Energy and nutrient recovery from agro-wastes: Rethinking their potential possibilities

Article information

Environmental Engineering Research. 2020;25(5):623-637

Publication date (electronic) : 2019 November 7

doi : https://doi.org/10.4491/eer.2019.269

Barkha Vaish ¹, Vaibhav Srivastava ¹, Prabhat Kumar Singh ², Pooja Singh ³, Rajeev Pratap Singh ¹^,

¹Department of Environment and Sustainable Development, Institute of Environment and Sustainable Development (IESD), Banaras Hindu University (BHU), Varanasi-221005, India

²Department of Civil Engineering, Indian Institute of Technology, Banaras Hindu University, Varanasi-221005, India

³Department of Science, Institute of Computer Science and Technology, SHEPA, Varanasi-221005, India

^†Corresponding author: Email: rajeevprataps@gmail.com, Tel: +91-9935912997

Received 2019 June 24; Accepted 2019 October 16.

Abstract

Agricultural sector has inescapably drawn attention towards itself due to growing concern over population explosion, changing climate and depletion of resources. Advancements in technology have caused rapid boost in farm productivity and hence huge production of agro-wastes. There have been massive deteriorating impacts of agro-wastes on existing environmental conditions, creating wide scale environmental pollution as well as biodiversity loss. Since, agro-wastes have huge potential in the form of energy and nutrient recovery (ENR), amalgamations of different technologies would promote sustainability. ENR not only adjoins an added profitable product but also facilitates to close environmental nutrient cycles. This provides a substitute to existing unsustainable approaches that are energy demanding and reliant on nonrenewable sources of energy. This review underpins why ENR from agro-waste must be key concern in production of array of higher value products.

Keywords: Agro-wastes; Energy; Nutrient Recovery (ENR); Technologies

1. Introduction

The energy predicament and dilapidation of environment are presently two vital issues for worldwide sustainable development [1]. The foremost source of potential energy is fossil fuels that are dominantly consumed by the most of developed and developing economies around the world [2, 3]. Dependence on traditional fuels has contributed to change in climate resulting in global warming [4, 5]. Continuous increase in consumption of energy has led to rapid exhaustion of existing natural sources [6, 7]. Therefore, the world is looking for an energy system that is accessible, sustainable and clean lesser carbon intensive for survival and welfare of living beings [8, 9]. Simultaneously, the inconsiderate reality of a mounting global population forces rampant utilization of inorganic fertilizers. It degrades soil fertility, depletes soil organic matter, poses risk of soil erosion and through agricultural runoffs causes eutrophication. In this context, agricultural wastes/agro-wastes that comprises straw, husk, stalks, cobs etc. embodies a profuse carbon-neutral feasible resource for bioenergy and nutrients generation as summarized in Fig. 1 [10].

Scientific advancement in genetics, biotechnology, engineering and process chemistry have yielded ‘biorefinery,’ a novel manufacturing concept. It converts renewable agro-wastes / biomass into biofuels, nutrients and by-products while integrating different technologies [11]. It offers the possibility for sustainable production of biopower and biomaterials [12, 13]. For biorefinery feedstocks, agro-waste provides potential substrate for recycling of nutrients in agricultural field which in turn maintain soil health, improves soil structure and reduces the risk of erosion. However, economic feasibility of the conversion processes is yet to be determined. Therefore, this review aims at better utility of agro-wastes, de-carbonization, biofuel generation, nutrient recycling and reduced pollutant emissions by actual application with holistic approach [14].

2. Biorefinery Approach

The increasing interest in biorefinery and the growing evidence for its benefits in terms of material savings and emissions reductions [14, 15], led to commensurate focus on biorefinery, particularly in those countries and regions with large agri-food sectors [16]. In the design and operation of biorefineries, biofuels and nutrient recovery technologies have been a prime consideration that is utilized for recovering a range of N, P or K rich products of environmental agricultural significance. A biorefinery may be considered as an integrated facility that employs extraction/conversion methods for the production of variety of platform chemicals, fuels, and energy that is somewhat parallel to a petroleum refinery [17, 18]. Sustainability is the key criteria promoting bio-refineries which utilizes conversion of low-value biomass (specifically agro-wastes material from agriculture) into a portfolio of higher-value products to fulfill future product demands and must also be considered as an integral part of environmental management approaches [19, 20] (Fig. 2). Biorefinery feedstocks usually contain biomass having an excessive amount of carbon viz., lignocellulosic biomass obtained from agro-wastes that comprises mainly of carbon-rich polymers such as cellulose, hemicellulose, and lignin. It also contains nutrients like N, P, and K recovered from any type of agro-wastes [17] (Table 1).

Fig. 2

Process of biorefinery concept with major substrates and products.

Table 1

Composition of Lignocellulose Component in Various Lignocellulogic Materials

Nutrients recovered from agricultural wastes can include complex organic mixtures, e.g, biomass; mixed residuals from thermo- chemical processes like char/ash; or separate N, P, K specific chemicals for e.g, NH₄⁺ or K based products [29]. Nutrient recovery technologies mainly include anaerobic digestion to produce biosolids, thermal processing to produce biochar or ash and aerobic composting to produce soil amendments. Char and ash are increasingly utilized in soil amendments which offer potential for heavy metal immobilization, carbon sequestration, and improved soil quality [30]. Potential technique for utilizing lignocellulosic agro-waste is by converting them into simple sugar (glucose) followed by fermentation to form ethanol [31, 32] (Fig. 3).

