Duckweed based bio-remediation of village ponds: An ecologically and economically viable integrated approach for rural development through aquaculture

M D Ansal*, A Dhawan and V I Kaur

Department of Aquaculture, College of Fisheries, Guru Angad Dev Veterinary and Animal Sciences University (GADVASU), Ludhiana-141004, Punjab, India
* ansalmd@gmail.com

Summary

Village pond is an integral part of rural India which is primarily constructed for harvesting rain water and bathing of domestic livestock. Through suitable scientific interventions these manmade water resources can be utilized for economic gains as well. But unfortunately, they are used only as dumping sites for disposing of human and animal waste which leads to pollution and ‘Eutrophication’ due to accumulation of excess nutrients (nitrates and phosphates). Although these ponds hold immense potential for producing high quality food through aquaculture for rural development in developing countries but due to poor management and deteriorating water quality, full aquaculture potential of these resources have not been realized so far. It is therefore, vital to reclaim and manage these water bodies to its optimum productivity status through some appropriate rural friendly scientific technologies.

Effective waste water treatments through ‘conventional methods’, which rely on heavy aeration, are expensive to install and operate. Hence, there is need to explore some ‘non-conventional’ methods which are not only economically viable and easy to operate but eco-friendly as well. For remediation of village ponds, the first step is to remove the excess nutrients dumped in it. For this purpose, plant based bio-remediation (phyto-remediation) technology is the most promising option. Any aquatic plant that is capable of recovering or extracting nutrients or pollutants and has a fast growth rate coupled with high nutritive value is an excellent candidate for bio-remediation of waste waters. Such plants grow very fast utilizing waste water nutrients and also yield cost effective protein rich plant biomass as a by-product.

Duckweeds hold immense potential for both nutrient recovery and utilization as fodder or feed for livestock including fish. Wastewater-duckweed-aquaculture is a perfect eco-friendly integrated package for converting the waste water nutrients into high quality fish protein and augmenting rural economy through generating employment opportunities and additional food security.

Key words: fish, non-conventional, nutrient recycling, nutrition, phyto-remediation, wastewater management

Introduction

Village ponds hold immense potential for producing high quality food through aquaculture for rural development in developing countries like India. Productivity of these manmade aquaculture resources is however, far below the actual potential due to their poor management. Punjab is one of the progressive states of India and as per Indian Census 2001, it has 12,278 inhabited villages (Statistical Abstracts of Punjab 2006). Each village has at least one village pond which is used for harvesting rain water, dumping sewage waste and bathing of livestock. Their importance in total fish production of the state can be assessed from the fact that out of the total 10,023 hectare (ha) area under fish farming, about 66.79% (6,695 ha) belongs to village ponds. But due to poor management and deteriorating water quality, full aquaculture potential of these resources has not been realized so far. At present, only 60% of the total village ponds in the state are being utilized for fish culture. Hence, there is still immense scope of utilizing these village ponds for additional fish production.

Village ponds are visited by the domestic cattle (mostly buffaloes) and also receive domestic waste from the village households (including sewage, kitchen waste and detergents), which not only pollutes the water affecting its productivity but also causes nuisance for the villagers due to foul smell and disease outbreak (Figure 1). Hence, it is vital to reclaim and manage these water bodies to its optimum productivity status through some suitable scientific interventions which are not only economically viable but easy enough to be adopted by the illiterate rural population as well.

Since village ponds are a rich source of nutrients (nitrates, phosphates and potassium), these could be utilized or recycled into some other suitable farming system which is capable of not only remediating the pond water but also converting the recovered nutrients into a much needed cost effective protein rich biomass as a by-product (Skillicorn et al 1993, Iqbal 1999).

Although in India domestic sewage-fed aquaculture is almost a century old technique but large-scale usage of sewage for fish culture began in the 1930s in its most thickly populated city Kolkata in West Bengal. It is a unique and the largest operational system in the world (Edwards 2005) to convert waste into consumable products. In view of waste water management, an urban scenario is different from that of a rural scenario. A well developed urban sanitary system carries the city sewage and waste water (including industrial effluent) to a common disposal site, whereas in rural areas waste water is disposed off in ponds constructed within the village itself due to poor sanitary system. Hence, the rural population is required to play a more active role in waste water management as compared to urban population. Although wastewater-fed aquaculture occurs in several countries in Asia, where it provides food, employment and income for millions of people, especially the poor,  the quality of fish raised on urban wastewater is a matter of concern because of the presence of industrial effluent contaminants. However rural waste water, being more organic based, is a more suitable water resource for producing consumer safe aquaculture products.

