Quantifying Total Phosphorus and Heavy Metals in Residential Septage

Abstract

Septic systems are used for wastewater treatment in rural areas. Septic tanks promote stratification of wastewater into solid (sludge and scum) and liquid layers. Pollutant concentrations in the layers of residential septic tanks may be highly variable, and thus septage pumped from tanks with different layer thicknesses may also be variable. The goal of this study was to quantify the total phosphorus (TP) and heavy metal concentrations and masses of residential septage. The solid and liquid layer thicknesses were measured in 37 septic tanks. Samples were collected from each layer for pollutant concentration analysis. The median TP concentration (10.6 mg L−1) was greatest in the sludge layer, followed by the scum (5.3 mg L−1) and liquid (1.8 mg L−1) layers. Concentrations of heavy metals were highly variable for each layer type. The masses of the TP, cadmium, copper, lead, nickel, and zinc contained median (range) values of 19.4 g (0.9–1041 g), <0.01 g (<0.01–1.99 g), 1.3 g (0.1–520 g), 1.8 g (<0.01–44.2 g), 1.3 g (<0.01–4.3 g), and 13.8 g (0.3–788 g), respectively. Since septage is typically applied on land as a soil amendment for crop growth, it is important that representative composite samples are collected to prevent excess buildup of TP and metals, which may harm yields or environmental health.

Evaluation of Phosphate and E. coli Attenuation in a Natural Wetland Receiving Drainage from an Urbanized Catchment

“…Four septic systems were repaired, and 15 septic tanks were pumped to help improve the performance of the systems. These efforts likely resulted in a one-time removal of 15 kg of phosphorus from the catchment (septage pumping) [64] and an annual reduction of 4.8 kg in phosphorus associated with the repairs of the malfunctioning septic systems [67]. Projects like this require gaining the trust of the communities and identifying where the practices may provide the most influence.…”

“…Septic tank effluent typically has concentrations of E. coli and phosphorus that exceed 42,000 cfu/100 mL [63] and 5 mg/L [12,26,64], respectively. In properly maintained and functioning septic systems, phosphorus and E. coli in septic tank effluent are removed in the soil beneath drainfield trenches via processes including sorption, mineral precipitation and immobilization for phosphorus, and filtration, predation, sorption, and die off for E. coli [65].…”

“…For example, excessive buildup of solids reduces the hydraulic residence time of wastewater in the tank, thus reducing settling/sedimentation and increasing the likelihood of clogging the effluent filter or pipe. Another benefit of pumping septage from a typical 3780-liter-capacity septic tank is that it may result in the removal of up to 1 kg of phosphorus [64] from the tank and the watershed.…”