Maturity and stability assessment of composted tomato residues and chicken manure using a rotary drum bioreactor

ABSTRACT

The efficiency of the composting process for tomato residues and chicken manure was estimated after monitoring of the rotary composting system. Physicochemical parameters and Compost Quality Index were evaluated. The tomato residues (leaves, stems, and some green and damaged fruits) were collected, cut into small pieces, moistened approximately (60–65%), and mixed with 20% chicken manure then distributed into three rotary drum bioreactors. The obtained results showed that, the temperature above 50°C was maintained for more than two days. Carbon: nitrogen (C:N) ratio was reduced from 30:1 to 19.13:1. The pH value ranged between 7 and 8.80 during the composting process, while the electrical conductivity (EC) ranged between 2.67 to 4.53 dS/m. Both compost quality parameters (Dewar and germination index) and (Solvita-CO₂ and Solvita-NH₃) indicated that, the final compost is stable and mature.

Implications: The idea of this research revolves around assessing the maturity and stability of the compost resulting from mixing tomato residues with chicken manure, using a rotary drum bioreactor which is characterized by reducing the time of the active phase to several hours or days instead of weeks or months. Several tests related to the maturity and stability of mixture have been used to judge its quality. Also, many parameters related to this topic were monitored and discussed with many previous researches to determine the importance of benefiting from mixing the different wastes together and obtaining a good fertilizer ready for application as an agricultural substrate or a soil conditioner.

Introduction

Organic waste generation is increasing worldwide and strategies must be developed and optimized for their environmental impact and cost. It is an urgent task to develop high efficient, inexpensive and environmentally friendly conversion technologies (Wang, Zeng, and Cao 2018). Molecules of biomaterials are promising materials, with low toxicity, good biodegradability, and environment friendliness (Zheng, Olayinka, and Bin 2018). One of the most widely used technologies is composting (Togun, Akanbi, and Adediran 2004). It has a high capacity for transforming various organic wastes into a valuable product useful for agricultural purposes (Jindo et al. 2012), as horticultural growth media to improve the soil structure by increasing soil organic matter and soil fertility (Gao et al. 2010; Togun, Akanbi, and Adediran 2004).

The rotary drum bioireactor is one of the promising devolved composting systems which have been established worldwide as having positive advanced features (Vuorinen and Saharinen 1997). It offers aeration, agitation and mixing of the compost to produce a reliable and uniform end product (Kalamdhad and Kazmi 2009).

Poultry plays a major role in developing countries, as is production relatively inexpensive and widely available (Windhorst 2008). The uncontrolled decomposition of chicken manure releases NH₃, N₂O, and CH₄ in the atmosphere, posing many threats to the environment. Also, these wastes can contaminate soil and water if applied periodically as a direct organic soil amendment. Kulcu (2014) investigated that, the optimum mixing ratio that can be applied in the fertilization process of greenhouse tomato residues, wheat straw and dairy manure for fertilization is 60% greenhouse tomato waste, 30% dairy manure and 10% wheat straw. Although the addition of mature compost to agricultural soil has several advantages, immature compost addition creates a suitable environment for anaerobic microbes because microbes deplete oxygen in the root area and continue to digest the organic materials available anaerobically (Butler et al. 2001; Mathur et al. 1993). Consequently, some intermediated toxic compounds are produced, such as hydrogen sulfide (H₂S) and nitrite (NO₂), which can limit the plant growth or adversely affect seed germination (Butler et al. 2001; Cambardella, Richard, and Russell 2003). Therefore, compost stability and maturity are often determined before any recommendation is made not only to assess the quality of the compost product, but also indirectly measuring the efficacy of the compost technology itself (Arnold and McCumber 2015). Also, successful composting process requires the monitoring of influential factors and control the composting progress such as compost temperature, moisture content, aeration C:N ratio, pH, and EC, where reflects the level of salinity in a compost product (Gao et al. 2010). Therefore, the aim of this study is to investigate the evolution of some physico-chemical, biological parameters and assess the maturity and stability of the composted tomato residues mixed with chicken manure using a rotary drum bioreactor.

