Hi

I am the process engineer of a wastewater treatment plant and I am quite new at this post.

We use anaerobic digestion as a means of stabilizing the sludge before it is dewatered.  The Digester is heated by means of a heat exchanger, whereby hot water from a boiler (app temperature of 70 degrees celcius) is recirculated to the heat exchanger.. sludge from the digester is passed to this heat exchanger and recirculated back into the tank.

The digestion is mesophilic so we require a temperature of 35 to 37 degrees.

We have recently been noticing that the temperature of this digester is dropping and has now reached 25 degrees.  We are trying to figure out the reason why. We checked for the following:

1. the water flow going to the heat exchanger is within design parameters and the temperature from boiler is correct
2. we opened the sludge part of the heat exchanger to ensure that there were no sludge or other deposits in the pipe which might be effecting the degree of heat exchange by forming an insulation layer... but no such sludge crust was found
3. we checked for air in the system that might be causing this lack of heat exchange through insulation... but no such air is present..
4. and we checked the feeding rate of the digester, which is also within design parameters.
5. the heat exchanger is equipped with a 4-way valve.. we checked this too and it looks like it is working properly..

we have no idea what could be causing the drop in temperature.. Have u ever encountered such problems?

your reply would be greatly appreciated.
Thanks!

mixing is by means of a gas lifting system (operating by a compressor which is on all the time) and the recurculated sludge itself.

Do you grab samples of sludge and check the temperature with a lab thermometer?

If your temperatures are going that low I would expect your digester performance to be rather poor.

Measure water temperature (both recircualted sludge and hot water) at inlet and outlet of heat exchanger.  If you know the flows of hot water and of sludge, you can calculate how much heat you are adding to the digester, and compare, both from loss in hot water, and in gain in recirculated sludge.  Depending on where you are located, you may have a greater heat loss in your digester than you are providing with the hot water, and you may have to consider using hotter water or a larger heat exchanger.  

When you look at heat required, you will have to look at heat requierd to heat influent, and heat loss from digester.

Hope this helps.

Peter

What is the shape of your digesters? Is it egg shaped or cylindrical?
In cylindrical digesters you might have mixing efficiency issues.

Where exactly is the temperature indicator located on the structure of the digester? Could you elaborate pls.
grit or sand deposits on the bottom could lead you to faulty values and assumptions. Withdrawal might be required.