Introduction

Increased per capita energy usage due to overexploitation of natural resources, combined with an ever-increasing global population, leads to instability in our ecological systems (Sadh et al. 2018). Every year, roughly 500 million tonnes of agricultural residue are produced, of which 18.4% (92 million tonnes) is burned in India, which is more than the total waste generated in Bangladesh, Indonesia, and Myanmar combined (NPMCR 2019; Jeff et al. 2017). Crop residues generated in large volumes after harvest pose major challenges for policy makers and farmers in India. Improper disposal of these crop residues have resulted in environmental pollution, greenhouse gas (GHG) emission, climate change, as well as a negative impact on human and animal health (Sadh et al. 2018; Bharathiraja et al. 2017; Bos and Hamelinck 2014). As a result, there is an imperative need to explore viable recycling options in order to address these issues. The agro-industry, particularly the food industry, contributes significantly to waste generation, resulting in environmental pollution, climate change, and economic development deterioration (FAO 2019). According to Caldeira et al. (2019), the European Union generates over 30 Mt of inedible food waste each year, which is projected to increase dramatically with fast growing food processing industry. Reduction of agro-industrial waste to 30–50% can enhance the available food supply to 15% (Wanga et al. 2016). The need of developing valorisation pathways is highlighted by the United Nations’ pledge on Sustainable Development Goals (SDGs) to halve per capita food waste in supply chains, retail, and manufacturing. According to Kurnik et al. (2018), the valorisation of agro-industrial waste is gaining momentum not just due to a necessity, but also due to recent breakthroughs in engineering approaches for turning waste into valuable products. Prasertsan et al. (2011) proposed four possible ways for agro-waste management: (a) waste minimization or waste reduction, (b) waste conservation, (c) waste segregation and (d) waste utilization (reuse, recovery/recycling). These management options, however, will necessitate not just engineering solutions, but also business models to implement them. There has been a significant shift in global perceptions towards the use of agro-industrial waste for resource conservation.

Given bio-technological interventions, the potential of agro-industrial wastes can be completely realised in a variety of ways, including the development of numerous value-added and by-products such as bio-fuel, bio-products, biodegradable plastics and organic fertilisers in the form of manure compost etc. As a result, this review article will concentrate on the valuation, classification, and environmental effect of agro-industrial wastes, as well as their present level of knowledge and possible applications. Therefore, focus will be on the valorization, categorisation, and environmental impact of agro-industrial waste, as well as their current state of art and potential applications.

Categorisation of agro-industrial wastes

Annually, India produces 187.7 million tonnes of milk, 296 million tonnes of fruits and vegetables, 515 million livestock, 15.2 million tonnes of fish, and 532 million tonnes of poultry. In India, about 45% of large and medium scale industries use agricultural products as raw materials (MOFPI 2020). Large volumes of stalks, shells, husks, rinds, scales and waste water generated during the processing and refining of raw materials are rarely used as animal feed or manure, can be further refined to make usable and marketable products.

Agro-based industries can be grouped into three different segments i.e. plant-based, animal-based food, and non-food based agro-industrial wastes (Table 1). Plant based wastes can be further divided into field residues and process residues. Field residues are the waste materials (leaves, stalks, seed pods, and stems) that remain in the field after the crop has been harvested. Some of these wastes are used on farms as animal bedding and feed, as well as for various horticultural uses. The processed residues, on the other hand, are the wastes generated after processing and refinement of raw produce. By-products of grain-flour mills, sugar industries, fermentation-based industries, food and fruit processing are the most common plant-based agro-wastes (Table 1). On the other hand, huge amounts of organic residues from the fish, poultry, and meat processing sectors are stacking up, containing perishable proteins that have the potential to cause public nuisance through foul smell. Non-food based agro-industries produce effluents that are largely biodegradable in nature. The effluents are non-toxic except in the tannery and textile industries. The majority of these wastes go unused or untreated, posing a threat to the environment as well as human and animal health.