Materials and methods

Collection of plant residues

Greenhouse-tomato, fresh plant residues (stems, leaves, and some green and damaged fruits) were collected from various farms of Riyadh city, Saudi Arabia. Preceding the composting, tomato residues were obtained at an average moisture content (MC) of approximately 90%. Then they were spread out on the ground to dry for three days before being chopped using a gas motor-powered shredder (Model FYS-76 Shredder, China Mainland) to promote better aeration and moisture control. In order to enhance the microbial degradation process, the residues were grinded to reach a suitable particle size (≈ 1–2 cm). The grinded residues were left on the floor for an additional two consecutive days for more drying (MC 15%), than tightly bagged and transported to the Biology Lab at the Educational Farm, King Saud University camps. For making compost tomato plant residue (80%) was mixed with 20% chicken manure, as a nitrogen source, and MC of the mixture was adjusted to be about 60%. The main chemical characteristics of the raw materials used in this study are listed in Table 1.

Table 1. Chemical characteristics of the raw materials used in the study (mean ± SD, n = 3)

		Measured value
Characteristics	Unit	Tomato residues	Chicken manure
Moisture content (MC dry basis)	%	05.53 ± 0.11	06.15 ± 0.06
Porosity	%	67.00 ± 0.06	53.20 ± 0.15
pH	-	06.96 ± 0.14	07.90 ± 0.10
Electrical conductivity (EC)	dS/m	00.35 ± 0.02	02.67 ± 0.04
Volatile solids (VS)	%	80.72 ± 0.21	58.34 ± 0.28
Ash	%	19.28 ± 0.22	41.66 ± 0.36
Total nitrogen (TN)	%	02.90 ± 0.18	4.32 ± 0.04
Organic matter (OM)	%	44.85 ± 0.17	79.41 ± 0.21
Organic carbon (OC)	%	48.45 ± 0.38	45.69 ± 0.20

Notes. SD: standard deviation.

Composting was performed in three identical pilot-scale rotary drum bioreactors (200 L each) at the Educational Farm, Agric. Eng. Dept., King Saud University. Each bioreactor was consisted of a steel barrel (Figure 1), designed to provide space for 50 kg (wet bases) of mixture plus 25% of the volume as head space. The barrel has an inner diameter of 58.5 cm, length of 91.4 cm, wall thickness of 0.09 cm and having a door of 50 cm × 40.5 cm that was made for loading, unloading, sampling, and cleaning. The outer surfaces of each bioreactor were insulated with a layer of 2.5 cm-thick glass wool. Each bioreactor rotated horizontally around a fixed axis (i.e., steel tube, 5 cm outer diameter) at 3 rpm by using a 0.25 HP MEZ electric motor (model no. 220–380-3, Zhejiang, China). The perimeter of the fixed tube in each bioreactor included on-line holes distributed longitudinally in the upper surface of the tube for measuring the compost temperature, and on the lower surface for aeration. The air was provided continuously from an air compressor to the central fixed tube, which the barrel rotated around, at the chosen airflow. The regulated air, then passed through a flow meter to control and measure the volume of the air entering each bioreactor. The flow meter has a scale that can control the air volume at 0.001 to 0.025 m³/min. Thermocouples (Copper-constantan-type T-Cole Parmer, Chicago, IL, USA) were used to measure the temperature inside and outside each bioreactor. The MC of the mixture was adjusted to about 60% at the beginning of the composting process, and C:N ratio at ≈ 30:1. After that, the adjusted mixture was transferred to three bioreactors (filled up to 75% of the total volume).

Figure 1. Schematic diagram of the constructed rotary drum bioreactor system; dimensions in cm, not to scale

