Archive for May, 2010

Anaerobic Digesters For Lagoons Part 2

May 27, 2010

Anaerobic Digester Lagoon With Methane Gas Recovery

Anaerobic lagoons are perhaps the most trouble free, low maintenance systems available for treatment of animal waste. This is particularly true in the southern U.S.where winter temperatures are mild, permitting anaerobic digestion the year around.  The effluent from the digester is a valuable source of nitrogen for plants that can be field applied for improved crop production.  Placing a cover over the lagoon for collecting biogas virtually eliminates odor from the lagoon.  The collected biogas, a byproduct of the digestion process, is typically 60 to 70 percent methane that can be utilized as a valuable energy resource.  Limited experience indicates that odor from field application of effluent from two cell covered lagoons is much reduced from what might be expected when applying untreated or uncovered lagoon effluent.  A properly designed, constructed and operated anaerobic digester is a low maintenance system that is very forgiving and not likely to create emergency situations that can be expected with many alternative waste management systems.  Adding methane recovery to the anaerobic digester increases maintenance, but even in the event of failure of the gas collection system, it will not interrupt the waste stream and digestion process.  It is well suited to the livestock industry.

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Anaerobic Digesters For Lagoons Part 1

May 25, 2010

What Is An Anaerobic Lagoon?

An anaerobic lagoon is an earthen impoundment receiving manure from an animal feeding operation in which manure is stored and stabilized by bacterial activity operating without oxygen (compare with an aerobic structure). The statute specifically provides that an anaerobic lagoon does not include a confinement feeding operation structure such as an earthen manure storage basin; a basin connected to unroofed operations (feedlots) which collects and stores runoff produced by rain or a system which collects and treats off gases.

Covered lagoon digesters are the simplest AD system.  These systems typically consist of an anaerobic combined storage and treatment lagoon, an anaerobic lagoon cover, an evaporative pond for the digester effluent, and a gas treatment and/or energy conversion system.  Figure 1 shows a typical schematic for a floating covered anaerobic lagoon.

Covered lagoon digesters typically have a hydraulic retention time (HRT) of 40 to 60 days. The HRT is the amount of time a given volume of waste remains in the treatment lagoon.  A collection pipe leading from the digester carries the biogas to either a gas treatment system such as a combustion flare, or to an engine/generator or boiler that uses the biogas to produce electricity and heat.  Following treatment, the digester effluent is often transferred to an evaporative pond or to a storage lagoon prior to land application.

Climate affects the feasibility of using covered lagoon digesters to generate electricity.  Engine/generator systems typically do not produce sufficient waste heat to maintain temperatures high enough in covered lagoon digesters in the winter to sustain consistently high biogas production rates.  Using propane or natural gas to provide additional heat for the lagoon contents is typically not an economically viable option.  Without that additional heat, most covered lagoon digesters produce less biogas in colder temperatures, and little or no gas below 39 FACE= “Symbol”>° F.  As a result, covered lagoon digesters are most appropriate for use in warm climates if the biogas is to be used for energy or heating purposes.

Complete mix digester systems consist of a mix tank, a complete mix digester and a secondary storage or evaporative pond.  The mix tank is either an aboveground tank or concrete in-ground tank that is fed regularly from underfloor waste storage below the animal feedlot.  Waste is stirred in the mix tank to prevent solids from settling in the waste prior to being fed to the digester.  The complete mix digester is essentially a constant-volume aboveground tank or in-ground covered lagoon that is fed daily from the mix tank.  Complete mix digesters with in-ground lagoons often employ covers similar to those used in covered lagoon digesters. In the digester, a mix pump circulates waste material slowly around the heater to maintain a uniform temperature.  Hot water from an engine/generator cogeneration water jacket or boiler is used to heat the digester.  A cylindrical aboveground tank, such as that shown in Figure 2, optimizes biogas production, but is more capital intensive than in-ground tanks.
Source: EPA. Manual for Developing Biogas Systems at Commercial Farms in the United States

Types of Geosynthetic Materials

May 20, 2010

Geosynthetic Materials

Geotextiles – Textiles in the traditional sense, they consist of synthetic fibers so that biodegradation is not a problem.  They make up one of the two largest groups of geosynthetics.  These synthetic fibers are made into a flexible fabric by standard weaving machinery or are matted together in a random, or nonwoven, manner.  The fabric is porous to water flow across its manufactured plane and within its plane.  There are at least 80 specific applications for geotextiles, but the fabric always performs at least one of five discrete functions: separation, reinforcement, filtration, drainage, or barrier to moisture.

