Sulfide and arsenic compounds removal from liquid digestate by ferric coagulation and toxicity evaluation

The liquid digestate has been regarded as a potential organic fertilizer for its benefit in nutrients recovery. However, the potential risk of hazardous substances remaining in the wastewater was still one of the main obstacles for the wastewater application in the circular agriculture. The pretreatment is important to remove pollutants with relatively satisfied results. Ferric coagulation was a feasible way to simultaneously remove various contaminants in the wastewater with few residuals of ferric ions under alkaline and neutral conditions. In special, it could reduce the residues of sulfide and arsenic compounds. We gained insights into the mechanism of ferric coagulation in removing sulfide and arsenic compounds. Redox reaction and precipitation were the reasons resulting in removing sulfide. The formation of precipitate by combining with iron(III) contributes to the removal of arsenic compounds. Toxicity tests using Scenedesmus obliquus and Chlorella pyrenoidosa showed an obvious reduction of toxicity for the liquid digestate after ferric coagulation. Besides, ferric coagulation could efficiently remove turbidity, reduce COD, and eliminate dissolved organic matters correlated with the fate of heavy metal and antibiotics. Therefore, this paper could give basic data and technique supports for the secure utilization and pollution control of liquid digestate. PRACTITIONER POINTS: Most sulfide and arsenic compounds were removed by 0.01 M ferric coagulation. Mechanisms on removing hazardous substances by ferric coagulation were discussed based on analysis of X-ray photoelectron spectroscopy and FTIR. The evaluation by two algae showed the toxicity of liquid digestate could be reduced obviously after ferric coagulation.