Complex Agent for Phosphate Sequestration from Digested Sludge Liquor: Performances and Economic Cost Analysis
Ana Paula Fonseca
Round 1
Reviewer 1 Report
I think this manuscriptis well structured, is a very interesting work with important information.
Author Response
Reviewer #1：
I think this manuscript is well structured, is a very interesting work with important information.
Response: We are grateful to reviewer #1 for his/her effort reviewing our paper and his/her positive feedback.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear Authors, below I send some corrections proposal which could improve your paper:
1) Please carefully chceck text formatting, interpunction and style.
2) At the beginning of the paper I suggest add shortcut list
3) Please format literature citations with MDPI standard
4) Introduction partly correspond the topic of the paper. Please describe some economic factors of described methods
5) In summaery of introduction part please add information about novelty of the work
6) In section 2.1 please add information about chemical properties of liquor (ie. heavy metals, dry mass etc.). In table 1 what mean +/- format
7) In chapter 3.1 Authors described turbidity. In methods section there is no information about this parameter. Also Fig. 1 in my opinion is not enough described and compared to literature
8) In chapter 3 Authors wrote about statistical correlations. Again in the previous chapter there is no information about test which were provided
9) In economic analysis please describe costs in USD or EUR
English language is fine.
Author Response
Reviewer #2：
Q1: Please carefully check text formatting, interpunction and style.
Response: Many thanks for the reviewer’s suggestion. The whole manuscript has been checked, with not only text formatting, interpunction and style revised, but also the structure, grammar, expression, reference format, and etc.
Q2: At the beginning of the paper I suggest add shortcut list.
Response: Many thanks for the reviewer’s suggestion. The list of acronyms has been added at the beginning of the paper:
Acronyms
AAO anaerobic-anoxic-oxic
CaP hydroxyapatite
DOM dissolved organic matter
DSL digested sludge liquor
EEM dimensional excitation emission matrix
PACl polyaluminum chloride
PFC polyferric chloride
P phosphorus
SNS superparamagnetic nano- sorbent
TP total phosphorus
WWTPs wastewater treatment plants
Q3: Please format literature citations with MDPI standard.
Response: Many thanks for the reviewer’s suggestion. All the literature citations have been checked and revised according with MDPI standard.
Q4: Introduction partly correspond the topic of the paper. Please describe some economic factors of described methods.
Response: Many thanks for the insightful comments. In the revised manuscript, a new section “Method of calculation on dosing cost” has been added as Section 1.4:
“1.4 Method of calculation on dosing cost
The cost on dosing commercial magnetic powder and coagulants should be product of dosage and the market uni-price. For SNS dosing, the cost should be product of dosage and the manufacturing cost (sum of the reagents involved and the main expensive processing). Lost amount should also be taken into account. As for complex agent, the cost should be calculated as the sum of the cost on SNS and coagulants dosing”.
Q5: In summary of introduction part please add information about novelty of the work.
Response: Many thanks for the reviewer’s suggestion. Information about novelty of the work has been improved in Introduction as below:
“By now, little information is available for the integration of PACl and SNS and little research has been conducted for simultaneous SS removal and P recovery by dosing this type of complex agent. The role of PACl in the complex agent has not been investigated from the perspective of P recovery.
In this study, complex agent by combining aluminum coagulant and SNS were prepared and used for P recovery from DSL. The novel agent was characterized in terms of P adsorption capacity, turbidity removal, and reusability”.
Q6: In section 2.1 please add information about chemical properties of liquor (ie. heavy metals, dry mass etc.). In table 1 what mean +/- format.
Responses: Many thanks for the reviewer’s suggestion. In the revised table 1, the parameter of “SS” representing the dry mass has been added. According to ICP-AES analysis, the content of Zn was detected as 0.3~0.5 mg/L while the concentration of other types of heavy metals, such as Cu, Pb, As, Cd, Cr, Zn was below the detecting limit.
As for the “+/- format”, all the values in Table 1 are in form of “mean ± standard deviation”. According to the reviewer’s suggestion. A “Note” for “mean ± standard deviation” has been added in Table 1.
