Global Worming

Charles Darwin’s final book, entitled "The Formation of Vegetable Mould, Through The Action of Worms, With Observations On their Habits" (1881), made aware the great importance of earthworms in the breakdown of organic matter and their release of nutrients to the surrounding soil. Included in this work is a quote that emphasized his respect of earthworms;

Although the conclusion may appear at first startling, it will be difficult to deny the probability that every particle of earth forming the bed from which the turf in old pasture land springs, has passed through the intestines of worms.

Yet it was necessary to wait almost 100 years until Darwin’s notion of worms was adopted by the environmental engineering community and their application towards waste management technologies began to root. More recently, vermicomposting has been implemented to successfully process sewage sludge and solids from wastewater, materials from breweries, paper wastes, urban residues, food wastes, and animal wastes as well as horticultural residues from industrial processes.

Vermicompost is the excreta of earthworms, which is rich in humus and essential plant nutrients. Vermicompost improves soil structure, texture, aeration, water retention, and prevents soil erosion. Additionally, vermicompost is rich in cellulose decomposing micro flora and phosphorous solubility agents which further improve the soil environment. Vermicompost is free from pathogens, toxic elements, weed seeds and minimizes the incidence of pest and diseases when compared to traditional composting methods. Nonetheless, while vermicomposting can be practiced cost effectively in outdoor windrows using batch or continuous flow processes, more efficient, higher yield methods require continuous maintenance and monitoring in addition to acquired skills depending on available feed stocks and environmental conditions.

VERMICOMPOSTING REQUIREMENTS

Like other biological processes, the selection of worm species to colonize organic wastes naturally is dependent on; high rates of consumption, digestion and assimilation, ability to tolerate a wide range of environmental factors, high reproductive rates with short hatching time, and rapid maturation. Additionally, species should be strong, resistant, and survive handling.  Of the 2700 species of worms, only a few possess all these characteristics with Eisenia fetida or red wiggler (Figure 1) being the most common species for management of organic wastes by vermicomposting.

Figure 1 - Red Wigglers

Compost worms like E. fetida have a specific range of environmental and ecological requirements that must be met for them to thrive; adequate bedding and a food source, moisture and aeration, as well as protection from temperature extremes and predators. Since worms breathe through their skin, aeration and moisture levels are necessary conditions for survival, therefore high absorbency bedding with good bulking potential is required. The bedding is consumed along with feed stocks as it breaks down, however high nitrogen levels can result in rapid degradation with concomitant heating, creating inhospitable conditions. In order to control overheating, bedding materials such as municipal paper waste (balanced C:N) and cardboard with much higher carbon content are generally found to slow the degradation process when mixed or by themselves and are considered good bedding materials for their absorption and bulking properties. A list of common bedding materials, absorbency and bulking properties, as well as C:N ratios is presented below.

Ideally, moisture-content in conventional composting systems is 50-60%, whereas ideal moisture-content for vermicomposting is 80-90%, with an optimum of 85% moisture. Moreover, the average worm weight in addition to other variables increased with moisture content. Worms can survive in moisture content down to 35% but will quickly die below this point, therefore, bed cover is usually provided to help retain moisture levels in addition to monitoring as required.

The actual amount of food that can be consumed daily by E. fetida varies with a number of factors including the state of decomposition of the food. Red wigglers will eat almost any organic material, however, beef and dairy manures are the most commonly used worm feed stock. Food sources such as manures consist of partially decomposed organic material which can be consumed more rapidly than fresh food, although pre-composted food stocks will have lower nutritional value than raw form. The general rule-of-thumb is that worms can manage half their weight per day. In addition to manures, other common feed stocks include fresh and pre-composted food scraps, human waste, grains from by-product, corrugated cardboard, as well as fish and poultry offal.

While composting worms do need oxygen, their requirements are relatively low. Worms are known to survive harsh winters inside windrows by living on the oxygen available in the trapped water, however they operate best when bedding material is well aerated. Through migration, worms aerate their bedding in addition to transporting materials or turning the bed, which is one of the major advantages to vermicomposting over conventional methods – Little or no additional aeration is required, even in static beds.

Composting worms are mesophilic and while Eisenia fetida can survive as low as 0ºC, it is generally considered necessary to keep the temperatures above 15ºC for efficiency. Composting worms will not reproduce below 10ºC, however cocoons can survive extended periods of deep freezing and remain viable for years. Compost worms prefer a temperature range in the 20s (ºC), but can survive up to the mid 30s (ºc). Temperatures above 35ºC are lethal and will cause the worms to leave the area if possible.

There are other important vermicomposting parameters including pH, salinity, urine, toxic substances, diseases, and predators. Worms can survive in pH range of 5 to 9, however, worms prefer a pH of 7 or slightly higher with an optimal pH of 8.0. Bedding pH tends to drop over time. Alkaline feed stocks will tend to moderate pH towards neutrality, otherwise, the pH of the beds can drop well below 7. In the case where the food source or bedding is acidic, pH can be adjusted upwards by adding calcium carbonate.