Geomembranes – These are the other largest group of geosynthetics.  In sheer sales volume, they are probably larger than geotextiles because their growth has been stimulated by government regulations enacted in 1982.  The materials themselves are impervious thin sheets of rubber or plastic material used primarily for linings and covers of liquid- or solid-storage facilities.  Thus, the primary function is always as a liquid or vapor barrier.  The range of applications, however, is very great, and at least 30 applications in civil engineering have been developed.

Geogrids – Plastics formed into very open, gridlike configurations, geogrids have at least 25 applications, but they function almost exclusively as reinforcement materials. They represent a rapidly growing segment within the geosynthetics family, says Drexel University Professor Grace Hsuan.

Geonets – Also called “geospacers,” these products are usually formed by a continuous extrusion of parallel sets of polymeric ribs at acute angles to one another.  When the ribs are opened, relatively large apertures are formed into a netlike configuration.  Their design function is completely within the drainage area, where they have been used to convey fluids of all types, explains Hsuan.

Geosynthetic Clay Liners – Rolls of thinly layered bentonite clay sandwiched between two geotextiles or bonded to a geomembrane, these products are seeing use as a composite component beneath a geomembrane or by themselves as primary or secondary liners.

Geopipe – Perhaps the original geosynthetic material still available today is buried plastic pipe.  Plastic pipe is being used in all aspects of geotechnical, transportation, and environmental engineering with little design and testing awareness, probably because of a general lack of formalized training.  The critical nature of leachate collection pipes coupled with high compressive loads makes geopipe a bona fide member of the geosynthetics family.  Its function is clearly drainage.

Geocomposites – Combinations of geotextile and geogrid; geogrid and geomembrane; geotextile, geogrid, and geomembrane; or any one of these three materials with another material (e.g., deformed plastic sheets, steel cables, or steel anchors) are geocomposites.  This exciting area brings out the best creative efforts of the engineer, manufacturer, and contractor.  The application areas are numerous and growing steadily and they encompass the entire range of functions for geosynthetics: separation, reinforcement, filtration, drainage, and liquid barrier.

“Geo-Others” – Innovations in geosynthetics have created products that defy categorization.  These “geo-others” include such products as threaded soil masses, polymeric anchors, and encapsulated soil cells.  The geo-other name is not one specific area, although similar to geocomposites, its primary function is product-dependent and can be any of the five major functions of geosynthetics.  The category is ‘temporary housing,’ if you will, for any new products.  When it is determined the appropriate family, it is moved to “permanent housing.'”

Biogas Production

May 18, 2010

Biogas Production

Biogas production using anaerobic (oxygen free) digestion is a biological treatment process to reduce odor, produce energy and improve the storage and handling characteristics of manure.  A biogas production system must be specially designed and requires regular attention by someone familiar with the needs and operation of the digester.  Associated manure handling equipment and gas utilization components are also required.  The digester does not remove significant nutrients and requires environmentally responsible manure storage and handling system.

A well designed and operated digester will require modest daily attention and maintenance.  The care and feeding of a digester is not unlike feeding a cow or a pig; it responds best to consistent feeding and the appropriate environmental (temperature and anaerobic- oxygen free) conditions.  The earlier a problem in operation is identified the easier it is to fix and still maintain productivity.

Stages Of Anaerobic Digestion Part 2

May 11, 2010

Stages Of Anaerobic Digestion 2

Anaerobic decomposition is a complex process.  It occurs in three basic stages as the result of the activity of a variety of microorganisms.  Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids.  Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

Anaerobic digestion, which takes place in three stages inside an airtight container, produces biogas. Different kinds of micro-organisms are responsible for the processes that characterize each stage.

