Week 10: Every stream goes somewhere
Week 10 — routing food waste, spent substrate, meat and dairy and fish sludge through compost and digestion, and closing the mass balance that is not banded
10 May 2027
The question#
Your household generates 380 kg of food waste a year, your mushrooms leave five kilograms of spent substrate for every kilogram picked, your fish tank produces sludge, and weeks 4 and 5 sent the meat and dairy away unprocessed.
Where does each of those go, and does the mass balance close?
The method: mass balance by stream, then check the temperature#
Every stream is tracked separately, in kilograms of dry matter, with a named destination for each. This course assesses waste-stream recovery from those balances — food-waste dry matter, greywater volume and excreta nitrogen, each routed to a productive use.
Waste-stream recovery is deliberately not banded. Every other indicator has a per-site threshold from a reference design; this one does not, because the model has no reference calculation behind it. Publishing a band without a reference would be inventing a number, so it is marked from your mass balances against the rubric instead. That asymmetry is honest, and it is on the method page too.
Hot composting is the pasteurisation route. The performance target is the Australian composting standard as described by a state environment regulator: core temperature above 55 °C for three consecutive days following each of at least three turns, and for manure, food and grease-trap feedstocks, fifteen consecutive days following each of at least five turns. The standard also puts the whole process at not less than six weeks.
Two scope notes you must carry. That is a commercial windrow standard, and no household anywhere is bound by it — this course uses it as the target a bay is designed against. And it is more demanding than it is usually quoted: fifteen days after each of five turns, not fifteen days in total.
WHO’s separate figure for composting excreta is above 50 °C for at least one week, with the caution that composting “is mainly recommended as an off-site secondary treatment at large scale”.
Anaerobic digestion is strongly temperature dependent. Screening factors run from 0.63 at Darwin to 0.07 at Canberra. And the biogas earns you nothing: it is energy, it belongs to the proposed sequel, and only the digestate’s nutrients count here. Record it anyway. An unrecorded output is the same mistake as an unrecorded import.
Worked example: Alice Springs, where the heat does the work#
Alice Springs has 90 days a year at or above 35 °C and an annual mean of 21.0 °C. That is a liability for the garden and an asset here: reaching and holding 55 °C in a compost bay is a question of managing moisture, not of adding heat.
Route the household’s 380 kg:
| Stream | Mass | Destination | Why |
|---|---|---|---|
| Plant-based food waste | 228 kg/yr | Insect unit, then frass to soil | Week 4’s substrate rule |
| Meat, dairy, cooked scraps | 152 kg/yr | Hot compost or digester | Excluded from larvae destined for fish by the course rule; also the feedstock class needing the 15-day pasteurisation |
| Spent mushroom substrate | ~5 kg per kg picked | Compost and soil | 1.46 % bioconversion by larvae makes the alternative arrow pointless (week 3) |
| Fish sludge | From the aquaponic system | Compost or digester | Vertebrate-derived; never to larvae under the course rule |
| Garden prunings | Site-dependent | Mushroom substrate, then compost | The one stream that passes through two subsystems |
The dry-matter total in must equal the dry-matter total out, plus the respiration loss you have stated as an assumption. If it does not, you have lost a stream.
Alice Springs’ particular trap: moisture. A bay that dries out stops composting and never reaches pasteurisation temperature, so the water that hot composting consumes is a genuine line item on the site’s most constrained resource. It is easy to write a compost plan here that quietly needs water the site does not have.
Five sites#
| Site | Composting | Digestion factor | The stream that causes trouble |
|---|---|---|---|
| Alice Springs | Fastest of the set; heat is free | 0.45 | Moisture, competing with irrigation |
| Darwin | Fast, but wet-season saturation | 0.63 | Volume: the highest garden output means the highest prunings load |
| Brisbane | Year-round | 0.57 | Odour and vermin next to neighbours. A strata site has the least tolerance for a badly run bay |
| Adelaide | Good most of the year | 0.34 | Seasonal: the glut from week 6 arrives all at once |
| Canberra | Winter bays stall | 0.07 | Digestion is close to pointless nine months a year. Design for compost, not gas |
What this week hands you#
A mass balance per stream, in dry matter, with a destination for every kilogram and a stated loss term. This is the input to the proposal’s waste-stream recovery assessment, and the one place in the course where showing your working is literally the deliverable.
Teaching team
- Noor Bakhshi — tutor