# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 2 Materials and Methods
### Septage Characterization and Sampling  
Before pumping began, the scum, liquid, and sludge layers were measured using a _Sludge Judge_ to determine the total thickness of each layer and the total depth of the septage in the tank. The _Sludge Judge_ is a wastewater sampling device consisting of a plastic tube with graduated depth markings, and at the base of the tubing is a ball valve. The _Sludge Judge_ was lowered into the septic tank until the bottom was reached, which engaged the valve and allowed septage to enter the sampler. The _Sludge Judge_ was removed from the tank, and the thickness of each layer was measured. Physicochemical parameters (pH, dissolved oxygen, temperature, specific conductance, and oxidation-reduction potential) were determined in the field using a _YSI-556_ multiparameter meter for each septage layer. Samples from the scum, liquid, and sludge layers were transferred to a calibration cup and threaded onto a _YSI-556_ multiparameter meter. When the readings stabilized, the values were recorded onto field sheets. For each tank, the scum layer was analyzed first, followed by the liquid and then the sludge layers, and the calibration cup was flushed with tap water between each layer. Wastewater samples were collected in clean polypropylene bottles and were lime-stabilized to kill any pathogenic microorganisms present. The lime consisted of pulverized dolomitic limestone comprised of a combination of calcium, magnesium, calcium oxide, magnesium oxide, calcium carbonate, and magnesium carbonate and did not contain any of the analytes discussed later in this subsection. The samples were transported on ice to the NC Department of Agriculture and Consumer Services laboratory, where they were analyzed for TP and heavy metals (Cd, Cu, Pb, Ni, and Zn). These heavy metals were the only metals included in the laboratory's suite of analytes at the time of the current study. This laboratory was selected based on its proximity to the monitoring sites and, more importantly, is commonly used by growers and land appliers to analyze septage samples to ensure compliance before land application.  
TP and metals were analyzed using acid digestion according to EPA Method 200.7 using inductively coupled plasma-atomic emission spectrometry [39; 40]. For TP, a 5.0-mL aliquot of liquid waste and a 0.5-g aliquot of dried and ground solid or semi-solid waste was digested in 10 mL of 15.6 N nitric acid. For heavy metals, liquid waste was analyzed in a similar manner to TP. However, for solid or semi-solid waste analysis, a 1.0-g aliquot of a dried and ground sample was digested in 15 or 10 mL of nitric acid, respectively. For TP and heavy metals, liquid waste was digested for 20 min at 180 \({}^{\circ}\)C in a Mars 6 microwave digestion system [41], whereas solid waste was digested for 30 min at 200 \({}^{\circ}\)C in the same microwave system [39; 40; 41]. The digested samples were mixed with deionized water up to 50 mL in volume and then filtered through acid-washed filter paper before measurement. After digestion and filtration, concentrations of TP and heavy metals were quantified  
Figure 1: Map showing the relative locations of septic systems within the studied high-density watershed located within Durham County, North Carolina. Approximately 33% of the subdivision agreed to volunteer, and their tanks were pumped.  
using a _Spectro Analytical_ Spectro Arcos EOP and Arcos II EOP spectrometer. The method detection limits for each pollutant in liquid and solid waste were the following: 0.22 and 1.10 mg L\({}^{-1}\) for phosphorus; 0.020 and 0.100 mg L\({}^{-1}\) for cadmium; 0.06 and 0.31 mg L\({}^{-1}\) for copper; 0.060 and 0.300 mg L\({}^{-1}\) for lead; 0.040 and 0.200 mg L\({}^{-1}\) for nickel; and 0.10 and 0.50 mg L\({}^{-1}\) for zinc.