Out of Sight
Out of Mind

A Wastewater Primer

  Sewer Systems

Some sewer outfalls, the pipes which carry wastewater into the Sound, discharge millions of gallons of treated water each day. Toxic chemicals that are not removed in the water treatment process enter the Sound.

In some areas of the Sound, elevated levels of toxic materials have contaminated the bottom sediments by binding to the fine mud particles. Animals living in the bottom muds and sand digest these toxic chemicals along with their regular diet. Shellfish in the area also ingest toxic chemicals as they filter the water for their planktonic food. These may seem like sad, but unimportant, facts until we understand that the worms, shellfish, and other bottom dwelling animals are lower links of a foodchain that often ends on our dinner plates.

Proper wastewater treatment, therefore, is very important. For example, before the establishment of METRO in King County in the early 1960s, most raw sewage was pumped directly into Puget Sound and Lake Washington. Much has changed since then, but much remains to be done.

Primary treatment removes most of the floating and settleable solids by passing the wastes through grates, screens, skimmers, and settling tanks. Little more than half of the materials suspended in sewage can be removed through primary treatment. The remaining water, the effluent, is usually treated with chlorine to reduce the risk of disease organisms being released into the environment.

Secondary treatment removes waste materials biologically. Bacteria and other organisms break down dissolved organic materials present in wastes. Special filters or activated sludge tanks provide suitable growing conditions for the organisms. Secondary treatment increases the removal of suspended solids to 85-95% and eliminates almost all disease bacteria. Secondary effluent is also usually treated with chlorine.

Tertiary treatment, while rarely employed, can further remove almost any undesirable component in the wastes including toxic substances, usually through additional biological action.

New treatment strategies including chemical systems and use of artificial wetlands are currently being examined to improve wastewater quality.

A Word About Pathogens...

Under nominal conditions, properly maintained municipal sewage systems release virtually no disease causing organisms as part of their wastewater stream. But in some systems, the pipes that carry wastewater from toilets, showers, sinks, etc. also carry stormwater from street drains.

During heavy rains, these combined sewer outfall (CSO) systems can receive more water than the treatment plant can handle. In these cases, raw sewage mixed with the rainwater runoff by-passes the treatment plant and is released untreated into the environment. These occurrences present both human health risks and a high risk of environmental damage.



The practice increases the flow of water throuigh your treatment system, decreases the treatment efficiency, and contributes to these by-passes of untreated wastewater. If your community wastewater system uses CSO's, support efforts to separate the two types of wastewater or to reduce the amount of overflow through other methods.

Pet droppings, another source of disease organisms, are often washed into the nearest storm drain or stream. Clean up after pets by burying wastes or flushing wastes down the toilet.

Septic Systems