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Duckweed Technology

Background

The disposal of untreated sullage water in ponds in rural areas is causing serious health hazards, as there in no proper drainage system. The waste water discharged to nearby ponds foul the environment and results in unsanitary conditions, thereby jeopardizing the community health. The available technologies for the treatment of waste water involve high capital as well as operational and maintenance costs and also offer no economic returns, hence, there is an urgent need for alternate technological options.

The duckweed based waste water treatment system in conjunction with pisciculture is one such technology has the potential of offering effective waste water treatment besides providing economic returns as well as generating employment opportunities in the rural areas.

The duckweed is a small free floating aquatic plant often seen growing in thick mats on nutrient rich fresh and brackish waters in tropical and semi tropical climate all over the world. It belongs to botanical family Lamnaceae consisting of four genera namely; Lemna, Spirodela, Wolffia and Wolffiella, of which first three genera are commonly found in india. The duckweed has the ability to bio accumulate up to 99% of the nutrients, dissolved solids and even heavy and toxic elements of waste as plants are harvested. Hence, it reduces Biochemical Oxygen Demand (BOD), Checmical Oxygen Demand (COD), suspended solids, bacterial and helminthic pathogens, some organic compounds, ions of potassium, nitrogen, phosphate and even heavy metals of waste water to a level quite safe for disposal.

Further, the duckweed plants can double their mass in about two to three days under ideal conditions of nutrient availability, sunlight and temperature. The average daily yield of fresh duckweed from a pond of one hectare is about 0.5-1.5 tons. It is highly nutritious feed due to high contentof proteins, fiber content and vitamins A & C, The prolific growth of duckweed can be harvested daily from the duckweed pond and can be utilized as fish feed. Hence, not only the duckweed can serve as complete feed for fish. Also interestingly, the duckweed species grow faster in Warm weather, when fish need more feed and more slowly in cold weather when the fish also do not require as much feed.

Methodology

  • To start a duckweed based sullage treatment unit in village pond, it is desirable to take up the duckweed project in villages where two separate ponds are available near by. The existing ponds are deepened, cut and dressed as per requirement. However, if the two separate ponds are not existing then a pond with a size of one hectare or more is partitioned into two equal parts (half hectare or more each) by construction of earthen bunds. Out of these two ponds, one is used for duckweed cultivation and other for pisculture. for the treatment of sullage it is important to give it proper tetention time (minimum 12-1 days) in the duckweed pond. Therefore, the depth of the duckweed pond from the invert level of imcoming drain is maintained to give the sullage the minimum required retention time for treatment.
  • The minimum viable size of pond for implementation of this technology is one hectare (to be partitioned into two equal halves) as quantity of sullage discharged by village having population of 4000-5000 can successfully meet the nutritional requirements of duckweed in half hectare pond and can thus be integrated with pisciculture.
  • Most of the village ponds are located in low lying area and in rainy season, the area around the pond gets flooded. for successful execution of the project, bunds of the ponds are raised so that the ponds does not get flooded during the rains. Alternatively, bypass arrangement on th einlet drains along with a shutter is provided to check the flow of excess water into the ponds.
  • The over flow pipes encased with wire mesh are embedded in the raised earthen bunds of fish pond to allow over flow of water in case of heavy rains. The proper stopes and compaction of the bunds is very important for the development of ponds. The minimum slope required for preparation of the bund is kept at 2:1 (2 horizontal and 1 vertical)
  • The sullage brought by the main drain of the village after passing through the inlet screen (Where all the suspended debris are removed) is allowed to enter in the duckweed pond. About 100kg of duckweed pond. A floating bamboo grid (details given below) is used to check the drifting of duckweed towards one side by wind in the duckweed pond.
  • For pisciculture, six varieties of fish viz. Catla, Rohu, Grass Carp, Common carp, Silver carp and Mrigal (suitable for flood water cultivation) cultivated in a separate fresh water village ponds. The sullage, after duckweed treatment is fit for pisciculture using the said varieties.
  • The treated sullage from the duckweed pond is allowed to flow to the pisciculture pond through culvert pipes encared with fine mesh as and when required and excess water if any is used for irrigation purposes.
  • The duckweed grown in the sullage pond is fed to fish through open bottom fish feeding stations which comprise 4m X 4m floating PVC grid. Four fish feeding stations are required in a fish pond of half hactare size.

