Population to treatment capacity.
Per-capita allowances and the conversion rule that turns water use into sewage flow, with the export figures for the UK, the United States and the Middle East.
Basis
GB 50014-2021 §4.1.14 (conversion rule); GB 50013-2018 Tables 4.0.3-1 and 4.0.3-3 (per-capita allowances); British Water Flows and Loads 4; US EPA 625/1-80-012 and 625/R-00/008; Saudi MEWA Design Guide for WWTPs. WHO does not publish a per-capita design flow — its guidance covers water quality and exposure risk, and no WHO per-capita allowance is used here.
Population to treatment capacity
Zone 1, Zone 2 and Zone 3 are the water-resource zones used by GB 50013-2018; the allowance is banded further by city size.
| Parameter | Value | Source | Evidence level |
|---|---|---|---|
| Combined domestic sewage allowance | Local water-use allowance × 90% | GB 50014-2021 §4.1.14 | National standard clause |
| Domestic per-capita water-use allowance, residents, highest day | Zone 1: 180–320; Zone 2: 110–190; Zone 3: 50–150 L/(person·day), in 7 bands by city size | GB 50013-2018 Table 4.0.3-1 | National standard table value |
| Combined domestic water-use allowance, highest day | Zone 1 from 250–480 L/(person·day); the later Zone 2 bands [to be confirmed with factory] | GB 50013-2018 Table 4.0.3-3 | National standard table value, partly pending |
| UK and Europe | 150 L/person·day; 60 g BOD/person·day; 8 g ammonia-N/person·day; a single dwelling of ≤3 bedrooms is designed for a minimum of 5 persons | British Water, Flows and Loads 4 | Industry-authoritative guidance |
| United States | Residential typical 45 gpcd (≈170 L/person·day); design is often taken as 75 gpd/person (≈284 L/person·day) | US EPA 625/1-80-012; 625/R-00/008 | Official document |
| Middle East | Design flow is derived from the served population plus current and future water demand; peaking factor 1.6 for large cities and 2.2 for villages and towns | Saudi MEWA, Design Guide for WWTPs | Official guide |
| WHO per-capita allowance | No such figure exists. WHO guidance covers water quality and exposure risk and does not give a per-capita design flow. Outward-facing documents must not claim a WHO per-capita allowance. | No source | Not applicable |
Worked example
Zone 2, medium city, 120 L/(person·day): combined domestic sewage allowance = 120 × 0.9 = 108 L/(person·day). Serving 10,000 people, the average daily flow is about 1,080 m³/day. Multiply by the peaking factor Kz from the peaking factor chart to reach the peak flow.
How to use and notes
- Take the local water-use allowance for the project region first; the combined domestic sewage allowance is 90% of it.
- Use the highest-day allowance for the design of the plant, not the annual average.
- The UK, US and Middle East figures are separate export conventions. Do not mix them with the national allowance bands.
- WHO does not give a per-capita design flow. Where a project refers to WHO guidance, that is a water-quality and exposure reference, not a sizing allowance.
- The later Zone 2 bands of GB 50013-2018 Table 4.0.3-3 have not been fully collected; use the value applicable to the project region: [to be confirmed with factory].
- Factory confirmation pending: the per-capita allowance used at selection, whether it is banded differently for domestic and export projects, and whether domestic 200 L and UK 150 L are presented as two parallel columns: [to be confirmed with factory].