Modified biochars prepared from straw, pig manure, and chicken manure biogas residues were used to adsorb and remove ammonium nitrogen (NH4+-N) and chemical oxygen demand (COD) in wastewater. The results showed that at 400–600°C, the adsorption performance of the biogas residue biochars increased with pyrolysis temperature, reaching the maximum at 550°C. Biochar from chicken manure biogas residue exhibited strong NH4+-N (20.3%) and COD (28.1%) adsorption capacity. Through FeCl3 modification, the Fe content on the surface of the biochar increased, and the adsorption data of the modified biochar conformed well to the pseudo-second-order kinetics and Langmuir model, and the reaction was dominated by chemical adsorption. The fitting calculation revealed that the FeCl3-modified biochar from chicken manure biogas residue showed maximum NH4+-N (55.29 mg/g) and COD (186.24 mg/g) absorption capacity. These results indicated that modified biochar from biogas residue could have potential application for the treatment and recycling of biogas slurry.