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Wastewater Treatment Works For You

The function of the wastewater treatment plant is to speed up the process by which water cleanses itself.

Have you ever wondered what happens to what goes down the drain or is flushed down the toilet?

Perhaps you think it magically disappears, never to be seen again.

- However, that is not the case.

Disposal of wastewater is an important issue in our modern culture. Although ancient Rome had sewers to remove foul-smelling water, it was not until the 19th century that large cities began to understand that they had to reduce the amount of pollutants in the used water that they were discharging to the environment. Populations had become so concentrated by 1850 that outbreaks of life-threatening diseases were traced to bacteria in the polluted water. Since that time, the practice of wastewater collection and treatment has been developed and perfected, using some of the most technically sound biological, physical, chemical, and mechanical techniques available. As a result, public health and water quality are better protected today than ever before.

Billions of dollars are spent each year by businesses, industries, and municipalities in treating wastewater so that it is safe to be disposed of in the environment. For this treatment to be effective it is important to know the content of the wastewater being treated to enable proper treatment to be applied. It is important that each of us takes some responsibility for what we put down the drain and storm sewers so as not to create hazards and injure the treatment system.

What happens in a wastewater treatment plant is essentially the same as what occurs naturally in a lake or stream.
The function of the wastewater treatment plant is to speed up the process by which water cleanses itself.

water treatment photo

water treatment photo

The Denton Wastewater Treatment Plant uses the activated sludge process with nitrogen conversion and phosphorus removal by chemical precipitation.

Included in the process is a head chamber, screening, grit removal, aeration (Biolac) reactor basins, denitrification (post anoxic) reactor basins, secondary clarifiers, chlorination, filtration, post aeration and dechlorination. Sludge handling consists of a sludge holding basin and sand/reed sludge drying beds. 

The Treatment Plant is designed for an average daily flow of 800,000 gallons and for a peak hourly flow of 2.67 million gallons. 

The Head Chamber is the first structure that raw sewage enters into. The purpose of this structure is to divert raw sewage flow to an emergency overflow basin.  

The raw wastewater flow passes through a Mechanically Cleaned Bar Screen via the influent channel. The mechanically cleaned bar screen is the primary device for removal of large debris from the wastewater flow stream. A manual bar screen is also provided for bypass of flow, for overflow or for maintenance down time of the mechanical screen. The screens protect the downstream plant equipment from being damaged by large debris in the wastewater stream such as rags, metal objects, sticks and other garbage. The screenings are collected in a dumpster and hauled off site for disposal.

The Grit Chamber is located downstream of the mechanically cleaned bar screen to remove grit from the wastewater stream. The grit is removed from the influent flow in a chamber containing a rotating paddle that includes a vortex settling grit to the bottom of the chamber. The grit is removed from the chamber by a vortex recessed impeller pump and is pumped to a grit concentrator/clarifier and grit washing screen. Dewatered grit is deposited into the screenings dumpster.

The headworks effluent flows by gravity to the Reactor Basin Influent Box where the flow is split equally to the two Reactor Basins (Biolac).  The Biolac reactors provide biological BOD, phosphorus, and nitrogen removal. Aeration to the reactors is accomplished using three (3) positive displacement type blowers located in the Operations Building. Air enters the basins through a series of pipe headers and is diffused by diffuser tubes attached to the aeration chains.

Mixed liquor from the Reactor Basins flows by gravity to the Post Anoxic Splitter Box where the flow is split equally to the Post Anoxic Reactor (PAR). The mixed liquor undergoes extended mixing in an anoxic condition further reducing nitrogen to meet ENR levels. Located at the outfall of the PAR are re-aeration zones used to release nitrogen gas that may be trapped in the mixed liquor. The PAR has an extensive real-time analyzing system for recording ammonium, nitrite, nitrate and ORP. The information gathered is used for trending and dosing supplemental carbon sources automatically via the PLC. 

Mixed liquor from the Post Anoxic Reactor flows by gravity to a splitter box where the flow is divided equally to the two Secondary Clarifiers and chemicals for precipitation of phosphorus are added. Each clarifier is equipped with a covered peripheral discharge weir. The covers serve to inhibit algae growth that could contaminate the secondary effluent.  Secondary sludge is withdrawn from the bottom of the clarifiers and pumped to the reactor basins (Biolac). The three (3) variable speed return activated sludge (RAS) pumps are in the Sludge Pumping Station which is located between the secondary clarifiers.  Excess sludge is also drawn from the bottom of the secondary clarifiers in the form of waste activated sludge. This sludge is pumped to the Sludge Holding Tank. The two (2) constant speed waste activated sludge (WAS) pumps are located in the Sludge Pumping Station as well.

Chlorine is added to the secondary clarifier effluent at the secondary clarifier effluent weir. The clarifier effluent flows by gravity to the Continuous Backwash Sand Filter and chemicals for phosphorus precipitation are added to reduce phosphorus to ENR levels.  The filter has an extensive real-time analyzing system for recording nitrates, nitrites, dissolved oxygen and phosphorus. The information gathered is used for trending and dosing coagulant automatically via the PLC.

From the filter, flow enters the mechanical post aeration system.  The system consists of two (2) reactors that implement fine bubble diffused air that is feed from positive displacement type blowers.  Air flow is regulated relative to the dissolve oxygen concentration.   Then, the wastewater flow from the filter enters the Dechlorination Tank by gravity. A final v-notch weir meter measures the effluent flow in the tank. Sulfur Dioxide is added to the stream in the Dechlorination Tank prior to the v-notch weir to dechlorinate the water.

A V-Notch Weir near the end of the Dechlorination Tank is used to add dissolved oxygen to the effluent stream.

In the Sludge Holding Tank, the sludge settles and thickens. The sludge is aerated by two blowers and coarse bubble diffusers located at the bottom of the tank. 

Sludge from the Holding Tank is deposited on sixteen Sand/Reed Sludge Drying Beds. By a combination of evaporation, filtrate removal, and water uptake by the reed plants, a very high percentage of water is removed from the sludge. This very dense sludge may be stored on the beds for several years before requiring removal to an off-site location.  

Town of Denton Department of Public Works, 650 Legion Road, Denton MD 21629 Phone: 410-479-5446 Fax: 410-479-5447

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