Desludging cycles with septic tank in Vinh Long City (Simulations)

The interview conducted with the two desludging operators at the end of their working day indicated that the first desludged site in an occupied house was the most frequent type of desludged septic tanks by contrast with to the second and third sites, which consisted of septic tanks in houses dismantled before reconstruction, hence with septic tanks easier to access. The fourth site being a beer factory was even rarer.

In line with this explanation, and to get a more accurate view of what the common desludging cycles in Vinh Long City are and how efficient FS collection would be under normal conditions, a simulation is made in Figure 5.3 in which the data used for the desludging cycles 2, 3 and 4 were those of the first desludged septic tank, except for the travel time (i.e., the travel time from and to the depot, the travel time to the sites to desludge, and the travel time from the desludged sites to the discharge site), the idle time and the vacuum truck cleaning time (both expressed in ‘Other’ in Figures 5.2 and 5.3). The travel time, idle time and vacuum truck cleaning time considered for both Figure 5.2 and 5.3 are therefore the same actual times measured during the field study in Vinh Long City for the 4 desludging cycles.

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Figure 5.2: Time for each type of motion measured during the 4 desludging cycles of the field

study in Vinh Long City*

Figure 5.3: Time for each type of motion measured as if the 4 desludged sites were

the same as the site observed in cycle 1 of the field study in Vinh Long City

*Same data as Table 4.2 and Figure 5.1

The result of the simulation shows that 4 desludging cycles with an average household septic tank, such as the one observed in cycle 1, would have required a total of 12 hours, 12 minutes and 30 seconds, which makes 3 hours and 50 minutes more than the observed 4 desludging cycles during the field study. It can therefore be assumed that, in the case of desludging only household septic tanks in occupied houses, 4 desludging cycles would not be possible within a working day of 8 hours. Even 3 cycles would not be attainable without overtime added to the normal working day as 3 cycles would require 9 hours, 25 minutes and 30 seconds. This estimate is actually consistent with what has been surveyed in Hanoi, Hai Phong and Ho Chi Minh City where respectively 2.7, 1.4 and 2.0 household septic tanks are desludged on average per day (Viet Anh et al., 2012).

In comparison to Figure 5.2, the motion that requires the longest time with the simulation made in Figure 5.3 is the preparation time. This motion includes the time needed to access the septic tanks (localization and floor breakage) and the time for assembling the sucking pipes and hoses in order to connect the vacuum truck to the septic tanks for FS pumping. As a result of the long time needed for these operations, particularly floor breakage, this motion alone would demand more than 4 hours if 4 desludging cycles were carried out. It confirms the observation made in the field and reported in sub-chapter 5.2 for motion No. 3 (preparation time), and clearly highlights the fact that this is the motion that requires the greatest attention and drastic

0 100 200 300

Travel Preparation Pumping Ending Discharge Other

0 100 200

Travel Preparation Pumping Ending Discharge Other

minutes minutes

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improvement measures in order to substantially increase the efficiency of the FS collection system in Vinh Long City.

The analysis of each type of motion clearly shows the motions that require most time, which, in the present case, are the preparation time and the travel time. The travel time is mainly affected by traffic conditions and the travel distance from the desludging site to the discharge site. While traffic conditions vary according to the time of day and routes, the distance to the discharge site cannot be changed unless it is moved to a location closer to the city center. In the case of Vinh Long City, the vacuum truck had to travel a rather long distance (15.2 km on average) to reach the discharge site from the desludging sites, however, this distance was covered at good speed (41.2 km/h on average). This indicates that the traffic conditions in Vinh Long City are satisfactory and, therefore, the travel motions can hardly be improved under the present conditions.

The reduction of the time needed for the travel motions could be achieved by using transfer stations (visual examples of two types of transfer stations are shown in Figure 5.7) placed in strategic locations throughout the city, in which sludge could be discharged without traveling long distance. An example of such FS collection system in one desludging area can be seen in Figure 5.4.

Figure 5.4: Example of FS collection system with discharge and temporary storage in transfer station

Source: Author

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With this research, by using the time and motion model developed, sensitivity analysis can be carried out which enables, by changing one or more variables, to conduct simulations and test FS collection systems in different scenarios. If for example the time needed to complete the motion No. 2 (travel from the sludge discharge site to the next site to desludge) and motion No. 6 (travel from the desludged site to the sludge discharge site) would have been reduced to 5 minutes for each of these motions in the 4 desludging cycles of the field study conducted in Vinh Long City, more than one and a half hours would have been saved during the full working day (6 hours, 32 minutes instead of 8 hours, 22 minutes and 30 seconds), which would have potentially enabled one more desludging cycle in the present case.

Following the same logic, other scenarios could be tested by simply selecting different times for the motions 2 and 6 as shown in Figure 5.5 and Table 5.1. For this simulation, the total time for 4 desludging cycles has been calculated using times for the motions 2 and 6 that increase by 5 minutes, ranging from 5 to 40 minutes. The total durations of 4 desludging cycles with these 8 scenarios have been divided by 4 (as there were 4 desludging cycles observed in Vinh Long City) to obtain the average time for 1 desludging cycle. With this information can be estimated the possibility of conducting one or more desludging cycles and to concretely understand what is likely to happen when the traffic conditions are bad or when the discharge site is remotely located from the desludging sites. By conducting such simulation and testing different scenarios, improvement measures, such as the installation of transfer stations in cities where traffic density is high, can be identified to increase the number of septic tanks desludged during a day. The profit generated by this increase of efficiency may be beneficial for both service providers and users. Indeed, it may encourage service providers to stop bad practices such as the dumping of untreated sludge into the environment (usually done because of the insufficient profit earned by private service providers) while it may also result in lower desludging fees for service users. In addition, the increase of revenue could also be redistributed to provide or improve treatment; another essential link of the sanitation and FSM service chain which will be under further stress if the volume of sludge increases as a result of improved FS collection efficiency.

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Figure 5.5: Relationship between cycle time and possible desludging cycle number Source: Author

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Table 5.1: Data of sensitivity analysis with various selected times for motions 2 and 6

Motions 1 + 10

Motions 2 + 6

Other motions

Total time for 4 cycles

Average time for 1 cycle 5 minutes for each of

the motions 2 and 6

31 minutes (5 x 3) + (5 x 4)

= 35 minutes

326 minutes 392 minutes 98 minutes

10 minutes for each of the motions 2 and 6

31 minutes (10 x 3) + (10 x 4)

= 70 minutes

326 minutes 427 minutes 106 minutes 45 seconds 15 minutes for each of

the motions 2 and 6

31 minutes (15 x 3) + (15 x 4)

= 105 minutes

326 minutes 462 minutes 115 minutes 30 seconds Results of field study

(approx. 20 minutes on average for each of

the motions 2 and 6)

31 minutes 145 minutes 30 seconds

326 minutes 502 minutes 30 seconds

126 minutes

25 minutes for each of the motions 2 and 6

31 minutes (25 x 3) + (25 x 4)

= 175 minutes

326 minutes 532 minutes 133 minutes

30 minutes for each of the motions 2 and 6

31 minutes (30 x 3) + (30 x 4)

= 210 minutes

326 minutes 567 minutes 141 minutes 45 seconds 35 minutes for each of

the motions 2 and 6

31 minutes (35 x 3) + (35 x 4)

= 245 minutes

326 minutes 602 minutes 150 minutes 30 seconds 40 minutes for each of

the motions 2 and 6

31 minutes (40 x 3) + (40 x 4)

= 280 minutes

326 minutes 637 minutes 159 minutes 15 seconds