Effective waste water treatments through ‘conventional methods’, which rely on heavy aeration, are expensive to install and operate. Hence, there is need to develop some ‘non-conventional’ methods, which are not only inexpensive and easy to install but also easy to operate and maintain. In view of this, “Phyto-remediation” is the most suitable bio-remediation method for village ponds (ecologically as well as economically) which combines two novel approaches – “Pollution Prevention” and “Re-Use”.

Phyto-remediation 

Phyto-remediation is an eco-friendly bio-remediation process of removing pollutants/nutrients from an environment (soil, sediment, water) by using any green plant based system which is not only an energy saving but also a resource recovering system.

Suitable aquatic plants for bio-remediation of village ponds 

Any fast growing aquatic plant of high nutritive value is an excellent candidate for bio-remediation of waste water. Many surface floating aquatic plants like water hyacinth (Eichhornia) and duckweeds (Spirodela, Lemna, Wolffia) are well known for their phyto-remediation qualities (Wolverton 1981, Debusk and Reddy 1987, Akcin et al 1994, Sinha et al 1994, Vajpayee et al 1995, Chandra et al 1997, Zhu et al 1999, Willett 2005). They grow naturally in the village ponds and other nutrient rich standing waters in tropical and sub-tropical climates and are capable of extracting nutrients from these waters which otherwise go waste and cause ‘Eutrophication’. The economic potential of any plant species for waste water treatment not only depends largely on its efficiency to remove nutrients under a wide range of climatic conditions and on its growth rate, but also on the possible application of the harvested plant biomass. Although as compared to duckweeds, Eichhornia has a higher nutrient uptake capability (Wolverton 1976, Reddy and DeBusk 1985) but no economically attractive application of the generated plant biomass has been identified so far due to difficulties in removal of the huge plant biomass from the system. On dry matter (DM) basis, Eichhornia has a fairly good amount of protein content (10-25%) but its high fiber content (17-20%) further reduces its potential for being utilized as animal fodder or feed (Iqbal 1999). In contrast duckweeds, being tiny surface floating plants, are easy to harvest and have a higher protein (15-45% DM basis) and a lower fiber (7-14% DM basis) content. Duckweeds hold an immense potential for both nutrient recovery from village ponds and utilization as animal fodder or feed due to its fast growth rate, efficient nutrient extracting capability, easy harvesting, high nutritive value and good digestibility (Leng 1999). In USA, duckweed-covered lagoon for tertiary treatment of waste water has been classified as an innovative/alternative technology by the U.S. Environmental Protection Agency (EPA), having great application in rural development in developing countries (Iqbal 1999).

Duckweeds 

Duckweeds are small green plants belonging to family lemnaceae and they grow densely on the water surface forming a mat like cover. Taxonomically they belong to monocotyledons and have four generas- Lemna, Spirodela, Wolffia and Wolffiella (Figure 2).

Spirodela	Lemna	Wolffia	Wolffiella
Figure 2.  Different type of duckweeds

About 40 species are reported worldwide (Les et al 2002). Biomass of duckweeds gets doubled in 2-3 days (Iqbal 1999, Sillikorn et al 1993) under ideal conditions of nutrient availability, sunlight, pH (6.5-7.5) and temperature (20⁰C-30⁰C) and can be cultured, harvested and sundried without much cost, labor and expertise.

Nutrient uptake/recovery by duckweeds

Due to their ability to propagate rapidly by consuming dissolved nutrients from water, duckweeds act as an excellent “Nutrient Sink” for harvesting nutrients over a short period of time and thus serve as a “Nutrient Pump” in waste water treatment absorbing various nutrients like nitrates, phosphates, calcium, sodium, potassium, magnesium, carbon, and chloride from the waste water. These nutrients are permanently removed from the system when the plants are harvested.

