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Sludge Cake Pumping to Sludge Silo

Sludge Cake Pumping to Sludge Silo

Hi everyone!

We have a 15 bar progressive cavity pump that is conveying lime-treated sludge to an 8-m high, 4-mΦ cylindrical concrete silo at the rate of 5 m3/h. We decided to inject the sludge cake at the bottom of the silo instead of the top as shown below:

Diameter of conveying stainless pipe is 150mm, sludge cake density is 1070 kg/m3. Conveying distance is ~5.0m.

Will this work?

RE: Sludge Cake Pumping to Sludge Silo

Hard to say with the information submitted. What is the water content?

Have you considered a shaftless screw conveyor. It would be the first choice for this application.

RE: Sludge Cake Pumping to Sludge Silo

If the sludge forms a crust on top because the liquefied material is going in and out of the bottom the silo may be exposed to 15 bar pressure? Can it withstand the pressure.
Four decades ago there was an accident in a margarine factory in the UK that killed a worker. The heating coil went into and out of the vat at the bottom. The margarine solidified on top and the tank was exposed to the thermal expansion pressure. The tank ruptured. From that time heating coils were fed from the top down inside the tank to allow expanding liquid to escape.

ôThe beautiful thing about learning is that no one can take it away from you.ö
---B.B. King

RE: Sludge Cake Pumping to Sludge Silo

Thanks for the answers guys!

@Bimr, water content after dewatering sewage sludge is 75%. We have considered shaftless screw conveyors, it required 8 motors (2 sets of twin conveyor, each pair of conveyor with 4 motors), operation guys think it will cause maintenance issues so we switched with sliding frame + 1 shafted screw conveyor.

@Stanier, that's a good info. Is that the reason why most sludge handling is top entry?. After searching net, I cannot find any silo that is bottom entry.

And what happens when pump is not running, and there is sludge inside silo, say 4m high, then pump runs again?

RE: Sludge Cake Pumping to Sludge Silo

@ Bimr, I believe I misunderstood the part about the shaftless screw conveyor. You were talking about using shaftless conveyor to convey sludge instead of piping. We do not have much space so we use piping. In my post above, I referred to the conveyors at the inside bottom of the sludge silo.

Hope you guys don't mind a follow-up question, will the screw pump really 'push-up' the sludge level in the silo? I am worried about pressure build-ups inside the piping, because solid sludge will eventually block the discharge point, and it's hard to predict whether the sludge will provide increasing resistance to flow, especially when it hardens.

RE: Sludge Cake Pumping to Sludge Silo

Are you designing this, or renovating?

If you push it in from the bottom of the bin, you will also push it through the screw conveyor, unless you have a slide gate.

I would not consider pumping this material. Unless you are pumping continuously, the material will set up in the piping.

Do you have a live bottom with screws covering the entire bottom. If not, the material will hang up.

Shaftless screws are generally preferred over shafted screws. You can convey the material horizontally, turn 90 degrees upward, then turn 90 degrees into the bin.

RE: Sludge Cake Pumping to Sludge Silo

We are still in design stage.

We will equip the bottom of bin with slide gate valve.

I attached some preliminary layouts. From belt press, it drops to a cavity pump equipped with a bridge-breaker. The breaker acts as mixer for the lime and sludge. The two sludge feed pipes will join to a common header leading to the bin.

It is live. See this product, we will be using it.

I asked my colleagues, Seepex cavity pumps are already approved. Had we used conveyors, would you say they are generally cheaper or more expensive in terms of OPEX/CAPEX?

RE: Sludge Cake Pumping to Sludge Silo

As a first cost, the sludge pumps will be less expensive. However, the conveyors will be less expensive to maintain and operate over the long run. The conveyor motors will be very small HP compared to the sludge pumps. The conveyors have UHM liners that are periodically replace and are made of SS that last forever.

One consideration is the bin construction as shown will be expensive, especially installed up in the air above the truck. That bin will give the structural engineers fits. It is more common to have a bin installed at grade. The bin bottom is angled with a shaftless screw pulling the material from the bottom.

Don't know what you are doing with the sludge, but in the colder climates, it is usually stored for months, with a much larger volume than would fit in a bin.

Unless you have a high capacity conveyor, it will take some time to fill the truck and most times the driver doesn't want to wait. The usual practice is to load into roll off containers or to a pile.

