Preparation of ceramsite from municipal sludge and its application in water treatment: A review

Municipal sludge is a solid waste material, and resource utilization is the optimal way to dispose of this material. The amount of municipal sludge produced in China is large, and it can be used in the preparation of ceramsite. The content of Al₂O₃ in drinking water treatment sludge is significantly higher than that in wastewater treatment sludge, while the content of K₂O, Na₂O and MgO in the two kinds of sludge is similar. When sludge is used to prepare ceramsite, the amount of sludge in most raw materials for ceramsite is less than 50%. The bulk density of the prepared sludge ceramsite is less than 1000 kg m^-3, and the highest water absorption rate is close to 40%. The leaching content of heavy metals in municipal sludge-based ceramsite is within the standard health safety limit, and heavy metals are better stabilized. The fitting effect of the pseudo-second-order kinetic equation of the dynamic adsorption of sludge ceramsite is obviously better than that of the pseudo-first-order kinetic equation. Sludge ceramsite used in bio-filter media and constructed wetland (CW) substrates is good able to purify wastewater. In the future, the preparation method of municipal sludge ceramsite and purification research of CW substrates based on sludge ceramsite need to be further improved.

Simultaneous removal of nitrogen and phosphorus by magnesium-modified calcium silicate core-shell material in water

In this study, a new core-shell material (CMCS) is prepared with magnesium oxide (MgO) around calcium silicate hydrate (CSH), and CSH is prepared by SiO₂ from the red mud. The CMCS simultaneously removes ammonia nitrogen (NH₄⁺) and phosphate (PO₄^3-) by chemical precipitation and it can achieve recovery of nitrogen and phosphorus. The removal process of NH₄⁺ and PO₄^3- is as follows. First, the shell of MgO is used to remove NH₄⁺ and a part of the PO₄^3- by the assisted adsorption and struvite (MgNH₄PO₄·6H₂O) precipitation method. Then the CSH is used to remove the residual part of PO₄^3- by chemical precipitation (Ca₅(PO₄)₃OH, CaHPO₄ and Ca₃(PO₄)₂). Furthermore, the MgO shell of CMCS not only removes NH₄⁺ and PO₄^3-, but also can control the calcium ions (Ca²⁺) spill from CSH and pH in the process of removing NH₄⁺ and PO₄^3-. The removal rate of NH₄⁺ and PO₄^3- can reach 76.63% and 87.18%, respectively, in the solution in 80 min, but in the actual wastewater the removal rate of NH₄⁺ and PO₄^3- is 61.40% and 62.83%, respectively. Finally, CMCS was recycled five times and its removal rates of NH₄⁺ and PO₄^3- are 21.01% and 24.99%, respectively. The aim of this article is to present CMCS, which has a good effect on removing the NH₄⁺ and PO₄^3- simultaneously.