Energy from Anaerobic Digestion of Phytomass An Enduring but

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					                                                                             Indian Chemical Engr. Section B Vol. 48, No. 2, April - June 2006



    Energy from Anaerobic Digestion of Phytomass:
         An Enduring but Unrealized Dream
                                S. Gajalakshmi, Tasneem Abbasi and S. A. Abbasi*
                                           Centre for Pollution Control and Energy Technology,
                                              Pondicherry University, Pondicherry 605 014



                        Paper received:18.1.06                                 Revised paper accepted: 7.5.06



In the early 1970s, it was widely expected that anaerobic                 metabolic process would keep producing biogas. A
digestion of phytomass will enable generation of clean energy             simple device such as a tube fastened to a small hole
(in the form of methane-rich biogas) on a large scale and                 in the container’s lid would tap this biogas and lead
also provide a means for utilizing weeds, crop wastes, and                it to the point of its use as fuel.
biodegradable municipal solid waste which otherwise cause                     So we had all set about making estimates of the
environmental pollution.                                                  enormous quantities of aquatic and terrestrial weeds
    But close to 35 years have elapsed and those dreams
                                                                          which were available for generating clean energy via
                                                                          anaerobic digestion. This material was not only
are yet to be realized. This write-up takes a look at what
                                                                          available, but was a major source of harm to water
happened and why.
                                                                          resources and agriculture. The dual prospects of
                                                                          controlling weeds and other biowastes plus
The dream                                                                 generating precious energy from the effort kept



I
    t has been known since 1880 [1,2,3] that animal                       thousands of scientists across the world, including
                                                                          these authors, hooked to the enchanting dream.
    droppings, if allowed to decompose out of contact
    of air, give off a combustible gas – named biogas                         Whatever came in sight, was assessed for its
– which can be used to fuel street lamps and boilers.                     ‘bioenergy potential’. We all keenly collected different
                                                                          species of plants, put them in batch reactors,
    About a century later, it was discovered that leafy
                                                                          inoculated the reactors and measured the biogas
biomass, when anaerobically digested, can also
                                                                          generated per unit mass of the plant (dry weight
release a gas similar in composition to the biogas
                                                                          basis). Such measurements kept adding to our
(about 65% methane and the rest mainly carbon                             knowledge of the bioenergy potential of different
dioxide). This finding launched a thousand dreams                         species of plants [4,5] and kept brightening our
and a good deal of activity as well.                                      dreams.
     Unutilized and apparently unutilizable,
                                                                          The disillusionment
phytomass is strewn everywhere. It is there in the
                                                                          Over 30 years have since elapsed. During this period,
form of aquatic weeds, such as, water hyacinth and
                                                                          technological advances have continued in leaps and
salvinia, which often cover lakes, ponds and canals
                                                                          bounds. The potential of anaerobic digestion as a
so completely that water is pushed out of sight. It is
                                                                          process for treating biodegradable wastewaters has
there in the form of amphibious weeds like Ipomoea
                                                                          been realized. But conversion of phytomass and other
carnea and terrestrials like Lantana, which seem to
                                                                          biosolids to biogas is still a dream being dreamt in
grow from nowhere to everywhere. Then there are                           scientific laboratories and seminar halls.
all forms of crop wastes, leaf litter, and other agro                     Why hasn’t the dream come true? Why are we still
wastes, full of organic carbon. Anaerobic digestion                       nowhere near controlling aquatic and terrestrial
had promised to turn the lot into clean energy with                       weeds by economically turning them into energy?
no strings attached. All that was apparently needed                       Why the municipal solid waste continues to litter our
was to put the waste in a container, add some sludge                      existence?
from another anaerobic digester to serve as inoculum,                         An eye-catching headline appeared in
close the lid and relax. The microbes would eat the                       Chennai–based newspapers on September 28, 2005 :
waste and breed, causing more eating and breeding,                        WASTE-TO-POWER PLANT NOW FEEDING
till most of the volatile solids would be utilized. The                   GRID.


