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.
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.