Stability and maturity measurement

Physical and chemical analyses of composted materials

Representative samples of mixture were taken every 2 and 8 days (during the active and curing stage respectively (6 days active stage and 30 days curing stage) for analyzing and measuring the following parameters: composts temperature, MC, pH, EC, total organic matter (TOM), and C:N ratio throughout the composting time. The temperature of the compost was measured every 10 min using three thermocouple sensors inserted inside each bioreactor, connected to a data logger; the average of the three bioreactors represented the evolution of compost temperature. The gravimetric method (BIS 1982) was used to determine the moisture content by drying the compost sample (at 105°C for 24 h) and weighing the loss. The pH was measured by using a Jenway digital pH meter, model 3510 (United Kingdom). EC was measured by filtering the compost sample through Whatman filter paper No. 42 using a digital conductivity meter. Organic carbon (OC) and organic matter (OM) contents were calculated as follows: OC (%) = OM (%)/1.8, OM (%) =100-Ash (%) (Abdullah and Chin 2010). Losses of organic matter (OM) and total nitrogen (N) were calculated from the initial (X₁) and final (X₂) ash contents and the initial (N₁) and final (N₂) concentrations, according to the following equations (Paredes et al. 2000):

(1) $\mathrm{O}\mathrm{M}\mathrm{L}\mathrm{o}\mathrm{s}\mathrm{s}(\mathrm{\%})=100-100\times \left[{\mathrm{X}}_1\left(100-{\mathrm{X}}_2\right)\right]/\left[{\mathrm{X}}_2\left(100-{\mathrm{X}}_2\right)\right]$(1)

(2) $\mathrm{N}\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}(\mathrm{\%})=100-100\times \left[{\mathrm{X}}_1{\mathrm{N}}_2\right]/\left[{\mathrm{X}}_2{\mathrm{N}}_1\right]$(2)

The content of ash was determined as the difference between the dry matter content and the gravimetric loss-on-ignition produced by ashing of the previously dried samples, at 550°C for 8 hrs. (EN 13,039 1999). Total nitrogen (%) was determined by the Kjeldahl digestion method (EN 13,654–1 2001).

California compost quality index protocol

Stability and maturity of compost cannot be described by a single parameter; at least, two or more specific tests will provide the highest level of confirmation. Generally, the evaluation of compost quality is done according to a procedure summarized by the California Compost Quality Index (Buchanan et al. 2001), where final product can be described as very mature, mature and immaturecomposts according to the reading of their maturity index. This is done specifically by three levels of decision as shown in Figure 2.

Figure 2. Compost maturity assessment process

Solvita maturity test

For Solvita test, moisture content of the samples was adequate as determined by a squeeze test. The sample carefully fills the Solvita jar to the fill line. The samples were incubated in 200 ml containers with a Solvita reactor for a 4-hour incubation period at 20–25°C following the manufacturer’s instructions (WERL-2018, Guide to Solvita Testing for Compost Maturity Index). Then, the extent of color change was measured using DCR (Digital CO₂ & NH₃ Color Reader, Solvita®) and the maturity index (MI) was calculated as described in the manufacturer’s protocol.

Dewar (self-heating) test

For Dewar test (WEL 2009), the samples were stored at 4°C until use. Each sample was kept in a flask for six days, then the maximum temperature and the corresponding room temperature were recorded, and consequently, the temperature difference was estimated for each sample.

Germination percentage and germination index

Germination index (GI) was determined using an adaptation method described by Zucconi et al. (1981b). Ten grams of screened sample was mechanically shaken with 100 ml of distilled water for one hour in the incubator shaker (IKA® KS 4000i control, Germany/Deutschland). The suspension was mixed for 15 min at 3000 rpm, and the supernatant was filtered through a Whatman filter paper No. 6. Ten seeds of white radish (Raphanus sativus) were placed on filter paper Whatman No. 1 in sterile Petri dishes (10 cm diameter). The seeds were scattered with optimum distance between them in each dish. About 4.0 ml (divided into two intervals) of the tomato composts extract was pipetted into a Petri dish. 4 ml of distilled water (added on two intervals) was used as a control and the aqueous extract was also sterilized through filtration at 0.45 µm pore size. The test was run in three replicates. Then the Petri dishes were covered with the lids and incubated at 25°C in the dark inside the incubator (Binder, USA) for 5 days. At the end of the 5th day, the percentage of seed germination in the tomato composts extract was compared with that of the distilled water control. The radish seeds were examined for germination, where germination was sure as any protrusion through the seed coat (Bazrafshan et al. 2016). Treatments were evaluated by counting the number of germinated seeds and measuring the length of the roots (Gao et al. 2010). The percentage of GI for evaluating phytotoxicity was calculated based on relative seed germination (SG) and root elongation (RE) was calculated according to (Zucconi et al. 1981b) as follows:

