IELTS Practice Test 7

Reading Passage A: Small Workers, Big Waste

A Every year a large share of the food grown for people to eat is never eaten. Much of it ends up in landfill, where it decomposes without oxygen and releases methane, a greenhouse gas far more potent over the short term than carbon dioxide. Composting and anaerobic digestion have long been offered as alternatives, but a growing number of engineers believe that a less obvious ally may be better suited to some kinds of waste: the larva of the black soldier fly. Unlike the housefly, the adult soldier fly does not feed at all, and so it neither seeks out human food nor carries disease from one kitchen to the next. Its larvae, by contrast, are voracious, and in roughly two weeks they can reduce a tray of mixed food scraps to a fraction of its original mass.

B The process used at the Harrowfield BioCycle plant, one of several pilot facilities built in the last decade, begins in the reception hall, where lorries deliver unsold produce from markets and leftovers from school canteens. On a typical weekday the hall receives around twelve tonnes of material. Packaging is removed by hand, because even small pieces of plastic would otherwise end up in the final products. The waste then passes through a shredder, which reduces it to a paste with particles no larger than a few millimetres. According to the plant's operations manager, Teodora Finch, this step matters more than visitors expect: young larvae have tiny mouthparts and simply cannot consume large lumps, which are left to rot and attract other insects. The paste is then tested for moisture. If it is too wet, the larvae drown; if it is too dry, they cannot feed. Dry material such as bran is therefore mixed in until the paste reaches the consistency of thick porridge.

C Meanwhile, in a separate room kept warm and brightly lit, adult flies mate and lay their eggs in narrow slots cut into strips of cardboard. Each strip can hold several hundred eggs, and the flies are given water but no food. The eggs hatch after about four days, and the newly hatched larvae are given a starter diet for their first week before being transferred to the main rearing area. There, they are spread over shallow plastic trays, each holding around 15 kilograms of paste, which are stacked on racks up to ten levels high. The temperature is held at between 27 and 30°C. Finch notes that the larvae generate a surprising amount of heat themselves, so that in summer the problem is usually keeping the room cool rather than warm. Over the next twelve to fourteen days, the larvae increase their body weight several thousand times.

D The most ingenious stage relies on the insect's own behaviour. When the larvae reach their final stage, they stop eating and instinctively search for a dry, dark place in which to transform into adults. Each tray is therefore fitted with a gently sloping ramp along one side. The mature larvae climb the ramp and drop into a collecting channel, separating themselves from the remaining residue without any machinery. At Harrowfield this happens mostly at night, when the larvae are at their most active. Most of the harvested larvae are then killed by brief heating, dried and pressed to extract their oil, and the remaining material is ground into a protein meal for fish and poultry feed. A small proportion is kept back and returned to the breeding room, so that the colony sustains itself. What is left in the trays, a mixture of insect droppings and undigested fibre known as frass, is sieved and sold to farmers as a soil improver.

E Supporters argue that the system turns a costly disposal problem into saleable products, and early figures from Harrowfield appear encouraging: the plant reports that each tonne of waste yields roughly 150 kilograms of fresh larvae and 250 kilograms of frass, with much of the remaining mass lost as water vapour and carbon dioxide. Finch adds that the protein meal has already found regular buyers among fish farms in the region. However, not everyone is convinced that the method can expand easily. The environmental economist Rafael Okonjo points out that the larvae perform best on a consistent diet, whereas municipal food waste varies enormously from week to week; a delivery dominated by citrus peel, for instance, can slow growth noticeably. Regulations also present an obstacle. In some regions, insects intended for animal feed may legally be raised only on plant-based material, which rules out the meat and fish scraps found in restaurant bins. Okonjo believes the technology will remain a useful niche rather than a replacement for composting, at least until feed rules are revised. Finch is more optimistic. She suggests that a network of smaller units, each attached to a supermarket or food factory with a predictable waste stream, could avoid the problem of variable inputs altogether.