It depends on the element. For instance is will not be affected the Hg if you use a constant temperature of 50 - 80 degrades Celsius. For others others heavy metals it is no problem to increase the temperature to 150 degrades Celsius, if the water is removed they will concentrate very fast. Ask another opinion.

Pollution with heavy metals is in municipal sludge is at so low concentration it doesn't give an observable effect on the heating value. I don't think you can find a reference to this.

In a case, because of discharging some industrial wastes to the sewage system, the concentrations of As, Zn, Al, Cu, Ni are rather high. I wonder how much these may reduce the typical heating values of sludge in incinerators.

Dear Shervin, like Henrik I think that these metals will have no impact on the heating value. But they for sure will have an impact of what you can do with the sludge later o, depending on their concentrations.

Dear Shervin,

I agreed to the views of Henrik, Olivier. You may not observe any change in calorific value of dried sewage sludge contaminated with heavy metals (in traces) BUT the ash handling will become critical in such cases. 

The ash generated should be disposed in secured landfill and should not reach to any water resources.

Regards

Pravin

and not only the solid ashes in the incinerator, Flying ashes should be precipitaded within the flue, unless you want to spread airborne metall pollution for hundreds or thousands of Km around the point source, i.e. the incinerator

Thanks you all,

However it seems heavy metal may have no effect on sludge heating value, I prepared some experiments to analyze this particularly with economical point of view.

Dear All,

Regarding the recent experimental results, it seems that heavy metals are effective on sludge heating values.

I have tested the heating values of sludge containing heavy metals and it is significantly lower than a sludge with low heavy metal content.

I guess this is due to the fact that when we sample from a dewatered sludge, we usually take a specific amount of sludge by its mass (1 gr, 10 gr, ...). The results also demonstrate how much energy contains (Kj) per Kilogram sludge (the sample). Here, Heavy metals make the samples heavier. They occupy some mass that has no heating value. Consequently, the ratio (Kj/Kg) may decrease.

Regards

What could be more important, in incineration than the impact of metals on the heating value  is the catalytic effect of some metals in terms of regenerating tricky organic compounds on the fly ash surface and the off-gas whilst crossing certain low temperature areas.

مرسی mersi!

@Shervin. I don't believe it is possible to measure an effect of heavy metals on the heating value of sludge. Am I right that the heavy metals in your sludge is a few mg/kg?

Shervin: notwithstanding all general opinions (which I definitely share) you went along , did your experiment and  found a measurable decrease of the heating value, which you claim to be due to the metal content, making up a significant part of the sludge's mass, so significant in fact, that you measured a lower calorific content of the high- metal-containing sludge, in comparison to a low sludge-containing one (presumably in an oxygen bomb calorimeter? or how else did you measure the heat output?)

Perhaps you would be so good and clarify what you mean by high vs low metal content? Thanks for sharing

Dear Domenico & Henrik

The highest concentrations in the disposed raw sludge belong to Cu, Zn, Cr, Al, and As that are respectively 120, 360, 100, 3770, and 455 mg/kg dry sludge.

I guess these are rather high in comparison with standard limits.

indeed, they seem fairly high which serves as a reminder on the cautions to be used when using this material for heat generation. However, the amount still seem too low to give a reproducible  (statistically significant) difference, especially given the usually fairly high variability of the inputs and outputs of treatment plants.

But while typing, I suddenly realized that we all followers of this question should habe started with a counter question: did you measure the organic and inorganic contents of your samples? perhaps there lies the difference you obtained when you measured their caloric output.

thanks for sharing

That is less than 5 g/kg of metals in the sludge, < 0.5 %.

I just don't believe it can explain any difference in the heating value.

If you like to prove it has any effect I suggest that you spike this amount of metals into the sludge and repeat the determination of the heating value.

Dear Shervin,

As a former operation manager of large hazardous waste facilities in Germany and the Middle East, I have a more practical input: Particularly Tank Bottom Sludge and Heavy Crude Residues, which commonly shows also a higher metal concentration, were thermally destructed abruptly in the incineration process, which was indicated by high temperature peaks (not good for the refractory). One interpretation was, that the fine distributed metals have a catalytic (increase of heat transmission) effect on the thermal conversion. - And maybe that is, what you are asking for!? Associated metals maybe do not increase the "calorific value" if  tested  in a colorimetric bomb for example, but they may increase the thermal conversion (means the process will run somehow faster).

Glück Auf!

Rüdiger

What you mention here sounds interesting Rüdiger. However, the discussion was on the amount (not the rate) of heat output. Shervin feels that those

Dear Ruediger

Thanks for your comment. That was very interesting. By my knowledge I could not come to this idea...However, the heat conversion is something I did not tested.

As Domenico truly said, I did some experiments to find the calorific value of sludge.

Dear Domenico

Because of rather high concentrations of heavy metals, I became curious about the heating value of sludge. Therefore, I tested the real secondary clarified sludge with 50%, 75% and 90% dry solids. It was 0, 6 and 10 MJ respectively. I guess these should be more than 8 to 15 MJ with typical sludge. Thats why I think heavy metals are effective.

I can do a synthetic test to add some heavy metals to a raw sludge but this would not be similar to the case study. Because I have to add heavy metals to a sludge with different conditions. By the way, the disposed MLSS is 2-3 gr/Lit.

Dear All,

This is the link of a research about incineration and heavy metals.

Article Evaluation of Sewage Sludge for Incineration (Case study: Ar...