Fig. 3

Process flow chart of lignocellulosic biomass into various useful products [33].

However, other solvents like butanol derived from biorefinery is equally useful or in some special cases superior to ethanol. But they have very limited public recognition. The superior qualities of butanol like low volatility, octane improving power, high energy content and miscibility could replace gasoline or diesel [34]. Economics is also favoring butanol fermentation with enhanced microbial stability and reduced product inhibition [35], and continuous improvements could be seen with efforts in research and development [36]. Therefore, possible efforts must be made to utilize each and every product of agro-waste. The experience gained must be used in building a modern mature technology for biofuel production which is economically and ecologically viable after petrol/oil era.

3. Energy Recovery Possibilities

Biorefineries are manufacturing services that transform bio-based materials into products like food, feed, fuels, chemicals, and energy [37]. These bio-based materials and chemicals could substitute products obtained from petroleum and natural gas [38, 39]. Moreover, they entail lesser investment and simple innovation technology while lessening air pollution from degradation of agro-wastes either when it is blazed in fields or when it is rotted in the countryside [40]. Moving towards renewable biomass resources is a progress of an industrial society that is sustainable and efficiently manages greenhouse gas emanations [13, 41]. The energy recovery potential of different agro-wastes adopted in different technologies is described in Table 2.

Table 2

Energy Recovery Potential from Different Agricultural Waste in Different Regions of the World

Although biofuels reduce fossil fuel driven energy problems and direct emissions [42], its increasing demand could cause an adverse environmental impact through changes in overall land utilization [43–45]. Production of biofuels might lead to loss of biodiversity either through straight displacement of natural habitat and/or meandering of ecologically valuable land [46, 47]. Therefore, biotechnological methods guiding biofuels production should be greatly accelerated that aptly use biomass recalcitrance agro-wastes while preventing biodiversity loss and taking care of sustainability criteria [48, 49]. However, huge demand of energy and fuel may not be fulfilled by biomass utilization due to relatively low energy content, seasonality and discrete geographic availability of biomass feedstocks [50, 51].

3.1. Biogas/Methane Recovery

Biogas consists of methane, carbon dioxide, hydrogen and hydrogen sulphide [52]. It is obtained in a bioreactor during biomethanation of organic substrates like sewage sludge, manure, agricultural residue, organic part of household and industrial wastes, along with energy crops [53]. Further, they are used as fuel for producing thermal energy and electricity [54, 55]. From a bioenergy point of view, the anaerobic treatment has an advantage over aerobic degradation of organic substrates as it yields high product (biogas) at low biomass input and results in less generation of waste sludge/ slurry [56, 57].

Agro-wastes are capable of biodegrading effortlessly owing to organic substance and low nitrogen content, making them appropriate for co-digestion and augmenting methane production with animal organic wastes from the fermenters [58, 59]. Significant studies on power generation from agro-wastes has been widely done. Considerable number of biogas plants are already running around the world. Developing nations like Brazil had already setup 85 MW biogas plant from agro waste; 36 MW from rice husk; 32 MW from elephant grass; 371 MW from wood residue [60]. Similarly, Piwowar et al. [61] studied agricultural biogas plants in Poland and found that production of biogas from agro-waste was increased by 137.29 million m³ from the year 2011–2014. Similarly, in developed nations like Germany, biogas plants are increasing continuously since last twenty years. There were approximately 140 biogas plants in 1992, while the number reached to 7720 at the end of 2013. For rural regions, development of market for establishment of biogas plants is crucial and must be analyzed for socio-economic and environmental aspects [62].

3.2. Bioethanol

The manufacturing of ethanol from agro-wastes derived sustainable resources is extended through the use of various microorganisms in fermentation systems [31]. These biofuels i.e. biomass to liquid are prepared from agro-industrial materials such as residues, straw, sawdust, reclaimed wood and low value timber [63, 64]. Bioethanol is a non-toxic substitute for traditional fossil fuel for transportation worldwide [65]. Along with energy generation, bioethanol is suitable for chemical feedstock and, as an industrial solvent [66]. There is also an increasing recognition that bioethanol provides environmental benefits to ease particulate emissions [67].

Recent studies have concluded that energy derived from agro-wastes has a completely positive energy balance to an extent that brings a prosperous impact on sustainability and security challenges [66]. For example, Dominguez-Bocanegra et al. [67] utilized coconut, pine and tuna as agricultural waste for promoting bioethanol production by yeast Saccharomyces cerevisiae CDBB 790. Their results showed that highest bioethanol concentration was in 22% (v/v) pineapple juice, 20% (v/v) coconut milk and least in tuna juice i.e. 12% (v/v). Similarly, Evcan and Tari [66] studied the apple pomace hydrolysate as agro-industrial waste co-culturing Aspergillus sojae, Trichoderma harzianum, and Saccharomyces cerevisiae. The highest ethanol and bioethanol concentration were 8.748 g L⁻¹ and 0.945 g L⁻¹, respectively. These preliminary studies suggest that bioethanol can be a treated as an alternative renewable feedstock for fossil fuel, as a feasible environmentally friendly solution for waste utilization.

