Technologies for Biological and Bioelectrochemical Removal of Inorganic Nitrogen from Wastewater: A Review
Round 1
Reviewer 1 Report
The submitted review presents biological and biochemical methods of nitrogen removal from the waste water. Protection of the environment nowadays has become one of the most urgent global challenges, The remediation of the waste water, among others, belongs to very crucial problems. Therefore, a subject of the review is very current and extremely important Despite recommended limited size of the Abstract, the principal subjects of the review are presented well and very comprehensively. The Keywords are also very informative and well chosen. The Introduction illustrates well the state of art and underlines the importance of the elimination of water pollutants generated by contemporary industrial civilization. Special focus is put on removal of nitrogen compounds. The Authors present the conventional biological methods (Chapter 2) such as Nitrification or Denitrification. The first one consists in oxidation of NH4+ to NO3- with aid of respective oxidizing bacteria. The above procedure could be also conducted with a contribution of microorganisms. In the latter one the nitrate is reduced (usually with some external organic compounds) to nitrogen gas. Denitrification can be also conducted with the presence of microorganisms (autotrophic and heterotrophic bacteria). In some cases both above processes can be also applied simultaneously (SND). The Anammox process (anaerobic ammonium oxidation) is considered very efficient and promising. The Anammox bacteria seem to consume ammonia without any presence of oxygen. Another two groups of methods (Chapter 3) comprise Bioelectrochemical Techniques. One is based on cathodic reaction in rhe microbioal fuel cell (MFC), and in the other one a microbial anodic reaction takes part in the electrolysis cell (MEC). The Figure 1 and included reactions schemes present very clearly the reactions in the above methods. The new microbioal findings (Chapter 4) allow to employ the bacteria enable to convert ammonium from waste water into N2 gas without any stage of nitrate formation (NOx , CANON, SHARON, OLAND). The above methods are clearly explained and illustrated with respective reactions. The nitrogen is indispensible component of proteins and its compounds (e.g. ammonia, nitrate, nitride) are broadly applied as the principal fertilizers. Thus, a recovery of nitrogen compounds from waste water plays an important role to support agriculture. The Authors present (Chapter 5) a recovery of nitrogen from water with aid of microalgae and cyanobacteria. Figure 2 illustrate very well the process and underlines its advantages (photosynthesis of oxygen, bio-energy, fertilizer, animal feed, etc.). Another presented method of nitrogen recovery consists in the chemical bonding ammonium compounds in a form of (Mg,NH4) phosphate precipitate (Struvite), which may be used as fertilizer. It is pity, that the Authors missed the information on zeolites, very broadly applied as ion-exchanging agents, very efficient in water remediation, soil fertilizers slowly-releasing the nutrient cations (e.g. K, NH4) . Literature on that is very abundant. Summing up, I think that presented manuscript is valuable and very well edited. It presents very important global problems. The Authors are competent both in chemical as well as microbiological (bacteria) problems. They quoted a great number (over 100) of literature data. The figures and reaction schemes are clear and nice looking. The work is very good and deserves the publication. Nevertheless, I think that it would be good, if the Authors could add some remark concerning great role of zeolites (several million tons annually, mostly natural and also synthetic) in water remediation, in agriculture as efficient fertilizers with long time release of nutrients (particularly ammonium, potassium ions) and control the soil moisture. Ammonia can be removed by means of an ion–exchange and afterwards NH3 can be recovered thermally from zeolite.
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Reviewer 2 Report
This manuscript focuses on various processes to remove and recover nitrogen. Generally it is informative and well written with sufficiency logic clarity. Please see below my comments:
My main common is that organic nitrogen is a part of wastewater contaminants, so why didn't the authors discuss organic nitrogen?
Line 83, long SRT is required as well.
Section 2.1.2. nitrification requires abundant oxygen; if insufficient oxygen is provided, we are looking at denitrification or other reduction processes. This section should focus solely on nitrification, rather than oxygen-controlled systems where other microbial processes take place other than nitrification.
Line 289 morphologically?
Section 5.1. How is the way microalgae remediate nitrogen different from other microorganisms, such as nitrified and dinitirfier?
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