Table 1 Potential by-products of various agro-based industries
Full size table

Environmental impact of agro-industrial waste

Untreated and huge amounts of agro-industrial wastes are a major concern in every country, and it is becoming worse by the year (Sadh et al. 2018). The agro-industrial waste and effluents are usually discharged on land or into water bodies. These have variable chemical characteristics and metal contents that may prove harmful to environment. Uncontrolled burning of agro-industrial waste releases toxic (nitrogen oxides, SO2, respirable particulate matter), carcinogenic (dioxins, furans, polycyclic aromatic hydrocarbons), and greenhouse gases (CH4, N2O) as well as smoke, contributing to significant haze, global warming and detrimental to human health (Sharma et al. 2020).

A few agro-industrial wastes, such as those from pulp and paper mills, textile mills, contain hazardous contaminants that can pollute air, water, and soil (Gupta and Shukla 2020; Paritosh et al. 2017) Organic and inorganic compounds present in the pulp and paper industry waste have been shown to have negative effects on aquatic ecosystems (mutagenicity, carcinogenicity, and endocrine disruption) (Servos 2020). Agro-industrial waste is normally high in nutrients, but if left untreated, it can become a source of pathogenic diseases (Ravindran et al. 2018), herbicide or pesticide residues, harmful faecal coliform bacteria (James 2020). The repeated application of such wastes without any treatment build ups pesticide and herbicide residue in soil, which can be harmful to beneficial soil microbes (Ramírez et al. 2019). Runoff from nutrient-rich wastes, such as those produced by the fertiliser industry, poultry, and aquaculture, can induce eutrophication, resulting in massive algal blooms and a disruption of the aquatic ecosystem (Smith et al. 2016; Withers et al. 2014; Merel et al. 2013). Those wastes are sometimes water soluble, end up in drinking water and eventually into the food chain, causing severe health problems in humans such as Parkinson's disease, cancer, birth defects, Alzheimer's disease, and reproductive problems (Kim et al. 2017). Antibiotic residues present in animal-derived wastes such as milk, meat, and eggs have a detrimental effect on community health as well as food safety in terms of carcinogenicity, drug toxicity, allergic responses and immune-pathological illnesses (Costa et al. 2019; Manyi-Loh et al. 2018). The pollution load of various untreated agro-industrial wastes is mentioned in Table 2. The solution to waste disposal and related environmental challenges is to repurpose waste to make valuable products (Yusuf 2017). As a result, effective disposal and economic exploitation of agricultural wastes not only mitigates pollution but also leads to long-term sustainability.

Table 2 Waste water and pollution load of agro-based industrial waste
Full size table

Valorisation of agro-industrial waste

Agro-industrial waste can be converted to valuable products majorly by either thermochemical or biochemical pathways. This section includes significant highlights for each of the major food groups, including meat, dairy, fish, eggs, fruits, vegetables, cereals, sugar crops, oil crops, and tubers.

To use agro-industrial waste as a feedstock, preliminary operations like drying (< 50% moisture) and chopping are inevitable, making the conversion process expensive. In combustion process, the feed stock rich in cellulose and hemicellulose is subjected to temperature range 800–1000 °C in presence of air. Fouling and slagging are typical in low carbon biomass due to the reaction of inorganic constituents. The exact mechanism behind fouling and ash slagging, however, remains unclear. Oxygenated compounds from combustion of cellulosic biomass were successfully used in few studies to run a compression ignition engines (Baumgardner et al. 2015). Thermal decomposition of biomass to solid biochar, liquid bio-oils and gas products in the absence of oxygen is known as pyrolysis. Based on heating rate and residence time, pyrolysis is of three types: slow, fast and flash pyrolysis (Biswas et al. 2017). Slow pyrolysis is conventionally used in production of low-grade charcoals from crop residues at 300–700 °C/h or even days. Fast pyrolysis is carried out (103–104 °C/s) with short residence time (Amutio et al. 2012). In terms of bio-oil yield, the conversion efficiency of fast pyrolysis is 15–20% greater in flash pyrolysis than in fast pyrolysis. Carbonization is used to relatively increase carbon content and calorific value of raw materials with 60–70% weight loss (Meyer et al. 2011). Carbonation is a slow process of heating biomass to drive out the moisture and volatile matter under an inert or low-oxygen. In opposite to carbonation, this process of gasification converts biomass into low calorific value combustible gases of energy 3.6–6.8 MJ/Nm3 by partial oxidation process at 700–800 °C. It is one of the suitable methods for production of producer gas. The selection of gasifier depends on chemical composition of biomass feedstock and energy density (Alauddin et al. 2010). Hydrothermal Conversion includes thermal depolymerisation of biomass at high temperatures and pressures. Water provides exceptional reaction environment as hydrothermal media acts as catalyst, especially under acid or base catalyzed conditions (Kruse et al. 2013). Depending on the severity of the conditions, it can be divided into distinct processes: (1) hydrothermal carbonization (below 247 °C)—it mimics natural coalification process in subcritical water giving out peat or hydro-coal (Toor et al. 2011), (2) hydrothermal liquefaction (247–374 °C)—a combination of hydrolysis and de-polymerization of macromolecules forming bio-oils (Toor et al. 2011), (3) hydrothermal gasification (above 374 °C) triggers the process forming higher fuel gas mixtures.