3.3. Briquette

Briquettes obtained from agro-wastes are a biofuel substitute to coal and charcoal and can be employed in domestic cook stoves, boilers and gasifiers as fuel [49]. Briquettes are compressed/compacted blocks of coal dust or any other added combustible biomass like wood chips, charcoal, peat, cobs, straw, sawdust, and paper [68]. The fine quality briquettes are environmentally safe biofuels and are produced at low cost from solid waste - formed from agricultural and industrial sources, through thermo-chemical conversion [69–71]. Scattered and untreated waste biomass of high density is dried, chopped and compressed to make briquettes [72]. The technological options available for making biomass briquette fuel, compress scattered and untreated resources into fuel (solid) of high density through drying, chopping and briquetting [68], thus. This reduces transportation and storage space costs and also enhances the combustion quality, while making its activation more purposeful [73]. Briquettes from agricultural residues can be employed in power generation through gasification, straight ignition, co-combustion, and in industrial boilers, furnaces, heating boilers and other combustion equipment [74, 75]. The prospective economic, environmental and social impacts of biomass briquetting required appraisal to ensure its eco-efficiency. Studies focused on the techno-economic aspect of biomass briquetting have revealed that appliance of agro-waste for sustainable construction materials provide a solution which proposes conservation of energy and natural resources [76]. The extent of wastes produced from agro-sources like sugarcane bagasse, rice husk, jute fiber, etc. is relatively high. Thus, it can be strategically reused as a sustainable building material which could provide an alternative to excessive cost of conventional building materials along with nuisance of pollution and land-filling. Briquetting technology requires densification of material aimed at improvising its handling characteristics and hastening the volumetric calorific value [72]. Practically, the lignin content in biomass is useful for binding of particles [76]. Investigations on binderless briquettes using biomass waste have employed combination of pearl millet, saw dust, coal fines, jatropha shell, spent coffee grounds, paper pulp, bagasse, cotton stalk, [77, 78], rye straw along with meadow hay or leaves in 1:1 ratio [73, 79], and were successfully reported to have approximate density ranging from 710 to 770 kg m⁻³. Therefore, briquetting may prove as an excellent and feasible option to refurbish the waste into valuable energy rather than leaving it to decompose at open dumps which may pollute the environment. Briquettes are easy to handle, can be easily stored or transported to long distances and do not cause any atmospheric crisis [68].

3.4. Hydrogen Production by Dark Fermentation

The environmental biorefinery is a unique concept where installations are designed to manufacture an extensive array of goods to increase biomass conversion. An unconventional fuel-hydrogen, is used as an energy carrier obtained from agro-lignocelluloses wastes so as to reduce dependency on fossil fuels, is being considered [80]. At present, the technology for biohydrogen production is not perfect as they are dependent on fossil fuels deviously through electricity generation [81]. Therefore, production of hydrogen by microbial action on wastes has immense potential for satisfying future energy demands through hydrogen production [82]. It is environment friendly as it decreases release of GHGs and other air pollutants. Biohydrogen can be employed as fuel for transportation in combustion engines and in fuel cells after purification for generating electricity. Its energy content per unit weight is as high as 142 kJ g⁻¹ and the only by-product yielded through oxidative combustion is water [83]. This produces hydrogen as an environmental benefactor and may certainly substitute fossil fuels. Currently 88% of the marketable hydrogen originates from fossil fuels like natural gas, coal or heavy oils [84]. Until now, hydrogen is not commercialized as an energy source, but it is used simultaneously as a chemical reactant during production of fertilizers and refining. However, in large part, the proposal for using utilization of hydrogen as energy resource has been is constrained by high energy costs, storage space demand and distribution methods [81].

To enhance the likelihood of biohydrogen as a feasible system, water electrolysis has been developed which currently supplies up to 4% of total H2 production around the world [81]. While the production of biohydrogen from biomass at a laboratory research level has garnered enormous attention, though substantial technical advancement is required for its market to turn out to be cost-effective [85]. The competent sources of biohydrogen entail water bio-photolysis through green algae and cyanobacteria, photo- fermentation via photosynthetic bacteria, and dark fermentation by anaerobic bacteria [86, 87]. Similarly, biohydrogen production through carbohydrate fermentation is a novel approach and has received significant concern in recent years [88]. The main force for exploring the hydrogen production as a substitute of methane is its superior economic value and vast relevance in the biological processes that are eco-friendly and could satisfy future hydrogen demands from various renewable and carbon-neutral biomass resources [82, 89].

Considering that agro-wastes are decomposed biologically via complex microbial ecosystems or dark fermentation using crop residues food and livestock waste.

4. Nutrient Recovery Possibilities

During last few decades, research on sustainable recovery possibilities of nutrients has got much importance. Nutrients like nitrogen (N), phosphorus (P), and potassium (K) are important for intensive agriculture and there are environmental concerns over their long-term availability and increasing costs [95]. Nitrogen is a renewable reserve whose extraction process (Haber-Bosch process) is energy intensive and its cost is dependent on availability of natural gas [96]. However, phosphorus rock is non-renewable reserve and also the chief source of phosphorus which is depleting at a progressively high rate. It has been speculated that the demand of phosphorus will surpass supply by the year 2033 [97]. Additionally, 90% phosphorus rock reserves are present in five countries viz., Algeria, China, Iraq, Morocco and Syria [88], and hence is a major challenge of food security for other nations. Similarly, prices for potassium-based fertilizers have increased four times during the years 2007–2009, and there are growing concerns for availability of potassium fertilizers in the developing economies. The reason is that their bulk distribution is found mainly in Canada and Europe and very limited distribution in rest of the world [98–99]. Therefore, other developing nations have little possibility to be self-sufficient in production of potassium through conventional fertilizers. In forthcoming decades, with the rise in global population, demand for food will increase that will create additional demand for alternative sources of nutrient recovery [100–102]. Therefore, there is an urgent need for integrated and holistic approach for recovery of nutrients. In this context, total amount of major nutrients (NPK) present in agro-wastes ranges from 40–100 kg tons⁻¹. United states alone have 9 million tons year⁻¹ of nutrients contained in agro-wastes and 74 million tons year⁻¹ around the world [99]. Table 3 [103] highlights the nutrient values of diverse potential feedstocks/agro wastes.