Preparation of Bamboo Grid

The duckweed is a free floating weed, hence it is easily dispersed by wind currents. The duckweed should cover the entire water surface for effective growth and treatment of sullage. If it gets collected at one end, it may start decaying. It is, therefore, essential to keep the duckweed floating on the entire surface of water by providing grid of bamboos of large diameter (75mm) supported by vertical bamboo guides. As such 10 m X 10 m grids of 75mm diameter bamboo are ideal.

The preparation of bamboo grid is carried out by joining the lengths of bamboo by giving a overlapping of 2 feet at the point of joining and nailing at the knots so as to prevent the twisting of the bamboos. The overlapped protion of the bamboos is tighthly joined with jute rope.

Preparation of Bamboo Grid

  1. The bamboos should be straight and of proper dia meter to avoid toppling of duckweed.
  2. The bamboos should not be cracked otherwise it will sink in water
  3. The joining of the bamboos should be carried out in a way (all bamboos should be joined at the same level) so that whole of bamboo grid floats on the water surface
  4. The bamboo should be nailed only at the point of knots to avoid crack in the bamboos otherwise it will sink in water
  5. The bamboo lengths should be joined by giving two feet overlapping and firmly tied by jute rope so as to avoid twisting of the bamboos
  6. At the points where bamboos cross, they should be cut and joined at the same level by a metal strip followed by joining them firmly with jute rope
  7. The verical bamboo grids used to support the bamboo grid should be held firmlu by embedding bamboo at least 1.25' in the bottom surface of duckweed pond.

DuckWeed Cultivation

Containment and wind buffering

Duckweed containment is essential to prevent disperal by water or wind current and to ensure the success cultivation of duckweed. Therefore, for duckweed cultivation, channel shape and ponds i.e. with narrow width are preferred.

Duckweed containment is a function of the three basic factors, wind diffusion, pond size and floating barrier grid size. The larger the pond and the greater the average wind speed, the smaller should be the grid size. The duckweed should cover the entire water surface for effective growth and threatment. this is achieved with the help of bamboos grid as mentioned above. In place of bamboo, the PVC or polyethylene pipes can also be used, however, besides being costly, when wind blows, these pipes bend, crack and finally sink in the pond.

Fast growing plants like banana, papaya should be planted all round the banks of duckweed at one end of the pond. Plantation may also be done all round the banks of fish pond. This would generate some income to the Panchayat.

Seeding duckweed

Seed stock should be taken from the available native species of duckweed growing near the project village or in the same region. These species will be well adapted to the local climate and water. Frequently, two or more duckweed species will be found growing together in wild colonies. Polyculture increases the range of environmental conditions within which the duckweed will grow.

The collected duckweed seed stock should be put into acclimatization ponds. The newly seeded crop may require a week or more to recover from the shock of handling and may grow slowly durind this period. After about a week, it starts growing, it should then be transferred to duckweed pond and about 1/8th protion of duckweed pond should be filled intially.

Nutrients for duckweed growth

The most economical sources of nutrients for duckweed growth are waste water effluents from homes or livestock feed lots. Solid materials such as manure from livestock, night soil or food processing wastes can also be mixed with water and added to duckweed pond.

