Removal of ammonium and orthophosphates from reject water generated during dewatering of digested sewage sludge in municipal wastewater treatment plant using adsorption and membrane contactor system

Recently the treatment of reject waters generated during dewatering of digested sewage sludge has become a matter of a concern in wastewater treatment plants. The problem of reject waters is that the concentration of ammonia and orthophosphates is relatively high compared to influent composition. In traditional treatment approach, side-streams are recycled to the main wastewater stream, sometimes causing its temporary overcharging. This study comes up with a novel two-step method of treatment real reject water. To this end, a membrane contactor system with a vacuum sweep mode for its ammonia removal efficiency was examined. In the second step, a new biocomposite, eggshells modified with ferric and zirconium oxide, as an effective adsorbent of orthophosphates was investigated. It turned out that application of the membrane contactor system can lead to a 98.9 ± 0.1% reduction in ammonium concentrations in the feed and producing pure ammonium solution concentrated almost three times as much. The investigation was performed at a temperature of 60 °C and an initial pH of the feed of 12.0. Subsequently, orthophosphates were removed from the pre-treated reject waters. For the adsorbent dosage of 15 g·L−1 with a pH of 4.38, the orthophosphate removal efficiency was 92.4 ± 0.1%. During adsorption the adsorbent was chemically and mechanically stable. The proposed mechanism of orthophosphate bound by ferric and zirconium oxide modified eggshells is based on a strong interaction between positively charged adsorbent surface and the negative orthophosphate ions in the solution. The present study confirmed that the membrane contactor process can be applied for effective removal of ammonium from reject water during wastewater treatment, while adsorption on metal oxide modified eggshells may be an attractive alternative to conventional orthophosphate removal processes. The level of the pollutants remained in the effluent after the process in the system was low in comparison to the initial side-stream, thus the effluent can be recycled to the main wastewater stream without causing its temporary overcharging.