Q7: In chapter 3.1 Authors described turbidity. In methods section there is no information about this parameter. Also Fig. 1 in my opinion is not enough described and compared to literature.
Response: Many thanks for the reviewer’s suggestion. We are sorry to say the parameter of turbidity was mentioned in section 2.2 which may not be noticed by the reviewer. This sentence “turbidity was assayed using turbidimeter (HACH TU5200)” has been highlighted in the revised manuscript. According to the reviewer’ suggestion, more description and interpretation has been added for Fig. 2 as below:
“The dramatic decrease in the turbidity removal efficiency was observed when dosing 5 and 10 mg/L of PACl, although the component SNS simultaneously dosed was high to >25 g. This could be explained by relatively high concentration of flocs in DSL and low concentration coagulant dosed which resulted in fewer opportunities for particle collision and finally a lower coagulation efficiency (Xue et al., 2019). According to previous literatures, magnetic nanomaterials could affect sludge properties and extracellular polymer behavior by aggregating colloids into large flocs and greatly improve the separation performance of sludge and water. As a result, magnetic nanomaterials combined with macroscopic magnetic fields could promote sludge dewatering effect and provide a relative clarified supernatant for P sequestration (Lakshmanan et al., 2014; Triques et al., 2020)”.
Q8: In chapter 3 Authors wrote about statistical correlations. Again in the previous chapter there is no information about test which were provided.
Response: Many thanks for the reviewer’s suggestion. As for the statistical correlations, in the revised Section 1.2, it has been added “each sample was assayed in triplicates. Results are shown as the means and standard deviations (error bars) of 3 replicates per point”.
Q9: In economic analysis please describe costs in USD or EUR
Response: Many thanks for the reviewer’s suggestion. All the costs in economic analysis have been converted to EUR.
Author Response File: Author Response.docx
Reviewer 3 Report
1. The first two authors are majoring in Finance and Economics. Just out of curiosity, who designed and implemented the actual scientific experiments for phosphate sequestration?
2. The texts in the same paragraph (for example, in the Abstract section) are written in different fonts and sizes. Please format the texts accordingly to be consistent.
3. The chemical formula, MgNH4PO4•6H2O (Page 2) does not confirm to the standard, the numbers need to be formatted as subscript.
4. "The specific objectives were to 1) prepare the complex...; 2) investigate the effect... and (iii) evaluate the economic...". The authors used more than one number styles.
5. The numberings 1.1, 1.2, 1.2, 1.3 under Section "2 Materials and methods" are incorrect. The authors should use appropriate numbering for the list.
6. The charge of the anion "HCO32-" in Table 1 is wrong. Please double check the negative charge of the bicarbonate ion.
7. The authors wrote "soda ash (NaOH)" in the Chemicals section on Page 4. However, the chemical formula of soda ash is not NaOH. Please use the correct formula.
8. The texts inserted in the Figures (especially Figure 2 and Figure 3) are very blur. Please use proper settings and make them more legible.
9. The authors mis-calculated the math equations in at least two places as follows:
1). "0.1 RMB/m3 (for PACL) + 11.2 RMB/m3 (for SNS) = 11.2 RMB/m3" on Page 10. This does not seem right.
2). "20 g/m3×1 ton/1,000,000 g÷15.6%×90, 000 RMB/ton=11.52 RMB/m3" on Page 10. Please double check the calculation.
The authors need to check the entire manuscript for grammar errors. To name a few:
1. "By adding or Ca2+ and Mg2+ to the sludge liquor..." on Page 2 is not correctly written.
2. "DSL was obtained from the lab-scale mesophilic digester (operating at 35℃), which were fed with excess sludge from..." on Page 3.
Author Response
Q1: The first two authors are majoring in Finance and Economics. Just out of curiosity, who designed and implemented the actual scientific experiments for phosphate sequestration?
Response: We can understand about the reviewer’s confusion. Actually, the first two authors are just starting their research on environmental economics and conducting the study on evaluation index for the resource reclaim from waste and green supply chain. The first two authors designed and implemented the actual scientific experiments in cooperation with the second corresponding author and other authors. They also did the economic analysis, drafted the manuscript and are responsible for the revisions.