A variety of factors affect the rate of digestion and biogas production.  The most important is temperature.  Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135°F (57.2°C), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic).  Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range.  However, the process is highly sensitive to disturbances, such as changes in feed materials or temperature.  While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction.  Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the biodigester must be kept at a consistent temperature, as rapid changes will upset bacterial activity.  In most areas of the United States, digestion vessels require some level of insulation and/or heating.  Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester.  In a properly designed system, heating generally results in an increase in biogas production during colder periods.  The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex.  Studies on digesters in the north-central areas of the country indicate that maximum net biogas production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of biogas output.  These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time.  Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly.  The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH; for example, when too much “green” or material high in nitrogen content is added.  It may be necessary to add water to the feed material if it is too dry or if the nitrogen content is very high.  A carbon/nitrogen ratio of 20/1 to 30/1 is best.  Occasional mixing or agitation of the digesting material can aid the digestion process.  Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters.  Complete digestion, and retention times, depend on all of the above factors.


Stages Of Anaerobic Digestion Part 1

May 6, 2010

Stages of anaerobic digestion

As with aerobic systems, the bacteria in anaerobic systems, and the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.  In an anaerobic system there is an absence of gaseous oxygen.  Gaseous oxygen is prevented from entering the system through physical containment in sealed tanks.  Anaerobes access oxygen from sources other than the surrounding air.  The oxygen source for these microorganisms can be the organic material itself or alternatively may be supplied by inorganic oxides from within the input material.  When the oxygen source in an anaerobic system is derived from the organic material itself, then the ‘intermediate’ end products are primarily alcohols, aldehydes, and organic acids plus carbon dioxide.  In the presence of specialized methanogens, the intermediates are converted to the ‘final’ end products of methane, carbon dioxide with trace levels of hydrogen sulfide.  In an anaerobic system the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.

Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective.  It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations, a process known as “seeding” the digesters, and typically takes place with the addition of sewage sludge or cattle slurry.
Stages

The key process stages of anaerobic digestion

There are four key biological and chemical stages of anaerobic digestion:

  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis

In most cases biomass is made up of large organic polymers.  In order for the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts.  These constituent parts or monomers such as sugars are readily available by other bacteria.  The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis.  Therefore hydrolysis of these high molecular weight polymeric components is the necessary first step in anaerobic digestion.  Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids.

Acetate and hydrogen produced in the first stages can be used directly by methanogens.  Other molecules such as volatile fatty acids (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilized by methanogens.

The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products.   The process of acidogenesis is similar to the way that milk sours.

The third stage anaerobic digestion is acetogenesis.  Here simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.

The terminal stage of anaerobic digestion is the biological process of methanogenesis.  Here methanogens utilize the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water.  It is these components that makes up the majority of the biogas emitted from the system.  Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8.  The remaining, non-digestible material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.

A simplified generic chemical equation for the overall processes outlined above is as follows:

C6H12O6 → 3CO2 + 3CH4

refer to “what is anaerobic digestion” article and “what do floating covers do?”

Introduction To Polymeric Geomembranes

May 4, 2010

Classifications For Common Geomembrane Types

It is important to know there are hundreds of different applications that require a liner system.  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 (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 (cross linked or cured and hence cannot be remelted without degradation) (see table).  Since thermoset geomembranes are cross linked, they can exhibit excellent long-term durability.

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

  • Choice of polymer
  • Type of fabric reinforcement
  • Color of upper ply (white to maintain lower temperatures for sun exposed applications)
  • Thickness
  • Texture (smooth or textured for improved friction angles)
  • Product life expectancy
  • Mechanical properties
  • Chemical resistance
  • Ease of installation

Main Plastic Classifications For Common Geomembrane Types

Thermoplastic geomembranes Thermoset geomembranes Combinations of thermoplastic and thermoset
HDPE, LLDPE, fPP, PVC, EIA, TPU, PVDF CSPE (crosslinks over time)
EPDM rubber
Nitrile rubber
Butyl rubber

Polychloroprene (Neoprene)

PE-EPDM

PVC (nitrile rubber)

EPDM/TPE
Polymer-modified bitumen