliser:
Biogas plant is generally regarded as a source for generating gaseous fuel. In the context of a developing country like India, biogas plant can also be viewed as a source of rich fertiliser. In China biogas plants are also sometimes referred to as ‘mini-manure factories’. Plan effluent can be basically used in three ways.
It can be applied to the field directly without any treatment, it can also be applied after enriching with liquid ammonia and phosphorus, or it can be applied after composting in a pit with straw and silt. Biogas plant effluent regardless of the, method of application leads to higher crop yield than obtainable from direct use of animal manure. Different crops respond differently to application of animal manure or biogas plant effluent.
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Crops which are more responsive cover vegetable crops like potatoes, tomatoes, sweet potatoes, water-melons, radishes, carrots, cauliflowers, onions, garlic etc.; fruit trees and vines such as orange, grape, apple, guava, mango etc.; and other crops like sugarcane, rice, jute etc. Crops which are less responsive include cereal crops such as jowar, bajra, wheat, ragi, barley and oats; oilseed crops such as groundnuts, linseed, sesame, castor beans, coconut; and other crops such as cotton.
Unlike chemical fertiliser, the effect of effluent is carried over a longer period of three years or more. Effluent is source of plant nutrients and provides an excellent soil conditioner as it has requisite quantities of different elements all essential for healthy soils and plants. Effluent has more nutrients than farmyard manure as the latter loses much nitrogen during air-drying.
Plant effluent contains more ammoniacal nitrogen than farmyard manure which is readily absorbed by crops. However, much of its ammoniacal nitrogen can be lost if digested slurry is allowed to dry for long in air. Like other compost materials effluent provides humus and supports microbiological activity in the soil.
Nitrogen in digested slurry is present in two forms-ammoniacal nitrogen and organic nitrogen the exact proportion of which depends on feed composition. M.A. Idnani and S. Varadarajan carried out experiments to study balance nitrogen left following anaerobic fermentation process.
According to their findings, more than 17 per cent of total nitrogen is in ammoniacal form and a small amount of 0.8 per cent is the form of volatile compounds. In another experiment, anaerobic digestion of rice straw with its lower nitrogen content of only 0.8 per cent produced 8 to 10 per cent of ammoniacal nitrogen. Thus, as much as 18 per cent of the original nitrogen can be lost if it is allowed to volatilize.
As only a very small amount of nitrogen in crop-residues and animal wastes is utilized by anaerobic bacteria to synthesize proteins, the greater is the nitrogen content of feed material, the greater will be the concentration of ammoniacal nitrogen in sludge. It is possible to keep nitrogen losses through volatilization minimum by proper storage of sludge and by following proper application technique.
To minimize losses, digested sludge should be stored in deep lagoons or tanks that provide a minimum of surface area for volatilization. Nitrogen can be conserved maximum if the sludge is injected below the soil surface a few days before crop planting or just prior to cultivation. If the sludge is spread on the soil surface and allowed to dry without interruption by rainfall, large proportion of ammoniacal nitrogen is likely to be lost by volatilization.
In many situations crops are specially cultivated for being used as feed in biogas plants. One of the benefits of harvesting crops for producing biogas is saving of valuable plant nutrients that are otherwise lost by leaching to depths below root zone and beyond reach for absorption by crops used as food crops.
Many nutrient elements of plants are transferred to soils to form sparingly soluble compounds and thus their availability to succeeding crop becomes less and less. Difficulties arising out of this kind can be alleviated if leaching is carried out within the root zone. For instance, some elements such as potassium can be easily leached from crop residues if these are left to decompose on soil surface.
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On permeable soils, these elements may be leached below the root zone of the next or the succeeding crop. Other nutrients, such as phosphorus and some trace-metal elements, mineralized from agricultural plant residues during composting or at soil surface, often become less available for plant absorption with time as they react with inorganic soil constituents to form compounds with relatively low solubilities.
Therefore, if agricultural crop residues are harvested and properly stored as feedstock for a biogas plant, many important plant nutrients are used more efficiently as they are transferred back to the soil to be used by next crop achieved by using plant effluent as fertiliser.
Several trace elements that are otherwise destroyed during microbial decomposition of crop residues in digesters, can be maintained in this state until they can be used by the succeeding crop. If exposure period of these elements to the soil environment is shortened by reducing storage period of sludge of which these elements are part, availability of plant nutrients to succeeding crop becomes much higher.
