Treatment of post-digestion liquors with the application of struvite precipitation and reverse osmosis

Abstract

Since the post-digestion liquors exhibited a high content of biogenic substances and organic matter, an attempt was made to develop an effective treatment of their treatment, allowing the treated liquors to be directly released into a natural reservoir. The liquors were treated with the application of reverse osmosis (RO) and chemical precipitation (struvite) as a pre-treatment step. The application of a unit RO process did not ensure an adequate removal of contaminants, mainly due to the excessive concentration of in the permeate. The treatment of post-digestion liquors with a struvite precipitation and a subsequent RO process turned out to be effective and allowed to decrease contaminants’ concentration below discharge limits, allowing direct release of permeate to a natural receiver. However, it was necessary to apply an increased ratio of magnesium and phosphates during struvite precipitation (Mg²⁺:: = 1.5:1:1.5), preceding reverse osmosis treatment. As compared to the MgCl₂·6H₂O, the application of MgO as a Mg²⁺ source allowed to receive an additional removal of COD. Struvite crystals were characterized by regular orthorhombic structure and contained insignificant amounts of heavy metals. Moreover, the struvite precipitation influenced the efficiency of a subsequent membrane process in a positive way.

Keywords:

• Post-digestion liquors
• Struvite
• Reverse osmosis
• Biogenic substances

Acknowledgement

Authors would like to express their gratitude to the National Science Centre in Krakow for the financial support of the research included in the article (Grant No. 7428/B/T02/2011/40).

Notes

Presented at the International Conference on Desalination for the Environment, Clean Water and Energy, European Desalination Society, 23–26 April 2012, Barcelona, Spain