Nutrient and greenhouse gas dynamics through a range of wastewater-loaded carbonate sand treatments

• Tested how different carbonate sand treatments removed wastewater nutrients.
• Nitrogen removal rate was six times higher in saturated treatments.
• Partial denitrification resulted in increased greenhouse gas concentrations.
• Addition of basalt sediments increased phosphorous removal from 72% to 95%.
• A biochar filter increased nitrogen removal from 8% to 42%.
• Modelling confirmed that denitrification occurs even with low DOC.

Most countries in the South Pacific rely on traditional soak pits which transfers wastewater down through the unsaturated carbonate sand sediment profile with little nutrient attenuation. With surface nutrients having an almost direct path to groundwater, this may have consequences for the sustainability of subsurface water reserves. This study examined the long-term removal and production of wastewater nutrients (including greenhouse gases) in tropical sediments and materials under different saturation regimes. The study showed that unsaturated carbonate sands alone are not an effective means of reducing nutrient loads to groundwater. There was no significant difference in removal rates of total dissolved nitrogen (TN) and phosphorus (TP) in unsaturated carbonate sand columns and columns amended with coconut husk and basalt sediment. Doubling of the effluent flow rate resulted in no significant decrease in the TN removal rate, but did halve the TP removal rate. The addition of a biochar filter to the unsaturated carbonate sand columns increased TN removal from ∼8% to ∼42%. The TN removal rate in the wastewater-saturated treatments (662.1 ± 49.4 mg m−3 day−1) was approximately six times higher than the unsaturated treatments (110.2 ± 27.2 mg m−3 day−1). TN removal in the fully saturated columns increased from 63% to 95% after the addition of an external carbon source (glucose). Higher nitrous oxide (N2O) and methane (CH4) concentrations in partially saturated columns and at the surface of the fully saturated columns were likely due to partial coupled nitrifier denitrification. The addition of basalt sediments to carbonate sands increased the total phosphorous removal from 72% to 95%. Sampling of groundwater below an in situ wastewater treatment system showed an almost complete removal of TN and TP within two meters of the aquifer surface, even with limited bioavailable carbon to facilitate denitrification. Modelling of NO3− and N2 concentrations for an idealized treatment system indicated reduced NO3− concentrations may be due to a combination of dilution with high volumes of groundwater (∼45% of removal at the farthest sampling point from the system) and denitrification.

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