7.1. Technological problems associated with scaling up of waste processing capacity
The major factor limiting the volume of organic waste which can be processed by fly larvae is the depth of substrate. Typically, the fly larvae do not burrow deeper than 7.5–10 cm into the substrate. This is caused by low levels of oxygen and resulting anaerobic conditions in deep layers of the media. Industrial-scale waste processing by fly larvae would thus require a large number of shallow trays or basins, which would increase handling and loading costs or space requirements of the facility. Eby and Dendy (1978) tried several different approaches to increase the depth of larval medium. Forced delivery of air into larval substrate through a system of copper pipes with air outlets set 7.5 cm apart allowed the larvae to process manure in the full depth of the incubation box (1.22 m). However, several problems were encountered with this system. The larvae, when ready to pupate, entered the air holes and migrated through the pipes all the way to the filter of air pump. Manure in the vicinity of air holes became dry and was not utilized by the maggots. Ultimately, loading and unloading of the system was problematic due to the large number of air pipes and there were no practical means to collect the larvae. A better approach proved to be periodic tumbling of larvae with substrate in standard cement mixers. Once the larvae matured, the spent manure residue with larvae could be easily unloaded on screen-bottom trays to allow light-induced separation of maggots. A specially designed closed culture tank capable of handling 1800-kg batches of medium however could not provide adequate conditions for larval development (Eby and Dendy, 1978). Still another option to increase waste batch volume is to use tall containers with perforated walls through which the larvae are inoculated (Ivanov et al., 1980). The mesh covering sides of the container can supply the maggots with sufficient amount of oxygen during their development and may serve as a convenient separator through which larvae migrate prior to their pupation.