Introduction

The energy is one of the important factors that drive development process of any country. It provides the stimulus and momentum to socioeconomic development of any developing society [1]. Therefore, the secure and accessible supply of energy is an inevitable need of modern society. The energy consumption pattern grows linearly with economic growth and industrial development [2]. The major proportion of energy demands is fulfilled by non-renewable sources such as natural gas, coal and petroleum derivatives [3]. Globally, the future energy demands will recklessly rise with population growth and increasing living standard of the society [4]. The excessive consumption of fossil fuel tends to diminish the availability of resource for future generations [5]. The fossil fuel-based energy is finite and produces by-products, such as greenhouse gases, that are detrimental to the environment [5–7]. The continued use of fossil fuels will set to face multiple challenges related to depletion of fossil fuel reserves, global warming and other environmental concerns, political and military conflicts and ultimately significant fuel price rise [7]. The necessity of proper management of existing energy resources and exploration of new energy resources is essentially needed to address. The transformation of energy economy based on vast, expensive supplies of coal, oil and natural gas to much cleaner and easily accessible wind, solar and biomass energy will help to mainstream energy demand [2]. Renewable energy is advocated to be the possible solution of growing energy challenges as they are abundant, inexhaustible and environmentally friendly [8]. The exploration of non-carbon energy sources such as solar, nuclear, biomass, wave, tidal and geothermal may provide sustainable solution for long-term availability of energy resources [4, 5, 9]. Biomass (including organic matter derived from living organisms) forms the most utilized sources of renewable energy and accounted for 10.2 % of global primary energy supply [1, 10, 11]. The biomass-based bioenergy production has several issues like availability of raw material, productive conversion processes, water and energy requirement, etc. [12]. In this process, the efforts are being made in searching viable plant resources for designing a cost-supportive biorefinery system [13]. On the other hand, the production of bioenergy (biofuels, biogas and bioelectricity) outsourced from renewable biomass feedstock is advocated as an energy alternative with low-emission potentials in terms of carbon budget [14]. The feedstock utilized for this purpose mainly includes post-consumer organic residues and by-products available from community wastes, agriculture and forest industries: crop residues, forest and wood process residues, animal excreta, sewage sludge, municipal solid wastes, food processing wastes, purpose grown energy crops (Miscanthus, Jatropha, Salix, Panicum, Eucalyptus) etc. [10, 15]. Worldwide, there are multitudes of plant species that can be used for several energy generation purposes like oil, heat, bioethanol, power production, etc. In this series, biofuels form the largest category of bioenergy derived from biomass. The principal fuels that can be manufactured includes ethanol, methanol, biodiesel and hydrogen; although, other categories of fuels such as biobutanol and dimethyl ether can also be produced from plant biomass [16, 17].

The thermal characterization of plant biomass is an important aspect of bioenergy production process. The proximate (moisture, ash, volatile, total carbon content) and ultimate (elemental composition such as C, N, H, S and O content) analysis of energy biomass play an important role in determining the energy potential of a plant feedstock [15]. The collaborative interaction between moisture contents and bulk density prescribed the heating value of the biomass and also decide the appropriateness of conversion process [1]. The total solids and volatile solid content of any liquid/solid material can have direct impact on the bioenergy production efficiency. The ideal biomass destined to be used for bioenergy production should have efficient solar energy conversion resulting in high yields, needs low agrochemical inputs, has a low water requirement and has low moisture levels at the time of harvest [14, 18, 19]. Conventionally used plant species may not address the issue of sustainability; therefore, the utility of plant species that demarks the direct interference with food security is critically needed.

Aquatic biomass is ubiquitous and grows in different environmental and climatic conditions [20, 21]. Water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), frog’s bit (Limnobium spongia), water fern (Salvinia minima), mosquito fern (Azolla caroliniana), water lily (Nymphaea mexicana), American lotus (Nelumbo lutea), water spinach (Ipomoea aquatica), duckweed (Lemna sp.), watercress (Rorippa nasturtium-aquaticum), etc. are few prominent species that offers flexibility in terms of water utility and environmental requirements for optimal growth [22]. However, the utility of local aquatic plant species strains other than exogenous species strain will helps to maximize the growth rate and ultimately bioenergy yield [23]. The adaptability of different aquatic plants to grow on a variety of wastewaters also diminishes the utility of freshwater resources for such purposes [24, 25]. Apart from having great energy values, the aquatic plants have been utilized effectively for on-site wastewater treatment approaches [25–27].

