# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 3 Results and Discussion  
### Physical and Chemical Characteristics of Septage  
The liquid layer contained the greatest median thickness (73.66 cm) across all pumped tanks, followed by the sludge (27.43 cm) then scum (2.54 cm) layers (Figure 2). In some cases (\(n=2\)), the scum and/or sludge layers were thicker than the liquid layer, likely from infrequent septic maintenance. The US EPA [46] suggests that homeowners should regularly pump their septic tank, typically every 3-5 years. However, depending on the size of the septic tank and the number of residents served by the system, the pumping frequency may need to be adjusted to maintain design performance. Households with smaller tank volumes and/or those with a greater number of occupants typically require more frequent pump-outs [4, 47]. In the current study, the septic tanks were typically 3785 L (1000 US gallons), and the average person per household in Durham County, North Carolina was 2.36 people [48]. Assuming the US Census estimate was accurate for the study area, then the pumping frequency should be approximately 3.7-5.9 years for year-round residences [47]. The median septic tank contained a total septage thickness of 103.63 cm, with the scum, liquid, and sludge layers comprising approximately 2%, 72%, and 26%, respectively (Figure 2). The layer thickness data suggest that most of the tanks pumped in the current study likely followed routine maintenance guidance. Septic systems should be pumped whenever the solids level is >33% of the liquid depth in any compartment (15A NCAC 18A.1961(a)(2) [3]). The median septic system contained solid layers (e.g., scum and sludge) that were approximately 29% of the liquid layer, suggesting that most septic systems were near the recommended threshold for pumping. At the time of pumping, the percentage of solids in the tanks ranged from 1% to 90%. There were 4 residences that contained solids levels that were \(\leq\)10%. Therefore, approximately 11% (4 out of 37) of the homeowners that participated in the study pumped their tank before they needed to with regard to solids accumulation. The study area has a predominance of expansive clays with very low permeability rates that often lead to hydraulic malfunctions and the surfacing of effluent for conventional-style septic systems [10]. It is possible some system owners may have pumped their tanks to relieve the hydraulic pressure from saturated drainfield trenches. The homeowners were informed that the grant would cover expenses to pump the tank, and thus they may also have been motivated to participate earlier than necessary to pump their tanks. There were 18 residences with solid levels that exceeded 33% of these residences and 11 tanks that contained solid levels that constituted >50% of the liquid volume. These findings suggest that homeowners may benefit from planning tools or guidance to develop a routine maintenance schedule. Figure 2: Boxplots of scum, liquid, and sludge with measured thickness (cm, **A**) and relative percentage (%, **B**) for each septage layer. The inset boxplot (**C**) displays and defines the summary statistics provided by the boxplots. The lower limit denotes the minimum value, the bottom border of the box denotes the first quartile (Q1), the bar denotes the median (or second quartile), the top border of the box denotes the third quartile (Q3), and the upper limit denotes the maximum value. The area between Q1 and Q3 is the interquartile range (IQR). The lower limit is calculated by subtracting the Q1 value from the product of 1.5 and the IQR, whereas the upper limit is calculated by adding the Q3 value to the product of 1.5 and the IQR. Asterisks (*) denote observations that fell outside the lower and upper limits and show the true minimum or maximum value for the range. Physicochemical parameters were similar between the scum, liquid, and sludge layers (Table 1).