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The following article will guide you about how to generate biogas from waterweeds.
Introduction:
Waterweeds provide widely available feedstock for biogas generation. Water-hyacinth and algae are two major categories of waterweeds. A number of experimental studies have been carried out in past both in India and abroad to estimate potential, and develop the related technology. Water-hyacinth regarded as neglected weed for many years is at present being increasingly utilised.
It is nowadays being used as a cleaning agent to purify industrial effluents and municipal refuse, as a biomass source to produce paper and board, and as an energy-resource to produce biofuels. Water-hyacinth is one of the fastest growing plants which grows on clean as well as polluted waters.
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It is a warm-weather plant which flourishes in tropical and sub-tropical regions alike. It grows in deltas, swamps and irrigation canals which often results in overrunning and clogging of waterways. If not frequently removed and put to use, water-hyacinth poses difficulties in shipping, navigation and hydropower generation besides offering breeding ground for insects.
Based on studies carried out in the USA and India and other parts of the world, seaweeds (algae) have emerged as valuable input material to generate biogas. Algae is one of the simplest and primitive waterweeds having no true roots, stems, and leaves, flowers and seeds. These are essentially photosynthetic plants which can be both aquatic and sub-aerial. Aquatic algae grows in water of low salinity whereas sub-aerial grows while being exposed to air.
Algae is also known by other names such as pond scum, water-moss or simply as seaweed. Algae is believed to be known to mankind as a useful resource since around 3000 BC. Industrial use of seaweeds began during the seventeenth century in Western Europe when they were used to produce soda and potash from ash of burnt brown seaweeds (kelp). Ash was also known to be used for producing glass and soap.
An Overview of Biogas Generation from Water-Hyacinth:
Water-hyacinth regarded as neglected weed for many years is a source of biogas and other products. Water-hyacinth (Eichhornia crassipes) consists of a fleshy vertical stem called rhizome from which roots, leaves and flowers sprout. The rhizome floats just below the water surface protected by shields of folded leaves. Being a warm weather plant, except during the period when water is in frozen state, temperature of around 10°C supports its growth.
Anaerobic fermentation of water-hyacinth generates adequate biogas yield. One metric ton of dry water-hyacinth following anaerobic digestion can generate 26,500 ft3 of biogas with a heat value of 600 BTU/ft3. There are, however, no fixed norms available about the rate of gas generation per unit weight of water-hyacinth fed as composition of feedstock varies from plant to plant.
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A hectare of water surface yields about 50 tonnes of water-hyacinth which is currently growing over several million hectares of water surface in India. Except for Gujarat, Punjab and Kashmir, it is abundantly available in several parts of the country particularly in water logged areas of West Bengal, Assam, Kerala and Andhra Pradesh. Growth of water-hyacinth in India is believed to have enormously increased from 1905 onwards.
In 1945, it covered 1.5 million hectares of water surface in West Bengal and was occasionally described as a ‘Bengal Terror’. Out of 1,23,000 hectares of water area in Assam 1,11,000 hectares are estimated to be infested with water-hyacinth.
About 10,360 hectares in Manipur and 3,850 hectares in Andhra Pradesh are infested with this weed. It is widely available in wastelands and neglected ponds in Kerala and most tank and rice fields in Madhya Pradesh. Fourteen out of 26 districts in Rajasthan find water-hyacinth as a growing menace.
Composition and Characteristics of Water-Hyacinth:
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A typical water-hyacinth plant contains 24.8 per cent root, 41.9 per cent stalk and 33.3 per cent leaf. Weights and proportions of water-hyacinth however vary from plant to plant and from region to region.
According to the results of a study carried at the Central Food Technological Research Institute, Mysore, fresh hyacinth contains 95.5 per cent water, 3.5 per cent organic matter, 0.04 per cent nitrogen, 1 per cent ash, 0.06 per cent phosphorus (P2O5) and 0.2 per cent potassium (K2O).
Dried plant normally contains 75.8 per cent organic matter, 1.5 per cent nitrogen and 24.2 per cent ash. This ash in turn contains 28.7 per cent K2O, 1.8 per cent Na2O, 12.8 per cent CaO, 21 per cent chlorine and 7 per cent P2O5. Water hyacinth contains 11-12 per cent crude protein, 15-18 per cent fibre, and 16-20 per cent ash.
As per the studies carried out at the National Technology Laboratory (NSTL) in Mississippi (USA), growth of plant in warm enriched domestic sewage generates about 18 tonnes of wet biomass per hectare per day. These plants grow at a rate of 215 tonnes of dry plant material per hectare. Water hyacinth is also appropriate in treating effluents mixed with toxic metals.
