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Production and characterizations of bio-fertilizer from tree leaves utilizing automated hot composting chamber with cyber-physical systems

Abstract— Composting is a reliable process to transform waste dry leaves into superior quality compost. In this research, we proposed a rapid composting technique for converting powdered dry leaves to matured compost in an automated rotary drum composter and monitoring the physico-chemical properties of the pile by cyber-physical systems. Here, the solar energy based equipments were utilized for grinding dry leaves and energizing heating coil inside the composter. The optimal temperature of the composter had been optimized and maintained 50-55 °C by using closed loop heating coils, and monitored by temperature sensors. During composting, the moisture content decreased with respect to the duration. Contrarily, the pH and electrical conductivity gradually increased as the dried leaves are being converted to the matured compost. The aim of this research is to automate the composter by using sensing devices, and monitored the physico-chemical characteristics in terms of pH, electrical conductivity, moisture content and temperature. Here, we anticipated a cost-effective, less maintenance and eco-friendly quick composting technique for obtaining good quality of compost from the leaves within 30 days.
Keywords— Rotary drum composting, physico-chemical parameters, dry leaves, process monitoring, compost quality, cyber-physical systems.
With respect to the crop production level in India, it is projected that the crops produce around 679 million tonnes of residues every year, and the anticipated N-P-K supply from the crop residues is 6.25 and 10.25 million tonnes, for the year 2011 and 2030, correspondingly 1-2. Unfortunately, around 90% of these wastes are used for land ?lling and uncontrolled dumping on outskirts of cities, which have serious environmental problems in global warming by the emission of green-house gases 2. Hence, an effective technology is required to compost the crops’ residue into prolific product in a short period of time. Composting is one of the effective techniques to convert crop residue to good nutritional quality compost 3-4. It is an exciting global process to turn wastes into useful product; however it depends on the consideration of cost, nature of the pile material and the amendment of the produced compost to the soil.
Composting is a biological decomposition and stabilization of the organic materials into a dark rich substance, which is stable, non-polluting, free of pathogens, and possesses significant value in agriculture 5. It is a vital process in waste management, owing to its robustness and obtaining valuable products with soil amendment potential. Furthermore, it is beneficial to the land and ecosystems, which include as soil conditioners, as fertilizers, addition of humic acids to the soil, as natural pesticides, erosion control, land reclamation, wetland construction and as landfill cover 4-5. Specifically, the quality of the compost depends upon source, composting facility design, composting technique and duration of composting process. Generally, the composting process occurs in two phases. Firstly, the decomposition phase (breaking of complex organic matter), which has three stages, namely, mesophile, thermophile and cooling. Secondly, the humification phase, which is related to the maturation phase characterized by the reorganization of the organic matter in the pile 6-7. In particular, the compost from the dried leaves has certain advantages such as economically feasible, eco-friendly and publicly acceptable 8.
Generally, the efficiency of the composting depends upon the pH, carbon to nitrogen ratio, moisture content, ionic exchange capacity, aeration in the chamber, bulk density, particle size and porosity of the composting piles 9-11. For rapid composting, the leaves must undergo size reduction in order to increase the surface area, which enhance the speed of biological oxidation with respect to the quantity of surface 6. Furthermore, the maturation is being carried out at ambient temperature, under the mesophilic microorganisms (bacteria and fungi) 6. The conversion of matured compost can be detected by a pleasant odor, growth of white fungi, low temperature, does not emit ammonia odor, granular, reduction of piles’volume and color change of the pile into brown color 12-13.
Conventionally, the composting is being carried out by using the waste organic materials and waiting for the materials to split into humus after several months 14. The conventional composting has certain drawbacks such as space utilized a long time, some nutrients leached out during rainfall and produce disease causing organisms and insects 15. In our research, we overcame the above drawbacks by utilizing fully automated rotary drum composter, which possess significant benefits, for instance, the system inexpensive, better moisture-holding capacity, superior air circulation and stirring of the composting materials effortlessly by rotating the chamber.
The rotary drum composting is traditionally carried out by cold process, which possess definite drawbacks such as uneven decomposition, nutrient leaching, pest attraction, anaerobic decomposition and very slow process (10-12 months) 16. In order to overcome the above drawbacks, we are going to employ hot composting process, which produces the matured compost in a short period of time. Furthermore, it has significant benefits such as killing weed seeds and disease causing pathogens, breaking down the organic materials into fine humus, need less space, no pests, degradation of toxin and more efficient process. However, the hot composting acquire certain difficulties, for example, it needs lot of manual effort, fire hazard and death of microbes at high temperature 17. In order to prevail over the mentioned drawbacks, in this research, we proposed a fully automated system with heating coils for optimizing and controlling the temperature inside the chamber, and monitored the physico-chemical characteristics (temperature, moisture, pH and ionic conductivities) by specific sensing devices, which are essential parameters for effective composting of the pile. In addition, the temperature and humidity of the atmosphere were also measured by sensors.
A. Sampling
Dry tree leaves (Millettia pinnata) were collected from our (AAME) college campus, kovilvenni, Tiruvarur, India. The leaves were cleaned in running water and dried in sunlight for a week. Subsequently, the dried leaves were chopped by using a grinder followed by sieving using mesh. The grinding of leaves was carried out by using specially designed system, which is shown in the Fig.1.

