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Vermicomposting of wastes from paper pulp industry (1996)

Antonín SLEJŠKA

Vermicomposting is a type of composting, which is typical by using some species of earthworms, that intensify the process. Differences between composting and vermicomposting are demonstrated in table 1.

The two most commonly used earthworms are epigeic species Eisenia fetida and Eisenia andrei. Other suitable species include Lumbricus rubellus, Eudrillus eugenie and Perionyx excavatus. The latter two species are from Africa and Asia and cannot withstand low temperatures (Edwards 1995). Between E. fetida and E. andrei is only small difference. E. andrei is uniformly pigmeted, while E. fetida is banded (Sims, 1983; Avel, 1937; Jaenike, 1982; André, 1963; Bouché, 1972).

E. fetida is a prolific breeder, how You can see from Fig. and Table 2.

Eisenia fetida and E. andrei can be used as organic waste decomposers, as bait, and are even considered to have potetial as a protein source (Reinecke 1991). Both species can consume organic residuals very rapidly by passing them through a grinding gizzard, an organ that all earthworms posses. The earthworms derive their nourishment from microorganisms that grow upon the organic materials. Laboratory studies have shown that only a portion of soil microorganisms can support weight gain of earthworms, whereas others do not support growth or cause toxic effects (Newhauser et al, 1980; Morgan, 1988; Flack, and Hartenstein, 1984). Edwards and Fletcher (1988) concluded that the selective microbial groups which are of nutritive value to earthworms are, in decreasing order of importance, fungi, protozoa, algae, bacteria and actinomycetes (=actinobacteria). At the same time, they promote further microbial activity in the residuals so that the fecal material, or “casts” that they produce, is much more fragmented and micobially active than what the earthworms consume. During this process, the important plant nutrients in the organic material - particularly nitrogen, potassium, and calsium - are released and converted through microbial action into forms that are much more soluble and available to plants than those in the parent compounds.

The retention time of the waste in the earthworm is short. Worms can digest several times their own weight each day, and large quantities are passed through an average population of earthworms. But in practise they biodegrade from a quoter to a half of their own weight per day. The amount of consumed substrate depends on environmental conditions and properties of substrate (pH, rH, temperature, humidity, nutritious value of substrate etc.) (Zajonc 1992).

In the traditional aerobic composting process, the organic materials have to be turned regularly or aerated in some way in order to maintain aerobic conditions. This often may involve extensive engineering to process the residuals as rapidly as possible on a large scale. In vermicomposting, the earthworms - which survive only under aerobic conditions - take over both the roles of turning and maintaining the organics in an aerobic condition, thereby lessening the need for expensive engineering.

The major constraint to vermicomposting is that, in contrast to traditional composting, vermicomposting systems must be maintained at temperatures under 35°C. Exposure of the earthworms to temperatures above this, even for short periods, will kill them. To avoid such overheating requires carefull management. Earthworms are active and consume organic materials in a relativly narrow layer of 15 to 25 cm bellow the surface of a compost heap or bed. The key to succesful vermicomposting lies in adding materials to the surface of piles or beds in thin, succesive layers so that heating does not become excessive. The heating, however, should be sufficient to maintain the activity of the earthworms at a high level of efficiency. Generally speaking, adding two or three cm of material every day or two is usually enough, depending on the feedstock (Edwards 1995).

The processing of organic materials occurs most rapidly at temperatures between 15°C and 25°C and at moisture content of 70 to 90 percent. Outside these limits, earthworm activity and productivity, and thus the rate of waste processing, falls dramatically.

Earthworms are very sensitive to ammonia, salts and certain other chemicals. However, salts and ammonia can be washed out of the organics readily or dispersed by precomposting. Earthworms have many natural enemies: thrushes, gulls, moles, shrews, ground beetles, earwigs, centipedes, mites and others (Zajonc 1992). But they can not be detrimental to earthworms with the right managing of vermicomposting technology.

The traditional methods of vermiculture are based on beds or windrows on the grounds containing materials up to 50 cm deep, but such methods have numerous drawbacks. They require large areas of land for large-scale production and are relatively labor intensive, even when machinery is used for adding materials to the beds. More importatly, such systems process organics relatively slowly, taking anywhere from 6 to 18 months to complete. There is good evidence that a large proportion of the essential plant nutrients in a relatively soluble form are washed out. A significant proportions of these same nutrients can volatilize during such a long processing period. Such nutrient losses are undesirable, particularly in relation to groundwater pollution, and result in a poor product.

