Reagent use efficiency with removal of nitrogen from pig slurry via struvite: a study on magnesium oxide and related by-products

Controlled struvite formation has been attracting increasing attention as a near mature technology to recover nutrients from wastewater. However, struvite feasibility is generally limited by the high cost of chemical reagents. With the aim to understand and control reagent use efficiency, experiments and equilibrium model simulations examined inorganic nitrogen (TAN) removal from pig manure via struvite with added magnesium and phosphate reagents. Four industrial magnesium oxide (MgO), a commercial product and three by-products from magnesite calcination, were tested with phosphate added as a highly soluble potassium salt. TAN removal extents with the MgOs ranged from 47 to 72%, with the highest grade MgO providing the greatest extent of TAN removal. However, model analysis showed that all the MgO reagents were poorly soluble (only about 40% of added magnesium actually dissolved). The model results suggested that this poor dissolution was due to kinetic limitations, not solubility constraints. A further set of additional reagents (termed stabilization agents) were prepared by pre-treating the MgO reagents with phosphoric acid, and were tested separately as a source of both magnesium and phosphate. Results showed that acid pre-treatment of moderate to highly reactive MgOs (soft to medium-burnt) primarily formed bobierrite as the stabilizing agent, whereas the pre-treatment of very low reactivity MgOs (dead-burnt) mostly formed newberyite. The newberyite stabilizing agents achieved very high TAN removal extents of about 80%, which is significant, considering that these were formed from dead-burnt/low-grade MgOs. However, the bobierrite stabilizing agents achieved a substantially lower TAN removal extent than their medium-to-high reactivity precursor MgOs. Again, model analysis showed that the bobierrite stabilizing agents were poorly soluble, due to kinetic limitations, not solubility constraints. In contrast, the model suggested that the newberyite stabilizing agents almost completely dissolved to very effectively form struvite. A mechanism was proposed by which conditions near a dissolving reagent particle surface causes unwanted struvite nucleation onto and overgrowth of the reagent particle, inhibiting further dissolution and markedly reducing reagent efficiency. The findings of the study could have implications for reagent efficiency with struvite in general, even when using other solid reagents such as magnesium hydroxide or other MgOs

Influence of MSWI bottom ash as unbound granular material on the corrosion behaviour of reinforced concrete

The reutilisation of MSWI natural weathered bottom ash (WBA) in many applications such as road and underground constructions, embankments or as an aggregate replacement is a common practice in many developed countries. Its potential environmental risk has regularly been evaluated from the point of view of the leaching of heavy metals and metalloids. Nevertheless, the influence over steel rebar when reinforced concrete is exposed to the contact with WBA has been poorly assessed before. In this study, it has been possible to evaluate in terms of days the probability of corrosion in a common case of WBA reutilisation, which is in contact with reinforced concrete formulated with conventional Portland cement. The corrosion monitoring indicates that the probability of corrosion of steel rebar is higher than 90 % with a corrosion rate (CI) estimated to be 11.6 lm year-1. The aggressive conditions imposed by WBA suppose an important withdrawal if the application of reutilisation involves contact with steel rebar.The authors would like to thank VECSA and SIRUSA for their financial support as well as the R+D National Plan for financing the project BIA2011-27182 and BIA2012-32320. D.M. Bastidas would like to thank the Ramón y Cajal Program from the Ministry of Economy and Competitiveness. The research group DIOPMA would also thank Judith Gómez and Jessica Giró for the technical support.Peer Reviewe