Characterization of Septic Tank Effluent from Coastal Residences.

Constituent concentrations in residential onsite wastewater systems become important input factors for resource management tools such as nutrient loading models, TMDL studies, ecosystem sustainability analyses, and also for resource managers and regulatory decision makers charged with setting and/or verifying compliance with established treatment standards.  For example, an estimate of total phosphorus (TP) concentration in septic tank effluent is a critical input value needed in TMDL studies for P-sensitive freshwater lakes.  Likewise, managers of nitrogen-sensitive coastal water resources may have a nitrogen treatment standard that includes a percent total nitrogen (TN) reduction criterion, as well as a TN concentration threshold.  However, verifying percent TN removal in some advanced onsite wastewater treatment technologies utilizing a single processing tank is daunting, because the influent raw wastewater is typically altered by mixing with recirculated flow.  The onsite wastewater industry also relies on constituent concentrations to determine the type of technology that should be designed for a particular application.  For example, the industry recommends that a high strength wastewater technology should be utilized for a waste stream that exceeds residential strength concentrations of BOD, TSS and FOG, placing the onus on the system designer to decide what actually typifies “residential strength”.  In all these examples, if an actual wastewater concentration value is not readily available, then typically a literature value (or an assumed value) is used for critical calculations which may have a profound influence on outcomes and decision making. 

A study of septic tank effluent constituents, from residential households located in Rhode Island coastal communities, was undertaken to help provide additional data for decision makers facing nutrient management and high strength wastewater issues.  Septic tank effluent from four full-time occupied homes was collected over a six-year period (36 observations) and analyzed for BOD₅, TSS, fecal coliform, TN, NH₄⁺, NO₃^-, TP, Cl^-, pH, alkalinity, dissolved oxygen, temperature, and actual hydraulic flow.

Hydraulic flows for the four study systems varied between 56 and 188 gpd per household, representing a per person flow between 28 and 65 gpd.  Household raw data mean BOD₅ concentrations ranged from 197 to 374 mg/L; mean TP concentrations ranged between 8 and 14 mg/L; mean TN concentrations ranged from 64 to 104 mg/L; and, most of the TN was in the NH₄⁺ form (means ranged from  57 to 95 mg/L), whereas, nitrate was below detection levels.  Low hydraulic flow in one study system appeared to yield elevated concentrations for all constituents analyzed.   Our presentation will report on raw data and trimmed data for each of the previously stated parameters, discuss similarities and differences in constituents among the study sites, and address the effects of carriage water volume on effluent concentrations.