Removal of ammonia nitrogen and phosphorus by biochar prepared from sludge residue after rusty scrap iron and reduced iron powder enhanced fermentation

Abstract The rusty scrap iron (RSI) or a mixture of rusty scrap iron and reduced iron powder (RSI-RIP) can be used as an exogenous additive to enhance the anaerobic fermentation of sewage sludge. In order to make rational use of the fermentation residue, the sludge after intensified fermentation was pyrolyzed to produce biochar in this study, which was used in the adsorption of ammonia and phosphorus from the anaerobic fermentation broth. The experimental results demonstrated that the pore structure of the sludge biochar was greatly improved after enhanced fermentation with RSI and RIP. Meanwhile, there was an increase in the proportion of metallic elements such as Ca, Fe and Mg. On the other hand, the RSI-RIP co-enhanced fermented biochar (ES600) prepared at 600 °C showed a higher adsorption capacity, which was comparable to the commercially activated carbon. Neutral or weakly alkaline environments were preferred during the adsorption process. At a suitable pH condition, the maximum removal efficiency of ammonia nitrogen (NH4+-N) and total phosphorus (TP) on ES600 reached 91.3% and 98.6%, respectively. In addition, the saturated ES600 was regenerated by simple washing with ammonia-free water. After three cycles, the removal efficiency of NH4+-N and TP remained at 71.3% and 83.2%, respectively. As a result, the biochar prepared from RSI-RIP enhanced fermented sludge can be used as a promising low-cost adsorbent.