Agro-industrial wastes are wet biomass and majorly containing cellulose, hemicellulose and lignin. They can act as constant source for production of value-added chemicals by hydrothermal conversion. Factors like composition of feedstock, temperature and hydrothermal media highly affects conversion efficiency of agro-industrial waste. Cellulose and hemicellulose constitute 25–40% of plant biomass. Cellulose rich feedstocks subjected to hydrolysis followed by depolymerization process was given by Lorby de Bruyn–Alberda van Ekenstein. The process is pH sensitive. Acidic environment generates more 5-hydroxy methyl furfural (5-HMF) and its acid derivatives; alkaline conditions lead to retro aldol production, hydration-dehydration, reshuffling of –CHO, to form simpler acids and alcohols (Yin and Tan 2012).

Factors influencing valorization process include (1) type of feedstock: hemicellulose with its simple structure and more side-groups are highly soluble and hydrolyzed both in acidic and alkaline conditions at 180 °C. Lignin in its complex structure contains p-hydroxy phenyl propanoids viz., sinapyl, coniferyl and coumaryl alcohols. Unlike cellulose and hemicellulose, it is more resistant to both enzymatic and chemical breakdown. Lignin undergoes hydrothermal hydrolysis in an alkaline environment and produces a variety of phenols; (2) temperature: based on type of the feed stock, the temperature varies in between 250 and 370 °C. Increase in the operating temperature results in depolymerization reaction and further dissociation of complex bonds (Dimitriadis and Bezergianni 2017); (3) solvent to feed ratio: it is important to note that solvents improve the stability and solubility of the fragmented macromolecules. Solvent to feed ratio is assessed in such a manner to cease gasification of all leftovers in the reactor. Some studies show that when a portion of water replaced with compounds like ethanol, methanol, acetone, 2-propanol, they act as a tarry material enhancing the ionic product of the mixture yielding higher bio-oil.

Table 3 primarily throws light on noteworthy research completed in recent past in the field of valorisation pathways for agro-industrial products/waste. This provides clear indication that there is scope for developing multi-feedstock bio-refinery in which similar value added product may be obtained from a wide range of resources reducing the pressure on supply chain and reliance on single source of food waste (Rood et al. 2017). Donner et al. (2020) presented circular business model valorisation pathways for agro-waste. The author’s value pyramid emphasised that new products should be inexpensive to customers and provide uppermost possible added value, as indicated in the value pyramid. Thus, there are diverse valorisation prospects in various sectors which can be developed as new products and applications with varying value addition (Fig. 1).

Table 3 Summary of valorisation for various key agro-industrial waste
Full size table
Fig. 1
figure 1

Schematic illustration of valorisation pathways for agro-industrial wastes, effect on soil health and co-benefits

Full size image

Approaches for value addition of agro-industrial waste

The agro-industrial waste are nutritionally very rich and contain vitamins, fibres, lignin and cellulose, proteins, lipids, polyphenols, pectin, and sugars (Barcelos et al. 2020). The majority of lignocellulose-rich agro-industrial waste are not properly valorized and are left to degrade (Ramírez et al. 2019). Lignocellulose residues are the abundant renewable resource on planet earth (Seidavi et al. 2019), with a potential to be transformed into biofertilizers, biofuels (Ansi 2017) and value added products. They can also be used to produce enzymes, mushrooms (Kumla et al. 2020), and as a substrate for many other important products. Currently, the production of bioethanol and related liquid fuels by microbes utilising agro-industrial waste as raw materials is triggering a snowball effect in biofuel production.

