Heat Transfer of open topped tanks

[Makeabrew]

I have a very basic understanding of heat transfer, so forgive my ignorance! I've been looking at the arrangement of secondary digesters on a sewage works I visit occasionally and would like to understand what is going on a bit better. Secondary digesters are basically holding tanks that are open to atmosphere and hold sludge for a number of days to allow pathogen kill. The particular arrangement at this site is 4 of these tanks arranged in series, so that incoming sludge, which is at 35 degrees C, flows into the first tank and displaces an equal volume of sludge into the next tank and so on. The sludge exiting the final tank should then be compliant with regards to e.coli numbers. However, these tanks are sensitive to atmospheric temperature and, out of curiosity, I measured the temperature in each tank. It turned out that the temp after the 2nd tank was insufficient to perform as well as expected, and so I was wondering if the 4 tanks were split up into two lots of 2 tanks this would help? In order to do this I need to work out how much heat is delivered into the system from the sludge (I think I can do this bit, using the flowrate and incoming temp), how it is lost to atmosphere (an equation that links the heat loss to the ambient temperature and temperature of the sludge in the tank). Once I know the equaitons I can hopefully work out the heat transfer if the 4 tanks were split into two lots of 2 and the flowrate divided between them. These tanks are about 8 metres in diameter and about 4 metres deep with no additional heating. I realise this is a very simple problem, but like I say, this is out of curiosity and I'm getting a bit confused looking on the internet for equations. Any help most appreciated.

 

[MintJulep]

Not so simple at all actually.

Through the tank walls the heat transfer path is essentially: Convection from the sludge to the tank wall. Conduction through the wall. Convection + radiation from the outside surface of the wall to the surroundings. It seems probable that there is a nice layer of "stuff" on the inside wall of the tank that you could either consider a layer of insulation, or use a "corrected" convection coefficient for.

Heat transfer through the tank bottom to the ground would be: Convection from the sludge to the tank bottom. Conduction through the bottom and into the ground. Again there is probably a "stuff" issue. The ground temperature across the 8 meter diameter is probably not constant.

From the free surface you have convection + radiation. Probably the easiest calculation, but it will not be steady state (neither will the side wall).

To further complicate things, depending on how viscous this stuff is, there may not be much mixing from convective circulation, so stratification is a possibility.

Probably not the help you were looking for, but you can at least take comfort that it's not easy.

 

[thermcool]

I think if you change 4 tank system to 2 tank system that will definately help you to reduce the heat losses to the ambient. According to my experience with the heat conservation in the large water bodies, i have seen that the losses through the ground are the most predominative. Therefore, if you are combining the two tanks into one, you are basically reducing the heat transfer (heat losses) from the ground by reducing the slurry contact area with the ground. definately, you will keep the contact area with the walls to be the same. But remember as MintJulep said, the heat losses from the wall is through natural convection and through ground is conduction. Here, the conduction heat loss through the ground will dominate as the natural convection losses to the ambient losses will be less.

if possible you can also use some sort of polyethylene ground insulation at the base and sides of the tank to minimise losses. Two water tanks which differ only in the insulation at the bottom (one have it and another dont have it) show wide difference in the heat conservation. The one with the insulation was able to conserve the heat contents much longer and at much higher temperature.

For your calculation of the heat losses to the ambient and ground, you can assume the ground temperature constant and follow the same approach as explain above. for the wall heat loss, if the walls are exposed to the ambient (i.e they are not underground) you can use the simple natural convection model (see, heat transfer by Holman, Incropera & Dewitt or Cengel and Turner, any book on heat transfer can give you this information). you want more precision also try to include the heat losses through radiation as most of the time they can be considerable when compared to the losses through the natural convection.

 

[Makeabrew]

Thanks for the help guys. I've had a go at this and I think I've got a reasonable answer for the heat transfered to the tank and lost at the surface. You've also given me enough so I know what I'm looking for now.

I think the best I can hope for is to get an idea of where the major heat losses occur and if changing the setup or adding insulation will help. I can then check this with somebody in the company that has a better understanding.

The tanks are concrete structures and buried underground, with the top being at ground level. They also mix the contents by passing compressed air through at set intervals, which complicates the situation. It may even be worth looking at increasing the temperature of the air used for mixing.

 

[thermcool]

Due to the concrete structure and buried underground, your major heat loss is going to the ground. I think putting some simple insulation like PE sheath will be a good idea to start with. Once you have done that you can evaluate the outcomes and accordingly go for anything further, if required (like providing hot compressed air supply or changing the number of tanks).

 

[Mattysdad]

Whoa a bit here ...

Digestion is a biological process in which there are many and complex reactions are taking place. There are bacteria eating other bacteria and producing gases.

There is digestion of primary sludge (primary digestor) which is essentially large organics, greases, oils, toilet tissue, bulk fibers, etc. from primary settling and/or screening of raw sewage.

There is also digestion of secondary sludge, which is essentially biomass (bugs) that have uptaken most of the various pollutants, both soluble and insoluble, from the wastewater after primary settling / screening. these bugs are typically aerobes (live in and proliferate in aertion tanlks in various activated sludge processes such as: aeratio0n basins, SBR's, RBC'as, triclikg filters, etc.

The wastewater exits the bioprocess and flows through a clarifier where most of the aerobes settle to ther bottom and produce a sludge (secondary sludge) blanket. Typically this sludge is 1-2% bugs and the balance water. the reason is that the bugs barely sink (some do escape with the exit wastewater and are filterewd out or killed in some manner.