In the duckweed ponds set up in Punjab, in addition to kitchen and bathroom waste water, cattle waste is also coming to the duckweed pond, as cattle are tied at the banks of the pond. The quantity of waste water coming to the pond should be sufficient to supply requisite nutrients to duckweed. However, if the qrowth of duckwwed is not satisfactory, especially during rainy season when the sullage will be diluted, other fertilizers like urea, triple super phosphate (TSP) and muriated potash may be added to obtain better growth of duckweed. These fertilizers dissolved in water and kept for 20 to 24 should be used in the ratio of 20:4:4 kg/hectare/day. 10 kg. Urea, 2 kg Triple super phosphate and 2 kg Muriated potash can be used at a time in 20,000 m3 of pond water, whenever required. It has been experienced that when 100 kg decayed cow dung in mixed with the fertilizers in the above meentioned proportion (after keeping the mixture for 20-24 hours) and applied twice a week per hectare of pond area, better results are obtained.

Unicellualr algea are the primary competitors of duckweed for nutrients and are among the few plants that will grow faster. One of the essential crop management techniques is to maintain a sufficiently dense duckweed crop cover to suppress algae by cutting off light penetration into the water column. Algae especially Blue Green Algaw dominance will result in a swing toward high pH and production of free ammonia, which is toxic to duckweed. species of microscopic algae also reduce duckweed growth by inhibiting nutrient uptake.

On the other hand, a dense crop cover of duckweed reduces dissolved oxygen in the water column and increase in number of anaerobic bacteria that enhance the denitrification process and swings the nitrogen balance further in favour of ammonium ions over nitrate. This tends to lower pH as ammonium ions are assimilated by duckweed. The ability to form a mat over the surface of the water is one of the competitive advantages of duckweed.

Harvesting of duckweed

The standing crop density or the weight of fresh palnt biomass per sqm. determines the amount and timing of harvests.

Measurement of standing crop density is done with a calibrated, fine mesh screen of 0.25 m2 that is used to lift a section of the duckweed mat growing in the pond. The procedure is to gently slide the screen beneath the surface and pick up duckweed above the screen. It is shaken gently to drain excess water, the fresh plants collected and the screen. It is shaken gently to drain excess water, the fresh plants collected and the screen are weighted. The standing crop density per square meter can then be estimated at four times the weight recorded.

Daily harvesting of the incremental growth of the duckweed-averaging approximately 100g/m2/day is suggested, not only to achieve the best production rate, but also to maintain a healthy standing crop. Fresh duckweed is used for feeding fish. A floating platform or a tyre tube filled with air can be used by the personnel to sit and harvest duckweed in the center of pond.

Improtance of pH in duckweed pond

  • Duckweed seedlings may be collected from nearby pond and kept for sometime in small ponds at site containing proposed duckweed pond water for acclimatization before introducing in the pond. Where space is not available for construction of acclimatization ponds, it may directly be inoculated first in small area of duckweed pond and after about 8 to 10 days, when good growth is observed, it may be transferred to ramaining area of the pond gradually.
  • On observing stunted growth of duckweed or in case of non-availability of requisite nuitrients in the duckweed pond water, nutrients such as decayed cow dung (@ 100kg/ha/day) and or chemical fertilizers like urea, potashand triple super phosphat (@ 15kg/ha/day, 10kg/ha/day and 10 kg/ha/day respectively) should be provided.
  • Ambient temperature between 10o and 35o is considered to be appropriate for duckweed growth. Sudden change in temperature is likely to effect the growth of duckweed. In winter, Lamena species of duckweed should be grown in duckweed pond.
  • Desludging of duckweed pond should be done, whenever required.
  • Removal of floating scum and floating macrophytes is required.
  • Remvoval of floating scum and floating macrophytes is required especially to prevent fly and mosquito bredding. In fish pond stocking of Gumbusia fish @ 2000 nos./ha is also suggested to avoid breeding of mosquitoes.
  • Periodic harvesting of duckweed should be done.
  • Only that much quantity of duckweed should be transferred to fish feeding station as would be consumed by fish. Duckweed be given to fish between 7 a.m. to 5 p.m. After sunset feeding should be stopped. Duckweed can survive across a pH range from five to nine, but grows best in the 6.5 to 7.5 range. This is because when the pH is below 7.0 ammonia can be kept in its ionized state as ammonium ion, which is the preferred form of nitrogen for the plants. An alkaline pH shifts the ammonium-ammonia balance toward the unionized state the results in the liberation of free ammonia gas, which is toxic to duckweed.