Table 3

Nutrient Content of Various Potential Feedstocks/Agro Wastes [103]

To recover energy from agro-wastes, biological, physical and chemical techniques have been utilized in different countries as described in Table 4 [104]. Chemical techniques are limited to phosphorus accumulation, whereas biological methods are utilized for nitrogen and potassium accumulation and physical techniques are utilized for recovering all the three nutrients. However, a number of specific studies exist regarding the assessment of different nutrient recovery and reuse systems under different scenarios, still more uniform and widespread investigations are required. Yet proper research and knowledge are lacking about life cycle energy costs of nutrient recovery and reuse systems (recovery, refining, transportation) and how such costs are compared to the use of mineral fertilizers (mining, processing, transportation).

Table 4

Nutrient Recovery Potential From Different Agricultural Waste in Different Regions of the World Nutrient Recovery

4.1. Phosphorus Recovery

Phosphorus, a vital plant nutrient is also a necessary component of fertilizers which is needed for healthy growth of plants. Phosphate rock-a finite reserve, is rapidly exhausting on a global scale as it is possessed by a few countries. Therefore, it is conclusive to recover P from agro-waste to close anthropogenic phosphorus cycle so as to promote agricultural sustainability [105, 106]. However, agro-wastes have rabeen explored for phosphorus recovery while at least 6.4% P wastage results from crop harvesting [107]. For this purpose, potential of microbes must be fully exploited as phosphorus recycling from agro-waste is in its infancy. In this direction, phototrophic and heterotrophic organisms work together to help in accumulation of vital nutrients [108]. Generally, proteobacteria like cyanobacteria, purple non-sulfur bacteria and polyphosphate accumulating organisms (PAOs) are utilized for accumulation of nutrients [109]. Presently, PAOs are most extensively utilized for accumulation of P i.e. 20–30% by weight [110] which has solids-retention time of 10 days that is called as polyphosphate [80, 111].

Another means to recover P is through struvite recovery [112]. It is helpful in recycling of vital nutrients. Different studies suggested that among different agro-waste, cattle manure is very successful in recovery of struvite [113, 114], followed by swine manure [108, 109, 115, 116], poultry manure [117], and lastly cattle urine [118]. Generally, farm wastes containing manures are abundant in P and NH₄⁺, which is advantageous for struvite recovery. However, the composition depends on various conditions like manure handling, rearing conditions, animal species, storage and treatment methods. In a similar study by Kataki et al. [119] stated that P concentration ranges from 90–600 mg L⁻¹ i.e. 90–200 mg L⁻¹ in swine manures, 100–460 mg L⁻¹ for dairy manure and 370–600 mg L⁻¹ in poultry manure.

Köse and Kivanç [120] conducted an experiment where calcium phosphate was recovered from calcinated waste eggshell. The recovery efficiency of phosphate was found to be 37.6%. From current literature, it was concluded that phosphorus can be recovered from agro-wastes at rate of 90% as calcium phosphate or 95% via struvite precipitation [121, 122]. Also, membrane technologies like osmotic membrane bioreactor (OMBR) have been utilized for biological phosphorus recovery from agro-wastes [123]. Padrino et al. [124] observed the highest rate of theoretical phosphorus recovery potential in agro-waste residues was obtained when subjected to pre-treatment with ionic liquid and thermophilic anaerobic digestion. Similarly, Yan et al. [125] studied the effect of wheat and rapeseed straw mulch on NPK recovery and was used in cultivation of hybrid rice. They found that wheat and rapeseed straw mulch increased total NPK accumulation in rice plants by 1.81–10.79%, 2.70–42.21% and 16.41–17.92%, respectively, consequently enhancing NPK utilization. Nonetheless, innovative ideas for nutrient recovery are considered for paradigm of the biorefineries, where the wastes from the one system must be entirely reinstated in the market as raw material [126].

4.2. Nitrogen Recovery

Excessive discharge of nutrients into water bodies leads to eutrophication [127] that causes water pollution and consequently leads to soil pollution. Therefore, to curb such environmental issues, recovery of nitrogen is a strategic measure. For this purpose, blue green algae (cyanobacteria) are most suitable for uptake of nitrogen [128]. Micro algae play a major part in nutrient recovery and adequate biomass is required for sufficient uptake. It is a well-established fact that uptake and reduction of NO₃⁻ is a photo-synthetically driven process in nitrogen sufficient conditions of cyanobacteria. This will implicate production of ammonium and carbon skeletons.

