On the understanding and control of the spontaneous heating of dried tannery wastewater sludge

•Dryed tannery wastewater treatment may self-heat, up to combustion.•SEM analysis revealed elemental S crystals.•FeS addition enhance the solids reactivity, while S prolongs it.•Ca(OH)2, Na2CO3, NaHCO3, FeCl2, EDTA, NaClO can limit or prevent self-heating.•The best prevention is reduction of S and Fe from the dried sludge.

We studied the spontaneous heating of dried sludge produced by treating wastewater mainly originating from tanneries. Heating up to burning has been observed in the presence of air and moisture, starting at ambient temperature. To understand and prevent the process we combined chemical and morphological analyses (ESEM) with thermal activity monitoring in insulated vessels. Selective additions of chemicals, either to amplify or depress the reactivity, have been used to investigate and identify both the chemical mechanism causing the sludge self-heating, and a prevention or a mitigation strategy. FeS additions accelerate the onset of reactivity, while S sustains it over time. On the contrary, Ca(OH)2, Na2CO3, NaHCO3, FeCl2, EDTA, NaClO can limit, up to completely preventing, the exothermic activity. All the experimental evidences show that the reactions supporting the dried sludge self-heating involve the Fe/S/O system. The total suppression of the reactivity requires amounts of additives that are industrially incompatible with waste reduction and economics. The best prevention requires reduction or removal of S and Fe from the dried solid matrix.