Rotary Drum Composting of Organic School Wastes and Compost Valorization

Abstract

Inappropriate waste disposal imposes significant health risks in densely populated urban environments and schools, necessitating sustainable waste management. Therefore, a study was carried out at Al-Jazaer School, Kuwait, to evaluate rotary drum composting (RDC) of organic school waste comprising used paper, dry leaves, and vegetable food wastes in a 1:4:20 ratio. Feedstock comprising 42% organic school wastes, 42% horse manure, and 16% sawdust produced mature compost with a C:N ratio of 20.55 on the 43rd day of composting. Distinct mesophilic, thermophilic, cooling, and curing phases were observed during composting. Mature compost recorded a moisture content of 54.3%, pH 8.56, EC of 2.71 mS/cm, total nitrogen of 0.77%, total organic carbon of 18.25%, carbon content of 15.86%, and sulfur content of 0.14%. Soilless growing media comprising peat moss, perlite, and rotary drum compost in three proportions (1:1:1, 1:1:2, and 1:1:3), and peat moss, perlite, and commercial organic compost in a 1:1:3 ratio were evaluated for greenhouse vegetable production. The performance of cucumbers (Cucumis sativus cv. Ramos) raised in the lowest proportion of in-house prepared rotary drum compost (1:1:1 ratio) was comparable with that raised in the highest proportion of commercial compost (1:1:3). The study revealed the potential of RDC for decentralized sustainable waste management at the small-community scale and the suitability of compost from school wastes for soilless culture.

1. Introduction

Sustainable and economical management of Municipal Solid Waste (MSW) is a serious challenge for many countries in the world. The challenge is more pronounced in the Gulf Cooperation Council (GCC) countries, characterized by a fragile desert ecosystem. Among the Gulf Cooperation Council (GCC) countries, the United Arab Emirates recorded the highest per capita MSW generation (2.1 kg/capita/day) in 2017, closely followed by Kuwait (1.5 kg/capita/day), whereas the average global per capita was 0.74 kg/capita/day [1]. Kuwait is ranked among the highest per capita waste generators in the world due to its increasing population, economic growth, improved lifestyle, demographic changes, and changing dietary preferences. The estimated amount of recycled waste in Kuwait in 2018 was 4.8 million tons per year, which is equivalent to an overall recycling rate of 11% of the generated waste, whereas countries with high rates, such as Germany or South Korea, recycle around 50% [2]. Almost all other waste (88%) is either landfilled or used for backfilling and landfill construction [2]. Despite several awareness campaigns conducted by the Kuwait Environment Public Authority to raise the public awareness towards waste sorting and recycling [3], the recycled amounts of household wastes in Kuwait are very low due to the reasons that the waste is often not separated at its source and that the recycling facilities cannot compete with the low costs for disposal at landfills [2].

Efficient Municipal Solid Waste Management (MSWM) systems and effective treatment technologies ensuring appropriate waste disposal amidst the growing population are an overwhelming global challenge in this century. The national and local governments and governmental agencies perform a pivotal role in the MSWM field of developing economies [4]. Notably, in many developing economies, MSWM policies are not effectively implemented despite their existence [4]. The current MSWM practices of most developing economies are either ineffective, inefficient, or limited. Hence, they are contributing to environmental, social, and economic negative impacts that can impede sustainable development [4,5]. On the other hand, most waste collection and treatment in developed nations are associated with high costs and are challenging for countries with low and moderate incomes [6]. Different techniques are applied for waste management, including biological treatment, anaerobic digestion, composting, vermicomposting, landfilling [7,8], thermochemical treatment, incineration, pyrolysis, and gasification [9]. Each technique has advantages as considered a route for the conversion of terrestrial waste into biogas, biofuels, and other converted products with a chain of disadvantages, including adverse environmental consequences, such as greenhouse gases, global warming, climatic change, and disturbance of the flora and fauna ecosystem [10]. The significant lacunae in waste management lie in the inefficiency of present technologies in promoting valuable waste utilization in reuse and recycling systems [11]. The current solid waste management policies are restricted to the collection, transportation, treatment, and disposal and lack large-scale valorization of organic-rich waste. Putrescible organic waste represents the most considerable portion of solid waste and is characterized by a high nutrient value [12,13]. Across the Arab region, the percentage of decomposable material in MSW is very high and varies from 30 to 70 percent; it consists mainly of fruits, vegetables and food scraps, while the proportion of wood is very low [14]. Composting using rotary drum composters of appropriate capacity will enable sustainable decentralized waste management of organic wastes at source at a relatively low cost, extending manifold benefits to the society. Implementation of rotary drum composting (RDC) minimizes health risks posed by inappropriate waste management, reduces waste-handling and transportation costs, and minimizes environmental issues. In addition, the compost generated can be used as a substrate component for soilless culture of greenhouse vegetables, transforming waste to wealth, a step towards sustainability. Capturing this inherent organic waste value could alleviate the environmental consequences to a greater extent and simultaneously serve food, fodder, or other products of commercial interest. Moreover, the transition from a large-scale centralized to a small-scale decentralized waste management system, viz. rotary drum composting, provides an adaptable system for waste management at source.