Other physico-chemical techniques for nitrogen recovery like ion exchange for NH₄⁺ and NO₃⁻ recovery, NH₄⁺ precipitation as struvite, NH₃ adsorption, stripping and distillation are well established [129, 130]. The best approach for N recovery depends on the concentration and form of N i.e. NO₃⁻/NO₂⁻ or NH₃/NH₄⁺. The only available technology to recover nitrate is anion exchange followed by precipitation as an inorganic salt of nitrate for application in agriculture [131]. Therefore, further research on less expensive promising technologies for nitrogen recovery definitely is considered on high priority. Conversely, other nitrogen recovery techniques are available like electrodialysis, and liquid gas stripping. Gas permeable membrane produces aqueous ammonia solution which is capable of utilization as manure or can be utilized for denoxification of emissions from exhausts in waste incinerators and power stations [132]. Further conversion of aqueous ammonia to solid inorganic fertilizers like NH₄NO₃ or (NH₄)₂SO₄ can also be achieved. In this respect, Hadas et al. [133] conducted an experiment to study the effect of decomposition rate of agro-waste (tobacco, rapeseed, rice hulls, corn and wheat residue) and nitrogen availability to soil. The results revealed that nitrogen recovery from tobacco was higher as compared to rapeseed residue. Rice, wheat and corn residues were comparatively recalcitrant; however, rice residue does not cause any nitrogen deficiency. Similarly, Fox et al. [134] conducted an experiment on dried mature tops of six legumes i.e. alfalfa (Medicago sativa L.), Fitzroy stylo (Stylosanthes scabra Vog., var Fitzroy), leucaena (Leucaena leucocephala Lam., deWit), round leaf cassia (Cassia rotundifolia Pers., var. Wynn), snail medic (Medicago scutellata L.), and vigna (Vigna trilobata L., var verde). They were introduced in soil @ 100 mg kg⁻¹ soil and found that net N mineralization after twelve weeks ranged from 47% of added N for alfalfa and 11% of added N with cassia. Two legumes i.e. Fitzroy stylo and cassia residues contained less than 20 mg kg⁻¹ of N for six weeks of the experiment. Similar study was conducted by Ladd et al. [135] on medic material residue (Medicago littoralis) that was amended with soil at 3 field sites in South Australia. Deliverables of around 189 kg N ha⁻¹ was achieved during fifteen months of decomposition. Of this 49.3% was taken up by test plant wheat), 19.7% was immobilized or persisted as fine root residue, 9.2% was left as inorganic N in the soil while 22.1% was accounted for soil-plant interaction or may be lost through inorganic N. Hence, around 6.5 kg inorganic N ha⁻¹ was provided by the amended soil with medic residues per 100 kg dry matter ha⁻¹ removed as wheat grain. However, the economic feasibility of nitrogen recovery is very low due to its high chemical cost for adjusting pH to increase free ammonia concentration i.e. NH₄⁺ to NH₃, the requirement of heat for decreasing ammonia gas solubility, ammonia stripping, and lastly because of comparatively low cost of ammonia products derived from Haber-Bosch process.

4.3. Potassium Recovery

Potassium is another important nutrient for increasing crop yield and enriching soil. It is well acknowledged that nitrogen use efficiency, productivity and resistance to drought, pests and diseases are improved by potassium. Most parts of the world are under agricultural practices that grow potassium demanding crops like sugar beet, fodder crops, vegetables, potatoes, and other commercial crops. Forms of potassium that are accessible to plants from soil are exchangeable potassium and soil solution potassium. Soil Organic Matter (SOM) present in surface layer is a crucial factor that retains sufficient quantity of potassium, but SOM is rapidly diminishing in soils of tropical region.

Adeoye et al. [136] stated that potential to utilize resource of potash from agro-wastes is available in ample quantity. They assessed thirteen agro-wastes for their potassium and other nutrient contents. They found that plantain waste, cocoa waste, water hyacinth and market waste are high in K contents. Different amendments of such wastes were applied at the rate of @ 0, 10, 15, 20, 25, 30 tons/ha in powder form together with recommended dose of NPK were applied in greenhouse experiment on Amaranthus cruentus L. used as test crop. The results obtained showed that cocoa waste and water hyacinth supported maximum crop yield and growth when treated with 10 tons/ha, respectively. Growth and yield of crop were significant (p < 0.05) with respect to inorganic NPK fertilizer. It was also detected that potassium obtained from water hyacinth was more easily available to plant as compared to other farm wastes. Potassium from water hyacinth and cocoa waste possibly prove to be economical for farmers of the developing countries to fulfill their required potassium demands.

4.4. Comlizers

Comlizers are obtained by mixing composted organic waste and ammonium sulphate fertilizers [143]. It was first developed in Ghana and has been tested on maize crop and compared with application of NPK (15-5-15) and ammonium sulphate. Plots that were amended with comlizers @ 91 kg N ha⁻¹ showed 11% higher uptake of nitrogen and phosphorus than ammonium sulphate treated plots @ 150 kg N ha⁻¹ and NPK treated plots (15-5-15). Also, the organic matter of soil amended with comlizer was 22 and 64% higher than the inorganic fertilizer and soil. Water use capacity was also 12% higher as compared to other treatments. Little threat was expressed from heavy metal and pathogens that must be considered for their elimination while practicing on larger scale. Therefore, application of comlizer improves nutrient uptake, crop yield, organic matter, and water use efficiency. Additionally, comlizers are relatively cheaper than other fertilizers available in the market [144].

