Stabilization step is the longest step in DSP method and it determines total producing time. Short producing time will promise higher productivity.
Therefore, stabilization time is one parameter to evaluate pros and cons o f different producing methods.
In Herhof Stabilat method, waste is kept in Herhof box with the lid closed for 7 days. Air pumped from the bottom o f stabilization box up-ward provides oxygen for micro-organism activity. After 7 days, waste is dry with water content is lower than 15%.
In D.N. Chau's research, composting time was decided by volume o f leachate.
Stabilization step ended when waste body stopped generating water. Result from D.N. C hau's study is varied from 18 to 51 days due to waste input characteristic and environmental condition. In this study, waste was taken out o f bio-box when water content was stable, around 6 weeks. Stabilization time o f this study in comparison with other study is shown in table 14.
Table 14: Stabilization time
Sample R l R2 R3 D.N. C h â u ’s
study [6]
R D F (H erhof Stabilat M ethod) [11]
Days 42 42 42 18-51 7
From table 14, time for stabilization step in this study and D.N. Chau's study is much longer than in Herhof method. There are several reasons for this. Main reason which can be suggested is temperature in compost process. In Herhof method, temperature in compost box can reach 60-70°C while in this study and D.N. C hau's study which has similar method, temperature is much lower. It will be explained more detail in next part o f this thesis.
When RDF is produced in practice - large amount o f waste input, higher temperature and shorter stabilization time is expected. Some other research suggested time for this step is from 2-3 weeks. [12]
3.1.2. Temperature
Temperature is an important parameter in RDF producing method. Required temperature o f composting step is over 60°C to ensure sanitariness o f RDF product. Furthermore, in high temperature condition, waste is dried faster which lead to shorten producing time.
As mentioned above, temperature was measured twice a week in all 3 barrels.
The result is shown in Figure 16.
Days
Figure 16: Temperature in stabilization barrels
As regards, temperature in all 3 barrels is higher than atmosphere temperature.
However, there was not much difference in temperature between 3 waste heaps.
The increase o f temperature is clearly seen in the fourth week. On the other hand, experiment was carried out in March when Hanoi's temperature raises (figure 16). Therefore temperature in 3 barrels also increases during stabilization process.
In theory, temperature o f composting waste starts increasing from day 10 to day 20 or 30 and the highest temperature changes from 45°C to 70°C depending on C:N ratio (figure 17). This temperature curve is built while atmosphere temperature is considered unchanged. However, in reality, atmosphere temperature range can be more than 20°C during stabilization process especially
M a ster Thesis 3. R e su lts a n d discussion
when the season changes. Therefore, differences between composting temperature and atmosphere temperature were evaluated in figure 18.
Carbon:Nitrogen Ratio Effects on Composting
Days of decomposition
Figure 17: Composting temperature depending on C:N ratio
Temperature curves from this study can be divided to 3 phases as expected from theory. Phase 1 was when micro-organism developed. Phase 2 was when micro
organism activities were strongest. It also leaded to highest temperature difference. And phase 3 was when micro-organism reduced. However, max temperature is expected to occur in the second week according to theory. In this research, temperature differences reached maximum in the fourth week. Other wise, maximum temperature in all 3 barrels are lower than 35°C while the maximum temperature in theory is over 40°C and it can be 70°C in ideal condition. This can be explained by following possibilities.
Firstly, due to research condition, waste amount was only 18 kg each barrel. It is quite small in comparison with other studies; hence the surface-to-volume ratio o f our batch is much higher, leading to more heat loss.
Secondly, the equipment used in this study was quite simple, it could not ensure good aerobic condition during composting, thus microbial activity was ineffective. Moreover, thickness and thermo-conduction o f barrel wall are also reasons for losing heat. A hn’s research has proven that the wall conduction accounted to 62% o f the heat loss. [3]
Figure 18: Temperature differences in stabilization barrels
In practice, large amount o f compost waste is expected to give better result;
however, further research in pilot scale is needed to prove it. In addition, wall conduction is required to be considered when producing RDF by DSP. Two- layer insulation wall is a suggestion which was used in Ha Nam. 114)
3.1.3. Leachate volume
In composting process, organic matter is broken down by micro-organism generating heat and leachate water. In the boundary o f this study, only volume o f leachate was monitored twice a week (Figure 19). Once a week, waste was taken out to mix with air. It resulted on micro-organism activities which can be observed by high amount o f leachate in the next days.
Figure 19 shows that leachate volume o f barrel 2 was not change much after air mixing. It can be explained by low bio-waste content in barrel 2 therefore microorganism activities did not change much. Barrel 1 and 3 contain higher percent o f bio-waste which result higher amount o f leachate.
M a ster Thesis 3. R esults a n d discussion
Figure 19: Leachate volume 3.1.4. Water content
Sample was taken once every seven days except day 35. Figure 20 shows water content o f 3 samples during composting step. All o f them had more than 60%
water content at the beginning.
During the first three weeks, percentage o f water in waste body showed almost no change. It is because there are 2 processes concomitantly occur. The first is drying process in which heat from micro-organism is expected to dehydrate waste body. Secondly, water was producing from degradation activity which added more moisture to waste body.
Water content dropped in the fourth week. This result is well-matched with temperature result. In the same period, higher temperature was observed which provided more heat for drying process.
In the last two week, water content in 3 samples was stable at the range o f 35 to nearly 50%. This is not good result when comparison w ith water content o f RDF product in Ha Nam and Germany (<15%) and Finland (25-35%). As explained above, low temperature will affect drying effect. Another reason is water content o f waste input. According to URENCO report, water content o f MSW o f Hanoi is from 50-60%. However, input waste o f this study contained more than 60% o f
water, especially waste input o f sample 3 (92%). Waste collection was taken place after some rainy day in Hanoi could be one reason. Therefore, water content in RDF product did not meet expectation.
Figure 20: Water content (%)
Although the same waste ingredients were mixed to make RDF sample, different input water content was observed. Different ratio o f waste fraction can be the reason but it can not explain why water content in sample 3 was higher than in sample 1 while bio-waste proportion in sample 3 was lower than in sample 1.
However, the moisture curves o f all three samples still followed the same trends during composting step.
In drying step, water content o f 3 samples reduced by 55%, 40% and 60%
respectively. It can be observed that in sample 1 which contained 78% bio-waste and sample 3 which contained 67% bio-waste had higher drying efficiency in comparison with sample 2 which contained only 50% o f bio-waste. The reason could be micro-organism working efficiency. Even if water content o f final products are considered, sample 2 still had higher water content than sample 1 and 3 in spite o f its lower water content at the beginning.
In the nutshell, bio-waste percentage plays an important role in water content reducing. A low percentage o f bio-waste can cause inefficient drying process.
However, too much o f bio-waste will lead to low quality RDF which will be discussed more in the next part.
M a ster Thesis 3. R esu lts a n d discussion