Struvite Phosphorus Recovery from Aerobically Digested Municipal Wastewater
Round 1
Reviewer 1 Report
General comments
The article deals with struvite phosphorus recovery from aerobically digested sludge derived from small municipal wastewater. This article is very interesting and useful for operators of Wastewater Treatment Plants. Hereby I recommend this paper to publish in Sustainability after some changes, which are listed below.
The Authors took into account mainly operating costs, and should also take into account investment costs, which are extremely important for the rate of return on investment.
As, among others, the exploiter of the wastewater treatment plant, I tested the Ostara Pearl® process and salinity increased enormously. The conductivity of wastewater increased as a result of a significant increase in chloride, which is not mentioned by the Authors. And this is essential for the infrastructure of the wastewater treatment plants, the sewage system for discharging treated wastewater to the receiver because both Cl- (generated with MgCl2•6H2O) and SO42- (MgSO4•7H2O) are corrosive, and additionally SO42- in reduction conditions can form H2S. It is highly important information for the exploiters of the wastewater treatment plant, it is recommended to complete it.
Please, evaluate the costs of the process in relation to the entire wastewater treatment plant (specify in %: constitute ...% of wastewater treatment plant operating costs).
As the last general comment, it is commonly used to give a concentration of P mg/dm3, kg/m3 or g/m3, and the Authors use the molar concentration, please consider whether to use mg/dm3 or g/m3 (like in Table 4).
Detailed comments
Page 1; Lines 36 – 39: ‘….. 33,000 Imperial million gallons water per day (IIMGD)….’; ‘…. Of 7.5 ppm P…’ etc. We should use the units of the SI system.
Page 4; Line 122-123: The abbreviation ‘IPA’ should be firstly explained in line 122 and then in line 123 it should be just used as ‘IAP’.
Page 9; Figure 6: The axis Y should have a description, not just a unit, because the axis X has.
Page 10; Lines 283-274: The description of the table no. 5 should explain the 5th and 6th column from the table no. 5.
Author Response
Author Response File: 
Author Response.docx
Reviewer 2 Report
Air sparge of wastewater results in an increase in pH, in the presence of aggressive CO2. Thus, in this case, both the costs and the increase in pH will depend on the concentration of affiliated and aggressive CO2. It would be worth giving it this time? The CO2 concentration will depend on the method of the process aerobic digestion in which aeration in a natural manner removes excess CO2.
The paper states that Al, Ca and Fe ions may interfere with the formation of struvite. (lines 109-111). In conclusion, there is no comment or in this case the concentrations of these ions were significant.
It would be useful to estimate costs in USD / m3. (lines 236-240; 267; table 5)
My question. Did stirring caused by the agitator and air sparging contribute to initiation of struvite and similar crystallizations. You can find papers on the subject.
Author Response
Author Response File: Author Response.docx
