In general the following vermicomposting systems are used the world over, for volume reduction, extraction of organic load, cost and energy reduction and rapid processing. Any of these systems may be adopted for vermicomposting depending on the availability of space, nature of waste or bedding material, quantity of waste to be processed etc.
A.Windrow system
This system deals with construction of windrows under shade to avoid direct sunlight. This method involves spreading out a layer of worms and bedding on the floor or ground to start(Fig.6).The first layer of a new windrow should be 10 to 15cm high. Earthworms can be reared at a production nursery or rectangular boxes prior to their inoculation in the windrows. The worms feed from the bottom till the top of the bed. The windrow has to be monitored daily and when signs of surface feeding are noticed, another 7 to 10 cm layer of feedstock can be added.
Thick layers of feedstock are avoided because they impede oxygen penetration into the windrow. This can cause the worms to migrate to the upper surface before lower layers are thoroughly digested, creating anaerobic fermentation. The windrows are irrigated with center post sprinkler up to twice daily to maintain optimum moisture content of 80% throughout the windrow. Now the material on top is removed to start a new row, and the material on the bottom is vermicompost. The new row can be started in a new location. Or it can be moved longitudinally by about 20' by dumping the worm inhabited material past the end of the row, digging out some vermicompost, and then shuffling some more worms over.
One of the problems with this method is that it requires some digging by hand. If the windrow is wide enough, it might be possible to drive into the side of it with a loader to remove some material off the top. But the rest of it will have to be forked off by hand because the loader will merely push it over and off the opposite edge of the row. No matter how careful you are, material will roll down the sides when digging off the top, and it takes some care to make sure that material does not end up getting mixed in with the finished product.
B. Wedge System
This is a modified windrow system and it maximizes space and simplifies harvesting because there is no need to separate worms from vermicompost. Organic wastes are applied in layers against a finished windrow at a 45o angle. The piles can be constructed inside a structure or outdoors if they are covered with a tarpaulin or compost cover to prevent leaching of nutrients. A front-end loader is used to establish a windrow 1.2 to 3 m wide by whatever length is appropriate. Spreading a 30 to 45 cm layer of organic materials the length of one end of available space starts the windrow. Up to 0.45 kg of worms is added per square meter of windrow surface area. Subsequent layers of 5 to 7.5 cm of organics are added weekly and preferably more addition in colder seasons. After the windrow reaches two to three feet deep, worms in the first windrow will eventually migrate toward the fresh feed. Worms will continue to move laterally through the windrows. After two to six months, the first windrow and each subsequent pile can be harvested. A variation of this method uses a migrating windrow. Here a row can be started as a windrow or as a layer and then grown into a windrow. A loader is used to feed one side of the row, keeping it the same height and length, but making it wider. After a while, a loader is used to remove vermicompost from the side opposite the one being fed. Later, the finished compost is removed from that same side again. The row migrates laterally as it is being fed along one side and harvested along the other. At any time, the feeding and harvesting sides can be switched to change the migration direction.
a. Pits,Tanks and Cement rings
Pits made for vermicomposting are 1 m deep and 1.5 m wide. The length varies as required. Tanks made up of different materials such as normal bricks, hollow bricks, asbestos sheets and locally available rocks were evaluated for vermicompost preparation. Tanks can be constructed with the dimensions suitable for operations. Scientists at ICRISAT have evaluated tanks with dimensions of 1.5 m (5 feet) width, 4.5 m (15 feet) length and 0.9 m (3 feet) height. The commercial biodigester contains a partition wall with small holes to facilitate easy movement of earthworms from one tank to the other (Nagavallemma, et al, 2004)(Fig.7)
Fig.7: Vermicomposting tanks made of ordinary bricks in a semi-commercial unit
Vermicompost can also be prepared above the ground by using cement rings. The size of the cement ring should be 90 cm in diameter and 30 cm in height.