Thus, this can lead to develop a coupled system for wastewater treatment and energy biomass harvesting. The use of aquatic plants especially weed species as bioenergy feedstock is often a remarkable alternative as it offers plant biomass harvesting, utility of wastewater resources, high energy return on investment, potentially greater economically efficiency, little or no conflict with food resources and much lower greenhouse gases (GHGs) emissions and other environmental effects [16]. This review summarizes the potential and feasibility of duckweed as bioenergy feedstock for sustainable urban habitat development.

Duckweed as a Resource for Bioenergy

Ecology and Morphology

The aquatic vascular monocotyledons angiosperms duckweed belongs to the family Lemanceae [21]. It is represented by 5 genera: Spirodela Schleid., Landoltia Les & D. J. Crawford, Lemna L., Wolffiella (Hegelm.) Hegel and Wolffia Horkel ex Schleid [28–30]. According to the reported literature, there are approximately 38 species of duckweeds distributed worldwide in freshwater ecosystems [31]. This plant inhabits in a wide range of aquatic ecosystems from tropical to temperate zones, and from freshwater to brackish estuaries, except arctic and antarctic zones. Duckweed occurs abundantly in still or slightly moving water, but flourishing growth is reported to occur in stagnant ponds, brackish water or ditches rich in organic matter, or near sewer outlets [31].

All five genus of Lemnaceae offer physiological plasticity and peculiar morphological and physiological features that account for their classification and differentiation [29]. Various morphological features (frond size, root pattern, flower pattern, etc.) are used, in general, to differentiate between different duckweed species [30]. In case of Wolffia, roots are absent, fronds are without nerves, the daughter fronds and flowers are originated from cavity or pouch present at the base of frond, scales are usually absent in flowers and seeds are smooth [29]. Whereas, in Lemna, roots are generally present and varies from 2–21 per frond, frond are with nerves, the daughter fronds originate from two lateral pouch present at frond base, flowers are surrounded with scales and seed are longitudinally ribbed [30]. Generally, plant body consists of fronds and with or without roots. Fronds are poorly differentiated and flattened, small leaf-like or spherical in shape [28, 32]. The number varies from one, two or to many and are cohering together at base. In size, it is 0.05–1.5 cm long (without stalk) and 0.03–1 cm wide, thin or thick. Roots are bottom lined and without root hairs. The major cellular constituents are chlorenchymatous type with large intercellular spaces. This helps to provide buoyancy and supports free floating nature [29]. To withstand unfavourable conditions and dormancy stage of lifecycle, turions are formed [29]. These are compact frond reduced in size and structure and filled with starch grains. Flowers are rarely present in plant. If present, 1–2 per frond in number, mostly perfect with no sepals and petals. Stamens are 1 or 2 and ovary 1, bottle-shaped characterized with short style and funnel-shaped stigma. Fruits are generally seeded (1–5) with dry pericarp. The chromosome number reported by counting somatic (root tip) chromosomes are 2n equal to 40 in Spirodela polyrrhiza and L. minor, 44 in L. triszlca, 64 in L. gibba [33]. The proliferation primarily took place through vegetative budding of new fronds from parent fronds. However, there occurs distinction both in size and longevity of plant body between generations. It may be expressed as cyclicity in the growth pattern of a colony [20, 29, 33]. The reproduction and growth pattern are found to be similar to microbial growth [34], and the doubling time is 20 to 24 h, which vary significantly among different species [35].

Potential of Duckweed Biomass in Renewable Energy Operations: Recent Approaches

The growing demand of energy can be dealt in best way by fostering the renewable energy technologies as long-term solution to energy problems mainly in developing countries [36] where conventional energy resources are either limited or explored indiscriminately. Plant resources especially non-edible plant species (wild plants, aquatic and terrestrial weeds, wetland plants, forest stand and their residues, etc.) can be a potential source of biomaterial and bioenergy. Not all plant biomass/products are suitable for energy generation in terms of energy values and outputs against invested cost in the energy harvesting technology. The chemistry is pivotal factor in determining the energy potential of any available biomass in energy production system. The biomass resources are the organic matters having chemical energy mainly synthesized through photosynthesis process in the presence of solar light [37]. Through complex metabolic activities, the plant synthesizes several active chemical ingredients which can be further harvested for biofuel purposes. So, in this process, the solar energy is stored in the forms of chemical bonds in plant biomass. The fixing of solar energy into bioenergy is illustrated in Fig. 1. The biochemical composition of duckweed as reported in scientific literature by various authors ensures the possibility of duckweed in renewable energy operations. The high rate of quality biomass (rich in carbohydrate, starch, protein and lipids) production in duckweeds advocates its utility in biomass-based biorefinery (Table 1). The quantity (dry weigh basis) of carbohydrate, starch, crude protein, crude fibre and lipid has been reported in the ranges of 17.6–35 %, 21–38 %, 16–41.7 %, 8.8–15.6 % and 4.5–9 %, respectively, by various researchers [24, 39–41]. However, concentration of lipid and starch seems comparatively lower in duckweed as compared to other plant biomasses that can be further enhanced considerably through plant growth conditions manipulations (like pH, temperature and nutrient concentration in culture substrates) [24, 40, 42–45]. The major constituents of carbohydrate in duckweed biomass are 20.3 % pectin (including galacturonan, xylogalacturonan, rhamnogalacturonan), 3.5 % hemicellulose (comprising xyloglucan and xylan) and 0.03 % phenolic compounds [46]. Lipid mainly comprises of long chain alcohol and acids and abundant in nitrogen contains compounds like pyrrole and indole [47]. In duckweed, the presence of few important fatty acids—palmitic acid, linoleic acid, linolenic acid and p-coumaric acid has been reported. Few authors have claimed the high content of palmitic acid and linoleic acid in duckweed biomass which can be further harvested for biodiesel production [48–50].

