Analysis of Overload Phenomenon in Garbage Decomposition

There is a phenomenon observed in various microbial waste treatment processes, in which the microbial degradation rate of organic substrates drastically decreases when the input of substrates to the process is too heavy. It is called "overloading". The overloading phenomenon in garbage treatment systems often causes serious problems. But the mechanism of overloading has not yet been clarified. This study intends to investigate this phenomenon by analyzing the microbial community as well as various physical parameters in the garbage treatment process.


Introduction
Kitchen waste accounts for about one-third of urban garbage (wet weight basis), which in Japan is mainly collected and incinerated by local governments, at present.This process is, however, a heavy financial and environmental burden on local communities.Thus, the development of a more 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 [1,2].For this reason, the microbial garbage treatment has been studied in our laboratory for a long time.One valuable result has been the development of a static-type small scale garbage treatment system [3].However this garbage treatment system has a problem called overloading, which is caused by excess garbage input.Once the garbage treatment system becomes overloaded, several unfavorable symptoms appear, such as low performance of garbage decomposition, the creation of a bad smell, a drop in pH and temperature, and a decrease of the total number of microbes.Although the overloading phenomenon is generally observed in many organic wastes treating processes using microorganisms such as activated sludge and, methane fermentation, a detailed mechanism of overloading has not yet been clarified.Therefore, analysis of the overloading phenomenon in garbage treating systems is not only practically useful but also has a high probability of broad application across various organic treating processes.This study intends to investigate this phenomenon in terms of the behavior of the microbial community [4,5,6]and physical parameters [7,8] such as temperature and moisture content (MC) in the garbage treatment process.

Experimental procedures 2.1 Materials and Method
Operating conditions of the garbage treatment system A static-type of garbage treatment system, developed in our lab, was used as a model process, in which 10 L of leaf mold (about 1.5 kg in weight) was set as a bacterial bed and dog food was input every day as a model substrate with adjusted MC.Mixing was performed by hand once a day when the model substrate was input in order to keep well mixed and maintain an aerobic condition.

Measurement of physical parameters
The physical parameters in the reactor measured before the input of dog food were as follows; total weight of the reactor, pH, moisture content (MC), temperature, and oxygen concentration in the reactor at a depth of 3 cm from the surface.And a smell test of the reactor was performed everyday by human sense.

Analysis of microbial community
Three grams of the garbage treating residue was sampled from the reactor, combined with 27 g of water, and homogenized for 10 min.before dilution by a factor of 10 6 ~10 8 .The number of colonies on the Trypticase-soy medium (T medium) was counted.

Control of temperature in the reactor
An electrical heater was installed under the bottom of the reactor for the purpose of keeping the temperature not lower than 30 ℃ to examine the effect of temperature on overloading.

Comparison of normal and overloaded conditions
The comparison of performance in terms of the weight reduction in the reactor between the positive control (loading:40) and the overloaded unit (loading:150) (unit of loading : g/(L･day)) is shown in Fig. 1.The rate of weight decrease in the normal unit was maintained at almost 80 % after day 3, whereas that of the overloaded unit dropped to lower than 20 % during the same time period.The moisture content and smell generation of the latter increased but bacterial number decreased in the reactor (data not shown), meaning that the overloading phenomenon could be clearly observed.

Change in physical parameters
The change in physical parameters, -temperature, pH, and moisture content (MC) in the reactor, -is shown in Fig. 2. Since the normal garbage decomposition process is an aerobic oxidation of organic compounds, heat generation can be observed when this exothermic reaction proceeds.That is why the temperature in the reactor is a good indicator of the process.As shown in Fig. 2 (a), the temperature in the OC (overloaded condition) rose rapidly on day 2 but dropped afterward, meaning that drastic but brief garbage decomposition occurred.On the other hand, the temperature in the PC (positive control) condition remained almost constant above 30 ℃ after day 2, showing that a stable condition could be maintained.The temperature in the NC (negative The change in pH is another important index of the decomposition process because various organic acids would be generated if anaerobic reactions dominate, which results in a decrease in pH, whereas pH would be constant or even increase slightly when aerobic decomposition occurs.Fig. 2 (b) shows that the pH in the OC (overloaded condition) decreased but those of the PC (positive control) and NC (negative control) conditions remained almost constant, meaning that between the input with fresh feed and output by evaporation, the water content remained in balance.The MC in the OC increased gradually, showing that water accumulation proceeded due to the decrease in the exothermal reaction, whereas those of the PC and NC conditions slightly decreased because the evaporation of water was greater than the input.

Analysis of microbial community
The microbial community was analyzed using the plate culture method on the negative control NC (0), positive control PC (40), and overloaded condition OC (150).Several examples of the colonies are shown in Fig. 3, in which the results of the NC condition was not shown due to the same appearance as (a) and (d).There was little difference in the observed colonies present between the PC condition and the OC as shown in Fig. 3.The change in the total number of bacteria that appeared in the culture plate is shown on Fig. 4. The tendency of change was similar to that of temperature; i.e. in the OC the total number reached about 10 11 CFU/g-wet sample on day 3 but decreased afterward, whereas in the PC condition the total number kept almost constant after day 2.Even in the OC the total number maintained a level of about 10 10 CFU/g-wet sample, meaning that there was little relation between the total number of microbes and the condition of the garbage decomposition process.

The effect of the changes in operating conditions on the overload condition
The effect of change in operating condition on garbage decomposition performance was investigated from the stand point of temperature and moisture control when a garbage treatment system was operated with an input rate of 150 g/(L･day) , overloaded condition.The results given in Fig. 5 show that the weight decrease rate could be maintained when the temperature was maintained over 30 ℃ ("heating"), but that without heating the decrease rate rapidly worsened after day 3. Also, the effect of MC was clearly shown.In particular, 50 % of MC was the best condition even for the overloaded condition.Since the pH decrease for the OC shown in Fig. 2 (b) suggested that the reactor was anaerobic, the oxygen concentration was measured during day 6 and 7 in the unheated OC reactor.The result is shown in Fig. 6, indicating that oxygen concentration decreased and reached almost zero after 12 hours; i.e. anaerobic conditions were generated in a very short time in this system.Thus it was confirmed that an anaerobic state was surely produced in the reactor in the OC.

Conclusion
It was indicated that an overloading phenomenon in garbage treatment systems could be constantly observed when the substrate input was about 150 g/(L･day) or more.But there was little difference observed in the microbial community analysis between the OC and PC conditions in terms of the colony variety and the total number of bacteria, meaning that another approach to the analysis of the microbial community, such as a molecular biological method, should be investigated.Since the decomposition performance of garbage changed with the conditions of moisture content and temperature maintained over 30 ℃ ("heating"), there might be a possibility that the decrease in decomposition performance could be ameliorate by controlling physical parameters such as temperature and moisture.This fact implies the importance of the physical conditions in microbial waste treatment processes.

Fig. 1 .
Fig.1.Comparison of weight decrease rate in the normal state and the overloaded state.

Fig. 4 .
Fig.4.Total number of bacteria in each loading rate

Fig. 5 .
Fig. 5. Comparison of weight decrease rate in each controlled conditions.

Fig. 6 .
Fig. 6.Oxygen concentration in the reactor in the overloaded condition during day 6 to day 7.