(3) $\mathrm{S}\mathrm{G}(\mathrm{\%})=\frac{\mathrm{N}\mathrm{o}.\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}}{\mathrm{N}\mathrm{o}.\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$(3)

(4) $\mathrm{R}\mathrm{E}(\mathrm{\%})=\frac{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}}{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$(4)

(5) $\mathrm{G}\mathrm{I}(\mathrm{\%})=\mathrm{S}\mathrm{G}(\mathrm{\%})\times \mathrm{R}\mathrm{E}(\mathrm{\%})/100$(5)

Statistical analyses

Statistical analysis was performed to compare variations in compost properties using SAS ver. 9.0 (SAS Institute, Cary, NC, USA). Data are presented as means (SEM) and analyzed by one-way analysis of variance (ANOVA), with the least significant difference (LSD) test at p_0.05.

Results and discussion

Temperature of the mixture

The temperature of the mixture started to increase rapidly after the creation of composting conditions and reached 53°C on the first 2 days, and reached its maximum value (63°C) after 66 hours of operation due to the faster breakdown of the available organic matter (OM) and nitrogenous compounds through microbial reactions (Petric, Helic, and Avdic 2012). Starting at ambient temperature, the compost can reach 40°C – 60°C in less than two days depending on the composition and environmental conditions (Young, Rekha, and Arun 2005). Then, it continued over 53°C for more than two successive days (Figure 3). This high temperature is important during the composting process, as theoretically, they could help destroy many pathogenic micro-organisms, weed seeds and weed propagules, thereby reducing the risk to human, animal and plant health (Brinton and Droffher 1994). The cooling stage started when the compost temperature started to decrease below 55°C after 80 hours. At the end of the cooling stage, temperature fluctuations became limited and temperatures remained largely constant until the end of the experiment. This is the final stage of the composting process and is called the curing or maturation stage; it takes a long time to produce humus and stable organic end product.

Figure 3. Average temperature changes of the mixture

The level of microbial heat generation due to tomato residue and chicken manure degradation was reduced obviously after the temperature reached 63°C. Hence, this temperature may have surpassed the optimum conditions for the thermophilic microorganisms and, as a result, microbial growth and possibly microbial activity declined, resulting in decreased bioreactor temperature (Ghaly, Alkoaik, and Snow 2006). Several researchers have tried to define the optimal temperature for composting. Mac-Gregor et al. (1981) and Bach et al. (1987) indicated that, the temperature optimum for biological degradation during the composting process in the range of 52–60°C. Leton and Stentiford (1990) proposed the optimum operating temperatures in the range of 45–55°C with an initial period above 55°C to sanitize materials. Also, it was recorded by Nakasaki, Shoda, and Kubota (1985) that, the optimal temperature for composting process as a whole is from 35°C to 55°C. Therefore, generally speaking, moderate is favored rather than extreme thermophilic temperatures during composting (Mac-Gregor et al. 1981). These results in agreement with the recommendations of the Guidelines for compost quality (Canadian Council of Ministers of the Environment 2005). As the OM became more stabilized, the microbial activity and the OM decomposition rate decreased, and the temperature in the compost gradually dropped, reaching 27°C (after 120 hours) owing to a reduction in bioavailable nutrients (Gao et al. 2010). Generally, decreasing temperature in the later stages of the composting process points to the stabilization of decomposed organic portions into compost (Manohara and Belagali 2014).

Moisture content

The optimum MC of the compost is a vital factor for the microbial decomposition of organic waste. MC remained approximately between 60 and 63% during the active period of composting, which considers the optimum range of microbial activity (Cabañas-Vargas et al. 2005; Haug 1993), while decreased to reach 57.5% during the curing period. In general, the optimum MC in an aerobic composting process is around 40–60%, which is favored during the composting process. The initial MC above 60% is proper for an acceptable composting (Elcik, Zoungrana, and Bekaraki 2016). However, excessive MC leads to a lower rate of oxygen, resulting in anaerobic conditions, which decrease OM degradation rate as well as promotes bad odor formation owing to the release of volatile compounds such as carbon and hydrogen sulfide (Manohara and Belagali 2014).

