Anaerobic Digestion Technology

Anaerobic Digestion Technology

ABSTRACT Anaerobic digestion is one of the oldest process used for the stabilization of sludge. It involves the demonstration of organic matter in the absence of molecular oxygen. Anaerobic digestion of biodegradable waste results in both, energy generation and reduction of greenhouse gas emissions. It would not only replace the use of fossil fuels in various applications but would also utilize methane, generated from the waste. A case study in milk processing unit in Maharashtra showed that anaerobic treatment can achieve saving to the tune of Rs. 25 million per year in Maharashtra alone (in 1997). This work reviews the current status of anaerobic digestion technology and various types of reactors used for the stabilization.

Anaerobic Digestion Technology

INTRODUCTION The generation and disposal of large quantities of biodegradable waste without adequate treatment results in significant environmental pollution. The degradation of waste in the environment can also lead to the release of greenhouse gases, such as methane and carbon dioxide. Some of the waste streams are treated by conventional means like aeration, which is both energy intensive and expensive, and generates significant quantity of biological sludge, which must then be disposed off. In this context, anaerobic digestion offers potential savings and is a more stable process for medium and high strength organic effluents. Apart from treating wastewater, the methane production from anaerobic system can be recovered. The anaerobic method of waste treatment offers numbers of significant advantages, over the treatment methods. This led to the development of range of reactors, suitable for the treatment of low and high strength soluble wastewater. Although the anaerobic treatment of wastewater has been adopted since the beginning of the century, yet conventionally, the anaerobic process is considered a slow process requiring digesters of large hydraulic retention time (HRT). Maintaining a high concentration of microbes in a reactor, and preventing them from escaping with the effluent can obtain high rates of conversion of organics into methane and carbon dioxide by the anaerobic treatment method .This concept is expressed by sludge retention time (SRT), being the ratio of the mass of biological solids in the system to that escaping from the reactor. This will however increase the reactor volume, which will increase the capital costs because minimal HRT minimizes the reactor volume.

Anaerobic Digestion Technology

Other requirements of the high rate systems are: 1) Intimate contact between incoming water and the detained biological solids in the reactor. 2) Maintenance of sufficiently warm temperatures. The various high rate anaerobic systems that have been discussed, are A) Standard rate digesters B) Fixed bed reactor C) UASB reactor D) Fluidized bed reactor. It is difficult to evaluate the advantages and disadvantages of each system in relation to other concepts, as generalization are not usually valuable in practice following considerations are important. 1) Purification rates 2) Loading rates 3) Investment costs 4) Energy balance 5) Space requirement 6) Operational costs

Anaerobic Digestion Technology

PROCESS DESCRIPTION In the anaerobic digestion process, the organic matter in mixture of primary, settled and biological sludge is converted biologically, under anaerobic conditions to methane (CH4). The process is carried out in an airtight reactor. The sludge is introduced continuously or intermittently, is retained in the reactor for varying period. The stabilized sludge withdrawn from the reactor is reduced in organic and pathogen content and is nonpotresible. The biological conversion of the organic matter in sludge occurs in three steps. In first stage the higher molecular mass compounds were converted into compounds suitable to use as a source of energy & cell carbon. One group of microorganisms is responsible for hydrolyzing organic polymers & lipids to basic amino acids & related compounds. A second group of bacteria ferments the breakdown product to simple organic acids, the most common of which in this digester is the ‘acetic acid’. These groups of microorganisms are often identified as acidogens or acid formers (Clostridium spp.) The third group of microorganism converts the hydrogen and acetic acid, formed by the acid formers to methane gas & carbon dioxide. The bacteria responsible for this conversion are strictly anaerobic and are called as methanogens / methenogenic. The principle genera of microorganisms that have been identified include the rod & spheres. The two types of commonly used anaerobic digesters are identified as standard rate and high rate. In standard rate digestion process the contents of the digester are unheated and unmixed. Detention time for standard rate digesters vary from 30 to 60 days .In high rate digesters the contents are heated and mixed properly. The detention time is typically 15 days or less. A combination of the basic process is known as the two-stage process. The function of the second stage is to separate the digested solids from the super liquor, however additional digestion and gas production may occur.

