Ammonia removal from wastewater by air stripping and recovery struvite and calcium sulphate precipitations from anesthetic gases manufacturing wastewater

Abstract

Anesthetic gas plant wastewater (AGPW) contains high concentrations of ammonia (NH3-N) and sulphate ions, which is known to results in severe environmental pollution in aquatic ecosystems without treatment. In the first stage of this study, a lab-scale investigation was done to improve removal of ammonia from AGPW by air stripping and struvite (or MAP: magnesium ammonium phosphate) precipitation processes, separately. The main operating parameters such as pH, air flow rate (L/min) and operating time were optimized for air stripping. The theoretical amount of removed ammonia via the air stripping process was calculated. The NH3-N removal efficiency reached 94 % at pH 12. On the other hand, the optimum ratio of [Mg2+]: [NH4+]: [PO43−] was determined to be 1:1.1:1.8 for the struvite precipitation. The concentration of NH3-N was reduced from 10,600 to 1680 mg/L at pH 8.0 and the corresponding removal efficiency was 84 % at optimum ration of MAP. The solution remained after the struvite precipitation was reacted with Ca(OH)2 at pH values in the range 10–13 for removal of sulphate in the second stage. The removal efficiencies at the optimum conditions were 98 % for sulphate, 31 % for COD and 20 % for TOC. Air stripping process and struvite precipitation could be considered an effective technology for removal and recovery of ammonia or ammonium ion (NH4+-N) from the AGPW. Surface analyses of the precipitated samples were characterized by SEM and XRD.