Archive for October, 2010

Anaerobic Digester Types and Designs

October 28, 2010

Anaerobic Digesters

Factors to consider when designing an anaerobic digestion system include cost, size, local climate, and the availability and type of organic feedstock material.

Anaerobic digesters—also known as biodigesters—are made out of concrete, steel, brick, or plastic.  They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface.  All anaerobic digestion system designs incorporate the same basic components:

  • A pre-mixing area or tank
  • A digester vessel(s)
  • A system for using the biogas
  • A system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters:

  • Batch

Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

  • Continuous

In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems.

Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas. They may be better suited for large-scale operations.

Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the biogas. They use it to run an engine/generator to produce electricity.

Floating cover applications

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

Floating cover advantages:

  • provide a true “floating” cover, keeping the cover on the water surface avoiding damage from wind due to an inflated cover without lateral floats
  • accommodate fluctuation in water level
  • can be installed without interruption in basin use
  • can be installed on tanks or lagoons
  • eliminates rainwater pooling problems
  • eliminates inflating the cover and gas ballooning
  • hatches provide access under the cover for equipment

Part 2 of Biogas and Anaerobic Digestion

October 26, 2010

Biogas and Anaerobic Digestion | Covered Lagoon

A covered lagoon digester is a large anaerobic lagoon (not a manure storage pond or basin) with a long retention time and a high dilution factor.  Typically covered lagoons are used with flush manure management systems that discharge manure at 0.5 to 2 percent solids.  The in-ground, earth or lined lagoon is covered with a flexible or floating gas tight cover.  They are not heated and considered ambient temperature digesters.  Retention time is usually 30-45 days or longer depending on lagoon size.

In climates that have elevated year round temperatures, such as southern and western U.S., these digesters can produce stable, reduced odor, nutrient rich effluent for application on fields and crops; pathogen and weed seed reduction and; produce biogas for farm energy use.  Heat recovery from the biogas can be used to heat nurseries on swine farms and warm milking parlors on dairy farms.  Very large lagoons in hot climates may produce sufficient quantity, quality and consistency of gas to justify use in an engine generator.  In areas with cooler climates, waste digestion, odor control and gas production will be less consistent and the low quality gas may need to be flared off much of the year for odor control and greenhouse gas reduction.

Part 1 of Biogas and Anaerobic Digestion

October 21, 2010

Biogas is formed solely through the activity of bacteria, unlike composting in which fungi and lower creatures are also involved in the degradation process.  Microbial growth and biogas production are very slow at ambient temperatures.  They tend to occur naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen, most commonly in the bottom sediments of lakes and ponds, in swamps, peat bogs, intestines of animals, and in the anaerobic interiors of landfill sites.

The overall process of anaerobic digestion (AD) occurs through the symbiotic action of a complex bacteria consortium as show in diagram.  Hydrolytic microorganisms, including common food spoilage bacteria, break down complex organic wastes.  These subunits are then fermented into short-chain fatty acids, carbon dioxide, and hydrogen gases.

Syntrophic microorganisms then convert the complex mixture of short-chain fatty acids to acetic acid with the release of more carbon dioxide, and hydrogen gases.  Finally, methanogenesis produces biogas from the acetic acid, hydrogen and carbon dioxide.  Biogas is a mixture of methane, carbon dioxide, and numerous trace elements.  According to some, the two key biological issues are determining the most favorable conditions for each process stage and how non-optimal circumstances affect each stage as a whole, and the governing role of hydrogen generation and consumption.

Geosynthetic Applications

October 19, 2010

Primary Functions Of Geosynthetics

Geosynthetics are generally designed for a particular application by considering the primary function that can be provided.  As seen in the accompanying table there are five primary functions given, but some groups suggest even more.

Separation is the placement of a flexible geosynthetic material, like a porous geotextile, between dissimilar materials so that the integrity and functioning of both materials can remain intact or even be improved.  Paved roads, unpaved roads, and railroad bases are common applications.  Also, the use of thick nonwoven geotextiles for cushioning and protection of geomembranes is in this category.  In addition, for most applications of geofoam, separation is the major function.

