asiaBIOGAS’ technical team has more that 50 years of experience in the fields of anaerobic digestion, bio-treatment technologies and environmental issues, and include PhDs in Chemical Engineering and Microbiology. They have developed and consulted on a wide variety of projects encompassing varied geographies, climates, wastewater types and technologies, including:

• UASB – Upflow Anaerobic Sludge Blanket
• CIGAR™ - Covered In Ground Anaerobic Reactor
• CSTR – Continually Stirred Tank Reactor
• Covered Lagoons
• Expanded Bed Reactors
• Hybrid Reactors

asiaBIOGAS will select the best solution for the circumstance of each project. Given that climate and geography of the company’s primary territory in Southeast Asia,  asiaBIOGAS has found that the Covered in Ground Anaerobic Reactor (CIGAR™) exceeds performance standards of other digesters in terms of:

• Higher conversion of BOD to biogas
• Greater overall biogas volume
• Higher methane gas content

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. 

KEY PRINCIPLES OF ANAEROBIC DIGESTION

Development of Anaerobic Treatment Systems

Anaerobic digestion of biodegradable wastes involves a large spectrum of bacteria of which three main groups are distinguishable.

1. The first group comprises fermenting bacteria that perform hydrolysis and acidogenesis. This involves the action of exo-enzymes to hydrolyze polymeric matter like proteins, fats, and carbohydrates into smaller units which can then enter the cells to undergo an oxidation-reduction process resulting in the formation of volatile fatty acids(VFA) and some carbon dioxide and hydrogen. The fermenting bacteria are usually designated as acidifying or acidogenic population because they produce VFA.

2. Acetogenic bacteria constitute the second group and are responsible for breaking down the products of the acidification step to form acetate. In addition, hydrogen and carbon dioxide are also produced during acetogenesis.

3. The third group involves methanogenic bacteria that convert acetate or carbon dioxide and hydrogen into methane. Other possible methanogenic substrates like formate, methanol, carbon monoxide, and methylamines are of minor importance in most anaerobic digestion processes.

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.

Key Principles of Anaerobic Digestion

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.

Mesophilic 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:

CODt (total)
BOD5 (fifth day)
Total Suspended Solids (TSS)
Volatile Suspended Solids (VSS)
Volatile Fatty Acids (VFA)
Fats, Oils and Greases (FOG)

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.  

Primary Objectives of Sustainable Wastewater Treatment Systems
 
These important aims of proper wastewater treatment systems were likely covered well by Dr. Parco in the morning sessions.  For anaerobic digester activity, the objectives that can be achieved easily with anaerobic treatment are in boldface.

1. 1. Be able to remove the chemical toxins as well as disease organisms.
2. 2. Be capable of producing secondary, tertiary and advanced tertiary effluents.
3. 3. Be reliable in producing safe, reusable water everyday all year round.
4. 4. Be ecologically sound, capable of improving rather than harming the local environment.
5. 5. Not add harmful chemicals or create toxic sludge during the treatment process.
6. 6. Produce valuable by-products from the waste nutrients to reduce net operating costs.
7. 7. Be cost effective, with low construction and operational costs, and the potential to achieve profitability from resale of water and products.

Be ecologically sound, capable of improving rather than harming the local environment

Anaerobic bacteria are nature’s methods to break down organic compounds to the base elements. In the process, suspended solids in the waste streams are converted into gases.  A treatment system that contains this activity improves both water (as organic contamination is the core environmental problem of organics in the waste streams as measured by CODt and BOD5) and air (as biogas usually has constituent gases that cause odor vectors).  Anaerobic reactors, if properly designed, usually are the best wastewater treatment units for breaking down VSS in the smallest available design footprint.

Produce valuable by-products from the waste nutrients to reduce net operating costs.

At PhilBIO our ‘motto’ is “Waste is just ‘natural resources’ out-of-place”.  The CODt, when removed via anaerobic activity, produces considerable energy value from each cubic meter of wastewater treated. As most of the energy is in the form of methane (at least 60% by volume), the COD removal efficiency is a key factor in the process, and to the economics of the wastewater treatment project.

