Wastewater anaerobic digester design 1

[JohnWeal]

Good evening,

We are currently designing two concrete digesters.
Unfortunately due to height planning constraints, we have a design that has a height which is approx half the diameter.
Diameter is 25m and height is 14m

The decision has been made to only have a 3 degrees angle of the conical base as the diameter and a 15 degree cone angle (from the horizontal) would make for an expensive excavation.
I have aexpressed my concerns as it is not normal to have anything less than say 10degrees. Most companies I have worked for work on 15 degrees.

Therefore, two questions;

1. what problems can develop with such a shallow base? The dry solids is 5 to 6 per cent of co-settled primary/ WAS sludge.

Also, as we have a requirement to raise the sludge from 5 to 39 degrees, but recirculation to raise the temeperature by 1 degree over a 24 hour period.
The feed sludge is 1/10th of the recirculation flow rate.

2. For a 25m diameter digester, is one heat exchanger normally enough? Suction and discharge ar at opposite sides of the digester and we have a gas mixing system.

Regards
John

 

[ashtree]

John,

You are correct about the angle of the base being a problem. I have seen a lot of digestors even steeper than the 15 degrees you are refering to.
In my experience it will come down to a couple of issues:
1) How well mixed the contents will be. This is not theoretical but actual. Maybe if you can get really good thorough mixing and no dead spots the proposal will be okay. Even with steeper slopes on the floor this comment applies but perhaps to a slightly lesser extent.
2) How efficent is the grit removal system. Grit in the digestor will be a problem and large quantities can accumulate in dead spots. Comment 1) above applies .
3) How well you can manage the solids content: Are you able to pump , mix , and recirculate 6% solids and will a flat floor impact on the flow into pumps and mixers etc. What happens if the solids content goes higher due to process problems.

It might still work okay, but it would seem to me that the cost of the extra excavation would be relatively small compared to the overall cost of the digestor.

I have attached the relevant excerpt from the Ten States Standards for your reference:See below

84.131 Slope

The tank bottom shall slope to drain toward the withdrawal pipe. For tanks
equipped with a suction mechanism for sludge withdrawal, a bottom slope not less
than 1 to 12 is recommended. Where the sludge is to be removed by gravity
alone, 1 to 4 slope is recommended.

That makes it approximately 5 degrees with a suction mechanism and 15 degrees for gravity.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[JohnWeal]

Many thanks Ashtree.
I didnt mention the upstream grit removal but sure will be looked at in the same detail.
False economy to skimp on good proven design anywhere on a treatment works. The operators will not thank us for grit accummulation in the digesters!!

Regarding the Ten States Standards 84.131, i assume this refers to a sibgle plane slope from one side to the other.
We do have a single slope and the discharge for pumping to the heat exchangers is at the low end. As this is pumping at 2 m/s velocity i was also concerned about long term erosion of the pipe and coaxial heat exchanger due to grit.
A lot of digesters are conical bottom in the UK with the pump suction pipe routed down to the bottom with the pump outside at grade. Plenty of positive suction head on the pump.

I will be speaking to the digester mixing company when I return to work 2nd Jan.

On a previous UK project we installed 3 heat exchangers around the digester each with its own suction / discharge piping system.

Do the Ten States standards also limit the height to diameter ratio?

Best Regards
John

 

[ashtree]

John,

The Ten State Standards give a minimum side water depth of 20 ft but make no other comment about dimensions.

Certainly many older digestors and particularly those of UK design have conical bases with at least a 30 degree slope on the floor and a central sludge draw off pipe. I have seen plenty of those clogged up with grit however.
The Ten State Standards do not state anything about the cone bottom so whilst that would be satisfactory the reference is to cross floor slope.

A 2m/sec design flow rate in the pipe work is higher than what i would go with, but i am sure that there will be people out there who would say that its okay. Personally i would aim for 1m/sec as the maximum.

The Ten State Standards also limits the design sludge concentration to 5%. Wording below.

