September 2010, Vol. 22, No.9

Small Communities

Conventional septic systems: anachronisms or low-tech decentralized options?

Anthony Smithson

As the “decentralized/distributed” wastewater industry becomes more sophisticated, so has the selection of treatment technologies offered to small communities. These technologies generally have many moving parts, often involve complex computerized control systems, and always rely on management of a sensitive biological population. On the other end of the spectrum, septic tank and drain-field systems are being excluded from the small community solutions — often simply because many designers (and regulators) consider them archaic.

These basic systems sit at a strange crossroads between being maligned by some for failing at tasks that they should never be expected to accomplish and being touted as the standard option for small communities without the facts to back up the reputation. However, these systems do protect the environment and public health when their abilities and limitations are properly understood.


Conventional septic

A conventional septic system — for this discussion, at least — may be defined as a system that uses passive processes, with no energy inputs (other than the flush), to treat and disperse wastewater in a subsurface soil environment. The components include a relatively quiescent basin (septic tank) and a simple physical device of some sort to attempt to divide and divert the flow into a series of pipes for distribution to the soil. These perforated pipes often run the entire length of a series of stone-filled trenches, where the effluent is deposited into the void spaces between the stones and, from there, drains by gravity to be absorbed into the soil. The problem with such systems is that nothing here works particularly well.

The septic tank is an anaerobic digester, but with a theoretical arbitrary retention time of anywhere from 24 to 48 hours, as determined by local code via prescribed tank size. Over the years, manufacturers have improved performance by splitting the tank into compartments or adding influent flow-directing devices or effluent screens and filters. Still, effluent is a variable, surging, and oxygen-demanding soup loaded with enteric organisms.

Balancing the outflow from the tank among five or seven inverts exiting a distribution box is virtually impossible, given the proclivity of liquid to escape any confinement at the slightest of opportunities. Studies of the effluent “distribution” piping dating at least as far back as the mid-1950s demonstrate that this piping is virtually useless. Also, recently void holding material — traditionally stone — has been replaced with furnace slag, shredded tires, plastic domes, shrouded pipe, and foam packing peanuts to better reserve empty space.

Still, the absence of controlled effluent distribution makes the concept of “design” an illusion. Required trench capacity is based upon a soil loading rate in gallons per square foot per day, but the effluent is just as likely to find its own way to only a few discrete spots throughout this subsurface maze, leading to an actual loading rate 20 to 50 times the design rate.

In many ways, this is not pretty. But ugly doesn’t matter; performance does. And despite these shortcomings, such systems, when properly sited, can sufficiently mitigate risks to public health and environment.


Soil conditions

The key to the system is making use of the surrounding soil conditions. The overall strategy for any soil-based wastewater treatment system is to use this marvelously complex matrix of air- and water-filled empty space, organisms, reactive ions, organic debris, covering vegetation, contact surfaces, and physical forces to attenuate wastewater constituents of concern and to return the water to its natural cycle.

Where Mother Nature has provided a generous supply of favorable soil — deep, well drained, with a medium degree of permeability — and in places where we have decided — either by careful planning or random chance — to preserve some space between homes, some amazing things can happen with conventional septic systems.

Given time to “mature,” a zone called the biomat develops to trap organic and inorganic solids and restrict permeability. With permeability restricted, effluent will redistribute itself within the system and move slowly into an aerobic and biologically active soil profile in a more favored pattern of unsaturated flow.

Microorganisms are captured (absorbed into the soil moisture or adsorbed to soil particles), chemically inactivated, and/or exposed to harsh living conditions and predation by higher life forms within soil. Nitrogen is readily oxidized to nitrate, and phosphorus is adsorbed or precipitated effectively on the soil’s surfaces (depending upon clay mineralogy).

Where the system’s capacity to sustain hydraulic performance over time has been correctly predicted by assuming flows and wastewater concentration, and by assigning to the soil a matching long-term acceptance rate, which accounts for the greatly reduced permeability of the biomat, conventional septic systems can protect the public health and natural resources in a sustainable natural system.


Advantages of conventional treatment

Conventional septic systems offer some advantages over more complex systems. With no energy inputs and no “moving parts,” so to speak, the cost of installation is lower, the time between maintenance interventions is longer, operational costs are low, practitioner skills for design and installation and maintenance are less critical, and regulatory oversight is less demanding.

These advantages are attributable solely to the immense capacity of an ideally appropriate — if rarely achieved — site to buffer the inherent inefficiencies and uncertainties in design. In light of its potential, those who focus mostly on complex and innovative decentralized and onsite system strategies almost certainly fail to give the old-fashioned, slow-going, conventional septic system its rightful due.


The danger of blind trust

Unfortunately, the primarily prescriptive regulatory structure most often responsible for regulating decentralized/onsite infrastructure can confound the entire issue. Conventional systems are often specified by prescriptive codes on sites that may not offer the capacity to buffer the treatment and hydraulic performance inefficiencies described above.

Conventional septic systems often enjoy a historical status as the onsite system “flag bearer” in small community settings. That status is founded upon decades of blind trust and the absence of defined performance standards for typical constituents of concern. After all, the normal standard for noncompliance is a complete hydraulic failure.

Since conventional systems are rarely subjected to routine monitoring, these performance limitations are viewed as episodic and not systemic, if they are even recognized at all. Worse, the widespread occurrence of episodic problems damages the reputation of decentralized/onsite strategies as a whole. Maybe most concerning, the performance assumptions granted to conventional septic systems create a profoundly unlevel playing field.

Because acceptable site conditions for conventional septic systems are often too broadly defined, the low cost of installation and the modest demands for maintenance put great pressure upon more efficient, robust, and complex technologies to minimize their cost and operations and maintenance requirements — meaning expenses.

This sets an unrealistic expectation in the marketplace. To compete economically with the conventional systems, “small” activated sludge treatment units, which require monthly monitoring and proactive sludge management, are marketed with a recommendation for only semiannual visits and no mention of sludge management, as discussed in “Managing activated sludge units in small decentralized systems” (May).


Navigating the crossroads

Where conventional systems have inherited an elevated status by default, in many governing codes, alternative approaches are measured against ghostly performance assumptions. Conversely, in the broader wastewater marketplace, alternative approaches that are lumped into the septic system category carry the baggage of a technology seen as suitable only for rural applications.

Consequently, advanced decentralized technologies and strategies are unfairly linked to conventional system costs on one hand and to conventional system reputations on the other. Often, the result is that centralized sewerage and treatment mistakenly is assumed to be a better choice wherever possible, even when it is immensely more costly.

To make the most of conventional and advanced onsite systems requires innovative individuals to rely on realistic assessments of performance requirements. Through creative means, small communities can manage conventional systems either as individual onsite units or as a cluster serving multiple lots. Caught up in this back-and-forth is the conventional septic system, with wonderful capabilities when employed within limited parameters.


Anthony Smithson is the retired director of environmental health for the Lake County (Ill.) Health Department, chair of the National Onsite Wastewater Recycling Association (Alexandria, Va.) Model Performance Code Committee, and past Onsite Wastewater Section chairman for the National Environmental Health Association (Denver).


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