Schools are sources of a substantial quantity of organic wastes which need to be handled in an environment-friendly and sustainable manner. Inappropriate management of wastes and their unscientific disposal can lead to significant health risks [15] and rapid transmission of contagious diseases in schools, where population density and human interaction is very high. Composting is a natural aerobic process by which microorganisms act upon the organic matter to transform it into humic substances with the release of by-products, such as CO₂, and H₂O [16]. The final product is free of viable human and plant pathogens and plant seeds that do not attract insects or vectors, which can be handled and stored without nuisance, and which are beneficial to the growth of plants [16]. Large-scale mechanized Municipal Solid Waste (MSW) composting plants promoted in the 1970s turned out to be financial failures in India [17]. A UNDP study [18], hence, recommended that instead of setting up single large mechanical compost plants, it would be beneficial and more effective to set up several small composting plants. Decentralized composting on a neighborhood or community scale allows small groups to pursue it at a relatively low cost and reuse organic waste where it is generated, thereby reducing waste quantities to be transported as well as transport costs [19,20]. An efficient and promising technique in decentralized composting is the rotary drum composter [17]. It is an excellent concept for smaller communities or in the case of projects that require a rapid and an enclosed pathogen-killing process [17]. Being a biological process, microorganisms play a crucial role in the composting process; therefore, factors affecting the growth and reproduction of these microorganisms should be taken into account. These factors include aeration or oxygen, temperature of composting, texture of raw materials, pH, moisture of substrate and the C:N ratio [21,22,23]. The relatively warm and humid ambience in rotary drum composters, coupled with the sufficient oxygen, carbon, and nitrogen sources available, promotes the proliferation of aerobic microbes and accelerates the decomposition of organic wastes [20]. The rotary drum provides agitation, aeration, and mixing of the compost to produce a consistent and uniform end product without any odor- or leachate-related problems [20]. Hence, rotary drum composting is an efficient and promising aerobic composting technique which can be adopted for waste handling at source, enabling a decentralized, more efficient, and cost-effective waste management in schools. It can be successfully used in compost product demanding areas, such as institutions, schools, vegetable markets, large dairies, and garden/park areas [20].

Protected Agriculture (PA), primarily evaporative cooled greenhouses employing soilless production techniques, is a prudent choice which improves and sustains domestic food production at desirable levels in constrained agro-ecosystems, where agricultural inputs are scarce and sustainability is of paramount importance. The practice of soilless culture has provided the flexibility to cultivate, even in regions where natural conditions to grow crops are unfavorable [24]. However, in addition to the ability of a medium to promote good plant growth and yield, the cost [25] and the availability of the soilless media are important factors in soilless culture. Various ingredients have been used to produce growing media for vegetable production throughout the world; the raw materials used vary based on their local availability [26]. Most of the prevalent soilless production systems are over-dependent on peat-based soilless substrates, leading to unethical exploitation and wastage of non-renewable natural resources. Research on alternative peat products of organic origin that have the potential to be used as substrates, therefore, appears necessary [27]. To reduce the expense of environment-friendly crop production, it is important to use relatively inexpensive and indigenous substrates, such as in-house produced compost, coco peat, or vermicompost as alternatives to the more expensive peat moss.

Although all materials of organic origin are suitable for composting, an optimum carbon–nitrogen ratio and consistency in the quality of feedstock are prerequisites for effective microbial action and the resultant quality of the produce [28]. School wastes, comprising used paper, vegetable food waste, garden waste, etc., of relatively consistent quality and characteristics, are suitable as feedstock for rotary drum composting. The establishment of an aerobic rotary drum composting unit in a school to recycle the organic wastes generated and the utilization of the produced compost as a substrate component for soilless culture of greenhouse vegetables can be effectively applied for sustainable waste management and food production in schools. In addition, gardening and agricultural activities in schools encourage children to become more active and enthusiastic learners and enable them to develop a more resilient, confident, and responsible personality. Garden-based learning has the potential not only to contribute to academic skills but also to address a child’s development in a social, moral, and practical or life skills sense [29]. The effect will be more pronounced in environmentally and climatically challenged regions, where children are less privileged to interact with nature. Therefore, this study was carried out at Al-Jazaer School in Kuwait, characterized by an arid, hot desert environment, to evaluate rotary drum compost from school wastes as a substrate component for soilless culture of greenhouse vegetables.