* Author to whom correspondence should be addressed
  E-mail : Prof. S.A.Abbasi@gmail.com

                                                                                                                                         '
Indian Chemical Engr. Section B Vol. 48, No. 2, April - June 2006


    It looked so impressive, so heart-warming! One                      6%, and 94 – 95% of the weed phytomass is
thought that the old dream is finally getting realized.                 comprised of water [3,10,11]. Thus, even the
It seemed so till one read the finer print. Then one                    whole plants represent a rather lean source of
learnt that the ‘waste-to-power’ plant is feeding all                   VS and if a weed-water slurry is made as digester
of 20 KW to the multi-megawatt grid ! Power not                         feed, it is even leaner in VS than the whole plants.
sufficient to even run a 4 KW heater for more than 5                    The effective space VS loading in digester of such
hours. What the ‘waste–to-power plant’ is feeding                       a feed would give a very poor energy yield per
the grid is akin to supplementing an elephant’s daily                   unit digester volume. But 75 - 85% of the cost of
diet with the skin of a rice grain !                                    any anaerobic digestion process is consumed by
   Moreover, the fine print revealed quite a few                        the reactor [2,12] and enhancing the methane
uneasy ‘ifs’ and ‘buts’.                                                yield per unit reactor volume is essential to make
                                                                        the process viable.
    To begin with, the Koyambedu market generates
110 tonnes of waste per day but the waste-to-power-                 The Abiding Hope
plant (WPP) can process only about 30% of it because                Hope lies eternal. Even after 30 years of abortive
the plant cannot handle all except ‘soft’ waste of low              attempts, to make biogas production from phytomass
fibre content.                                                      economically viable, the scientists have not given
      So at the best of times the WPP will leave more               up. One of the reasons is that solid wastes, including
than two-third of the Koyambedu waste problem                       weeds, are squeezing us tighter and even more tighter
unsolved. At the best of times, that is. But at present             with each passing day. Another reason is that no
it is able to handle only 15 tonnes of waste which is a             other option has yet been found. And yet another
mere 10% of the market muck.                                        reason is that even now anaerobic digestion appears
     More worrisome is the prospect of the long-term                so very attractive if only the engineering problems
viability of even this small-scale processing capability.           can be solved …
It is one thing to start anaerobic digestion and run it                 So we continue to try. One of the recent efforts
cleanly for a month or two. It is a totally different               [10] which has rekindled our hopes is based on a
thing to maintain such a digester in steady state for               three-stage process (Fig. 1). In the first stage, volatile
long because solids accumulate and other                            fatty acids (VFAs) are extracted from aquatic weeds
destabilizing influences set in.                                    using inexpensive contraptions. In the second stage,
   As for other phytomass, particularly aquatic                     the VFAs are used along with cowdung in
weeds, the engineering problems hitherto                            conventional biogas digesters. The third stage
unsurmounted, are:                                                  comprises of composting – vermicomposting the
                                                                    ‘spent’ phytomass after VFAs have been extracted.
a) The weeds cannot be fed to the conventional
                                                                    As only carbon is lost from the weeds during the
   fixed-dome and floating-dome biogas digesters,
                                                                    process of VFA formation, the C:N ratio of the ‘spent’
   which are extensively used in most of the Third
                                                                    phytomass is lesser (and more fovourable) than the
   World countries to generate biogas from animal
                                                                    C:N ratio of the unprocessed weed. This facilitates
   dung-water slurry. This is because the weeds do
                                                                    composting as well as varimicomposting. The overall
   not flow out of the digester exit along with water,
                                                                    system promises to fully utilize the weeds, partly in
   as the animal dung-water slurry does, but,
                                                                    generating energy (in the form of biogas), and, partly
   instead, accumulate in the digester to eventually
                                                                    in producing fertilizer (in the form of vermicompost).
   clog it. Even when fed as partial feed supplement
   along with animal dung slurry, the weeds                             Another of the attempts involves the use of ‘solid-
   eventually clog the digesters [6,7,3,8,9]. As a                  feed anaerobic digesters’ (SFADs). In these systems,
   result, the digesters become non-functional a few                weeds are fed in their solid state, either as chopped
   weeks after start–up with weed–feed. In a like                   pieces or in air-dried form in specially designed
   manner, weeds also clog the continuously stirred                 SFADs [11]. Some energy is, thus, saved which
   tank reactors (CSTR) used in most developed                      otherwise would have been needed in mincing
   countries for anaerobically digesting piggery and                (shredding) the weed, if the feed were to be cowdung-
   dairy wastes.                                                    like slurry. The SFADs produce VFA-rich leachate,
                                                                    which is converted to biogas in anaerobic filters. The
b) Aquatic weeds contain less than 4% volatile solids
                                                                    SFADs can generate upto 2 m3 of biogas per m3 of
   (VS). Their total solids content is rarely above


