Liquid purification or separation – Structural installation – Geographic
Reexamination Certificate
2002-01-26
2003-11-25
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Structural installation
Geographic
C210S196000, C210S220000
Reexamination Certificate
active
06652743
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a system and method for removing pollution from water.
BACKGROUND OF THE INVENTION
It has long been desirable to remove pollutants from water in a safe, efficient and cost-effective manner. Agricultural, industrial, and street runoff, among other polluted water flows, require treatment before being released into the environment. The high concentration of pollutants in these wastes can overwhelm self-purifying mechanisms in the receiving environment. When this occurs, the result is contaminated ground water and/or surface water.
Public wastewater treatment systems typically serve high-density population areas. However, in less densely populated areas or where public sewage treatment is not available, many homes and businesses use a septic system, implemented on-site, for the treatment and disposal of wastewater or sewage.
A typical on-site waste fluid treatment, or septic, system includes a mound or drainfield and a septic tank. Waste fluids such as household sewage may include wastewater from washers and dryers, showers and bathtubs, toilets, disposals, disposal waste, sink wastewater and wastes from various commercial operations. In a typical on-site septic system, the wastewater generally drains into a septic tank before being routed to the mound or drainfield. A septic tank typically separates sewage into solid and liquid fractions, then introduces the separated liquid fraction (effluent) back into the ecosystem with significant levels of undesirable nutrients and other pollution. Undesirable compounds present in the effluent are then decomposed and utilized by soil organisms in the drainfield or mound as the effluent moves (percolates) through the underlying soil profile.
The average life of a conventional on-site wastewater treatment (septic tank and drainfield or mound) system is typically only seven to ten years. A failing septic system can lead to public health concerns and non-point source pollution. Another related concern is the difficulty of quickly and accurately assessing whether the underground septic system is functioning properly. If the conventional system is not functioning properly, untreated, hence polluting, wastewater is likely being released into the ecosystem with little or no surface indication.
A primary concern with any on-site septic system or wastewater treatment system is to ensure that nutrients and other pollutants are removed from the wastewater before the wastewater enters a surface or subterranean body of water. If the treated wastewater is not sufficiently pollutant-free, the effluent will create water quality problems by contaminating surface or subsurface bodies of water.
The ability of wetland plants to remove pollutants from wastewater is known. To this end, natural wetlands have been used as wastewater discharge sites for a long period of time. Thus far, however, constructed wetlands have made only limited use of the potential of wetland vegetation to purify (detoxify) wastewater.
Existing constructed wetlands, including both surface and subsurface flow systems, utilize only wetland plants and atmospheric diffusion to transfer oxygen into (oxygenate) the wastewater being treated (the water column). These naturally aerated (aerobic) zones support populations of oxygen-requiring bacteria. Other areas within the constructed wetland, which are not oxygenated, are anaerobic and support populations of bacteria which do not require oxygen. It is known that aerobic metabolic pathways are much more efficient than anaerobic pathways in decomposing certain types of pollutants. Consequently, aerobic bacteria are capable of consuming, and thus removing, more pollutants than anaerobic bacteria for a given treatment cell size.
In existing constructed wetlands, aerobic zones are typically found only at the top of the water column and in the immediate vicinity of wetland plant roots. The top of the water column is usually a region where there is sufficient gas exchange, via atmospheric diffusion. In the immediate vicinity of wetland plant root hairs, oxygen—translocated by wetland plants into their root systems—diffuses out through the root membranes. These naturally occurring aerobic zones occupy only a small portion of the wetland liquid volume. Thus, the ratio of aerobic activity to anaerobic activity is usually extremely small in natural wetland systems. This lack of aerobic capacity thus limits the overall treatment capacity of the wetland, particularly in subsurface flow constructed wetlands.
FIG. 1
depicts a conventional on-site septic system, and
FIG. 2
depicts a constructed wetland treatment system. In
FIG. 1
, the conventional, on-site septic system is depicted generally at
50
. The conventional system
50
includes a sewer line
52
conveying sewage from a house
54
to a septic tank
56
. In the septic tank
56
, the solids are allowed to settle out of the sewage. The separated liquid wastewater effluent flows from the septic tank
56
to the drainfield (or mound)
58
via a sewer line
59
. In the septic tank
56
, the wastewater is treated to a limited extent when compounds present in the settled solids and effluent undergo predominantly anaerobic decomposition. However, levels of pollutants present in the wastewater being conveyed from the septic tank
56
are usually too high for direct release into the environment (e.g., direct release into a body of water such as a stream, lake, or aquifer). The final disposal and treatment of wastewater occurs in the drainfield
58
, which includes a series of perforated pipes
60
. Thus, the wastewater is conveyed by the sewer line
59
from the septic tank
56
to perforated pipes
60
within the drainfield
58
. The partially treated wastewater seeps from the perforated pipes
60
into the soil profile underlying the drainfield
58
. In the underlying soil profile, the wastewater effluent undergoes a final series of purification steps as it percolates predominately down as discharge
62
through the soil profile.
These purification steps are accomplished by soil organisms—mostly soil microflora. Thus, whether the wastewater effluent will ultimately be purified to an acceptable level before entering a body of water depends on the ability of the soil profile to accommodate the liquid flow and to harbor soil microflora. The texture of the soil profile must permit the wastewater to enter the soil profile from the perforated pipes
60
and percolate generally downwardly, e.g., without pooling the wastewater. Soils with high levels of clay or organic matter generally have low capacities to hydraulically convey wastewater in this respect. Coarse textured soils have higher proportions of sand and silt particles and possess higher degrees of hydraulic conductivity. The discharged wastewater (discharge
62
), whether or not adequately treated, percolates down to aquifers or can also be conveyed somewhat laterally into exposed bodies of water
64
. Thus, one limitation of the conventional system depicted in
FIG. 1
is that the drainfield
58
cannot be located within a specified distance from to a surface body of water or cannot be used at all if the water table (aquifer) underlying the drainfield or mound is sufficiently high. Another disadvantage of the system
50
is that the soil profile underlying the drainfield
58
will slowly lose its hydraulic conductivity. The loss of hydraulic conductivity is due to such factors as unsettled solids conveyed by the wastewater, soil bacteria, substances associated with soil bacteria (e.g., polysaccharides), and solidified wastewater components. These substances slowly fill the spaces (pores) between the soil particles. When these pores become filled, the soil becomes incapable of conducting the wastewater through the soil profile and the wastewater cannot be exposed to the soil bacteria. In time, the conventional disposal system
50
will fail to purify the sewage and will itself become a source of pollution. Present remedies for failed disposal systems of this nature include replacing and/or relocating some
Lambrecht Paul E.
Wallace Scott D.
Barry Chester T.
North American Wetland Engineering, Inc.
Patterson Thuente Skaar & Christensen P.A.
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