Anaerobic Digestion Technology

The reactions involved in the process are: 1) 4H2 + CO2 -------------- CH4 + 2H2O 2) 4HCOOH

------------ CH4 + 3 CO2 + 2 H2O

3) CH3COOH

------------- CH4 +CO2

4) 4CH3OH

------------ 3CH4 + CO2 + H2O

5) 4(CH3)3N + H2O -------- 9CH4 +3CO2 + 6H2O + 4NH3

Anaerobic Digestion Technology

The figure shows the steps in Anaerobic digestion

Digestion with energy flow.

Anaerobic Digestion Technology

GAS PRODUCTION The volume of methane gas produced during digestion process can be estimated using equation Uch4 = (0.35m3/kg) (10-3) -1.42 (p-x). Uch4 = volume of methane produced 0.35 = Theoretical conversion factor for amount of methane produced from conversion of 1 kg of BOD. E

= Efficiency of waste utilization.

Q

= Flow rate (m3/d)

So

= ultimate BOD

1.42

= conversion factor for cell tissue to BOD

Px

= Net mass of cells tissue produced per day (kg/d) The total production is usually estimated from the volatile solids loading

of the digester or from the percentage of volatile solids reduction. Gas production can fluctuate over a wide range, depending upon the volatile solids content of sludge & biological activity in the digester. Excessive gas production rate sometimes occur during startup period, and

may cause foaming, and escape of foam & gas from around the edges

of floating digester covers. If stable operating conditions have been achieved and the foregoing gas production rates are being maintained, it is assured that the result will be a well-digested sludge

Anaerobic Digestion Technology

TYPES OF REACTORS USED FOR TREATMENT PROCESS & DIGESTION TECHNIQUES. Although anaerobic treatment of waste water has been adopted since the beginning of the century , yet conventionally, the anaerobic process is considered to be a slow process. Maintaining a high concentration of microbes in a reactor & preventing them from escaping with the effluents can obtain high rates of conversion. The digestion techniques are classified as: 1) Standard rates digestion. And 2) High rates digestion. (A) Standard rates digestion:-

In standard rate digestion, the

functions of digestion sludge i.e. thickening & supernatant formation are carried out simultaneously. In the single stage process, untreated sludge is added to the zone where the sludge is actively digested. As gas raise to the surface it lifts sludge particles & other materials like grease oil and fats, ultimately giving rise to formation of scum layer. As a result of digestion, the sludge becomes more mineralized & it thickens because of gravity. Because of these limitations standard rate process is used for small installations. Example:- CSTR (B) High rate digestion process: In this process, the sludge intimately mixed by gas recirculation, pumping or by mixers and the temperature is maintained to obtain high digestion rates. With the exceptions of high loading rates and improved mixing there are few differences between standard rate digestion and high rate digestion process. The high rate digestion reactors are being developed, some of them are

Anaerobic Digestion Technology

1) Fixed film reactor 2) Upflow anaerobic sludge blanket 3) Fluidized bed reactor 4) Hybrid reactor

These are shown as:

Anaerobic Digestion Technology

Anaerobic Digestion Technology

Anaerobic Digestion Technology

Industries were this technique could be implemented Due to increased, human and industrial activities most of the Indian rivers are polluted and made them unfit for designated use. Total waste generation from major water consumption industries such as agro based industries, refineries, petrochemicals, fertilizers and industrial chemicals were estimated to be about 3000 million liters per day. Table above shows the share of industrial out put to water pollution.

The central pollution board has selected 17 categories of major polluting industries for prior action. Table 2 shows characteristics of wastewater from various sectors of industries. More than 50% of the units in four categories, namely distillery, pulp and paper, textile, and steel industries are yet to attain satisfactory performance.

Anaerobic Digestion Technology

Pollution control systems in some major polluting industries are shown in above table:

Anaerobic Digestion Technology

Constraints in development and popularizing anaerobic reactors .

