Microbes Environ. Vol. 20, No. 1, 1–13, 2005 http://wwwsoc.nii.ac.jp/jsme2/ Minireview Microbiology of Fed-batch Composting TAKASHI NARIHIRO1 and AKIRA HIRAISHI1* 1 Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi 441–8580, Japan (Received December 6, 2004—Accepted January 11, 2005) Repeated fed-batch composting (FBC) processes, which are modifications of traditional composting technolo- gy, have in recent years attracted attention not only in their biotechnological aspects but also from ecological viewpoints. FBC processes differ from the conventional batch system in that the biodegradation of solid organic waste proceeds without definitive thermal stages including the thermophilic phase under self-heating conditions. Mesophilic FBC processes for treating household biowaste are also characterized by low moisture contents, alka- line pH and the presence of high population densities of microorganisms under steady state conditions. Kinetic and microbiological studies of the FBC process have increasingly been conducted using commercially available composters as well as laboratory- and bench-scale reactors. Information from recent studies on FBC processes has provided new insight into our understanding of compost microbiology. This article reviews current knowl- edge of the FBC process with a special emphasis on microbial diversity, succession and activity in garbage com- posters. The potential application of FBC technology for bioremediation purpose is also discussed. Key words: fed-batch composting, compost, microbial community, solid-phase bioreactor ature is increased by self-heating as a result of vigorous mi- Introduction crobial activity. In the thermophilic phase, the temperature Composting provides a good model of microbial commu- reaches 80LC, which not only stimulates the proliferation of nities to study ecological issues such as diversity, succes- thermophilic microorganisms but also prevents the growth sion and competition during the biodegradation and biocon- or survival of mesophilic microorganisms, including meso- version of organic matter with thermal gradients. The philic pathogens. After the thermophilic progression of typical batch composting process proceeds via four major waste decomposition, the microbial activity lowers due to thermal stages, i.e., the mesophilic, thermophilic, cooling the limited availability of degradable organic substances. and maturation phases, each of which has a particular mi- This cooling phase leads to a decline of temperature and al- crobial community structure developing in response to tem- lows mesophilic microorganisms to predominate again. perature and other environmental conditions. In the first Eventually, solid organic waste turns into stable end com- stage, organic substances are decomposed by mesophilic pounds, including humic-like substances. In addition to the microorganisms at moderate temperature. Then, the temper- traditional batch process, sequencing-batch or semi-contin- uous composting systems are being widely used for the * Corresponding author; E-mail: [email protected], Tel: 81– treatment of various organic wastes. In sequencing-batch 532–44–6913, Fax: 81–532–44–6929 composting systems with a long type of field-scale reactor, Abbreviations: FBC, fed-batch composting; FUSBIC, flowerpot-us- ing solid biowaste composting; PCDD/Fs, polychlorinated diben- a thermal gradient occurs during the continuous flow of 25) zo-p-dioxins/dibenzofurans; PLFA, phospholipid fatty acids; composted material from the way in to the way out . SCM, solid waste-compost mixture. Details of physicochemical and microbiological features 2 NARIHIRO and HIRAISHI of the conventional batch composting process have been reviewed8,19,20,23). Composting technology has in recent years been modi- fied for fed-batch treatment of solid organic waste using rel- atively small-scale reactors. In the repeated fed-batch com- posting (FBC) process, reactors are daily or repeatedly loaded with fresh organic waste and always operated under relatively nutrient-rich conditions, unlike the conventional batch composting system. Therefore, FBC reactors have a much lower and narrower range of temperature, usually less than 50LC, and provide conditions most favorable for growth of mesophilic microorganisms, unless otherwise in- cubated forcibly. In Japan, various types of electric FBC re- actors are commercially available. In particular, personal composters for the treatment of household garbage have come into wide use. A flowerpot-using solid biowaste com- posting (FUSBIC) system has been studied as a model of 33,35,41,42) Fig. 1. Schematic illustration of a representative of personal garbage the self-heating FBC process . composters available commercially. Common specifications