Journal of Environmental Biotechnology（環境バイオテクノロジー学会誌）
Vol. 3, No. 1, 33–36, 2003

Original paper (regular paper)

Acidulocomposting, an Accelerated Composting Process of Garbage
under Thermoacidophilic Conditions for Prolonged Periods
TOKUZO NISHINO1*, TORU NAKAYAMA1, HISASHI HEMMI1, TAKEFUMI SHIMOYAMA1,SATOSHI YAMASHITA1, MINORU AKAI1, TAKAHIRO KANAGAWA2 and KATSUJI HOSHI3
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University,Aoba-yama 07, Sendai, Miyagi 980–8579, Japan
2National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305–8566, Japan
3Star Engineering, Co., Ltd., Hitachi, Ibaraki 316–0022, Japan1* TEL: +81–(0)22–217–7270 FAX: +81–(0)22–217–7270* E-mail: nishino@mail.cc.tohoku.ac.jp(Received; 22 April 2003/Accepted; 12 July 2003)
We propose a new, highly practical process of accelerated high-temperature composting of garbage. This process, which
we named acidulocomposting, is autonomously sustained under thermoacidophilic conditions (at pH 3.5–6.5 and 50–70°C)
for prolonged periods (more than 2 years). This is in striking contrast with known accelerated composting processes that generally proceed under slightly alkaline conditions (pH 7.5–9). This process emitted only low levels of odor, in contrast to otherknown composting processes. Thus, acidulocomposting may be a low-maintenance process that can overcome problems associated with conventional accelerated-composting processes, such as low sustainability and odor emission.
Key words: acidulocomposting, garbage, composting, odor, high temperature, thermoacidophilic

1.

Introduction

Large quantities of garbage generated by homes, restaurants and food industries cause environmental problems,such as the formation of dioxins, when incinerated5). Thus,
the establishment of alternatives to treat garbage is an urgent issue, especially in small, overpopulated areas.
Composting, which produces fertilizer, is an effectivemeans of recycling garbage without the formation of dioxins
and is an alternative way of garbage management1,3,6,9). To
date, a variety of apparatuses have been designed to compost garbage from kitchens of homes, restaurants, feeding
facilities for school lunch, etc., and they are currently available from many commercial sources in several countries,
including Japan. These apparatuses are generally equippedwith heating and agitating devices that promote biodegradation of organic materials by thermophilic microorganisms
under neutral to alkaline conditions at moderate-to-hightemperatures (40–60°C)1,3,6). The development of such accelerated high-temperature composting (AHTC) systems
has been expected to promote efficient bio-recycling of garbage6). Nevertheless, several problems associated with thesesystems remain. First, composting with these apparatuses
tends to emit foul odors6). Second, the composting activity
by most AHTC gradually diminishes with a decrease in pHof the compost, and, finally, typically 2 to 6 months after

the start of the process, the composting activity stops andputrid odor is emitted6).
To overcome these disadvantages, the AHTC conditions
to compost garbage have been examined extensively, andan efficient AHTC process has been observed that proceeds
autonomously under acidic conditions for prolonged periods. We here describe this process, which we name here“acidulocomposting”, and its general properties to show
its practical advantages in the efficient recycling of garbage
from a wide variety of sources.

2.
2.1.

Materials and Methods

Microorganism

Alicyclobacillus sendaiensis strain NTAP-18) was a stock
culture of Nishino Laboratory, Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku
University. The cells were grown at 55°C and pH 4.5 for
48 h with shaking in the ATCC 573 medium containing
0.13% (NH4)2SO4, 0.037% KH2PO4, 0.025% MgSO4•
7H2O, 0.007% CaCl2,0.002% FeCl3, 0.1% glucose and 0.1%
yeast extract, and were used as a starter microorganism foracidulocomposting. The cells of strain NTAP-1 were stored
at –80°C in broth cultures supplemented with 15% (w/v)
glycerol.

NISHINO et al.

34

2.2.