Q2: The texts in the same paragraph (for example, in the Abstract section) are written in different fonts and sizes. Please format the texts accordingly to be consistent.
Response: Many thanks for the reviewer’s suggestion. The format of the text, including the fonts and sizes, have been revised to be consistent.
Q3: The chemical formula, MgNH4PO4•6H2O (Page 2) does not confirm to the standard, the numbers need to be formatted as subscript.
Response: Many thanks for the reviewer’s suggestion. We revised it as “struvite (magnesium ammonium phosphate, MgNH4PO4·6H2O)”. The chemical formula of “MgNH4PO4·6H2O” was revised according to the references as below:
Doyle, J. D. and Parsons, S. A. Struvite formation, control and recovery. Water Res. 2002, 36(16): 3925-3940;
Shu, L.; Schneider, P.; Jegatheesan, V.; Johnson, J. An economic evaluation of phosphorus recovery as struvite from digester supernatant. Bioresource Technol. 2006, 97(17): 2211-2216;
Hanhoun, M.; Montastruc, L.; Azzaro-Pantel, C.; Biscans, B.; Frèche, M.; Pibouleau, L. Temperature impact assessment on struvite solubility product: A thermodynamic modeling approach. Chem. Eng. J. 2011, 167(1): 50-58.
Q4: "The specific objectives were to 1) prepare the complex...; 2) investigate the effect... and (iii) evaluate the economic...". The authors used more than one number styles.
Response: Many thanks for the reviewer’s suggestion. The numbering style has been revised as “1), 2) and 3)”to be consistent.
Q5: The numberings 1.1, 1.2, 1.2, 1.3 under Section "2 Materials and methods" are incorrect. The authors should use appropriate numbering for the list.
Response: We are sincerely sorry for the error. All the number of the sub-sections of section 2 has been revised as “2.1, 2.2, 2.3, 2.4 and 2.5”.
Q6: The charge of the anion "HCO32-" in Table 1 is wrong. Please double check the negative charge of the bicarbonate ion.
Response: We are sincerely sorry for the mistake. "HCO32-" has been revised as “HCO3-”.
Q7: The authors wrote "soda ash (NaOH)" in the Chemicals section on Page 4. However, the chemical formula of soda ash is not NaOH. Please use the correct formula.
Response: We are sincerely sorry for the mistake. It has been revised as “NaOH”.
Q8: The texts inserted in the Figures (especially Figure 2 and Figure 3) are very blur. Please use proper settings and make them more legible.
Response: Many thanks for the reviewer’s suggestion. In the revised manuscript, both Fig. 2 and Fig. 3 (Fig. 3 and 4 renewed in the manuscript) have been improved as below:
Fig. 3 Phosphate adsorption isotherm with complex agent and sole SNS
a
b
Fig. 4 The variation of P (a) and turbidity (b) removal performance of complex agent during 10 cycles of treatment/reuse with complex agent
Q9：. The authors mis-calculated the math equations in at least two places as follows:
1). "0.1 RMB/m3 (for PACL) + 11.2 RMB/m3 (for SNS) = 11.2 RMB/m3" on Page 10. This does not seem right.
2). "20 g/m3×1 ton/1,000,000 g÷15.6%×90, 000 RMB/ton=11.52 RMB/m3" on Page 10. Please double check the calculation.
Response: We are sincerely sorry for the mistake. In the revised manuscript, “0.1 RMB/m3 (for PACL) + 11.2 RMB/m3 (for SNS) = 11.2 RMB/m3” has been revised as “0.14 EUR/m3 (for PACL) + 1.4 EUR/m3 (for SNS) = 1.54 EUR/m3”; "20 g/m3×1 ton/1,000,000 g÷15.6%×90, 000 RMB/ton=11.52 RMB/m3" has been revised “20 g/m3×1 ton/1,000,000 g÷15.6%×11,400 EUR/ton=1.46 EUR/m3”. All the calculations have been checked.
Q10: The authors need to check the entire manuscript for grammar errors. To name a few: By adding or Ca2+ and Mg2+ to the sludge liquor..." on Page 2 is not correctly written; "DSL was obtained from the lab-scale mesophilic digester (operating at 35℃), which were fed with excess sludge from..." on Page 3.
