Anaerobic digestion is a major category of biological treatment systems, referring to bacteria that operate optimally in the absence of oxygen.These microbial cell formations are the building blocks for decomposition of volatile organic compounds (VOCs), and can be found in highly moist environments at operating temperatures in excess of 10 degrees C. up to temperatures of 90 degrees Celsius.
Anaerobic bacteria can be part of a full system of biological treatment, including aerobic bacteria activity in activated sludge or other aerobic biological process.
Development of Anaerobic Treatment Systems
Anaerobic digestion of biodegradable wastes involves a large spectrum of bacteria of which three main groups are distinguishable.
In addition to these three main groups, hydrogen consuming acetogenic bacteria are always present in small numbers in an anaerobic digester. They produce acetate from carbon dioxide and hydrogen and, therefore, compete for hydrogen with the methanogenic bacteria.
The synthesis of propionate from acetate, as well as production of longer chain VFA, occurs to a limited extent in anaerobic digestion. Competition for hydrogen can also be expected from sulphate reducing bacteria in the case of sulphate containing wastes (DeZeeuw 1986).
It was a long accepted belief that anaerobic digestion was feasible only for the treatment of concentrated wastes such as manure and sewage sludge with long retention times.
Around 1950, anaerobic treatment of wastewater was attempted and the concept of high rate systems received importance with the use of mixing devices. The latter helped to break scum in the digester and increase contact between organisms and the substrate. Special reactor types for wastewater treatment such as the anaerobic contact processes were also developed. This recognition of the need to optimize anaerobic contact was a major advance in the understanding of how to design anaerobic reactors effectively.
Advanced methods such as the upflow anaerobic sludge blanket (UASB) process and various fixed film reactor types based on the principle of sludge immobilization were introduced at this point. At present, anaerobic digestion is a popular option and is a widely used wastewater treatment method for a number of wastewater treatment applications.
Temperature: Anaerobic microbial matter works in temperatures ranging from 10 degrees C to 90 degrees C. There are two broad categories of bacteria: Mesophilic, or bacteria that work optimally at human body temperature; or thermophilic, a bacteria that operates optimally in high temperature conditions above 40 degrees C., and nearly to boiling point.
return to topMesophilic bacteria are robust and adaptable to changes in conditions. They are easy to control and cultivate in an anaerobic climate of at least 30 degrees C. Thermophilic bacteria are susceptible to changes in conditions. They are difficult to grow and sustain. However, the ability to convert VSS are several times better in thermophilic conditions than in Mesophilic conditions. The key is in the knowledge to control their environment.
The nature of the bacteria: A designer of anaerobic systems must decide upon the most efficient method for the reactor to manage its microbial biomass. Anaerobic bacteria are water-loving bacteria, and thrive in an intensive hydrogen environment. These bacteria can be developed as flocculant bacteria that readily leave the reactor and are either replaced or recirculated to the digester; or a sludge mobilization process that maintains the bacteria in a sludge blanket along the bottom and walls of the cell.
ph: Neutral pH is the rule of thumb for healthy anaerobic environments. If acid-forming bacteria are too prevalent in the digester tank, the wastewater conversion of VSS will diminish substantially. Other competitors for hydrogen and carbon dioxide within the reactor will become more aggressive, reducing biogas yields. Before the operator is aware of it, the acid-forming bacteria have caused the methanogenic and other types of anaerobic microbes to become dormant. Almost always this is a result of intolerably low pH levels in the tank over the course of time.
Chemical Composition of the Wastewater Stream
Proper testing and analysis of the wastewater stream is necessary in determining the key design characteristics for an anaerobic reactor. In the analysis, the key elements of the chemical composition of the organics in the wastewater are:
To determine biogas yields, the most important factor is CODt. The degree of presence in the wastewater of factors will determine the ability of anaerobic microbes to break them down. For example, a high VFA may inhibit biogas conversion, even if CODt levels are relatively high and uniform. Frequent wastewater sampling and testing will verify the characteristics of the wastewater, ensuring the designer enough data to develop an appropriate reactor type and size.
return to topTo portray graphically the bio-engineering principles of anaerobic reactors v. aeration, please see the following graphic presentation. Note that to break down a kg of BOD, at least 1 kWh or electric power is required if aerobic bacteria are used. In the case of anaerobic bacteria, the chemical reaction leads to the generation of a fuel, methane gas at .35 cubic meters per kg of BOD removed from the digester.
Figure 1. Bio-Engineering Principles | Source: Jurgen Thiele
Design characteristics are numerous. However, the important considerations for anaerobic reactors are the following:
A key element in the design to determine gas yields and the most economic utilization of the gas yields.
With most biogas, there are constituent components that may cause corrosion (H2S) or additional wear and tear on burners, boilers and engine generator sets. Once the biogas quality is determined through gas analysis, then the decision can be made. Usually, a boiler will be able to withstand contaminants such as H2S in the biogas. Electric power plants usually require less than 800 mg/liter of H2S in the system to maintain optimal performance of the small power production plants.
The following pie graphs illustrate the industrial process wastewater applications for anaerobic digestion in major industries in the United Kingdom.