Composting

Figure 1 shows the composting apparatus for acidulocomposting used in this study. It has a maximum capacityof 5 kg (wet wt) of garbage a day. Before the first additionof the substrate (garbage), the culture of A. sendaiensis(see section 2.1), typically 1 liter, was mixed with the basematerial (cedar-wood saw dust, 32 liters) in a vessel by running the machine for 1 h with no substrate. Considering
the importance of the practical feasibility and the general applicability of the process to compost garbage, whose
composition varies with the source and other factors (e.g.,seasons), we did not use “standard garbage” with a specificcomposition3,6) as a substrate for composting; instead, weused garbage from a wide variety of sources for evaluation,such as restaurants, homes of different countries, feeding
facilities for school lunch, and hospitals. Typical added substrates were food refuse and raw residual materials generated during cooking. All edible materials, fruit rinds, animalskins, fish bones, and eggshells are basically good substrates
for acidulocomposting. However, large animal bones, shells,
and wood materials were not suitable substrates. Excess water in the fresh garbage was drained by using a basket, anda specific quantity of the substrate was added daily to thevessel of the composting system, which was operated as described in the legend to Fig. 1. Through repeated additions
and composting of the substrate, the volume of the productcompost in the vessel gradually increased. Excess productwas removed to maintain the volume of the compost in thevessel at less than 27 liters. The temperature of the compostin the vessel was routinely measured at 30 cm depth fromthe surface before each addition of the substrate. Compost(1 g) was routinely sampled and was vigorously suspended
in 10 ml distilled water by using a vortex mixer. The resultant suspension was allowed to stand for 30 min at room
temperature, and the pH of the supernatant was measured.
2.3.

Measurement of odor levels

The odor emission from the acidulocomposting system was quantitatively analyzed by using an odor meterequipped with an electrochemical sensor based on a ZnO2
semiconductor (KALMOR-Σ, Karumoa, Co., Tokyo, Japan). The Σ value, which could be directly measured byusing the odor meter, is an arbitrary value showing a linear
relationship with the odor level2), was measured at the exhaust outlet of the deodorizer and at a position 3 m fromthe system and was compared with the values measured in a
typical kitchen of feeding facilities for school lunch (in Hitachi, Japan).

3.

Results and Discussion

Figure 2 shows the results of long-term monitoring of
the acidulocomposting process. Garbage from a university
restaurant was added every weekday (an average of 4 kg wetwt/day) to the system (Fig. 1) and was treated as describedin section 2.1 and legend to Fig. 1. The pH and temperatureof the compost were monitored for 2.5 years. The cumulative input of the substrate garbage to this process was 2.2 tons
(wet wt; Fig. 2A), from which approximately 0.41 tons ofFig. 1. (I) A cross-sectional drawing (side view) of the composting apparatus used for acidulocomposting. The apparatus had a composting
vessel (a) of 500 mm×300 mm×500 mm (the maximum depth) with a top cover (b) and five bars for agitation (c) (see also photographB). The compost (d) was heated with a sheet-shaped heating device placed below the vessel (e), with a thermostat set at 85°C. The agitation rate was set at 2 rpm, and the direction of rotation of the agitation bars, which are indicated by a double-headed arrow, was reversed
every 2 min. An 8-min agitation was alternated with a 24-min interval, and the agitation and heating devices were automatically stopped
when the top cover remained closed for over 30 h. Air (arrows) was introduced by suction by using a fan (f) at a flow rate of 0.5 l/min
from an air-inlet window (g) into the vessel and escaped from an air-outlet window (h, see also photograph C) through a pipe (i) into adeodorizer unit (j) containing moist wood chips (k, see also photograph D). (II) Photograph of (A) a general view, (B) the empty composting vessel (with agitation bars, top view), (C) air-outlet windows, and (D) deodorizer unit containing moist wood chips (top view).

Acidulocomposting of Garbage

35Fig. 3. Comparisons of compost pH and durations of different
AHTC systems. An identical composition and amount of garbage, from a feeding facility for school lunch, was subjectedto the conventional systems3,6) from three different commercial
sources (maximum capacity, 1 kg/day; indicated by ○, □, and△) and six, identical, independent acidulocomposting systems(maximum capacity, 1 kg/day; indicated by ●, ■, ▲, ◆, ▼, and×). The composting stopped at the time indicated by arrows.

Fig. 2. Long-term monitoring of the course of acidulocomposting of garbage generated from a university restaurant. Thisstudy started on January 11, 2000. The substrate garbage (anaverage of 4 kg wet wt/day) was added every weekday to thesystem and was treated as described in the text; at intervals,
the excess compost was removed. The cumulative input of
substrate during this study is plotted in panel (A). The pH andtemperature of the compost at a 30-cm depth from the surface
are shown in panels (B) and (C), respectively. For further details, see the text.

