Time taken for faecal sludge tankers to discharge?

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  • kevintayler
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Re: Time taken for faecal sludge tankers to discharge?

Hello everyone

In response to Elisabeth's question about 'pressure off' I think that Andrew is referring to whether or not the vacuum tanker operators use the pump on the vehicle when discharging. The 'pressure off' option is when the septage is discharged by gravity.

Overall, the responses suggest that discharge times are rather longer than suggested by my calculations based on orifice theory.

I am currently working on finalising the first draft of my book - the draft is due by the end of this month. Elisabeth, after that I will be able to find some time to update the Wikipedia page.

Regards
Kevin
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Re: Time taken for faecal sludge tankers to discharge?

(note to Forum novices: This discussion thread has two pages of discussion so far. Access the previous page by using the buttons at the end of this page)


Dear Andrew,

Just a small clarification question, what do you mean by:

Time to discharge depends on whether or not you can pressure off. If you pressure off you can dump in about 10 minutes.

What does "pressure off" in this context mean? Does it mean to pump? And what determines if you can pump or not?

Regards,
Elisabeth

P.S. It would be nice if someone would take the information provided in this thread and use it to update the Wikipedia page on vaccum trucks (en.wikipedia.org/wiki/Vacuum_truck) and on fecal sludge (en.wikipedia.org/wiki/Fecal_sludge_management). This way, we and others would have it handy & at our fingertips next time that we need it.
Any volunteers? I could help but would like to do it together with someone else.
Dr. Elisabeth von Muench
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  • Janexi
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Re: Time taken for faecal sludge tankers to discharge?

Good day Sir,

I work with a 4200l truck it only takes 7 to 15 minutes and it also depends with what type of effruent tht you are working on

Kind regards

Elvis
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Re: Time taken for faecal sludge tankers to discharge?

Dear All

Thanks for the responses that are coming in - they are all helping to build the picture. In addition, I have just come across a paper: www.bonnecherevalleytwp.com/userfiles/fi..._Report_Mar_2010.pdf which gives a time of 30 - 45 minutes to discharge a 9m3 tanker by gravity through a 75mm diameter hose.

The hoses on the tankers look to be 100mm diameter, which I guess is normal.

I calculated the flow rate using the equation for an orifice with a short tube outlet, which is q = C x A x sqrt(2gh), where q is the flow in m3/sec, C is a composite coefficient, A is the area of the outlet in square metres, g is the acceleration due to gravity and h is the head in metres.

For a 100mm outlet, this gives a flow of about 27 litres/sec at a head of 1 metre, which suggests something like 37 seconds per cubic metre of capacity. This discharge rate takes just over 6 minutes for a 10m3 truck and just over 9 minutes for a 15m3 truck but actual emptying time is presumably longer because of the influence of the end of the discharge cycle, when the level in the tanker is very low. (I am assuming that a typical tank is about 2 metres deep and that the total head is equal to the depth of water in the tank). This is a simplified approach but it does seem to give emptying tims of the same order (albeit rather shorter than suggested by the information from Seattle and Bonnechere Valley) than some of those quoted from the field). It is beginning to give me an idea of peak flows, which I will use for the design of septage receiving and screening facilities, indicating that it will be best to check actual figures in the field.

I will also go through the information on septage characteristics in the various publications referenced by Daniel and Moritz. One of the interesting things about the Kampala results is that some of the tanks and pits are being emptied very frequently. I presume that this is because of percolation problems and it would be interesting to see whether it would be possible to link emptying frequency to septage strength. My initial quick review of the Kampala results suggests probably not.

I will aim to share drafts of part of the publication that I am working on as they are ready in draft form - it will then be really useful if I can get feedback.

Regards to all

Kevin
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Re: Time taken for faecal sludge tankers to discharge?

Hi Kevin,

There is a company in Seattle that I work with that provided the following information:

The trucks vary in size but most of our trucks are 4000 gallons but we have a couple smaller trucks for getting in tight places that are only 2000 gallons.

Time to discharge depends on whether or not you can pressure off. If you pressure off you can dump in about 10 minutes. If you gravity discharge it takes from 20-30 minutes depending on how the thick the material is, how much garbage is on the material and how often the driver has to stop to clean the dump screen.


I hope that helps a bit.

Andrew
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Re: Time taken for faecal sludge tankers to discharge?

Dear all,

Really interesting discussion. I spent some much time on faecal sludge treatment plants but have no feeling of how long it takes to discharge a truck. I sent this discussion to a few colleagues with more field experience (like Daniel) who can maybe also contribute. I got a question recently on the maximum slope that a faecal sludge truck can manage, also an interesting info for faecal sludge transport logistics.