Response: We are sincerely sorry for the grammar mistake. In the revised manuscript: The sentence of “By adding or Ca2+ and Mg2+ to the sludge liquor...: has been corrected as “By adding or Ca2+ and Mg2+ to the P-bearing sludge liquor, struvite crystals would form and could be separated from the wastewater for later agricultural use as slow-release fertilizer”; The sentence “DSL was obtained from the lab-scale mesophilic digester (operating at 35℃), which were fed with excess sludge from...” has been revised as “DSL was obtained from the lab-scale mesophilic digester (operating at 35℃), which was fed with excess activated sludge in the WWTP treating domestic sewage in Shandong Jianzhu University”.
The authors went through the entire manuscript, the errors that have been corrected and highlighted in the manuscript.
Reviewer 4 Report
This study is interesting in preparing the complex agent and investigating the performance of turbidity removal and phosphorus adsorption, the reusability, and economic cost. However, the results are lacking in the study and analysis of the characterization of materials to prove and confirm the mechanism in the treatment process. Therefore, please consider the following observations:
1. The abstract should include the key findings of the study.
2. Figure 1 shows a dramatic decrease in the removal efficiency of turbidity when PACl is dosed at 5 and 10 mg/L. Please discuss the reason for this decrease and the implications of this finding.
3. Recheck the numbering of subtitles in page 4, they are two section 1.2.
4. Section 1.2: Desorption and regeneration experiments in page 4. Please explain the regeneration process in detail, including the use of 2M NaOH solution.
5. Section 1.2: Adsorption and clarification batch experiments (page 4). Please provide the conditions of the adsorption experiments, both batch and isotherm.
6. Figure 3: The variation of P (a) and turbidity (b) removal performance of the complex agent during 10 hours. All abbreviations should be identified, and the quality of presentation should be improved. The amount of removal efficiency should also be included.
7. This study only demonstrates 1 table and 5 figures. They are not sufficient to present the results and discuss the experiments in detail. Please provide more information about the materials, such as characterization of materials varying of ratio of PACl:SNS with various techniques.
8. References in the discussion should be provided to compare and confirm the findings and cite them in manuscripts.
9. Economic analysis, please provide details in the table form for explaining well in terms of costing analysis.
10. In conclusion, the authors mentioned the number of cycles in the treatment/reuse process (adsorption/desorption). Please identify the optimum number of cycles for this process using these materials.
Please check the grammar of the text and rewrite some of the sentences as follows:
1. In page 2, first paragraph “By adding or Ca2+ and Mg2+ to the sludge liquor containing.
ammonia nitrogen and phosphate, struvite crystals form and could be separated from the
wastewater for later agricultural use as slow-release fertilizer”.
2. In page 2, first paragraph “Therefore, the large amount of organic matter in the suspended substances in the sewage might interfere with the crystallization process, producing low quality P end-product, which is often criticized due to the lack of market motivation”.
Author Response
Q1: The abstract should include the key findings of the study.
Response: Many thanks for the reviewer’s suggestion. The abstract has been revised and improved a lot, with key findings as below
“Phosphorus (P) management in the “water-energy-resource-nexus” in a wastewater treatment plants (WWTPs) remains a longstanding challenge. P adsorption from the P-enriched digested sludge liquor (DSL) is a comparatively more practically and economically viable approach for P recovery in WWTPs. However, high concentration of impurities in DSL might pose negative and interferential effect on the P adsorption, hindering the application of sorbents or precipitation methods. Given such a situation, highly efficient and cost-effective sorbent towards P reclamation from DSL is highly needed. Therefore, this study aims to develop a novel complex agent containing aluminum coagulant (PACl) and superparamagnetic nano-sorbent (SNS) which can be used in magnetic seeding coagulation for P recovery. The complex agents with different PACl:SNS ratios showed varied turbidity removal rates and P recovery efficiencies and the optimal ratio was 15mg PACl:15g SNS. PAC and SNS showed significant interaction since PAC could enhance the P adsorption by shielding the interferential effect of colloidal impurities. Besides, the complex is highly regenerative, with turbidity and P removal rate stably maintained at 70%-80% after 10 adsorption/regeneration cycles. The cost benefit analysis of dosing complex-agent showed a dosing cost of 0.154 EUR/m3, admittedly much higher than the conventional magnetic seeding coagulation, could be probably covered by the profit if reclaiming expensive and rare P product. This work indicated that the complex agent was superior due to its high adsorption capacity, easy separation, repeated dosing and therefore had the potential for P sequestration and recovery from DSL.
