Global pollution of surface waters from point and nonpoint sources of nitrogen

A review of ammonia removal using a biofilm-based reactor and its challenges

Extensive growth of industries leads to uncontrolled ammonia releases to environment. This can result in significant degradation of the aquatic ecology as well as significant health concerns for humans. Knowing the mechanism of ammonia elimination is the simplest approach to comprehending it. Ammonia has been commonly converted to less hazardous substances either in the form of nitrate or nitrogen gas. Ammonia has been converted into nitrite by ammonia-oxidizing bacteria and further reduced to nitrate by nitrite-oxidizing bacteria in aerobic conditions. Denitrification takes place in an anoxic phase and nitrate is converted into nitrogen gas. It is challenging to remove ammonia by employing technologies that do not incur particularly high costs. Thus, this review paper is focused on biofilm reactors that utilize the nitrification process. Many research publications and patents on biofilm wastewater treatment have been published. However, only a tiny percentage of these projects are for full-scale applications, and the majority of the work was completed within the last few decades. The physicochemical approaches such as ammonia adsorption, coagulation-flocculation, and membrane separation, as well as conventional biological treatments including activated sludge, microalgae, and bacteria biofilm, are briefly addressed in this review paper. The effectiveness of biofilm reactors in removing ammonia was compared, and the microbes that effectively remove ammonia were thoroughly discussed. Overall, biofilm reactors can remove up to 99.7% ammonia from streams with a concentration in range of 16-900 mg/L. As many challenges were identified for ammonia removal using biofilm at a commercial scale, this study offers future perspectives on how to address the most pressing biofilm issues. This review may also improve our understanding of biofilm technologies for the removal of ammonia as well as polishing unit in wastewater treatment plants for the water reuse and recycling, supporting the circular economy concept.

Fertilizers and nitrate pollution of surface and ground water: an increasingly pervasive global problem

Nitrate pollution of ground and surface water bodies all over the world is generally linked with continually increasing global fertilizer nitrogen (N) use. But after 1990, with more fertilizer N consumption in developing countries especially in East and South Asia than in the industrialized nations in North America and Europe, nitrate pollution of freshwaters is now increasingly becoming a pervasive global problem. In this review it has been attempted to review the research information generated during the last two decades from all over the world on different aspects of nitrate pollution of natural water bodies. It is now evident that not more than 50% of the fertilizer N is directly used by the crops to which it is applied. While a small portion may directly leach down and may reach ground and surface water bodies, a large proportion ends up in the soil organic N pool from where N is mineralized and is taken up by plants and/or lost via leaching during several decades. Present trends of nitrate pollution of freshwaters, therefore, reflect legacies of current and past applications of fertilizers and manures. Tools such as simulation models and the natural variation in the stable isotopes of N and oxygen are now being extensively used to study the contribution of fertilizers and other sources to nitrate enrichment of freshwaters. Impacts of agricultural stewardship measures are being assessed and nitrate enrichment of water bodies is being managed using modern digital models and frameworks. Improved water and fertilizer management in agroecosystems can reduce the contribution of fertilizers to nitrate pollution of water bodies but a host of factors determine the magnitude. Future research needs are also considered.

Agricultural non-point source pollution management in a reservoir watershed based on ecological network analysis of soil nitrogen cycling

The Miyun Reservoir plays a pivotal role in providing drinking water for the city of Beijing. In this research, ecological network analysis and scenario analysis were integrated to explore soil nitrogen cycling of chestnut and Chinese pine forests in the upper basin of the Miyun Reservoir, as well as to seek favorable fertilization modes to reduce agricultural non-point source pollution. Ecological network analysis results showed that (1) the turnover time was 0.04 to 0.37 year in the NH₄⁺ compartment and were 15.78 to 138.36 years in the organic N compartment; (2) the Finn cycling index and the ratio of indirect to direct flow were 0.73 and 11.92 for the chestnut forest model, respectively. Those of the Chinese pine forest model were 0.88 and 29.23, respectively; and (3) in the chestnut forest model, NO₃^- accounted for 96% of the total soil nitrogen loss, followed by plant N (2%), NH₄⁺ (1%), and organic N (1%). In the Chinese pine forest, NH₄⁺ accounted for 56% of the total soil nitrogen loss, followed by organic N (34%) and NO₃^- (10%). Fertilization mode was identified as the main factor affecting soil N export. To minimize NH₄⁺ and NO₃^- outputs while maintaining the current plant yield (i.e., 7.85e0 kg N/year), a fertilization mode of 162.50 kg N/year offered by manure should be adopted. Whereas, to achieve a maximum plant yield (i.e., 3.35e1 kg N/year) while reducing NH₄⁺ and NO₃^- outputs, a fertilization mode of 325.00 kg N/year offered by manure should be utilized. This research is of wide suitability to support agricultural non-point source pollution management at the watershed scale.