Abstract: Disclosed herein are embodiments of a system for treating water. The system comprises one or more inlets for introducing biochar and polyamine to the water, such as a biochar inlet and a polyamine inlet, or a biochar/polyamine mixture inlet. The system may optionally also include a metal salt inlet, ozone inlet, an additional organic carbon compound inlet, or any combination thereof. The biochar and polyamine may optionally be premixed prior to addition to the water. The system also comprises a filtration device, such as a reactive filtration device. The system produces a treated water stream and a reject stream, which may be further separated into a recycled water stream and a solid product. The solid product may be suitable as a soil amendment for application to agricultural land, or for recycling. A method for using the system to treat water, particularly nitrate-contaminated water, also is disclosed.