Municipal Sludge Tank Mixing

[sewerfix]

I'm looking for a tank mixing tank system suitable for a solids content of between 4- 7% w/w solids content.

We have two sludge stream coming into a blend tank and the mixing system should blend and keep the solids in suspension before being processed through our new sludge thickening facility.

I beleive submersible mixers would not suitable due to the high viscosity of the sludge and I was looking at a hydraulic nozzle jet system using a recirculation pump. In order to effectively blend I was going to pipe the two sludge streams into the discharge of the recirculation pump.

I have received one quote for such a system but it was just to expensive.

Has anyone had experience with municipal sludge (blend) tank mixing systems?

Thanks
Andrew

 

[oxilume]

Andrew,

Ensure that your sludge mixing system has as few moving parts as possible. As a former Naval Officer/Engineer and present day wastewater process developer/inventor I am inclined to suggest a typical propeller type mixer. However, in the instant you get filamentous type bacteria or any type of stringy material you will regret using a propeller type mixer.

The system is currently being demonstrated for mixing drilling mud that weigh close to 18 pounds/gallon.

 

[Berko]

Andrew,

Hidrostal do sludge tank mixers using V02 Aerators or versions not using entrained air. They would be able to handle your solids concentrations, you didn't say what sort of volumes you are dealing with ?

I believe they are used extensively & successfully in the UK.

Where in the world are you located ?? Hidrostal UK should be able to direct you to your local agent (they have an excellent website).

 

[waterexpert]

Sewerfix,

have you considered inline mixing? I've done a few thickening systems in the UK which blended sludges at the inlet to a progressing cavity pump.

I've succesfully used propellor, and recirc. pump type mixers for this sort of application, with sludges of this DS content.

I'd be a little cautious about mixing by aeration in this application: it could yield nuisance odours, if any of the sludges were already septic.

Hope this helps

Seán

http://www.wastewatertreatmentplantdesign.co.uk

 

[Berko]

Hi Andrew,

Looking back at your thread of last year, how did you get on with your mixer application ?

We are currently looking at a sludge tank mixer system in Fiji.

We have narrowed down our choice to submersible mixers (ABS or ITT) or pump recirculation systems (Vaughan or Hidrostal).

We have used submersible mixers on jobs in the past with varying levels of success.

In the past we have used Hidrostal's on sewage & sludge only with good results, but never used their mixers.

The Vaughan system we have no experience of at all, but as it uses a chopper pump we are sceptical (many of our customers, i.e. councils, water authorities etc will not accept chopper type pumps due to past problems associated with wear, blockages etc).

Any feedback you can give me would be appreciated.

Berko

 

[sewerfix]

Hi Berko,

From my research I have come to this general conclusion - submersible mixers are only reliable with sludges of up to 5% solids. Submersible mixer suppliers are very reluctant to provide a process / mixing guarantee beyond this characterisitc. Top entry agitator suppliers I find size their equipment on rule of thumbs and when you question their design in regards to the application they upsize their equipment in an attempt to squash your concerns. They also will not provide a mixing guarantee without being pushed. For the project in question - building a massive supporting bridge above the tank was not economical.
In the end we went for the Vaughan (Rotomix) system. This 500m3 tank utlised one recirculation pump which then discharged through 3 floor mounted pedestal nozzles assemblies. Each assembly consisted of twin nozzles - one discharging tangentially with the tank radius and the other facing inwards to induce a rolling motion of the sludge as it circulated the tank. The unit has been in continuous operation since December without any issues of blockages, wear. We tendered for a mixing system and Vaughan were the only supplier that offered CFD analysis as verification of their design. Although it was a very expensive and long lead time item - their package offered the lowest risk to our project.


Andrew

 

[Berko]

Hi Andrew,

You seem to have a pretty good handle on various mixing systems.

How easy was the Vaughan system to install ?

Have you any experience of the the other recirc. pump mixing system we are considering, the Hidrostal tank mixing system ?

We are leaning towards this, as we have used their pumps on arduous wastewater duties in the past (heavily rag laden sewage & viscous sewage sludges) without any issues.

This said, we have never used one of their mixers, but like the simplicity of the system. If you are not familiar with this product, not only are the pumps external to the tank, but the mixing barrel can be installed & removed via the outside of the tank.

