Feeding Emitters for Microirrigation with a Digestate Liquid Fraction up to 25% Dilution Did Not Reduce Their Performance

Round 1

Reviewer 1 Report

Aim of the study in "Digestate liquid fraction up to 25% dilution did not reduce the performances of the emitters for microirrigation, which released a solution with constant traits" was to test the efficiency of a commercial compensating emitter when injected with the liquid fraction of the effluent from an agricultural biogas unit previously treated with a hydrocyclone filtration system. Although the topic is interesting, it requires clarification before accepted for publication. The specific comments are given below.

1. Complete page numbering.
2. Line 78-81: Remove from the introduction. The test procedure is described here. This should be included in the methodology.
3. Line 218: When the p-values were lower than 0.05, F and p were shown in bold. Is bold marked in the Table 2?
4. Line 221: Why are the double standard deviations in Figure 5 adopted? Please correct.
5. The authors are aware (line 48-49) that wastewater may contain pollutants and pathogens for the plants and animals. Has it been investigated? Were bacteriological tests performed?
6. Explain why for the control test (100% water), turbidity and pH were measured only before starting the test and no samples were collected during the test.
7. Provide the most important numerical values in your conclusions.

Author Response

Reviewer 1: Complete page numbering.

Authors: Done.

Reviewer 1: Line 78-81: Remove from the introduction. The test procedure is described here. This should be included in the methodology.

Authors: We disagree; this sentence is aimed to highlight the lack of knowledge in the topic we are studying and does not describe the procedure we are using, but solely the topic. Removing it would leave the reader disoriented on why making this experiment.

Reviewer 1: Line 218: When the p-values were lower than 0.05, F and p were shown in bold. Is bold marked in the Table 2?

Authors: Corrected. Thank you for the suggestion. This correction was not made in tracked change mode because the table styles were overwhelmed by track change marks, which in turn reduced the readability of the manuscript.

Reviewer 1: Line 221: Why are the double standard deviations in Figure 5 adopted? Please correct.

Authors: As specified in the figure captions, these are not double standard deviations, but standard error. This is needed to help the reader understand the differences among treatments. For the control treatment, the standard error was given in bold only for visualization purposes, since it overlapped with other treatments.

Reviewer 1: The authors are aware (line 48-49) that wastewater may contain pollutants and pathogens for the plants and animals. Has it been investigated? Were bacteriological tests performed?

Authors: Such problem was taken into account in lines 53-54. Also, the Chemical Oxigen demand (COD) was provided in table one. The wastewater used in the present experiment was provided by Consortium Bietifin SRL (Bologna), which undergoes recurrent analyses by the Water & Life Lab (Lab n° 0081 L accredia), which did not find any pathogen in the filtrate. The information on the society of the consortium was provided in the M&M section, subsection 2.1, L98-101.

Reviewer 1: Explain why for the control test (100% water), turbidity and pH were measured only before starting the test and no samples were collected during the test.

Authors: These variable did not vary during the irrigation from a previous test on the system, which highlighted that they did not vary during the test. This information was added in Lines 160-161.

Reviewer 1: Provide the most important numerical values in your conclusions.

Authors: Added in lines 310-313.

Reviewer 2 Report

A brief summary

The authors designed an experiment to observe the behaviour of a small microirrigation system when tap water is mixed with digestate liquid fraction. They prepared an indoor setup and they run it for a short time. They measured the water volume emitted by the drippers and water quality parameters in order to assess the microirrigation system performance.

Broad comments

The authors focused their research on studying the short term behaviour of a microrrigation system when tap water is mixed with disgestate liquid fraction.

The introduction is written in order to known the state of the art of microirrigation using wastewater. As the authors say the use of digestate liquid fractions allows saving water, but it is very important the fact that this digestate could be used as a fertilizer, as shown in table 1. So, the general objective of the paper should be to assess the possibility of using the digestate liquid fraction as a fertilizer in a microirrigation system. In this sense, obtained results by the authors could give information about the possibility (from the hydraulic point of view) of using the digestate liquid fraction for crop fertirrigation.

The experiment was equipped by a hydrocyclone filtration system, the authors should justify, maybe at the introduction section and using references, the use of this filtration system instead of other filtration alternatives.

The objectives of the experiments are written as:

“Aim of the present study was thus to test the efficiency of a commercial compensating emitter when injected with the liquid fraction of the effluent from an agricultural biogas unit previously treated with a hydrocyclone filtration system. The traits of the fluid released by the emitters during an 8-hours irrigation cycle were also measured. The system was built in order to avoid losses of pressure head, deposition of suspended solids, and salts precipitation before the filtrate was injected in the system and samples of water released were taken hourly.”

