Hi Alice

It cost us 330 USD to make the P Lite here (excluding the data logger). Much of that was the cost of labour - the materials required are not expensive - so the cost will depend on where you are. Jamie Radford gave us the specifications for fabrication and he would be very happy I am sure to work with you if you want to make one and collect some data with it. He is particularly interested in people around the world using the P Lite so he can build up more data on pit sludge using the P Lite as a standard means of measurement.

Regards

Dave

Hello Dave,
thank you for the very interesting post.

Can you give us an estimate of how much the P-Lite will cost?

Best reagrds,
Alice

Dave wrote: No, I would like to try out a liquid ring pump, but I have not been able to locate one here. Are those pumps very robust? The pump we are using is I believe the right size for what we are doing. If it was smaller one would at times miss the extra power, and if it was bigger then you would need a bigger motor and the whole objective of keeping the machine portable and able to fit through narrow spaces would be harder to achieve.

Yes they are quite robust, especially if you make sure the working fluid (usually water) is buffered again for example CO2 dissolving (=lowers pH and can cause rusting), but I guess they are somewhat less efficient than rotary vane pumps. I don't have experience with them for pit emptying, but as far as I know they are quite common in desludging trucks.
I just looked into these kind of pumps a while ago for a small scale vacuum sewer prototype.
But international ecommerce (Alibaba etc.) should allow ordering these to more or less any place on earth. I was actually surprised how smooth ordering something similar in Uganda (from South-Korea) was a few years back.

Dave wrote: We did try using the exhaust to feed an air lance which was meant to stir up the sludge, but we did not find it particularly effective.

Yes, I guess you would need a small air-tank to build up sufficient pressure for it to be of any use (in bursts). Could also be useful to push back any solids blockage instead of having to pull out the suction pipe to remove it.

Dave and all,

Like Elisabeth, I very much enjoyed your post. It was rich in information and very clear.

Vacuum systems seem to not deal with trash well. Has anyone tried something like in the attached sketch, a rounded inlet shaped like a trumpet bell, with the outer diameter 2-3 times the hose diameter? It would seem that soft trash should get pulled in easily since there are no edges to catch on, and even hard trash like bottles might get sucked in.

The rounded inlet ideally would be flexible to fit into a small opening (drop hole) but stiff when in use so that the suction forces didn't distort its shape when in use. One way to do this might be an inflatable toroid around the end of the hose, as in the second page of the attached document. I would imagine there are other ways to accomplish the same thing.

I've tried building a rough version of this to test it, but my initial test idea didn't work. I do sanitation work in my spare time in my back yard and have limited resources, and I've not yet figured out a way to test this idea with my limited resources. Perhaps others have tried this and can share their results.



This might also work on a full-size vacuum truck. Of course, any time you suck trash into a chamber you have to worry about how to get it out again.

Dale Andreatta, Ph. D., P.E.
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Hi Kris

No, I would like to try out a liquid ring pump, but I have not been able to locate one here. Are those pumps very robust? The pump we are using is I believe the right size for what we are doing. If it was smaller one would at times miss the extra power, and if it was bigger then you would need a bigger motor and the whole objective of keeping the machine portable and able to fit through narrow spaces would be harder to achieve.

The kind of problem shown in the manual with the rust is not common - we didn't really know what we were doing when that happened. The way the eVac is built now you only ever get a few droplets of liquid finding their way through to the moisture trap in an average day's work, and those droplets end up on the floor of the trap, well away from the top where the air is sucked through to the pump.

We did try using the exhaust to feed an air lance which was meant to stir up the sludge, but we did not find it particularly effective. As the machine is presently designed, you can use the exhaust to provide extra force to blow sludge out of the vacuum tank when you are filling the drum. That is not necessary with wet sludge, but might be helpful with very thick sludge.

Have you worked with a number of different pit emptying machines?

Regards

Dave

Dave wrote: We only make one size. It is, I believe, right sized for the job (wheelbarrow like chassis which can go through a doorway etc, and two horsepower motor), but one of the reasons why we have not been able to offer different sizes is that we have only been able to find one vacuum pump (the De Laval VP18M, which has an airflow of 345 litres per minute at 1700 RPM) that meets our requirements. Most vacuum pumps on the market are a lot bigger than that. This pump moreover can be powered by a single phase electrical pump (1.5 kW), which means you can run it off any normal household electrical socket or small generator.

Did you at all look into single stage liquid ring pumps? I believe the smallest units start at about 150l/min and have the big advantage that they usually require much less maintenance or at least are more resistant to the rusting problems of the vane pump shown in the manual (with the right working fluid this can be mostly eliminated).

Other than that: what about partially feeding back the air exhaust to the front of the suction pipe for difficult to pump sludge? With an air pressure vessel in between it might even be possible to push short burst of high pressure air or run some sort of rotary cutter?

Hi Elisabeth

The cost does vary a little according to exchange rate variations, but the last unit we sold came to 3000 Euros. The cost for shipping/airfreight depends on the mode (sea/air) and the destination, but that can add a further 700 to 1000 Euros. The buyer must deal with all admin and costs related to customs on their side, and that cost will vary from country to country.