2. Materials and Methods

2.1. Rotary Drum Composter

Rotary drum composter of 200 L capacity was used for the study (Figure 1). The composting system comprised a high-density polyethylene (HDPE) cylindrical drum with lid and lock, which can be secured tightly to avoid spillage during turning, mounted on a steel frame, and equipped with a central axle and handle to enable easy rotation. Perforations of 10 mm in diameter were provided to the drum at 150 mm spacing to maintain aerobic conditions and to facilitate drainage of excess water during composting. The drum was rotated manually to provide appropriate mixing and aeration of the feedstock during composting.

2.2. Waste Collection Points

Three designated waste collection points, attractively labeled to capture student’s attention, were established in the school premises to collect school wastes. HDPE bins of 100 L capacity, with lids, were used for this purpose. Separate bins were allocated to deposit the waste materials of different types, viz. paper, plastics, and vegetable/fruits/leaf wastes, in order to enable the segregation of wastes at their point of origin (Figure 2). The organic waste materials collected in two-day intervals were transferred to the composting area and subjected to mechanical shredding prior to feeding to the rotary drums.

2.3. Feedstock Material

A mixture of easily available and inexpensive waste materials of consistent quality, comprising dry leaves, used paper, and green vegetable wastes from the waste collection bins placed in the school, as well as horse manure and sawdust were used as the feedstock [19,30]. Canteen food waste, comprising processed food and a variety of ingredients, was excluded to maintain consistency of the feedstock. Fresh horse manure was used as starter culture to enhance the microbial action and to retain the moisture content of the feedstock at desirable levels. Sawdust/wood shavings was added to make up the desirable volume and maintain the friability of the feedstock. Feedstock components on wet mass basis are presented in

Table 1. Composition of feedstock used in rotary drum composting.

Composition of Feedstock	Mass Used (kg)
Dry leaves	4.0
Paper	1.0
Vegetable wastes	20.0
Sawdust	10.0
Horse manure	25.0
Total	60.0

.

The collected waste mixture was shredded mechanically to achieve <2 cm² particle size for better aeration, moisture management, and mixing inside the composter drum. The feedstock was moistened to 55–60% prior to loading in the composter. The optimum initial moisture content should vary between 50 and 60% [31].

2.4. Loading of Feedstock and Composting

The shredded feedstock was loaded into the drum up to 70% of the total volume [32]. Lid of the rotary drum composter was kept closed, except for sampling and moistening the feedstock, to provide relatively warm and humid ambience congenial for aerobic microbial activity. The feedstock was provided with adequate aeration by intermittent turning of the composting drums at 24 h interval (five turnings daily), throughout the course of composting. Frequent turning apparently inhibits the profuse development of molds and actinomycetes, characteristics of material disturbed less often [32]. In frequent turning, these organisms only develop sporadically, whereas in material allowed to remain undisturbed for one day, they form a thick continuous layer, which provides a favorable condition for growth and biological activity of microorganisms [30]. Temperature and moisture content of the feedstock were recorded daily using hand-held analog thermometer and a probe-type moisture meter. Tap water was sprinkled in quantum sufficient as and when required to maintain the moisture content of the feedstock at a desirable range (55 to 60%) during the composting period. A composting unit comprising eight rotary drum composters was established and operated in an area protected from direct sun and rain for the ecofriendly management of organic wastes produced in the school. Grab samples of feedstock collected from the eight rotary drums were composited, air-dried, ground, and subjected to periodic laboratory analysis during the entire course of composting, which was stopped at maturity when the compost exhibited C:N ratio of 20.

2.5. Crop Production

The rotary drum compost produced from school wastes was evaluated as a substrate component in soilless growing media, with salad cucumber as test crop. Soilless cultivation is a method that permits good control of plant growth and development and is currently in practice all over the world [33,34]. Soilless culture techniques have several advantages over the traditional soil-based cultivation systems [35,36,37]. This technique allows the achievement of high yields without jeopardizing quality product [37]. The study was carried out in an evaporative cooled single-span polycarbonate greenhouse measuring 16 m × 6 m × 4 m (L × W × H), during September to December 2021. The greenhouse was equipped with a double door entry and a disinfectant footbath (2% Clorox) to reduce contamination and provide adequate sanitation. The experimental treatments comprised rotary drum compost from school wastes mixed with peat moss and perlite in three different proportions. A commercial compost (organic compost, manufactured by Gulf Palms Company, Kuwait, Kuwait) mixed with peat moss and perlite was used as control for comparison.