Fig. 1
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Fixation of solar energy into energy rich biomass

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Table 1 Techniques for biomass resource recovery [38]
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Traditionally, combustion had been utilized as major practice to harvest energy from dry biomasses. Apart from that other processes like anaerobic digestion, incineration, pyrolysis, gasification, oxidation, etc. are also used efficiently for energy resource recovery from plant biomass [38, 51]. The choice of mechanism to harvest energy from biomass is governed mainly by various factors: the type and quantity of biomass feedstock; the desired form of the energy, i.e. end-use requirements; environmental standards; economic conditions and project specific factors. The possible end products of biomass conversion processes are depicted in Fig 2. The quality of end products and their further processing is also concerned in terms of energy values of end products, total cost of operation and emission controls. The chemical characterization of available duckweed species has been estimated by various authors through proximate and ultimate analysis which suggests the feasibility of duckweed-based feedstock in other energy operations like pyrolysis, gasification, biogas, incineration, etc. [1, 24, 39, 42, 52].

Fig. 2
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Option to convert duckweed biomass into energy resources

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The duckweed biomass can be converted into three main products: two related to energy—power/heat generation and transportation fuels—and one as a chemical feedstock [53]. Table 2 describes the multitude of techniques that can be employed in biomass energy resource recovery. But for optimum productivity of the duckweed-based energy units, the protocol for the maximum biomass harvesting and process design need to be explored and designed. Recently, few studies on optimization have claimed the enrichment of duckweed biomass with energy rich substances. Like alternation in physical environmental conditions and/or nutrient loads manipulations leads to fast metabolism of energy value substances in duckweeds. Studies have shown the starch contents in duckweed biomass in the ranges of 3 and 75 % (dry weight basis) under manipulated conditions [28–31]. Tao et al. [52] have studied the interrelationship between nutrients uptake and metabolic activities in duckweed Landoltia punctata and found that the quality of biomass enhances under starvation conditions. The prolonged nutrient limitation regulates the different metabolic activities in plants. Data on transcriptomics, enzymatic assay and physiological characterization have revealed that the collaborative effect of enzyme regulating pathway helps in fast and high starch accumulation and low lignin percentage in aquatic weeds [59]. The load of nutrient in culture media also helps in chemical quality enhancement of weed biomass. Cheng and Stomp [24] recorded the increase in starch content (45.8 %) in duckweed S. polyrrhiza biomass when the plants were transferred from a nutrient rich solution (anaerobically treated swine waste) to a solution made up of tap water. The temperature is an important physical environmental condition which directly governs the metabolic process in plants. Cui et al. [42] have observed the increased starch accumulation in S. polyrrhiza cultured at low temperature (5 °C) than high temperature (with 15 and 25 °C) cultures.