pH

During the active stage, pH value was about 7 in the first two days, and decreased less than 7 due to the production of organic acids and high level of CO₂ derived from the intense fermentation carbohydrates (Estevez-Schwarz et al. 2012). Afterward, the pH began to rise and reached 9.5 in the thermophilic period. This increase results from the release of ammonia due to the start of the proteolytic process (De Nobili and Petussi 1988). It decreased thereafter at curing stage to reach 8.8 at the end of the composting process due to the formation of NH₃ (Bazrafshan et al. 2016; Gao et al. 2010; Liao et al. 1996) and mineralization of organic nitrogen (Bustamante et al. 2008; Wong, Mak, and Chan 2001). This finding is in accordance with the recent study by Elcik, Zoungrana, and Bekaraki (2016) who reported that the pH increases over time as a result of microbial activity that caused the production of ammonia during ammonification and mineralization of organic nitrogen. Young, Rekha, and Arun (2005) indicated that, the pH of the compost has been observed to vary with time, showing an initial drop and then increasing to between 8.0 and 9.0, finally leveling off between 7.0 and 8.9. During the curing stage, the pH remained in the alkaline range may due to the mineralization of organic N that leads to NH₃ emission and depletion (Bazrafshan et al. 2016). Generally the pH tends to be alkaline in a mature compost (Song, Li, and Jia 2014; Young, Rekha, and Arun 2005). As long as the compost stays aerobic, the buffering has been proved sufficiently within the compost to allow the pH to stabilize at an alkaline level (Young, Rekha, and Arun 2005).

Electrical conductivity (EC)

Electrical conductivity (EC) is one of the most important physical parameters identifying conductance of liquid solutions. It is an indicator of soluble salt content, where it reflects the level of salinity in a compost product which suggests the possibility of phytotoxic or phyto-inhibitory effects (Gao et al. 2010). High levels of salt in a compost can result in a reduction in crop yields when salt concentrations near the root zone accumulate to a level at which the root can no longer extract sufficient water from the soil-compost solution. So it is very important to measure the EC in compost because, high salt levels may injure plants, with seedlings and transplants being most susceptible to injury (Sullivan et al. 2018). On the other hand, more soluble salts are soluble nutrients, so compost with a high salt concentration may be a good source of nutrients when applied at a low rate (Sullivan et al. 2018). An acceptable EC for compost used in field situations depends on soil EC prior to application, the compost application rate, depth of compost in corporation (tillage), soil texture, and irrigation water management (Sullivan et al. 2018). EC during the composting process ranged from 2.67 to 4.53 dS/m. The increasing in EC in the curing stage might be due to the net loss of weight and release of soluble salts through decomposition activity and degradation of organic matter during the composting process (Avnimelech et al. 1996; Bazrafshan et al. 2016; Singh and Kalamdhad 2013). As well it can be considered due to the increase in mineral cation concentration that it is not attenuated by salt leaching or by binding to stable organic complex and majority due to loss of organic matter (Francou, Poitrenaud, and Houot 2005; Gao et al. 2010). Some other studies believe that, because the mature compost is dark black due to its high carbon content and the carbonaceous materials are conductive materials. This result in agreement with Wang et al. (2019a) when they reported that, the ultra-black materials with excellent performance are carbon materials with high EC, and when these materials are heated, their temperature rises, causing changes in their electrical resistance (Chen et al. 2020; Zheng, Olayinka, and Bin 2018). Also, Wang et al. (2019b) reported that, graphene or carbon based materials have unique properties, such as high EC and superior mechanical flexibility. While, Zhou, Feng, and Chia-Yun (2019), reported that, the EC may increase due to the addition of some nanomaterials or decrease the resistivity by adding some chemical elements to the substrate (Rui et al. 2019). As well, the result of this study is in agreement with Hemidat et al. (2018) and Khater (2015) who reported that the EC varied from 2.58 to 4.14 dS/m, and Brinton (2000) who reported that EC varied from 2.0 to 4.0 for growing media. Irvan et al. (2018), reported that, EC increased to reach 7 dS/m at day 20 before to decrease and become constant at 4.72 dS/m at day 40.