5. Energy and Nutrient Recovery: An Economic Assessment

Currently, reconsideration of the effective exploitation of alternative uses of renewable resources through clean technologies is crucial. Also, it is essential to overcome the huge dependency on petroleum for liquid fuels. The selection of suitable feedstock and processes for generating clean energy and essential nutrients must be such which consume lesser electricity, generate less emission in total, and have low adverse human health impact [145]. Considering these factors and to fulfill the present-day energy and nutrients’ need, lignocellulosic biomass obtained from agro-waste is considered as a most potential material. Furthermore, from the biorefinery concept, discovery and characterization of new enzymes through advanced technologies led to low-cost transformation of lignocellulosic biomasses into bio-fuels, bio-chemicals and bio-fertilizers [146–148].

5.1. Economic Perspective on Energy Recovery

The manufacturing of any product must be economically viable otherwise; marketing is not possible even if they are made from renewable resources. Time to time economic analysis has been carried out by many researchers and scientists on the potentiality of agro-residue in terms of energy recovery and valuation of their bio-products. Ethanol and bio-diesel are the most well-known forms of agro-fuels for gasoline and diesel substitution, respectively. In Brazil (2010), Portugal-Pereira et al. [60] reported that 8% of total electricity consumption was fulfilled by the bioenergy which is approximately equal to 39 TWh. Around 9 GW energy productions were estimated from agro-wastes in Brazil considering the average annual availability factor of 50%. On the contrary, another study by Dassanayake and Kumar [149] indicates that electricity generation in Alberta by direct combustion of triticale straw is not reasonable at any plant scale as compared to coal based power generation.

They concluded that even at very low power cost of $ 71.57/MW, triticale straw-based energy plant is not considered economically viable. Moreover, the presently available conversion technology is less efficient and logistics causes the triticale straw-based bio-energy generation costlier in comparison to fossil fuel-based energy; however, it could become competitive with carbon credits.

Seabra et al. [150] carried out the economic analysis that involved evaluation of the minimum ethanol selling price (MESP) of sugarcane residual biomass. The total investment calculated for the biochemical conversion plant is around 152 M$ leading to final MESP values of 318 $ m⁻³ for biochemical conversion while for thermochemical conversion the total value cost was 127 M$ and MESP value of 329 $ m⁻³ was achieved. Thermochemical conversion. Total electricity generation from biochemical conversion is 50 kWh t⁻¹ of cane of electricity surplus while from thermochemical conversion the values are reduced to 32 kWh t⁻¹ of cane for electricity. However, the residues that cannot be converted into ethanol through biochemical conversion could lead to provide power at 557 kWh m⁻³ ethanol. Via biochemical route, the high amount of residues that cannot be converted into ethanol leads to a high potential to export power, at 557 kWh m⁻³ ethanol. Electricity would be an important co-product of this biorefinery plant, for bio chemical conversion, with surplus of about 50 kWh t⁻¹ of electricity from cane.

Similarly, Yoosin and Sorapipatana [151] stated that the production costs of the bio-ethanol are quite variable due to highly volatile nature of the raw materials. and concluded that in general, of the total bio-ethanol production costs feedstock contributes 60–75%. The cost of bio-ethanol from sugar cane is US$ 0.23–0.29 per liter [152], whereas sugar and corn-based bio-ethanol in EU and the United States are reported to be $ 0.29 per liter [153] and US$ 0.53 per liter [154], respectively. The bio-ethanol production costs are commonly higher based on the energy content. Sugars and corn-based ethanol plants affect food production and also its production cost is higher which makes it difficult to become cost competitive with fossil fuels. Because of this serious economic concern, now majority of the industries are focusing on scientific research to develop economically viable lignocellulose-based biofuel energy plant.

5.2. Economic Perspective on Nutrient Recovery

The sustainable production of fertilizers, particularly NPK, are quite challenging in this century. To avoid toxic effect of chemically produced fertilizers on soil health, the biologically treated organic wastes are the best option which could augment the soil fertility. The best way to utilize large volume of nutrient rich agro-wastes is by recycling through appropriate biological processes like composting, vermicomposting or anaerobic digestate [155, 156]. Bekchanov and Mirzabaev [157] evaluated that recycling of agro-wastes through composting causes reduction of expenses from US$ 357 to 197 million in total agro-waste management and fertilizer usage cost. However, investment costs of operating an anaerobic sludge composting plant (capacity of 7.12 × 106 kg) is around € 462,646 and annual cost ranges between € 250,000 – 360,000. Similarly, Dockhorn [158] assessed operating and maintenance costs of a phosphorus recovery plant from struvite (capacity 350,000-person equivalents (PE)). They concluded that with 50 mg L⁻¹ PO⁴-P concentration, € 2800 ton⁻¹ of struvite was achieved; however, with 800 mg L⁻¹ PO⁴-P concentration, only€ 520 ton⁻¹ was attained. In a similar study, Battistoni et al. [159] estimated total profits between € 7,800–89,400 year⁻¹ from struvite production at operating costs of € 0.19 – 0.28 m⁻³ digestate. However, Blumenstein et al. [160] reported that from an investors’ perspective composting of grasses from semi-natural grassland of Germany, Wales and Estonia was not profitable in comparison to generation of solid fuel and biogas from the biomass. In a precise case study, capital and operational costs for pre-treatment of anaerobic digestate, were estimated between 5.40 and 6.97 € m⁻³ by Bolzonella et al. [140]. However, the economic valuation of any plant is a complicated process due to many uncertainties on investments and operating costs. Further, research to evaluate risk and optimization of combination of technologies is needed.

