Method development and application for measuring representative & potential denitrification rates during wastewater reclamation in soil treatment units

Abstract

Onsite wastewater systems can be designed and managed for robust wastewater treatment, reclamation and potential reuse. A typical onsite wastewater system treats wastewater utilizing a septic tank connected to a soil treatment unit. Although soil treatment units provide significant removal of many contaminants such as suspended solids, pathogens and organic content; mass loading of nitrogen remains a top concern. Previous research focused on quantifying and modeling nitrogen fate in and below soil treatment units has identified the large variability in denitrification rates as a major obstacle. A method for quantifying representative and potential rates of denitrification in soil subject to wastewater application was developed and implemented. Four columns were packed with soil classified as Seffner fine sand obtained from Central Florida. The soil had sand-silt-clay percentages of 95-1-4, respectively (% weight). This fine sand had an effective size (D10) of 0.12 mm, a uniformity coefficient of 2.3, a pH of 5 and a fraction of organic carbon ranging from 0 - 0.05 (% weight). All four soil columns were dosed two times per day at a hydraulic loading rate of 2 cm/d. Half of the columns were dosed with septic tank effluent and the other half were dosed with nitrified intermittent sand filter effluent. Effluent applied to the columns and percolate exiting the columns was characterized to facilitate a nitrogen mass balance. After 68 days of operation, two of the soil columns were deconstructed and subject to numerous denitrification rate measurements using both static core acetylene inhibition (representative rate based on actual conditions) and denitrification enzyme activity (potential rate based on optimal conditions) methodologies. Prior to application of wastewater effluent the native Seffner fine sand was subject to numerous potential denitrification rate measurements; potential denitrification rates were not detectable. After ten weeks of operation, results from the mass balance and the denitrification rate measurements indicated that the Florida soil had a low capacity for nitrogen removal. The average potential denitrification rate for the soil column receiving septic tank effluent was 0.037 mg N L-1 d-1 and the average representative rate was 0.0020 mg N L-1 d-1. The average potential denitrification rate for the soil column receiving nitrified effluent was 0.026 mg N L-1 d-1 and the average representative rate was below reporting levels. After ten-weeks of effluent application the Seffner fine sand demonstrated a statistically insignificant nitrogen removal of 0 - 16%. This removal range was accurately predicted using the nitrogen fate model STUMOD. The limited decrease in nitrogen documented by the mass balance corresponds with the low measured values for representative and potential denitrification rates. The Seffner fine sand showed an increase in potential denitrification rates after dosing with either type of effluent quality. Future research may elucidate how denitrification rates change during longer operational time periods and ultimately, techniques for optimizing denitrification in soil treatment units.