The soilless growing media evaluated were as follows:

• T1—peat moss: perlite: rotary drum compost in 1:1:1 ratio (v/v);
• T2—peat moss: perlite: rotary drum compost in 1:1:2 ratio (v/v);
• T3—peat moss: perlite: rotary drum compost in 1:1:3 ratio (v/v);
• T4—peat moss: perlite: commercial compost in 1:1:3 ratio (v/v).

Hybrid seeds of Cucumis sativus cv. Ramos produced by Quantum Seeds, Riverside, CA, USA, were sown in 8 cm polyethylene nursery pots filled with a potting mixture prepared by mixing SAB potting soil and perlite in 3:1 ratio. The seeds were sown on 6 September 2021, and the nursery attained transplanting maturity in 20 days. The experimental crop was raised in open containerized system, arranged in double rows at 1 m × 0.5 m spacing (row × plant). Five-gallon PVC pots were used as the growing container. Uniform, healthy seedlings at 3–4 leaves stage were transplanted to the pots filled with the growing media under study. The plants were raised under drip irrigation and standard cultural practices for greenhouse cucumber cultivation. Growth stages of the experimental crop are presented in Figure 3.

Nutrient management of the experimental crop during the course of the study was accomplished using three commercially available inorganic water-soluble fertilizer formulations, viz. Kristalon 18+18+18+TE, Kristalon 12+12+36+TE, and Calcinit (calcium nitrate, Yara brand). Irrespective of the growing media treatments, the experimental plants were uniformly fertigated with Kristalon 18+18+18+TE at the rate of 2 g/plant/week during the early vegetative growth stage. In addition, the experimental plants were fertigated with Calcinit (calcium nitrate) at the rate of 2 g/plant/week during the early flowering stage. Starting from the reproductive growth stage (flowering and fruit initiation), the crop was provided with a combination of 18+18+18+TE, and 12+12+36+TE, at the rate of 2 g each/plant/week and Calcinit at the rate of 4 g/plant/week. The crop exhibited satisfactory vegetative and reproductive growth during the period of study, and the fruits were harvested twice a week at harvestable maturity.

The experiment was laid out in completely randomized block design. The treatments were quadruplicated, and each treatment was allocated to 32 plants with a total of 128 plants in the experiment. Observations on vegetative growth parameters, viz. plant height, number of leaves, and chlorophyll index, were recorded at 15 d intervals. Leaf chlorophyll index/content, which is a key indicator of leaf greenness and often used to investigate leaf nutrient deficiencies and changes in chlorophyll, was recorded using SPAD 502 Plus Chlorophyll Meter (Spectrum Technologies, Inc., Montreal, QC, Canada). Data generated during the course of the experiment were tabulated and statistically analyzed and are presented in the results and discussion section. All the results reported are the means of four replicates. The results were subjected to analysis of variance (ANOVA) using Statistical Package for the Social Sciences (SPSS) software (Version 26).

3. Results

3.1. Composting

The feedstock was subjected to periodic testing of temperature, moisture content, pH, EC, total organic carbon, nitrogen, carbon, sulfur, and C:N (carbon:nitrogen) ratio during the course of composting. Ambient atmospheric temperature and the variation in temperature of the composting material with time, recorded during the aerobic digestion process, are presented in Figure 4.

The mixture of raw materials remained at environmental temperature (28 °C) during the initial hours of composting, indicative of the mesophilic phase. After a few hours from the beginning of the course, aerobic microbial activities started, and the feedstock underwent a high rate composting and developed a steep rise in temperature (49 °C) that extended to 8 days, depicting the thermophilic phase. Periodic rotation of the drum composter resulted in the mixing up of the outer mesophilic layer and the inner thermophilic layer yielding a uniform matrix of the compost material. Further to the thermophilic phase, a cooling phase and maturation phase that extended until the 20th and 43rd day, respectively, were observed during the course of the composting. The mesophilic, thermophilic, cooling, and curing phases were distinct and clearly depicted during the course of the study.

Eleven samples were analyzed in the laboratory, starting on day one to day fifty of the composting procedure. Results of laboratory analysis of the feedstock are presented in

Table 2. Laboratory analysis of rotary drum feedstock/compost.

