# Quantifying Total Phosphorus and Heavy Metals in Residential Septage
## 1 Introduction  
Many rural and suburban areas across the United States rely on septic systems as their primary method of wastewater treatment. For example, approximately 25% of US residents and half of the population of North Carolina utilize septic systems [1]. Conventional-style septic systems collect raw wastewater and discharge septic tank effluent to the subsurface. Pollutants in the effluent are treated in soil via various processes (e.g., adsorption, filtration, precipitation, and/or dilution), and the treated effluent eventually mixes with groundwater and may drain to adjacent surface water [2, 3]. Primary treatment occurs in the septic tank,where raw wastewater stratifies into 3 layers--scum, liquid, and sludge--and collectively, these layers are called septage [3]. The scum and sludge layers gradually thicken overtime and, without routine maintenance (e.g., pumping the tank every 3-5 years), can potentially cause sewage backup or premature failure of the drainfield via clogging with solids [4]. In the United States, septic system malfunction rates are estimated to be <7-13% [5], whereas other work has estimated this range as typically being between 10 and 20% [3], and some communities have reported up to 70% failure rates [6]. Poorly operated or maintained septic systems increase the risk of hydraulic malfunctions, which may result in ponding of effluent at the surface. Common reasons for hydraulic malfunction include effluent loading rates that exceed the infiltration capacity of soils, presence and thickness of the biomat layer under drainfield trenches and the infiltration rate through the biomat, septic tanks that are damaged or overloaded with soils, and clogged drainfield trenches [5; 7]. Hydraulic malfunctions resulting in surfaced effluent can greatly reduce the treatment of nutrients, since these failures bypass subsoils beneath the drainfield where most of the treatment occurs [2; 7; 8; 9; 10]. Septage pumped from tanks is often lime stabilized and later applied on agricultural, forested, pastoral, or disturbed lands for use as a soil amendment [11; 12; 13]. Land application of septage replenishes soil nutrients (e.g., nitrogen and phosphorus), organic matter, and micronutrients (e.g., nickel, zinc, and copper) and improves soil texture and water holding capacity [12]. To maximize the soil fertility benefits from land application, biosolids should be applied to agricultural or pastoral lands before or during the growing season [14]. In addition to organic compounds and nutrients, septage can contain trace elements including heavy metals [15; 16; 17; 18; 19; 20], microorganisms, and micropollutants [15], which may pose an exposure risk for humans if attenuation of the pollutants does not occur in the soil beneath the application fields, or sediment-bound contaminants may be transported to local surface waters via overland flow. Repeated application of biosolids on agricultural lands can eventually result in the buildup of phosphorus and metals in the soil and increase the potential for leaching of these elements to groundwater [18; 21; 22; 23]. Toxic concentrations of heavy metals in soil may also lead to elevated concentrations in crops, which can be harmful for humans upon consumption [15]. Therefore, it is important that the mass of phosphorus and metals in septage is accurately measured, especially for lands that have received biosolids for many years. Wastewater contains elevated concentrations of total phosphorus (TP) [3; 24; 25; 26; 27; 28; 29], where 76-99% of TP occurs as orthophosphate (PO\({}_{4}\)-P) [24; 28; 29; 30; 31]. Previous studies [3; 24; 25; 26; 27] have shown that TP in raw wastewater and septic tank effluent can range between 2.90 and 39.5 mg L\({}^{-1}\), though the concentrations in septic tank effluent tend to be lower than those of raw wastewater due to the primary treatment [29]. These studies characterized phosphorus concentrations in the liquid layer of septic tanks.