Biorefineries

Bioeconomy and biorefinery are the new concepts emphasized by agro-industries where one industry runs by using the waste material of another industry (Acevedo et al. 2020; Ravindran and Jaiswal 2016). Biorefinery naturally includes a complex, integrated network of physical and chemical conversion processes, such as mechanical and physical biomass pre-treatment, pyrolysis, catalytic and enzymatic reactions and downstream purification procedures. They facilitate sustainable conversion of biomass into energy and chemicals viz., pharmaceutical constituents, plastics and food additives. For instance, a lignocellulosic feedstock (LCF) biorefinery that produces ethanol, succinic acid, acetic acid and power has been found to be cost effective and eco-friendly (Luo et al. 2010). The lignin-enriched residue in the refinery may be utilized as a feedstock for chemicals and materials or for on-site electricity generation by integrating geothermal heat into a biochemical lignocellulosic biorefinery (Sohel and Jack 2010).

The biological nature of these approaches makes them require far less energy inputs, reduces maintenance costs, and minimizes ecological disturbance. From a sustainability viewpoint, this is a far better option than other mechanical or chemical treatment methods. This requires social cohesion and integration among policy makers, researchers, technology developers, project developers, and society (Li et al. 2018).

Biofuels

Conversion of biomass to fuel is not a new concept, and direct combustion of solid biomass is the most common approach, but it is also inefficient owing to low density and incomplete biomass combustion. Various types of agro-industrial residues e.g. rice straw, sweet potato waste, sawdust, potato waste, corn stalks, sugarcane bagasse, and sugar beet waste have been used for the production of biofuels (Kumar et al. 2014, 2016; Duhan et al. 2013). The major one of them is bioethanol, which is produced almost entirely by sugarcane molasses in India. In today's world, biofuels are produced in one of two ways: first-generation or second-generation, with the latter being preferred owing to the added benefit of waste utilisation (Ramos et al. 2016). For example, research into cultivation of Rhodococcus and Yarrowia on agro-waste/industrial biomass pre-treatment waste streams to produce second generation biodiesel is underway (Le et al. 2019).

Bioethanol is produced in billions of litres across the world due to technical advancements and raw material availability (Agrawal et al. 2019). Availability of waste is in abundant in the developing countries for the production of biofuels. Bioethanol can be produced from vegetable wastes such as potato peels, carrot peels, and onion peels by fermenting them with Saccharomyces cerevisiae (Mushimiyimana and Tallapragada 2016); or by treating banana pseudo stems with Aspergillus ellipticus and Aspergillus fumigatus (Ingale et al. 2014). Maiti et al. (2016) utilized Clostridium beijerinckii to produce butanol from agro-waste (Maiti et al. 2016). The use of wastes such as spent coffee grounds to produce biofuel reduces investment and production costs (Banu et al. 2020).

Microbial enzymes

Many successful efforts have been made utilizing biotechnology as a tool to get valuable products, such as enzymes, from lignocellulosic materials for industrial uses at a low cost (Leite et al. 2016). Enzymes including chitinase, amylase, phytase etc. are biocatalysts used in variety of industrial processes and can be produced from microbial fermentation of agro-industrial waste. For example, enzyme keratinase produced using keratinous waste from meat producing sectors (Preczeski et al. 2020). The use of solid state fermentation technology (SSF) for bioconversion of agro-industrial waste has been proved to be both eco-friendly and economical (Marzo et al. 2019). In this technology, lignocellulose residues are hydrolyzed enzymatically into fermentable sugars, and then into a marketable product. SSF is advantageous over conventional fermentation technology due to its closeness to nature and simplicity (Ravindran et al. 2018a). Various studies have been reported using agro-industrial waste as a substrate (Carbon source) in SSF with different micro-organisms, such as wheat bran for production of gibberallic acid using Fusarium monoliforme (Prema et al. 1988); oat straw for lignin degradation using Polyporovs spp. (Bone and Munoz 1984); corn and soya for mycotoxin production using Aspergillus flavus (Hesseltine 1972) and various moulds (Bhumiratna et al. 1980); oat cereal using Rhizopus oryzae for lactic acid (Koutinas et al. 2007); Agro distiller grains using Aspergillus niger for citric acid (Prado et al. 2004) etc.

