(Published May 2004)
Responsible disposal of human wastes is a necessary, albeit unpleasant, part of everyone’s life. Here’s how it works. “Primary” sewage treatment removes solids from the “stuff” that goes down the drain, usually by settling, and directs the remaining liquid on to the next, or “secondary” stage. Septic tanks serve as the “primary” stage of sewage treatment in the on-site systems that most of us use. If the solids are not trapped in the septic tank, they escape the tank outlet and will likely clog the drain field, which constitutes the “secondary” stage of most on-site systems. In addition to trapping solids, the septic tank provides an oxygen-free environment in which microbes can begin to slowly digest the solid organic material. The products of digestion are either released to the wastewater stream as dissolved substances, or as gasses that escape to the atmosphere out the vents in our roofs. The longer the solids remain in the tank, the more reaction products escape to the atmosphere to be widely disseminated, rather than entering the water which discharges to the drain field and then on to the nearest waterway via the groundwater.
These two purposes of a septic tank, trapping solids and encouraging microbial processing of the solids, provide guidance as to how we should maintain our septic systems so that they function efficiently and inexpensively. First, minimizing the amounts of solids that go down the drain puts less stress on the system. Garbage disposals typically produce about 15% of the solids produced by normal household practices, and should probably be banned. Obviously you should wash the dirt off your turnips and your hands out in the yard, not down the drain. Secondly, minimizing the amount of water that goes down the drain makes it less likely that solids will be flushed out into the drain field. Truth be told, nothing except the toilet (assuming there is no garbage disposal) should enter the septic tank. The remainder of household water, called “grey water,” containing only small amounts of soap and other soluble substances, should be directed to a dry well or a second tank between the septic tank and the drain field. Unfortunately, Virginia law currently precludes this practice. Finally, harsh chemicals should never be poured down the drain or else they might kill the hard-working microbes in the septic tank. We should all follow the adage seen in the heads (toilets) on some boats “Except for small amounts of toilet paper, if it hasn’t gone into you, don’t put it into the head.”
The purpose of the “secondary” stage of sewage treatment, the drain field in most of our on-site systems, is to oxidize the water and kill pathogenic organisms. Nitrogen and phosphorus exit a properly functioning drain field as nitrate and phosphate. As long as the pipes stay open and in an oxidizing environment, the drain field is maintenance free. But if the pipes get clogged with solids that escape the septic tank, or they get crushed by vehicles, or clogged with the roots of trees, then trouble can be expected as the primary effluent backs up. Drain field replacement is extremely expensive, so an ounce of prevention is worth a ton of cure. Another problem, typical where soils are muddy or near the coastline, is that after heavy rains the water table rises and floods the drain field. Saturated or waterlogged conditions preclude oxidation and lead to the escape of pathogenic organisms into the environment.
Conventional on-site sewage treatment systems, and too many municipal sewage treatment systems, stop with “secondary” treatment. Solids are removed as they accumulate in the “primary” treatment stage, and are disposed in landfills, by incineration or by land application. The water from the “secondary” stage is discharged into the nearest waterway, or in the case of our septic systems into the groundwater that flows underground to the nearest waterway. Sewage treatment was originally designed to reduce odors and destroy pathogenic organisms, and those two goals are usually achieved by the first two stages of treatment. But a third goal has now become necessary. The effluent from the “secondary” stage contains nitrate and phosphate, microbial breakdown products of the material that entered the “primary” stage. The fertilizers used to produce the organic material that went down the drain (your food) are processed by microbes and released back to the environment by the septic system. In order not to pollute the environment with excess fertilizer, Chesapeake Bay’s biggest problem, we must remove as much nitrate and phosphate from the wastewater stream as possible. In the case of municipal plants this involves adding “tertiary” treatment technology. At last, Maryland and Virginia seem to be slowly moving in this desirable direction. But what about on-site septic systems? How can homeowners reduce the amounts of nitrate and phosphate that exit the septic system? Here are some things we all can do:
Minimize the amount of solids that go down the drain.
Minimize the amount of water that goes down the drain. Give the microbes in the septic tank more time to digest the solids so that gasses like ammonia escape to the atmosphere.
Do not pour harsh chemicals down the drain.
Do not crush your drain field or allow roots to clog the pipes.
Grow trees between (never over) your drain field and the nearest waterway. The deep roots of trees tap the nutrient-laden groundwater and microbial reactions in the root zone convert nitrate to harmless nitrogen gas. This process is called “denitrification” or Biological Nitrogen Reduction (BNR) and is one of the mainstays of municipal tertiary sewage treatment. Nature denitrifies for free.
If possible, establish marsh grasses along your shoreline and “garden” marshes by pruning so that they receive full sunlight. Marsh plants also promote denitrification.
Have your septic system inspected at least every five years and the tank pumped if necessary. It is the (unenforced) law! It is the environmentally and economically responsible thing to do.
All citizens need to be aware that laws may change so that if your current system fails, you may be required to replace it with a more modern, expensive, and maintenance-intensive system. A word to the wise!