We only make one size. It is, I believe, right sized for the job (wheelbarrow like chassis which can go through a doorway etc, and two horsepower motor), but one of the reasons why we have not been able to offer different sizes is that we have only been able to find one vacuum pump (the De Laval VP18M, which has an airflow of 345 litres per minute at 1700 RPM) that meets our requirements. Most vacuum pumps on the market are a lot bigger than that. This pump moreover can be powered by a single phase electrical pump (1.5 kW), which means you can run it off any normal household electrical socket or small generator.

Regarding copying/design/patents etc. Although we do have some IP in the machine, having worked out all the finer details of how to get it to work reliably, and how to operate and maintain it, I don't believe there is anything in there that is not already in the public domain - we started by learning from the Vacutug and the MAPET, and were given advice by people like Manus Coffey. When we make the machine in South Africa, which has relatively expensive labour compared to many countries where this machine might be of interest, the cost is certainly not going to be less than it could be made for in those countries. Then by the time you have added the costs for our overheads, shipping and customs at destination, you have doubled the cost. Given that we are competing with manual emptying where the entry costs are very low, cost is a factor in the viability of the machine. My thinking is that we would rather sell a few machines to any interested party, offer training in the use and maintenance of the machine, and investigate local fabrication options. If the customer wants to make their own, as an additional service we can provide a very detailed list of all the parts and all the drawings for manufacture. Pit Vidura in Kigali have gone the local production route, and that seems to be working for them.

We probably should take out copyright on the name eVac - I don't mind people copying it and replicating it, but it would be nice if they acknowledged where the idea came from, and we wouldn't want them to make the machine badly.

Re. quantification of sludge density/trash and affect on extraction methodology. Jamie Radford has done some work on that. He has produced a lightweight easily portable and inexpensive tool called the P-Lite for measuring sludge shear strength. Using this he has come up with a method and some figures with guidance on what machine works where, which he will present at FSM5. On the trash side, one picks up a feel fairly quickly with field experience. In Lusaka the emptying teams liked the way we could use the eVac to screen the larger trash items out at the source (saving them time and hassle screening at the FSM plant). That way it would be possible to charge the pit owner one rate for sludge removed, and different (higher) rate for trash removed. With a differential rate for trash it should be possible to change users' behaviour over a period of time, with less trash being disposed of in pits as a result.

I hope you are keeping well, and I look forward to seeing you at FSM5, assuming we both make it there.

Regards

Dave

Dear Dave,

Thanks so much for your informative post! I love it when an old post gets resurrected (Steve's post was from 4 years ago!).

Please do keep us informed about progress on this.
I have some easy questions for you:
- How much does one of the units cost?
- Do they come in different sizes?
- You don't mind if someone copies your design and builds their own? Do you even actively encourage that?
- The design is not patented or IP protected in any way? (why not?)
- Is there any way of quantifying your statement of "I must note that if a pit has a great deal of trash and if the sludge is also very thick then the eVac is no faster, and could even be slower, than manual emptying"? Has anyone tried to come up with a measurement to determine how much trash is "too much" or how thick is "too thick"?

Thanks so much.

Elisabeth

For anyone interested, the eVac has evolved since Steve Sugden posted the first post here in April 2014. The user manual for the latest version is attached.

Pit Vidura in Kigali have been using eVac since 2015. They have bought two machines from us, and then they have made their own eVac, which they call a Dovac, which is in regular use. They have just purchased two DeLaval VP18M vacuum pumps from us so that they can make another two Dovac for themselves. To me this is the acid test that proves that we are on to something with the eVac - it has been used in Kigali more or less continuously for nearly three years now, and the agency that bought the first machine is now making their own. I should mention that the Kigali city authorities actively discourage manual emptying, so I suppose it is an ideal city for the machine.

In November 2017 I was asked to demonstrate the eVac in Lusaka on behalf of GIZ. The tests went pretty well (see my report attached). I must note that if a pit has a great deal of trash and if the sludge is also very thick, then the eVac is no faster, and could even be slower, than manual emptying, but for wetter, less trashy pits, it works very well. The pit emptiers in Lusaka certainly seemed to like it.

BORDA in Dar es Salaam have ordered one on these machines from us and, just as soon as we can get through the incredibly obstructive Tanzanian customs barriers, we will be sending it to them. I'm hoping to have the opportunity to go to Dar for a week to train the Borda emptying teams. Meanwhile another NGO in Dar, People's Development Forum, have expressed interest in buying 2 eVacs.

The eVac is a simple, portable vacuum pumping machine. A key design feature is that sludge does not come into contact with any moving parts. The pump has a capacity of approx. 330 litres per minute, which is well matched to the 45 litre vacuum tank. The pump is driven by a 1.5 kW single phase electric motor. That's equivalent to 2 horse power, more than adequate to empty a pit. Yes, the vacuum tank is small (because it needs to be portable), but you can fill it and empty it very quickly.

If any readers of this post are interested in getting hold of an eVac to try out, do please let me know. You can contact me direct at This email address is being protected from spambots. You need JavaScript enabled to view it..

Not sure where else to post this on Susana, but anybody interested in emptying and prefers something as little less organic than a Gulper may be interested the E-vac created and developed by Dave Still of Partners in Development in South Africa. It pushes miniaturizing the principles of a vacuum tanker to the limits and the result is a cleaner pit emptying process.