טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentTal Marina
SubjectExtraction of Phosphorus Compounds from Wastewater for
Reuse as Struvite Fertilizer, via Dosage of
Cheap Seawater-Based Mg2+Ions, Separated
Using a Novel NF-Based Method
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Ori Lahav
Full Thesis textFull thesis text - English Version


Abstract

The problem of low P reservoirs has induced the notion of recovering P from wastewater in wastewater treatment plants (WWTP). A promising P-recovery approach relies on precipitating struvite (MgNH4PO4), a solid with high utility value. However, the method is not widely employed due to its high cost, the largest cost component stemming from the Mg chemicals used in the process.

The goal of the present research was to develop an alternative method for extracting solid struvite from WW supernatants using Mg2 solution separated selectively from seawater (SW) by nanofiltration (NF) membranes. Mg2 ions (present at high concentrations in SW) are effectively rejected by NF membranes, a fact which allows obtaining low-cost Mg2-rich concentrates.

However, apart from Mg2, other SW constituents are also rejected by NF membranes with the potential of contaminating the WW stream.  For example, Cl- and Na ions increase the salinity of the WW, while Ca2 ions may promote precipitation of unwanted calcium-phosphate solids thereby reducing the purity of the struvite solid product. This study showed that rejection of Mg(II) by NF membranes deteriorated with the recovery value, as could be expected, but the ratio between Mg(II) and Cl(-I)/Na(I) increased with the recovery ratio as a consequence of differences in the extent of the reduction of the rejection for both Mg(II) and the monovalent ions.

Further, a theoretical simulation model enabling to determine the best operational conditions for the process was developed. The calculations allowed assessing the feasibility of the proposed struvite precipitation method considering not only associated costs, but also various process scenarios and different Mg2 sources.

Using this concept a theoretical simulation was carried out before each struvite precipitation experiment from anaerobic digestion supernatants.  A fluidized bed (FB) reactor was fed with a mixture of supernatant and NF brine solutions; different scenarios were tested by varying the total ammonia and orthophosphate supernatant concentrations, Mg2 dosage in the NF brine, pH, and the hydraulic retention time. Struvite purity notably increased at lower pH values, where the competition with other potential solid precipitants was minimal.  The results of dissolution experiments and XRD analyses demonstrated that at the relatively low pH conditions (pH~7.5) struvite product of high purity (~95%) was obtained. Moreover, it was shown that the use of NF brine led to a three- to five-fold cost reduction as compared to the process operated with traditional Mg2 sources, making the method attractive for industrial implementation.