Towards modeling and design of vermicomposting systems: Mechanisms of composting/vermicomposting and their implications

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1 Indian Journal of Biotechnology Vol 8, April 2009, pp Towards modeling and design of vermicomposting systems: Mechanisms of composting/vermicomposting and their implications Tasneem Abbasi, S Gajalakshmi and S A Abbasi* Center for Pollution Control and Energy Technology, Pondicherry University, Puducherry , India Received 29 April 2008; revised 29 September 2008 ; accepted 2 December 2008 Several studies have been reported, and are continued to be done by scientists especially in Asia, in which earthworms are added to one or other substrate undergoing composting. The concerned authors call it vermicomposting. Other authors use the term vermicomposting to denote processes in which earthworms are made to feed upon one or other substrate, to generate a useful product (vermicast). Present review has embarked on a series of efforts aimed at clearly defining the mechanism of vermicomposting process and to model it. This, in turn, is envisaged to be made the basis for developing rational criteria with which vermireactors are to be designed and operated in a manner that maximizes the process efficiency and minimizes the production cost. During the course of these efforts, authors have conducted a detailed analysis of the steps associated with composting and vermicomposting. Based on an analysis of the experiments done earlier by authors, as also on the work published by others, it is now reported that composting and vermicomposting are essentially different types of processes involving different bioagents, process conditions, reactor operation strategies, and process control parameters. Hence, to achieve optimal results, the two processes should be run in isolation, composting should always precede vermicomposting, and never in combination. It is also suggested that the term vermicomposting should be used only to denote the process in which reactor systems are used to transform biodegradable substrates into vermicasts. Keywords: Composting, vermicomposting, modeling, design Introduction Vermicomposting, which involves the use of earthworms to convert biodegradable solid waste into a useful product (vermicast), has a unique position in the domain of environmental engineering: it is the only pollution control bioprocess which has a multicellular animal as the main bioagent. Indeed, it is the only bioprocess outside the ones involving one or other type of animal farming in which a multicellular animal is used in reactor systems to generate a product other than the animal offspring. All the other engineered bioprocesses, except a few which are based on plants (botanical species), revolve around the use of enzymes, bacteria, microfungi, or microalgae in mobile or immobilized modes. Such processes have been intensively studied, modelled, designed, and engineered 1-3. In contrast, no design or operation criteria based on bioprocess engineering principles has been developed for vermicomposting. Considerable work has been going on on the science of vermicomposting, especially in terms of biotic and *Author for correspondence: Mobile: ; Tel: /98 prof.s.a.abbasi@gmail.com abiotic factors, which influence vermicast production, earthworm growth and fecundity 4,5. Studies are also being increasingly reported on the vermicompostability of newer substrates and newer earthworm species 6,7. But the aspects of vermicomposting process design, control, operation, and optimization are by-and-large still unaddressed. Definition of Vermicomposting The term vermicomposting is generally believed by many authors, including the present ones, as denoting the process in which earthworms are made to feed upon certain substrates in order to convert the substrates into vermicast 4-8. Of the three main categories of earthworms epigeics (humus feeders, surface dwelling), anecics (geophytophagous, soildwelling which construct vertical tunnels), and endogeics (geophagous, soil-dwelling which construct horizontal branching burrows) the epigeics are most suited to vermicomposting 4-7, but several species of anecic earthworms are also very effective, especially in the vermicomposting of phytomass and waste paper However, several authors also use the term vermicomposting to denote processes in which earthworms are added to raw or partly decomposed

2 178 INDIAN J BIOTECHNOL, APRIL 2009 waste and the act of natural degradation of the waste at ambient temperatures (usually 30±5 C), purportedly assisted by the worms is deemed vermicomposting. The end product, which is usually a mixture of still incompletely stabilized waste and vermicast, is dubbed as vermicompost These authors have conducted extensive studies on composting 7 as well as vermicomposting 6,9-15,24-26 from the point of view of not only the biological and chemical reactions occurring during these processes but also the process engineering. The aim has been to identify the essence of the two processes in terms of abiotic and biotic factors that are involved, the nature of biological, biochemical and chemical reactions taking place, process conditions, reactor operation strategies, and process control