Improving soil aquifer treatment efficiency using air injection into the subsurface

Abstract

Soil aquifer treatment (SAT) is an effective and sustainable technology for wastewater or stormwater treatment, storage, and reuse. During SAT, the vadose zone acts as a pseudo-reactor in which physical and biochemical processes are utilized to improve the infiltrated-water quality. Dissolved oxygen (DO) is necessary for aerobic microbial oxidation of carbon and nitrogen species in the effluent. Therefore, to enhance aeration, SAT is generally operated in flooding and drying cycles. While long drying periods (DPs) lead to better oxidizing conditions and improve water quality, they reduce recharge volumes. As the population grows, the quantity of effluent directed to SAT sites increases, and increasing recharge volumes become a concern and often a limiting factor for SAT usage. In this study, direct subsurface air injection SAT (Air-SAT) was tested as an alternative to long-DP operation. Six long-column experiments were conducted (2 m column) that aimed to examine the effect of air injection on the soil's water content, oxidation–reduction potential (ORP), DO concentrations, infiltrated amounts, and ultimate outflow quality. In addition to basic parameters, such as dissolved organic C (DOC) and N species, the effluent quality analysis also included an examination of three emerging water contaminants: ibuprofen, carbamazepine, and 1H-benzotriazole. Pulsed-air-injection experiments were conducted during continuous flooding using different operation modes (i.e., air pulse durations, frequencies, and airflow rates). Our results show that Air-SAT operation doubled the time during which infiltration was possible (i.e., the infiltration was continuous with no downtime) and allowed up to a 46 % higher mean infiltration rate in some cases. As a result, the infiltration volumes in the Air-SAT modes were 47 %–203 % higher than conventional flooding–drying operation (FDO). A longer air pulse duration (60 min vs. 8 min) and higher airflow rate (∼2 L min−1 vs. ∼1 L min−1) led to a higher mean infiltration rate, whereas a high pulse frequency (4.5 h−1) led to a lower mean infiltration rate compared with low-frequency operation (24 h−1). Air injection also allowed good recovery of the ORP and DO levels in the soil, especially in the high-frequency Air-SAT experiments, where steady aerobic conditions were maintained during most of the flooding. Consequently, the mean DOC, total Kjeldahl N (TKN), and ibuprofen removal values in these experiments were up to 9 %, 40 %, and 65 % higher than those with FDO, respectively. However, high-frequency Air-SAT during continuous flooding also led to significant deterioration of the mean infiltration rate, probably due to enhanced biological clogging. Hence, it may be more feasible and beneficial to combine it with conventional FDO, allowing a steady infiltration rate and increased recharge volumes while sustaining high effluent quality. While these results still need to be verified at full scale, they highlight the possibility of using air injection to minimize the DP length and alleviate the pressure on existing SAT sites.