Q2: Figure 1 shows a dramatic decrease in the removal efficiency of turbidity when PACl is dosed at 5 and 10 mg/L. Please discuss the reason for this decrease and the implications of this finding.
Response: Many thanks for the reviewer’s suggestion. In the revised manuscript, reason for the decrease has been explained as below:
“The dramatic decrease in the turbidity removal efficiency was observed when dosing 5 and 10 mg/L of PACl, although the component SNS simultaneously dosed was high to >25 g. This could be explained by relatively high concentration of flocs in DSL and low concentration coagulant dosed which resulted in fewer opportunities for particle collision and finally a lower coagulation efficiency (Xue et al., 2019)”.
Q3: Recheck the numbering of subtitles in page 4, they are two section 1.2.
Response: We are sincerely sorry for the mistakes. All the number of the sub-sections of section 2 has been revised as “2.1, 2.2, 2.3, 2.4 and 2.5”.
Q4: Section 1.2: Desorption and regeneration experiments in page 4. Please explain the regeneration process in detail, including the use of 2M NaOH solution.
Response: Many thanks for the reviewer’s suggestion. Desorption and regeneration experiments in Section 1.2 has been explained in more detail and use of 2M NaOH solution has been revised as below.
“After 12h agitation and 15min sedimentation, 1 ml supernatant was taken to determine the turbidity and orthophosphate content, with P adsorption capacity (q0) and turbidity removal rate calculated. The SNS in the exhausted agent were separated with a magnet, rinsed with deionized water and regenerated in 100 mL 2M NaOH solution for 12h. 2M NaOH solution as generative agent was prepared in advance and OH- in NaOH cause the outer- sphere complexation (phosphate and SNS) to proceed in converse, leading to new “adsorption sites” for phosphate in next cycle of adsorption (Zhao et al., 2020). Then the amount of phosphate in the supernatant, i.e. the desorption amount was determined, with P recovery rate calculated. Afterwards, these SNS was rinsed, dried and mixed with 15 mg PACl before dosing into another 100 ml DSL. Thus, the cycle 2 of adsorption started, with P adsorption capacity in cycle 2 (q1) being calculated.”
Q5: Section 1.2: Adsorption and clarification batch experiments (page 4). Please provide the conditions of the adsorption experiments, both batch and isotherm.
Response: Many thanks for the reviewer’s suggestion. The conditions of the adsorption experiments have been described in more details.
The procedures of batch test have been revised as “The complex agents, with PACl and SNS ingredients at different mass ratio (25mg:5g, 20mg:10g, 15mg:15g, 10mg:25g and 5mg:30g), were respectively dosed into 5 jars containing 100 ml DSL at room temperature. These jars were capped and shaken vigorously in a shaker at 120 rpm for 12 h to reach saturation (pre-determined in the previous P adsorption-equilibrium test). Afterwards, the supernatant was decanted through a magnet, filtered with 0.45 μm filter and the phosphate, TP concentration was determined according to the standard molybdenum blue method (APHA, 1998) and turbidity was assayed using turbidimeter (HACH TU5200). The P removal performance was determined by calculating the ratio of the P loading amount (mg P/g) to the original P amout in DSL”.
The procedures of isotherm test have been revised as “Adsorption capacity for phosphate at different equilibrium concentrations could be described by adsorption isotherms. The isotherm test was conducted by exposing 0.5g-2.5g dry mass of complex agent (with the PACl:SNS ratio as 15 mgPACl:15g SNS) respectively to a 100ml aqueous solution containing phosphate with concentration of 20 mg P/L at room temperature. After 12h agitation in a shaker at 120 rpm, qe (mg/g), i.e. the amount of phosphate loaded per unit mass of the complex agent at the equilibrium, i.e. the maximum adsorption capacity can be determined.”