Besides nutrient extraction, duckweeds has been found to reduce total suspended solids (TSS), biological oxygen demand (BOD) and chemical oxygen demand (COD) in waste water significantly. Korner and Vermaat (1998) reported that depending on the initial concentration of nutrients, duckweed covered systems can remove nitrates (N) and phosphates (P) at rates of 120- 590 mg N/m²/day (73-97% of initial concentration) and 14-74 mg P/m²/day (63-99% of the initial concentration) in three days. Removal efficiencies of 96% and 99% by duckweeds have been reported for BOD and ammonia (NH₃), respectively (Alaerts et al 1996). Reddy and DeBusk (1985) recorded N and P uptake rate of 0.15 g/m²/day and 0.03 g/m²/day by Spirodela polyrrhiza in Florida. whereas, Alaerts et al (1996) found it to be 0.26 g/m²/day and 0.05 g/m²/day, respectively in Bangladesh. Cheng et al (2002) reported maximum N and P uptake of 0.955 mg N/liter/hr and 0.129 mg P/liter/hr by Spirodela punctata. Fat duckweed – Lemna gibba have also been found to reduce TSS, BOD, COD, N, NH₃, P, phytoplankton crop and fecal coliform counts in waste water by 96.3%, 90.6%, 89.0%, 100%, 82.0%, 64.4%, 94.8% and 99.8%, respectively in 8 days (El-Kheir et al 2007).

Nutritive value of duckweeds

The protein content of duckweeds is one of the highest (up to 45%, on DM basis) in the plant kingdom (Fasakin 1999) and has a better array of essential amino acids than most plant proteins and more closely resembles animal protein (Hillman and Culley 1978). Further, its amino acid spectrum especially with regard to lysine (7.5% of total protein) and methionine (2.6% of total protein) is much higher as compared to other plant feed stuffs (Rusoff et al 1980 and Mishra 2007). Duckweeds are highly variable in their composition and it depends on the nutrient status of the water on which they grow (Table 1). They grow slowly on nutrient poor waters and under such growth conditions have low protein content associated with high fiber, ash and carbohydrate content. In contrast, they grow rapidly on nutrient rich waters and have a high protein content associated with high ash and low fiber content.

Protein content of duckweeds growing on nutrient poor and nutrient rich waters varies between 15-25% and 35 - 45% (DM basis), respectively (Mbagwu and Adeniji 1988). Root length is a useful indicator of whether pond conditions are appropriate (with respect to nutrients) for production of high protein duckweeds or not (Rodriguez and Preston 1996, Le Ha Chau 1998). Roots less than 10 mm in length indicate higher protein content in duckweeds than roots more than 10 mm in length and the reverse is true for the fiber content which can be observed very easily under field conditions. Duckweeds are also a rich source of carbohydrates (30-35%), vitamin-A and pigments, particularly beta-carotene and xanthophylls. They contain 92-94% of moisture and harvested biomass can be easily sundried within a period of 24-48 hrs during the dry summer months and 4-6 days during winter.

Sundried as well as pelleted forms of duckweed have been observed in storage for 13 years without any sign of fungal growth and nutrient loss (Mbagwu 2001) and it has been attributed to the presence of a wax coat on the upper surface of plants which acts as a barrier for fungal growth. A recent finding by Effiong and Sanni (2009) of decreased mold infestation in duckweed (Lemna pausciscostata) incorporated pelleted fish feeds also highlights its value addition potential with great application in feed storage.

With 30-40% of protein content (DM basis) nutritive value of duckweeds is comparable to that of soybean. With an annual duckweed yield of 20 t dry weight/ha/yr and protein content of 35% (DM basis), protein productivity of 7 t/ha/yr can be achieved which indicates that relative annual protein production per unit area through duckweeds is about 10 times higher than that of soybean (Skillicorn et al 1993, Khateeb 2004). Nutritionally also duckweeds have been found to substitute soya and fish meal in feeds of farmed animal like chickens, goats, pigs, ducks and fish (Hillman and Culley 1978, Culley et al 1981, Edwards 1990, Leng et al 1995, Men et al 1995, Anh and Preston 1997, Leng 1999, Iqbal 1999, Landesman et al 2002).