Worms are very sensitive to salts and prefer salt contents less than 0.5%. Manure high in salt content may need to be pre-leached by running water through the material for a period of time. By the same token, manure from animals raised on concrete floors often contains excessive urine which should be leached before use. Excessive urine can react with chlorine gas to produce toxic gases in the bedding. Additionally, different feeds can contain a wide variety of potentially toxic components. Some of the more notable are de-worming medicine, detergent cleansers, industrial chemicals, pesticides, and tannins from trees, such as cedar and fir which have high levels of these naturally occurring substances.

While compost worms are not subject to diseases caused by micro-organisms, they are still subject to predation by animals, insects, and to a disease known as “sour crop” caused by environmental conditions. Earthworms are a natural food source for moles and birds and can be a nuisance if outdoor windrows are used. It can be prevented by putting some form of barrier, such as wire mesh, paving, or a good layer of clay, under the windrow. Centipedes eat compost worms and their cocoons. Ants and mites can consume the feed meant for the worms. Also red mites are parasitic to earthworms whereby they feed on the blood or body fluid from worms and they can also suck fluid from cocoons. In these cases, the best prevention is to make sure that the pH stays at neutral or above. This can be done by keeping the moisture levels below 85% and the addition of calcium carbonate, as required. Sour crop or protein poisoning is the result of too much protein in the bedding. This happens when the worms are overfed. Protein builds up in the bedding and produces acids and gases as it decays. Keeping the pH at neutral or above will preclude the need for these measures.

VERMICOMPOSTING METHODS

There are two common vermicomposting methods in practice using either batch or continuous flow-through reactors. Reactors can be constructed as windrows, bins or beds, and can be modified to operate in a combination of continuous or batch modes. In batch systems, the bedding and food are mixed on beds or windrows up to 18 inches deep, worms are added, and then left to consume the materials until the process is complete. By contrast, in continuous-flow systems, feed and new bedding are incrementally added to the top layers on a regular basis and composted materials are harvested at the bottom layers.

Windrows can be constructed to operate as either batch or continuous flow systems. In batch operations, static piles of mixed bedding and feed are inoculated with worms and allowed to stand until the processing is complete. Static windrows do not need to be turned, however, they do need to be covered to retain moisture and guard against pests and predators. In continuous operation, food is placed on top of windrows and covered. Eisenia consume food at the food/bedding interface then drop their castings near the bottom of the windrow. Over time, a layered windrow is formed, with the finished product on the bottom, partially consumed bedding in the middle, and the fresher food on top.

Like windrows, bins and beds can be set up to operate as either batch or continuous flow systems. In a batch process, material is pre-mixed and placed in the bin, worms are added, and the bin is stacked for a pre-determined length of time. Continuous operation is similar to a top-fed windrow, however, bedding is contained within four walls, a floor, and is protected to some degree from the elements by cover. Stacked bins address the issue of space by adding the vertical dimension to vermicomposting and alleviate pest and predatory losses, however bins must be small enough to be handled on a regular basis and more maintenance is required. Since its development in the 1980s, the flow-through concept developed by Dr. Clive Edwards in England has been adopted and modified by several companies in North America to successfully treat a wide variety of materials mentioned above. In the flow-through system, worms live in raised boxes. Material is added to the top, flows through the reactor, and is harvested out through the bottom through screens. Using this type of process, it has been demonstrated that a 1000 square foot surface area can process 2 to 3 tons of organic waste per day.

BENFITS OF VERMICOMPOSTING

In addition to much faster decomposition rates, there are several other reasons that make vermicomposting a preferable method over standard methods. With vermicomposting, there is little to no need of aeration or turning unlike conventional methods. The end product of vermicomposting has greater soluble nutrient levels as well as higher microbial populations when compared to traditional methods.

Vermicomposting can be practiced indoors and outdoors, and is well suited for both urban and rural areas making it a good choice for homeowners looking to divert their organic waste from landfills despite available space or climate. In fact, in our case study, 250g of worms were allowed to reproduce under ideal conditions until the population was doubled at which point, the colony was transferred into a hybrid batch reactor maintained under conditions conducive with higher metabolic rates. Consequently, the system is capable of handling roughly 250g of organic household waste per day. While this is slightly less than our household produces, mature worms are regularly harvested and transferred back into a reproduction bin with subsequent reintroduction of the hatchlings into the reactor once mature. Eventually, the reactor will be able to manage all of a normal household’s waste from organic kitchen scraps, pet hair, dryer lint, eggshells, and we are continuously finding new materials to introduce into the process.

On an industrial scale, vermicomposting has been practiced as an in-situ soil remediation process whereby worms mine heavy metals from the soil or treat hydrocarbon contamination. Additionally, vermicomposting has been effective at treating municipal bio-solids and wastewater as well being capable of processing animal manures and other by-products from paper, distillery, and others. What’s more, worms can destroy pathogens including salmonella and E-coli, as well as parasitic worm eggs without further treatment. It’s no wonder, as Darwin pointed out over a century ago, worms remain the most effective natural cleaning agent known and will continue to find new applications towards landfill diversion, climate change, and waste treatment problems to name a few.