Quantifying Nutrient Concentration and Masses in Residential Septage.

Septic tanks are designed to separate raw wastewater into 3 layers: scum, liquid, and sludge; but few studies have quantified the nitrogen and phosphorus concentrations and masses within each of the 3 layers and in the septage overall. Septage may contribute to eutrophication and water quality degradation in some watersheds if nutrient management plans underestimate masses of nutrients in septage, leading to excess nutrient applications on receiving fields. The goal of this study was to quantify nutrient concentrations and masses in each layer and overall in septic tanks. Septic systems were selected (n= 37) based on volunteer participation from homeowners within nutrient sensitive watersheds of the North Carolina Piedmont. Each septage layer was measured using a sludge judge to estimate layer thickness and a sample was collected from each layer. Samples were transported on ice to a certified lab for analysis of total kjeldahl nitrogen (TKN) and total phosphorus (TP). Physical and chemical parameters including pH, dissolved oxygen, specific conductance, and temperature for each layer were measured in the field during sample collection. Results indicated that concentrations of TKN (>1000 mg/L) and TP (>4 mg/L) were elevated in each layer. Mean TKN concentrations were greatest in the sludge layer (1333 mg-N/L), while TP concentrations were greatest in the scum layer (98 mg-P/L). The liquid layer contained the greatest TKN mass (2.77 kg). Mean masses of TP were similar between sludge and scum layers (0.04 kg). The mean, composite mass of TKN and TP for septage in the tanks was 4.7 kg and 0.09 kg, respectively. Septage represents a potentially significant source of nutrient pollution, if not appropriately managed. Septic tank pumping can also represent significant exports of nutrients from a watershed, if the septage is land applied in a different watershed.