Sewage I – Introduction

We all want to "flush and forget". We trust that our wastes will just disappear. There are three stewardship issues, however, which require us to understand some aspects of the waste disposal process and to make some decisions. Our own waste, after all, is our own responsibility.

1 – The liquid effluent from many municipal sewage treatment plants discharges into the Chesapeake Bay or into one of its tributaries. The discharge pipes are “point sources” of immense quantities of nutrients (nitrate and phosphate). Scientists agree that "over-fertilization" of the Bay is the primary cause of degradation of the marine ecosystem. As we seek to de-list Chesapeake Bay from EPA's "impaired waters list" by 2010, or face mandated Federal controls, nutrients added by the effluent from sewage treatment plants (including our own) must be reduced.

2 - Sewage treatment plants also produce solids, called "sludge". We must decide whether or not to work toward legislative changes that would permit local elected officials to regulate sludge imported from urban and industrialized areas, which currently can be applied to agricultural lands without our consent.

3 - Most of us not connected to a sewage treatment plant use a septic tank and absorption field (or equivalent) to handle our waste. For those people who have always lived in urban areas, moving to the Northern Neck may be their first experience with a septic system. The effluent from our septic systems contributes to the high nitrate concentration in shallow groundwater in the County, which averages 5 milligrams of nitrate per liter of water (5 ppm). “Normal” nitrate concentrations in groundwater are less than 1 ppm. Large amounts of nitrate and phosphate from commercial agricultural practices and from many people using too much fertilizer also contribute nutrients to our shallow groundwater, which ultimately discharges into local waterways. Our waterways are commonly turbid and/or green because so many algae grow in the water, fertilized by the nutrients we supply year-round. We are over-fertilizing our local waterways just as surely as the treated effluent from sewage treatment plants in urban areas is over-fertilizing the entire Bay.

It is critical to understand that our wastes are not destroyed when they are "treated", they are just converted to a different form. Organic material is extremely complex chemically, but is mostly composed of the elements carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S). Plants convert inorganic substances like water (H₂O), carbon dioxide gas (CO₂), dissolved phosphate ion (PO₄^-3), dissolved nitrate ion (NO₃^-) and dissolved sulfate ion (SO₄^-2) into complex organic molecules. A simple way to write this reaction (leaving the N, P and S out of the equation) is: CO₂ + H₂O à CH₂O + O₂, where the formula CH₂O is “shorthand” for organic material. The sun provides the energy to drive this reaction, which is called photosynthesis. Animals eat the organic material (and sometimes each other) and then convert some of the complex organic molecules back into inorganic substances: CH₂O + O₂ à CO₂ + H₂O. This reaction, the opposite of photosynthesis, is called respiration. The energy to drive respiration comes from the organic material itself (it is “burned”).

Sewage treatment is a way to contain and manage the decomposition of organic material by respiration and by other microbial reactions. Sewage treatment is a natural biologic (microbial) process, and it takes place slowly. The ultimate products are the same inorganic substances from which the organic material was formed in the first place, including dissolved nitrate and phosphate ions. It is these two “nutrients” or fertilizers, which enter our waterways from pipes or via the shallow groundwater, that cause the Bay’s biggest problem - nutrification. Sewage systems were originally designed to reduce odors and to keep pathogenic organisms out of contact with people. They were never designed to reduce nitrate and phosphate released to the environment. Only recently have we become concerned about the release of nitrate and phosphate, and we are still in the process of trying to figure out how to economically reduce the quantities of these nutrients released to the environment by our sewage treatment systems.

Sewage treatment typically takes place in two, and ideally three, stages. The primary stage will be discussed in the next article, secondary and tertiary stages will be covered in a third article, and sludge will be discussed in a fourth and final article in this series.