NOVELTY - Removal of heavy metal from sludge includes pretreating sludge by low-frequency ultrasonic wave; conducting biological leaching treatment using Thiobacillus acidophilus and ferrous sulfate; separating in a sedimentation tank; processing liquid supernatant with electric flocculation treatment; dehydrating the sludge and adding alkali to neutralize and obtain fertilizer; and returning the filtering solution generated by sludge dehydrating device back to the electric flocculation device to be treated.    USE - Removal of heavy metal from sludge (claimed).    ADVANTAGE - The method utilizes ultrasonic pretreatment to accelerate the mass transfer velocity of sludge system and accelerate the use of substrate by T. acidophilus. The leaching cycle of microorganism is shortened; and the efficiency is improved. The method overcomes the secondary pollution, so that heavy metal problem in sludge is solved and harmless recycling is realized.    DETAILED DESCRIPTION - Removal of heavy metal from sludge comprises: (A) pretreating sludge by low-frequency ultrasonic wave, comprising conveying the sludge in a sludge adjusting tank into a biological leaching tank by a sludge pump; and processing low-frequency ultrasonic pretreatment on the sludge in the biological leaching tank for 1-15 minutes by an ultrasonic generator with low frequency of 10-60 kHz and amplitude of 20%; (B) conducting biological leaching treatment comprising inoculating T. acidophilus into a biological leaching tank; adding ferrous sulfate with the concentration of ferrous ion of (Fe2+) of 10 g/L and 8 g/L sulfur powder, as leaching substrate; starting an air blower for blowing oxygen; conducting biological leaching treatment for 2-6 days until the pH is reduced to 1.5-1.9; and dissolving the difficult-to-dissolve metal; (C) separating in a sedimentation tank comprising pumping the sludge in the biological leaching tank into a sludge sedimentation tank by a sludge pump; naturally depositing for 2-5 hours; separating solid and liquid and making the liquid supernatant enter a collecting tank; and conveying the sludge into a sludge dehydrating device; (D) processing the liquid supernatant by pumping the liquid supernatant containing the heavy metal into an electric flocculation device by wastewater pump; processing electric flocculation treatment according to current electric flocculation technology; adding alkali into the electric flocculation device to process the wastewater; and discharging the water processed by the electric flocculation device after meeting the requirement; and (E) dehydrating the sludge by sludge dehydrating device and adding alkali to neutralize and obtain fertilizer; and returning the filtering solution generated by the sludge dehydrating device back to the electric flocculation device to be treated. An INDEPENDENT CLAIM is included for a special device for removing heavy metal from sludge.