Day of Sampling	Moisture %	pH	EC (1:5) mS/cm	TOC %	N %	C %	S %	C:N
1	57.33	7.41	5.02	30.77	0.65	26.62	0.16	40.85
4	57.25	7.5	3.13	-	-	-	-	-
8	57.91	7.5	3.47	25.04	0.60	23.89	0.15	39.77
11	57.86	7.64	4.09	-	-	-	-	-
15	57.82	7.86	3.92	22.48	0.79	24.61	0.16	31.22
18	58.88	7.83	3.15	-	-	-	-	-
22	56.77	7.89	3.11	19.44	0.80	19.43	0.16	24.15
29	59.75	8.41	2.97	20.31	0.91	22.58	0.17	24.70
36	56.55	8.44	2.77	17.21	0.78	18.14	0.14	23.35
43	54.30	8.56	3.05	18.25	0.77	15.86	0.14	20.55
50	57.33	8.62	2.71	17.76	0.71	15.34	0.14	21.53

. Perusal of Table 2 provides clear evidence of effective biochemical reactions and the resultant chemical changes that have taken place during the composting process.

Moisture content of the feedstock was maintained within the range of 54% to 60%, throughout the course of study. The pH of the feedstock recorded a steady increase from 7.41 on day 1 to 8.62 on day 50, whereas the electrical conductivity dropped from 5.02 mS/cm to 2.71 mS/cm. The total organic carbon content of the feedstock was reduced from 30.77% to 17.76% during the composting process. The bacterial activity during composting contributed to an increase in the nitrogen concentration by 0.26% on the 29th day and a reduction in the carbon content by 10.76% on the 43rd day, which in turn contributed to a preferable C: N ratio of 20. Fluctuations observed in different parameters may be attributed to the changes in ambient environmental conditions during the progress of composting. Maturity of the compost was determined upon the C:N ratio of 20.55 [15] at 43 days of composting.

3.2. Crop Production

3.2.1. Laboratory Analysis

Laboratory analyses of the three soilless growing media prepared using different proportions of rotary drum compost (T1, T2 and T3) and that prepared using the commercial organic compost (Control—T4) for crop production studies are presented in

Table 3. Laboratory analysis of experimental soilless growing media.

Trt.	pH	EC (1:5) mS/cm	N %	C %	PO4-P mg/kg	K mg/kg	CO3− mg/kg	Cl- mg/kg	Ca++ mg/kg	Mg++ mg/kg	Na+ mg/kg	HCO3− mg/kg
T1	5.68	1.38	1.03	21.87	463.00	193	<10	1313.0	695	135	134	554
T2	6.55	1.68	1.02	18.19	401.00	233	<10	1774.5	710	192	167	1207
T3	6.71	1.74	0.996	15.19	272.00	251	<10	1859.0	575	174	180	1534
T4	7.06	7.25	1.88	15.86	301.00	1256	<10	6469.5	2100	1077	426	1814

T1—Peat moss: perlite: rotary drum compost in 1:1:1 ratio (v/v), T2—peat moss: perlite: rotary drum compost in 1:1:2 ratio (v/v), T3—peat moss: perlite: rotary drum compost in 1:1:3 ratio (v/v); T4—peat moss: perlite: commercial compost in 1:1:3 ratio (v/v).

. The analysis revealed that the initial pHs of the growing media treatments T1, T2, T3, and T4 were 5.68, 6.55, 6.71, and 7.06, respectively. Electrical conductivity of the growing media comprising the three proportions of rotary drum compost ranged from 1.38 mS/cm to 1.74 mS/cm. The soilless growing medium prepared using the commercial compost (T4) exhibited a neutral pH, high electrical conductivity, and the highest concentrations of anions and cations compared to the growing media prepared using in-house compost (T1–T3).

3.2.2. Crop Growth and Yield

Table 4. Average plant height (cm) of cucumber (Cucumis sativus cv. Ramos) grown in four selected soilless growing media.

Treatment	Days after Planting (DAP)
	1	15	30	45	60
T1	32.88a	84.03b	170.72b	253.59a	299.75a
T2	33.25a	68.13a	154.47a	251.94a	300.47a
T3	33.22a	68.91a	146.38a	240.44a	276.78a
T4	33.25a	122.84c	199.59c	258.97a	297.50a
Significance	NS	**	**	NS	NS

Identical letters within a single column indicate differences that are not significant at p ≤ 0.05; NS: non-significant; **: significant at p ≤ 0.01. T1—peat moss: perlite: rotary drum compost in 1:1:1 ratio (v/v); T2—peat moss: perlite: rotary drum compost in 1:1:2 ratio (v/v); T3—peat moss: perlite: rotary drum compost in 1:1:3 ratio (v/v); T4—peat moss: perlite: commercial compost in 1:1:3 ratio (v/v).