The NSTL studies revealed that these plants rapidly absorb gold, silver, cobalt, strontium, cadmium, nickel, lead and mercury. They also absorb or metabolise phenols and other organic substances which are often present in water supply systems of large cities. Results of experiments carried out at the NSTL to evaluate the effectiveness of water-hyacinth in purifying the canal waters are given in Table 8.2.
Results of studies carried out by the NSTL to determine the pollution-abatement capabilities of effluent by water-hyacinth grown in domestic sewage lagoons are given in Table 8.3. It was found that half hectare lagoon filled with water-hyacinth can purify daily night-soil of 1000 heads to acceptable levels with a minimum sewage retention time of two weeks. Water-hyacinth can reduce pollution levels by 75 to 80 per cent.
As per the NSTL studies, a hectare of water-hyacinth fed on sewage nutrients can yield 0.9-1.8 tonnes of dry plant material per day. This biomass can produce 220-440 m3 of biogas having heat value of 7-14 million BTU. In addition, the sludge that remains after fermentation is a rich fertiliser, following its retaining most of nitrogen, phosphorus and other mineral contents.
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The Regional Research Laboratory at Jorhat in Assam analysed composition and characteristics of water-hyacinth which were obtained by following standard procedures prescribed by the US Technical Association of Pulp and Paper Industry.
H. Siregar, et al., reported that water-hyacinth when fed in batch system yielded more gas than semi-continuous system. Furthermore, with the batch system, the gas yield was maximum when water-hyacinth was submerged to a depth of one-third of the digester height. In contrast in continuous system the highest gas yield was obtained when water-hyacinth was submerged to two-thirds of the digester height.
Studies on Biogas Yield from Water-Hyacinth at CMERI (Durgapur):
According to studies carried out at the Central Mechanical Engineering Research Institute in Durgapur, 1 kg of dry water-hyacinth can produce 4 to 6 litres of gas following fermentation at temperature between 30-35 °C. About 10 cm long chopped stem and evenly crushed material with moisture-content ranging between 50 to 60 per cent is appropriate for biodegradation.
Chopping or crushing of lower parts of stems is helpful in quick drying to the desired level of moisture-content. Gas yield is maximum after 13 to 15 days of anaerobic fermentation during summers and after 18 to 20 days during winters. Maximum yield occurs from 15th to 20th day of fermentation. For 3000 litre capacity biogas plant installed at the CMERI, 18 quintals of coarsely chopped semi-dried water-hyacinth is required to be fed.
CMERI’s laboratory-scale units consisted of 200 litres capacity empty diesel drums which were able to give a yield of 50 litres of gas per day during summers when temperature ranged between 31 to 35°C. In the full-sized plant for obtaining the yield of 3000 litres of gas per day, 40-45 kg of semi-dried and coarsely chopped water-hyacinth was needed to be fed daily from 20th day onward after initial charging.
As for the availability of enriched fertiliser from water-hyacinth fed plants, the CMERI’s 3000 litre capacity plant produced 5 tonnes of compost on dry basis having 2.05 per cent nitrogen, 2.5 per cent K2O and 1.1 per cent P2O5.
In other words 1 tonne of dry compost should contain 20.5 kg of nitrogen equivalent to about 105 kg of ammonium sulphate, 25 kg of K2O equivalent to about 50 kg of muriate of potash, and 11 kg P2O5 equivalent to about 62 kg of super-phosphate. This resulting compost is also found to be a superior soil-conditioner due to its having fibrous characteristics and moisture retention property.
Power Generation from Water-hyacinth Based Plant:
K.V. Gopalakrishnan and B.S. Murthy carried out studies at IIT, Madras for analysing the economics of power generation from water-hyacinth based biogas plants. Efficiency of the system can be improved by recovering part of heat generated during the process, and gainfully utilising it for warming up digester-contents for obtaining higher gas yield.
Complete plant economics worked out as follows:
In other words this is equivalent to saying that one hectare of water-hyacinth plantation can generate 400 kwh/day and support a refrigeration capacity of 2.55 tons. The proposed system is shown in Fig. 8.1. Water-hyacinth can be grown in ponds, tanks or natural lakes.
The inputs to the growth area for water-hyacinth cultivation would include raw sewage and carbon dioxide derived from biogas plant digesters of the system or the power plants used in the system. Water hyacinth is regularly harvested by a suitable mechanical means, slurred and fed to biogas digesters.
Findings of a JNTU (Hyderabad) Study:
A pioneering study with water-hyacinth as feed was carried by N. Sreeramulu and B.N. Bhargava at the Jawaharlal Nehru Technological University which was sponsored by the Department of Science and Technology.