Reinforcement is the synergistic improvement of a total system’s strength created by the introduction of a geotextile, geogrid or geocell (all of which are good in tension) into a soil (that is good in compression, but poor in tension) or other disjointed and separated material.  Applications of this function are in mechanically stabilized and retained earth walls and steep soil slopes; they can be combined with masonry facings to create vertical retaining walls.  Also involved is the application of basal reinforcement over soft soils and over deep foundations for embankments and heavy surface loadings.  Stiff polymer geogrids and geocells do not have to be held in tension to provide soil reinforcement, unlike geotextiles.  Stiff 2D geogrid and 3D geocells interlock with the aggregate particles and the reinforcement mechanism is one of confinement of the aggregate.  The resulting mechanically stabilized aggregate layer exhibits improved load bearing performance.  Stiff polymer geogrids, with rectangular or triangular apertures, in addition to three-dimensional geocells made from new polymeric alloys are also increasingly specified in unpaved and paved roadways, load platforms and railway ballast, where the improved load bearing characteristics significantly reduce the requirements for high quality, imported aggregate fills, thus reducing the carbon footprint of the construction.

 

Identification of the Usual Primary Function for Each Type of Geosynthetic

 

Filtration is the equilibrium soil-to-geotextile interaction that allows for adequate liquid flow without soil loss, across the plane of the geotextile over a service lifetime compatible with the application under consideration.  Filtration applications are highway underdrain systems, retaining wall drainage, landfill leachate collection systems, as silt fences and curtains, and as flexible forms for bags, tubes and containers.

Drainage is the equilibrium soil-to-geosynthetic system that allows for adequate liquid flow without soil loss, within the plane of the geosynthetic over a service lifetime compatible with the application under consideration.  Geopipe highlights this function, and also geonets, geocomposites and (to a lesser extent) geotextiles.  Drainage applications for these different geosynthetics are retaining walls, sport fields, dams, canals, reservoirs, and capillary breaks. Also to be noted is that sheet, edge and wick drains are geocomposites used for various soil and rock drainage situations.

Containment involves geomembranes, geosynthetic clay liners, or some geocomposites which function as liquid or gas barriers.  Landfill liners and covers make critical use of these geosynthetics.  All hydraulic applications (tunnels, dams, canals, reservoir liners, and floating covers) use these geosynthetics as well.

Benefits and Challenges of Biogas Technology

October 14, 2010

Biogas Technology

Anaerobic digestion can convert organic wastes into profitable byproducts as well as reduce their environmental pollution potential. Anaerobic digestion offers the following benefits to an animal feeding operation and the surrounding communities:

  • Electric and thermal energy.
  • Stable liquid fertilizer and high-quality solids for soil amendment.
  • Odor reduction.
  • Reduced groundwater and surface water contamination potential.
  • Potential revenue from sales of digested manure (liquid and solids) and excess electricity and/or processing off-site organic waste.
  • Reduction of greenhouse gas emissions; methane is captured and used as a fuel.
  • Revenue from possible reuse of digested solids as livestock bedding.
  • Potential revenue from green energy and carbon credits.

The cost of installing an anaerobic digester depends on the type and size of system, type of livestock operation, and site-specific conditions (EPA AgStar, 2006).  In general, consider the following points when estimating installation/operating costs:

  • Estimate the cost of constructing the system.
  • Estimate the labor and cost of operating the system.
  • Estimate the quantity of gas produced.
  • Estimate the value of the gas produced.
  • Compare operation costs to benefits from operation (include value as a waste-treatment system and the fertilizer value of the sludge and supernatant).

The main financial obligations associated with building an anaerobic digester include capital (equipment and construction and associated site work), project development (technical, legal, and planning consultants; financing; utilities connection; and licensing), operation and maintenance, and training costs.