Duckweed based rural model for bioremediation 

The focus on duckweed as a key step in waste recycling is due to the fact it forms the central unit of the recycling engine which is driven by photosynthesis making it energy efficient, cost effective and eco-friendly. Hence, duckweed based rural bio-remediation model is an effective, cheap and simple way of reclaiming polluted village ponds. For this, divide the village pond into two ponds i.e., duckweed culture pond and fish culture pond, by erecting earthen partition. Only the duckweed pond receives the village waste and duckweed is cultured in surface floating frames (made up of either PVC pipes or split bamboo sticks) to mitigate wind action which can disturb the growing duckweed mat and carry it in the direction of wind (Figures 3 and 4).

Remediated water from duckweed pond is released periodically into the fish culture pond. In case it is not possible to divide the village ponds into two parts as desired, it is suggested to culture duckweed in enclosed pens (constructed by erecting partitions made up of bamboo poles and fine mesh net) near the periphery of the ponds (Figures 3 and 5).

In Punjab, a pilot duckweed project for bio- remediation of village pond was initiated in 2001 by the State Government in collaboration with Punjab State Council for Science and Technology (PSCST) in villages Sanghol and Chanarthal kalan in District Fatehgarh Sahib (Singh et al 2003). Under this project, village ponds were divided into a duckweed pond and a fish culture pond. Bio-remediated water from the duckweed pond was used for poly-culture of carps (Indian major carps and exotic carps) in the fish culture pond and harvested duckweed biomass was utilized to feed the fish. Encouraging results in terms of enhanced fish production from the bio-remediated village pond has lead to continuation of the project till date.  For the first couple of years the project was operated and maintained by  ‘Sulabh International’ (an Indian based social service organization which works to promote human rights, environmental sanitation, non-conventional sources of energy, waste management and social reforms through education), but now this project is operated and maintained by the Gram Panchayat (elected village administration) of the villages itself. After the success story of first duckweed pilot project in Punjab, another project was taken up by PSCST in village Sandhua in district Ropar in 2003 and many other are in pipeline (PSCST 2005).

Duckweed harvesting schedule

Regular harvesting of duckweed helps in regular extraction or recovery of nutrients from the village ponds (Figures 6-7).

A well planned harvesting schedule is required to maintain vigorous growth of duckweed and nutrient removal. It should be designed according to the growth rate of duckweeds, usually having biomass doubling times ranging from 2 to 3 days. Hence, removal of half of the duckweed biomass or cover on every third day is a practical option which not only ensures development of full duckweed cover over the pond surface within a short period but also helps in blocking the sunlight from entering into the waste water. This is required for preventing growth of unicellular and filamentous algae in the waste water, which otherwise grow very fast and compete for nutrients affecting growth and quality of duckweed. Duckweed productivity from 10 to 50 t (dry biomass)/ha/yr has been reported (Gijzen and Khondker 1997) from different parts of the world. Fresh duckweed yields in the range of 0.5 to 1.5 t/ha/day have been reported in Bangladesh which corresponds to production of 185 to 550 t of fresh or 13 to 38 t of dry duckweed biomass/ha/yr (Skillicorn et al 1993).

Utilization of duckweed biomass

Huge biomass of duckweed harvested from the village ponds can be utilized in both fresh and dried form (Figures 8-9) by the destitute rural population for economic gains through one of the following options.

Duckweeds as fish feed

Prior to 1988, duckweeds had been used only in commercial applications to treat wastewater in North America. In 1989 staff of a non-governmental organization based in Columbia, Maryland, The PRISM Group initiated a pilot project in Bangladesh to develop farming systems for duckweed and to test its value as a feed for herbivorous/omnivorous fishes like carps and tilapia. The results of the pilot operations were extremely promising and dried duckweed meal provided an excellent substitute for expensive conventional feed ingredients like soybean and fish meal (Iqbal 1999). Fresh duckweed is converted efficiently to live weight by fish. Feed conversion ratio of 1.2 to 3.3 for Spirodela in carps and 1.6 to 3.3 for Lemna in tilapia has been recorded by Gijzen and Khondken (1997). Duckweed incorporated dry diets have also been found to support growth in not only herbivorous or omnivorous fishes like carps and tilapia but in high protein demanding carnivorous fishes like catfishes and snakeheads as well (Table 2).   

