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Peer-Review Record

Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer?

Materials 2020, 13(4), 954; https://doi.org/10.3390/ma13040954

by Jakub Pulka¹

, Piotr Manczarski²

, Paweł Stępień³

, Marzena Styczyńska⁴

, Jacek A. Koziel⁵

and Andrzej Białowiec^3,5,*

Reviewer 1: Anonymous

Reviewer 2: Anonymous

Reviewer 3: Anonymous

Materials 2020, 13(4), 954; https://doi.org/10.3390/ma13040954

Submission received: 31 December 2019 / Revised: 6 February 2020 / Accepted: 17 February 2020 / Published: 20 February 2020

(This article belongs to the Special Issue Advances in Carbonized Refuse-Derived Fuel)

Round 1

Reviewer 1 Report

Reviewer’s comments

This paper discusses sewage sludge torrefaction and the characterization of the produced biochars. This work has a lot of experimental data that are of interest to the reader. There are however several aspects of the manuscript that the authors need to improve; the results must be discussed in depth to elucidate the parameters’ effects. From the discussion it is unclear if the biochars are suitable for use as fertilisers as some key data such as heavy metals analysis are missing. Moreover, the authors tend to repeat their data in various forms (figures, tables, experimental results and discussion sections) rather than presenting new findings or conclusions. Given that sewage sludge thermochemical conversion is a relatively mature subject, the authors need to present their data in a way that highlights the new knowledge and innovative findings of their work. Some more specific comments follow as suggestions to improve the manuscript.

Major comments

In paragraph 2.2 the authors should add at which point they measured the reaction temperature. A schematic diagram would also help the reader understand their experimental setup, otherwise a few more details should be added; for example how many crucibles were simultaneously torrefied, how was CO2 introduced in the reactor etc. In figures 1, 2, 3 and 5, the equations do not offer anything concrete to the reader and make the figures unnecessarily complicated. Paragraph 3.1 should be shortened; the authors should integrate some of their conclusions together for brevity. For example, it makes sense that when C content increases so will the HHV of the biochars. The authors should not repeat data already provided in the figures and tables but focus on describing the general trends and providing the reader with data not given in the corresponding figures. On page 12 of the manuscript, the authors begin the discussion with a comparison of their biochars, derived from sewage sludge, with lignocellulosic derived ones. They try to explain why their results are different when clearly the starting material is the main cause. This part should be shortened and rewritten with a focus on comparing their findings to literature findings with the same starting material (sewage sludge). The paragraph between lines 389 and 404 is confusing; the parameters’ effects on HHV are unclear from this discussion. The authors should avoid using acronyms that make it hard for the reader to follow the text; for example on line 416 the reader has to remember what WWTP stand for. On lines 418-420 the authors state: “The maximum value was 7% higher 419 than the values characterizing dry SS. Such a small increase in calorific value is associated with increased ash content in the biochars in relation to the dry SS.” Please rephrase this, as it is written it attributes the increase in HHV to the higher ash content. A significant part of the discussion section is repeating the data given in the previous paragraphs; this should be shortened and more on point. On page 16 please report the prices in euros, not PLN. On page 16 on lines 528-529 the authors state: “It is worth mentioning that the required energy for torrefaction and fertilization value will increase significantly with the decrease of SS moisture.” They need to rephrase this as with decreasing moisture less energy will be required for drying and torrefaction overall. The authors indicate the increase in alkali metals content which is beneficial for use of the biochar as fertilizer. They do not however report if the original material and biochars were measured for heavy metals content which would forbid their use as fertilisers.

Author Response

Our responses are included in the attached file.

Author Response File: Author Response.pdf