|
|
FYI
|
|
Wastewater is mostly water (99.9 percent). Various kinds of solids make up only 0.1
percent of sewage. Solids include organic and inorganic materials from feces, urine, washing, laundering, and, in some cases, from food processing, agricultural,
and industrial operations. It is this tiny amount of solids that can contaminate receiving waters.
|
|
(Source: Texas Engineering and Extension Service, Texas A&M
University System, Basic Wastewater Operation [College Station, 1994], 51.)
|
|
Wastewater can undergo several levels of treatment, each of which makes the
water progressively cleaner. The first level, called primary treatment, removes solids, usually by mechanical means. During secondary treatment, water is
aerated and microorganisms are used to remove remaining small particles. Tertiary treatment removes nitrates and phosphates, and the water is then
chlorinated. Some wastewater treatment plants use alternatives to chlorination or even dechlorinate the water after chlorination to prevent problems associated with chlorine by-products.
Due to the high construction and operating costs of conventional systems, as
shown in the diagram, many smaller communities have begun looking into so-called innovative and alternative wastewater treatment plants.* Alternative
collection systems often use smaller-diameter pipes that follow the topography of the land to reduce excavation costs and take advantage of gravity flows.
Innovative treatment systems may also utilize natural processes to reduce effluent contaminants to acceptable levels. These processes include using the
wastewater to irrigate land, which uses the soil's biological activity to treat the wastewater. Or, the system may rely on specially constructed wetlands, which
physically filter the water and break down contaminants through biological and microbial activity. Communities such as
Beaumont and Johnson City have utilized constructed wetlands for secondary and tertiary treatment.
|
A TYPICAL WASTEWATER TREATMENT PLANT
|
|
|
|
Primary Treatment:
Separates large suspended solids from water through sedimentation. Solids thickened into sludge. Sludge sent to sludge treatment and disposal.
Secondary Treatment (Aeration Basin): Uses aeration and microbial bacteria to rid remaining solid particles.
Secondary Clarifier:
Remaining solids are settled out.
Final Step: Chlorination. Some plants also require dechlorination before final release into stream.
|
|
Source: Texas Extension Educational Service, Texas A & M University System, Basic Wastewater Operations
(College Station: Texas Engineering Extension Service, 1994), 3-5.
|
|
|
AN EXAMPLE OF AN ALTERNATIVE SECONDARY TREATMENT WASTEWATER PLANT:
CONSTRUCTED WETLANDS
|
|
|
|
Source: Tennessee Valley Authority, General Design, Construction and Operation Guidelines:
Constructed Wetlands Wastewater Treatment Systems for Small Users Including Individual Residences.
|
|
(1) Wastewater enters the constructed wetland where it is distributed evenly across
the width of the first cell by a series of plastic valves or PVC tees.
(2) The first cell contains gravel. A waterproof liner is used on the sides and bottom
of the first cell to conserve water and provide more effective treatment.
(3) Cattails and bulrushes are usually planted in the first cell. The roots of these
marsh plants form a dense mat among the gravel. Here chemical, biological, and physical processes take place which purify the water.
(4) Water from the first cell passes into the second cell through a perforated pipe embedded in large stone.
(5) The water level within each cell is regulated by swivel standpipes in concrete tanks at the end of each cell.
(6) Wastewater is distributed evenly across the second cell through another perforated pipe.
(7) Cell 2 has a layer of gravel covered with topsoil and then mulch.
(8) This cell is planted with a variety of ornamental wetland plants such as iris, elephant ear, and arrowhead.
(9) The water in Cell 2 eventually seeps into the soil below or passes into another perforated pipe where
(10) it is released into a drainfield similar to those used with conventional septic tanks.
|
|
|