,

Table 5. Average number of leaves in cucumbers (Cucumis sativus cv. Ramos) grown in four selected soilless growing media.

Treatment	Days after Planting
	1	15	30	45	60
T1	3.34a	10.06b	17.28b	20.84a	17.38a
T2	3.34a	8.88a	15.78a	21.31a	17.78a
T3	3.28a	8.84a	15.13a	22.59a	17.13a
T4	3.22a	12.66c	20.81c	20.91a	18.13a
Significance	NS	**	**	NS	NS

Identical letters within a single column indicate differences that are not significant at p ≤ 0.05; NS: non-significant; **: significant at p ≤ 0.01. T1—peat moss: perlite: rotary drum compost in 1:1:1 ratio (v/v); T2—peat moss: perlite: rotary drum compost in 1:1:2 ratio (v/v); T3—peat moss: perlite: rotary drum compost in 1:1:3 ratio (v/v); T4—peat moss: perlite: commercial compost in 1:1:3 ratio (v/v).

and

Table 6. Leaf chlorophyll index in cucumber (Cucumis sativus cv. Ramos) grown in four selected soilless growing media.

Treatment	Days after Planting (DAP)
	15	30	45	60
T1	29.47b	30.34bc	34.52b	32.08a
T2	28.39ab	28.31a	33.35ab	32.03a
T3	27.46a	28.66ab	32.74ab	31.07a
T4	36.38c	31.01c	32.50a	30.73a
Significance	**	**	NS	NS

Identical letters within a single column indicate differences that are not significant at p ≤ 0.05; NS: non-significant; **: significant at p ≤ 0.01. T1—peat moss: perlite: rotary drum compost in 1:1:1 ratio (v/v); T2—peat moss: perlite: rotary drum compost in 1:1:2 ratio (v/v); T3—peat moss: perlite: rotary drum compost in 1:1:3 ratio (v/v); T4—peat moss: perlite: commercial compost in 1:1:3 ratio (v/v).

present data recorded on vegetative growth parameters, viz. plant height, number of leaves, and chlorophyll index, of Cucumis sativus cv. Ramos grown in the four selected soilless media. Though uniform seedlings were transplanted in the four selected growing media treatments, a significant difference was observed in average plant height starting 15 DAP (Table 4). Despite the comparable plant height observed in cucumbers under T2 and T3 treatments, plants under T4 treatment emerged with the significantly highest plant height, followed by those grown under T1 treatment, at 15 and 30 DAP. However, the experimental plants recorded comparable heights, starting 45 DAP till 60 DAP, irrespective of the growing media treatments.

The average number of leaves in cucumbers grown under the four soilless growing media treatments are presented in Table 5. The table shows that the growing media influenced leaf production in cucumbers during the vegetative growth stage, recording a significantly high number of leaves in plants grown under T4 treatment, followed by T1 plants at 15 and 30 DAP.

From 45 DAP, the experimental plants recorded comparable number of leaves, irrespective of the growing media treatments. However, cucumbers grown under T2 and T3 treatments produced comparable numbers of leaves throughout the course of the study.

Table 6 presents the leaf chlorophyll index of Cucumis sativus cv. Ramos grown in the four selected soilless growing media, measured using the SPAD chlorophyll index meter.

Cucumbers raised in the growing medium containing commercial organic compost (T4) recorded the significantly highest chlorophyll index at 15 DAP, followed by T1 plants, whereas at 30 DAP, both the T4 and T1 plants exhibited a significantly high chlorophyll index record. However, the leaf chlorophyll index of T4 plants declined with time and recorded the lowest index at 60 DAP. Irrespective of the growing media treatments, all plants recorded a comparable chlorophyll index at 60 DAP.

The yield attribute records of cucumber grown in the four soilless growing media under study (

Table 7. Average fruit yield and number of fruits record of cucumbers (Cucumis sativus cv. Ramos) grown in four selected soilless growing media.