According to some entomologists and pathologists, recourse to under-ploughing of agricultural crop-residues is beneficial for decreasing the incidence of insect and plant pathogen infestation in succeeding crop. On the other hand, where soils are subject to severe erosion by water or wind, soil conservationists recommend the ploughing should be delayed as long as possible before planting, or it be eliminated altogether.
Anaerobic fermentation of agricultural crop residues can be viewed as a way of controlling some insects and diseases more effectively than what is possible if these crop residues are allowed decomposition aerobically. For example, according to some scientists, fungi that tend to cause plant disease can be more effectively controlled by anaerobically digesting crop residues than by ploughing them under the soil.
As the anaerobic surfaces of cultivated soils contain abundant supplies of various kinds of micro-organisms, readily degradable organic constituents of digested sludge are rapidly decomposed to carbon dioxide and water or are transformed to humus compounds.
There occurs some partial degradation of lignin and lipid components of sludge when it is applied to soil as fertiliser. According to the results of a study when anaerobically digested municipal sludge was annually applied for 6-7 years at various rates and on different types of soil, the soil organic matter was found to reach a more or less dynamic equilibrium state.
About 20 to 40 per cent of the organic matter in sludge decomposed each year depending on difference in soil type. Within 5 years after sludge application began soil organic content reached a more-or-less stable level, maximum soil organic matter content at equilibrium depended on amount of sludge applied annually.
In other words, the greater the amount applied each year was, the greater soil organic matter content was found on reaching equilibrium.
M.A. Idnani, et al., proposed enrichment of biogas plant effluent for improving its fertiliser properties. It can be enriched in nitrogen and phosphorus content by taking 11 kg of urea and 31 kg of superphosphate and dissolving them in requisite quantity of water. This solution is then absorbed in 48 kg of dry low grade manure thoroughly mixed and spread out in the sun to dry.
The enriched manure would then contain 5 per cent nitrogen and 5 per cent phosphoric acid in addition to its original strength of these plant nutrients. These impregnated manures which have been found to be highly effective in field experiments can provide impetus for effective utilisation of biogas plant effluent as improved quality fertiliser.
Results of Studies Carried out at the Guangdong Province Agricultural Institute at Canton (China):
Effluent from a biogas plant is an efficient fertiliser as following anaerobic fermentation, nitrogen content in animal and agricultural wastes is transformed into ammonia which crops can more easily absorb. According to result of studies carried out at the Agricultural Institute of Sichuan Province in China ammonia content of organic fertiliser fermented for 30 days in a pit increases by 19.3 per cent and the useful phosphatic content by 31.8 per cent.
Sealing this organic fertiliser in pits prevents evaporation and loss of ammonia. As per the studies carried out at the Guangdong Province Agricultural Institute at Canton, ammonical content of common fertiliser stored in these pits was found to increase by 144.7 per cent in 30 days, whereas the same manure if stored in traditional heaps and unsealed pits would undergo a loss of 84.1 per cent in ammonical content. Manure that was fermented in pits was found to substantially increase agricultural yield.
According to the results of experiments carried out by the fourth work-team of the third brigade of Yungxing commune in Mianyang County of Sichuan Province, the maize yield was found to increase by 28 per cent. Similarly according to the results of experiments carried out by the seventh brigade or Qingdao commune in Mianzhyu county, rice yield increased by 10 per cent.
In Pengsi County, the Agricultural Science Station of Dongfen commune reported that the wheat yield increased by 12.5 per cent and according to Jinghua brigade of Chunguan Commune in Gunnan County, Hubei Province, the cotton yield increased by 24.7 per cent.
Experimental results comparing yield of four crops when fertilised with unfermented excreta and digested slurry are summarised in Table 12.2. The fertiliser value of cow wastes as determined by nitrogen effectiveness index is basically the function of its freshness and interval after which it is applied to fields.
Results of Studies Carried Out at the IARI, New Delhi for Analysis of Manurial Efficiency of Effluent:
A.B. Ghosh and M.N. Dutt carried out studies at the Division of Soil Science and Agricultural Chemistry, IARI, New Delhi for analysing manurial efficiency of biogas plant effluent vis-a-vis other fertilisers. In their experiments, 15 kg of identical soil, slightly alkaline (pH 8.2), medium in phosphorus and potassium contents and low in nitrogen and organic matter was used for observing plant growth under varied conditions.
Various fertiliser samples used in their experiments included biogas plant effluent containing 0.12 to 0.14 per cent nitrogen, another effluent sample with 2.03 to 2.08 per cent nitrogen and sun-dried farm yard manure containing 0.56 to 0.61 per cent nitrogen, and ammonium sulphate containing 20.8 per cent nitrogen.
