Sustainable Horticultural Waste Management: Industrial and Environmental Perspective

Horticultural crops are highly nutritious and shared lion portion of our daily diet. These items are consumed in different ways according to their nature and processing processes. These days, a crucial concerning issue is arising globally to ensure nutrition security for huge population that leads to focus on production increase, quality improvement, food safety assurance, and processing strategies. Consequently, a large amount of waste generates in the processing industries, household kitchen, and supply chain of horticultural commodities that has led to a significant nutrition and economic loss, consequently creating environment pollution with extensive burden of landfills. However, these wastes showed magnificent potentiality of re-utilization in several industries owing to as rich source of different bioactive compounds and phytochemicals. Therefore, sustainable extraction methods and utilization strategies deserve the extensive investigations. This review paper extensively illustrates the horticultural waste generation options, sustainable recycling strategies, and potentiality of recycled products in different industries for betterment in population with the assurance of green environment and sustainable ecology.


Introduction
Nowadays, the horticultural field is exploring with its various utilization. An ever-demanding market is going on with its various options. Vast cultivation in field level with fabulous export potential makes the horticultural product market more outstanding. The global horticultural market value about 20.77 billion USD was estimated in 2021 and targeted to 40.24 billion USD till 2026 [1]. From the kitchen to processing industry, the uses of horticultural product are remarkable today. With its flexibility of uses, problems also arise with various means. The by-products or wastes in horticultural point of view is getting worst day by day for lack of proper utilization not taken. Both developing and developed countries such as Bangladesh, Cambodia, India, Indonesia, Malaysia, Philippines, Thailand, and Vietnam are suffering from various environmental pollutions in concern with water, soil, and air pollution. The increasing trend of population is found to be the major cause of waste generation [2]. Increased population make wastes more usual in homestead generally produced by unnecessarily.
From the production of horticultural foods, various factors involve for the hazardous environmental appearance. Uncontrolled uses of pesticide and residual effects of various chemicals initiate the primary threats to nature by creating bad impact on the wildlife, soil, human, and animal communities [3]. Sometimes these chemicals (about 5-15%) that introduced in the global market for field management are counterfeit in nature [4]. The food wastes evolved in kitchen is noteworthy in many countries such as China, containing solid food wastes between 88% and 94% [5]. Processing industries greatly influence the environment pollution as so many by-products discarded to the environment from this sector while these by-products contain some high quantities of phytochemicals that can be reusable enough for the better disposal [6]. Massive climate issues arise from the undisposed waste available in the environment [7]. Greenhouse gases increases are also introduced in nature by the inappropriate waste disposal method. So, proper and alternative process of waste disposal is compulsory for economic viability and environmental stability.
Increasing the recycling and developing various disposal methods can ensure the proper mitigation of environmental pollution. The industrial and various sources of horticultural wastes are getting importance for its valuable compositions. Recycling can develop the new opportunities with commercial benefits. Biofuels, enzymes, vitamins, antioxidants, and various important chemicals are manufactured from the industrial wastes today. Waste to wealth can be the modern thought of waste disposal. The management of these wastes can be supervised by the government with its regular monitoring, because the waste disposal with its economic benefits can bring the sustainability for both environment and industrial concern [8].

Nature and generation of horticultural waste
The affluence of horticulture in industrial and environmental perspective is greatly significant today. Whereas there are some phenomena arising day by day with management and utilization of horticultural wastes. The nature of wastes in horticultural end is multidisciplinary. Some create chemical hazard; some are alarming for their biological and thermal point of view also. Postharvest handling and storage occur about 54% of wastes that is upstream, while 46% happens "downstream," at the processing, distribution, and consumption stages [9]. These wastes disposal is our major concern in case of sustainable waste management.

Wastes evolving during horticultural production chain
The pragmatic scenario of waste evolving is associated with the increasing of population. With higher population, increasing rate demands the higher agricultural produce. In other words, more food demand may arise with the population increasing rate.
Agricultural production nowadays is more than three times than the last five decades [10]. With technological advancement, the productivity may increase in horticultural sectors also. On the contrary with the increasing productivity, it generates the higher quantities of wastes also. Some of them are green wastes, and some are recyclable solid wastes. Mainly this sector may generate waste most in quantities. The whole process demands lots of intercultural operations such as training, pruning, thinning, earthing up, etc., of various fruits and vegetables can provide some wastes. For example, leaf residues, debris, dead leaves. However, these wastes sometimes added the additional organic matter in the soil. But if the maintenance is not sound enough in horticultural production chain, then the waste becomes burden for the environment. If we enlighten on the data ( Table 1) given by Gmada et al. in 2019, according to their own supervision in the farm of Almeria, there are high amount of wastes distribution in various horticultural production systems. Greenhouses have the higher wastes in 39,215 ton out of 90,738 tons of total wastes, which is 43% of the total waste. Another approach of waste getting is disinfection having the second highest waste getting percentage that is 23%. So, there are different steps and period of waste getting and without waste management of horticultural products we the environment will be depleted day by day at the negative manner.

