Week 11: The nitrogen has nowhere to go
Week 11 — sizing excreta and greywater systems, the uptake limit that caps every site, and the greywater rule that is the law in a state none of you is in
17 May 2027
The question#
The household excretes 13.6 kg of nitrogen a year, about 88 % of it in urine. A garden can take up roughly 12 g of nitrogen per m² per year before the surplus becomes a pollution problem rather than a fertility one.
Multiply those out for your site. What do you do with the rest?
The method: an uptake-limited nitrogen balance#
Absorbable N = production area (m²) × 0.012 kg N/m²/yrAgainst 13.6 kg, no site in this course absorbs even half.
| Site | Growing area | Absorbable N | Share of household N |
|---|---|---|---|
| Darwin | 500 m² | 6.0 kg | 44 % |
| Canberra | 450 m² | 5.4 kg | 40 % |
| Alice Springs | 400 m² | 4.8 kg | 35 % |
| Adelaide | 250 m² | 3.0 kg | 22 % |
| Brisbane | 130 m² | 1.6 kg | 11 % |
So between 56 % and 89 % of your household’s nitrogen has to go somewhere that is not your garden. That is a design problem, not a design failure, and a proposal that quietly applies all 13.6 kg to the beds will be marked down for the arithmetic, not the ambition.
Storage, and why Canberra pays for being cold#
WHO’s guidance on the safe use of excreta, Table 4.5, dry excreta with no new material added:
| Ambient temperature | Storage time |
|---|---|
| 2–20 °C | 1.5–2 years |
| Above 20–35 °C | Over 1 year |
| Alkaline treatment | pH above 9 for over 6 months |
Canberra’s annual mean is 13.1 °C and Adelaide’s is 17.0 °C, so both sit in the 1.5–2 year band. Alice Springs, Brisbane and Darwin are above 20 °C and need one year. Canberra therefore needs about 75 % more chamber capacity than Darwin for the same household — a climate-driven sizing burden with nothing to do with how well the design is thought out. It is built into the reference designs, so the bands already absorb it.
Urine, WHO Table 4.6: at 20 °C, one month’s storage for crops that will be processed, six months for all crops.
And the scope the draft of this course originally missed. Immediately above that table, WHO writes: “For individual one family system and when the urine is used solely for fertilization on individual plots, no storage is needed.” A one-month withholding period before harvest applies to vegetables, fruit and root crops eaten raw. So the course’s storage requirement is deliberately more conservative than WHO’s own household position, and you should know that, rather than believing you are meeting a requirement.
Greywater: a rule that is the law somewhere none of you lives#
Ask anyone who has looked into greywater and you will hear the same three rules. Untreated greywater goes subsurface, at least 100 mm down. It is never stored. It never includes kitchen water.
That is New South Wales law. None of this course’s five sites is in New South Wales.
| Requirement for untreated greywater | ACT | NT | Queensland | South Australia |
|---|---|---|---|---|
| Subsurface mandatory | No — surface and bucket diversion contemplated | Yes, “below the surface of the ground” | No — the Code covers “subsurface or surface irrigation” | Yes, “a subsurface land disposal system” |
| A depth figure in the instrument | Only for treated greywater | Definitional only (100–150 mm), not stated as the device criterion | None | None in the Code |
| Kitchen water excluded | “Not recommended” | Prohibited | Included in the legal definition | Generally excluded unless treated |
| Storage | ≤24 h, as guidance | ≤24 h | — | ≤24 h |
| WaterMark device | — | Not required | Required | Required |
| Instrument | Guidance only, no greywater-specific regulation | Prescribed code | Code plus local approval | Regulation plus code |
The course design rule is the strictest column: subsurface at 100 mm or more, no storage, no kitchen water. Every design follows it. But you must also state what your own jurisdiction actually requires, and where the course is stricter, say so.
The point is not that regulation is inconsistent, though it is. The point is that a designer who cannot tell a remembered rule from a cited one will eventually apply the wrong jurisdiction’s rule to a real site and be confident while doing it.
Two Australian standards that carry relevant depth and plumbing requirements are paywalled and were not read for this course. No depth figure anywhere on this site is sourced to them.
Five sites#
| Site | Excreta storage | Greywater instrument | The hard part |
|---|---|---|---|
| Canberra | 1.75 years | ACT: guidance only, no regulation | Cold storage burden and the least regulatory guidance to design against |
| Adelaide | 1.75 years | SA: regulation plus code | Same storage burden; 22 % uptake means most N leaves |
| Brisbane | 1.0 year | QLD: code plus council approval | 11 % uptake. Nine-tenths of the nitrogen has to leave a 300 m² townhouse block |
| Alice Springs | 1.0 year | NT: prescribed code | Dry soils and 35 % uptake; subsurface irrigation in sand behaves differently |
| Darwin | 1.0 year | NT: prescribed code | Wet-season saturation makes subsurface disposal fail exactly when there is most of it |
What this week hands you#
A sanitation system sized to your site’s storage band, a greywater scheme that meets the course rule with your own jurisdiction’s requirement stated beside it, and a nitrogen disposal plan for the majority of your household’s nitrogen that the garden cannot take.
Teaching team
- Eun-Ji Marchetti — guest
- Harriet Oyelaran — convenor