If you have any experience of this system, your feedback would be appreciated.

Berko

 

[sewerfix]

Berko,

The biggest headache with the Vaughan system is the floor piping (and capital cost). Any internal tank piping is going to be an obstruction to the mixing application, especially if it is compounded by heavy sludges.
To optimise the mixing efficiency we installed piping just above the floor of the tank and then concrete encased (benched) the floor so we had riser pipes connecting to the nozzle pedestals.
Obviously this is a very expensive means of getting to the nozzles compared to the Hidrostal system. Keep in mind that the Hidrostal is typically one pump with a single point of discharge on the tank perimeter, whereas the Vaughan is one pump with multiple discharge points. I only have experience with the Vaughan system, not the Hidrostal.
I have had a project where Hidrostal recommended two pump units (one on each side of the tank) whereas for the same pump kW Vaughan were doing it with one pump, again because they can achieve multiple discharge points.
You also have to look at the dynamics of the blend tank. From my experience if you have floor mounted nozzles then you can achieve very good mixing (looks like a fast flowing river of sludge) say 2m above the nozzle. Above that the energy is dissipated to such an extent that the surface layers (boundary) velocity is significantly reduced and depending on the influent characteristics you run the risk of forming a scum layer. My point being that if you can keep your blend tank low (even level controlled), you can achieve good mixing, short retention times in the tank which then mitigates generation of odours - keep the sludge moving through the process, not in the process.
Vaughan have a high level nozzle to combat this problem but I have never used it.

I can send you some photos if you want; randy73@bigpond.com

Andrew

 

[Berko]

Hi Andrew,

Thanks for the reply,

You have given me much food for thought (decisions, decisions !!!)

Will let you know how we get on.

Berko

 

[sawtry]

Hi Berko

Have been reading the forum thread with much interest and hope I can offer you some enlightenment. Here in the UK we had a presentation by Vaughans representatives using a mobile scale presentation tank that was scaled to an equivalent 10m 30' tank. We understand there is at least 130 of these systems installed in the UK. We were also told thatone water company in the South had frameworked the system for it to be first choice because of its success over 5 years.
This is why we were prepared to look at the demonstration of the scale tank.

In the tank they had a Rotamix system and another set of connections to mimic the Hidrostal system. (As back-ground I had been responsible for sludge mixing and have both the Vaughan and Hidrostal systems; I have had regular pump problems after about 9 months with Hidrostal and have to dig the tanks out every couple of years).

When they started the mixing on Hidrostal all the solids rotated and ammassed in the middle, creating a heap. It was then that I realised what happens; as solids continue to enter the tank the 'Tea-Cup' effect causes solids to amass and build in the center, as the heap grows so solids are gradually pushed out to the side and thats when the Hidrostals began to block.

Their rep turned off the hidrostal system and started Rotamix. The effect was dramatic, we saw the solids drawn through the central take-off and distributed evenly through-out the tank.

We now have both systems in our group and so far the homogenous mix that rotamix provides has increased our sludge through-put and reduced blockages down stream on the PC pumps. It has not skipped a beat and we've done nothing to it except check it on a quarterly basis.

My veiw is Vaughans Rotamix is as their rep said 'A Cut Above The Rest'

As an aside, we have had the Vaughan Chopper pumps on digester re-circulation for about 2 years now and have had no heat exchanger blockages or pump failures since they were put in, our sister site has PC pumps and Screw pumps and they are still sufferng with blocked heat exchangers and pumps, we are planning this year to change all to Vaughan

Regards

Sawtry

 

[Berko]

Hi Sawtry,

Thanks for the info. It would be interesting to see the Vaughan demo model in action.

We are seriously looking at the Hidrostal mixers, as the end user has a preference for a system where all the mixing components can be maintained externally of the tank. Also the end user has had issues with chopper pumps in the past.

I am intrigued about the "Tea Cup" effect you had in your tank. To help us in our decision process, we have contacted various end users of the Hidrostal system for comment. Not one we talked to seemed to have such solid build up issues, rather they remarked how the system kept the floor clean.

A couple of questions for you:

What are the dimensions of the sludge tank in question (Diameter & height)?

What type of sludge are you mixing ?

How many mixer barrels are operating in the tank ?

What was Hidrostal's response to your solids build up issues ?

