Archive for September, 2010

Advantages and Disadvantages Of Commonly Used Synthetic Geomembranes

September 28, 2010

Advantages & Disadvantages of Common Geomembrane Types

Geomembrane quality begins with base polymer resin selection.  It is important to select or specify high-grade polymer resins that have been manufactured to meet the specific, unique demands encountered by geomembranes.

Polymeric geomembrane properties are a function of the chemical structure of the base polymer resin, the molecular weight, the molecular weight distribution and the polymer morphology (e.g. the crystallinity).  Next, it is necessary to select the right combination of additives to protect the geomembrane, such as premium carbon black as well as antioxidant additives and stabilizer to ensure long life even in exposed conditions.  Finally, it is necessary to select the most appropriate geomembrane manufacturing & installation method.

Geomembrane

Advantages

Disadvantages

HDPE Broad chemical resistance
Good weld strength
Good low temperature properties
Relatively inexpensive
Potential for stress cracking
High degree of thermal expansion
Poor puncture resistance
Poor multiaxial strain properties
LLDPE Better flexibility than HDPE
Better layflat than HDPE
Good multiaxial strain properties
Inferior UV resistance to HDPE
Inferior chemical resistance to HDPE
fPP Can be factory fabricated and folded

so fewer field fabricated seams
Excellent multiaxial properties
Good conformability
Broad seaming temperature window

Limited resistance to hydrocarbons

and chlorinated water

PVC Good workability and layflat

Behavior
Easy to seam
Can be folded so fewer field

fabricated seams

Poor resistance to UV and ozone

unless specially formulated
Poor resistance to weathering

Poor performance at high and low

temperatures

CSPE Outstanding resistance to UV and

Ozone
Good performance at low

Temperatures
Good resistance to chemicals, acids

and oils

Cannot be thermally welded after

ageing

EPDM Good resistance to UV and ozone
High strength characteristics
Good low temperature performance
Excellent layflat behaviour
Low resistance to oils, hydrocarbons

and solvents
Poor seam quality

Butyl rubber Good resistance to UV and

weathering
Good resistance to ozone

Relatively low mechanical properties
Low tear strength
Low resistance to hydrocarbons
Difficult to seam
Nitrile rubber Good resistance to oils and fuels (but

not biodiesel)

Poor ozone resistance unless properly

formulated

Poor tear strength

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Floating Covers For Potable Water

September 21, 2010

Floating Cover Systems For Potable Water

Floating covers prevent losses through evaporation.  Odor and taste free, they help protect potable water supplies by minimizing reservoir contamination from dust and dirt, leaves, insects, animals, etc.  Literally tons of such debris can collect at the bottom of uncovered reservoirs.  This can actually trap chlorine, and make it difficult, if not impossible, to maintain a sufficient amount in the water itself.

Floating covers also exclude sunlight.  This reduces problems with algae growth and chlorine depletion.  Floating cover systems are customized for each client’s lagoon or tank size and shape.

The American Water Works Association and many states recommend covering all reservoirs containing potable water that will be delivered directly to customers.  Some states even require it.

Floating cover applications:

  • any type of gas collection from water basin
  • keep rain & snowmelt water separate from wastewater under the cover

Landfill Liner Systems

September 17, 2010

Environmental Containment Systems

Liner systems are containment elements constructed under the waste to control infiltration of contaminated liquids into the subsoil or groundwater.  The contaminated liquid, or leachate, may be part of the waste itself or may originate from water that has infiltrated into the waste.

Liner systems consist of multiple layers which fulfill specific functions.  The description presented below refers specifically to landfill liner systems.  However, the main characteristics of liner systems are similar for other applications.  Landfill liner systems may consist, from top to bottom, of the following functional layers:

Protective layer
This is a layer of soil, or other appropriate material, that separates the refuse from the rest of the liner to prevent damage from large objects.

Leachate collection layer
This is a high-permeability layer, whose function is to collect leachate from the refuse and to convey it to sumps from where it is removed.  Frequently the functions of the protective layer and the leachate collection layer are integrated in one single layer of coarse granular soil.

Primary liner
This is a low-permeability layer (or layers of two different low-permeability materials in direct contact with each other).  Its function is to control the movement of leachate into the subsoil.

Secondary leachate collection layer or leakage detection layer
This is a high-permeability (or high transmissivity, if geosynthetic) layer designed to detect and collect any leachate seeping through the primary liner.  This layer is used only in conjunction with a secondary liner.

Secondary liner
This is a second (or backup) low-permeability layer (or layers of two different low-permeability materials in direct contact with each other).  Not all liner systems include a secondary liner.