parameters. Based on these studies and that of vermicomposting as reported in literature where earthworms were used to assist composting, it is found that composting not only entails totally different process control parameters and operational strategies than vermicomposting, but it also significantly differs in terms of the governing biological and physico-chemical aspects. If the two are attempted in a single reactor, both proceed suboptimally. Composting Process Composting is a quintessential batch process and can not be operated even in semi-batch mode; let alone in a continuous fashion 7, It comprises of the following steps: i) Setting up windrows in which the substrate to be composted is laid out in layers about 15 cm thick, alternated with thinner layers (5 cm) of microorganism-rich material such as cow dung or sewage sludge 30. ii) Providing adequate moisture (60% of the reactant mass) through sprinkling of water over the windrow, and a passive or active means of aeration. In the former, open-ended, perforated pipes are partially inserted into the windrow at different points. Air flows into the open ends of the pipes and through the windrow because of the chimney effect created as hot gases rise upward out of the windrow. In the latter, the outer ends of the pipes are connected to aeration devices with which air is forced into the composting pile. iii) Covering of the windrow with layers of cohesive clay or open plastic sheets. This sets the aerobic decomposition of the substrate in motion; the process being exothermic, it gradually lifts the temperature of the pile to 55 C or higher. Then, as the availability of the substrate in the aerated zones as also oxygen in those zones declines, further decomposition is reduced and the temperature begins to fall. In forced air composting systems, the aeration is shut off once the temperature goes beyond 55 C and, instead, a blower is used to push off hot gases so that the pile temperature does not shoot up so high as to kill most of the microorganisms other than thermophiles. If that is allowed to happen, the composting would be incomplete and the product unsatisfactory. iv) Turning the windrow to rejuvenate the process by bringing in previously un-decomposed or partially decomposed substrate in contact with microorganisms and air. This is done mechanically either manually with showels or through turning machines. After a freshly turned windrow, in which the moisture has been replenished is covered again, the aerobic decomposition generates another process wave in which the temperature gradually rises to a peak and then begins to decline after a brief plateau. In forced air composting system, the substrate isn t turned but aeration is restarted once the blower cools the pile to near ambient temperature. This leads to restart of composting and the generation of the next temperature wave. With the passage of time, during which the substrate gets more and more stabilized, the peak of temperature attained by each wave gradually comes down. In d, depending on the nature of substrate and efficiency of process operation, the peak pile temperature falls to ~40 C. This is illustrated in the results of the composting of acacia leaf litter by the authors (Fig. 1A). Fig. 1 Typical patterns of temperature waves in the composting process (A), illustrated here with reference to the composting of the acacia leaf litter. When the same substrate is vermicomposted the reactor temperature remains more or less constant (B), fluctuating within a narrow range of ±2 C.

3 ABBASI et al: MODELING & DESIGN OF VERMICOMPOSTING SYSTEMS 179 v) Completion of the predominantly microbial decomposition phase, which occurs after the steps ii to iv have been repeated 3-4 times or more and the pile temperature becomes more or less constant. vi) The curing phase wherein the compost pile is left undisturbed for 2-4 wk. The curing phase provides the time required for (i) degradation of the more refractory organics, (ii) overcoming the slowing effects imposed by kinetic rate limitations, and (iii) re-establishing lower temperature microbial populations, which may be beneficial in maturing the compost, metabolizing phytotoxic compounds, and suppressing plant diseases 31. Mechanization of the composting process and the variations in terms of particle size and chemical characteristics of different substrates have brought in different means of aeration and some pre-composting steps are also used for some substrates, such as the drum system, in which the substrate is fragmented and anaerobically digested to make it more compostable. But, in essence, the composting process remains as described in steps i to vi above. The distinguishing features of the composting process are: a) It is a truly batch process and can not be made even semi-continuous. Once composting has begun in a pile, no fresh substrate should be added to it as it would only hinder the ongoing process. b) Mechanical turning of the reactor contents at periodic intervals is absolutely essential for the composting to be complete. c) The process accompanies sharp rise in temperature which is essential as it destroys most of the pathogens and seeds of weedy plants. Vermicomposting Process In contrast to composting, the