Nutrients are also made available through the anaerobic process. Most wastewater contains the building blocks for plant activity including high nitrogen. Potassium and Phosphorous are found in large quantities in many wastewaters, especially those from animal farming.

Be cost effective, with low construction and operational costs, and the potential to achieve profitability from resale of water and products.

Anaerobic reactors usually provide a cost effective method of wastewater treatment through higher COD removal rates than aerobic wastewater treatment methods per cubic meter of reactor volume.  This will greatly reduce the construction costs of the anaerobic reactor.

Anaerobic reactors also impact operational costs positively, as methane gas generated in the process can be utilized in the process to reduce energy costs. If anaerobic reactors are the primary treatment in a system, the chemical reaction of anaerobic microbial activity will lead to reductions in the secondary treatment, or aeration.  The aerators, dissolved air flotation (DAF) or other types, may be reduced or even eliminated if anaerobic reactors are operating well.

If the heating value of the biogas is of a certain level (usually 450 BTU per pound or above), the use of biogas for process steam (fuel for the boilers), or electric power conversion or cooking gas may actually result in a return on invested capital in the system.

Nutrients can be harvested as compost (if solids are preserved), or as liquid fertilizer (if maintained as totals suspended solids) have value in the marketplace.

The Dynamics of Bio-Engineering

To 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.

Graphic 01: Bio-Engineering Principles | Source: Jurgen Thiele

BASIC DESIGN CONSIDERATIONS

Design characteristics are numerous. However, the important considerations for anaerobic reactors are the following:

1. The Total Wastewater Flow
2. The kg of VSS per cubic meter of wastewater flow
3. The homogeneity of the wastewater flow
4. Changes in the wastewater flow from day to day
5. The composition of the wastewater in terms of
6. BOD and COD, the keys to energy value
7. Inhibiting factors to anaerobic activity
8. High TSS loaded to the digester (usually above 500 mg/liter)
9. High Sulfates, as sulfur molecules sometimes lead to poor conversion of the COD
10. While the anaerobic microbes do not convert Potassium, high levels may lead to low conversion rates
11. Low pH inhibits the process and may cause loss of anaerobic sludge in the process
12. The most difficult volatile organic compounds are fats,oils and greases.

1. Examine Optimal Mixing Requirements for wastewater as there needs to be maximum contact between the microbial matter and the VSS. If this is not happening readily, mixing may be required.
2. Mesophilic or Thermophilic: Depending on the temperature of the wastewater and the size of the footprint, a choice must be made on what kind of bacteria are to be used.
3. The average organic loading rates: Most digesters need to be fed within a certain range of organic solids (TSS and TDS). Anaerobic reactors are much like any other life form, if nutrients levels are too little or too much, they will underperform. In some cases, the anaerobic sludge may be lost from the system.

Biogas Utilization

• 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.

PRACTICAL APPLICATIONS

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):

WASTEWATER TREATMENT BENEFITS

• Naturally occurring waste treatment process
• Usually requires less land than aerobic treatment
• Reduces total organic matter significantly over aerobic treatment

ENERGY BENEFITS

• Net energy producing process
• Generate high quality renewable fuel
• Biogas proven in numerous end-use applications

ENVIRONMENTAL BENEFITS

• Significantly reduces Greenhouse Gas Emissions
• Eliminates odors
• For municipal solid waste applications, produces
  a sanitized compost and nutrient-rich liquid fertilizer
• Maximizes recycling benefits

ECONOMIC BENEFITS

Is more cost-effective than other treatment options from
a life-cycle perspective

Among the major applications for anaerobic digestion are:

Sanitation Wastewater and Sewage Sludge

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.

Agricultural Wastewater

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 Fraction of Municipal Solid Waste (OFMSW)

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.

Industrial Process Wastewater

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.

The 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.

The following pie graphs illustrate the industrial process wastewater applications for anaerobic digestion in major industries in the United Kingdom.