טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentFalah Somaya
SubjectDeliberate Struvite Precipitation from the Supernatant of
Sludge Dewatering Facilities for both Phosphorus
Recycling and Decrease in Nutrients Load
on Wastewater Treatment Plants
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Ori Lahav
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Struvite precipitation is being considered as means of reusing phosphate minerals from wastewater as agricultural fertilizers. Since other potential phosphate-containing minerals obtained from wastewater are insoluble and thus impractical as fertilizers, the quality of the struvite precipitate, and thus its price, is dependent upon its purity. In this study, the performance of a continuous completely-stirred-aerated-reactor to attain homogeneous struvite precipitation was studied within a wastewater treatment plant (WWTP).

The objective of this work was to define the most favourable reactor operation conditions required to attain deliberate homogeneous struvite precipitation, in addition to relieving ammonia and P flux on the WWTP. To this end, a laboratory-scale continuous completely-mixed reactor fed with the supernatant of a sludge belt filter press system was operated at different hydraulic retention times (one hour, 30 minutes and 15 minutes) and pH values (7.4, 8.0 and 8.6). 10 mM MgCl2 was dosed into the influent reactor in a few experiments to improve precipitate quantity and homogeneity.

The results indicate that short retention times and pH<7.5 are favourable conditions for the attainment of the most homogeneous struvite precipitants. Despite the fact that increasing the HRT resulted in a small increase in the mass of precipitant formed the increase consisted of unwanted calcium phosphate and magnesium phosphate insoluble minerals. It was further found that an increase in pH beyond pH7.45 improves reactor performance for struvite precipitation however the improvement is too small to justify the added cost. At the favourable conditions, Ca2+ showed a low and approximately constant removal percentage at all the HRTs and pH values studied, indicating that the precipitate was composed of some calcium minerals but at a low percentage.

SVI results (settling performance) of precipitates collected at different reactor operation conditions indicated that the most advantageous precipitate was obtained when the reactor was operated at a HRT of 0.25 h. The best dewatering properties (based on SRF results) were obtained for precipitates collected at HRT of 0.25 hours and pH >8.0.