SSF based enzyme production is yet another microbial biotechnological intervention (Adrio and Demain 2014) that is gaining popularity and lowering production costs (El-Naggar et al. 2014). SSF can produce xylanase and exo-polygalacturonase (Marzo et al. 2019), lignocellulosic enzymes (Kumla et al. 2020), polygalacturonase and pectin methylesterase (Patidar et al. 2018), cellulase (Abdullah and Greetham 2016) etc. Cellulases, xylanases and ligninases and cinnamoyl esterases are produced using agricultural and agro-industrial by-products through SSF processes (Abdullah and Greetham 2016). SSF was used for Aspergillus awamori to produce amylase and glucoamylase, with field residues such as rice bran and wheat bran as substrates (Suganthi et al. 2011). A. niger MTCC 104 has been reported for production of α-amylase employing SSF (Duhan et al. 2013; Kumar et al. 2016). Buenrostro et al. (2013) produced Ellagitannase, an enzyme used for biodegradation of ellagic acid and ellagitannins, using four different agro-industrial wastes, including sugarcane bagasse, corn cobs, candelilla stalks, and coconut husks, where corn cobs delivered the best results as a substrate. Oil cakes, particularly palm kernel oil cakes, were also employed as substrates for the manufacture and optimization of lipase enzyme by Aspergillus ibericus (Oliveira et al. 2017). Similarly, Saharan et al. (2017) studied the release of polyphenols and antioxidants using various enzymes e.g. α-amylase, xylanase, and β-glucosidase during SSF of cereals. Likewise, fermentation of peanut press cake with Aspergillus oryzae led to a significant increase in key enzyme activities for industry, such as α-amylase, β-glucosidase, lipases, and xylanase (Sadh et al. 2017). Table 4 gives an overview on production of enzymes from various agro-waste.

Table 4 Use of agro-waste to produce various enzymes from micro-organisms
Full size table

Single cell protein production (SCP)

Bioconversion of agro-industry wastes produces a very high quality of protein which is economical and nutritionally valuable (LaTurner et al. 2020). The cost of SCP production can be dramatically reduced by using low-value agro-wastes, such ascitric waste, yam peels, pineapple cannery effluent, corn stover, whey concentrates, soy molasses, rice effluent and hydrolyzed sugar cane bagasse (Aruna et al. 2017). Mondal et al. (2012) studied the synthesis of SCP by S. cerevisiae from fruit waste fermentation waste, especially cucumber and orange peels, with cucumber peels producing more protein. Several studies have shown the production of protein-enriched feed using SSF technology (Rompato and Somoza 2015; Mahawar et al. 2012), as well as silage making and nutrient enrichment of agro-industrial waste (Silva et al. 2016; Alemu 2013; Nasir and Butt 2011). On the one hand, this can handle the problem of animal feed, while on the other hand, it can address protein enrichment. If the right technology are put in place, questions about agro-industrial waste valorization and protein energy malnutrition (PEM) can be addressed.

Antibiotic production

Agricultural wastes, such as maize cobs and sawdust, are used to make a variety of antibiotics. The utilisation of a low-cost carbon source derived from agro-industrial waste led to a significant reduction in the production cost of antibiotics, such as neomycin and rifamycin (Vastrad and Neelagund 2011). Streptomyces rimosus was used to make oxytetracycline using groundnut shell as a raw material (Tobias et al. 2012). Synthesis of extra cellular rifamycin B using oil cake as a raw material and Amycolatopsis mediterranean MTCC 14 by SSF was studied. Coconut oil cake and ground nut shell are reported to show maximum antibiotic production in comparison to other agro-industries waste (Arora et al. 2017).

Biodegradable plastics

Plastics have a wide range of applications in today’s world due to their versatility and flexibility. Many single-use plastics, like disposable bags, cutlery, and wet wipes, are among the worst environmental offenders. Biodegradable plastic can be produced using various kinds of agro-industrial wastes, such as banana/fruit peels, cassava starch, cellulose, corn, wheat straw and rice straw (Mostafa et al. 2018; Broeren et al. 2017). Cassava is a useful source of starch for bioplastic synthesis since it is non-toxic, biocompatible, low-cost, and renewable carbon-rich organic raw material (Nanang and Heru 2018). Other starch-rich raw materials for biodegradable plastic production include potato starch (Karana 2012), potato peel (Spiller et al. 2020; Ezgi and Duygu 2019), and maize (Nasir and Butt 2011). Because they are readily available, nutrient-rich, and easily decomposable by bacteria, they are the best options for bioplastic production. Bacillus licheniformis and Bacillus megaterium produced polyhydroxybutyrate utiling wheat straw (Gasser et al. 2014).

