Removal and recovery of phosphorus during anaerobic digestion of excess sludge by the addition of waste iron scrap

In the current investigation, the feasibility of phosphorus removal in the anaerobic digestion of excess sludge by the addition of waste iron scrap (WIS) was studied. The results showed that the removal efficiency of phos- phorus increased with increasing amount of WIS, and the maximum removal efficiency of 39, 93 and 99 % could be reached at WIS dosages of 1, 2 and 3 g L - 1 , respectively. Sterilization greatly decreased the removal efficiency of phos- phorus, being only -6, 53 and 64 % at WIS dosages of 1, 2 and 3 g L -1 , respec- tively. This means that iron-reducing bacteria and hydrolytic bacteria enhance 45 , 40 and 35 % of the phosphorus removal at WIS dosages of 1, 2 and 3 g L -1 , respectively. The first and most important mechanism of phosphorus removal using WIS involves hydrolytic bacteria, which reduce the pH of the sludge to corrode the WIS, followed by precipitation of phosphorus by ferrous iron gene- rated by iron-reducing bacteria. Phosphorus adsorption onto the WIS is the second probable mechanism. The removed phosphorus is recovered by up to 56 % using magnet. This method is characterized by high removal efficiency, easy and steady operation, low cost, recovery and reuse, making it better than other physical and chemical treatments.