Table 2 Biochemical composition of various duckweed species biomass (DW, dry weight basis)
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The high starch content (≥75 %) in duckweeds is comparable to other starch rich crop, i.e. corn (starch ≥65 to 75 %) which is popularly being used in bioethanol production [60]. Few earlier workers have demonstrated that duckweed carbohydrate can be converted to ethanol and butanol efficiently [58, 59, 61]. Cheng and Stomp [24] have reported 25.8 % (of biomass) bioethanol produced though fermentation of duckweed-derived hydrolysates. L. punctuate has been reported as promising feedstock for bioethanol production as glucose yield in this plant is about 218.6 ± 3.10 mg/g dry matter [61]. A study by Perniel et al. [61] has claimed 30.8 ± 0.8 g/l ethanol production from L. punctuate biomass with 90.04 % fermentation efficiency using Saccharomyces cerevisiae. However, the starch yield in duckweed-based system is highly appreciated by few works. For example, Zhao et al. [41] have estimated, based on the accumulation rate of 2.06 g starch/m2/d, an annual starch yield of 7.52 tons per hectare. Such high quantity of starch can be further utilized for bioenergy purposes. Ge et al. [56] have reported 20.3 % (w/w) total glucan and 32.3 % (w/w) proteins; trace hemicellulose and undetectable lignin in duckweed biomass harvested from wastewater. Their study further confirmed the release of about 96.2 % (w/w) glucose from both the cellulose and starch fractions of biomass without prior thermal-chemical pre-treatment. These hydrolysates, fermented through two yeast strains (self-flocculating yeast SPSC01 and conventional yeast ATCC 24859, yielded ethanol of 0.485 g/g glucose [56].

The duckweed can be a potential source of feedstock in anaerobic digestion process which is utilized as major mechanism in biogas production. This process occurs in four main stages namely: hydrolysis, acidogenesis/fermentation, acetogenesis and methanogenesis, thereby require various groups of microbes to complete each of the stages [43]. Due to certain sets of limitations, the duckweed biomass cannot be utilized in a single feed in anaerobic digestion; therefore, the co-digestion of duckweed with any other conventional sludge can solve this issue. Few previous studies suggested the utility of duckweed biomass in anaerobic digestion to enhance the anaerobic process. Clark and Hillman [62] investigated the impact of adding iron rich duckweed as a supplement to chicken manure in batch and semi-continuous lab-scale anaerobic digesters. The results validated an increased gas production rate of about 44 %. Undergoing anaerobic digestion, the accumulated biomass can be used as a source of energy and at the same time the digestate can be used as a low quality feed or a soil conditioner.

The thermochemical conversion (gasification, liquefaction and pyrolysis) of weed biomass also produces few products of benefits. The utility of duckweed biomass in gasification processes is less studied due to several shortfalls associated with these processes. Pyrolysis is another major biochemical conversion process which is significantly studied by various authors in case of duckweed biomass. The major gaseous products of duckweed pyrolysis are H2, CO, CO2 and CH4, with minute amounts of C2H6 and C2H4 hydrocarbons. However, in this process, the CO2 was regarded as a major component of the gaseous mixture amounting to about 60 vol% [48] which can be harnessed for energy purposes. The liquefaction process yields in some products of energy values. But liquefied oil obtained from liquefaction process contains high carbon and hydrogen content and low nitrogen content than the oil obtained through pyrolysis process [63]. So, in this process, there is possibility of upgrading process in order to maximize the quality yield of the system. Duan et al. [57] in their study upgraded the liquefaction process coupling it with pyrolysis system to convert Lemna sp. biomass into alternative energy products. Temperature and retention time are important factors in the completeness of the thermochemical liquefaction process. Temperature range between 260–340 °C increases oil yield, and the process get completed within 60 min even in the absence of catalyst [58]. The elemental composition of obtained liquefied oil is reported as 73 % carbon, 9 % hydrogen and 5 % nitrogen with average heating value of 34 MJ/kg [56]. Thus, properties of this oil were very similar to that of liquefied oil obtained from manure. The slow thermolysis of duckweed biomass results in 10.7 % of the bioleum, 45.0 % of char and 19.44 % of gases [62]. Muradov et al. [64] has also reported yield of 38 % bio-oil, 44 % char and 17 % gas during slow pyrolysis of L. minor (at higher reaction temperatures of 500 °C) biomass. The physico-chemical properties of bio-oils derived from duckweed through hydrothermal processing were dark-brown, viscous and with smoky odour. Few other workers have reported the heating values of duckweed oils around 32–36 MJ/kg, which seems slightly lower than the petroleum-derived fuels (42 MJ/kg) [61, 65]. Biochar is another value-added product obtained from duckweed biomass. Biochar acts by enhancing moisture retention and nutrient holding capacity of the soil (which means less dependence on chemical fertilizers). The pyrolysis temperature and sweep gas flow rate determines the yield of L. minor derived biochar by changing pore development pattern, surface area and textural characteristics [66].

Apart from the chemically rich, the duckweed is one of the fastest growing weeds which make this as suitable candidate for rich biomass production. A small plant size, limited life cycle, high duplication rate, adequacy to grow on different culture medium and lack of waxy cuticle (to prevent water loss) of the duckweed are few of the characteristics that have been used to evaluate the efficiency of biomass generation and ensure its utility as bioenergy feedstock [38]. Duckweed biomass has a high surface area to volume ratio that obviates the need for biomass milling or grinding, also absence of waxy cuticle quantified water availability in biomass. Both of these characteristics enhance duckweed as a viable biomass option as it can be dried quickly with low energy inputs. Table 3 depicted the bioenergy potential of harvested duckweed biomass.