Chemical wastes during cultivation
These wastes are generated from the continuous use of pesticides, insecticides, and herbicides during the cultivation. These are mainly solid wastes such as pesticides containers, bottles. The activities of using these types of chemicals in developing countries are mostly handled by the rural uneducated farmers. So, the disposal of these types of solid wastes usually gets ignorance by the farmers or the users. Such types of ignorance result in the degraded mode of the environmental balance. About 2% of pesticides usually remain unused in the containers, and then the disposal of these hazardous material is done by the throwing these into the nearest ponds or on the open field condition; the ultimate environmental issues may arise by this as food poisoning, water pollution, air pollution, etc., by this type of ignorance [12].

Postharvest wastes
Postharvest food loss is any loss in physical weight, edibility, nutritional quality, caloric value, consumer adequacy happens between the period of reap and the time it reaches the consumer, while food waste is a subset of the food losses [13], and this might occur through human activity or inaction such as discarding produce, not consuming accessible food before its expiry date, or taking serving sizes beyond one's ability to consume [14]. Horticultural crops are highly perishable products. As it is perishable so that handling and the maintenance are really tough. For this kind of phenomena, the developing countries are the real sufferers of this type of problems. Postharvest loss and wastes of perishable commodities in horticulture are up to 60% depending on the seasons, commodity, and the region of production [15]. So, the wastes after harvesting threaten the sustainable environmental security with environmental pollution. Postharvest loss is not the issue of reduction of food availability for the consumers; it may cause negative externalities to the societies with the increasing cost effect of waste management, greenhouse gas production, and loss of scarce resources used in production [16].

Unconsumed waste foods and kitchen waste
One part of the world's population is struggling every day with the hunger and scarcity of food, whereas in some parts of the world, people waste food without thinking about the food security. Horticultural foods such as vegetables, fruits, and grain crops are wasted daily in our home and appear as the kitchen wastes. In

Biowaste Bioactive compounds
Avocado peel and seed Phenols, carotenoids America, horticultural wastages consist of fruits nearly 20%, vegetables 30%, others 25% [17]. General estimation of food waste annually is about trillion US dollars [18]. The whole world scenario is also alarming in this concern.

Industrial horticultural wastes
There are lots of food processing industries. These industries use some hazardous materials for food processing such as coloring agents, dyes, by-products such as banana peels, coconut husks, and other extraneous bioactive compounds phenols, flavonoids, flavanols, anthocyanins always evolved in the processing industries as by-products ( Table 2). Heavy and rapid disposal is required for this kind of wastes. If it is not disposed with the time, then it will appear as a biggest threat for human survival and environmental balance. Higher emissions of pollutant make the environment more vulnerable. So, without utilizing these types of compounds it can be delectable enough to make our environment polluted.

Horticultural wastes as environmental concern
Environment day by day is threatened with the undisposed wastes derived from various sources of horticultural sectors, from both farming and industrial perspective. Air, water, and soil are major three components of the environment that get affected by the pollutants derived from the horticultural wastes.

Impact of waste on air quality
Sometimes, we disposed some waste with the burning. But these types of waste management are not fruitful always as burning of crop stubbles possesses some hazardous emissions of many harmful gaseous components. As a result, the atmosphere represents monoxide, nitrogen oxide, nitrogen dioxide, sulfur dioxide, methane associated with other toxic hydrocarbons. These types of dangerous gases and particulate matters make a negative impact on air and are harmful for both human and animal health ( Table 3) [21][22][23].
Besides burning of crop stubbles, nitrous oxide is derived from microbial processes in cultivated soil and manures. Machineries used in crop cultivation require  Table 3.
Major air pollutants emitted during crop residue burning [24].

Farm activities surface water Groundwater
Tillage/plowing Sediment/turbidity: sediments carry phosphorus and pesticides adsorbed to sediment particles; siltation of river beds and loss of habitat, spawning ground, etc.

Fertilizing
Runoff of nutrients, especially phosphorus, leading to eutrophication causing taste and odor in public water supply, excess algae growth leading to deoxygenation of water and fish kills.
Leaching of nitrate to groundwater; excessive levels are a threat to public health

Manure spreading
Carried out as a fertilizer activity; spreading on frozen ground results in high levels of contamination of receiving waters by pathogens, metals, phosphorus and nitrogen, leading to eutrophication and potential contamination Contamination of groundwater, especially by nitrogen.