Biocompounds: biostimulants, biocomposites and bioactive peptides

Many phytochemicals used in food, cosmetics and medicines have been identified in the barks, shells, husks, leaves, and roots of various fruits, vegetables and crops (Shirahigue and Antonini 2020; Usmani et al. 2020; Yusuf 2017). One such example are polyphenols, such as flavonoids, tannins, anthocyanins, and alkaloids. Phenolic compounds found in the plants have been used as food preservatives. Simple phenols like cresol, hydroquinone, and gallic acid are found in wood smoke used for food preservation (Quinto et al. 2019). Lampronti et al. 2013 extracted polyphenols (apigenin, oleuropein, and cyanidin chloride) from olive mill waste water and demonstrated their efficacy in cystic fibrosis cells by inhibiting NF kappa B/DNA complexes. NF kappa B activity is responsible for inflammation in cystic fibrosis, osteoporosis, rheumatoid arthritis and cancer. It is feasible to extract phenolic compounds using green technologies such as microwave or ultrasound assisted extraction (Panzella 2020). Another class of compounds generated from waste are biostimulants. The use of biostimulants improves plant nutrient use efficiency, potentially reducing the need for chemical fertilizers. These originate from organic waste streams, comprising vermicompost, sewage sludge, protein hydrolyzates, chitin and chitosan. If the valorization chain is well established, bio stimulants produced from by-products can be more widely employed (Xu and Geelen 2018).

Many biocomposites derived from agro-industrial wastes are structural compounds that can well replace wood, plastic, or glass-based materials in automobile components on one hand, and film and soft tissue scaffolds in the food, cosmetic, and medical industries on the other (Johnson et al. 2017). Bioplastics such as polylactic acid (PLA) can be made from maize or sugarcane, and their properties can be improved by adding cellulose nanofibers. These nanocomposites are used to make packaging films (Lau et al. 2010). Biocomposites of silk fibres (sericin and fibroin) with other resins like polyurethane, polyvinyl alcohol (PVA), chitosan or alginate are of great use in tissue engineering (Santos et al. 2020; Lau et al. 2010). Such biomaterials can be employed in regenerative medicine, as drug delivery vehicles, and wound dressings.

Bioactive peptides are yet another type of compounds that have potential for use in foods and pharmaceuticals. Bioactive peptides have various therapeutic properties, including cholesterol-lowering, antiprotozoal, antiviral, antithrombotic, antioxidant, antihypertensive and antimicrobial activities (Lemes et al. 2016). Table 5 shows some biocompounds produced from agro-industrial wastes.

Table 5 Biocompounds from waste valorization from agriculture based industries
Full size table

As discussed in previous sections, the core notion of agro-industrial waste management lies in the adoption of new technologies that are both cost effective and ecologically safe. Table 6 lists a few cutting-edge studies along with their environmental advantages.

Table 6 Innovative agro-industrial waste based products with their environmental impacts
Full size table

Agri-industrial waste utilization for improving soil health

Organic matter has long been recognised for its favourable effects on soil and the environment (Atalia et al. 2015). Farmyard manure (FYM) has traditionally been utilised as the primary organic amendment by farmers in many countries to improve soil quality and fertility (Lakhdar et al. 2010). However, in present scenario, using agro-industrial wastes as soil amendments has become an unconventional alternative to improve soil health and quality (Mandal et al. 2016) in a cost effective way (Zoghlami et al. 2016; Lakhdar et al. 2010). Zoghlami et al. (2016) reported that soil organic matter content and fertility were restored after two successive annual amendments with urban sewage sludge.