vermicomposting process can be operated in batch, semi-batch, and even continuous modes. The time taken to complete the composting of a substrate is of the order of 6-8 wk, whereas the vermicomposting is accomplished as quickly as the time it takes for a feed to be ingested by an earthworm, digested, and excreted just a few hours! Vermicomposting involves the following steps: i) Ingestion of the substrate particles by the earthworm. ii) Physical size-reduction of the ingested particles by the action of the earthworm gizzard, which is located next to the worm mouth. iii) Digestion of the substrate as it passes through the earthworm body and is acted upon by the microorganisms and enzymes present in the earthworm gut. iv) Exit of the subtracts as vermicast a few hours after the ingestion. The number of hours depends on the nature of the substrate, the worm species, and the length of the worm body. In general, earthworms of shorter body length take lesser time to deliver the vermicast than longer bodied earthworms, and epigeic earthworms process their feed faster than the anecics or the endogeics. In contrast to composting, vermicomposting does not accompany exothermic reactions; hence, there is no measurable rise (or, for that matter, fall) in temperature in the vermireactors (Fig. 1B). Nor do vermireactors need supplementary aeration. In fact, most species of earthworms are not able to thrive at temperatures exceeding 40 C; hence, if put in an under-composting substrate pile, most worms would perish when the pile temperature rises (as it is must to ensure proper composting) to greater than 55 C. Also, in contrast to composting, there is no necessity for the periodic turning or mechanical mixing of the substrate. Rather, the natural moving and burrowing activities of the worms accomplish the task of keeping the substrate well-mixed. These activities of worms also keep the substrate adequately aerated and, hence, prevent anaerobic regions from developing in the vermireactors. All things considered, composting requires labour and machinery to accomplish fragmentation, mixing, and aeration, while in vermicomposting these tasks are by-and-large accomplished by earthworms. Vermicomposting of Compost Composting of a substrate prior to vermicomposting facilitates the subsequent vermireactor operations and improves product quality in several ways: The thermophilic phase, which is an essential component of the composting process, kills most of the pathogenic organisms. This not only ensures survival and growth of the earthworms when the compost is subjected to vermicomposting but also ensures a much more pathogen-free vermicompost. The thermophilic phase also kills seeds of weedy plants; an advantage which is carried over to the vermicomposting stage.

4 180 INDIAN J BIOTECHNOL, APRIL 2009 On the other hand, the mesophilic digestion that takes place during the post-thermophilic curing phase bestows upon the compost a rich and diverse community of microorganisms. These microorganisms, which are known to be beneficial to soil and plants, have the ability to kill several pathogens due to the former s ability to generate pathogen inhibitors. In vermireactor, these microorganisms are taken as feed by the earthworms digesting some of the species and making other species proliferate before excretion as part of the vermicast. Composting of any substrate is accompanied by a loss of carbon (which is converted to CO 2 by microbial action), but little or no nitrogen is lost; neither other nutrients are lost. This not only improves (reduces) the C:N ratio of the substrate but also, effectively, enhances the concentration of nutrients in the compost relative to the uncomposted substrate. This situation favours the subsequent vermicomposting as greater concentration of nutrients now exists in the substrate and in better bioavailable forms. The brisk microbial action as also the periodic turning that is done during composting breaks down larger lumps in the substrate, making it more easily ingestible by the earthworms. Most significantly, composting stabilizes all the rapidly biodegradable parts of the substrate. When the substrate happens to be vegetable waste, poultry waste, or slaughterhouse waste, which have a large proportion undergoing rapid biodegradation, anaerobic zones set in which prove very hazardous to the earthworms 32. Moreover, rapidly biodegrading fruits, vegetables and animal wastes generate volatile fatty acids and other decomposition products, which severely stress the worms, killing many. By and large, the environment within any rapidly biodegrading substrate is hostile to earthworms. This situation gets rectified to a significant extent if such substrates are first stabilized through compositing. In several studies reported earlier, the authors and coworkers have used well-stabilized composts of municipal solid waste, and different weeds as substrates for generating vermicompost. Earthworms not only feed voraciously on compost to produce vermicompost but display good growth and fecundity. Moreover during its passage through the earthworm gut, the compost acquires microflora, enzymes and hormones known to be beneficial to plants. Apparently, the complex organic matter which survives composting