for Since microorganisms fulfill the key role of waste degra- composters: reactor size, 450–760 (H)P400–575 (W)P335–415 dation in the FBC process as well as the conventional com- (D) mm; working volume, 20–43 L; amount of wood chips added posting system, the study of the resident microorganisms is as the solid matrix, 12–20 L; maximum capacity for waste load- ing, 1.0–1.5 kg (wet wt) day1. Every hour and just after the addi- prerequisite not only for elucidating the microbiological ba- tion of waste, the solid waste-compost mixture (SCM) is stirred sis of the process but also for improving it from a biotech- with an impeller for 1 to 5 min, this mechanical mixing being the nological point of view. Information from recent studies on only way to maintain aerobic conditions. Reactors are equipped FBC processes has provided new insight into our under- with a fan and a heater regulated with a thermistor. This heating system works only when the core temperature declines to 30LC. standing of compost microbiology. This article summarizes current knowledge of the FBC process with a special em- phasis on microbial diversity, succession and activity in per- sonal garbage composters. It also deals with the potential application of FBC technology for bioremediation purposes. General characteristics of FBC process The characteristics and performance of the FBC process have been studied mainly using commercially available composters and laboratory- and bench-scale reactors. A typ- ical commercial composter used for garbage treatment is il- lustrated in Fig. 1. When FBC reactors are operated with household biowaste under self-heating conditions, they ex- hibit common physicochemical characteristics with respect to temperature, pH, moisture content, conductivity and so on. Changes in these parameters commonly found during the start-up operation of FBC reactors are shown in Fig. 2. Fig. 2. Changes in physicochemical characteristics of SCM com- As mentioned above, the biodegradation of organic waste monly found during the start-up operation of personal garbage in traditional batch composting processes is achieved composters at a waste-loading rate of 0.7 kg (wet wt) day1 under through different thermal stages. In general, maximal mi- self-heating conditions. Line a, core temperature; line b, pH; line crobial activity during composting is found at around c, moisture content; line d, conductivity. The figure was made based on information from references 68 and 71. 60LC67,88). On the other hand, waste decomposition in FBC reactors takes place in lower ranges of temperature under Microbiology of Fed-batch Composting 3 self-heating conditions. During start-up, the temperature in FBC reactors rises at the beginning but becomes stable usu- ally between 30 and 45LC at the fully acclimated stage33,35,41,42,68,71). In each batch cycle in the steady state, temperature fluctuates with the degradation of biowaste within the range noted above26,33). In this respect, the FBC process is similar to the mesophilic or cooling phase of the conventional batch composting system. However, the former process is different from the latter in that the resident microorganisms are repeatedly supplied with fresh biowaste while they proceed with the biodegradation. This is the main reason why the FBC process has a narrower and lower Fig. 3. Mass reduction efficiency of personal garbage composters operated at a waste-loading rate of 0.7 kg (wet wt) day1 under range of temperature than the conventional batch system. In self-heating conditions. Left: (A), cumulative amount of biowaste some cases, the temperature in FBC reactors rises over 50LC added; (B), increased mass of SCM. Right: relationship between by self heating26,33). For accelerating the biodegradation, mass reduction and net solid reduction efficiencies. The figure FBC reactors are operated under slightly thermophilic con- was made based on information from references 33, 35 and 68. ditions by on-off control of a heater30,72,75,81). Also, FBC re- actors operated forcibly at different temperatures (10–60LC) CO2 and H2O). FBC reactors showing high mass reduction have been reported45). rates can be operated for a long time without the removal of The mesophilic FBC process for garbage treatment is SCM. However, long-term use brings about the accumula- also characterized by low moisture contents and alkaline tion of excess amounts of minerals in the reactor as judged pH. The moisture content of the solid waste-compost mix- from increasing conductivity (see Fig. 2). In the case of ture (SCM) in commercial FBC reactors in the steady state commercial electric composters, it is desirable to remove is kept between 30 and 40% at a waste-loading rate of 0.7 excess SCM from the reactor every 2 months.