6. Strategic Action Plans for ENR

6.1. Regulatory Coherence and Policies

Society is well aware of the problems generated due to anthropogenic activities. To tackle these problems, collective legislative actions and policies need to be framed. There are considerably large number of directives and regulations concerning ENR from agro-wastes. The legal framework on energy guarantees certainty to relevant stakeholders, administrative authorities, waste producers, disposal companies and citizens to know their roles and obligations. Appropriate framework is required to reach the objective of sustainability. Moreover, regular monitoring and legal binding is necessary for regulating emission standards and other environmental prescriptions.

However, there is a great range of corresponding regulation at national and regional level which can hinder entrepreneurial impetus, investment and knowledge transfer. Similarly, for greater uptake, recovery and reuse of nutrients and development, regulations governing the sectors need certain coherence between the member states. For instance, the European Union (EU) has created a significant assembly of regulation impacting nutrient use. The main elements that are to be regulated are consequential treatment of organic waste, their utilization in crop and livestock farming, nutrient market, and secondary raw materials arising from this complex chain [161].

Owing to substantial financial risk, poor implementation, and gaps in regulation requires primary concern. However, the problems have patently not been resolved despite collection of attempts have been made to deal with them. Therefore, full techno-economic assessment of local, regional and global recovery of energy and nutrients must be backed up by decision assistance tools.

Regulatory policy scheme of many countries promotes renewable energy technologies (RETs) and it is getting great importance now-a-days. Two most standard regulatory policies are Feed-in-tariff (FIT) and Renewable Portfolio Standard (RPS). Mostly, FIT and RPS are emphasized because they denote most widespread regulatory promotion policies [162, 163]. Theoretically, FIT is based on the price regulation policy in which producers of power is paid a fixed rate or premium for their electricity, irrespective of how much they can generate. Generally, for a definite duration of time the prices are fixed, however, the tariff may change with time. RPS is based on quantity regulation policy and valuation depends on market condition which decides the price for power generated. In this connection, governments fix targets or quotas to confirm that a certain market share of capacity or generation of electricity produces from renewable energy sources [164]. Earlier research work concluded that FIT is a better promoter of power capacity development than RPS [162, 164, 165 ]. However, these research conclusions are made on the basis of comparative case studies that lack vigorous empirical test.

Langniss and Wiser [166] studied the implication of national and sub-national policies. In the way of power development projects, they emphasized that FIT can make a stable and profitable market whereas in case of RPS there is a lack of surety in the market assurance and industrial profitability, henceforth, it provides fewer progresses in power generation. However, in previous studies by Mitchell [167] and Menanteau et al. [168] indicated that RPS is more liable to incentivize competition between different renewable technologies whereas FIT does not inspire innovation since it promises stable prices to producers. Other studies from the environmental and economic aspects, market-based approach is considered as superior to a non-market system in promotion of technological change [169–172]. From the energy point of view, RPS is categorized as market-based policy whereas FIT is focused on price control by government. Therefore, RPS appears to be more ideal according to this standard of comparison.

6.2. Recommendations

Following recommendations regarding framing of policies can be adopted:

Technologies to lower the cost of implementation of energy recovery plants must be encouraged.
The cost and financial aspect must be critically assessed to encourage energy plants in developing nations.
Advancement in emission control technologies from energy generation through wastes should be taken into prime consideration.
Technical specifications, emission standards, suitable waste, extraction process for energy as well as cost benefit analysis must be clearly defined.
Frequent interaction with environmentalists, stakeholders, municipality and local people must be done for operational health and safety measures.
Adequately qualified and skilled staff members must be provided for effective enforcement and regular inspections.
Manage nutrient cycle in a resource efficient way by promoting sustainable management of agricultural ecosystems.
Governments need to end harmful farming practices of intensification and switch to sustainable agricultural practices.
Tax must be levied for excessive pesticides and fertilizers usage.
Polluter pays principle must be implemented for ensuring cost recovery.
For overall release of nitrogen and other nutrients, local targets must be set keeping in view the ecological limits.
Regular monitoring of quality standards, both for energy and nutrient recovery strongly implemented.

7. Conclusions

The recovery of energy and nutrients from agro-waste must be ensured that they are consistent in competition with current technologies [175]. Amalgamation of different technologies may help in enhancing the use of agro-wastes for centralized ENR. However, socio-economic, logistical, regulatory, political etc. barriers create a gap between the sustainable, technical and economic potentials of energy and nutrient recovery. Presently, for fertilizer industries, new prospects arise to move from inorganic fertilizers, to organic fertilizers that augment soil health. Synchronization of small and medium farmers into cooperatives as well as application of energy service companies (ESCOs) for financing in ENR would lessen operational costs. Also, commercial incentives such as FIT and RPS would reassure stakeholders to expand their activities together with bioenergy in their business portfolio. Overall, development of respective technologies for ENR from agro-wastes must be implemented for the secondary market. Thus, this paper outlined the importance of energy and nutrient recovery of higher quality from agro-wastes that counteracts present problems of wasteful energy use, environmental degradation and nutrient resources depletion.

Acknowledgement

The authors are thankful to the Dean & Head, Department of Environment and Sustainable Development and Director, Institute of Environment and Sustainable Development, Banaras Hindu University, for providing necessary facilities. RPS is thankful to Department of Science & Technology for providing financial support (DST-SERB P07-678). BV is also thankful to Council of Scientific & Industrial Research for awarding Senior Research Fellowship.