Fig.8: Cement rings used as Vermicomposting tanks
(Source: ICRISAT, Hyderabad, India)
b. Commercial model
The commercial model for vermicomposting consists of four chambers enclosed by a wall (1.5 m width, 4.5 m length and 0.9 m height). The walls are made up of different materials such as normal bricks, hollow bricks, asbestos sheets and locally available rocks. This model contains partition walls with small holes to facilitate easy movement of earthworms from one chamber to another. Providing an outlet at one corner of each chamber with as light slope facilitates collection of excess water, which is reused later or used as earthworm leachate on crop. The four components of a tank are filled with plant residues one after another. The first chamber is filled layer by layer along with cow dung and then earthworms are released. Then the second chamber is filled layer by layer. Once the contents in the first chamber are processed the earthworms move to chamber 2, which is already filled and ready for earthworms. This facilitates harvesting of decomposed material from the first chamber and also saves labor for harvesting and introducing earthworms. This technology reduces labour cost and saves water as well as time.
Fig.9: A view of a semi-commercial vermicomposting unit
c. Beds or Bins
1 Top-fed type
A top-fed bed works within four walls and (usually) a floor, often within a building. If the bins are fairly large, they are sheltered from the wind, and the feedstock is reasonably high in nitrogen, the only insulation required may be an insulating “pillow” or layer on top. These can be as simple as bags or bales of straw. The reader should note that these beds were designed for vermiculture, rather than vermicomposting. Harvesting vermicompost can be most easily accomplished by taking advantage of horizontal migration. To harvest, the operator simply stops feeding one of the beds for several weeks, allowing the worm’s time to finish that material and then migrate to the other beds in search of fresh feed. The “cured” bed is then emptied and refilled with bedding, after which feeding is resumed. This is repeated on a regular rotating basis. If the beds are large enough, they can be emptied with a tractor instead of by hand.
2. Stacked type
One of the major disadvantages of the bed or bin system is the amount of surface area required. While this is also true of the windrow and wedge systems, they are outdoors, where space is not as expensive as it is under cover. Growing worms indoors or even within an unheated shelter is an expensive proposition if nothing is done to address this issue. Stacked bins address the issue of space by adding the vertical dimension to vermi- composting. The bins must be small enough to be lifted, either by hand or with a forklift, when they are full of wet material. They can be fed continuously, but this involves handling them on a regular basis. The more economical route to take is to use a batch process, where the material is pre-mixed and placed in the bin, worms are added, and the bin is stacked for a pre-determined length of time and then emptied. This method is used by a number of professional vermicompost producers.
The initial cost of setting-up a stacked-bin system is high. It requires a shelter, bins, a way to mix the bedding and feed, and equipment to stack the bins, such as a forklift. On a smaller scale, of course, this could all be done by hand. Another disadvantage arises when it comes time to harvest. As with the batch windrow systems, the worms are mixed in with the product and need to be separated. That requires either a harvester or another step in the process, where the material is piled so that the worms can migrate into new material.
D. Continuous Flow Systems
This system originally developed by Dr.Clive Edwards of the Rothamstead Agricultural Research Station is gaining popularity and has been adopted by many mid-scale operations. The efficiency savings offered by their continuous flow design increases with the amount of material processed.
This system design is now almost ubiquitous in commercial mid to large-scale vermicomposting systems. Each of these systems uses a relatively deep top-fed container, in which the composting mass sits upon a raised floor made from a widely spaced wire mesh. Worms are added to the system and food waste is added gradually, layered with bedding material. The system is continually fed until the bin is nearly full. The worms generally move upward through the feedstock/bedding layers and vermicompost is harvested from below by scraping or cutting a thin layer of finished material from just above the grill using a rake or a manually or hydraulically-operated blade.
Continuous flow systems offer several advantages to medium to large scale composting operations. They are relatively straightforward to construct and operate. They are labour-efficient in terms of operation and harvesting finished material. They avoid the need for expensive equipment associated with technical ‘in-vessel’ systems and the turning and screening of windrowed material. It should be noted that, despite the recent and increasing interest in this design, windrows are still the most common large-scale vermicomposting system in use. Continuous flow vermicomposting designs are arguably the most efficient systems available, in terms of time and labour savings. However, regardless of efficiency or ease of operation, there is no design that eliminates the need for careful monitoring and good system management, which may require considerable initial experimentation and familiarization.
