# Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects
## 2 Analysis of Trends in Phosphogypsum Use
### Phosphogypsum Recycling in Agriculture
The possibility of using biotechnological methods of drilling wastewater treatment based on biodestruction of organic pollutants was substantiated [78; 79]. The efficiency of a treatment depends, to a great extent, on the activity of microorganisms-destructors, and the presence of macro- and microelements in the environment. The efficiency of purification depends to a large extent on the activity of microorganisms-destructors, and the presence of macro- and microelements in the environment. A lack of one of the most important elements--phosphorus and calcium--is a limiting factor in the process of drilling wastewater bio-treatment, so the use of phosphogypsum as a cheap source of these elements is promising. In the study by [PERSON] et al. [81], it was found that, during three days of cultivation in the experiment with the addition of 1.0% wt. of phosphogypsum, the degree of destruction of carboxymethyl cellulose and polyacrylamide in drilling wastewater was 66.8% higher than in the control, as 99.8% [81]. However, this method requires further development and improvement as the implementation on an industrial scale requires activation of the biological component of the process considering all physical and chemical properties of biodegradable material. Another area of phosphogypsum use is its application in biotechnological processes of sewage sludge neutralization. The paper by [PERSON], 2014, describes a method of sewage sludge treatment in systems of anaerobic fermentation with heavy metals deposition into complex sulfide fraction with the help of biogenic hydrogen sulfide--a product of life activity of sulfate reducers, where phosphogypsum is a mineral additive for intensification of bacterial culture development [82]. At the same time, for intensification of bacterial culture development, it would be appropriate to use the process of its bacteria immobilization on a mineral carrier to reduce biomass export from a bioreactor and to transfer the process of treatment into a continuous one, which would increase the productivity of the bioreactor. Therefore, anaerobic fermentation with sulfate reduction process is a promising alternative for phosphogypsum utilization. It was investigated during our study [83] and confirmed in the study by [PERSON] et al. [84], where strong sulfide formation was observed from enrichment culture inhabited by marine sediments collected near the wastewater discharge point of the Tunisian fertilizer plant (Sfax, Tunisia). The authors [84] confirmed that _Desulphovibrio_ species are potential candidates for phosphogypsum bioremediation by immobilization of metals and metalloids through sulfide production, which has also been studied in our work [85]. In accordance with our previous studies [20; 85], the biotechnologies of processing of complex organic substances allow in anaerobic as well as in aerobic conditions to carry out the decomposition of organic wastes with the formation of volatile fatty acids and carbon dioxide, which, in the liquid phase, is represented by carbon dioxide. The development of the biochemical process of converting phosphate raw materials into phosphate fertilizers is a promising field of study. The way of the combination of recycling areas of already accumulated phosphogypsum and implementation of new technological solutions for processing phosphorus raw materials will reduce the level of technogenic impact from phosphogypsum on the environment. Therefore, we have formed a model reflecting an integrated approach to environmentally friendly production and recycling of phosphogypsum (Figure 9). It is important to note that the application of complex compost based on mixing phosphogypsum, elementary superphosphate, manure, and plant residues in the reclaimed soil can reduce the content of mobile forms of heavy metals (Co, Mn, Cu, Ni, Pb) by 60-70% or more per year owing to the transformation of heavy metals into forms of organic complexes and metal salts that are hardly accessible to plants [86].