The higher bioconversion of the liquids (4.1–6.6%) compared to the sediments (≤ 0.4 %) shows that the dissolved nutrients in the liquids are more readily available than the non-dissolved nutrients in the sediments (although insufficient to promote larvae growth). Dumitrache et al.  reported that the carbohydrates in the secondary cell wall of poplar lignocellulose hydrolyze during fermentation. As the hydrolysis progresses, recalcitrant lignin appears at the surface, which eventually stops the hydrolysis, and thus, the bulk of the lignocellulose remains undigested. It is reasonable to assert that fermentation dissolves part of the PPBS, makes available nutrients that are otherwise unavailable, and thus increases digestibility. On the other hand, the bulk of the matter remains undissolved, and most of the nutrients are therefore unavailable to the larvae. Thus, a low bioconversion of the sediment occurs. The bioconversion of the liquid (4.1–6.6%) is substantially higher than that of previously reported data for municipal sewage sludge (0.2–2.3%)  and in the same range as coconut endosperm waste (CEW) (6%)  but lower than for fermented CEW (8–11.5%) [19, 21]. On the other hand, the bioconversion of the sediment (≤ 0.4%) is in the lower range of municipal sewage sludge (0.2–2.3%) , and the bioconversion of both the sediment and the liquid is lower than for food and feed waste (12.8–15.2%) and manure (7.1–11.3%) .
The concentration of VFA (Table 2) in the liquid of the fermented PPBS (1.9–2.6 g/l) increased compared to untreated PPBS (≤ 0.09 g/l). However, the weight of the larvae (0.7–0.8 mg) was as low as that of those reared on untreated PPBS (0.4 mg). Pang et al.  reported a substantial increase of prepupae weight at a VFA concentration of 15–26 g/l compared to 0 g/l. A possible explanation could be that the VFA concentration of the liquid used in this study is too low to affect the weight of the larvae. Furthermore, the concentration of sugars (Table 2) in the sediments of fermented PPBS (< 0.04 g/100 g) was as low as for untreated PPBS (< 0.04 g/100 g).
The individual dry weight of the larvae reared on the sediments and the liquids of fermented PPBS (≤ 1.0 mg, Fig. 1) was as low as for the larvae reared on untreated PPBS (0.4 mg) but much lower than for larvae reared on self-fermented CEW (30 mg) . Similar low larvae weights were found in an earlier study on larvae reared on untreated PPBS . The fact that fermentation of PPBS did not increase the weight of the larvae indicates that the fermentation applied here did not significantly increase the amount of available nutrients to a level necessary for growth of the larvae, even though some volatile fatty acids were produced. Another possible reason for the lack of growth may be that the PPBS contains substances that are growth inhibitory and that such substances still persist in sediment and liquid after fermentation.
A factor contributing to the low nutrient availability of PPBS may be the content of lignocellulose material of plant origin . Lignocellulose is recalcitrant to biodegradation  because lignin provides protection against microbial attack and oxidation . The lignocellulose is therefore largely intact after the fermentation, and the nutrients are therefore unavailable for the larvae [10, 16]. Thus, the dry weight of the larvae that received sediment or liquid from fermented PPBS was much lower than previously published weights of prepupae (22.6–48.0 mg DS) that received highly nutritious chicken feed, chicken manure, or four types of food-processing by-products [16, 28, 29]. Fermentation of PPBS as carried out in this study does not increase the PPBS reduction rate.
The fact that the larvae reared on untreated PPBS had a similar survival rate (51.9 %) to those reared on the reference diet (54.8 %) implies that potential pathogens and toxins present in the untreated PPBS play an insignificant role in larvae survival. The low survival rate of the larvae reared on the liquid (26.3–30.9%) may be attributed to the small amount of dry feed (0.8 g). The small amount of feed is because the bulk of the lignocellulose remains undigested . Previous studies of leachate as feed for insects reported that leachate is nutritiously poor, thus causing feed shortage . Thus, it is reasonable to assert that the small amount of feed in the liquid of the fermented PPBS caused the low survival rate. The survival rate of both the larvae that received liquid and sediment (26.3–52.6%) was much lower than the survival rate (72–86%) for prepupae receiving chicken feed, chicken manure, or four types of food processing by-products [16, 29]. The survival rate of 47% for the larvae receiving sediment is in line with published data for sewage sludge . The result for the PPBS reduction rate (Fig. 1) is consistent with the individual dry weight and survival rate.
Another factor that may affect PPBS nutrient availability is texture . Nyakeri et al.  observed that mixing fecal sludge with coarse textured food waste increased prepupae yield compared to mixing it with fine textured brewer’s waste. The authors argue that a coarse texture facilitates larvae movement, allowing them to seek food, thereby improving prepupae yield. It is reasonable to assert that the fine texture of fermented PPBS impedes movement of the larvae and access to food, thus contributing to low nutrient availability.
The content of protein and fat in larvae have not been analyzed in this study. However, previous studies on bioconversion of fermented CEW have reported the following values: protein 15–39% [18, 20, 22] and fat 44–58% [18, 21, 22]. Analysis of protein and fat in larvae reared on PPBS is recommended for future studies.
The final weight of the larvae and the PPBS reduction rate are both low. However, a minor part of the PPBS is digested during fermentation, and those nutrients are readily available in the fermentation liquid. The bioconversion of the liquid is therefore substantially higher than for the sediment, which illustrates the need for further research on improved fermentation of PPBS. Important fermentation factors not tested in this study are water content [9, 18,19,20,21], process duration [9, 19, 20], and co-fermentation  as well as inoculum concentration [19,20,21,22]. Several methods to improve fermentation of lignocellulose by pre-processing have been reported: physical using microwaves, ultrasound, steam explosion, or heating; biological using microorganisms or fungi (i.e., white root fungi); and chemical using strong acids, alkalis, organic solvents, or ionic liquids .
Further research should focus on methods to increase degradation of lignocellulose by improving the fermentation process including pre-processing using physical, biological, and chemical methods . Combinations of pre-processing and fermentation of PPBS should be assessed with respect to nutrient availability. Methods to improve the texture of fermented PPBS need further attention.
The cause of the high bioconversion of the fermentation liquid should be investigated. The characteristics of lignocellulose metabolites such as palatability and anti-nutritive effects may be part of the explanation and should be assessed . Substances that affect palatability include rutin, sinigrin, gossypol, gamma amino butyric acid, waxes and plant secondary compounds, sucrose, ascorbic acid, sugar, amino acids, vitamin C, Mg, and K . Examples of antinutrients are protease inhibitors, lectins, phytic acid, reactive oxygen species, trypsin and chymotrypsin inhibitors and other noxious-tasting substances . The presence of these substances in fermented PPBS and their effect on nutrient availability should be assessed.