Maintaining Continuous Flow
Continuous flow vermicomposting systems provide an ongoing flow of vermicompost that is easily removed from the system without disrupting the worm activity or requiring complex or time-consuming harvesting methods. Because of their operating efficiency, these system designs are becoming almost as popular as windrows for large-scale applications. However, like all vermicomposting systems, the continuous flow model poses several challenges.
In order to simplify some of the technical terminology, the worms most often used in vermicomposting are usually referred to as "surface feeders.”
They are generally presumed to only be active at or just below the surface. However, this is not always the case. Earthworms are oxygen breathing, moisture-loving animals that require organic material to be bacterially active before they eat it. In their natural environment, this is usually top few inches of soil or surface organic litter, such as leaves. In any system with a free flow of oxygen, monitored moisture level and abundant supply of decomposing organic material, earthworms may spread throughout the material unless the system is carefully managed. Earthworms may therefore be found anywhere within the continuous flow systems which meets their requirements.
One of the advantages to the continuous flow design is in the ease with which a continuous supply of vermicompost can be removed from the system. However, harvesting of the finished material should not begin until the system is nearly full of material. Many operators have found that, along with appropriate loading rates, a minimum depth of material in the system of between 12"-18" will help to ensure that few, if any, worms will be low in the bed and drop through, or fall out with the harvested vermicompost. Once fully charged, vermicompost then needs to be removed at a rate that maintains a relatively constant level of material in the system.
Feeding Rates
The precise loading rate (at which raw feedstock can be added to a worm bed to encourage the worms to concentrate at or near the surface) will vary depending on the feedstock being used, temperature, moisture levels and the density of the worm population. Proper loading rates require that new feedstock is not added until the majority of the previously added feedstock has been decomposed. Adding new feedstock too early means there can lead to a build-up of unprocessed material within lower layers. There will therefore be sufficient available food deeper within the container, instead of being concentrated immediately below the surface. The worms will then spread into all the available food areas. Worm movement in the lower levels of a flow-through system often causes vermicompost to drop through the mesh floor before it has been sufficiently decomposed. Also, when the system is harvested, worms remaining low in the material will fall through with the vermicompost and will either need to be separated using labour-intensive screening methods, or will be lost to the system. Most operators of continuous flow systems find that frequent additions of thin layers of feedstock (1"-2" deep spread across the surface) produce the best results. Feedstock is sometimes mixed with bulking agents like compost, shredded leaves, cardboard, paper or straw, or covered with an equally thin layer of these materials. Paper products are a preferred feedstock for earthworms, as they provide an easily accessible and digestible form of carbon.
Excessive Heating
Another of the challenges to any vermicomposting system, irrespective of size, is the potential for heating in the feedstock. Bacteria are the primary decomposers of raw organic matter and in oxygen rich system, water, carbon dioxide and heat are produced as a result of microbial activity. When raw material is added to the system, particularly in large volumes, the mass can support the activity of billions of bacteria. Bacterial activity can produce significant amounts of heat, which may be trapped within the system. Even a small volume of raw material can result in heating if it contains sufficient energy to support high levels of bacterial activity. This potential for heating complicates the assessment system loading rates. It should be recognized that a worm bed may contain thousands of different species of invertebrates and microorganisms, all of which playa vital role within the vermiculture ecosystem. The loading rate cannot therefore be based solely on the needs or capacity of a single organism in that system. Bacterial activity may have as much impact as the worm activity, as bacteria will have access to the feedstock first.
Overfeeding (in relation to the design capacity, the type of feedstock and/or the level of system activity) may generate sufficient heat to deter worm activity. Unless design modifications can be made, such as installing fans to remove excess heat, the loading rate will need to be decreased to a point where heating is not a problem, even if that means feeding less material than the worms are capable of processing.