Carps

In India carp poly-culture system contributes more than 80% of the total aquaculture production where Indian major carps (Catla catla, Labeo rohita and Cirrhinus mrigala) and exotic carps (Cyprinus carpio, Ctenopharyngodon idella and Hypophthalmichthys molitrix) are cultured together. Among these species grass carp (Ctenophryngodon idella) is the primary consumer of aquatic plants (herbivorous) including duckweeds. Catla (Catla catla) and common carp (Cyprinus carpio) also compete aggressively for available duckweed feed and consume it directly. In Bangladesh, about 10t/ha/yr fish productivity has been reported from duckweed fed carp poly-culture ponds (Iqbal 1999). Another exotic carp, Thai silver barb- Barbodes gonionotous (Puntius gonionotous) has also been reported to grow fast in duckweed fed poly-culture system (Azim and Wahab 2003).

Incorporation of dried duckweed, Lemna minor at 40 % in the supplementary diet of common carp, Cyprinus carpio (Devaraj et al 1981) revealed higher specific growth rate besides lowering the feed cost significantly. In Bangladesh, higher fish yields have been recorded in a poly-culture system, comprising Indian major carps, Chinese carps and tilapia, when fed with diets containing 60% sewage-grown mixed duckweeds and 40% mustard oil cake (Iqbal 1999). Guru and Patra (2007) also reported higher specific growth rate in Labeo rohita fingerlings fed with diets having 13.2% dried Lemna powder. Das et al (2007) recorded 205% higher weight gain and about 105% higher food conversion in  Labeo rohita fed with diets containing 20% dried Lemna minor powder and also saved about  20% of feed cost. Central Inland Fisheries Research Institute (CIFRI), India reported increased growth rate in grass carp when fed with diets containing 10% Wolffia (Verma 1989). Studies conducted at Guru Angad Dev Veterinary and Animal Sciences University (GADVASU), Punjab, India also revealed significantly higher weight gain in carps like Labeo rohita (20.60%), Cirrhinus mrigala (26.80%) and Cyprinus carpio (70%) fed with diets containing 20% sundried Spirodela and saved up to 50% on feed cost (Ansal and Dhawan 2007, Ansal et al 2008) by 100% replacement of animal protein supplement in the traditional diets.

Tilapia

Nile Tilapia, Oreochromis niloticus, being extremely flexible in its feeding habits, readily consumes Lemna and Wolffia species along with phytoplankton and detritus. Skillicorn et al (1993) reported that when fresh duckweed was used as a single nutritional input for tilapia in earthen ponds, fish production reached 7.5 t/ha/yr in Bangladesh. Fasakin et al (2001) developed a low cost feed for tilapia, Oreochromis niloticus L. by utilizing solar-dried duckweed (Spirodela polyrrhiza) up to 30% dietary inclusion as a replacement for fishmeal in practical diets. Dry duckweed has also been reported to replace up to 50% of the commercial tilapia feed without adverse effects on fish performance (Essa 1997, Tavares et al 2008).

Catfishes

Robinette et al (1980) obtained weight gain and food conversion equal to that of the standard feed in catfish (Ictalurus punctatus) fed with feed containing 20% dry duckweed. Effiong et al (2009) also reported 10% fish meal replacement with duckweed meal (Lemna pauciscostata) in African catfish- Heterobranchus longifilis diet.

Snakeheads

Incorporation of duckweed, Lemna minor at 50% in the supplementary diet of snakehead, Channa striatus also resulted in higher specific growth rate and weight gain besides lowering the feed cost significantly (Raj et al 2001).

Other species

In Australia, Jade Perch (Scortum barcoo) has been reported to actively consume and gain weight (with 100% survival) solely on fresh duckweed harvested from effluent treatment plant (Willett et al 2003). Presence of good amount of carotenoids and pigments in duckweed can stimulate crustacean growth (Landesman et al 2002). Fletcher and Warburton (1997) have found that decomposed Spirodela is as effective as commercial pelleted feed for culturing redclaw crayfish, Cerax quadricarinatus.