Other system use bins or larger containers, often stacked in racks. Although container methods and other small-scale systems are widely used in homes and domestic projects (Appelhof 1982), they have drawbacks on a larger scale. They require considerable handlihg and lifting machinery, and also involve problems in adding water and aditional layers of materials in ways essential for maximum productivity.

Much more satisfactory techniques use containers raised on legs above the ground. These allow feedstock to be added at the top from mobile gantries and collected mechanically at the bottom through mesh floors using breaker bars. These methods range from relatively low technology systems using manual loading and collection, to completely automated and hydraulically driven continuous reactors.

Vermicomposting can break down organic residuals into valuable, finely divided plant growth media with excellent porosity, aeration and water holding capacity, rich in available nutrients with superior plant growth characteristics. In side-by-side plant growth trials at Rothamsted and elsewhere, involving 25 kinds of vegetables, fruits or ornamental plants, vermicomposts outperformed both traditional composts and commercial plant growth media in almost every experiment (Edwards 1995). This may be explained partially by circumstantial evidence that vermicomposts have a better structure and may contain plant growth hormones, enhanced levels of soil enzymes, and high microbial populations.

The major drawbach of vermicomposts is that organics do not go through a high temperature phase, so if materials containing pathogens are used, they may need an additional precomposting phase or sterilization process, to ensure that the pathogens are killed. There is, however, considerable scientific evidence that human pathogens do not survive the vermicomposting process (Edwards and Bohlen, 1995).

Apropriate substrate for vermicomposting is usually made by mixing of different materials with the aim of countrebalancing of negative properties of single ingredients. Suitable materials for vermicomposting are for example: cow manure, pig slurry, rabbit manure, poultry slury, straw, activated sludge, paper sludge etc. Manures are natural inoculants of macro and microorganisms and contribute some elements scarse in paper sludge. Activated sludge can be mixed with paper sludge, as well as manures, with similar results.

In experiments with vermicomposting of paper sludge enriched with some manures (Elvira 1995) biotransformation of the pulp/paper mill sludge developed in two stages. In the first, called the destabilization stage, polysaccharides (hemicellulose and cellulose) degraded into CO₂, biomass of macro and microorganisms and unassimilated carbonate intermediaries and simple products which easily break down. These hydrolysis processes resulted in a reduction of the total organic carbon causing an incrrease in the concentration of nitrogen and other nutrients, and consequently decreasing the C:N ratio to < 20:1. The fiber content decreased (10 to 20 percent final crude fiber) making the product rich in digestible components. In tight relation to these processes, the amount of extractable carbon increased, not only the phenolic part (carbohydrates and other intermediate metabolites) but also the humified part (fulvic and humic acids). As a result, the final product had a higher humification rate (10 to 20 percent humified organic materiel to total carbon).

In the second or stabilization stage, simple compounds completed their mineralization or gave rise to a higher amount of biologicaly active and humic stable substances through the processes of biosynthesis or neoformation. The products obtained in this way were suitable for application in agriculture as organic amendment or fertilizer.

The presence of earthworms is not required for the destabilization stage. However, when the microorganism inoculant, naturally added with organic additive, is supplemented by introducing earthworms, optimization of the transformation has been observed. In other words, the activity of earthworms speeds up the breakdown of polysaccharides, favoring the humifying process and improving the quality of the final product.

A great problem of paper/pulp wastes are heavy metals. For example in biosludge from Štětí paper mill average amounts of cadmium and zinc exceeds the maximum limits for composts of the first class. In paper sludge only zinc overlaps his limit (Johanovský 1996; ČSN 46 5735). This fact is even more serious, because during composting the relative amount of heavy metals is increasing, owing to decrease of organic matter. There are two ways of lessening of quantity of heavy metals in substrates. The first eliminates heavy metals from substrate by means of electrokinetic processes (e.g. electrophoresis, electroosmosis, electrolysis and diffusion) (Jizba 1995). But these methods are for this case too expensive. The second way rests in addition of material with low content of the problematic element.

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