Septic tank FS = Septage COD and TSS concentrations:

www.tandfonline.com/doi/full/10.1080/09593330.2016.1165293
washdev.iwaponline.com/content/5/1/72
www.ncbi.nlm.nih.gov/pubmed/16042250
www.ircwash.org/sites/default/files/Kengne-2011-Vertical.pdf
www.eawag.ch/fileadmin/Domain1/Abteilung...gies/novel_plant.pdf
www.eawag.ch/fileadmin/Domain1/Abteilung...ization_FS_Ouaga.pdf

I hope this helps.

Cheers,
Moritz
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Re: Time taken for faecal sludge tankers to discharge?

Dear Kevin,
Please have a look at this report from a faecal sludge characterisation & quantification study in Kampala, Uganda which actually has photos of the samples and the characteristics (www.eawag.ch/fileadmin/Domain1/Abteilung...analyses_kampala.pdf) and this one from a similar study in Hanoi, Vietnam (www.eawag.ch/fileadmin/Domain1/Abteilung...terization_Hanoi.pdf). You can also see more publications about Sandec's work on characterization here www.eawag.ch/en/department/sandec/public...stewater-management/ (under the tab "Faecal Sludge Quantification and Characterization").
I hope they can provide some insights for your work.

Regards,
Daniel
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Re: Time taken for faecal sludge tankers to discharge?

Hi Doreen

Thanks a million for the information - its a really good start. The 7 minutes for the 10,000 litre truck checks reasonably well with my theoretical calculations based on the approximate depth of the tank and an assumed 100mm diameter outlet. You mention the size of the pumps - I was not clear about this as I assume that they are discharging by gravity. The mention of the problems with solids from pit latrines was also interesting. I think that there is a good argument for moving towards water-sealed pour-flush pans in order to reduce problems with solids dumped into pits, assuming that water is available. My understanding is that such pans are still quite expensive in Africa, unlike in South and East Asia where they are cheap. However, I would guess that people are working on that

I will keep in touch with you, Daniel and others who might be interested as I work on this.

Thanks again

Kevin
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  • Doreen
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Re: Time taken for faecal sludge tankers to discharge?

Dear Kevin,

I have spoken to my colleagues who were recently present during desludging of a 10,000 litre exhauster and they mentioned that it took about 7 minutes (sludge was from a septic tank)

I have also spoken to one exhauster operator based here in Nairobi and he mentioned about 20 minutes however due to the content of the sludge (presence of solid waste which gets stuck). He primarily empties pits.

Below is a picture that I took at the discharge site in Nairobi. Note the size of the pumps too.



Best regards,

Doreen
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Re: Time taken for faecal sludge tankers to discharge?

Dear Doreen

Thanks a lot for this - could you let me know the size of the tankers as well - approximately will do and the operators should know.

On another point, I am trying to get together information from different places on septage characteristics - in particular BOD or COD and TSS. Any information that you or Daniel have would be really useful. I have a fair amount of information from Indonesia and other places but the more examples that I can give, the better. I know that the strength of septage varies widely but it will be good to get an idea. I will make sure that all the sources of information are identified and credited.

Thanks again

Kevin
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Re: Time taken for faecal sludge tankers to discharge?

Dear Kevin

I actually plan on contacting a few exhauster operators tomorrow and next week here in Nairobi to inquire about licenses and permits. I could ask them the approximate time they take for them to discharge and let you know next week!

Best regards,

Doreen
Doreen Mbalo

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Re: Time taken for faecal sludge tankers to discharge?

Daniel

Thanks for the reply. As you say, there are so many variables. My main interest is in assessing the time that a typical tanker takes to discharge - it should then be possible to make some reasonable assumptions about time taken to get into position ready to discharge and then to move out again after discharging. As you say, this will depend on the local configuration but I would aim to look at a situation in which some thought has been given to ensuring that tankers can discharge without too many problems due to geometry/steep slopes etc.

Your recollection of up to 6 minutes to empty ties in with my recollections although I was looking at the situation in Indonesia, where tankers tend to be much smaller than those often found in Africa. Like you, I was concerned with other things at the time and did not keep a record of the emptying time. I have calculated flow rates from an orifice with a varying head, which is theoretically similar to the situation when a tanker discharges and initial flow rates can be surprisingly high. If I get no definite information through the forum, I will contact tanker manufacturers to see whether they can give me some guidance.

I want the information to be able to provide design criteria for treatment plant elements, particularly preliminary treatment (screening and perhaps grit removal) for which it will be desirable to design for peak flows..

Thanks again for taking the trouble to reply

Best wishes
Kevin
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