At the first sentence, add information explaining that the injection rate of digestate is variable. Rewrite the second sentence in order to make it as an objective. Think on moving the third sentence to the methodology section, is not an objective.

From my point of view there is a lack of information at the methodology section. In example there is no information of the general schedule of the trails, one irrigation cycle per day? Did the authors change the drip emitters for new ones between each cycle? How long it takes the storage of the liquid digestate before using it at the irrigation setup? It seems that the hydrocyclone was not placed at the irrigation setup, explain where was it placed? There is no information about the water mixture preparation.

The increase of released water in the control is a very important result, as it is considered control and the performance of the other treatments are referred to it. At the discussion section you must justify why the increase of the released water has to be accepted as a control. Another fact is that the measured flow at control treatment, after the first hour, is 3.07 l/h, 53.5% higher than the nominal flowrate. Try to explain why this flowrate is so different from the nominal and expected flowrate.

At the conclusions, it is important to remark that this results came from a short term experiment.

Specific comments

My specific comments for the manuscript are:

Title

Think on changing the title. It’s like a very short summary of the paper.

Abstract

Line 30 It is written "aroung”

1 Introduction section

Line 52. Use the units with the International System

Line 59. Explain what is considered a “high quality drip tape”

Line 67. It is said “Digestate from crop biomass and manure is increasingly being used” Justify where and why.

Line 68 It is said “its liquid fraction was indicated as a potential source for a wastewater fermentation”. What is meaning of the sentence? Maybe it would be said wastewater irrigation? The 17 reference seems to be talking about the possibility of using the digestate as a fertilizer.

Materials and methods

Line 91-92. It is said: “A water pump of 0.75 kW power (Pedrollo company, model: JSWm 2CX, San Bonifacio, Verona, 91 Italy) was used to pump the digestate liquid fraction”. Where was the water pumped to?

Line 98-100. Move the sentence to a better place, near the begging of the 2.1 section.

Line 98-100. It is said that “the focus of the study was the effect of the hydrocyclone filter on the emitter efficiency” It is true? From the text it seems that the focus was to assess the emitter performance depending on the filtrate dilution.

Line 105. Repeated information of the 200 kPa? It is the same pump? Choose kPa or MPa unit.

Line 112. Add information about the compensating range

Line 116-117. It is said that water flow (L h^-1) is calculated and it is not true. Results are presented in g h^-1

Line 118-120. Add information about model, city and manufacturing country of the thermometer and turbidity sensor.

Line 185. It is written “showeda”

Discussion

Line 308. It is said “Nonetheless, these differences can be due to the high-frequency flushing in our experiment (one every 8 ours)”. It lacks an h at “hours”. There is no information of flushing procedures in the experiment at methodology section. It is a very important issue of the experiment. The behaviour of the emitter in each trial must not be affected by the results of a previous trial. It is supposed that the drip emitters are changed by new ones at every irrigation cycle.

Author Response

Reviewer 2: The introduction is written in order to known the state of the art of microirrigation using wastewater. As the authors say the use of digestate liquid fractions allows saving water, but it is very important the fact that this digestate could be used as a fertilizer, as shown in table 1. So, the general objective of the paper should be to assess the possibility of using the digestate liquid fraction as a fertilizer in a microirrigation system. In this sense, obtained results by the authors could give information about the possibility (from the hydraulic point of view) of using the digestate liquid fraction for crop fertirrigation.

Authors: We warmly thank the reviewer for the suggestion to have multiple aim, some of which are meant to be established in further works. However, the aim of the work was not << to assess the possibility of using the digestate liquid fraction as a fertilizer>>. Indeed, this could have been done but would have required the presence of the soil or a substrate or a plant (either under soilless conditions or not). The aim of the work was indeed pointing to the agricultural engineering aspects of their use. Nonetheless, some of the aspects pointed by the reviewer were also referred in the intro and discussion, with not a strong deepening given that such aspects are out of the aim and topic.

Reviewer 2: The experiment was equipped by a hydrocyclone filtration system, the authors should justify, maybe at the introduction section and using references, the use of this filtration system instead of other filtration alternatives.

Authors: Filtration of a wastewater liquid fraction is a prerequisite for its use in any irrigation system and strong filtration able to discard low molecular weight compounds can be made by centrifugation (with hydrocyclone, in our case). Information on these aspects were added in lines 225-227 of the discussion. The hydrocyclone filtration was out of the aim of the work and was reported as supplemental information. Indeed, the treatments implied only a comparison by dilution, not by presence and absence of the filtration.

