Kitchen waste accounts for about one-third of urban garbage (wet weight basis) which is mainly collected and incinerated by local government at present in Japan. However, this process is a heavy financial and environmental burden on local communities. Thus, the development of an effective garbage treatment system would have great social significance. Among several possible options, the microbial decomposition type of garbage treatment system is regarded as one of the most environmentally friendly options (Golueke, 1972Golueke, C. G. (1972). Composting a study of the process and its principles. Emmaus: Rodale Press.; Kasinski, Slota, Markowski, & Kaminska, 2016Kasinski, S., Slota, M., Markowski, M., & Kaminska, A. (2016). Municipal waste stabilization in a reactor with an integrated active and passive aeration system. Waste Management, 50, 31–38.10.1016/j.wasman.2016.02.012; Suzuki, 1984Suzuki, S. (1984). Rural refuse composting plant. Journal of Irrigation Engineering and Rural Planning, 5, 48–56.). That is the reason why the microbial garbage treatment has been studied in our laboratory for a long period. One of the valuable results was the development of the static type of small scale garbage treatment systems (Matsuda, Iwata, & Uhara, 2012Matsuda, S., Iwata, R., & Uhara, Y.(2012). Development of an effective and ecological garbage treating system. Journal of Advanced Research in Physics, 3, 1–6.). This system has a simple structure but has sufficient performance in treating garbage by microbial decomposition with small generation of bad smell, and in particular, little consumption of electricity, and the system is classified as one of “passive aeration system” (Karnchanawong & Suriyanon, 2011Karnchanawong, S., & Suriyanon, N.(2011). Household organic waste composting using bins with different types of passive aeration. Resources, Conservation and Recycling, 55, 548–553.10.1016/j.resconrec.2011.01.006; Kasinski & Wojnowska-Baryla, 2013Kasinski, S., & Wojnowska-Baryla, I.(2013). Passive aeration allows stabilization of waste without regulation of moisture. Compost Science & Utilization, 21, 147–155.10.1080/1065657X.2013.859530; Ogunwande & Osunade, 2011Ogunwande, G. A., & Osunade, J. A.(2011). Passive aeration composting of chicken litter: effects of aeration pipe orientation and peroration size on losses of compost elements. Journal of Environmental Management, 92, 85–91.10.1016/j.jenvman.2010.08.026).
1.2. Basic concept of the “static type” of garbage treatment
The conditions required for an excellent garbage treatment system are as follows:
|(a)||High performance in garbage treatment (=large reduction in weight and volume of total garbage input can be stably maintained for long periods)|
|(b)||Small emission of bad smells: one of the most important factors of the system|
|(c)||Little electricity consumption: also an important point|
Continuous internal mixing with forced ventilation had been adopted in almost all conventional types of garbage treating machines. However, it was speculated from our accumulation of experience that this operating condition is one of the essential defects in this type of garbage treating system (Matsuda, Arai, & Muro, 2006Matsuda, S., Arai, K., & Muro, G.(2006). Development of ecological garbage treating system (pp. 449–455). Proceedings of 5th international conference on global research and education, Iasi, Romania.; Matsuda, Sugita, Kinoshita, & Amakata, 2002Matsuda, S., Sugita, S., Kinoshita, K., & Amakata, M. (2002). The development of garbage treating system without electricity. Journal of Chemical Industry and Engineering China, 53, 97–98.). As an improved technology, we tried to adopt a “static” type of treatment system, in which mixing is done only when the garbage is input (usually once or twice a day). Although proper mixing is absolutely necessary for micro-organisms to contact with their foods (substrate = garbage) in a solid–solid reaction system like a garbage treating system because microbes cannot move freely in the reactor, frequent or continuous mixing is not indispensable after micro-organisms are fully distributed to their substrates. On the other hand, mixing also works as an air supply system to aerobic micro-organisms in the reactor. This is the reason why continuous internal mixing was always required for conventional garbage treatment machines, since in most cases the reactor wall was made of steel or plastics through which air could never pass. Thus, the property of the reactor wall, especially the degree of permeation of air and moisture (we use the term “breathability” here), is very important if frequent mixing is avoided. We found out by several preliminary experiments that there are two major factors for a good static type garbage treatment system: i.e. high breathability of the reactor wall, and materials used in the reactor (bulking agent and/or bed for micro-organisms). Then, we investigated thoroughly the best combination of reactor wall material, materials used in the reactor and the depth of the heap (i.e. size of the reactor).
1.3. The difficulty and complexity of microbial garbage decomposition
Microbial garbage treating systems are classified as a “complex system” in contrast with their simple appearance, in which many factors, such as moisture content, temperature, and physical properties of the content in the reactor, interact with each other in diverse ways. For example, temperature in the reactor, which is one of the most important factors for microbial activity, is the cause of garbage decomposition performance, and is at the same time as the result of microbial activity because the decomposition process is essentially an oxidation reaction of organic compounds using O2. Other operating factors act in similar states, meaning that many factors cannot be fixed independently at the same instant, which is the essential difficulty in researching and developing a microbial garbage decomposition system. Important considerations needed for the optimal design and operation of a new type garbage treating system can be summarized as follows:
|(1)||Reactor: Shape (horizontal and vertical size), Wall Material, Mixing System (intensity, frequency and method)|
|(2)||Operating factors (correctable): Moisture content of the garbage, as well as in the reactor at the start, Organic loading to the reactor, Bulking agent (kind, quantity), Accelerator for Micro-organisms (kind, quantity)|
|(3)||Operating condition in the reactor (not always correctable: as a result): Temperature, Moisture content, pH, ORP, Physical properties of the reactor content (Density, Porosity, Viscosity, etc.), which were found to be important factors in composting (Agnew & Leonard, 2003Agnew, J. M., & Leonard, J. J. (2003). The physical properties of compost. Compost Science & Utilization, 11, 238–264.10.1080/1065657X.2003.10702132; Ahn, Richard, & Glanville, 2008Ahn, H. K., Richard, T. L., & Glanville, T. D. (2008). Laboratory determination of compost physical parameters for modelling of airflow characteristics. Waste Management,28, 660–670.10.1016/j.wasman.2007.04.008; Ahn, Sauer, Richard, & Glanville, 2009Ahn, H. K., Sauer, T. J., Richard, T. L., & Glanville, T. D. (2009). Determination of thermal properties of composting bulking materials. Bioresource Technology,100, 3974–3981.10.1016/j.biortech.2008.11.056).|
The garbage treating performance would appear as the result of the interaction among the factors above, and be expressed as the reduction rate of weight and volume of the input garbage as well as smell emission.