Looking forward to your reply,

Berko

 

[Berko]

Hi Sawtry,

Just wondering if you have had a chance to get the info requested about your Hidrostal mixing system.

I did a bit of investigation work myself & asked for Hidrostal's comment to your feedback.

The response from Hidrostal was: -

1. They have heard of isolated instances of solids settlement in tanks using their mixer system. The causes were typically found to be due to incorrect nozzle positioning at installation stage or that the sludge being mixed had a much higher dry solids content than that originally specified when the system was selected.

Could either of these scenarios have occurred in your installation ?


2. Vaughan demo model: -

Here Hidrostal questioned the Vaughan mixing model.

As they effectively custom design mixer systems on an individual basis (in terms of pump & mixer nozzle selection, nozzle orientation, suction pipe positioning, etc) it would be difficult to generalise about the mixing flow pattern with a model.

They also asked whether the model took into account that typically Hidrostal's mixing philosophy is high flow versus low head systems, whereas Vaughan tends to use systems with lower flows & higher heads ?

Looking forward to your reply,

Berko

 

[wwchick]

Why is mixing so important here? Sounds like a breading ground for filamentous bacteria and trash accumulation around the mixer. I have operated a similar system without mixing with out any problems.

 

[poopusher]

Hi guys,

A friend at work suggested I join Eng.tips & I am glad I did.

I am an Asset Management Engineer for a large UK water authority, who looks after waste water treatment equipment.

This mixer thread caught my eye & after reading it, I thought I would put my two bobs worth in.

I have actually witnessed the demo model of the Rotamix system & was totally unimpressed with what was presented.

To stand up & rubbish the competition is pretty unprofessional & the model presented was a "smoke & mirrors" affair that in my opinion could not be taken seriously.

We couldn't have got rid of the Vaughan sales rep quick enough after such a display.

In our area we have a supply agreement with Hidrostal & use their pumps for all types of applications, anything from Primary Sludge (upto 9%), WAS, RAS, Digested sludge, Raw Sewage, Storm Water & also tank mixers.

The lads at Hidrostal UK have probably lost more orders than they have won by putting generous safety factors on their equipment selections. They never forget that they are engineers first & that to me is refreshing in todays market.

The equipment they provide is robust & comes at a price, but hey the sewage industry isn't easy on plant equipment. You get what you pay for.

In terms of Hidrostal mixers, ours never miss a beat. They keep the sludge moving & we have never experienced solids settling out.

One thing we have learnt (through bitter experience) about waste water pump selections is that the cheap ones tend to cost us more in the long term. Vaughan's hydraulic efficiency is woeful (approx. 60%), so Hidrostal's 70%+ is music to our accountants ears !!

Our experience of Vaughan chopper pumps is their performance drops off markedly once the solids content in a sludge goes above 4% DS. Hidrostal don't seem to have problems here until about 6% DS & then the de-ration is marginal.

Am I a Hidrostal fan you ask, hell yes, they certainly make my job easier, in my opinion they are a "fit & forget" product.

I would be keen to hear anyone elses comments.

Until the next time,

Poopusher

 

[Berko]

Hi Poopusher (like the nickname),

Thanks for your input.

I tend to agree with you regarding Hidrostal & their safety factors. For our application Vaughan & Hidrostal offered identically sized motors, even though Hidrostal had an additional 10-11 % hydraulic efficiency point advantage.

When questioning both suppliers about this, Hidrostal explained that they had already allowed a sizeable power safety factor to take into account the viscous sludge we want to mix, Vaughan on the other hand revised their offer & gave us the next motor size up ??

All along Hidrostal have taken the stance that due to the inconsistent nature of sludge, it is better to be over-cautious, on the flip-side Vaughan seems to offer borderline selections.

This is typical of dealings with Hidrostal in the past, if they are not totally convinced that they will offer an effective solution they would rather not win the work.

Their view is that selling a potential problem could damage their reputation as pump engineers. In todays market, their are few suppliers that would do that.

As said previously in this forum, I haven't seen the Rotamix model, but I find it pretty distasteful when suppliers openly attack a competitor.

If a supplier talks more about the competitions disadvantages than his owns advantages, what does it tell you about the product they sell ?

We are making our final decision by the end of the month & will let the forum know how we get on. The ball is now in the court of our customer in Fiji.

Cheers,

Paul B (Berko)