Pisciculture

Liming in fish pond

Liming should be done in the pond before filling water in the fish pond. 100 kg per hectare loquid lime is spread throrughly over the entire pond surface. Subsequently, water is filled in the pond after a week.

Stocking of fingerlings in fish pond

Carp species are the most widely cultivated family of fresh water fish. Polyculture increases the efficiency of carp production by maintaining top-feeding, mid-feeding and bottom-feeding carp species in the same pond to extend productivity through out the three zones. Carp polyculture is designed to make maimum use of available oxygen and nutrients. Efficient use of oxygen is the key to maximum carp production.

It has been observed that grass carp is the primary consumer of duckweed in polyculture. however, both catla and common carp also complete aggressively for available duckweed feed and consume it directly. these top-feeders directly absorb about 50% of duckweed nutrients in their digestive system. Their faeces contain the balance of the original duckweed nutrients and furnish a relatively high quality detritus consumed directly by detritus feeders or indirectly through plankton and other organisms which also feed on this detutus and in turn all consumed by bottom carp varieties. Thus duckweed is a complete dietfor top, mid and bottom feeders. Mrigal is a bottom-deeder and is tolerant of the lowe oxygen levels at the bottom. They help in keeping the pond bottom clean. Howeever, although growth of Mrigal is poor in comparison to other varieties, yet these are stocked in relatively high numbers so that fecal matter from top feeders could be consumed before contributing to pond B.O.D.

Rohu and silver carp are two phytoplankton feeding species used in the duckweed fed poluculture. Carp fry and fingerlings feed on Zooplankton but they start eating Wolffia as soon as their mouths are big enough.

Before introducing fingerlings in the pond, it should be ensured that there are no cat fish or any other variety of carnivorous fish in the pond as they feed on vultivated fish varierties.

The fingerlings of above said varieties of fish should be brought from nearest fisheries station or any ohter source and should be stocked in fishpond in order that mortality is low, it is preferable to stock 3 to 5 cm long fry instead of fingerlings. The quantity needed per hectare of pond area is as follows.

Grass carp (Ctenopharyngodon idella)   6,000 Nos.
Silver carp ( Hypophthalmichthys molitrix)   6,000 Nos.
Common carp (Cyprinus carpio)   3,000 Nos.
Rohu (labeo rohita)   4,500 Nos.
Catla (Catla catla)   4,500 Nos.
Mrigal (Cirrhinus mrigala)   6,000 Nos.
Total    30,000 Nos.

In ponds, which have mewly constructed earthen bund, stocking of common carp is not suggested, as it damages the earthen bund. In that case, proportion of fish stocked should be as under:

Grass carp   7,500 Nos.
Silver carp   6,000 Nos.
Rohu   4,500 Nos.
Catla   6,000 Nos.
Mrigal   6,000 Nos.
Total    30,000 Nos.

Fish feeding stations

A fish pond of one hectare size is provided with six floating fish feeding stations (3m X 3m) that are open bottom enclosures made of PVC pipes joined with elbows. The feeding stations are required to be provided for rabit making in fish to feed on duckweed. Freshly harvested duckweed is brough in baskets to the pond and distributed evenly among "feeding stations". The later also prevent duckweed from dispresing over the pond surface also accumulation at one corner.

Duckweed as feed

Duckweed is a complete meal for the fish and is the main source of nutrition. The fish can be fed with as much of duckweed as they eat. Fish feed consumption varies with fish size and water temperature. Carp may eat little during the coldest months but in summer, it can daily eat as much as its own weight. Uneaten or poorly digested feed results not only in low productivity but also contributes to oxygen depletion. Any uneaten duckweed will be visible floating in the feeding station. The quantity to be fed should be adjusted accordingly. Fish should be fed duckweed throughtout the day. Serveral light feedings (less quantity) during the day are preferable as compared to two ot three heavy feedings.