For commercial vegetable crop production, the obtained EC result of tomato compost is suitable, where most of the commercial tomato and cucumber cultivars are sensitive to moderate salinity levels up to 2.5 dS/m, without significant reduction of yield (Abu-Zinada 2015; Singh, Sastry, and Singh 2012). On the other hand, higher EC values might reveal the degree of compost salinity, indicating that its possible phytotoxicity influences the growth of vegetable plants if applied to the soil (Madan et al. 2012).

Carbon to nitrogen (C:N) ratio

The C:N ratio is used to describe the organic waste decomposition and compost quality with respect to OM and N cycling. Several investigations indicated numerous ideal C:N ratios ranging from 12:1 to 25:1 for compost quality (Estevez-Schwarz et al. 2012; Tiquia 2010). For compost with a high C:N ratio (more than 40), the microorganisms take much time to break down waste for N deficiency, reducing composting performance (Escobar and Solarte 2015), while compost with a low C:N ratio causes ammonium toxicity (Bazrafshan et al. 2016). In the present study, the C:N ratio of mixture decreased through the active stage from 30.01 to 28.22. While, in the curing stage it decreased gradually to reach 19.13 at the end of composting period. According to Singh et al. (2009), C:N ratio of the final compost within 10–15 represents the stabilized compost and the C:N ratio between 15 and 20 is commonly accepted (Kayhanian and Tchobanoglous 1993). While, C:N ratio less than 20 represents matured compost (Heerden et al. 2002), which was achieved in the present study. According to Bernal et al. (1998); Chefetz et al. (1998); and Bazrafshan et al. (2016), the reduction in the C:N ratio during the composting process of the mixture was attributed to the transformation of carbon to CO₂, followed by a decrease in organic acid concentration and an increase in the N content per unit material. Generally, the C:N ratio of compost should be about 20 to avoid N immobilization and to simplify the release of mineral N for crop use when the compost is applied to the soil (Allison 1973). Compost characteristics of the studied parameters during composting stages are shown in Table 2.

Table 2. Compost characteristics during active and curing stages (mean ± SD, n = 3)

Compost parameters	During the 6 days (Active stage)	After 30 days (Curing stage)	Compost criteria (Limit values)
Temperature of mixture, °C	43– 53	(Optimum) 53 ± 0.01	52– 60
Moisture content (MC), %	60– 63	57 ± 0.12	50– 65
pH	7.0– 8.8	8.8 ± 0.03	6.5– 8.5
Electrical conductivity (EC), dS/m	2.67– 2.84	4.53 ± 0.01	2– 6
Carbon/Nitrogen ratio (C/N ratio)	30:1– 28:22	19.13:1 ± 0.89	12:1– 25:1
Solvita test (CO2)	-	CO2 = 7.23 ± 0.10	7–8 = Mature
Solvita test (NH3)	-	NH3 = 4.17 ± 0.20	3 = medium NH3, 4 = low NH3
Solvita index (Intersection of both tests)		7	7 = Mature
Dewar net – temperature, °C	-	1.6 ± 0.08	<10 is very mature
Germination index for radish seed, %	-	74.23 ± 11.07	80–90% mature, 50–80% mature, low ammonia, moderate phytotoxicity
Curing period		Cured for ≥ 21 days	Maturity tests acceptable

Solvita testing guide

The result of the Solvita test of the composted sample of the mixture was 7.23 out of 8 for CO₂, and 4.17 out of 5 for NH₃. The maturity index (MI = 7) is calculated by reading both probes and determining the interrelation of the two indicators according to the manufacturer’s protocol. According to WERL (2018) and Buchanan et al. (2001), the interpretation of the Solvita test result represents that the compost is very mature with low ammonia. More details of the Solvita test are shown in Table 5.

Dewar self-heating test

The Dewar or self-heating test is a valuable method for testing the maturity of compost (Estevez-Schwarz et al. 2012). The results of Dewar flask temperature verses time (Table 3) showed that, there were small differences between the temperatures inside and outside of the Dewar flask during the 6 days, ranged between 1.2 to 1.6°C (˂ 10°C). These results indicated that the compost product is finished and mature (WEL 2009).