Water hyacinth used in this study was collected from Hussain Sagar, an open lake situated between the twin cities of Secunderabad and Hyderabad in Andhra Pradesh. Leaves used in the experiments were first cut to 2 to 3 cm long pieces for feeding into the digester.
In the beginning leaves were kept under mud slurry in a container for a period of three months which provided necessary anaerobic conditions for fermentation to start. Initially about 60 g of starter-seed thus prepared was blended with 1000 ml of distilled water.
The sediment was allowed to settle for 30 minutes and the supernatant was then decanted into digesters. Digester was sealed to the atmosphere with a single-holed rubber stopper fitted with a glass tube. Digester was connected with a rubber tube to another sealed container fitted with double-holed rubber stopper.
The container was filled with water which was acidified with sulphuric acid. Digester was covered with aluminium foil to prevent exposure to light. Additional feedstock was added to digesters only when earlier feed practically ceased to yield further gas. Displacement of water in the second container by resulting biogas generated in first one provided a convenient method for monitoring gas yield.
Biogas yield was analysed in wet and partially dried conditions by carrying out batch type experiments at ambient temperature (26 ± 4)°C. It was found that green leaves caused higher yield as compared to dry or partially dried leaves. With green leaves, the yield varied from 51 to 64 litres per kg of feed whereas in case of partially dried leaves it varied from 47 to 58 litres.
Higher temperatures caused more significant increase in the yield in case of semi-dried leaves than wet or fully green leaves. In contrast as per the NSTL studies, biogas yield was merely 11 to 18 litres per kg of green hyacinth plant. Comparative higher yield reported vide JNTU studies can be attributed to much higher ambient temperatures prevalent at Hyderabad than Mississippi.
If we compare the gas yield from water-hyacinth and cowdung which are 50 to 64 litres per kg and 37 litres per kg respectively, we find it is much more in case of former. Complete results of these studies are summarised in Tables 8.6 to 8.9.
Volatile-solid content of water-hyacinth which is a main contributing factor for gas yield compares very closely with that of cowdung and night-soil. Heat value of gas obtained from water-hyacinth and cowdung are more or less same. Sludge obtained from water-hyacinth based digester compares well in richness with that from cowdung and night-soil based plants.
Findings of an RRL (Jorhat) Study:
B.G. Unni, et al., working at the Biochemistry Division of the Regional Research Laboratory in Jorhat carried out a study to analyse the kinetics of biogas yield from water-hyacinth digested in unstirred conditions. Agitation of water-hyacinth based slurry is particularly desirable to prevent stratification which if not checked creates problems in digestion.
Initially the fermentation was started with animal dung slurry which was subsequently mixed with increasing quantity of water-hyacinth in powdered form and finally with freshly chopped water-hyacinth. Laboratory scale digester consisted of 5 litre capacity digester bottle. Digestion was carried out for a period of 65 days with ambient temperature varying from 28° to 32°C and pH value from 6.5 to 6.9.
Experiments were carried out for varying retention periods and volatile solid concentration of influent. Total solids, volatile solids and volatile acids were measured using standardised procedure prescribed by the American Public Health Association vide its manual ‘Standard Methods for Examination of Water and Water Wastes, 1971.’
Cellulose, hemicellulose and lignin were analysed by following the method described by H.K. Goering and R.J. van Soest in their book ‘Forage Fibre Analysis, US Dept of Agriculture, Washington, D.C.’ Carbohydrates were analysed by acetone reaction and proteins by the Kjeldahl nitrogen estimation procedure.
Effect of influent VS concentration and retention period on per cent VS digestion is given in Table 8.11. Volatile solids digestion rises with increase in retention period. For a given retention period, digestion increases with increase in influent VS concentration.
Biogas yield corresponding to varying retention period and for different volatile solid concentrations in influent is given in Table 8.12. Gas yield was analysed to increase with increase in influent VS concentration and decrease in retention period. Proportion of methane in biogas was found to increase with rise in retention period. Gas productivity was analysed to be linearly related to the VS loading rate.
A Weed Harvester Developed by CIFT (Cochin):
Central Institute of Fisheries Technology, Cochin, developed a harvester which is manufactured by a Maharashtra firm, Sri Vardhman Industrial Engineering Pvt. Ltd., Kolhapur, Application of the machine makes weed removal faster.
It is particularly appropriate for harvesting floating weeds like water-hyacinth (Eichhornia sp.), salvinia (Salvinia sp.), water lattuce (Pitia sp.), submerged weeds like Hydrilla, Vallisneria and Naias and rooted weeds like loture (Nymphea) and lilies (Nelumbium).