If a pile storage is used, front end loaders are used to quickly fill trucks seasonally.

Also, electrical rooms generally have two access doors.

RE: Sludge Cake Pumping to Sludge Silo

Very helpful and informative post, thanks heaps bimr!

You are right about structural engineers complaining about this, and I wish I can still change the sludge silo part, but it looks like this is where we are heading.

Theoretical sludge production is 30 m^3/d. Conveyor capacity is 15m^3/h. So that's two 20m^3-trucks 2h per day.hairpull2

I'll come back to this post once this plant is operational, hopefully a few years from now.

RE: Sludge Cake Pumping to Sludge Silo

Re top entry being the standard method. I have no idea but it seems logical. There must be a textbook somewhere that refers to it. Also very difficult to maintain if the pipe was to block on bottom entry.

ôThe beautiful thing about learning is that no one can take it away from you.ö
---B.B. King

RE: Sludge Cake Pumping to Sludge Silo

The reason for top entry is simply that the "oldest" sludge shall be discharged first.

For such a short distance and low solids concentration progressive cavity pumps are a good choice. Make sure that they are well sized (low speed). The lime will not help to reduce wear and it will also increase the friction loss. But 15 bar should nevertheless be o.k. for this distance. Generally I would never dare to disagree bimr, but there is no danger of set up of the sludge in the pipe. At 25 % solids it is a homogenous cake. Nothing will settle and even over 2-3 weeks the moisture will not dissapear.

RE: Sludge Cake Pumping to Sludge Silo

Thanks again guys! 'Oldest sludge gets discharge first', my engineering manager had similar reasoning, but project manager wanted to inject at bottom to save on piping/fittings. Sludge hauling is frequent anyway so I guess we are ok. Also what will happen if pump stops, then there is sludge inside the silo, say about 3m high, then restart the pump?

@bimr, we have mixer, sorry it is not clear in the drawings, but the cavity pump is equipped with a bridge breaker, this acts as a mixer. We only need to add another hopper between the belt-press and this rectangular hopper of pump.

RE: Sludge Cake Pumping to Sludge Silo

Also what will happen if pump stops, then there is sludge inside the silo, say about 3m high, then restart the pump? Yes, why not. It can only become a problem if the pump fails and must be repaired. You should therefore make sure that you can isolate the pump from the silo.

Your installation and application is nothing new or unique. I do not know what PC pump (brand) you are going to use but the manufacturer should have experience with these applications. I recommend you should address all your (reasonable) questions to him, too.

RE: Sludge Cake Pumping to Sludge Silo

Thanks a lot micalbrch! Btw, it is PC Pump is Seepex.

RE: Sludge Cake Pumping to Sludge Silo

The biggest problem I see is that you're working against gravity by loading from below, than if you just loaded the tank from the top. By bottom loading, you have to push up all of the material inside the tank, whereas if you are loading from the top, you're only pushing up the material inside the pipe.

Additionally, from a maintenance POV, I would think that the top loading would be easier to fix/replace than if you're trying to fill from the bottom. A top loading auger would only have to deal with the material in the auger. Bottom loading, you'd have to worry about everything in the silo coming out during repairs.

Top loading is the standard way. Heck, even at farms the do top loading for their bins/silos. I wouldn't be surprised if your PM is being penny wise and pound foolish by wanting the bottom loading design.

RE: Sludge Cake Pumping to Sludge Silo

Thanks Zelgar! PM said he've seen many times and I trust him. Also, pump supplier didn't gave any comment regarding this.

For working against gravity, does the pump really push-up all those materials inside? At 3.0m high, that's about ~40T of sludge! What about pressure at the bottom of the bin? Can we use equation for hydrostatic stress here?

Hard to imagine what the sludge pile would look like inside. It's easy to imagine if it's liquid (non viscous) since it offers negligible resistance to shear. But sludge cake is what?

RE: Sludge Cake Pumping to Sludge Silo

There will be more pressure to push up from the bottom vs. loading from the top. Solids work a little differently than liquids about pressure at the bottom of a tank/silo, but using the hydrostatic stress (e.g., p = density x height of materials) would be a good place to start. The only way you wouldn't be pushing the tank contents up would be if the unloading operations is creating voids in the tank the new materials would be filling.

