Phosphorus (P) is an essential nutrient, which in excessive concentrations from sewage treatment systems can cause eutrophication of waterways. The use of adsorption substrates is becoming a common method for P-removal from wastewater. To achieve sustained P-removal from passive wastewater treatment systems, such as constructed wetlands, it is essential to select substrates with a high capacity to retain P which can be replaced when saturated. An enriched hemp biochar substrate was added to experimental constructed wetland cells to treat domestic wastewater. Hemp feedstock was treated with hematite, melanterite and dolomite prior to pyrolysis at 400 °C. The amount of P which was present in the wastewater from the biochar wetlands cells after treatment was compared to gravel control wetland cells. During the 7-month study period, the wetlands containing the enriched biochar consistently reduced PO4-P concentrations in primary treated sewage to lower levels than in the control wetlands, with an average inlet P concentration of 15.5 mg/L, to below 2 mg/L. Various analytical methods were used to characterise the changes in the biochar substrate. Concentration of P in the substrate increased by 77% over the test period. X-ray photoelectron spectroscopy (XPS) analysis revealed iron phosphate formation. Scanning electron microscopy together with energy dispersive X-ray spectroscopy (SEM–EDX) showed P captured on the biochar surface was associated with aluminium, silica, iron, magnesium and calcium-rich mineral phases. This study showed that an enriched biochar can be used as a substrate to capture phosphorus in passive wastewater treatment.