EDT domain: d03_water_waste_circular
Definition
Anaerobic digestion (AD) breaks down organic matter (effluent, manure, food waste, blackwater) without oxygen, producing biogas (~50–65% methane) for heat/cooking/electricity and a nutrient-rich digestate usable as biofertiliser. In the Neobiome frame it sits across three domains — energy (D01), sanitation/circular (D03), and food nutrients (D02) — but is most defensibly a sanitation + nutrient-recovery technology with energy as a co-benefit. CR_039
Verdict — Conditional (community scale only)
- Household scale: Out for NZ. Unheated digesters sour below ~20 °C internal — roughly half the NZ year — and heating them consumes much of the gas produced. A passive household unit is not a reliable year-round energy source anywhere in mainland NZ. CR_039
- Community/farm scale: Conditional, viable only where (a) a concentrated, year-round organic feedstock already exists (dairy effluent, piggery, aggregated food-waste/blackwater), (b) the digester is heated/insulated to hold ≥20 °C through winter, and (c) the output is valued primarily as process heat + digestate fertiliser, not grid electricity. CR_039
- Independent-geography corroboration (Malawi, Robin & Ehimen 2024). A full fixed-dome-digester techno-economic model for rural Malawi reaches the same shape from a warm-climate, low-labour-cost economy: cow-dung-alone and household-only configurations are not viable (negative 20-yr NPV), and AD only pencils out with co-digestion (cow dung + grass or maize residue) in the largest reactor — best case 15.09 m³ / 6 cows, NPV +
8,962.58, payback ~4 yr, LCOE0.06/kWh. Because LCOE falls with reactor size (0.48 →0.09/kWh across 3.28 → 15.09 m³), the authors recommend sharing one reactor across 2–3 households or running it at institutional scale (schools, dispensaries) rather than one-per-household — reinforcing this page’s “household Out / community Conditional” verdict. ⚠ Costs are Malawi 2023 USD and do not transfer to NZ; the transferable content is the method + literature-value design parameters (CHP η_elec 39% / η_thermal 45%, biogas 21 MJ/m³, feedstock CH4 yields). The paper even notes efficiency drops at 10–13 °C overnight — the everyday NZ cool-temperate case, so it strengthens rather than softens the NZ caution. LIT_076 - Field-study corroboration of the household-scale verdict + cold-climate seasonality (Bozeman MT, Ebel et al. 2025). A peer-reviewed 12-household case study in a cool-continental US town (pop. ~53,000) independently reaches this page’s shape from an off-shore geography: household-scale AD processed food waste but was “not functional for over 6 months of the year due to cold temperatures” (mesophilic optimum 35 °C vs 15–21 °C ambient summer; digesters had to be disassembled in winter to prevent the digestate freezing), and the units were run for digestate biofertiliser, not gas — half the commercial Home-Biogas users could extract no biogas at all. Despite this, 87% of participants would still recommend/continue, favouring the simpler custom-built unit. This is the retrievable primary behind CR_039’s energy-vs-nutrient framing, and it independently corroborates the cold-climate seasonality (CR_039’s own “≈6 months/yr” figure traces to Castaño et al. 2014, not this study). ⚠ Montana nutrient/temperature values are context-specific, not NZ inputs — the transferable content is the operational pattern (seasonal, fertiliser-first) + user-acceptance evidence. LIT_087
Cost & performance (NZ, indicative)
- Capex ~NZD 400k–5M (community/farm scale, depending on CHP); per-cow ~NZD 1,500–3,000. [low–medium] CR_039
- Output — a NZ dairy case (410-cow System-5 farm, Awakeri BoP) covered the farm’s hot-water demand only (~221 MJ/day): 558 MJ/day from the solid fraction (42-day BMP) and 176–861 MJ/day from the liquid fraction (176 BMP → 861 CSTR-model). Enough via solids-only, or liquid+solid together, but not liquid alone; a farm producing 30,000 L liquid effluent/day could generate ~320 kWh/day of heating. A 900-cow farm ~10 TJ/yr (~2.8 GWh/yr, via CR_039/NZ Herald). Best directed at heat. [verified — MJ/day primary; lab-BMP+model, single farm] LIT_086 CR_039
- Scale threshold — energy-led economics need ~500 cows; NZ’s ~450-cow average herd is borderline. CR_039
Operation & safety
Daily controlled feeding (over-feeding sours it, under-feeding starves it), periodic desludging, and gas handling. Biogas is flammable and contains toxic/corrosive H2S — non-trivial safety management for a self-operated community asset; cold-climate add-ons push it toward needing a competent operator. See Regulatory & safety (NZ) below. CR_039
Regulatory & safety (NZ)
No single NZ “biogas code” — four overlapping pillars: (1) WorkSafe occupational H₂S — WES-TWA 5 ppm, WES-STEL 10 ppm, IDLH ~100 ppm + confined-space duties; (2) Gas (Safety and Measurement) Regulations 2010 with AS/NZS 5601.1 (names biogas) → Building Code G11; (3) Building Act 2004 consent; (4) RMA discharge-to-air consent (regional; small-scale often permitted below council thresholds, e.g. ~10 m³/day in Greater Wellington — council-specific). CR_041
Digestate as biofertiliser
Digestate is a low-analysis but high-availability N fertiliser — total N ~3.3–4.4 g/kg with 82–90% as plant-ready ammonium-N (vs ~45% for cattle slurry), modest P/K, plus soil-health organic matter (UK figures, transferable — LIT_060 Table 3). NZ status: compliant digestate meets the fertiliser definition under the ACVM Act 1997 → exempt from ACVM registration if accredited under the BANZ scheme (Technical Guide 8 / DBPAS-05, built on UK PAS110, tested per NZS 4454:2005); otherwise it’s a biosolid under regional-plan rules. A community applying its own digestate on its own land falls under regional-plan land-application rules, not commercial accreditation. OT_080 CR_041
Relevance to Neobiome
Not a modelled engine cell — AD’s verdict here is Conditional, and its role is as D03 evidence. Its strongest contribution is closing the sanitation/nutrient loop (effluent/food waste → stabilised digestate fertiliser, cross-linking the composting-toilet theme and the Gullberg digestate-recovery figures on d03_water_waste_circular), with biogas heat as a co-benefit where a year-round feedstock exists. CR_039
Research targets
- NZ dairy-AD MJ/day verified via LIT_086 (RT_269); capex still open (RT_269 capex leg); cold-climate ops evidence (RT_273); any NZ community-digester deployment (RT_272).
- (Resolved → CR_041: NZ AD regulatory/safety framework (RT_270); digestate fertiliser value + NZ status (RT_271).)
Connections
Links to
Referenced by
Sources (7): CR_039 · CR_041 · LIT_060 · LIT_076 · LIT_086 · LIT_087 · OT_080
EDT domains (3): D01: Renewable Energy & Storage Systems · D02: Smart Food Systems & Agriculture · D03: Water, Waste & Circular Systems