In case duckweed production is unable to meet the feed requirement of fish, it be supplemented with conventional feeds like rice bran, oil cake etc.

Although dcukweed does not allow mosquito breeding in the pond, yet Gumbusia affinis fish, which feeds on mosquito larvae, should be stocked in the ponds to prevent mosquito breeding.

Drag-netting

To check health and growth of fish, drag netting should be done every month after 3 months of introduction of fingerlings. For this, water mixed with potassium permanganate is sprinkled on the fish caught in the net @ 5 ppm and this lot of fish is released back to the water to allow potassium permengnate to spread throughout the fish pond.

Harvesting of fish

Harvesting of fish should begin about 6 months after the pond is stocked weekly harvesting is the preferable pattern. Largest fish and the smallest fish should be harvested. The rationale is the assumption that the largest fish will have a declining growth rate and that the small fish are simply poort performers. In harvesting, care should be taken to maintain the desired ratios of species in the pond.

In order to achieve high production, the fish harvested should be replenished by stocking fry of each variety in the same proportion and number after each harvesting. In case fry are not availables, harvesting should be done June/July and fingerlings/fry should be stocked in July/August.

Importance of oxygen in fish pond

Dissolved oxygen not only supports the fish and their food but it also supports the denaturing of toxins such as ammonia, which can limit productivity. Even brief periods of anoxia can be disastrous to the fish crop in a pond. Hence monitoring is dissolved oxugen level in pond is very important. The facility to get it tested is not available in the rural areas. Hence periodic testing be got done whenever need is felt. However, monitoring of pond oxygen can roughtly be performed bu observing the activities and behaviour of fish. Fish with adequate oxygen exhibit considerable vigor during eating duckweed, drag netting and harvesting. When oxygen level falls below 4 mg/l it can be increased by aeration, adding fresh water in the pond, reducing the feed inputs or as a last reort by reducing the stock of fish.

Operation and Maintenance

The major operational concern with duckweed project is essentially to ensure good and healthy production of duckweed and fish. The salient points on which the success of the suytem depends upon:

  • The pond should be kept cleared off all algal and vegetation growth necessary dressing of sides of the pond should be done to keep them to proper shape.
  • Algae may interfere in the growth of duckweed. Necessary precautions should be taken so that algae does not interfere with the functioning of the duckweed pond. Blue green algae like Spirulina increases the pH of pond water. Complete duckweed cover will not allow algae to grow. In case algae devolp, it should be removed manually by cloth utilized as poultry or cattle feed. Composting of harvested duckweed plants is also feasible.
  • Before stocking of fish in ponds, the exsisting carniorous fish should be taken out after dewatering of ponds. Liming @ 100 kg should be done. After 20 days warer should be filled in the pond.
  • Five varieties of fish seed viz. Grass carp, Silver carp, Catla, Rohu and Mrigal may be stocked @ 30,000 nos. of seed per hectare in the ratio of 2.5:2:1.5:2:2.
  • pH and dissolve oxygen(D.O.) of fish pond water should be checked at least twice a month. pH of the fish pond should be in the range of 6-8. pH of the pond mainly increases due to appearance of blue green algae. On increase of pH, addition of fresh water in the pond and removal of algae manually by cloth should be done. D.O. should be more than 4 ppm.
  • Regular netting should be done and potassium permegnate @ 5 ppm be used as disinfectant for maintaining food health of fish.
  • Every third year, fish pond may be dewatered followed by raking, liming warer filling and stocking of fresh fingerlings.
  • To enhance fish yield. as experienced in Bangladesh, mixed feed system using duckweed and conventional feed, may be adopted. Fish yield may increase to 15 to 18 tons/ha/year.
  • Strict vigil should be kept in a duckweed project associated with pisciculture because there are great chances of theft and pilferage of fish.