Treatment	No. of Fruit	Average Yield (gm)
T1	26.63a	1926.59a
T2	22.34a	1571.88a
T3	22.75a	1654.97a
T4	26.44a	1744.28a
Significance	NS	NS

Identical letters within a single column indicate differences that are not significant at p ≤ 0.05; NS: non-significant. T1—peat moss: perlite: rotary drum compost in 1:1:1 ratio (v/v); T2—peat moss: perlite: rotary drum compost in 1:1:2 ratio (v/v); T3—peat moss: perlite: rotary drum compost in 1:1:3 ratio (v/v); T4—peat moss: perlite: commercial compost in 1:1:3 ratio (v/v).

) reveal that, irrespective of the treatments, the cucumbers recorded a comparable number of fruits and fruit yield during the course of the study.

This is in high correlation with the records that the vegetative growth parameters of the experimental cucumbers remained comparable during the later stages of growth (from 45 DAP), irrespective of the growing media treatments. However, growing media treatments with higher proportions of rotary drum compost (T2 and T3) did not impart a significant difference in the growth and yield of the experimental cucumbers.

4. Discussion

The mesophilic, thermophilic, cooling, and curing stages, indicative of an ideal composting process, were clearly depicted during the rotary drum composting, despite the relatively low ambient atmospheric temperature (16–30 °C). The sharp rise in temperature from an initial 28 °C to 49 °C through the fourth day is clear evidence of the rapid establishment and proliferation of an aerobic microbial population, which accelerates decomposition of the feedstock in the composter [38]. The temperature of the composting material remained at 49 °C for a period of eight days, sufficing the requirement for the destruction of pathogens [31]. The longer thermophilic phase may be attributed to the presence of a sufficient carbon source in the feedstock. The temperature records were in near confirmation with the findings of Mohee and Mudhoo [39] that temperatures from 52 to 60 °C are considered to maintain the greatest thermophilic activity in composting systems. The temperature of the composting material recorded a gradual drop during the cooling stage, which extended for 10 days, followed by the curing period that persisted until the 43rd day of composting.

The moisture content of the composting material ranged between 59.75% and 54.30%, indicating normal composting conditions. During the initial days of composting, the feedstock was occasionally sprayed with tap water to maintain a desirable moisture content. Moisture loss during the composting process can be viewed as an index of the decomposition rate since the heat generation which accompanies decomposition drives vaporization [19,40]. However, leachate formation was not observed during the composting period.

The pH of the composting material exhibited a steady increase from the start to the end of the composting process. The results are in correlation with previous studies reporting composting proceeds most efficiently at the thermophilic temperature when the pH is approximately 8 [39,41]. The increase in pH during rotary drum composting may be attributed to the production of ammonia during the microbial decomposition of organic matter. The increase in the pH level during composting resulted from an increase in the volume of ammonia released due to protein degradation [40].

The peak value of EC at the beginning of the composting dropped up to eighth day, and then exhibited a slight increase, followed by a steady drop until the end of the composting. This is in high correlation with the results reported by [19]. The initial peak could be due to the release of mineral salts and ammonium ions through the decomposition of organic matter [42]. The volatilization of ammonia and the precipitation of mineral salts could be the possible reasons for the decrease in EC at the later phase of composting [43]. The EC value of the mature compost, which is the measure of the salinity of the compost, was in the acceptable range for use as a substrate component for soilless growing media.

Microbial decomposition of the organic matter during rotary drum composting resulted in a decrease in the total organic carbon content (TOC) of the composting material. The carbon content of the composting material was reduced to 61%. Available carbon was utilized as an energy source for micro-organisms [19], and the content of TOC decreased as the decomposition progressed. The total nitrogen content of the composting material increased during composting and recorded its peak on the 29th day of composting. This may be due to the net loss of dry mass in terms of carbon dioxide as well as the water loss by evaporation due to heat evolution during oxidization of organic matter [42]. Nitrogen-fixing bacteria might also have contributed to the increase in nitrogen in the later stage of composting [44]. The decrease in nitrogen and sulfur content in the composting material during the curing stage may be attributed to the loss of nitrogen due to the escape of ammonia [45] and other volatile gases from the compost material to the atmosphere.

The change in the C:N ratio from 40.85 on the first day to 21.53 on the fiftieth day of composting reflects the organic matter decomposition and stabilization achieved during composting. The desirable C:N ratio of mature compost is preferred to be less than or equal to 20 [20].

Organic manure is widely used in substrate culture for plant production at a commercial level in soilless culture. Composted organic wastes are also recommended as nutrient sources for plants grown in substrates [46,47,48,49,50,51]. Containers, along with the substrates, affect air and water status and are critical factors affecting the productivity of the crops. A study by Al-Naddafa et al. [52] employing growbags with composted pig manure (CPM) alone or mixed with perlite, with a bag height of 10 cm, was found to have a strong impact on the agronomic performance of cucumbers, which are associated with water availability.

