NOVELTY - Performing co-fermentation of henna plant biomass and sludge, involves (a) crushing henna plant biomass using crusher to obtain henna powder, (b) dehydrating sludge collected from city sewage treatment plant and industrial wastewater treatment plant, and (c) uniformly mixing the dehydrated concentrate and henna powder to obtain mixture, adding the mixture to fermentation tank or anaerobic reactor, performing co-fermentation, adjusting pH value of mixture in the reactor, adjusting oxidation-reduction potential, and stirring the mixture using stirrer.    USE - The method is useful for performing co-fermentation of henna plant biomass and sludge (claimed) for producing volatile fatty acids.    ADVANTAGE - The method eliminates pre-processing of raw materials.    DETAILED DESCRIPTION - Method for performing co-fermentation of henna plant biomass and sludge, involves (a) crushing henna plant biomass using crusher to obtain henna powder having particle diameter of less than 3 mm, and drying the powder at 50-70 degrees C, where the henna plant biomass comprising mixture of flower, stem, seed or leaf, (b) dehydrating sludge collected from city sewage treatment plant and industrial wastewater treatment plant, and performing biochemical treatment of dehydrated concentrate, and (c) uniformly mixing the dehydrated concentrate and henna powder to obtain mixture, where the mixture has 5-70% henna powder, adding the mixture to fermentation tank or anaerobic reactor, performing co-fermentation process of added materials at 15-55 degrees C, adjusting pH value of mixture in the reactor to 6-11 using phosphate buffer solution, sodium bicarbonate buffer solution or alkaline solution, adjusting oxidation-reduction potential to less than 150 mV, and stirring the mixture using stirrer at a rotating speed of 50-300 revolutions/minute for 1-30 days, where the anaerobic reactor is anaerobic sequencing batch reactor or anaerobic digestion pool, and reflux ratio of the anaerobic reactor is 100-200%.