Accounting for Dilution of SARS-CoV-2 in Wastewater Samples Using Physico-Chemical Markers
, Peter Kille 2,3, Andrew Weightman 2,3, Davey L. Jones 4,5, Gareth Cross 6 and Isabelle Durance 2,3,*Round 1
Reviewer 1 Report
The manuscript from Wilde et al reported a method to estimate the wastewater overflow and dilutions using three common physico-chemical markers including electrical conductivity, ammonia, and orthophosphate. The problem is well defined and important for the field, given the high interests in wastewater-based epidemiology. Writing is clear and enjoyable. The developed method is interesting and helpful for wastewater treatment plants that do not have flow rates data available, however, it also relies on a historical data collection in low-flow days, about five months in this paper. Overall, the method-estimated flow shows a good association with the measured flow data, temporally and geographically (six wwtps), suggesting the effectiveness of this method.
Specific comments
1. Ln 176, section 2.4 Using proxies for dilution. The authors seems to ignore the recent method, i.e. using PMMoV as a reference for wastewater dilution, that the field are using to normalize SARS-CoV-2 in wastewater. The authors may need to discuss related papers including but not limited to mSystems 2020, PMID: 32694130; Sci Total Environ 2022, PMID: 34534872.
2. The authors need to clarify the data source, where are the ammonia, electrical conductivity, ammonia, orthophosphate, and SARS-CoV-2 data from? Maybe with a brief introduction about how they were quantified.
3. Ln 265-268: it is not clear how the equation (2) was derived. Descriptions about ms can also be improved.
4. Figure 2: It seems there are strong associations between ammonia and electronical conductivity. Is it legit to use the two parameters for dilution estimation?
5. Ammonia and electrical conductivity-based estimation have good associations with the measured flow data in Figure 4, orthophosphate has the biggest deviation. The authors may want to discuss the reason.
Author Response
Please find attached a word document with our response to reviewer 1.
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Author Response.docx
Reviewer 2 Report
MAJOR COMMENTS
- The approach of giving a tolerance limit combined with qualification for abandoning flow measurements for the calculation of volume and using chemical markers to estimate volume provides a better estimate for those data sets that are diluted with other sources and/or where the flows are capped. This is the strength of the paper.
- Provide additional evidence to substantiate for the discrepancy of the recovered volume estimate. For example, is it possible to match precipitation data with the doubling of volume estimate for Cardiff Bay for early Nov 2021 and slight increase in late December? This overestimate is reflected in the slight elevation of orthophosphate values early Nov which is also smoothened out in the overall estimate in Fig. 4. This is a good example that adds veracity to your proxy records that can be highlighted better. As a reader, the next question in mind is the explanation about sources of parasitic water in this area can increase phosphate and not ammonia and electrical conductivity. It would bode well for this paper to expand the discussion and explanations more as this is a paper about using chemical proxy record to estimate volume, but in the eventual pursuit of commenting on the integrity of the SARS-CoV-2 loading. A good contrasting example can be underestimates in volume and how this was reflected in the individual and overall estimate of marker/s for SARS-CoV-2 viral loading. effects to example is the large increase increase in volume estimate for Llangefni for late January to late February 2022.
- It is recommended to publish this paper after addressing comments as the problem on estimating dilution of wastewater and integrity of SARS-CoV-2 viral loading are major concerns in wastewater-based epidemiology.
MINOR COMMENTS
- - introduction must be improved to include in the last paragraph insert the markers used and the general conclusion of which ones are most effective in which scenario and that the approach to use more markers are more effective.
- Note typographic error Line 266 – the minus sign should be an equal sign
- Comments on figures: please make fonts bigger
- Reference or source and date of population survey for table 1
Author Response
Please find attached a word document with our response to reviewer 2.
Author Response File: Author Response.docx
