PILOT STUDY OF CONSTRUCTED WETLAND TREATMENT SYSTEMS DESIGNED FOR REMOVAL OF AMMONIA IN OILFIELD PRODUCED WATER

Pilot-scale constructed wetland treatment systems (CWTSs) were designed to create biogeochemical conditions needed to promote pathways for treating ammonia in simulated post-reverse osmosis oilfield produced water. The treatment goal was to meet stringent discharge or beneficial use requirements (<1.2 mg/L as N). Objectives of this research were to (1) identify the biogeochemical conditions required for microbial nitrification and denitrification, (2) construct a pilot-scale CWTS with amendments designed to provide the required treatment conditions, (3) determine ammonia removal performance of the amended CWTS, and (4) compare ammonia removal of the amended CWTS with a CWTS containing no amendments. Based on biogeochemical conditions required for microbial nitrification and denitrification (e.g. dissolved oxygen, pH, and alkalinity), a free water surface (FWS) pilot-scale CWTS containing Typha latifolia (broadleaf cattail) was designed and constructed. Amendments included mechanical aeration to increase dissolved oxygen concentrations, metered sucrose additions as a source of electron donors, oyster shell additions to stabilize pH and alkalinity, and an 8-day hydraulic retention time (HRT). Ammonia removal and related chemical parameters from the amended CWTS were compared with an unamended Typha FWS CWTS operating on a 4-day HRT. Results indicate that the amended CWTS was capable of lowering ammonia-N concentrations from approximately 20 mg/L to non-detectable levels (<0.1 mg/L) versus 12.6 mg/L for the unamended CWTS. Calculated rate coefficients for the amended CWTS ranged from 1.35 to 1.39 d^-1 versus 0.071 to 0.111 d^-1 for the unamended CWTS. Explanatory parameters related to ammonia removal (e.g. dissolved oxygen, pH, and alkalinity) for the amended CWTS were within the required range, indicating that the selected amendments were capable of promoting biogeochemical conditions needed for ammonia removal. Pilot-scale Typha CWTSs designed to promote microbial nitrification and denitrification pathways can successfully treat ammonia in water generated from oil production to concentrations required for reuse or discharge.