Effects of Wastewater Filtration on the Properties of Rice Husk Biochar and Its Reuse as Soil Amendment After Wastewater Filtration

Biochar is valued for its soil‐enhancing properties but often shows limited benefits for crop production due to low inherent nutrient content. Combining biochar with fertilizers or manures can greatly improve its effectiveness. This study explored the potential of enriching rice husk biochar (RHB) with nutrients by using it as a filter medium in domestic wastewater treatment. The agronomic effects of the treated rice husk biochar (TRHB) were then compared to those of the pristine RHB in a screenhouse pot trial at the University of Dschang, Cameroon. RHB and TRHB were applied at 10, 20, and 30 t/ha, with and without mineral fertilizer in a completely randomized design with four replications. Agronomic effects on lettuce ( Lactuca sativa ) were evaluated over two cropping cycles by measuring the number of leaves, leaf surface area, shoot height, and aboveground biomass. Wastewater filtration significantly altered biochar properties. Compared to RHB, TRHB showed increases in bulk density (+45.1%), organic carbon (+21%), total nitrogen (+170%), and total phosphorus (+106.7%). However, there was a decrease in the pH (−29.4%), ash content (−27.9%), cation exchange capacity (−10%), and base cations (K: −49.8%, Na: −44.6%, Ca: −22.2%, Mg: −13.6%). In unfertilized treatments, TRHB increased aboveground biomass relative to the control (up to 87.9% and 244.1% at 30 t/ha in the first and second cycles, respectively), but these gains were not statistically significant at rates below 30 t/ha. In contrast, RHB produced significantly higher aboveground biomass increases of 337.7% and 436.4% at 30 t/ha. Fertilization significantly enhanced aboveground biomass production ( p < 0.05) across treatments. Although wastewater filtration achieved partial nutrient enrichment, the substantial loss of ash and base cations reduced the liming capacity and overall agronomic performance of TRHB compared to RHB on the acidic oxisol. These findings highlight the trade‐offs in dual‐use biochar systems and the need for optimized filtration strategies to enhance its value as a sustainable soil amendment.