Table 3 Bioenergy production from different species of duckweed biomass
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It is therefore suggested that conversion of duckweed biomass into renewable energy bioresources is an important carbon-negative technology. The duckweed plant represents an attractive non-food aquatic crop that has a high potential to turn nutrients from wastewater to economically sound bioenergy feedstock, which can further transform into a biofuel/biogas/biochar. It can be used as a promising alternative feedstock for bioenergy production.

Biomass-Based Energy Production and Duckweed: Challenges and Opportunities

Bioenergy production is considered to be an ecologically attractive option. The sustainability of bioenergy production processes with respect to ecological services and land use, emissions, resource efficiency and waste management is often a critical issue to deal with [1, 69]. The use of conventional energy crops and trees (Miscanthus, Jatropha, Salix, Panicum, Eucalyptus, etc.) will pose serious ecological challenges such as loss of biodiversity, loss of natural habitats and wildlife and soil degradation [10, 17]. The increase in biomass for energy plantation will reduce the amount of land available for agriculture. This situation is aggravated in recent years due to unparalleled government incentives to agriculture and bioenergy, favouring the latter. Additionally, the requirement for water resources and land availability also creates problem [1, 17].

The use of aquatic plant duckweed diminishes the possibility of any shortfalls. Relying on duckweed biomass for bioenergy could be associated with carbon neutral process and provides numerous benefits related to energy security, economics and the environment [70]. The integration of wastewater remediation using duckweed species not only mainstreams water resource availability but also results in accumulation of abundant biomass [58]. This biomass can be further harvested to see its utility in bioenergy productions. Utilization of duckweed substrates as raw materials for bioenergy should minimize the potential conflict between food and fuel as these can withstand and offer a viable option in place of recently used bioenergy feedstock which are mostly food-industry based (such as corn) [10]. Besides having several benefits, the production and utilization of duckweed feedstock also admit several challenges that need to get target for better resource utilization. An improved biomass collection network and storage is the main challenge for the establishment of commercial bioenergy feedstock platform [57]. Transportation, storage and logistic requirements need address for economically viable bioenergy production. The inconsistencies in quality and quantity of duckweed produced, moisture content of duckweed materials may also contribute to unproductive conversion processes [70]. The technological improvements could help to improve the system efficiency and provide value-added co-products, which will reduce the production cost [43]. It can be concluded that a critical evaluation has been needed to address various issues regarding selection of any process for the bioconversion of substrates [24, 39]. The realization of current research challenges helps to mainstream the production of bioenergy using abundant duckweed biomass. It is important to derive primary cost-benefit analysis to ensure the practicability of this novel technology [71]. The feasibility aspects help to enhance the energy balance and also reducing the emissions and production costs, to become true alternatives that complete the biomass-based energy future scheme [39]. The by-products resulting from various conversion methods (such as hydrothermal processing/upgrading and liquefaction, thermochemical and biochemical conversion) may result in environmental problem and therefore need to be properly disposed of [47]. The fate of phosphorus present in biomass is unanswerable as it remains in the processed solids residue and liquid stream. The additional research helps to reveal the efficient ways to deal with relevant problems associated with treatment before disposal. The characterization of waste products obtained will be helpful to analyse the recovery, reuse and recycle option.

Conclusions

Energy demands will aggravate with time in this fast growing industrial society. The use of biomass for bioenergy purpose will provide a competent option towards meeting energy needs. Various plant species (food and non-food) are accounted for this purpose; however, their utilization depends on availability, growth dynamics, etc. Also, the associated food crisis equally demarks their use as potential energy feedstock. The use of aquatic plant duckweed represents an attractive non-food plant that can be efficiently utilized for bioenergy production. The integrated duckweed-based treatment system supported with wastewater will bring significant environmental benefits (such as water purification, biomass generation) and seems to be an ecologically friendly and cost-effective solution for water pollution and bioenergy-based feedstock production. The harvested plant showcases significantly high content of crude protein, amino acids, starch and lower content of fibre. These biochemical characteristics of duckweed supported their candidature for excellent feedstock for bioenergy production. The varieties of products were reported to be produced from duckweed such as biofuel, biogas, biochar, etc. This advocates that a plant has huge potential towards nutrient recovery and its conversion to efficient energy resource. The available feedstock is cost-supportive, easily harvestable and can be processed into energy form efficiently.