4.1. Temperature and volume evolution during composting
Temperature patterns along the 92-day period were similar for the three composting types and very close to the ambient temperature. The observed temperatures were relatively low (12 to 26 ºC), probably because the experiment was carried out in winter. The low waste volume disposed in the digesters (about 14 L) can also account for these results, since surface/volume ratio of composting waste is inversely related to composting temperature . It is worthy to remark that the volumes tested in the present study are suitable for medium- to small-scale composting systems.
The maximum temperature observed was 28.8 ºC, which is about 20 ºC below the expected for a common thermophilic phase, in which high temperatures contribute to eliminate pathogens. However, the invertebrates are temperature sensitive, and the temperature range from 20 to 28 ºC must be appropriate for vermicomposting . On the other hand, vermicomposting has been shown to reduce pathogens when compared to traditional high-temperature composting, which appears to be related to earthworm digestion . Therefore, the temperatures obtained in our study were appropriate to the survival of the invertebrates and, at the same time, to produce humus and leachates of high quality and maturity.
Volume loss was higher in vermicomposting (51%) than millicomposting (43%), indicating higher efficiency of earthworms in reducing organic waste. Similarly, previous works had related volume losses of 60% and 40% for organic waste treated by vermicomposting and millicomposting, respectively [20, 32]. These results reinforce the potential use of this type of composting in households, which waste a large part of the food produced for consumption [13, 14].
Volume profiles in the three systems were also very similar. At the beginning, volume loss was rapid, and the temperature inside the digesters was higher than the ambient temperature thanks to the activity of microorganisms in decomposing easily degradable organic matter. After volume stabilization in approximately 40 days, there was a brief volume increase, maybe related to the manual revolving of the waste to promote aeration. From the 72nd day on, volume continued to slowly decrease, reflecting the intensification of humification and final maturation of the composts. The performance of the invertebrates was more evident at the end of the 92-day period, resulting in the highest volume loss in vermicomposting, highest leachate maturity in millicomposting, and gains in chemical quality, which will be discussed below.
4.2. Nutritional content of solid compounds (humus)
The nutritional content was higher in V, when compared to those of M and C. However, the three composting types produced nutrient-rich organic compounds of neutral pH and C/N ratios < 20, indicating their potential as fertilizers. In fact, these compounds comply with the Brazilian legislation for organic fertilizers .
Calcium contents were higher in V and M, when compared to C. This Ca content increase may be explained by the secretion of calcium carbonate granules produced by earthworm calciferous glands around their esophagus during vermicomposting [33, 34]. In millicomposting, the incorporation of calcified parts of the exoskeletons, such as millipede cephalic capsules, may respond for Ca increase in M products . This addition of exoskeleton parts (exuviae) may occur during ecdysis and after death.
Excretion of calcium-rich feces by microorganisms in the intestines of the invertebrates can also contribute to the Ca increase in the humus . Calcium is probably in the form of carbonates and/or oxides. Earthworm secretions, for example, contain calcium carbonate, calcite, aragonite, vaterite and amorphous calcium carbonates [33, 34]. These Ca-rich substances are suitable for agricultural use, especially when applied to acid soils, such as the Oxisols, which are common in tropical areas and predominate in Brazil.
Mg and K2O contents were higher in V than in M, whereas P2O5 and S were higher in V than in C. Nitrogen contents were similar in the three composting types. Such results differ from those reported by Thakur et al. , who found higher K2O and P2O5 contents in the milli- than vermicomposting, and higher N and P2O5 contents in milli- and vermicomposting than in traditional composting . These apparent contradictions seem to be related to the type of materials used in composting, since these authors used a mixture containing animal manure (soil, cow dung, vegetable skins, leaves, grass and rice straw), which are rich in N, probably resulting in low C/N ratios. Regarding the invertebrates, they used different species, such as the diplopoda Harpaphe Haydeniana for millicomposting and the earthworm Eudrilus eugeniae for vermicomposting. In addition, this previous study did not replicate the composting experiments, and the conclusions were drawn from a single set of experiments. Therefore, it is not possible to check whether the differences reported by them are significant.
4.3. Potential effects of the types of waste in composting with invertebrates
There are some evidences of the influence of the type of waste in the efficiency of composting with invertebrates  compared millicomposts (prepared using millipedes Arthrosphaera magna) to vermicomposts reported in the literature. They observed that final products of the Areca waste vermicomposting yielded higher pH values and a lower organic carbon content; however, these differences did not occur when composting coconut waste.
Differences in composting mediated by earthworms and millipedes may be related to their food preferences, which in turn, depend on the use of resources from which they evolved. Earthworms are adaptable to a wide range of environments, since there are more than 3,500 species described. Epigeic earthworms, as Eisenia foetida, are “soil-formers” living at the interface between the forest floor and the soil surface; they can consume decomposing organic matter (vegetal debris and animal feces) . Therefore, they are able to consume waste with high N concentration, such as animal manure. In symbiosis with their intestinal microorganisms, they produce and excrete coprolites, which are stabilized-organic-matter rich feces (high humic substances content) [15, 35].
Diplopodes, on the other hand, are litter transformers living on the forest floor they are saprophages specialized in the consumption of vegetal debris, such as leaf, grass and wood litter [23, 39] of high C/N ratios and structural carbon contents in the form of cellulose and lignin. During digestion, millipedes crush, moisten and inoculate the material with microorganisms. In this case, microbial activity in the feces after excretion is important to the complete detritus degradation .
Both types of invertebrate act as catalysts during composting and respond to the quality of the substrate. For example, there is a greater biomass of both earthworms and millipedes in litter and soil in forest plantations of low C/N ratios than in plantations of high C/N ratios . However, millipedes are almost restricted to the consumption of litter layers, while the earthworms access a wide variety of resources both on the litter and on the soil surfaces. Thus, millipedes may be more sensitive than earthworms to the palatability of the decomposing materials, which would partly explain our results.
