October 2008, Vol. 20, No.10

Small Communities

Septic Tanks: More Than Your Average Primary Clarifier

Victor D’Amato

Utilities considering their carbon footprint, sustainability, cost-effectiveness, and how to provide the most appropriate level of service to their customers are embracing and implementing distributed water management, whereby several different treatment systems are employed, as appropriate, across their service area. In the process, a technology that may have been thought of as temporary and archaic — the septic tank — has become relevant and appropriate.

Large, centralized treatment plant operators know that preliminary and primary treatment are of critical importance because of the potential impact of solids on downstream treatment units. This is even more critical for decentralized systems, which often use filtration and soil dispersal to treat effluent to tertiary standards in a passive, energy-efficient manner. The vast majority of decentralized wastewater systems include a septic tank and a grease trap, or both, for primary treatment. Properly sized and designed septic tanks may operate for as many as 10 or more years with only periodic inspection but no solids removal or external energy inputs, epitomizing the sustainability of unit processes associated with well-managed distributed wastewater infrastructure.

While these primary treatment units are of relatively simple design and operation, septic-tank function is quite complex, with primary settling, solids storage, and partial anaerobic digestion co-occurring in a single reactor. Septic tanks are horizontal-flow reactors in which primary settling occurs, and aerobic, anaerobic, and facultative organisms perform complex biochemical processes that may take 2 or more years to mature. Anaerobic processes usually dominate due to the lack of oxygen typically in the liquid mass.

Septic tanks and grease traps primarily remove solids (mostly the settleable fraction) and floatable fats, oils, and grease from influent wastewater. Organic carbon is cycled through physical settling to the solids layer and flotation to the scum layer, with subsequent periodic resolubilization. A portion of the influent organic carbon is biologically transformed in septic tanks, partially as a result of biological activity in the supernatant, or clear, zone. Nitrogen undergoes sedimentation in its organic influent form but is mostly returned to the clear zone as it is transformed to soluble ammonia. Phosphorus removal in septic tanks is primarily a physical process with precipitation reactions contributing to remove soluble orthophosphate. Again, cycling back to the clear zone limits ultimate reductions in total phosphorus. The fate of other influent constituents is primarily controlled by physiochemical processes, including sorption and settling.

Design Considerations
Ideal settling theory has been used to describe the settling behavior of particles in centralized wastewater treatment processes for both primary and secondary clarification. In conventional primary and secondary clarification, solids are removed via wasting or recycling on a regular or semicontinuous basis, resulting in low hydraulic and solids residence times, compared with septic tanks. Primary treatment processes are predominantly physical in nature, and their design generally assumes that biological processes are negligible. Septic tanks, with their long detention times and lack of regular solids removal, are somewhat different, requiring a more nuanced application of ideal settling theory to assessments of their performance and design.

Type 1 and Type 2 settling characteristics — discrete particle and flocculent settling — are the predominant settling phenomena in septic tanks and are functions of surface overflow rate, rather than depth per se. That is, a greater settling efficiency (more particles settled) can be achieved by maximizing the surface area (hence, reducing the liquid depth for a particular tank volume). In reality, because the contents in a properly operating septic tank stratify into relatively distinct zones and because a design objective is to draw effluent from the clear zone, there is a tradeoff between shallow depths (for greater surface area) and deeper depths (to maximize vertical extent of the clear, effluent zone).

While septic-tank performance may deviate from strictly ideal settling conditions due to several factors — including inlet and outlet turbulence, eddy currents, thermal effects, solids resuspension from organic solids gasification, scouring, short-circuiting, fluctuations in influent flow rates, the presence of a significant scum layer, and a lack of continuous solids removal — the proper application of settling theory can inform engineering evaluations of alternate septic-tank designs.

Compartmentation. In most cases where biologically mature units have been studied, two-compartment septic tanks have been shown to have better solids removal efficiencies than comparably sized single-compartment tanks. This is generally attributed to the physical separation of the main settling and digestion compartment from the smaller outlet chamber, where loadings are low, solids digestion is limited, and scour is minimized, thus minimizing these negative impacts on sedimentation that are more likely to occur in the first compartment or in single-compartment tanks.

Sizing. Septic-tank sizing typically is based on allocating volume for solids storage, clarified supernatant, a floating scum layer, and air space for surge storage and venting. Hydraulic residence times of 1 to 3 days are common, and sizing is at least as important with regard to pumping frequency as it is in terms of settling efficiency, with larger tanks taking longer to reach biological maturity but also ultimately having larger capacities for solids digestion and storage.

Hydraulic and settling efficiency. Hydraulic surges can be managed by maximizing surface area and restricting intercompartment transfer and effluent flow rate. Increased surface area also improves the settling efficiency of septic tanks. Settling efficiency, however, may be adversely affected by the resuspension of solids as a result of biogas ebullition from the digesting solids layer. As such, compartmentation (which also may improve hydraulic performance) and the use of effluent screening devices can mitigate the negative impacts of solids digestion. Hydraulic efficiencies also should be improved by the use of tanks with long, relatively narrow aspects. Data that isolate the effects of specific dimensional criteria, as well as inlet, intercompartment transfer, and outlet design, are generally lacking.

Seasonal effects. The well-understood dependence of biological reaction rates on temperature has been clearly observed in laboratory and field observations of operating septic tanks. Numerous researchers have anecdotally described a “spring turnover” or “boil” and often an increase in effluent solids concentrations during warmer months. Settling and solids-removal efficacy increase during the cooler months coincidently with a growth in the amount of accumulated solids. In the warmer months, digestion and solids reduction are maximized, reducing the amount of accumulated solids, but gas ebullition during the increased digestion may hinder settling and solids-removal efficiency.

Solids accumulation and removal. Solids- and scum-accumulation rates have been established by several researchers over the years with results in fairly good agreement with one another. Although several factors affect the rate of solids accumulation, these relationships can be used as a general guide for designers and planners. The consistent theme from these investigations is that the typically recommended pump-out interval of 3 to 5 years is often quite conservative and may affect performance by restricting biological development and associated process efficiencies. While this conservatism acknowledges the historical lack of oversight and maintenance afforded to onsite decentralized wastewater systems, accurate, periodic measurement of accumulated materials is the only way to determine specific tank pump-out needs.

Properly designed, installed, inspected, and maintained septic tanks can be reliable elements of sustainable distributed wastewater infrastructure. They are remarkably efficient primary treatment units, requiring no energy for routine operation and relatively minimal oversight and maintenance while producing a reasonably consistent clarified effluent for subsequent treatment and dispersal. Septic-tank design is based on applying sound engineering principles to what is ultimately a relatively complex treatment process incorporating solids separation, storage, and digestion in a single reactor. As the concepts of sustainability, carbon footprint, and cost-effectiveness become increasingly relevant, ongoing efforts to develop systems that recover nutrients and energy from primary treatment units in decentralized systems will make septic tanks an even more important part of the distributed wastewater management paradigm.

Victor D’Amato is a senior engineer in the Research Triangle Park, N.C., office of Tetra Tech Inc. (Pasadena, Calif.) and a member of the Water Environment Federation (Alexandria, Va.) Small Communities Committee

Victor D’Amato is a senior engineer in the Research Triangle Park, N.C., office of Tetra Tech Inc. (Pasadena, Calif.) and a member of the Water Environment Federation (Alexandria, Va.) Small Communities Committee

Additional Information More information may be found in the Research Digest and at www.werf.org (Water Environment Research Foundation [Alexandria, Va.] Project 04-DEC-7).