Wastewater Treatment
December 12, 2008
QUESTION: What is wastewater?
Wastewater is liquid waste. It is animal, vegetable, mineral or chemical matter in solution or suspension that residents and businesses flush down their toilets and pour down their sinks and drains. Raw sewage contains a variety of substances in addition to human wastes – dirt particles, food fragments, oil and grease, soaps, detergents, bleach and other cleaning agents, solvents, paint, pharmaceuticals, and cosmetics. Even human waste contains surprising amounts of trace metals such as copper, zinc, iron, manganese, etc. because they are essential elements in human nutrition.
In areas served by sewage treatment plants (STP’s), wastewater drains into a network of pipes or sanitary sewers that feed the waste by gravity or a series of pumping stations to the STP. Treatment of our wastewater is an essential process that prevents pollution, contamination, and destruction of our waterways and our natural water resources like the Shenandoah River. To make wastewater acceptable for reuse or for returning to the environment, the concentration of contaminants in it must be reduced to a nonharmful level, usually a standard based on scientific study prescribed by the U.S. Environmental Protection Agency.
QUESTION: How does a wastewater or sewage treatment plant work?
There are 3 levels of treatment: primary, secondary and tertiary or advanced.
Primary treatment is the first process used as the wastewater enters the STP. Mechanical screens remove large objects such as cans, rags, sticks, plastic, etc. The screened wastewater flows into a primary settling tank or clarifier where solid particles settle out and fats, oils, and grease are skimmed from the surface.
The remaining wastewater is then sent on for secondary treatment where it is mixed with a controlled population of bacteria either in an oxygen rich tank or on a series of trickling filters. The bacteria digest the fine suspended and soluble organic materials, thereby removing them from the wastewater. The effluent is then moved to secondary clarifiers where the bacteria and biological sludge are settled by gravity. As with the primary clarifier, the sludge is moved to a digester where more bacteria convert it to water and methane gas which can be used for heating. A wet soil like material called “biosolids” is produced. Water is then squeezed from the biosolids which then can be used as fertilizer or be trucked to the landfill. Historically, the effluent water was then returned to the environment, usually a lake or stream.
This is generally no longer the case as the effluent still retains more nitrogen, phosphorus and organic material than the receiving water can absorb without exceeding EPA standards. In our area standards for nitrogen, phosphorus and organic materials have become more restrictive as government agencies try to reduce nutrients in order to clean up the Chesapeake Bay. Therefore, STP’s in our region like those in Woodstock and Strasburg are adding more advanced or tertiary treatment to the process. This can include a variety of physical, chemical or biological treatment process targeted at the specific pollutants still remaining. Finally, just before releasing the clean effluent back into the environment, it is sterilized using ultraviolet light, clorination/declorination, or ozone.
With present and future population growth in the Shenandoah Valley, the need for increased wastewater treatment capacity and technology will only continue to grow, but well managed STP’s have become a solution, not a cause, of water quality issues in the environment.
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