Effect on soil physical and chemical properties and nutrient availability

Many studies have shown that agro-industrial waste application increases soil organic carbon levels significantly, thereby improving soil tilth, aeration, pH, cation exchange capacity and microbial activity (Erana et al. 2019). Das and Dkhar (2012) reported press mud and bagasse (bio wastes from sugar cane industries) contains substantial amount of plant nutrients and bio degradable organic matter which improve soil physico-chemical properties. According to Raju et al. (2016), the press mud generated through sulphitation process, contains CaSO4, which acts as a soil amendment in alkaline soils (Raju et al. 2016). Similarly, improved water holding capacity and electrical conductivity of soil with sugarcane industrial effluents (Raju et al. 2016); cotton ginning and paper mill discharge (Narasimha et al. 2009) have also been reported.

Using agricultural waste in the form of vermicompost enhances soil structure, making it more porous and permeable to air and water (Rekha et al. 2018). Walker and Bernal (2008) found that combining by-products from olive industries with poultry manure in the form of compost improved soil pH, soluble and exchangeable-K+, and soil CEC even under saline soil environment. Furthermore, Wang et al. (2014) reported that combining with furfural residue, sedge peat, pig manure and rice straw decreased EC, ESP, and bulk density of treated soil while increasing total porosity and organic C. Application of cassava-industrial waste compost (Oo et al. 2015) and gin crushed compost (Chattha et al. 2019) yielded similar results.

Researchers have further demonstrated the release of different organic acids during the decomposition of organic residues (Dotaniya et al. 2016), are responsible for the mobilization of P from fixed locations and its availability to plants (Dotaniya et al. 2016). P availability in soil and its availability to plants improved by application of bagasse and press mud from sugarcane industry (Dotaniya and Datta 2014); rice straw + pressmud (Dotaniya and Datta 2014). Rice straw and pressmud are the potential sources of silica and organic carbon and have been shown to greatly increase nutrient content (Baiyeri et al. 2019; Ghorbani et al. 2019; Hossain et al. 2018).

Effects on heavy metal immobilization in soil

Application of organic amendments generated from agro-industrial wastes help in the immobilization of toxic metals in soil, lowering their bioavailability to plants (Alvarenga et al. 2015; Khan et al. 2015; Sabir et al. 2013; Alamgir et al. 2011). This immobilizing capacity of organic residues is mainly due to the presence of acidic groups that can bind a wide range of metal (loids) viz., lead (Pb), cadmium (Cd), chromium (Cr), and copper (Cu) (Khanam et al. 2020; Lwin et al. 2018; Alvarenga et al. 2015). The most commonly used soil amendments to immobilise toxic metals in soil include biosolids, composts, and manures from various bio wastes, rice husk, straw, saw dust, and wood ash (Sabir et al. 2013).

Soil pH influences the in-solubilization and precipitation of toxic metals and also affects the formation of insoluble organic complexes (Walker et al. 2004). It is also the main parameter for monitoring changes in toxic metals in agro-waste treated soils (Huang et al. 2017). Secondly, soil organic matter is considered as an important absorbent for metal ions in soil with 4–50 times higher cation exchange capacity compared to clay (Hamdi et al. 2019). The strong negative charge generated through the dissociation of organic acids strongly binds the positively charged metal in soil. Researchers revealed that the application of organic matter by wastes application enhances the fixation of toxic metals and reduces their mobility, phytotoxicity (Hamdi et al. 2019) and the bioavailability (Huang et al. 2017; Fleming et al. 2013). Many researchers reported variable responses of heavy metals to compost application in soil. For example, the affinity of organoc matter (OM) for arsenic (As) was less compared to other cationic metals; Cu availability reduced, but As availability was raised (Fleming et al. 2013). In contrast, Clemente et al. (2010) reported increased solubility and mobility of As and Cu in soil when treated with green wastes. Application of olive husk compost reduced the availability of Pb by forming complex with humic substances (de la Fuente et al. 2011). Similar results were reported by Zhou et al. (2012) in alkaline soil environment. However, direct application of untreated agro-industrial wastes or immature composts may have a deleterious effect on crop growth as they contain relatively high soluble OM content (Huang et al. 2017).