The secondary is digested by further aeration / detention in some cases and is an aerobic process. the heat balance would need to be a more complex material and energy balance rather than just a heat balance to get any meaningful results.

More often a secondary sludge digestor operates in the absence of air in which case it is an anaerobic process. This process uses a diffeent type of bugs. The eat the existing bugs thus reducubng the sludge volume and increasing the sludge concentration. Anaerobic organisms produce a waste gas which is a mixture of Methane, CO2 and misc. minor other constituents. Again the material and energy balance must be considered, not just conservation of energy heat balance.

Further, there a two (or more) common types of anaerobic processes; mesophillic and thermophillic. The mesophillic operates at relatively low temps. (don't quote me but maybe 80-100° F)and thermophillic operates at relatively higher temps. (maybe 120-130°F). Thermophillic is more robust and produces muchg more methane but also requires more careful control of operating parameters and maybe addition of heat to maintain temperature.

Selection of type of anaerobic process takes several items into consideration, but has a lot to do with whether your goal is more to reduce sludge volume / disposal cost or to produce gas for energy production.

Back in the late 70's / early 80's there was great interest in mathane production via thermophillic digestion, but that waned with energy price drops in the late 80's and beyond. Perhaps the conditions are right for a resurgence !!!

Do note, you have a little bio-chemical factory in the digestor and the bugs also produce heat as a result of the mtabolic processes, much like we do. So, any energy balance must also include the production of heat via chemical changes.


Advise if this does not make sense or you want more information.

Regards, AWA

"disconnectllc@comcast.net"

 

[Makeabrew]

Thanks for the reply,

The setup we use is that everything gets fed into the primary digester: primary settled sludge, surplus activated sludge from the aeration lanes and any sludge imports. The secondary digesters were added to reduce pathogen levels in the sludge and the sludge is literally held in huge open top tanks with occasional mixing (usually by compressed air). They are usually effective when the ambient temperature isn't too low as digestion continues (albeit at a reduced rate than in the primary digesters)as long as the bulk temperature of the sludge is above about 20 degrees C. I just want to quantify the majority of the heat losses and to work out the holding temperature of the tank if insulation is added to see if it can be maintained at above 20 degrees during colder weather.

I'm aiming to simplify this problem as much as possible, due to me having no background in heat and mass transfer, although I don't want to overlook processes that will afect any calculations. My plan at the moment is to produce a very basic analysis and then take it to someone in the company who does this kind of thing for a living. Unfortunately, the secondary digesters are a low priority as this is very much a winter problem and we have huge holding tanks that non-compliant sludge can be held for months until the e-coli levels eventually reduce enough to allow the sludge to be put to land. The problem I have with this is that if simply insulating the tanks or increasing the temperaute of the primary digesters in winter (to increase the thermal input into the secondarys) will stop us wasting money tankering non-compliant sludge around, then it's worth investing money in.

 

[Mattysdad]

What sort of a climate are you in ?

Is this domestic sewage or industrial wastewater ?

Are tanks concrete, steel, fiberglass ?

Are tanks in ground or above grade ?

What is typical sludge concentration ?

If it were simply heat transfer through sidewalls and bottom of tank, how would you explain that sludge temp is above both ambient temp and wastewater temp. ?

Consider that evaporation of water (which has an extraordinary high latent heat of vaporization) requires approx. 1,000 btu's / lb. This leaves the remaining water slightly coller as in any evaporative cooling process (cooling tower, sweating human, etc., etc. As quickly as you can conserve heat losses through the sidewallls (say with insulation), you will lose what is conserved via additional evaporation.... Unless you cover the tank to minimize / eliminate evaporative losses.

Those little buggers are metabolizing the O2 you put in via aeration (as dissolved oxygen) and respiring CO2. This is where the heat (metabolism) is coming from.

Yes, you will have much greater microbiological activity as temperature in tank increases, as a function of seasonal conditions. But, your biological activity will drop to minimal, even at high temperature, if you do not aerate, which will force evaporation and mixing. UNLESS YOU CONVERT TO AN ANAEROBIC PROCESS - which does not require aeration and must have a top on the tankage.

ANAEROBIC DIGESTOR CAN ALSO BE A NET ENERGY PRODUCER - think buried organics being digested by bugs in the absense of air: in deep lakebeds, swamps, ponds, oceans, underground, etc. This is where natural gas comes from - it is the byproduct of anaerobic microbilogical digestion.

Converion will allow you to potentially operate in the thermophillic (140°) range all year with high activity, reduced sludge residence time and high gas productions rates, some of which is used to maintain digestor temperature.

Maybe you need to look at the Civil Engineering forums, since civils / sanitary engineers design most of these (even though they are poorly trained in reactor design, process kinetics, etc.

Regards;

 

[Makeabrew]

I'm in good old England! So it can get cold in Winter.

The sewage is domestic waste.

Concrete tanks.

Tanks are in the ground (the top is pretty much level with the ground).

Sludge concentration is about 3% dry solids.

The sludge temp is above ambient as the secondary digester tanks recieve heated sludge from the primary digesters.


Wouldn't the evaporation rate depend on the surface area to volume ratio?

The secondary digesters are not used to harness any gas, they just hold what comes out of the primary digesters for a duration to increase pathogen kill. The conditions are far from ideal for anerobic digestion, but it is usually enough to kill enough remaining e-coli to make the sludge compliant.

The primaries are anaerobic and mesophillic, with the gas being burnt in a CHP engine to provide the heating. The temperature of the primaries struggles to maintain 35 degrees C in winter and so thermophilic is not really an option.

Thanks,