# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 1 Introduction
Phosphorus and metal concentrations in the sludge and scum layers of the septic tank are currently underrepresented in the literature. Septic tanks are designed with baffles, sanitary tees, or effluent filters to retain sludge and scum. However, these solids are applied to agricultural fields via land application, along with the liquid portion of the wastewater [11; 12; 13]. Septage is periodically sampled prior to land application to assess the nutrient and metal contents and to gain perspective on the loading rate of phosphorus and metals via land application. If significant differences between phosphorus and metal concentrations are observed when comparing the liquid layer to the scum and sludge layers, then protocols for obtaining a representative composite sample will be important for accurate estimates of nutrient and metal loading to soil via land application of septage. Therefore, quantifying the nutrient and metal concentrations in these layers and the thickness of the layers may help improve the management of septage. Septic tank pumping is one of the most important routine maintenance activities associated with owning a septic system [32; 33]. The frequency that a tank needs to be pumped is based upon several factors, including the size of the septic tank and wastewater generation rates [4]. As the solid layers thicken, the liquid capacity is reduced, and the residence time of wastewater in the tank decreases, potentially reducing the primary treatment efficiency. Septic systems with inadequate clarification in the tank (i.e., primary treatment) may experience hydraulic malfunction as solid waste exits the tank and clogs the drainfield trenches. Pumping the tank removes most of the solid waste and re-establishes volume for the settling of solid waste in the tank, thereby facilitating more effective primary treatment. End-of-life management of pumped septage can also result in a net reduction in nutrients and metals at the watershed scale. For example, if a septic tank is pumped in one watershed, and the septage is land-applied to a different watershed, then there is a net reduction in nutrients and metals in the watershed of origin. Thus, septic tank pumping may be considered a best management practice for reducing nutrient and metal loading in instances where the septage is applied to lands in different watersheds. The goal of this study was to improve understanding of phosphorus and metal characteristics in residential septage. Few studies have focused on quantifying phosphorus and metal concentrations in the scum and sludge layers in residential septic systems [34; 35], and those that did were published decades ago. More specifically, the objectives of this study were to (1) determine the thickness of the scum, liquid, and sludge layers in residential septage, (2) quantify concentrations and masses of TP and metals in the scum, liquid, and sludge layers, (3) estimate the total mass of TP and metals present in residential septage within the studied sub-watershed to Lick Creek, and (4) discuss the broader implications of land application of septage. This project provided funds for various septic system maintenance activities including septic tank pumping, replacing broken system components, and cleaning sanitary tees and effluent filters.