Effects on soil biological properties

Soil microorganisms are very essential for long-term sustainability of agricultural systems as they can control various soil processes which are important for soil formation and nutrient cycling (Li et al. 2017). Soil microbes are the living entity of soil and also important aspect of soil quality (Jacoby et al. 2017; Osman 2013). Addition of organic matter from agro-industrial waste, produces manure which greatly affects the activity and diversity of soil microbes as well as soil quality (Liu et al. 2010). The diverse group of soil microorganisms mineralize soil organic matter (SOM) to recycle the organic carbon (C) and nutrients from their unavailable form to available form, thus, they not only act as a source but also a sink for available C and nutrients in soil (Sharma and Garg 2019). Sole application of manure compost enhances soil respiration and enzyme activities (p < 0.01), by increasing the number of cultivable microorganisms as well as microbial biomass. In combination with bacterial bio-fertilizers this can improve the structure and diversity of microbial community in degraded soils (Zhen et al. 2014). Further, combined application of compost and inorganic fertilizer for 32 years increased microbial biomass carbon (MBC) by 89% (Nayak et al. 2007). When compared to composted swine manure, composted cattle manure increased the species richness. Cattle manure commonly decomposes complex organic compounds in a better way and play important roles in plant growth and lignocellulose degradation (Das et al. 2017). Crushed Cotton gin compost (CCGC) is prepared from the by-product of cotton industry, and used to recover degraded soils in semiarid regions (Tejada et al. 2006).

It was also found that vermicompost based on agro-industrial waste has the potential to be used as a substitute of FYM to improve and maintain the microbial activity even in an alkaline calcareous soil of Mediterranean region of Turkey (Uz and Tavali 2014). The relative abundance of beneficial fungi increases, and pathogenic fungi decreases in vermicompost amended soil compared to the other organic amendments (Zhao et al. 2017). In rice–wheat system, crop residue retention with green manuring and zero tillage significantly increased organic carbon, MBC, basal respiration, microbial quotient and mineralization quotient in soil (Saikia et al. 2020). The vast number of by-products generated by sugarcane industries, such as press mud, bagasse, vinasse, and molasses, have a storage problems across the countries (Dotaniya et al. 2016). Among the by-products, press mud alone contains 21% organic C and other macro- and micro-nutrients, which promote the growth and activity of soil microbes (Dey et al. 2020) and also functions as substrate in the bio-composting processing (Chand et al. 2011). Several researchers documented the vital role of sugarcane by-products like filter cake (Chattha et al. 2019); pressmud and bagasse (Dotaniya et al. 2016); molasses (Boopathy et al. 2001) in improving soil organic carbon, MBC and microbial diversity and population (Singh et al. 2009). The populations of bacteria, fungi and actinomycetes, biomass C and N contents increased in press mud and vinasse amended soils compared to the chemical fertilizer treated soil. Besides, the activity of different enzymes such as phosphatase, cellulase, and aminopeptidase were higher with press mud treatment compared to the chemical fertilizer. Hence, press mud and vinasse can be used as a potential substitute to chemical fertilizers. These not only improve soil health and sugarcane productivity, but they can also be disposed off without polluting the environment (Yang et al. 2013).

Conclusion

The valorisation and subsequent value addition of agro-industrial wastes not only enables waste recycling in eco-friendly manner, but also helps to reduce production costs while promoting economic development and societal upliftment. This paves the way for a circular economy model in the agriculture sector, in which waste becomes transient phase and reintegrated into the economy. There are numerous ways to utilise agro-industrial wastes for the production of value-added products such biofuels, microbial enzymes, single cell protein, bio compounds, bioplastics and soil amendments. The use of existing products to produce new products reduces investment and production costs. Undoubtedly, regulatory policies, markets and waste specific research are required to convert agro-industrial wastes into valuable commodities. Soil health is of paramount importance in agriculture sector. The use of agro-industrial waste for improving soil health or treating the soil is also taking momentum and providing better and sustainable alternative to the current practices. Handling of agro-industrial waste adds a significant cost to an agro-based industry in terms of space, transportation and safe disposal. Their sustainability depends on gradual adoption of innovative approaches that are both cost-effective and eco-friendly. The rice burning and associated issues are such an example. In future there is potential of further research in the field of naturally rich wastes containing vitamins, fibres, lignin and cellulose, proteins, lipids, polyphenols and pectin, sugars, etc. These factors make agro-industrial wastes very promising candidates for development of future circular economy. Among many other agro-industrial wastes, lignocellulose residues are considered as the biggest renewable resource on planet earth and have huge potential applications as biofertilizers and in biofuel generation.