Figure 2. Applications for Anaerobic Digestion
(left) Industries using AD for wastewater pretreatment
(right) Types of AD systems used for wastewater pretreatment
With respect to applications of anaerobic digestion, there are a number of benefits for nearly every kind of organic wastewater available, as well as for the organic fraction of municipal solid waste (OFMSW):
AD is more cost-effective than other treatment options from a life-cycle perspective. Among the major applications for anaerobic digestion are:
Digestion of sanitation wastewater and sewage sludge provides significant benefits when recycling the sludge back to land. The digestion process provides sanitization and also reduces the odor. Typically between 30 and 70% of sewage sludge is treated by anaerobic digestion in industrialized countries. The energy generated powers the sewage treatment works. At larger plants, there is excess biogas for export from the plant. The technology for sewage sludge digestion is well established.
Farm scale digestion plants treating principally animal wastewater have seen widespread use throughout the world, In rural communities small scale units are typical, and Nepal has some 47,000 small-scale digesters. In China, estimates reach 6 million digesters, again mostly backyard type. These plants are generally used for providing gas for cooking and lighting for a single household.
In more developed countries, farm scale anaerobic digestion plants are generally larger. Biogas is used to generate heat and electricity to run the farm and for export. The countries with the largest concentration of anaerobic digesters for animal wastewater include Germany, where renewable energy mandates from the Federal Government allow for the direct export of electric power to the electric grid at wholesale prices.
These farm scale digestion plants range from continuously stirred tank to covered lagoon designs that use long retention times to provide the treatment required. Modern developments in agricultural waste digestion have developed the concept of centralized anaerobic digestion (CAD) where many farms co-operate to feed a single larger digestion plant. The wastes provided to this will be principally agricultural manures and some food wastes. CAD is most prevalent in Denmark.
Organic wastes from commercial and residential areas provide potential feedstocks for anaerobic digestion. Options exist for treating clean source-separated fractions for recycling both the energy content and the organic matter. We term this the organic fraction of municipal solid waste (OFMSW). Alternatively the unsegregated wastes can be treated to gain the biogas from the waste as well as stabilizing leachate to prevent further problems in landfill. As the ineffectual disposal of collected food waste by cities is arguably the single biggest environmental problem facing the Philippines, anaerobic digestion could be a practical part of the solution.
Organic solid wastes from industry are increasingly being controlled by environmental legislation. As the Philippines has joined these ranks recently with the ‘Clean Water Act’, all industries are under notice to treat effluent properly or face fines and legal action. Treatment of these wastes by anaerobic digestion allows additional value to be gained through providing products and reducing the cost of disposal. In addition the sensitive treatment of wastes can be used to aid the environmental image of the industries concerned.
Anaerobic digestion of industrial wastewater is becoming a standard technique in many parts of the world. Whilst anaerobic digestion is only an initial stage in the treatment of high quality water discharge, it can significantly reduce the cost and size of plant compared to wholly aerobic treatments.
return to topThe following offer a general catalog of the ‘name brand’ anaerobic digesters with proven performance records. Please note that all digesters are wastewater specific. Prior to final design, considerable testing of the wastewater should be undertaken to ensure that the anaerobic digester technique matches the key wastewater stream characteristics.
Many digesters fail because a technology for one wastewater (an example would be brewery wastewater and UASB) is advanced for a second wastewater with totally different characteristics (such as distillery wastewater and the same design for the UASB).
As mentioned, OFMSW is an appropriate application for anaerobic digester technology. In Germany, complete mix reactors are often sited at sanitary landfill sites to treat the food wastewater formed in the process.
In the United States and Canada, some waste-to-energy systems combine waste streams. In many cases there are significant advantages to co-digest the OFMSW with other clean solid waste streams. Successful co-digestion candidates include organic industrial wastes (OIW), such as food processing wastes, and agricultural wastes, such as manure.
An important feature of the OFMSW is that it is generally available in constant quantities throughout the year, and acts as a stable base feedstock.
Anaerobic digestion is ideally suited for many of the concentrated wastewaters typical of many industrial processes today. Over 30 industries have been identified with wastewaters amenable for anaerobic digestion treatment, including processors of beverages, chemicals, food, meat, milk, pulp and paper, and pharmaceuticals,among others.
The types of anaerobic digesters commonly used in industrial applications include low-temperature covered lagoons, continuously stirred tank reactors, complete mix tanks, anaerobic filter reactors, upflow anaerobic sludge blankets (UASB), and fluid bed reactors.
The advantages of these technologies compared to aerobic processes include low sludge production, high loading rates, low nutrient requirements, low maintenance, and, significantly, the production of biogas.
An Ingersoll Rand a microturbine system is a complete biogas-to-energy system ready for hookup and operation. An Ingersoll Rand onsite energy system incorporates one or more rugged Ingersoll Rand microturbines, a matched fuel conditioner designed specifically for agricultural digester gas, and all necessary switchgear. Everything is completely configured at the factory for reliable operation and skid mounted for easy installation and hookup — with or without a connection to the grid.
Farms that now flare digester gas — literally wasting a valuable energy resource — may be able to simply redirect the gas from flaring to fueling a microturbine. Although anaerobic digester gas is essentially a free or lowcost fuel for a microturbine, some pretreatment of the gas is required. The integrated Ingersoll Rand fuel conditioner is designed for a wide range of biogas conditions and effectively removes hydrogen sulfide (a highly corrosive and toxic gas) and other contaminants from the gas stream. The fuel-conditioner compressor also permits the microturbine to operate at sites with low available pressure.
Ingersoll Rand Microturbine Systems