Vermicomposting of duckweeds

As fresh duckweed is characterized by high amounts of nitrogen and phosphorus, compost made from duckweeds is also expected to be rich in these macronutrients. Kostecka and Kaniuczak (2008) developed a high quality macronutrient rich (N, P & K) vermicompost from duckweed (Lemna spp.) biomass by using Eisenia fetida (SAV.) earthworms. Hence, vermicomposting of harvested duckweed biomass further corroborates its potential for utilization in environmental reclamation including aquaculture as well as agriculture.

Production of value added products from duckweeds

Besides quality protein resource, duckweeds are also a good resource of starch. Hence, there is great scope of production of value added products like protein concentrate and ethanol from duckweeds. About 64.4 % crude protein content has been reported in leaf protein concentrates prepared from Spirodela polyrrhiza (Fasakin 1999), which can be used as feed supplement not only in animal feeds but also for human consumption. Spirodela polyrrhiza grown on anaerobically treated swine waste water has been found to have a starch content of 45.8% on DM basis and its enzymatic hydrolysis yielded a hydrolysate with a reducing sugar content corresponding to 50.9% of the original duckweed biomass. Further, fermentation of the sugar hydrosylate by yeast gave an ethanol yield of 25.8% of the original dry duckweed biomass which reflects an additional scope of harvested duckweed biomass in ethanol production (Cheng and Stomp 2009).

Suggestions 

Most of the village ponds are constructed without proper planning (location and layout) and drainage system. Ponds located in the middle of the inhabited area of the village are difficult to drain out and have little scope of alterations required for developing duckweed based bio-remediation models. However ponds at the outskirts of the village are easy to manage as its nutrient rich water can be utilized for irrigating the adjoining agricultural fields. Hence, for full scale commercial utilization of duckweed based bio-remediation models, construction of village ponds are required to be well planned with respect to location, layout and drainage facility. State government also needs to promote duckweed based rural models for bio-remediating village ponds through educating the people and introducing some rural welfare schemes integrated with village ponds for employment and income generation. Variety of animals like cows, buffaloes, goats, sheep, pigs, chicken and fish are being reared in rural India for milk, meat, wool and eggs. Utilization of duckweeds as fodder or feed ingredient for these animals is also required to be popularized among the rural population. Increased local demand will certainly promote duckweed aquaculture in village ponds.

Conclusions 

• Although polluted, village ponds are rich source of nutrients like nitrate and phosphate which can be recovered by phyto-remedition. It is an affordable technology utilizing plants as environmental cleansers in wastewater management. On one hand manure and fertilizers are getting costlier day by day and on the other hand we have resources like village ponds where the much needed nutrients are lying free of cost. Therefore, recovering this valuable nutrient resource and recycling into some productive system makes sense both ecologically and economically.

  
  

• Hence, an eco-friendly approach of duckweed culture in nutrient rich village ponds will not only help in free of cost extraction of nutrients (which otherwise pollute the water and go waste) in the form of protein rich duckweed but also bio-remediate the village ponds and make them a more suitable water resource for aquaculture. Wastewater-duckweed-carp poly-culture makes the perfect integrated package for pollution control and re-use of recovered nutrients. Bio-remediation will not only augment contribution of village ponds to total aquaculture production of any developing countries like India but also generate employment opportunities and additional food security for its rural population. Moreover in view of increased pressure on land over the years for production of food and fodder (due to ever increasing population, urbanization, industrialization etc.), utilization of an alternate resource for the purpose makes sense. In this direction, village ponds hold ample scope not only for high quality food production through aquaculture but also releasing pressure on the underground water resources.

  
  

• Although proven but full scale commercial application of duckweed based rural models for bio-remediation of village ponds has not achieved a major breakthrough so far. It is possible only through educating the rural population regarding its economic and environmental benefits and by providing them the required technical guidance and financial assistance.

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Received 1 February 2010; Accepted 16 May 2010; Published 1 July 2010

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