Drainage layer
In cases where the liner system is close or below the water table, a high-permeability (or high transmissivity, if geosynthetic) blanket drainage layer is generally placed under the liner system to control migration of moisture from the foundation to the liner system.

Subbase
This layer is generally of intermediate permeability. Its function is to separate the liner system from the natural subgrade or structural fill.

These layers are normally separated by geotextiles to prevent migration of particles between layers, or to provide cushioning or protection of geomembranes.

As indicated above, liner systems may have a primary liner only or may include primary and secondary liners.  In the first case it is called a single-liner system, and if it has a primary and a secondary liner it is called a double-liner system.  Also, each of the liners (primary or secondary) may consist of one layer only (low-permeability soil, geomembrane, or GCL) or adjacent layers of two of these materials, in which case it is called a composite liner.  There are multiple combinations of these names, some of which are given below as examples (obviously there are many more combinations):

• Single synthetic liner: primary liner only, consisting of a geomembrane.

• Single soil liner: primary liner only, consisting of a low-permeability soil layer.

Performance Factors That Influence Geomembrane Materials

September 15, 2010

Polymeric Geomembranes

The large number of commercially available geomembranes (or polymeric geosynthetic barriers) can make it challenging to select which geomembrane has the most appropriate combination of performance properties for a given application.  Each type of geomembrane material has different characteristics that affect its installation procedures, durability, lifespan and overall performance. It is therefore necessary to match the project performance criteria with the right combination of properties of a particular geomembrane.

Geomembrane materials are generally selected for their overall performance in key areas of chemical resistance, mechanical properties (elastic modulus, yield strength, puncture/ tear resistance), weathering resistance, product life expectancy, installation factors and cost effectiveness.  The properties of polymeric geomembranes are determined mainly by their polymer structure (architecture of the chains), molecular weight (i.e. the length of the chains) and the crystallinity (packing density of the chains).  Polymer crystallinity is one of the important properties of all polymers.  Polymers exist both in crystalline and amorphous forms.

Common geomembranes can be classified into two broad categories depending on whether they are thermoplastics (i.e. can be remelted) or thermoset (i.e. crosslinked or cured and hence cannot be remelted without degradation) (see Table).  Since thermoset geomembranes are crosslinked, they can exhibit excellent long-term durability.

When selecting a geomembrane for a particular application, the following aspects need to be considered:

Main plastic classifications for common geomembrane types

Thermoplastic Geomembranes Thermoset Geomembranes

Combinations of thermoplastic and thermoset

HDPE, LLDPE CSPE (crosslinks over time) PE-EPDM
fPP EPDM rubber PVC-nitrile rubber
PVC Nitrile rubber EPDM/TPE (Trelleborg)
EIA Butyl rubber Polymer-modified bitumen
TPU, PVDF Polychloroprene (Neoprene)
  • choice of polymer
  • type of fabric reinforcement
  • color of upper ply (e.g. white to maintain lower temperatures for sun exposed applications)
  • thickness
  • texture (e.g. smooth or textured for improved friction angles)
  • product life expectancy
  • mechanical properties
  • chemical resistance
  • ease of installation
Source: Guide To Polymeric Membranes

Floating Cover Systems For Odor Control and Gas Collection

September 13, 2010

Odor Control | Gas Collection

Odor Control & Gas Collection Covers are specifically designed for each client utilizing a variety of material options.  Cover applications can be used with any type of gas collection from water basin and keep rain and snowmelt water separate from wastewater under the cover. Advantages of a cover include installation without site interruption, use on tanks or lagoons, elimination of rainwater ponding problems, elimination of gas ballooning, provides high buoyancy and rigidity, hatches can provide access to in-basin equipment, improved quality with pre-manufactured panels and are fabricated at IEC’s plant, so field welding is not required.

Modular Cover System comprised of a series of individual casings connected together to form a complete floating cover system.  Each individual casing consists of a panel of closed cell insulation encapsulated between two sheets of durable geomembrane.  The result is a unique floating cover system that provides insulation values ranging from R-2 to R-30; and is engineered and manufactured to specific dimensions/basin requirements.

The Modular Cover System offers the following advantages over conventional covers systems:

  • maintenance free
  • can be installed on tanks or lagoons
  • adepts to varying water levels
  • individual casings are removable
  • installed without site interruption
  • shorter installation time, no field welding required
  • installation requires less heavy equipment
  • eliminates rainwater ponding problems
  • eliminates gas ballooning
  • high buoyancy and rigidity
  • hatches can provide access to in-basin equipment