Trouble-Shooting Stubborn Settling with Coagulants

[Erik_V]

I am working on a strategy for solids removal, from an effluent stream that is derived from fermented food. The food is de-watered first through a mesh vibratory screen, filtering out particles above the smallest sieve size (I believe this is 40 microns or larger, however I can check detail again if the answer if needed). Because the stream is also acidic, I have picked hydrated lime as a combined neutralizing agent and coagulant. Despite this, the settling is still hindered. I would describe the mixture as being relatively high in dissolved minerals, acetic acid, lactic acid, glucose, along with other characteristics attributable to a food derived leachate.

I understand that this problem could have a multitude of causes (dissolved mineral content is high, for instance, so surface charges are likely abundant on the dissolved material). Could anybody recommend a coagulant that would still allow the settled solids to be land applied? We are a private business, and the bulk of our dried solids will be applied as a soil conditioner, including content from the settling tank. I've considered Iron coagulants, but ultimately this is an area where my experience is only theoretical.

Thanks in advanced for any help.

- Erik

 

[bimr]

Does the waste contain oil? Is it possible that you are dealing with an oil water emulsion? If so, a pH adjustment will break the emulsion.

 

[ashtree]

Without having a full water analysis it is a little difficult to give you a good answer.

However a couple of general suggestions:

1) Under the circumstances you describe the lime dose required is likely to very high. I think a dose of 1000mg/l would not be unreasonable.
2) Try adding soda ash or sodium bicarbonate to the mixture as well as lime. This will be a source of carbonate which will combine with the calcium to form a CaCO3 floc. If you can form a floc you may enmesh other particles and improve the overall clarity.
3) You will need to properly mix the lime with the waste and then continue to mix for several minutes afterwards to allow any reactions to take place , floc to form etc. It is difficult to give a definitive mixing time though.

Do you know the concentration of suspended solids as there will be a limit as to how concentrated to will be able to get the solids.
Most likely 0.5 to 1% solids will be the limit?
What is the nature of the solids? Is the solids SG(specific gravity) greater than 1.0. If it is not than it will be difficult to settle and a flotation process may work better.


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