# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 4 Conclusions  
The goal of this study was to measure the layer thicknesses of the scum, liquid, and scum layers and quantify concentrations and masses of TP and heavy metals in septage collected from residential septic tanks. The median percent thickness of the liquid layer was 72%, suggesting that most septic tanks received adequate maintenance. The sludge and scum layers contained a median percent thickness of 25% and 3%, respectively. Median concentrations and masses of TP were greatest in the sludge layer, containing median values of 10.6 mg L\({}^{-1}\) and 35.6 g, respectively. Except for zinc, the median concentrations of heavy metals were similar across all layers. The layer thickness, pollutant concentration, and pollutant masses all exhibited large variability, suggesting that household behaviors, wastewater strength, the volume of wastewater generation, and frequency of maintenance may have differed substantially between tanks. These factors in turn have the potential to affect TP and heavy metal concentrations observed in septage. Masses and watershed estimates indicate that the septic tanks in the current study may have been a substantial source of TP, zinc, and copper.  
The broader implications of this work highlight the importance of residential septage management. Land appliers base application rates on a combination of soil fertility, crop needs, and the concentrations of pollutants in the septage. Therefore, it is imperative that compliance samples are collected after homogenizing septage to ensure that a solid-liquid mixture is sampled and analyzed. Otherwise, TP and metal concentrations may be vastly underestimated. Underestimating these concentrations may endanger environmental and public health through the accumulation of TP and heavy metals in the topsoil of arable lands. Future work should consider focusing on quantifying other traditional water pollutants (e.g., nitrogen and pathogens), other heavy metals with established ceiling concentrations by the US EPA (e.g., arsenic, mercury, molybdenum, and selenium), organic pollutants (e.g., pesticides, plasticizers, detergents, etc.), pharmaceutical and personal care products (e.g., estrogen compounds, caffeine, lofepramine, etc.), and other emerging contaminants (e.g., per- and poly-fluoralkyl substances) in residential septage. Additionally, partnering water quality data with septic system age data and household wastewater behaviors and practices would provide further insights into how these factors impact solid accumulation rates and pollutant concentrations in residential septage.  
Conceptualization, C.P.H.J., M.O. and G.I.; methodology, G.I., J.J., B.S., C.S. and C.P.H.J.; software, G.I.; validation, G.I., C.P.H.J., M.O., J.J., B.S. and C.S.; formal analysis, G.I. and C.P.H.J.; investigation, G.I., J.J., B.S., C.S. and C.P.H.J.; resources, C.P.H.J. and M.O.; data curation, G.I.; writing--original draft preparation, G.I. and C.P.H.J.; writing--reviewing and editing, G.I., C.P.H.J., M.O., J.J., B.S. and C.S.; visualization, G.I.; supervision, C.P.H.J., M.O. and G.I.; project administration, C.P.H.J. and M.O.; funding acquisition, C.P.H.J. and M.O. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the North Carolina Department of Environmental Quality 319 Non-Point Source Program, contract 6201.  
Not applicable.  
The authors acknowledge and thank the North Carolina Department of Environmental Quality for their support and sponsorship of this research. Furthermore, we wish to extend our gratitude to the following individuals for their assistance in project development, field support, laboratory analysis, and any other contribution or support given: [PERSON], [PERSON], and [PERSON].  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.