Dry Composting Loo (UD): Urine-Diverting, Sawdust Cover, 12 V Vent, Sealed Bins
Operational, low-water sanitation for cabins, vans, off-grid, or SHTF: odor-managed, easy to service, compatible with external curing. (Check your local regs if you plan reuse.)
Targets
- No odor in room: continuous negative pressure via quiet 12 V fan (25–40 dB).
- Keep solids dry: urine diverted; add carbon cover after each use.
- Hands-off service: sealed solids bin; separate sealed urine can.
- Safe handling: gloves, dedicated tools, exterior curing bin; urine stored before reuse.
Specs
| Footprint | Cabinet 16–18″ W × 22–24″ D × 18–20″ H (seat height ~17–18″) |
|---|---|
| Vent | 12 V DC fan 80–120 mm → 1.5–3.5 W → 25–40 CFM out; 40–50 mm duct |
| Bins | Solids: 20–30 L HDPE w/ gasket lid; Urine: 10–20 L jerry (HDPE) |
| Power | 12 V battery (or 24→12 V buck), inline 1 A fuse |
Bill of Materials (parts to buy)
Core
| UD seat/pedestal | Urine-diverting seat (polypropylene) with 32–40 mm outlet barb |
|---|---|
| Solids bin | HDPE 20–30 L with gasketed gamma-style lid + compostable bag liners (heavy-duty) |
| Urine can | HDPE 10–20 L jerry with cap; extra cap with bulkhead fitting + 6–10 mm hose barb + carbon vent |
| Cover material | Fine sawdust / wood shavings / coco coir (dry); small scoop |
| Vent fan | 12 V DC fan 80–120 mm, 0.15–0.3 A; weather cap + insect screen |
| Duct & fittings | PVC or ABS 40–50 mm (1.5–2″) pipe, 90° elbows, wall thimble |
| Cabinet | Birch ply 12–15 mm or HDPE sheet; stainless hinges, latch, piano hinge for seat panel |
Power & Hardware
| 12 V supply | 12 V AGM/LiFePO₄ (7–20 Ah) or 24→12 V buck (3–5 A) |
|---|---|
| Protection | Inline blade fuse holder + 1 A fuse (fan), 5–10 A (buck) |
| Switch | SPST rocker (optional; fan usually runs 24/7) |
| Hose | 6–10 mm ID urine hose + clamps; check-valve (urine-safe); grommets |
| Sealants | Silicone (sanitary), neoprene foam tape, aluminum tape for duct seams |
| PPE | Nitrile gloves, eye protection, sanitizer, paper towels |
Wire gauge: 18 AWG is sufficient for a single fan run; upsize for longer runs per ampacity/voltage-drop tables. Label everything.
Cut & Drill List (cabinet path)
| Top panel | Seat cutout per UD seat template; pilot holes for hinge/latch |
|---|---|
| Front panel | Service door; magnetic catch; optional viewing slot (covered) |
| Rear panel | Vent hole Ø 48–52 mm; power grommet Ø 8–10 mm |
| Side panel | Urine hose grommet Ø 10–12 mm (if external jerry) |
| Internal baffle | Short baffle behind seat to direct airflow upward to vent |
Flow & Wiring
AIR: Room air → bowl → solids bin headspace → rear baffle → 40–50 mm duct → 12 V fan (pulling) → outside (weather hood + insect screen)
URINE: Seat UD outlet → hose (6–10 mm) → bulkhead in jerry cap → jerry (sealed) → small carbon vent on spare cap (or run a tiny separate vent line)
POWER: 12 V (+) → 1 A fuse → (optional switch) → fan (+) ; fan (−) → 12 V (−)
If using 24 V system: 24 V → buck (12 V) → 1 A fuse → fan.
Fan at the end of the duct (pull configuration) maintains negative pressure in cabinet & bowl.
Assembly — Step by Step
- Build the cabinet. Cut panels; assemble a rigid box. Add neoprene tape around lid/seat panel for a soft seal.