Indeed, literature indicates that millicomposting should be performed only with vegetal waste, simulating what happens in forest soils [20, 21, 32, 37]. Since millipedes’ food preferences are plant waste, their use can be optimized in urban composting of pruning waste, which has similar characteristics to litter. Despite the material used in the present study was of vegetal origin, half of it was composed of fresh vegetable waste from local fruit and vegetable markets. This type of material (water- and N-rich, of low C/N ratios) may have favored the activity of earthworms to the detriment of millipedes. On the other hand, the material of high C/N ratios used in this study was rich in cellulose, hemicellulose, and lignin (mixture of sugarcane bagasse and sawdust), being more difficult to degrade. Apparently, this material was unattractive to the invertebrates, explaining the leftovers found in the digester boxes at the end of the experiments.
Further studies should evaluate the potential use of earthworms and millipedes in degrading urban solid waste of different chemical characteristics (C/N ratios, proportions of cellulose, lignin, fibers and polyphenols), to understand their waste-dependent efficiency. Vermicomposting is considered an efficient method to produce high quality composts from different types of animal manure [7, 15, 16]. It could contribute to the degradation of waste containing feces of domestic animals. On the other hand, the composting of pruning waste can be optimized by reducing the particle size and/or adding waste of high energy content, such as food waste . Therefore, the use of millipedes plus food waste could be a good technical solution for the composting of pruning waste.
4.4. Degree of maturity and quality of vermi- and millicomposting
Stability and maturity of composting products evolve together, although they are not quite the same thing. Stability is related to the activity of decomposing microorganisms, while maturity refers to the potential of the composting product to promote plant growth, which, in turn, depends on the stabilization of organic matter. Maturity can be evaluated by pH (from > 7.0 to ≤ 8.0) and C/N ratios (from < 10/1 to > 20/1) [7,36). The pH of matured composts tends to be neutral to alkaline, reaching values higher than 8.0 due to the formation of humic acids that react with basic chemical elements forming alkaline humates . The C/N ratio considered as indicator of maturation vary: the Brazilian legislation adopts C/N ratios ≤ 20 for organic composts . Despite the lack of consensus on the ideal minimum value, this parameter reflects the capacity of microorganisms to degrade organic material.
In composting, stability begins after the initially fast mass loss. At the beginning of the stability phase, the transformation of materials that are difficult to degrade, such as lignin, still occurs, giving rise to the complexification of organic matter. Thus, small mass loss oscillations may occur in the stability phase, during the maturation of the composts. In this process, humic acids increase, fulvic acids decrease and the C/N ratio increases . This improves the quality of the compost. Therefore, long maturation phases tend to generate higher quality composts, with more stable organic matter. It is not possible to indicate in this study when exactly composts reached maturity, because the humus quality was only assessed at the end of the experiments. However, we can affirm that three months was time enough to achieve maturation and to produce high-quality composts, appropriate for use as fertilizers. The humus obtained in this study complies with the maturity degree parameters pH (7.1–7.7) and C/N ratio (15–16) which, according to Bernal et al. , they must be around 6.6 and 7.8 and 10.8 to 19.3, respectively.
4.5 Degree of maturity and chemical quality of liquid compounds (leachates)
The species of invertebrates used in composting affected the quality of leachates. The M leachates yielded higher N (in the form of NO3−) contents, higher maturation indexes (lower NH4+/NO3− ratios), and lower alkalinity. The V leachates yielded higher K+ contents and a very alkaline pH value (9.2). Besides, V leachates yielded very low N contents and almost half in the form of NH4+, which increased the NH4+/NO3− ratios of V leachates relatively to those of M and C leachates. Furthermore, four of the six V leachate samples did not yield any NO3−, probably because in such alkaline conditions, nitrogen was completely lost . These differences may reflect the influence of the metabolism of each invertebrate, related to processes occurring in their digestive tracts, as discussed in Sect. 4.3.
The V products (humus and leachates) were the most alkaline. The pH values obtained for the three types of leachates were moderately alkaline, but higher than those reported in the literature, which varied between 6 and 7.9 [8, 9, 43]. This can be related with the type of composting material, since in those previous studies waste of animal origin was used. Humus and leachate pH and C/N ratios are usually higher in composts derived from vegetal waste [8, 9, 16]. Although pH increases indicate maturity, very high pH can be harmful for vegetal growth. Liquid fertilizers have optimal pH values varying from 5.5 to 8, because for pH > 7.5, nitrogen losses can occur due to NH4+ volatilization . Consequently, high pH values can explain the low N levels obtained for their leachates. Considering that NO3− is the most available N form for vegetation, our results point out that M leachates are more adequate as liquid fertilizers, since they yielded higher NO3− contents and lower (almost neutral) pH. Nonetheless, all the leachates obtained in this study are for use as agricultural fertilizers. Provided that it is diluted, V leachates can be used as fertilizer. Indeed, there are reports of their positive effects on germination, plant growth, and fruit quality [9–11, 43]. On the other hand, high concentrations can have inhibitory effects because of excessive alkalinity and salt concentrations, and dilutions above 10% are not recommended .
Potassium concentrations were low in solid composts and high in leachates, thanks to its extreme mobility. The K contents are 2 to 4 times higher in leachates than those reported in the literature, while phosphorus contents were lower [8, 9, 44]. This probably reflects the high K content of the sugarcane bagasse used in this study , and the fact of using waste exclusively of vegetal origin. Previous studies support this explanation, since they report differences in chemical quality according to the type of waste used in composting, suggesting that initial C/N ratios are decisive [8, 9, 46].