In making a decision to install a digester, one must realize that the system will require continuous monitoring and routine maintenance and repair that should not be underestimated.  Components should be maintained as recommended by the manufacturers because manure and biogas can be corrosive on metal parts.  In fact, the majority of digester failures over the past few decades were the result of management, not technological, problems.

Why Use Floating Covers?

October 12, 2010

Floating Cover Systems

Floating Cover Systems are successfully used in several commercial and municipal applications.  Some examples include:

  • evaporation and algae growth prevention
  • potable water protection from pollution and contamination
  • odor and emission control
  • biogas recovery for power generation or flaring
  • protection of birds and waterfowl from contact with hazardous liquids
  • remediation contamination

Why Use Floating Covers?

The best engineered floating cover systems cost 75% to 85% less than most every acceptable rigid roof structure.  A single floating cover can exceed over a million square feet / 93,000 square meters of surface area and be viable.  Saving of natural resources is another large factor that should be considered.

Floating cover systems prevent water loss due to evaporation; greatly reduce algae growth and treatment chemical demand resulting in improved water quality.  They also provide barriers against contamination by dead animals, airborne particles such as pollen and bird droppings.

In potable water contamination applications, another advantage is treatment chemical cost reduction and positive health impact as considerably less chlorine is required in covered reservoirs.  Using less chlorine in potable water enhances safety by reduced production of trihalomethane (TTHM) (a methane-derived compound that contains three halogen atoms, e.g. chloroform, formed especially during the chlorination of drinking water) type compounds like chloroform that result from the combining of organic substances with chlorine.

Floating Cover Systems have been used for about 35 years.  Service life’s of 20 years or more have been recorded in potable water applications.  Gas collecting floating cover systems can be expected to perform for about ten years.

Research On Geosynthetic Materials

October 5, 2010

Geosynthetic Applications

Geosynthetics are sheet polymeric materials used in civil engineering.  They have been used since the 1970s in geotechnical (soil) structures for functions such as separation, reinforcement, drainage, filtration, liquid containment and as gas barriers.  In practice this has included applications as diverse as reinforcement in the walls of the Pentagon, reservoir liners, canal liners, road reinforcement, retaining walls, sports fields, dams, landfill liners, embankment stabilization, tree containers, chemical tank liners, and as base and roofing membranes for new buildings.  There is an increasing trend to use recyclates in geosynthetics, particularly PET from bottle recovery.

Geosynthetics often play critical roles in civil engineering and it is important that the materials in use can withstand the physical and chemical pressures of the environment. These range from resistance to leachates from landfill to resistance to root damage in soil liners, as well as standard properties such as resistance to creep, oxidation and UV light, and tensile strength.  This has resulted in sets of test standards being developed by the EU, ISO, BSI and ASTM.

There are several main categories of geosynthetics: geotextiles, geomembranes, geosynthetic clay liners, geogrids and geonets.  This review discusses the polymers used in each type, production methods, test methods and applications.

Geotextiles are permeable fabrics comprising around 75% of all geosynthetics.  Globally, 1,400 million square metres are used each year and the trend in consumption is upwards.  Polypropylene comprises the bulk of this with polyester as the second most commonly used material, Polymer properties and economics decide on material choice. Natural fibers are being used where durability is less important.

Geomembranes are thin flexible sheets with very low permeability.  They are used as barriers to the passage of gases of liquids.  Butyl rubber was the first material used, but now PVC and polyethylene are the most common materials.  Uses include landfill odor control, facing of dams and reservoir liners.

Geosynthetic clay liners are structures containing a clay layer and used as water barriers.  Thus the main component is a clay mineral, bentonite.  They can be used instead of geomembranes or as a second line of defense to geomembranes.

Geogrids are sheets of tensile elements with a regular network of apertures, usually constructed of polyethylene, polypropylene or polyester.  The most common use is for reinforcement of unstable soil and waste masses.

Geonets are composite grid constructions used for drainage capabilities.  Usually a geotextile is used as the drainage core with an upper and lower section of geomembrane.