compost was obtained. The appearance of the productcompost was a generally brown pulverulent material. Visualinspection of the compost suggested that the degradation of
the added substrate was completed within 24 h at high temperatures (50–70°C, Fig. 2C). Most noticeably, the pH of
the compost was autonomously maintained between pH 3.5
and 6.5 throughout the process (Fig. 2B; see Fig. 3 for other examples), in striking contrast to known AHTC processes
that generally proceed under alkaline conditions (pH 7.5–9; Fig. 3)1,3,6). The acidification of the conventional AHTC
process tends to cause malfunctioning of the process3,4,6).
The process of this study was sustained for a prolonged
period (more than 2 years) with no supplementary addition of starter microorganisms. In comparison, most known
AHTC processes malfunction within 2–6 months after thestart of the process (Fig. 3); in such cases, starter microor-ganisms must be supplemented to increase the composting
activity to restore the process, or the total content of thecomposting materials must be removed so that the process
can be restarted6). In our new process, when the substrate
was added beyond the capacity of 5 kg/day, biodegradationof organic materials markedly decreased. In such cases,
the pH remained at 4–6, and the compost became blackish
brown, was slimy, and contained oily substances. However,
the ability to degrade garbage and the pulverulent nature of
the compost was autonomously restored by operating thesystem for 1–2 days with no addition of a substrate. Thesecharacteristics have generally been identified when using
this system to compost garbage from a variety of sources,
including Japanese, Korean, Chinese, European and fastfood restaurants, homes of different countries, feeding facilities for school lunch and hospitals. These results suggestthat this process should not be specific for the garbage fromparticular sources, but may be generally applied to garbage
from a wide variety of sources and other organic wastes.
Thus, this unique AHTC process is named “acidulocomposting” with emphasis on the distinctive acidophilic natureof the process.
Unsanitary vermin, such as cockroaches, flies, and maggots, were not found in the compost product, named acidulocompost. Nitrogen, organic carbon, phosphorus (as P2O5),and potassium (as KO2) of the acidulocompost were measured by official protocols7): typical values were 3.4, 39, 0.95,
and 0.78 w/w%, respectively. Although these values mayvary with substrate garbage, the C/N ratio of the compost
was usually 10–12 independently of the source of garbage.
For comparison, the nitrogen content and C/N ratio of
the composts produced from conventional processes frommunicipal wastes are mostly 1.5–2% and 20–30, respectively3,6). The water content of the acidulocompost during

NISHINO et al.

36
Table 1. Measurement of odors

Site of measurement
Outlet of deodorizer
3 m from the system inopen air
KitchenOpen air4.

Σ value
Measurement 1

Measurement 2375

340

250

220330230350210

composting was 25–40%; however, it decreased to 10%
when the garbage was not newly added for several days.
The acidulocompost was mixed with soil and was subjected
to secondary fermentation for several days, as for the compost produced by traditional non-accelerated compostingprocesses. The resultant soil-mixed compost can be used tocultivate vegetables, flowers, and fruits (T. Nishino and K.
Hoshi, unpublished results).
Olfactory examination showed that acidulocompost generally emits a weak burning and caramel-like smell and does
not emit an ammonia-like odor. This is in striking contrast
to the odor of compost produced by conventional AHTC
processes, which is ammonia-like. Moist wood chips are
inexpensive, effective deodorizing materials for the processof this study; thus, the acidulocomposting system is regularly equipped with a deodorizer unit containing them (Fig.
1). The odor emission from this system was quantitativelyanalyzed by using an odor meter equipped with an electrochemical sensor based on a ZnO2 semiconductor (Table 1).
The results showed that the Σ value of the exhaust fromthe system, measured at the outlet of the deodorizer, wascomparable to the value obtained in the kitchen. The Σvalues measured at a position 3 m from the system in open
air were mainly the same as those in the outdoor air, wherealmost no odor was detected in an olfactory examination.
These results show that acidulocomposting emits only very
low levels of odor.
The fact should be noted that detailed microflora analyses of the acidulocomposting process by polymerase chain
reaction-denaturing gradient gel electrophoresis, combinedwith fluorescent in situ hybridization, are currently underway and show the participation of lactic acid producing
bacteria during the process. These observations may provide
a clue to understand why this process is stably sustained
under thermoacidophilic conditions for prolonged periods
with very low odor emission. Details of the results of theseanalyses will be reported in future.

Conclusion

Acidulocomposting is a novel method to recycle garbage.
It is autonomously sustained under thermoacidophilic conditions for more than 2 years with very low emission ofodor and does not require tedious procedures to maintainthe process. Therefore, acidulocomposting is a generally
efficient and relatively maintenance-free process applicableto bio-recycling garbage from a wide variety of sources.

Acknowledgments
This work was supported in part by the industrial technology research grant program in 2001 from the New Energy and Industrial Technology Development Organization(NEDO) of Japan (to H. H.).

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