Reviewer 2: The objectives of the experiments are written as:

“Aim of the present study was thus to test the efficiency of a commercial compensating emitter when injected with the liquid fraction of the effluent from an agricultural biogas unit previously treated with a hydrocyclone filtration system. The traits of the fluid released by the emitters during an 8-hours irrigation cycle were also measured. The system was built in order to avoid losses of pressure head, deposition of suspended solids, and salts precipitation before the filtrate was injected in the system and samples of water released were taken hourly.”

At the first sentence, add information explaining that the injection rate of digestate is variable. Rewrite the second sentence in order to make it as an objective. Think on moving the third sentence to the methodology section, is not an objective.

Authors: The injection rate of the digestate liquid fraction is not variable. Injection rate was constantly kept to 0.2 MPa (L. 111). We only varied the % of digestate liquid fraction in the solution used. We agree that the second and third sentences are not aims but methodological explanation, were thus deleted. These sentences were not moved since these aspects are carefully detailed in the M&M section

Reviewer 2: From my point of view there is a lack of information at the methodology section. In example there is no information of the general schedule of the trails, one irrigation cycle per day? Did the authors change the drip emitters for new ones between each cycle? How long it takes the storage of the liquid digestate before using it at the irrigation setup? It seems that the hydrocyclone was not placed at the irrigation setup, explain where was it placed? There is no information about the water mixture preparation.

Authors: Thanks for the request of clarification. We added the relevant information in lines 93-96; 98-100; 115-118; 122-124; 159-160.

Reviewer 2: The increase of released water in the control is a very important result, as it is considered control and the performance of the other treatments are referred to it. At the discussion section you must justify why the increase of the released water has to be accepted as a control. Another fact is that the measured flow at control treatment, after the first hour, is 3.07 l/h, 53.5% higher than the nominal flowrate. Try to explain why this flowrate is so different from the nominal and expected flowrate.

Authors: The mean flow at 3.07 L/h is normal, since the nominal rate is declared, as usual, at 1 atm, whereas we used it at 2 atm (see lines 255-256 in the previous version an 274-275 in the present tracked change). Increase from one to 2 atm does not double the rate. With regards to the increase of the water flow in the control, we can assume that it is actually a control since the system was tested before the experiment. As displayed in the discussion, such increases are also normal as seen by Bodole et al. [26]. Specification of the reason to consider it as normal was given in lines 260, where we specified that it occurred in all treatments, despite at variable extent, and may be due to usury of the system. An additional specification was added in in lines 265-266 (tracked change version) to clarify it.

Reviewer 2: At the conclusions, it is important to remark that this results came from a short term experiment.

Authors: Done (L310-313).

Reviewer 2: Title. Think on changing the title. It’s like a very short summary of the paper.

Authors: Thanks. Changed to “Emitters for microirrigation had a constant performance when fed with a digestate liquid fraction up to 25% dilution”.

Reviewer 2: Line 30 It is written "aroung”

Authors: Corrected.

Reviewer 2: Line 52. Use the units with the International System

Authors: No data were present in L.52.

Reviewer 2: Line 59. Explain what is considered a “high quality drip tape”

Authors: In the work by Hills and Brenes [15] and Puig-Bargués et al. [14] cited in this point, various drip tapes were tested and referred with high and low quality.

Reviewer 2: Line 67. It is said “Digestate from crop biomass and manure is increasingly being used” Justify where and why.

Authors: A reference was added [ref. 17]. There is no need of specifying where, since it is occurring in any place where it is produced. Also, the reason for using it was specified earlier (it is safe and it is a resource).

Reviewer 2: Line 68 It is said “its liquid fraction was indicated as a potential source for a wastewater fermentation”. What is meaning of the sentence? Maybe it would be said wastewater irrigation? The 17 reference seems to be talking about the possibility of using the digestate as a fertilizer.

Authors: Thanks for letting us note the mistake. We ment irrigation and not fermentation. The ref. 17 displays also the properties of the liquid fractions. The sentence was corrected.

Reviewer 2: Line 91-92. It is said: “A water pump of 0.75 kW power (Pedrollo company, model: JSWm 2CX, San Bonifacio, Verona, 91 Italy) was used to pump the digestate liquid fraction”. Where was the water pumped to?

Authors: As clearly indicated, it was pumped in the irrigation system. A specification was added in line 102.

Reviewer 2: Line 98-100. Move the sentence to a better place, near the begging of the 2.1 section.

Authors: Done (L151-153).

Reviewer 2: Line 98-100. It is said that “the focus of the study was the effect of the hydrocyclone filter on the emitter efficiency” It is true? From the text it seems that the focus was to assess the emitter performance depending on the filtrate dilution.