Anaerobic Digestion Technology

This section discusses the major barriers faced in the implementation of anaerobic digestion in industries, including distilleries, pulp and paper, and dairy industries. A high strength effluent producer, the distillery sector has to meet stringent pollution control regulations. That’s why this sector has most suitable condition for implementation of this technique, but only 145 of 254 industries in the country have adopted anaerobic digestion method for wastewater treatment. This is partly because of the fact that high rate anaerobic digestion technique was established in India in early 1990’s. Most of the plants installed prior to this period were based mainly on aerobic process. There is some hesitation to change over to anaerobic technique partly due to lack of information on installations. Moreover, it is difficult to obtain reliable plant scale data because distilleries are unwilling to divulge the digester performance due to fear of reprisals from the pollution control authorities. Although a significant potential exists for the energy generation from anaerobic technology, most of dairy industries employ aerobic plants for effluent treatment. Thus installing anaerobic plant after demolishing the existing aerobic unit is not acceptable due to additional cost. Moreover being in the small-scale sector most of them do not find it economically feasible to install the treatment plant. Although pulp and paper mill effluent is a prime candidate for this technique, instance of its use is limited .One, reason is the presence of toxic components in the effluent, which results in non-adaptation of the microorganisms. Identifying cultures that can adapt toxic components in the effluent may be one of the solutions for the above problem.

Anaerobic Digestion Technology

Government policies for promotion of anaerobic digestion Technology To meet the growing energy needs of the country, the government of India has placed growing emphasis on new and renewable sources of energy. the government of India, under the chairmanship of secretary MENS, setup the national bioenergy board for providing policy guidelines and directions for harnessing bioenergy from urban municipal and industrial waste. MENS has initiated a project for development of high rate bio methanation processes as a means of reducing GHG emission .It has a total out lay of Rs. 400-million .The objective of the project are: 1) Development of institutional framework at national level to generate necessary awareness and capabilities to provide impetus to the bio energy development programme utilization high rate bio methanation processes. 2) development of requisite expertise and capabilities in the national and state level institutes,R&D organizations, and universities to assimilate and adapt technologies, improve applied R&D skills in the field of high rate methanisation processes , and to provide technical know how and bio assistance in setting up plants using bio methanisation process.

Anaerobic Digestion Technology

Besides several incentives and subsidy’s, the following tax benefits are applicable to bio methenation projects: 1) Accelerated 100% depreciation in the first year. 2) Tax holiday for five years. 3) No excise duty on gas engine and municipal waste conversion device producing energy. 4) Concessional custom duty of 10% for all goods, imported by the manufacturer for manufacturing and supply of machinery and equipments to a power generation plant. 5) Concessional custom duty (20% custom duty + 13% countervailing duties) on project import. 6) Interest subsidy, i.e. 10% on the lone amount for the entire loan repayment period, subject to maximum capitalized amount of Rs 10 million per megawatt electricity.

Anaerobic Digestion Technology

CONCLUSION Though anaerobic digestion system have been operational in certain industries like distillerie, there is still some apprehension regarding adopting this technique of anaerobic digestion in other sector, since large number of industries in India are yet to establish adequate waste water treatment facilities, the government of India is actively promoting anaerobic digestion technology. Apart from funding pilot studies, financial & fiscal intensives are also provided for establishing full-scale plants. There is urgent need for more intensive interaction between research organizations and suppliers of anaerobic systems. This supported by the fact that over the last 15 years, even though MNES has supported more than 90 researches oriented projects on biogas production there is still lack of sufficient design information. Out of nearly 60 demonstration projects, only one has been selected for commercial application. Still number of industries have not attaining the sufficient performance and had wide scope in industrial sector for implementing anaerobic digestion technology. There is wide gap between R&D efforts and technical implementation pointing to the need for more efforts towards technology adoption aspects.

Anaerobic Digestion Technology

REFERENCES 1. Metcalf & Reddy inc. Waste water engineering ,Mc Graw Hill New York 1972. 2. Environmental pollution control engineering By: C.S. Rao 3. Waste and waste water technology By: M.J. Hammer

Websites :1) www.energy.ca.gov/dev/anaerobic.html 2) www.teriin.org/digestion.html 3) www.climatetechnology.com 4) www.synargo.com 5) www.epa.gov/workshop.html