Land Application of Biosolids-Derived Biochar in Australia: A Review

Round 1

Reviewer 1 Report

The manuscript consists of a comprehensive review of several articles about biochar made of biosolids. Biosolids disposal represents one of the most problematic urban issues worldwide, and the study is critical and can potentially set light on the subject. Some key points are missing in the text, but the authors listed them as “key research gaps” as no data is available. The manuscript shall be shortened because it is a bit too long. Please see below some minor remarks.    

“Thermal processes including pyrolysis, gasification, and hydrothermal technology can be employed to sustainably process biosolids intended for land application [8]. The  materials that result from these processes offer several advantages compared with wet biosolids, including”.

The review then evaluates the physicochemical properties and contaminant fate of biosolids-derived biochar to assess its potential forland application compared to wet biosolids.

Figure 1: why do soil high porosity and low bulk density increase heavy metal imobilisation?

(i.e. the ability a phenomenon strongly controlled by type of or- 95 ganism, type of exposure and metal availability) something is missing here.

Consequently, the existence of POPs in land-applied 120 biosolids may result in ecosystem contamination with potential for bioaccumulation in plants 121 and animals [26] and risk of human and animal toxicity [27].

Okoffo et al., (2020) [34] further projected that around 4,700 Mt of plastics 143 are released into the Australian environment through biosolids end-use, of which 3,800 144 Mt is released onto agricultural land. In wich period?

Please define nanoplastics

“The pathogen load depends on the treatment and stabilization processes used to produce the biosolids [19]".” It also depends on the feedstock.

 Figure 3 is blurry.

Biochar pH influences the mobility of macro- and micro-nutrients, and heavy metals in soil?

 A temperature x variables graphic from table 1 data can show temperature influence on elements.

 Thomsen et al. (2017a) [98] measured an increase in total P from 4% in DBS to around 8% in BDB formed at 600 ° C and to 11% in BDB formed at 750 ° C. This increase could be due to the increased contact of Ca, Mg and P upon the transformation of organic matter in the biosolids, which would lead to the formation of insoluble Ca-P 306 and Mg-P compounds [59]. What about P relative concentration due to the loss of N and otther volatile elements and substances?

“There are no legislative standards available in Australia that prescribe limits for the 415 concentrations of heavy metals in biochar intended for soil application.” “However, there are currently no legis- 427 lative guidelines for the application of biochar in Australia;” Repeated information in the same paragraph.

The bulk density of biochar is lower than that of mineral soils, suggesting that the application of biochar can alter soil hydrology and further increase soil porosity, which can result in long-term impacts on soil aggregation [121, 125]. That’s not the way. Biochar density does not matter for increasing soil porosity.

 Although there is no fixed application rate, the manuscript does not discuss it. Is there a minimum biosolids-derived biochar application rate to influence soil properties?

Author Response

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Reviewer 2 Report

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Comments for author File: Comments.pdf

Author Response

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