Authors: Thank you for letting us note it, we deleted the sentence.

Reviewer 2: Line 105. Repeated information of the 200 kPa? It is the same pump? Choose kPa or MPa unit.

Authors: Corrected.

Reviewer 2: Line 112. Add information about the compensating range

Authors: We are sorry not to be able to provide it. As stated in the previous Line (now 127), the emitter is Stocker company N°26085, Bozen, Italy, but do not provide additional specification on the compensating range.

Reviewer 2: Line 116-117. It is said that water flow (L h^-1) is calculated and it is not true. Results are presented in g h^-1

Authors: We are sorry to letting the reviewer note that we did not state that the water flow is calculated and not true. As clearly specified, we weighted the liquid released once per hour and thus calculated the water amount released per hour. Since it is a measurement of weight and not volume, g and L were provided. Also, we can assume that, since the concentration of dispersed solids and solutes is very low, density of both the control water and DLF dilutions are 1000 g/L at TPS, but we wanted not to state it since we did not measure it and also temperature was varying (and we provided it).

Reviewer 2: Line 118-120. Add information about model, city and manufacturing country of the thermometer and turbidity sensor.

Authors: The information required were provided soon after stating it. The thermometer is a DS18B20 digital thermometer Maxim IC, the the turbidity sensor is a SKU:SEN0189. City and manufacturer were provided.

Reviewer 2: Line 185. It is written “showeda”

Authors: Corrected.

Reviewer 2: Line 308. It is said “Nonetheless, these differences can be due to the high-frequency flushing in our experiment (one every 8 ours)”. It lacks an h at “hours”. There is no information of flushing procedures in the experiment at methodology section. It is a very important issue of the experiment. The behaviour of the emitter in each trial must not be affected by the results of a previous trial. It is supposed that the drip emitters are changed by new ones at every irrigation cycle.

Authors: We specified now in M&M that emitters were not changed. Tests were made from the purest (control) to the most concentrated (50%) dilution. At the end of each experiment, a flushing of 15 min with tap water was performed. Now it is specified in lines 122-125.

Round 2

Reviewer 1 Report

Accept in present form

Author Response

Thanks.

Reviewer 2 Report

I would like to thank the authors for answering all my reviewing comments. I think that with the introduced changes the manuscript improved its quality.

I would like to say it was difficult for me to track the changes because of the no coincidence of your line number cite and my line numeration.

My last specific comments for the manuscript are:

Line 60. In my first review version there was a “kPa” and “bar” unit at Line 52. Now this is at Line 60. Choose the international system unit

Line 172 and 175. First, it should appear the cite 21 instead of 22.

Line 283. If the used emitter is pressure compensating the flow must be constant for a range of working pressures. Some of commercial pressure compensating emitters had a compensating pressure range that goes from 40 kPa to 400 kPa. It should be nice to provide this technical information of the used emitters. Maybe this information would have made easier to explain the “normal flow” measurements at 200 kPa.

Author Response

Reviewer 2: I would like to thank the authors for answering all my reviewing comments. I think that with the introduced changes the manuscript improved its quality.

Authors: Thank you very much for your kind advice.

Reviewer 2: I would like to say it was difficult for me to track the changes because of the no coincidence of your line number cite and my line numeration.

Authors: Sorry, this time we will better indicate the changes

Reviewer 2: My last specific comments for the manuscript are: Line 60. In my first review version there was a “kPa” and “bar” unit at Line 52. Now this is at Line 60. Choose the international system unit

Authors: Done (now line 55).

Reviewer 2: Line 172 and 175. First, it should appear the cite 21 instead of 22.

Authors: We updated the Mendeley desktop field that were used to compile the citation and it automatically corrected the citation numbers. See lines 136 and 139. This correction was not made in track change because the mendeley desktop would have corrected all the citations, including the correctly placed ones.

Reviewer 2: Line 283. If the used emitter is pressure compensating the flow must be constant for a range of working pressures. Some of commercial pressure compensating emitters had a compensating pressure range that goes from 40 kPa to 400 kPa. It should be nice to provide this technical information of the used emitters. Maybe this information would have made easier to explain the “normal flow” measurements at 200 kPa.

Authors: We are sorry to be unable to add this information. We made efforts and looked for the technical aspects of the emitter used, but the company does not provided it. Please see https://www.stockergarden.com/en/product/soaker-hose-25-m-1-2/ (or the Italian version https://www.stockergarden.com/prodotto/tubo-gocciolante-per-irrigazione-25-m-1-2/). To be more clear, we deleted the “pressure compensating” and avoid confusions. See lines 79, 115, 117, 246. This strengthens the discussion.