The CIFT developed two kinds of machines, one meant for removal of only submerged weeds and the other for removal of both submerged and floating weeds. It is designed to clear about 1.5 hectares of area in 8-hours shift operation. The machine consists of a shallow draft and self-propelled barge made up of mild steel tanks. It has a cutter to chop the weeds as they pass through the cutter.
A releaser releases the weeds from the carrier rake which then fall into the hopper of a roller crusher wherein weeds are crushed to smaller sizes. Crushed weeds are kept in tilting type trays. Barge is driven by paddle wheels with independent clutch control which is also used for steering the barge. All machine controls are located at one place so that one person can handle them conveniently.
An Overview of Biogas Generation from Algae:
Algae is another major waterweed which can be utilised for biomethanation. Several studies have been carried out both in India and abroad on use of algae as feedstock. A study was carried out at the University of California. Slurry consisted of 5 per cent algae and 95 per cent water. Algae was cultivated in large shallow ponds and stirred occasionally.
Harvesting was done either by centrifuging or by adding aluminium sulphate which helped in settling it. Results of studies showed that composition and volume of gas produced per kg of digested algae was more or less the same as from sewage sludge. A period of 11 days was taken as digestion period. One acre pond with algae cultivation was estimated to generate 11 kW of electric power.
Growth of algae in water depends on many factors including salinity and alkalinity of water, concentration of dissolved oxygen and carbon dioxide, presence of pollutants and contaminants in air and water, ambient and water temperature, wind velocity and intensity of light penetration in water. Several species of algae form chains or filaments that aggregate to generate colonies. However, the common species known as phytoplankton grow as single cells that are suspended in water.
Common Algae Varieties:
There are several ways in which algae can be categorised. One common way is to classify them as micro or macro-algae depending upon size. Micro-algae comprises several microscopic cells which range in size from less than 2 µm to 1 mm. Some microalgae varieties which have high protein content provide valuable food ingredients for both livestock and human-beings.
Mass cultivation of micro-algae as a food ingredient is commercially practised in Mexico and several Far East countries, chlorella and spirulina are the species which are commonly grown for this purpose. Spirulina has about 74 per cent protein, 14 per cent insoluble acids and 8.5 per cent carbohydrates. Algae can also be classified into five groups on the basis of their pigment content and certain biochemical characteristics.
These groups are blue green algae (myxophyceae), green algae (chlorephyceae), red algae (rhodophyceae), brown algae (phacophyceae) and golden algae. Aquatic weeds have higher moisture which sometimes can be as high as 90-95 per cent. Submerged plants have higher proportions of crude-protein emergent plants.
Aquatic algae found in suspension are called planktonic and those living on bottom as benthic. Planktonic algae generally consists of flora, fauna, and fungi of organisms in suspension.
Benthic algae on the other hand comprise attached and bottom-dwelling organisms and are known by different names depending on the place of location. Algae living on stones are called epilithic, those attached to mud or sand are called epipetic, those living on plants and vegetation are called epiphytic, and those attached to animals are called epizoic.
Harvesting of micro-algae is somewhat difficult. Single-cell species offer more difficulty in harvesting than multi-cell ones. It is more convenient to cultivate and harvest filamentous blue-green algae like Spirulina, Ocillatoria or colonial green algae than single cell species.
Planktonic algae which are found in suspension can be harvested by drawing a plankton net through water. This net is usually made of silk of fine mesh of about 180 pores per square inch. Silk net acts as a strainer which helps in filtration and accumulation of planktonic algae and other fine species.
Estimates of Region-Wise Algae Availability in India:
Algae is abundantly available in different parts of India. Y.A. Doshi and A.V Rao made estimates of seaweeds (algae) availability in different coastal areas of India. Although coastline of India extends over 5700 km, seaweeds are mostly limited to narrow littoral and sub-littoral belts of country’s marine environment.
Seaweed resources are limited to rocky coral formations in Gujarat, Andhra and Tamil Nadu states, in Karwar, Ratnagiri, Varkala, Visakhapatnam, Chilka and Pullicat lakes; and islands of Laccadives, Andaman and Nicobar. Agarophytes and alginophytes are found in commercially viable quantities on east and west coasts.
Seaweeds can be used for producing variety of chemicals, minerals, vitamins, amino acids, and proteins. They contain a variety of polysaccharides which constitute an industrially important group of phycocolloids. This group forms 10-65 per cent of dry weight of harvested seaweeds. Among polysaccharides, agar-agar and carrageenan obtained from red algae, and algin from brown algae, have considerable economic importance.
