Impacts of Biological Additives, Part 2: Septic Tank Effluent Quality and Overall Additive Efficacy

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Introduction

People living in subdivisions, rural areas, and other suburban areas depend upon individual onsite wastewater treatment systems for household wastewater treatment. The number of onsite wastewater treatment system users increases every year due to land development resulting from increasing rural populations, continuous urban sprawl, and cost efficiency

The increasing popularity of onsite wastewater treatment systems has led to widespread production and use of septic system additives. More than 1,200 septic system additives are available on the market (National Small Flows Clearinghouse, 2002).

Unfortunately, very little peer-reviewed, published, and replicated field research exists regarding the efficacy of biological septic tank additives. Additive effectiveness assessments have, up until now, typically relied on laboratory or benchtop studies (Jantrania, Sack, & Erap, 1994), or on literature and process assessments (Scow, 1994), often by product manufacturers, which have not been substantiated in the field via independent, third-party, replicated experiments. Pradhan and co-authors (2008, 2011), however, recently reported results of comprehensive, replicated field experiments using 48 full-scale septic tanks as the experimental units. Those studies included statistical evaluation of additive impacts on total microbial concentrations as well as digestion of sludge and scum. Those studies were conducted across a comprehensive cross section of septic tanks including three prior-maintenance levels (i.e., well-maintained tanks, pumped 2-3 years prior to study; poorly maintained tanks, not pumped within 15-20 years prior to study; and an intermediate prior-maintenance level, with a pumping schedule between the two extremes).

A benchtop study using scaled-down tanks conducted by Jantrania and co-authors (1994) revealed that under stress conditions sludge accumulation rates were significantly different in additive-treated tanks than in control tanks. Microbial conditions within laboratory benchtop anaerobic reactors, however, may not be fully representative of the more diverse biological conditions (e.g., including larger and more complex biological organisms) that occur within full-scale systems serving individual residences. In addition, laboratory-scale benchtop reactors do not represent the variability in total daily flow, flow regime, and solid addition patterns throughout the day or variability in biochemical oxygen demand (BOD), organisms, and food additions that occur from one septic tank to another. These variations often result due to differences from home to home in how families use water and dispose of waste solids as well as in various wastewaters generated.

A recent study by Pradhan and co-authors (2008) found no significant impacts of three additives on total bacterial concentrations in septic tanks. Additionally, no generally positive additive effects (for additives as a generic group) on sludge and scum decomposition were observed by Pradhan and co-authors (2011) across a range of septic tank maintenance levels. Positive impacts (reductions in sludge accumulation rate) for two out of the three additives evaluated, however, were observed under a specific set of conditions. Those reductions in sludge accumulation rates (i.e., sludge depth continued to increase for the treatment, however, at a slower rate than for the control) only occurred within highly maintained septic tanks. While those findings were potentially positive, they also led to a concern about whether reductions in accumulated sludge were truly a positive benefit of these additives. If sludge reduction also was associated with increased five-day BOD ([BOD.sub.5]) or total suspended solids (TSS) concentrations in the septic tank effluent being delivered to the drain field, then sludge reductions due to additives in the tank would be a net negative effect, rather than a positive impact.

BOD and TSS contents are common indicators of wastewater strength. …