The soilless growing media with proportions of rotary drum compost as the substrate component exhibited the most congenial pH and electrical conductivity for crop production. The ideal pH for most of the substrates is 5.5 to 6.5 for rooting, seedling growth, and containerized production in large containers. A higher EC indicates excess fertility or higher amounts of impurities in the substrates [53]. A pH range of 5.5 to 6.8 is suitable for most crops as all essential nutrients are available to plants at this pH range [46,54]. The availability of phosphorus, iron, manganese, zinc, and copper decreases with an increase in pH above 6.5 for substrates [53]. The soilless growing media with commercial organic compost as the substrate component (T4) exhibited substantially high electrical conductivity, which may be attributed to the chemical composition of the composting material. Several waste-derived substrate components, such as poultry and litter composites or spent mushrooms, may be high in salt components [53].

Plant height, number of leaves, and chlorophyll index of the experimental plants recorded towards the later stages of crop growth are indicative of the comparable performance of the growing media tested, hence their suitability for use in greenhouse cucumber production. Significant correlations between leaf chlorophyll index/content and leaf nitrogen have been reported in many agricultural crops [55,56]. Chlorophyll index/content, leaf area index, and leaf dry weight are positively influenced by fertilizer application, especially nitrogen [57]. This trend was reflected in the yield performance of cucumbers also. The average fruit yield in cucumbers grown in the soilless growing medium prepared by mixing the lowest proportion of rotary drum compost, i.e., peat moss: perlite: in-house compost in a 1:1:1 ratio (v/v) (T1), was on par with those grown in the soilless growing medium prepared by mixing peat moss: perlite: commercial compost in a 1:1:3 ratio (v/v) (T4). Various components in the medium and their structure and texture could impart direct and indirect effects on plant growth and development [53].

It is evident from the study that rotary drum compost can be effectively used as an alternative to commercially available compost to grow vegetables in a protected agriculture (PA) system. These results are also encouraging regarding the fact that household waste after composting can be utilized for vegetable production and not end up in the landfill as an absolute waste. Researchers and the growing media industry are working to use peat more wisely and sparingly [58] by partially or totally substituting it with other renewable or sustainable organic materials (e.g., coir, bark, wood fibers, green composts) [59,60]. In addition to the ability of a medium to promote good plant growth and yield, the cost [16] and the availability of soilless media are important factors in soilless culture. Different media for soilless culture have different water- and nutrient-holding capacities and aeration. For this reason, the most suitable media should be selected for different species. The physical and chemical characteristics of the medium, together with the growing techniques (e.g., fertigation) employed, will determine the yield and quality of the vegetables that are produced [61]. An early review by Gruda [35] strongly suggests changes in the quality parameters of many vegetables in response to the growing medium used. Albaho et al. [62] reported that peat moss, perlite, vermicompost, and coco peat or a mixture of these substrates in different ratios had significant effects on cultivars’ heights, number of leaves, chlorophyll index, and total yields of peppers.

5. Conclusions

Rotary drum composting of organic school wastes at the Al-Jazaer school, Shamiya, Kuwait, yielded mature compost with a C:N ratio of 20.55 on the 43rd day of composting. The composting material underwent distinct mesophilic, thermophilic, cooling, and curing phases. The pH of the composting material recorded a steady increase from 7.41 to 8.62, and the electrical conductivity exhibited a drop from 5.02 mS/cm to 2.71 mS/cm during the composting process. The total organic carbon content of the feedstock was reduced from 30.77% to 17.76% during composting. The nitrogen concentration increased by 0.26% on 29th day and carbon content was reduced by 10.76% on the 43rd day, resulting in the desirable C: N ratio. The influence of seasonal variations and the resultant ambient environment during the composting process as well as the effect of feedstock volume may be evaluated to optimize the rotary drum composting technique. Selected proportions of in-house prepared rotary drum compost, when used as a soilless substrate component, produced promising results in terms of vegetative growth and yield of greenhouse cucumbers. The study portrayed rotary drum composting as a viable option for the sustainable management of school wastes, and the compost as a substrate component suitable for the soilless culture of greenhouse vegetables. Hence, rotary drum composting can be recommended for decentralized sustainable MSW management at source on a small-medium community scale and for compost valorization. The results may also impart tangential influence on promoting agricultural activity and gardening in schools, which may help students to become more active and enthusiastic learners and to develop a more resilient, confident, and responsible personality.