Wastewater from chemical fertilizer factories contains many pollutants such as NH, inorganic acids H2SO4, H3PO4, soluble salts, and suspended solids. In particular, wastewater from chemical fertilizer factories also contains large amounts of nutrients such as Nitrogen (N) and Phosphorus (P). These substances in high concentrations can cause excess nutrients in rivers and lakes, affecting the ecosystem. To treat this wastewater to meet environmental standards, most factories are applying physical and  chemical technology to treat the above wastewater, especially wastewater containing a lot of N and P.  This technology requires businesses to spend a large amount of chemicals and energy for treatment, in addition to causing secondary pollution.

NPK fertilizer is produced from struvite

Meanwhile, P reserves in mines for phosphate fertilizer production are decreasing. Therefore, reducing chemicals for wastewater treatment and recovering nutrients N, P from wastewater is always  the technology that environmentalists and businesses aim for. However, in the country, there is currently no complete research on  N and P recovery technology from fertilizer factories, as well as research on using recovered products to produce fertilizers for agricultural production. Therefore, a research group at Ho Chi Minh City University of Technology has recovered struvite (MgNH4PO4.6H2O) from fertilizer factory wastewater to produce slow-release fertilizers for agricultural use .

Wastewater was collected at Binh Dien, Ha Lan, and Ca Mau Fertilizer Plants to analyze and evaluate the content of substances in the wastewater such as N, P2O5, K2O, and pH. From there, the authors proposed parameters to recover Struvite (Magnesium ammonium phosphate hydrate – MgNH4PO4.6H2O) from wastewater with high concentrations of N and P.

According to Associate Professor, Dr. Le Minh Vien, Head of the project, struvite is a common crystal in nature, in an insoluble form (very slightly soluble). Struvite is slightly soluble in water and solutions, so the slow release of struvite has created an  effective  source of N, P and Mg for plants fertilized through leaves or soil. Using fertilizer from slow-release struvite can reduce from 20-30%, or even more, compared to conventional fertilizers while still having equivalent productivity. The results of research on a laboratory scale show that technological parameters  such as pH, molar ratio Mg (Magnesium)/P, N/P, and reaction time affect the efficiency of struvite precipitation. In which, the most influential parameters are pH, molar ratio Mg/P, N/P.

With the technological parameters  to achieve recovery efficiency of over 80%, the molar ratio Mg/P=1.0 and N/P=1.2; pH=8.3, time is 60 minutes at ambient temperature. By precipitation method, the product obtained is struvite in powder form with particle size from 13-22 micrometers, nutritional content (Mg, N and P) suitable for making slow-release fertilizer for agricultural production.

The chemical technology to treat wastewater from fertilizer plants often requires the use of large amounts of chemicals (NaOH, HCL), without obtaining any by-products, even causing secondary environmental pollution. With the precipitation method to collect, the authors used struvite to produce fertilizer. Compared to commercial fertilizer, fertilizer from struvite dissolves about 20% slower after 60 days.

Source: NPD (synthesis)