Week 4: The scraps-to-larvae-to-fish loop does not close

Week 4 — threshold temperatures, bioconversion on a dry-matter basis, and the arithmetic that shows the most-drawn diagram in permaculture is mostly imports

15 March 2027

Open slides — week 4

The question#

Search for a closed-loop household diagram and you will find this arrow: kitchen scraps feed black soldier fly larvae, larvae feed the fish, fish feed you. It is drawn as a closed circle roughly every time.

Size it. What fraction of the fish’s feed do your own scraps actually cover?

The method: thresholds and dry-matter bioconversion#

Black soldier fly larvae have a lower developmental threshold of 10.4–12.3 °C; below it, development stops. Egg to adult takes 91 days at 19 °C, 42 at 28 °C and 32 at 31 °C, with best population growth near 30 °C. This course screens each site with a linear productive factor ramping from 12 °C to 27 °C — a simplification, and labelled one.

Conversion is measured on dry matter: on food waste, larvae achieve about 52 % substrate reduction and 28 % bioconversion over 14 days. This course uses 20 % as a conservative course assumption.

Worked example: Darwin, the best site there is#

Darwin’s productive factor is 0.97 — larvae are thermally productive essentially all year. Start from the household’s 380 kg of food waste:

Now the demand side. A 4 m² aquaponic grow bed at the FAO feed-rate ratio of 50 g/m²/day, running twelve months:

50 g/m² × 4 m² × 365 d = 73 kg of feed per year

Allowing larvae to make up at most 90 % of the ration:

11.1 ÷ (73 × 0.9) = 17 %

Seventeen per cent, at the best site in the set, with the whole household’s plant-based scraps committed. An 8 m² bed halves it to 8 %. Canberra, with a thermal factor of 0.19, would manage 3 % outdoors.

The other 83 % is bought feed. Under this course’s closure definition, fish grown on bought feed are bought fish, so a Darwin excellent design counts 0.27 kg of fish protein a year against a household requirement of 67.5 kg.

That is not a flaw in your design. It is the finding.

The fairness lever#

Rearing indoors at about 20 °C gives a productive factor of roughly 0.53 anywhere. It converts a climate advantage into an energy cost, which is exactly the trade this course wants you to make explicitly rather than accidentally.

Substrate: a course rule, and what the law actually says#

The course rule. Insect substrates are plant-based. Larvae destined for fish are heat-treated before feeding. Meat, dairy and fish sludge go to compost or the digester, never to larvae.

What the law actually says. Australian restricted-animal-material rules prohibit feeding vertebrate-derived material to ruminants, and mammal products to pigs. Swill feeding is prohibited nationally and expressly covers pet and hobby animals. None of it mentions fish, aquaculture or insects. A household feeding its own fish is not captured.

The claim that insect substrates must exclude meat and catering waste appears in one academic review and is published by no Australian regulator.

So the course is deliberately stricter than the law, for a biosecurity reason worth understanding: the pathway a rule closes matters more than whether the rule happens to bind you. Learning to tell a design rule from a legal requirement is the transferable skill here, and week 11 tests it again on greywater.

Five sites#

What this week hands you#

A feed mass balance with the import line drawn in and labelled. If your diagram still shows a closed circle after this week, it is wrong, and week 12 will find it.

The waste tonnage behind that balance is a standard per-household figure, so the larval yield is a scoping estimate. What this house throws out, and in which months, goes on the measurement list for Assignment 1.

Back to week 4 overview