Q6: Figure 3: The variation of P (a) and turbidity (b) removal performance of the complex agent during 10 hours. All abbreviations should be identified, and the quality of presentation should be improved. The amount of removal efficiency should also be included.
Response: Many thanks for the reviewer’s suggestion. All abbreviations in Fig. 3 (Fig. 4 in the revised manuscript) have been corrected and the quality/resolution has been improved with removal efficiencies included.
a
b
Fig. 4 The variation of P (a) and turbidity (b) removal performance of complex agent during 10 cycles of treatment/reuse with complex agent
Q7: This study only demonstrates 1 table and 5 figures. They are not sufficient to present the results and discuss the experiments in detail. Please provide more information about the materials, such as characterization of materials varying of ratio of PACl:SNS with various techniques.
Response: Many thanks for the reviewer’s suggestion. We did SEM for the complex agent recently, which has been added in the revised manuscript (shown below).
Fig. 1 SEM image of complex agent (a) and SNS (b)
Correspondingly, materials and methods for SEM testing has been added in Section 2.1 while SEM pictures have been added in NEW Section 3.1, along with the interpretation for SEM characterization results.
The authors did quite of a lot characterization test for sole SNS, i.e. primary component of the complex agent, including SEM, SEM-mapping, TEM, XRD, XRF, FTIR, EDS. These results have been released in previous work and confirm the P removal performance.
Q8: References in the discussion should be provided to compare and confirm the findings and cite them in manuscripts.
Response: Many thanks for the reviewer’s suggestion. References in the discussion have been check and revised in order to compare and confirm the findings and cite them in manuscripts.
Q9: Economic analysis, please provide details in the table form for explaining well in terms of costing analysis.
Response: Many thanks for the reviewer’s suggestion. In the revised manuscript, the comparison of cost and environmental benefit between magnetic seeding coagulation process and complex agent are shown in Table 2 as below:
Table 2 Comparison of dosing cost and environmental benefit between magnetic seeding coagulation and complex agent
|
|
Magnetic seeding coagulation |
Complex agent |
|
Cost (EUR/m3) |
0.015 |
1.54 |
|
Profit (EUR/m3) |
0 |
1.46 |
|
Net cost (EUR/m3) |
0.015 |
0.08 |
|
Environmental benefit |
Low |
High |
Q10: In conclusion, the authors mentioned the number of cycles in the treatment/reuse process (adsorption/desorption). Please identify the optimum number of cycles for this process using these materials.
Response: Many thanks for the insightful comment. It has been added in Section 3 and conclusion that “The 5th cycle might be optimal cycle for this process using these materials since there seemed relative significant reduction in turbidity removal rate after cycle 5”.
Q11: Comments on the Quality of English Language: Please check the grammar of the text and rewrite some of the sentences as follows: 1) In page 2, first paragraph “By adding or Ca2+ and Mg2+ to the sludge liquor containing. ammonia nitrogen and phosphate, struvite crystals form and could be separated from the wastewater for later agricultural use as slow-release fertilizer”; 2) In page 2, first paragraph “Therefore, the large amount of organic matter in the suspended substances in the sewage might interfere with the crystallization process, producing low quality P end-product, which is often criticized due to the lack of market motivation”.
Response: We are sincerely sorry for the grammar mistake. In the revised manuscript: The sentence of “By adding or Ca2+ and Mg2+ to the sludge liquor...: has been corrected as “By adding or Ca2+ and Mg2+ to the P-bearing sludge liquor, struvite crystals would form and could be separated from the wastewater for later agricultural use as slow-release fertilizer”; The sentence “therefore, the large amount of organic matter in the suspended substances in the sewage might interfere with the crystallization process, producing low quality P end-product, which is often criticized due to the lack of market motivation” has been revised as “Therefore, the large amount of organic matter in the sludge liquor might have adverse effect on the crystallization process, producing low quality of P end-product”.
Author Response File: 
Author Response.pdf
Round 2
Reviewer 2 Report
I thank Author's for revise their paper.
Reviewer 4 Report
Thank you for your response and correction.
Thank you for your correction.