Table 3. Result of Dewar or self-heating test of the final compost product (mean ± SD, n = 3)

Day	Flask temp. (^oC)	Ambient temp. (^oC)	Net rise temp. (^oC) (Flask–Ambient)
1	22.2 ± 0.26	20.9 ± 0.10	1.3
2	22.6 ± 0.15	21.1 ± 0.30	1.5
3	22.7 ± 0.10	21.1 ± 0.26	1.6
4	22.8 ± 0.21	21.4 ± 0.10	1.4
5	23.0 ± 0.26	21.5 ± 0.17	1.5
6	23.3 ± 0.26	22.1 ± 0.20	1.2

Germination percentage and germination index

Germination index (GI) is one of the sensitive parameters for evaluating the toxicity and the degree of compost maturity (Tiquia, Tam, and Hodgkiss 1996; Zucconi et al. 1981b). The germination percentage of white radish seeds (Raphanus sativus) in the control and in the compost runs during the 5 days are presented in Table 4. Most of the seeds had germinated in the control and in the compost runs, particularly over the test period. The relative reduction in germination seeds during the earlier germination periods might be owing to the release of some concentrations of ammonia, which influences the seed zygote’s sensitivity during its earlier germination period (Asgharipour and Sirousmehr 2012; Ells, McSay, and Workman 1991; Wong 1985). To assess the response of white radish seeds to the toxicity of the compost extract, a GI based on relative seed germination and the relative root elongation index was used (Tiquia, Tam, and Hodgkiss 1996). They reported that, GI is a sensitive indicator of compost maturity and phytotoxicity, where GI values lower than 50% indicate high phytotoxicity, values between 50 and 80% point to moderate phytotoxicity, and values more than 80% indicate nonappearance of phytotoxicity. However, phytotoxicity may occur with stable or mature composts due to substances which are not removed in the composting process, like heavy metals, and persistent herbicides (Research report 2005). The GI value of white radish seeds, which germinated on the tomato compost at the end of the germination period, is 74.23% (Table 4), indicated that, the tested sample of the compost is nottotally free of phytotoxicity but low phytotoxicity is present. This result is in agreement with the earlier findings obtained by the Zucconi research group (Zucconi et al. 1981a, 1981b; Zucconi, Monaco, and Forte 1985) as well as the later findings of Emino and Warman (2004), Barral and Paradelo (2011) and Estevez-Schwarz et al. (2012).

Table 4. Radish seeds germinated during the germination period

Sample No.	No. of seeds	Germinated seeds seeds	SG (%)	Radicle (mm)
1	10	9	90	42.25
2	10	10	100	45.19
3	10	10	100	62.73
Mean	10	9.67	96.67	50.06
SD	0	0.58	5.8	11.07
Control	10	9	90	72.46
GI (Final sample) according to Eqs (3(3) $\mathrm{S}\mathrm{G}(\mathrm{\%})=\frac{\mathrm{N}\mathrm{o}.\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}}{\mathrm{N}\mathrm{o}.\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$(3) , 4(4) $\mathrm{R}\mathrm{E}(\mathrm{\%})=\frac{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}}{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{r}\mathrm{o}\mathrm{o}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}\times 100$(4) and 5(5) $\mathrm{G}\mathrm{I}(\mathrm{\%})=\mathrm{S}\mathrm{G}(\mathrm{\%})\times \mathrm{R}\mathrm{E}(\mathrm{\%})/100$(5) ):			74.23%

Notes. SG: Seed germination, GI: Germination Indix, SD: Standard deviation

Table 5. Quality indicators results of the compost

			Solvita index		Dewar test (^oC)
Sample No.	(C:N) ratio	MC (%)	CO2	NH3	Ranged from	SG (%)
1	19.70	57.3	7.20	4.20	1.2 to 1.6	100
2	18.11	57.5	7.15	4.35	Max. value	100
3	19.58	57.6	7.34	3.95	10	90
Mean	19.13	57.5	7.23	4.17	1.4	96.67
SD	0.89	0.12	0.10	0.20	-	5.77

