The nitrogen has nowhere to go#
SLOP4761 · Week 11 · Sanitation, nutrient limits and choosing
Where we are#
Second last week, and the last graded checkpoint. Two halves: the constraint that caps every site, then the options grid Assignment 2’s recommendation comes out of.
This week’s question#
The household excretes 13.6 kg of nitrogen a year, about 88 % of it in urine.
A garden absorbs 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?
By the end you can#
- Build an uptake-limited nitrogen balance and find a destination for the surplus
- Size an excreta chamber to your site’s WHO storage band and say why the band moves
- Compare an existing condition with three alternatives and mark the dominated ones
An uptake-limited balance#
Absorbable N = production area (m²) × 0.012 kg N/m²/yr
| Site | Growing area | Absorbable N | Share of 13.6 kg |
|---|---|---|---|
| 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 % |
Between 56 % and 89 % of the household’s nitrogen has to go somewhere that is not your garden.
A design problem, not a design failure.
Storage, and why Canberra pays for being cold#
WHO guidance on the safe use of excreta, Table 4.5 — dry excreta, 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 13.1 °C and Adelaide 17.0 °C sit in the long band. Alice Springs, Brisbane and Darwin need one year.
Urine, WHO Table 4.6: at 20 °C, one month for crops that will be processed; six months for all crops.
So Canberra pays for being cold#
About 75 % more chamber capacity than Darwin, for the same household.
A climate-driven burden with nothing to do with how well the design is thought out. The reference designs already absorb it, so the bands do too.
Then: the options grid#
Write the existing condition down first. “Do nothing” is a legitimate baseline that sometimes wins on one axis.
| measure | existing | A | B | C |
|---|---|---|---|---|
| cost, $ incl. contingency | 0 | |||
| peak-month energy, kWh/day | ||||
| water closure / garden satisfaction | ||||
| client targets met | ||||
| maintenance, h/week |
Dominated, and cheapest feasible#
A dominated option is worse on every row than another. Strike it out, with one sentence of reason.
Then find the cheapest option that meets your client’s stated targets.
Anything dearer has to justify itself in Assignment 2 — and this grid is where you find out what it would have to justify.
Your tutorial task#
Part one, 45 minutes: chamber capacity from your band; the nitrogen balance; then find your jurisdiction’s greywater instrument and write two sentences — what the law says, and where this course is stricter. Four of five sites will find the course rule is not their law.
Part two, 45 minutes: the grid, the dominated options marked, the cheapest feasible option identified.
And say what you are declining, with the number behind it. That is a result.
Before you leave#
- Does the balance show absorbed and surplus separately, with the surplus going somewhere?
- Have you cited an actual instrument?
- Does the grid include the existing condition as a column?
- Is the cheapest option meeting the client’s targets identified?
Full citations in the week 11 notes.