                                                                         Indian Chemical Engr. Section B Vol. 48, No. 2, April - June 2006


                                                                    3.    Abbasi, S. A., and Ramasamy, E.V., Biotechnological
                                                                          Methods of Pollution Control, Orient Longman
                                                                          (Universities press India Ltd) Hyderabad, (1999).
                                                                    4.    Abbasi, S. A.,: Renewable Energy from Aquatic Biomass,
                                                                          In : Terol, S. (Ed), Proceedings of the International
                                                                          Congress on Renewable Energy Sources, Madrid, CSIS, p
                                                                          60-69, (1987).
                                                                    5.                    .
                                                                          Abbasi, S. A., P C. Nipaney, and G. D. Schaumbery :
                                                                          Bioenergy Potential of Eight Common Aquatic Weeds,
                                                                          Biogical Wastes 34 359-366, (1990).
                                                                    6.                             .C.,                  .,
                                                                          Abbasi, S. A., Nipaney, P and Ramasamy, E. V : Use of
                                                                          Aquatic Weed Salvinia(Salvinia molista, Mitchell) as Full/
                                                                          Partial Feed in Commercial Biogas Digesters, Indian
                                                                          Journal Technology, 30 451-457, (1992a).

Fig. 1 : The VFA Extraction-biogas Generation-                      7.                             .                  .,
                                                                          Abbasi, S. A., Nipaney, P C., Ramasamy, E. V : Studies on
Vermicomposting System for Water Hyacinth                                 Multi-phase Anaerobic Digestion of Salvinia, Indian
Utilization                                                               Journal Technology, 30 483-490, (1992b).

reactor volume which represents an economically                     8.    Bouallagni, Y., Touhami., R. Ben Cheikh, Hamdi, M.,:
                                                                          Bioreactor Performance in Anaerobic Digestion of Fruit
viable rate of energy generation by anaerobic                             and Vegetable Wastes, Process Biochemistry 40 989-995,
digesters [13].                                                           (2004).
    Will this type of unconventional approaches                     9.                                                         .,
                                                                          Santosh, Y., Sree Krishnan, T. R., Kohli, S., Rana, V :
                                                                          Enhancement of Biogas Production from Solid Substrates
finally usher in the era of biogas generation from
                                                                          Using Different Techniques – A Review, Bioresource
phytomass ? Lets hope it does !                                           Technology 95 1-10, (2004).
Acknowledgement                                                                           .,                .,
                                                                    10. Sankar Ganesh, P Ramasamy, E. V Gajalakshmi, S., and
                                                                        Abbasi, S. A., : Extraction of Volatile Fatty Acids (VFAs)
Authors thank the Department of Biotechnology, Ministry of
                                                                        from Water Hyacinth Using Inexpensive Contraptions, and
Science and Technology, Government of India, for support
                                                                        the Use of the VFAs as Feed Supplement in Conventional
under the project BT/PR 4741/AGR/21/182/2004. SG thanks                 Biogas Digesters with Concomitant Final Disposal of Water
the Council for Scientific and Industrial Research (CSIR), New          Hyacinth as Vermicompost, Biochemical Engineering
Delhi, for Research Associateship.                                      Journal, 27 17–23, (2005).
                                                                    11. Ramasamy E. V., Gajalakshmi, S., Sankar Ganesh, P        .,
References
                                                                        Abbasi, S. A., : Solid-feed Anaerobic Digestion of Ipomoea
1.   McCarty, P.L., : One hundred years of Anacrobic                    (Ipomoea carnea) Environmental Technology (2006)
     Treatment, In : Anaerobic Digestion 1981, Hughes, D.E..            (communicated).
     Stafford, D.A., Wheatley, B.I., Wentworth, R.L. (Eds),
                                                                                       .,
                                                                    12. Selvagenesan, P : MPhil Thesis, Pondicherry University,
     Elsevier Biomedical Press, Amsterdam, 3-22, (1982).
                                                                        Pondicherry, (1999).
2.   Abbasi, S. A., and Nipaney, P    .C., : Worlds Worst Weed
                                                                    13. Abbasi, S. A. and Abbasi, N., : The Likely Adverse
     (Salvinia) – Its Impact and Utilization. International book
                                                                        Environmental Impacts of Renewable Energy Sources.
     Distributors, Dehradun, (1993).                                    Applied Energy, 65 121-144, (2000).




                                                            Corrigendum

     The authors (Baisali Sinha, Partha K Sarkar, S N Roy and S Bandyopadhyay) of the paper, Yeast Separation
     by Ceramic Membrane and Flux Recovery after Chemical Cleaning, (published in ICE, Vol. 48, No. 1, January
     – March 2006), are affiliated to Ceramic Membrane Division of Central Glass & Ceramic Research Institute,
     Kolkata. The affiliation was wrongly published as Chemical Membrane Division. The error is regretted.




                                                                                                                                     

				
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