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Lignocellulogic Materials	Lignin (%)	Hemicellulose (%)	Cellulose (%)	Ref.
Wheat Straw	16.00 – 21.00	26.00 – 32.00	29.00 – 35.00	[21]
Sugar cane Bagasse	20.00	25.00	42.00	[22]
Corn Cobs	15.00	35.00	45.00	[23]
Sweet Sorghum	21.00	27.00	45.00	[22]
Corn Stover	19.00	26.00	38.00	[24]
Hard Wood	18.00 – 25.00	24.00 – 40.00	40.00 – 55.00	[25]
Rice Straw	18.00	24.00	32.10	[23]
Soft Wood	25.00 – 35.00	25.00 – 35.00	45.00 – 50.00	[25]
Nut Shells	30.00 – 40.00	25 – 30	25–30	[26]
Grasses	10.00 – 30.00	25.00 – 50.00	25.00 – 40.00	[25]
Bagasse	23.00 – 33.00	16.52	54.87	[24]
Banana Waste	14.00	14.80	13.20	[28]
News Paper	18.00 – 30.00	25 – 40	40 – 55	[26]
Sponge gourd Fibre	15.00 – 46.00	17.44	66.59	[27]

Energy Recovery

Country	Region	Income	Population	Biomass Waste generation (tonnes/day)	Biomass energy source	Units	Potential Energy/bioelectricity		By the year	Ref.
Algeria	MENA	LMI	19,225,335	23,288	Biomass	Mtoe	3.7		-	[90]
Brazil	LCR	UMI	144,507,175	149,096	Sugarcane biomass	TWh	62 – 93		2009	[91]
Finland	OECD	HIC	3,301,950	7,030	Agricultural solid waste	PJ/yr	91		2025	[92]
Germany	OECD	HIC	60,530,216	127,816	Agricultural solid waste	PJ/yr	200		2025	[92]
Norway	OECD	HIC	3,605,500	10,082	Agricultural solid waste	PJ/yr	17		2025	[93]
India	SAR	LMI	1,358,137,719	97,192	Rice straw	PJ/yr	311.6		2002	[94]
Thailand	EAP	UMI	69,266,999		Rice straw	PJ/yr	237.5	At 100% collection efficiency
Philippines	EAP	LMI	104,256,076		Rice straw	PJ/yr	141.8	At 100% collection efficiency

Feedstock/Agro Waste	Nitrogen (%)	Potassium (%)	Phosphorus (%)
Cereals (wheat)	0.1	0	0
Corn Cob	1.38	0.46	0.09
Corn Grain	2.15	0.42	0.34
Corn Leaves	1.30	1.48	0.21
Corn Silage	1.30	0.9	0.16
Corn Stem	0.84	1.23	0.09
Rapeseed Crop Biomass	3.3–5.4	0	0
Sugarcane Bagasse	0.73	0	0
Wood	0.07–0.39	0	0
Water Hyacinth	1.96	0	0
Pine	0.1	0	0
Peach Leaves	0	2.45	0.14
Giant Brown Kelp	1.22	0	0

Country	Region	Income	Population	Biomass waste used	Proportion Used	Method used for nutrient recovery	Nutrient recovered	Year	Ref.
Czech Republic	ECA	HIC	10,628,906	Apple pomace waste	25%, 50%, and 75%.	Vermicomposting	N = 2.8%, P = 0.85%, K = 2.3%,	2014	[137]
India	SAR	LMI	1,358,137,719	Coconut husk	80%, 90%, 100% coconut husk with poultry manure and pig slurry	Vermicomposting	N = 1.27 g kg⁻¹ P = 1.4–9.7 g kg⁻¹ K = 7.7–10.5 g kg⁻¹	2016	[138]
Iran	MENA	UMI	82,565,364	Crop Residue (Rice, corn, wheat and sugarcane)	5 kg of substrate included with 200 adult epigamic earthworms	Vermicomposting	-	2015	[139]
Italy	ECA	HIC	61,838,227	Agriculture waste	-	Anaerobic digestate: drying with acidic recovery, stripping with acidic recovery and membrane separation	Drying: N = 6.8 gN/L; P = 0.8 gP/kg Membrane Technology: N = 3.35gN/L, P = 1.64 gP/L, Stripping process: N = 3.6 gN/L P = 0.5 gP/L.	2017	[140]
Germany	OECD	HIC	82,392,809	Digestate	10 – 50 L of digestate	Membrane-based treatment chain	N = 4.4 kgNtotal·t⁻¹ P = 1.95 kgP₂O₅ t⁻¹ K = 3.98 kgK⁺	2018	[141]
Australia	EAP	HIC	24,987,583	Agro-industrial residues	-	-	N = 246 kt N, P = 88 kt P, K = 359 kt K TOC = 32.2–52.8% decrease	2010	[95]
India	SAR	LMI	1,358,137,719	Rice straw	1 Kg	Vermicomposting	N = 10.56 and 18.58 g/kg K = 1.1–2.6 times higher with increment of 169% P = 36–166%	2018	[142]
Germany	OECD	HIC	82,392,809	Agriculture waste	-	Membrane filtration	8 kg t⁻¹ N total, 5.5 kg t⁻¹ P₂O_5, 10 kg t⁻¹ K₂O	2018	[141]