Magnesium Ammonium Phosphate By Thermal Decomposition Of Renewable Ammonia Precipitant Process Is A Comparative Study

Magnesium ammonium phosphate(MAP) chemical precipitation techinique for ammonia nitrogen removal has many advantages,such as simple process,free from limit of temperature and heavy metal ions,and reuse for controlled release fertilizer.However since cost of the precipitator is high while MAP is still not marketable,practical application of this method is affected.Based on the study of process condition optimization of MAP chemical precipitation,three reagent-reuse ways,which were direct pyrolysis of MAP,pyrolysis of MAP with NaOH and pyrolysis of MAP with Mg(OH)₂ were compared.Effects of the pypolysis conditions on the NH₃-N removal capability of pypolysate were investigated,such as reagent ratio,heating temperature,as well as heating time.Meanwhile effects of NH₃-N adsorption conditions were studied,which include adsorption reaction temperature,adsorption reaction time,pH and so on.MAP was heated directly at the conditions:T=100℃,t=3h,and produced MgHPO₄·3H₂O was used to treat 800mg/L NH₄⁺ wastewater at 25℃,pH=10,reaction time 40min.The NH₃-N removal rate reached 95%. It proved that MHP had a good ability of absorbing NH₃-N.The pyrolysis conditions of MAP dosing NaOH were optimized: the reagents mixed as n(MAP):n(NaOH)=1:1 were heated at 100℃for 2h,the pyrolysate was MgNaPO₄,which can reduce the NH₄⁺ concentration from intial 800mg/L to 1.6mg/L at 25℃,pH=9,reaction time 10min,and the residual phosphate concentration was up to 185.6mg/L.It is clear that MgNaPO₄ is excellent at absorbing NH₄⁺,but the phosphate concentration is too high.The pyrolysis conditions of MAP dosing Mg(OH)₂ were optimized: the reagents mixed as n(MAP):n(Mg(OH)₂)=1:1 were heated at 140℃for 2h.The pyrolysate was Mg₃(PO₄)₂·10H₂O.It is used to treat intial NH₄⁺ 800mg/L wastewater at 25℃,pH=8.5,reaction time 10min.As a result,the ammonia removal rate reached 97.8%and the residual phosphate concentration was 30mg/L.It is obvious that Mg₃(PO₄)₂·10H₂O has a feature that it is excellent at absorbing ammonia nitrogen,and the residual phosphate concentration is low.Finally pypolysates of MAP from three methods above were used to treat the leachate in Shenzhen Xiaping Landfill for removing high ammonia nitrogen,and the NH₃-N removal rates all exceeded 80%. Especially,the NH₃-N removal rate with the pyrolysate of MAP dosing NaOH reached 91.4%.It is proved that it is possible to use the pypolysate of MAP as precipitator to remove NH₃-N from leachate.Data of the simulation and practical wastewater test indicates that the pypolysate of MAP dosed with NaOH is the best for removing NH₃-N according to its NH₃-N removal rate,but the residual phosphate concentration is high.The pypolysate of MAP dosed with Mg(OH)₂ has a lower NH₃-N removal rate but the residual phosphate concentration is low.