For the success of such projects in rural areas, community involvement is very neccessary. the village Panchayat should actively participate in the implementation of the project. However, the Panchayat would not be able to operate and maintain the system properly unless necessary infrastructure is created. There is risk of theft and pilferage of fish. Day and night chowkidars will have to be kept for watch and ward and lighting arrangement may also be necessary to keep vigil in the night. Creation of infrastructure just for this work may not be economical; the expenditure incurred will not commensurate with the revenue earned. Therefore, the Panchayat should give the operation and maintenance of the system to a suitable person on contract after inviting bids as per existing practice of giving village panchayat ponds on lease for pisciculture. The contractor (Fish farmer) will be interested to see that the sustem functions well so that he may earn maximum possible income. Fish farmer's family will be the most cost-effective source of labour and can be gainfully employed year round. Hired labour would be neccessary during times such as fish harversting, pond cleaning etc. By giving ponds on lease, the Panchayat would earn revenue from the project and proper operation and maintenance will also be ensured.

Water analysis

For duckweed production and pisciculture, water analysis for the following parameters is required to be done. To keep down the cost, testing kit should be purchased and local person be trained to carry out the tests at site wherever possible.
Parameters for testing Testing frequency

Duckweed pond

pH, Phosphate, Postassium, Ammonical Nitrogen, Total Kjeldahl Nitrogen, BOD

pH, twise a month (This test can be done at site). Ohter parameters whenever found necessary (twice a year). These test be got done in the laboratory of state Pollution Control Board.

Fish pond

pH, Dissolved Oxygen

Twice a month (This test can be done at site)

Conclusion

The duckweed, that is daily harvested from the half hectare duckweed pond, being highly nutritious can be utilized to feed the fish cultivated in half hectare pisciculture pond, thus saving the costs involved in purchase of fish feed. The income generation from pisciculture pond will motivate the beneficiary to adopt the technology for waste water treatment. Besides effective sullage water treatment, the project will also generate self employment as the ponds can be given on lease by the Panchayats to the local residents of the village.


Cost Estimates** for Implementation of Duckweed Technology

Sr. No. Particulars Amount (in INR)
1 Dewatering   10,000.00
2 Desilting of pond   30,000.00
3 Deepending of Pond   30,000.00
4 Repairing of demanged earthen bunds   20,000.00
5 Raising of side bunds   30,000.00
6 Preparation of dividing bund   80,000.00
7 Work involved for diversion of drains at one point   80,000.00
8 Sedimentation Tank   20,000.00
9 R.C.C pipes for culverts   10,000.00
10 Bomboo Grid   30,000.00
11 Plantation of banana & papaya trees among the periphery     5,000.00
12 Miscellaneous (Fish feeding stations/Dragnet/Chamical analysis kit)   10,000.00
Total 3,55,000.00
Contingencies @ 3%   10,650.00
Sub Total 3,65,650.00
Say   3.7 Lakhs

**The cost estimates given here are tentative and has been calculated assuming pond size of one hectare. The actual cost will depend upon the distance from which the soil is to be brought for construction of bunds and other site conditions.

Operation and Maintanence Cost Per Annum

Sr. No. Item Budget (in INR)
1 Purchase of Fingerlings     4,000.00
2 Casual labour one person @2000/month   24,000.00
3 Conventional feed for fish     2,000.00
4 Miscellaneous     5,000.00
Sub Total   35,000.00

Economic Return

The pond area proposed for pisciculture is 0.5 hectare, the sale of fish is expected to fetch about Rs. 0.75 lakhs (Considering the annual production of fish as 4.5 tons per hectare and the sale price of fish to be taken as Rs. 30 per kg). Therefore, the net annual profit will be to the tune of Rs. 0.40 lakhs.

The panchayat may give the opeeration and maintenance of the system to a suitable person on contract. The contractor will be interested to see that the system functions well so that he may earn maximum possible income. Some income will be generated through the sale of banana and papaya trees.

 

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