also gets digested to a significant extent during vermicomposting. Due to all these reasons, vermicompost has turned out to be a better fertilizer than the parent compost during several controlled experiments Pointers for Modelling, Design and Operation of Both Processes From the foregoing, it is clear that composting is to be modeled as a strictly batch process in which the contents are homogenized intermittently (at gaps of 7-10 d) and the oxygen deficit caused in the course of the aerobic fermentation is made good by surface aeration effected during the turning of the reactants or/and by pumping in air. Vermicomposting, on the other hand, approximates a continuously stirred tank reactor (CSTR) wherein the contents are mixed and kept aerobic by the tunneling and burrowing action of the earthworms as also the stirring of the substrate caused due to the movement of the worms towards substrate particles. The species of earthworms (epigeic and anecic) which are usually employed in vermireactors deposit their cast (which is the reactor output the vermicompost) either at the top of the reactor contents (as Eudrilus eugeniae or Eisenia fetida do). In either case, the product is fairly well-separated from the substrateworm zone. This behavior contributes to the CSTRlike operation of the vermireactor. There is another facet unique to the vermireactors, i.e., in a vermireactor the earthworms function as reactors within a reactor in the sense that the entire processing of the substrate from the reactant to the product occurs within each worm. It follows that the boundary condition for the minimum time a vermireactor will require to produce vermicast is the time taken by the concerned species of earthworm to digest a substrate. A number of medium and large scale vermicomposting units are in operation across the world Of these, some are run in batch mode but several are supposed to be continuous flow units 41,42. Yet all of these are based on solid retention times (SRTs) of atleast 1 month; more commonly 2-6 months. This is surprising because the longest duration that the species of earthworms normally used in vermicomposting take to generate vermicompost from any substrate is less than 24 h. Moreover, if the vermicast is harvested early and air dried, it becomes fairly stable with little subsequent biodegradation. This being the case, it may not be in the interest of

5 ABBASI et al: MODELING & DESIGN OF VERMICOMPOSTING SYSTEMS 181 product quality and process efficiency to keep the vermicast (which is produced daily) unharvested for long durations as is being done in the existing largescale systems at present. The corollary of this conceptual model is that other forcing factors being equal, the vermireactor efficiency will be controlled by access-to-food of earthworms. Lesser the distance each earthworm must travel before reaching the food and lesser the competition it faces each time it sets out to ingest a food particle, closer will be the time taken for vermicast production to the minimum time it takes for the given species to process the given substrate (time taken from ingestion to excretion). But the rate of vermicast production, i.e., mass fraction of the substrate converted to vermicast per day, is a function of earthworm density because more the number of worms simultaneously feeding upon the substrate, greater the rate of vermicast production. But the earthworm density can be increased only upto the level below which competition for food does not become a restrictive factor. Hence, it is important to optimize the earthworm density and the surface-tovolume ratio of vermireactors to ensure easiest possible access to substrate by the largest possible number of animals. Studies 43,44 carried to test this concept have revealed that vermireactors of high surface-to-volume ratio yield more vermicast per unit time than rectors of lower surface-to-volume ratio, evidently because shallower rectors prevent crowding and also reduce the vertical movement needed by each worm to deposit its vermicast. It also follows that highly unstable and rapidly selfdegrading biowaste, such as, the one generated in vegetable and fruit markets, restaurants, food processing industries, etc, should be first composted properly and then subjected to vermicomposting or else the high oxygen demand of the waste would stress and even kill the worms by creating anaerobic zones within the substrate mass. Even if significant earthworm mortality does not occur in such systems, the incompletely composted fraction of the substrate in a substrate-vermicast mixture can be harmful to plants. This is borne out by several studies in which inhibition of seed germination, root destruction, and inhibition to plant growth have been observed when immature compost was applied 45,46. Moreover, as detailed above, several aspects of the composting process are antagonistic to the vermicomposting process; hence, the operation of the two simultaneously in the same reactor will make both of them inefficient and leave both the processes eventually incomplete. Acknowledgement Authors thank Department of Biotechnology, Government of India, New Delhi, for support through their project grant. 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