Pesticides
Runoff of pesticides leads to contamination of surface water and biota; dysfunction of ecological system in surface waters by loss of top predators due to growth inhibition and reproductive failure; public health impacts from eating contaminated fish. Pesticides are carried as dust by wind over very long distances and contaminate aquatic systems thousands of miles away (e.g. tropical/subtropical pesticides found in Arctic mammals).
Some pesticides may leach into groundwater causing human health problems from contaminated wells.

Feedlots/animal corrals
Contamination of surface water with many pathogens (bacteria, viruses, etc.) leading to chronic public health problems. Also, contamination by metals contained in urine and feces.  fuel combustion resulting in the rapid production of CO 2 . The ultimate result of air pollution leads to the temperature rising, ecological disbalance, and degradable sustainability of the environment.

Water contamination through horticultural pollutants
Only the industrial solid wastes with heavy metals are not the headache for water contamination today. Agricultural wastes, more specifically the horticultural wastes within horticultural cultivation system and processing by-products, can hamper the water quality in various ways. Fertilizer and other pesticide chemicals are responsible for both ground and surface water contamination. Toxic trace elements make the essential nutrients unavailable, and beneficial soil-borne microorganisms become extinct. Water pollution pyramid (Figure 1) stated that groundwater use has become unsafe for the toxicity of the chemicals. Erosion, sedimentation, salinity are the typical after-effects of continuous cultivation system.
Only agricultural field makes the water polluted about 70% worldwide [26]. Besides, different intercultural operations have the major influence in water contamination ( Table 4).

Impact on soil
Long-term fertilization and indecomposable plastics solid waste make the soil barren for the crop cultivation. Besides, some plant residues contain the toxic chemicals (secondary metabolites, volatile terpenes, phenolic compounds), which can suppress the growth and production of other crops. This type of phenomenon is addressed as crop-crop allelopathy. Postharvest residues are mainly the source of this kind of allelopathic effects [28].

Technique of horticultural waste management
Horticultural waste such as the peels, seeds, and other constituents of vegetables and fruits that contain high amount of phytochemical compounds and essential nutrients are used to produce different industrial products. It can be utilized to extract as well as obtain bioactive compounds that can be used in food, textile, and pharmaceutical industries as shown in Figure 2.
The techniques of horticultural wastes management consist of different applied strategies for different kinds of wastes. Generally chemical, biological, biofuels, and thermal strategies are followed throughout the world ( Table 5).

Pectin, starch, cellulose as biopolymers
From various by-products we find some starch, cellulose; where starch is a white granular, organic compound with soft, tasteless powdery appearance insoluble in cold water, alcohol or different solvents, and cellulose found by peeling of horticultural crops as it is available in primary cell wall of green plants [30] Amylose and amylopectin are the branched form of starch, whereas linear polymer is the simplest one [31] Starch nowadays is produced from banana peels, corn, pea, potato, cassava roots. Banana peels can be processed for bioplastic production and sometimes sodium metabisulfite used as antimicrobial agent, glycerol used for more flexibility. Degradation of bioplastics produced from starch starts after 3-4 months Pectins date of production [32] By this, starch-producing strategies can give the commercial aspects for many emerging entrepreneurships. This is also nonhazardous for the environment because it is readily disposable after a short period of time. Not only banana but also cassava will completely be degraded on the ninth day after production of bioplastic [33] On the contrary, plastic materials are nondegradable products that can hamper the balance of the environment. Cellulose sometimes coverts into starch or glucose by decomposing called cellulolysis with the help of microbes such as Trichoderma reesei and Aspergillus terreus. Then it is used as a material for bioplastic production ( Table 6) [34].

Animal feed
Animal feed can be the good approach for productive waste disposal. That can provide manures, which influence reduction of synthetic fertilizer use tendency. Fermentation industries are established on the basis of wastes types ( Table 7). This provides the ultimate economic outcome without creating the environment hazards in nature.

Chemical
Biological Biofuels Thermal  Table 5.

Figure 2.
Utilization of fruits and vegetable peel-based waste into novel industrial products [29]. The color and smell of the sample were affected and strawberry flavor was added to improve them.

Chocolate
Grapes pomace Powder (drying-> grinding-> sift) Dietary fiber and prebiotic compound: lignin, cellulose, oligosaccha-ride 3-5% At a higher dosage there is a slightly bitter taste due to phenols. The greatest impact on the product occurred in the particle size.
Water activity and stability increased.