Findings of a CSMCRI (Bhavnagar) Study:
S.J. Tarawadi and V.D. Chauhan carried out experimental studies at the Central Salt and Marine Chemicals Research Institute, Bhavnagar, for producing biogas from Sargassum tenessium, a brown seaweed that was collected from the coastal Gujarat and stored in dried conditions. Before drying, the collected brown algae was cleared off extraneous materials such as sand and salt by soaking in water. Measured characteristics of dry algal powder used in the experiments are summarised in Table 8.17.
Two strains obtainable by decomposition of Sargassum were identified which could cause about 28 per cent breakdown of seaweeds in about 45 days fermentation period. These strains were used to cause primary biodegradation of seaweed for yielding biogas. In a separate experiment, effect of adding bacterial strains to cow dung slurry on gas yield was also studied.
In a batch type laboratory digester, 250 g of cow dung was added to 1 kg of Sargassum used as a feed for anaerobic fermentation. With gas production having started from 13th day onwards, 49.3 litres of gas yield was obtained out of a total of 60.6 litres of gas generated. In the second experiment in which no bacterial strain was added, gas yield was 21.7 litres out of a total of 28.7 litres of gas produced with gas generation having started from the 21st day onwards.
Effect of variable feed rate on gas yield was also analysed. For this purpose two laboratory-scale digesters of 6 litre capacity with facilities for feeding, discharging and gas collection were set up. Varying feed rates of 1, 2 and 2.5 kg VS/m3 day were used to determine the optimal rates for biogas yield.
It was observed that a feed rate of 1 kg VS/m3 day caused maximum gas yield with gas formation having started from the seventh day onwards, and accordingly further investigations were carried out by varying other parameters but keeping feed rates fixed at this value. Effect of daily feed versus on alternate on gas yield was also analysed.
In case of daily feeding, a feedstock comprising 6 g algal powder, 850 g seaweed, 50 g ulva (green seaweed), 100 g cowdung and 100 ml of bacterial strain produced 3.7 litres of biogas per day. In case of alternate day feeding but keeping all other parameters same, resulting biogas yield was 3.58 litres per day.
Under experimental conditions, maximum yield of biogas was found in the range of 400-500 litres per kg of dry algal powder. Production of gas in semi-continuous mode was greater than in batch-operation. Semi-continuous digester operation resulted in biogas yield in range of 470-500 litres per kg of seaweed.
In batch-type digester at room temperature 26-31 °C, biogas yield was 49 litres per kg of seaweed. Carbon-nitrogen ratio was kept at 23 : 1 and pH was maintained in the range of 6.8-7.4. Comparative performance of seaweed digester and cow dung digester showed that biogas yield per kg of volatile matter feed was 0.083-0.36 m3/day for marine algae whereas for cow dung based digester it ranged between 0.09-0.2 m3/day.
Comparison of heat value of gas obtained from different feedstock showed that it was quite high in case of marine algae (4900 kcal/m3) being only marginally less than that of pure methane (5320 kcal/m3). Salinity of water more than 2 per cent if used in slurry preparation had detrimental effect on gas yield. Sludge obtained from seaweed digestion could be used as an enriched fertiliser and soil-conditioner.
Findings of an IARI (New Delhi) Study:
G.S. Venkataraman and B.D. Kaushik carried out studies to analyse the effect of adding algae in feed on gas yield from a biogas plant primarily running on cowdung slurry as feedstock.
Algae used in these experiments was collected from Pirana sewage oxidation ponds in Ahmedabad city which included several species like oscillatoria chalybea, euglena, scenedesmus, spirulina, and merismopedia in which however oscillatoria chalybea formed the largest proportion.
Laboratory-scale digesters used in the experiments consisted of a battery of 10 litre aspirator bottles each with slurry containing 4 kg of fresh cowdung and four litres of water. Experiments were carried out at ambient temperatures of 16-18°C, 20-22°C and 30-31°C.
The resulting biogas was collected over saturated brine solution and measured by liquid displacement method. For analysing the effect of algae, in one experiment, feed comprising cowdung slurry was supplemented with 3 per cent dried algae.
Results of experiments were analysed for two different variants, namely, with and without algal supplementation at different ambient temperatures which are graphically shown in Fig. 8.2. It was found that with increasing fermentation period, gas yield increased initially but slowed down later.
Addition of algal powder was reported to stimulate yield which was about 1.64 times more (3114 cm3/kg of feed) compared to the yield of 1900 cm3/kg obtained from cowdung slurry alone. Addition of algae powder however did not significantly affect composition and combustibility of gas produced. Addition of algal powder is thus an effective way of improving gas yield which in small amounts can be grown in sewage oxidation ponds.