Enhancing struvite crystallization from anaerobic supernatant

Nutrient recovery in the form of struvite from anaerobic supernatant will not only eliminate the operation and maintenance nuisance caused by struvite formation, but can also generate a valuable agricultural fertilizer. However, the struvite crystallization process is generally slow. In this research, different Mg²⁺ supplement chemicals — Mg(OH)₂ and MgCl₂ — and different seeding materials — sand and struvite — were tested in an effort to speed up the struvite precipitation/crystallization process. The research results showed that (1) both seeding materials were instrumental in enhancing the reaction rate, with struvite being better than sand; (2) the more surface area provided by the seeding material, the faster the precipitation is; (3) both Mg(OH)₂ and MgCl₂ were beneficial in speeding up the precipitation process, but MgCl₂ was more effective than Mg(OH)₂; (4) compared with simple aeration to remove CO₂, pre-acidification helped speed up the precipitation and could lower the final phosphate level; and (5) if Mg(OH)₂ is to be used, pre-acidification can be eliminated but the Mg(OH)₂ needs to be mixed with the wastewater at an earlier stage in the treatment process.Key words: struvite, crystallization, anaerobic supernatant, centrate, filtrate, sludge dewatering.