Fig.1. Image of the grinding system
B. Design of composting chamber
The composting process was carried out in a plastic drum of 125 L capacity in batch mode process, and the drum was suitably customized for air circulation. Additionally, black color paint was applied on outer surface of the rotary drum for retaining the heat from the sun light also. A sampling window at middle part of the chamber was used to feed the powdered dry leaves (~15 kg) and used to collect the samples for physico-chemical analysis, periodically. For draining out leachate, a hole was made under the drum. During composting, rotation of the composting chamber had been carried out from time to time. The image of the rotary composting chamber is shown in Fig. 2.

Fig.2. Image of the composting chamber
C. Physico-chemical parameters of composting materials
Physico-chemical properties such as temperature, moisture, electrical conductivity, and pH (1:10 w/v) were measured during composting process by using specific sensors, digital thermometer, humidity, electrical conductivity and pH meters, respectively. The samples were manually collected from three spots in the chamber, and mixed thoroughly to make a homogenized sample for analysis. Additionally, moisture content of the powdered dry leaves to the matured compost was found out by using Gravimetric method 18. Furthermore, solar radiation, temperature and humidity of the atmosphere had been measured by specific sensors every day. Specifically, we employed a photovoltaic system, which comprising of solar cells used to generate electrical power for grinding the leaves and heating the coils inside the chamber. The schematic representation of the proposed composting process is shown in the Fig. 3.

Fig.3. Schematic of the proposed composting process
A. Moisture Content
Moisture content (MC) is a critical factor, which in?uences the oxygen uptake rate, free air space, microbial activity and temperature, and the optimum MC for effective composting mainly depends on the type and nature of the composting material. For optimizing and enhancing the composting process, the decomposition of the organic materials by the microbial activities is significant one and the composting works well if the moisture content of the pile is around 50 % 18-19. Specifically, very low MC cause early dehydration and seize the composting, which produces biologically unstable product 19. Contrarily, high MC produce soggy mass, clumpy and anaerobic conditions as a result of water logging and high transportation cost, which prevent the composting process 19. In this research, the MC gradually decreased from 71.3 to 65.1 %, from the powdered dried leaves to the matured compost, respectively (Fig.4), owing to maintain the optimal temperature of 50 – 55 °C during the composting process.

Fig.4. Changes in moisture content during composting period.
B. Influence of temperature
Generally, the temperature of the chamber maintain around 50 – 55 °C for effective composting process. The optimal temperatures are used to achieve proper sanitation, rapid degradation, water evaporation, and humification during composting 11, 19. In particular, high temperature should be avoided, because they slow down the biological activity and cause undesirable chemical modifications of the composting pile. Additionally, the heat also generated by the respiration of the microorganisms by break down of the organic materials 6. In this research, the compost chamber had been frequently rotated to prevent the pile from getting too hot 20. Because, turning the pile is not only used to reduce overheat, but also used for well aeration, which is essential to decompose organic materials in the chamber. During composting, thermophilic phase (> 45 °C) occurs for a longer time in the composting process as it attributes towards the killing of pathogens and reduces biodegradable carbon 6. Furthermore, the development of eumycetes and actinomycetes, which are the main decomposers of long-chain polymers, cellulose and lignin, had been well functioned at optimum temperature (50 – 55 °C) 6. In this research, we optimize, maintain and monitor the temperature by using digital temperature sensor, maintained around 50 – 55 °C throughout the process by using closed loop heating system.
C. Electrical conductivity
The electrical conductivity (EC) of the dried leaves was 2.4, and the matured compost was 3.3 S/m (Fig. 5.), and the EC measurement is used to identify the presence of soluble salts during composting. The enhancement of the EC was observed during thermophilic phase of the composting material owing to the decomposition of organic matter into the nutrients 6. Significantly, augment of the EC, during composting was mainly due to the increase of concentration of cations in the chamber 7. In particular, the humic compounds formed during composting possess high capacity of swap over positively charged ions, which are easily exchanged with other cations. The EC tends to increase during composting, as the substrates are humified, and formed carboxyl and phenolic functional groups 21. The EC values reflect the degree of salinity, and illustrated its possible phytotoxic effects on the plant growth, when applied to the soil.

Fig.5. Variation of electrical conductivity during composting.
D. pH measurements
The pH is a vital factor, which influences the degradation process of the organic matter and affects the activities of microbes during composting 10. Generally, the optimal pH values are in between 5.5 and 8 for effective composting process, and the pH values around neutrality are most favorable for the development of microorganisms in the pile 6. Furthermore, the pH may be considered as good indicators for compost stability and maturity 14. In this research, the pH varied from 6.2 to 7.6 for the dried leaves and matured compost, respectively. The pH of all the trials at regular time intervals was in the range of 6.2 – 7.6. It might be due to the dried leaves possess high nitrogen content, which favors the conversion of nitrogen bound organic compounds in to ammonia at the final stage of composting process 6, 21-22. Here, the improvement of pH was due to the activity of proteolytic bacteria releasing ammonia, by broken down the proteins and nitrogenous bases. Further, the pH enhancement depends on the conditions of airflow in the chamber, which allows degradation of the dried leaves at higher pH 10.

Fig. 6. pH variation during composting process.
The obtained compost was dark brown colour and an earthy odour. The moisture content decreased during the composting process owing to maintain optimal temperature range during the composting process. Contrarily, the pH and EC slightly increased from the dried leaves to the matured compost. Specifically, the grinding of the leaves into fine particles and maintain the temperature by using heating coils inside the composting chamber were powered by solar. The optimal moisture content – 65.1 %, pH – 7.6 and electrical conductivity – 3.3 mS/cm were observed for the matured compost after 29 days. Hence, the proposed rotary drum composter is an effective technique for rapid composting of dried tree leaves, competently.
Authors would like to thank the Tamilnadu State Council of Science and Technology for the financial support for this research.
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