UBC Theses and Dissertations
Ammonia removal and recovery using heated struvite as an adsorbent Novotny, Chad
The removal and recovery of aqueous ammonia from BNR plant centrate or supernatant is an important research area. Stringent ammonia discharge requirements must be met to avoid fish toxicity in the receiving waters. Completely removing ammonia from the process reduces the load recycled back to the head of the plant and minimizes operational problems associated with high return loads. The controlled recovery of phosphate and ammonia into struvite with a dedicated fluidized bed crystallizer has led to the production of commercially valuable struvite pellets for the fertilizer industry. However, excess ammonia remains after struvite crystallization. The purpose of this research was to initiate the development of a nitrogen removal technology. This was achieved using the isothermal decomposition of struvite to remove ammonia. The decomposed pellets were subsequently placed into an ammonium solution for removal of excess aqueous ammonium. Struvite was shown to decompose into a mixture of magnesium phosphates and struvite. Satisfactory decomposition was achieved with a minimum of 100°C for 30 minutes. Ammonium removal reached up to 99% for a solution pH 8. Effective ammonia-N removal required a minimum reaction duration of between 15-30 minutes with a 66.7g/L dose. Struvite heated at higher temperatures worked as a better substrate to remove aqueous ammonia-N. Molar ratio comparisons show that the ammonia removed from solution is likely incorporated into newly formed fine struvite, rather than being incorporated into the heated struvite. This provides evidence in favour of a dissolution-reformation mechanism, whereby heated struvite acts a source of magnesium and phosphate. Heated struvite is more soluble than unheated struvite, because water of hydration and ammonia are removed from the pellet. Mass balances were reasonable but were complicated by natural adsorption of atmospheric water onto heated pellets. The economic viability of this technology may be unfavourable. A trade off exists between high ammonium removal and dissolution of pellets, which are worth up to $3000 per tonne in the United States. The total daily cost to remove nitrogen using this technology is estimated to be about 15 times greater than using side stream nitrification.
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