- Install the UD seat. Follow the seat template; ensure a snug, sealed fit. Route the urine outlet barb through a grommet toward the urine can.
- Set the solids bin. Place the 20–30 L HDPE bin inside; add a compostable bag liner; snap on the gasket lid with a center cutout matching the seat drop. (Keep a few spare liners.)
- Vent system. Fit 40–50 mm duct from the bin headspace to the rear. Mount the 12 V fan at the exterior end (puller), blowing out. Add insect screen + weather hood outside. Seal duct joints.
- Urine line. Run 6–10 mm hose to the jerry cap bulkhead fitting; add a one-way check valve close to the can. Keep hose slope downward. For odor control, add a small carbon vent on a spare cap.
- Power. Wire 12 V → inline 1 A fuse → (optional rocker) → fan. Confirm polarity. Tie and label harness. Fan is meant to run 24/7.
- Commissioning. Smoke-match at bowl edge (incense): smoke should be drawn inward. No leaks around seat. Add an initial layer (2–3 cm) of dry sawdust to the bin.
- User rule. After each use, add a scoop of dry cover material to keep the top layer dry and odor-free.
Fan Runtime Calculator
| Power draw | — W |
|---|---|
| Est. runtime | — hours |
| Tip | A 12 V-7 Ah gel battery typically runs a 0.18 A fan ~30–35 h; 12 V-20 Ah runs ~85–95 h. |
Maintenance Plan
Daily
- Add cover after every use until the surface looks dry.
- Check urine level; empty before full; rinse can with a little vinegar-water.
Weekly
- Wipe seat & splash areas with mild cleaner (avoid mixing bleach with urine residues).
- Inspect fan intake for lint/dust; confirm airflow.
Solids Handling
- When bin is ~⅔ full, close liner, cap, and move to an external curing bin.
- Let cure per your local guidance; do not apply to crops without meeting safety targets.
If planning reuse: follow WHO’s risk-reduction approach (time/temperature barriers and safe end-use). For urine reuse, store 2–6 months (alkaline pH & free ammonia inactivate many pathogens); verify local legality. For solids, true thermophilic composting requires managed conditions—this simple loo is a collector, not an in-bin composter.
Hazards & Mitigations
- Biological risk: Wear gloves; wash hands; disinfect surfaces; keep tools dedicated. Transfer solids in sealed liners.
- Ammonia: Stored urine can off-gas; ventilate while emptying; never mix with chlorinated bleach (toxic gases may form). Use a lid and small vent filter.
- Odor/back-draft: Fan must pull out; seal leaks; keep cover material dry and plentiful.
- Leaks: Hose clamps tight; check-valve in urine line; secondary tray under jerry.
- Regulatory: Some jurisdictions require NSF/ANSI 41 listed systems. If portable/temporary, verify local rules.
Quick-Start (print)
- Switch fan ON (should draw inward at bowl).
- Urine jerry connected, valve open, hose sloped down.
- Base layer of dry sawdust (2–3 cm) in solids bin.
- After every use: add one scoop of dry cover.
- Empty urine before full; keep floor dry; wipe surfaces.
- When solids ~⅔ full: seal liner → move to curing bin.
Printable Diagram
Sources & References
- UDDT technology & design review — SuSanA/Eawag. PDF
- UDDT construction manual — WECF. PDF
- NSF/ANSI 41 (overview) — NSF. Link
- EPA Wastewater Tech Fact Sheet: Composting Toilets (pathogens, performance). PDF
- WHO Guidelines: Safe Use of Wastewater, Excreta & Greywater (risk reduction). Volume 4
- Urine pathogen inactivation & storage time — Frontiers review. Link
- Vent-fan norm in compost toilets — CompostingToiletsUSA (odor/moisture). Link
- Cover material guidance — Humanure Handbook cover material note. PDF
- Ammonia exposure precautions — CDC/ATSDR. Link
Local codes vary; some areas require listed systems for permanent installations. Check with your health department before permanent use.