Notes. C:N ratio: Carbone: Nitrogen ratio, MC: Moisture content, SG: Seed germination, SD: Standard deviation

The summary results of the final compost quality indicators are shown in Table 5. It is clear that the C:N ratio 19.13:1 meets the first judgment decider of compost quality as stated by Buchanan et al. (2001). Moisture content (MC) is also in the optimum range for microbes. The second decider of compost quality is the stability test (Dewar). The result of Dewar test indicated that the compost is a finished and mature (max. temperature ˂ 10), while the third decider is the maturity test (GI). The result of the GI test indicated that moderate phytotoxicity is present. According to the compost quality index protocol, tested compost quality is “Good”. A similar conclusion is also reached using the results of Solvita tests. Stability test (Solvita-CO₂) and maturity test (Solvita-NH₃) showed that the final compost is mature. Therefore, our recommendation is to extend the maturation period to increase the quality of the final compost.

Conclusion

The final results showed that the maximum temperature of the compost reaches 63°C by the 5th day and it continued at over 50°C for more than two days. The C: N ratio decreased from 30:1 to 19.13:1 at the end of the composting time. The pH value ranged between 7.0 to 8.8, while EC ranged between 2.67 and 4.53 dS/m. The Solvita tests for CO₂ and NH₃ were 7.23 and 4.17 respectively. The Dewar test temperature ranged between 1.2–1.6°C. The presence of a small amount of ammonia inhibited seed germination and root elongation (GI = 74.23%). The physiochemical and both compost quality parameters (Dewar and GI) and (Solvita-CO₂ and Solvita-NH₃) were indicated that, the final compost is mature and has low phytotoxicity. Therefore, according to the results obtained in this study, the final compost product is stable and mature and needs a further maturation period to become ready for application as an agricultural substrate or a soil conditioner.

Acknowledgment

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project No. RGP-VPP-134.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the The Deanship of Scientific Research at King Saud University [Research group project No. RGP-VPP-134].

Notes on contributors

Mohamed A. Rashwan

Mohamed A. Rashwan, Assistant Professor, Department of Agricultural Engineering, College of Food and Agricultural Sciences, King Saud University, Saudi Arabia. Associate Professor, Department of Agriculture and Biosystems Engineering, Faculty of Agriculture, Alexandria University, Egypt. I did postdoctoral studies at a University of Alexandria, I hold a doctorate from Kagoshima University, Japan, in 2005 in the field of agricultural engineering. I am interested in working in environmental engineering fields such as compost production and greenhouse cooling. I also have an interest in solar energy and its uses in agriculture. I supervise a number of Master’s theses. I have published and judged many research papers and authored many books in the field of agricultural engineering.

Fahad Naser Alkoaik

Fahad Nasser Alkoaik, Professor and head of department of Agricultural Engineering, College of Food and Agriculture Sciences, King Saud University. The Main Investigator of a project titled “Design and Feasibility Study of Farm Scale Composting Unit for the Sustainable Utilization of Horticultural Wastes in the Production of Alternative Growth Substrates for Vegetables in Greenhouses.” Supervising of a number of Master’s and Doctoral Theses. Publishing and judging many research papers and authoring many books in the field of agricultural engineering. Attending many conferences at home and abroad.

Hesham Abdel-Razzak Saleh

Hesham Abdel-Razzak Saleh, Professor of Vegetable Crops Department, Faculty of Agriculture, Alexandria University, Egypt. Ph.D. in Agricultural Sciences (Dept. of Biotechnology and Biochemistry, 2002), Faculty of Agriculture, Miyazaki Univ., Miyazaki, Japan. He worked for 7 years at The College of Food and Agriculture Sciences, King Saud University, Vegetable Crops Department. Co-Investigator of project under National Science, Technology and Innovation Plan, King Saud University, Kingdom of Saudi Arabia. Supervising of a number of Master’s and Doctoral Theses. Publishing and judging many research papers and authoring many books in the field of agricultural engineering.

Ronnel Blanqueza Fulleros

Ronnel Blanqueza Fulleros and Mansour Nagy Ibrahim work as a technician in the laboratory, where they make various laboratory experiments and measurements and help in writing scientific papers.