These samples were applied separately under identical conditions to IR-8 variety rice, and berseem crops and their growth observed first during a period of three years and later these were extended for a period of further two years. Comparative nutritive value from fresh and fermented manure for varying combinations of fertiliser application rates is given in Table 12.5.
Relative manurial efficiency of fresh and dried biogas plant effluent, farm yard manure etc. measured in terms of productivity of rice and berseem crops grown with their applications is given in Table 12.6.
From the results summarised, it is to be seen that both wet and dried effluent compared very well with farmyard manure, caused 15 to 37 per cent yield of rice grain, and 15 to 44 per cent higher yield of berseem during the first three years vis-a-vis when no manure was applied.
Oil cake and ammonium sulphate was found to produce much higher yield of rice but their effect on berseem yield was found to be considerably less as compared to that of dried effluent and farmyard manure. As for the relative performance of fresh and sun-dried effluent, the latter was found to achieve 20 to 28 per cent higher productivity of the two crops.
All the fertiliser samples used were found to increase crop uptake of nitrogen. The soil tested after three years of manuring with the effluent showed a significant improvement in total nitrogen and available phosphorus content.
In case of dried effluent, nitrogen and phosphorus uptake changed from 0.055 to 0.066 per cent and 18 to 20 kg per hectare, respectively; and in case of fresh effluent from 0.055 to 0.07 per cent and 18 to 22 kg per hectare, respectively; and in case of FYM from 0.055 to 0.069 per cent and 20 kg per hectare, respectively.
No such improvement was observed in case of groundnut-cake whereas in case of ammonium sulphate-phosphate it was observed in decline substantially. These experimental results thus confirmed that direct and residual manurial value of biogas plant effluent is generally superior to farmyard manure, a fact which is widely underscored during campaigns for biogas programme.
Results of Studies Carried out at the Institute of Soil and Fertiliser in Sichuan (China):
Scientists at the Institute of Soil and Fertiliser (ISF), Academy of Agricultural Science Research (AASR), Sichuan, during late seventies carried out extensive experiments to analyse the fertiliser value of biogas plant effluent. Studies were inspired by the fact that plant effluent as fertiliser improves soil fertility more than farmyard manure.
Advent of biogas programme in China has greatly revolutionised the system of collecting and producing manure in the countryside. Sludge consists of all main nutrients of influent and organic matter. There is no fixedness with regard to proportion of nutrients present in effluent which largely depends upon feed composition.
Proportion of nutrient elements present in digested slurry (effluent) and sludge, as analysed by the ISF scientists are given in Table 12.7. ISF scientists also studied manurial efficiency of anaerobically treated digester effluents and aerobically treated farmyard manure. Both these fertilisers were applied to the rice, maize, wheat, cotton and rape crops under identical conditions.
Experiments were also carried out for determining the relative manurial efficiency of applying digester sludge after air-drying, and on continuous basis directly, keeping the amount of sludge applied same in both the cases. Crops whose growth were studied under two variants were sweet potato, rice, maize and cotton. Effect of adding a chemical fertiliser such as ammonium bicarbonate to plant effluent were also studied which had a beneficial effect to improve crop yield and minimise soil erosion.
In another experiment to alleviate problem of low sulphur content in certain soils which reduces crop productivity, a new kind of fertiliser called biogas phosphohumate was developed which when applied to soils improved their sulphur content. This chemical was obtained by mixing air-dried sludge with phosphorite powder in proportion of 10 to 20 : 1 and composting it for 1 to 3 months.
Effect of applying this fertiliser on purple soil and yellow clay which are common in China are summarised in Table 12.12. Application of plant effluent as fertiliser helps to improve soil properties. With successive cultivation soil nutrients are increasingly diminished which lower soil fertility. Plant effluent as fertiliser which consists of most of the lost nutrients of soil, helps to make up the deficiency of these elements in soils.
Chemical and physical properties showed significant improvement after 1 to 2 years application of digester sludge. As a result, organic matter content, total nitrogen, total phosphorus, available phosphorus and porosity increased whereas soil density showed some decline. Following improvement in soil properties, total yield of some crops went up by 20 per cent.
Techniques of Effluent Usage as Fertiliser:
Considering enormity of agricultural operations in a country like India; chemical fertilisers alone cannot meet the total demand. Demand for fertilisers is required to be met by alternative sources of supply. Wastes obtained from animals, agricultural plants and human beings can provide six to eight times more nutrients than available from chemical fertilisers. Among organic fertilisers, effluent from biogas plant provides enriched fertiliser.
