Metal/P ratio

When a mixed liquor taken from a municipal plant was treated with alum on a jar tester followed by settling and supernatant filtering with 0.45 micron syringe filters, an Me/P ratio of 3.12 was required to achieve 3 mg/L phosphorous in the supernatant (Song, 2008). It is noteworthy that the mixing time in this experiment was only 5 minutes.

In full scale MBR, much lower Me/P ratio is required to achieve much lower phosphorus concentration than 3 mg/L as summarized in Table 1. The long SRT employed by MBR, e.g. 12-30 days, allows orthophosphate to contact with metal hydroxide for extended period of time and the slow ion exchange reaction can move toward the equilibrium (see here for detail). According to the table, the lowest phosphorus concentration in MBR effluent is reportedly around 0.05 mg/L at Me/P ratio of 2.6.

The discrepancy of Me/P ratio between bench scale and full scale experiment can be explained by vastly different contact time of metal hydroxides and mixed liquor. Due to the slow ion exchange reaction of metal hydroxides as explained here, it takes a few days or longer to utilize the full capacity of metal hydroxides. Therefore, the longer the contact time (or SRT ) is, the lower the Metal/P ratio is to achieve same phosphorous removal. Since mixing is performed a few minutes to a few hours in typical jar tests, jat tests tend to overestimate Me/P ratio required to achieve a target phosphorus removal efficiency in full scale MBR.

The mixing intensity at the point of coagulant addition is a conceptually important factor. With intense mixing at the point of coagulant addition, more metal ions can bind with orthophosphate before they form metal hydroxides because the chance of collision between metal ions with orthophosphate ion increases. Although many literatures emphasize the intense mixing at the point of coagulant addition, the efficacy of this method is not fully supported by data obtained from practical situations to the author’s knowledge. Likewise, multipoint injection is conceptually beneficial since it increases the chance of metal ions’ collision with orthophosphate before they form metal hydroxides, but again the efficacy of this method is not fully proven. There is still a possibility that the extended contact time in MBR under a long SRT overwhelms all other kinetic factors and coagulant injection strategies do not matter.

Table 1. Phosphorus removal efficiency of various municipal MBRs

Anaerobic Tank TP in effluent (mg/L) Removal rate (%) Metal / P (mole/mole) Remark
No 0.23 98 0.53 Brepols, 2011
No 0.29 97 0.58
Yes 0.50 ? 1.0 (Alum) Pilot MBR, Hirakata, Japan (Trivedi, 2004)
0.10 ? 1.0-2.0 (Alum)
0.05 ? 2.6 (Alum)
Yes <0.1 >98 1.0-2.5 (Alum) Broad Run, VA, USA (Daigger, 2008)
Yes 0.38 94 1.2 (FeCl3) Traverse City, MI, USA (Daigger, 2008)

 

© Seong Hoon Yoon