84.7 Anaerobic Digestion Sludge Production

For calculating design sludge handling and disposal needs, sludge production
values from a two-stage anaerobic digestion process shall be based on a maximum
solids concentration of 5 percent without additional thickening. The solids
production values on a dry weight basis shall be based on the following for the
listed processes:

Primary plus waste activated sludge - at least 0.12 lb/P.E./day [0.05
kg/(P.E.·d)].

Primary plus fixed film sludge - at least 0.09 lb/P.E./day [0.04 kg/(P.E.·d)].

If you are going to change that design loading you probably need to go back and have a good look at the whole sludge design assumptions, and if there is an existing plant in place it would be good to verify and test those assumptions. Likewise the loadings per person differ a little between the UK and the US so you need to consider this also. But i would suggest that the above numbers could at least be used as a sanity check.

Overall it seems like everything is being shaved to the bare minimum or below. This is what typically happens for the sludge management side of the process, which will ultimately have process impacts down the track. Many plants that would have otherwise been quite good have been scuttled by having inadequate sludge digestion capacity.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[ashtree]

John,
With regards to your heater question, what will happen if the heater has to go off line or fails. Even assuming that you can adequately heat and mix the whole tank with a single inlet and outlet and there is no short circuiting, the performance of that digestor will deteriorate very quickly if the temperature drops from 39 degrees to something like 5 degrees. Instead of having perhaps a 20 day SRT per stage to stabilise the sludge you may well now need 180 days. Gas production will drop , and supernatant will become dirty.
I would suggest that you should have a 100% capacity standby system particularly if your digestor will be fully loaded. I would suggest that you also look at the mixing capacity to make sure that the digestor is truly fully mixed , otherwise you will get short circuiting between inlet and outlet. Plug flow factor on circular tanks with inlets and outlets opposite one another (but unmixed) is 0.1.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[JohnWeal]

Hello Ashtree,
Some digesters have a grit removal facility which is a two valve arrangement that relies on the static head to push the grit out. But if these have been ignored (due to manual operation) then they end up never being used. I’m sure you have heard many stories like this!!

The digester design is normally concrete with insulation sufficient to allow the digester to drop only 1 degree Celsius per 24 hour period based on an ambient temperature of -5 degrees C.
Most of the calculations are very similar to Metcalf & Eddy for heat losses to the ground etc.

Regarding the recirculation through the heat exchanger, the suction from the digester is sized at 300mm which is a 1 m/s velocity and the discharge is approximately 1.7 m/s in a 250mm pipe.
The heat exchanger company have sized their proposed heat exchanger with the sludge side tube inside diameter as 100mm giving a 2m/s velocity. (Gives a 0.9 bar pressure drop)
So it isn’t the complete pipeline that is sized for 2 m/so I gave you incorrect information there.
But I suspect that the 2 m/s gives better turbulence for heat transfer.
A company in the U.K. we use is HRS and they use tubes that have been like swaged to deliberately create extra turbulence.

I find it very interesting how different wastewater companies have their own particular standards. Even in the U.K. regions vary. So it is nice to have a perspective from the US.

If only it was easier to visit I would attend some of the wastewater exhibitions / conferences.

I have found the document you were referring to ‘Recommended Standards for Wastewater Facilities’
Is the 2004 Edition still current?

Best Regards
John

 

[ashtree]

John,

I am actually based in AU but have spent some time in US plants. I use the Ten State Standards as a useful reference if nothing else is specified or applicable, as they offer a very practical standard based on hard won experience.

You are correct about the need for turbulence in the heat exchanger but the manufacturer would be better than me to advise on recommended flow velocities. The Ten State Standards require a full standby capacity for the digestor heating system.

Whilst the digestor itself may be well insulated there will be significant heat losses when new sludge is bought in or digested sludge or supernatant run off. I don't believe that you can rely on keeping the digestor warm in lieu of a standby heater if you need the digestor to be operating in the mesophilic range to achieve the required capacity.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[ashtree]

John,

I forgot to say that there is a 2014 version of the Ten State Standard but i think that in all the respects that we have been discussing it is identical to the 2004 version.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"