# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 3 Results and Discussion
### Total Phosphorus and Metal in Septage
#### 3.2.2 Metal Concentrations and Masses
4 times greater than that of the sludge layer (\(p\) = 0.04) and 13 times greater than the scum layer (\(p\) < 0.01), both of which were statistically significant differences. Furthermore, the median mass of zinc in the sludge layer was approximately 3 times greater than the scum layer (\(p\) = 0.04). Similar trends were observed in median masses of lead, but differences between the septage layers were not significantly different (\(p\) = 0.07). Copper masses tended to be elevated in the sludge layer relative to the liquid and scum layers (Table 2), but this difference was not statistically significant (\(p\) = 0.18). The median mass of nickel was identical between the liquid and sludge layers, both of which were elevated relative to the scum layer, but this difference was not statistically significant (\(p\) = 0.06). Cadmium masses were negligible in all septage layers and exhibited no statistical differences based on the layer (\(p\) = 0.76). In the current study, the liquid layer contained median concentrations and masses of all metals (except cadmium) that were greater than the values reported by past studies that quantified trace metals in liquid wastewater (Tables 2 and 3). The main mechanism in reducing metal concentrations in primary effluent is through sedimentation, which is controlled by factors that affect metal solubility and settling rates [57; 58]. The elevated median concentration of zinc in the liquid layer and similarity of lead across all layers may be indicative of inadequate sedimentation in septic tanks. Copper, lead, and zinc can form fine particulates (0.2-35 um), which may be too fine (typically <35 um) to be removed during primary sedimentation [59]. Past research has found that heavy metal concentrations and masses tend to be elevated in sludge and septage relative to liquid wastewater regardless of the wastewater treatment approach (Table 3). Both septic and municipal wastewater treatment approaches contained a high degree of variability in pollutant concentrations in sludge and septage. Municipal wastewater treatment facilities contain pollutant concentrations that were typically greater than septic systems, which likely occurred due to differences in scale and wastewater characteristics (Table 3). Municipal wastewater treatment facilities generally receive higher volumes of wastewater given the larger communities that these facilities serve relative to residential septic systems. Furthermore, some municipal treatment plants receive industrial wastewater, which can substantially increase the concentrations of heavy metals in sludge or septage [60]. The current study did not find apparent differences in the median metal concentrations between scum, liquid, and sludge layers. This was not expected since past research has found that solid layers typically contain greater concentrations and masses of pollutants relative to the liquid layers (Table 3). However, the mean concentration for each pollutant in the sludge and scum layer was elevated compared with the mean concentration reported in the liquid layer (Table 2). This trend likely occurred because of the large variability in metal concentrations between septic tanks, which is discussed in greater detail in the next section.