Very little of NPK in plant slurry is lost during anaerobic fermentation and whatever little occurs is in the form of gaseous ammonia. It is possible to keep this loss minimum by using proper storage methods such as tanks or lagoons. As far as application methods are concerned, when sludge is injected from below rather than spread on soil surface, loss of nitrogen is found to be much less.
Generally 50 to 70 per cent material of plant slurry which are biodegradable are decomposed during anaerobic fermentation and only a small proportion of 10 to 20 per cent of carbon is converted to cellular matter. Thus problems arising from using sludge on soil are much smaller than those from using aerobically treated wastes.
Effluent from a biogas plant can be utilised in a number of ways. It can be utilised in liquid form which can be removed daily from outlet chamber or in the form of semi-dried sludge which settles at bottom and can be taken out when digester is periodically emptied. Liquid form of effluent can be taken in buckets and used for added productive growth of vegetable and fruit trees.
Liquid digested slurry can also be applied in sufficiently diluted forms which can be carried to fields after mixing with water. Biogas plant effluent is, however, normally used after composting. It is quite easy to compost digested slurry by filling it in usually 2 ft 8 in (80 cm) deep pits. Plant effluent is very effective composting material having necessary micro-organisms which cause it easy break-down into stable matter.
Composting is carried out in a particular way. Inside the pit, first a layer of crop residues comprising straw, leaves etc., is spread on which the first layer of effluent is poured. This is followed by another layer of crop residues and then by one more layer of effluent. Proceeding in this way alternate layers of crop-residues and effluent are built one over another till the composting pit is completely filled in.
Pits need to be in sufficient number of requisite size so that they can hold entire compost before being put to use. When effluent is obtained from dome type biogas plant (Janata plant), it is directly used without composting. However, even in this case the sludge obtained twice a year when digester is emptied, is preferred to be composted in the above manner.
Application of Plant Effluent as a Protein Source for Farm Animals:
Promotion of non-conventional energy sources has led to the installation of large number of biogas plants in India and many other developing countries. These plants not only produce biogas but also generate rich fertiliser which apart from use as fertiliser can also be used to supplement animal feed. This application is particularly well-suited for countries which are chronically short of animal feeds and fodders.
K.K. Saxena has carried out a number of studies at the Indian Veterinary Research Institute, Izatnagar concerning use of slurry as feed to ruminants in limited amounts. Plant slurry used in his experiments was produced from dung of concentrate-fed animals which contained 8-9 per cent crude protein (CP). It was shown that fermentative processes in the biogas plant enhance the protein content of dung from 8 per cent to about 15 per cent in the digested slurry.
Animals subsisting on poor quality fodders without any concentrate yield a dung of only about 5-6 per cent CP. K.K. Saxena showed that even low CP dung when digested in a biogas plant can be upgraded to a digested slurry of high protein content. For effective results this however required mixing of digested slurry with molasses, wheat straw, mineral mixture and salt.
Ration of this composition could be made easily available during scarcity conditions when concentrate feeds and high quality fodders are not available. Ability of animals under study during the short period of 21 days confirmed the feasibility of using such rations during periods of scarcity.
Biogas plant effluent can be used as valuable feed for fish, poultry and cattle. Balasubramanian and R. Kasturibai working at the School of Energy Sciences, Madurai Kamaraj University, and Madurai analysed the nutrient status of cattle dung before and after biomethanation.
According to their findings, ammonia content in effluent increased by 70 per cent following anaerobic fermentation. On account of these changes, effluent can prove valuable fertiliser for aquatic plants and feed for fish culture which tends to improve yield.
Studies at the Madurai Kamaraj University covered several varieties of fish culture namely Tilapia fish (Oreochromismossambicus), Rogu (Labeorohita), Mrigal (Cirrhina mrigala) and the Common Carp (Cyprinus Caris). These fish were grown in biogas plant effluent without any supplementary feed thereby avoiding the cost of additional feed and fertiliser. Fish weight was found to increase five to seven times in this effluent than through conventional production with recommended doses of chemical nutrients.
Selection of Pit Size for Composting:
Volume of pit is equal to product of volume of animal wastes added per day and average period for which it remains in pit before being removed. Area of the pit can be determined by dividing the pit volume by average height of the pit.
Assuming depth of pit to be 80 cm, and if compost pit is emptied every 7 month and biogas plant is fed with 120 kg (120 litres appx.) of animal wastes per day, area of composting pit can be calculated in the following manner-