I think it's a little funny that both you and your engineering manager have concerns about the bottom loading design, but you're basing your engineering design only on the PM's indications that it'll work. If I were you, I'd try and find a similar design tank and see if you could talk to people who use it and see what type of problems they have with the system (if any).

To be honest, I think the system you're designing will result in a situation of Last In - First Out. This may result in an extremely wet material leaving the tank and possibly having the material at the top of the tank drying out.

p.s. I do think it's a little funny that the tanks shown on the Saxlund website that makes the bridge breaker show top loading for the tanks in the biomass, wastewater treatment and cement industries.

RE: Sludge Cake Pumping to Sludge Silo

Saxlund states the same in the brochure.

"Because of this bulk material flow the Saxlund Sliding Frame is a “First in, First out” technology, which is an added benefit for materials that can degrade over time such a sewage sludge cake."

It will be inexpensive to install an extra flanged connection on the upper side of the bin and allow space for the piping. Then, if the design does not work as planned, a field modification would be easy.

The PM's comment "but project manager wanted to inject at bottom to save on piping/fittings" is strange considering the overall design with the silo supported by the structure. The structure is significantly more expensive than a few feet of pipe.

If the pump has greater capacity than the outlet conveyor and the solids crust over in the bin, you may put pressure on the outlet conveyor and break it. Would not think the conveyor manufacturer would have anticipated the loading from the pump.

RE: Sludge Cake Pumping to Sludge Silo

I'm really glad I posted here, your comments are very helpful!

Originally, I objected to the bottom loading but all my reasoning is theoretical-based. I only have 4 years exp. in WW Treatment while PM has more than 2 decades, so I said I defer to his experience. All this I have on email. EM is a bit soft and she will not impose the top loading.

I'm actually having a hard time finding a similar design (bottom-loading), that's partly why I'm here, to see if it is common. Seepex did not comment on this so I concluded it is ok (funny thing is that all their brochures show top loading as well). We are still waiting for Saxlund to comment on the matter.

RE: Sludge Cake Pumping to Sludge Silo

Does the PM have engineering experience? If not, I wouldn't take their advice that it'll work with much confidence and design it the way you think it should be (you're the engineer). Either way, I would talk to the PM and bring up the issues (e.g., increase load on equipment, drying out of material in silo, Last in = First out) of the bottom loading.

RE: Sludge Cake Pumping to Sludge Silo

Yes, PM is also an engineer, double-degreed. He is actually pretty good in process stuff. I already brought this up and we had a long written correspondence. He said he does not agree with my interpretation. See attached rough sketch I sent 2 weeks ago. I also added that there might be pressure spikes in the piping since the Positive Displacement Pump is continuously pumping material, and we've got something blocking at the end. I said that it's like running the pump and gradually closing a valve downstream.

RE: Sludge Cake Pumping to Sludge Silo

I'd be concerned if the bridge breaker is enough for mixing. Look at this video:

What you see is sewage sludge at 20% (later 45% with added lime and wood shavings) TS pumped with a piston concrete pump. Note the action in the twin auger feeder of the pump early in the video. Confirm with Seepex that the bridge breaker is up to the task.

You'll also see that the pressure rises up to 20 bar on the stroke of the pump, at rather low flowrates. Since ths sludge in the video is 20%TS and you have 15%, I think you are still save. Viscosity (and non-Newtonian behaviour) seems to rise sharply (> linear) with TS content.

I second checking out similar installations, preparing a flange for top feeding operation and seriously looking at an auger.

RE: Sludge Cake Pumping to Sludge Silo

Personally, what I would do is design it both ways. I'd present a list of pro's & con's for both designs. Unfortunately, a lot of the con's for the bottom loading design are hard to quantify (e.g., How much of an increase load is it for pump? How much of an increase load on the tank due to the bottom loading? Will the material dry out on the top? How difficult is it to replace/repair the feed line into the tank? How often could it fail?)) One big advantage of the top loading design is that most of the con's of the bottom loading design are nullified.

In the end, you need to satisfy your PM. One important thing if you do provide the two different designs is you put the responsibility of the final decision of the design of the tank with your PM. You minimize your risk if the bottom loading design fails because you have a paper trail indicating that you had a lot of concerns with that design.

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