Composting for waste disposal
Compost is most demandable nutrient source in the crop field. This approach allows growers to spend less money for their initial cultivation inputs. Also, higher yield will be observed by using compost instead of synthetic fertilizer [36]. Different types of composting methods are used on the basis of grower's choice and wastes types (Figure 3).

Biofuels (bioethanol, biogas)
Waste can be disposed through converting the wastes and by-products into biofuels. Bioethanol and biogas production nowadays appears as the most sustainable waste management program, which has some significant economic values.
Bioethanol can be processed through horticultural by-products such as carrot peels, banana peels, and other crops parts, which previously can be dried in the sun. The product can be ground and sieved for further processing. After that, the products can be pretreated with 1 N NaOH for 2 hours. Then draining or in other words alkali subsequent washing can be done. Enzymatic hydrolysis by cellulase enzyme leads to saccharification. Then the inoculation of Saccharomyces cerevisiae is required. Then the fermentation and distillation are done for bioethanol production (Figure 4).
The other way for wastes disposal as the biofuel source is biogas production. Nowadays we can see the rapid adaptation of this disposal system in our rural areas also.
In case of biofuel products, horticultural by-products can be utilized in effective way. Different content of organic matters can yield the sufficient amount of methane gas ( By-products of oil industry Citrus by-products (molasses, citrus-activated sludge) By-products of sugar and starch industry Anthocyanins By-products of fruit and vegetable industry Effluents from biogas production Plant by-products (husk and pods) Dairy industry Table 7.
Horticultural wastes were used in animal feed [19].

Incineration
Combustion of wastes materials to achieve waste to energy is called incineration. High-temperature thermal treatment converts wastes into ash, flue gas, and heat. It requires localized combined heat and power facilities to encourage its heating process. Japan, Denmark, Singapore, and Netherlands follow this technique usually to dispose the wastes [40]. This method can reduce wastes up to 90%; but this is one type of waste reduction process rather than the disposal process as it is associated with the fire disaster and production of greenhouse gases [41]. Energy produced as coal could save about 2.26 MT of CO2 eq/year [42].

Pyrolysis
Waste composition can determine the effectiveness of pyrolysis. It has several advantages comparing the incineration process. Lower temperature is preferable, and the plant for pyrolysis more flexible enough; product derived from pyrolysis can be converted through alternating the temperature and heating performance [43]. The pollutant emissions are lower in this disposal process as there is absence of oxygen and with low processing temperature, although emissions of other compounds simultaneously could increase with lower oxygen ratio [44]. Biochar production can be done by fruits and vegetable peels and other residues from, for example, spinach, bananas, peas, and tomatoes [45]. There are two types of pyrolysis, i.e., slow and fast. For biochar and gas production, slow pyrolysis is preferable, and bio-oil can be produced better in fast pyrolysis (Figure 5).

Prospects of horticultural waste management
It is obvious that waste disposal is not an easy task as it requires bigger margin of resources and right methods to minimize its after-effects. All the techniques or methods of repealing wastes are not efficient enough always. Food recovery hierarchy published by US EPA showed that there are different methods or approaches are  [46][47].
proficient at different level. Landfilling and incineration (combustion of the waste materials) are the last resort of wastes disposal, because sometimes it is harmful for our environment. Soil pollution and abundance of toxic gases are visible by this kind of disposal system. Then the composting creates a nutrient-rich soil amendment. It requires specialized area away from the home, and it requires more time for disposal. Industrial uses of wastes are just above from the composting in that pyramid as it provides waste oils for rendering and duel conversion and food scraps for digestion to recover energy. Lots of commercial industries are developing today with the new hope with horticultural by-product establishing. Although some wastes also are considered developing the industries but commercial exposure may be spread rapidly. Main effective approach will be the source reduction and sometimes we waste food more than we consume so that extra food can be donated to food banks, shelters can reduce the possibility food wasting. Public awareness is the big thing for food waste management in horticultural sectors also (Figure 6).

Conclusion
Various environmental concerns and some economic benefits demand the appropriate disposal of horticultural wastes. Minimizing of wastes can maximize the environmental stability. However, people are not so aware about the impact of horticultural wastes. So, the proper awareness with the effective implementation of wastes is a crying need for today. Meanwhile, sustainability can be brought through adapting the modern disposal methods with longer effects and economic flexibility. In addition, the growth of the wastes disposal industries also gives the new dimension for the sustainable waste management. Finally, it can be enunciated that waste management provides green ecology, which can serve environmental stability with industrial prosperity. Food recovery or management hierarchy [48].