Sizing a commercial sewage system isn't a case of picking a big tank and hoping. Undersize and the plant fails under peak load; oversize and you've paid for capacity you don't need and often treat less effectively. This guide walks through the factors that drive the right specification for pubs, holiday parks, farms and rural estates.
Population equivalent (PE)
Commercial sizing works in population equivalent: one PE is the design load of one resident producing roughly 150–200 litres of wastewater per day. Different premises convert differently to PE, and using the right conversion is the first discipline.
| Premises | PE conversion |
|---|
| Pub, seated diners | 0.3 PE per cover per day |
| Restaurant, full service | 0.4 PE per cover |
| Hotel / B&B room | 1 PE per room occupant |
| Holiday park pitch / lodge | 1 PE per berth |
| Office worker (no canteen) | 0.2 PE per employee |
| School pupil | 0.2 PE per pupil per day |
These figures are a starting point; BS EN 12566 gives the full table and your specialist will select the right conversions for your exact operation.
Peak vs average load
A treatment plant must handle peak daily load, not average. A 50-cover restaurant on a quiet Tuesday is running at 20% capacity; the same premises on Saturday night is at 100%. A system sized to the average chokes at peak.
Peak-day rule of thumb
Size to the busiest 95th-percentile day in the year. For seasonal businesses, that's the peak summer week; for wedding venues, it's a full-capacity reception.
Seasonal variation
Holiday parks, campsites and seasonal pubs bring another challenge: biological systems need a steady flow to keep the bacteria population healthy. A plant sitting nearly empty for six winter months and then shocked to full load in July often underperforms for the first few weeks of peak season.
Options include two-stage plants (a smaller primary plant kept live year-round with a secondary brought online seasonally), aerated plants with recirculation, or modular packaged systems that scale more gracefully.
Discharge route
Surface-water discharge from a commercial treatment plant almost always requires an Environment Agency bespoke permit (rather than operating under the General Binding Rules), which sets discharge quality limits and sampling frequency. Permit applications typically take 3–6 months and fees depend on discharge volume.
Ground-discharge to a drainage field is regulatorily simpler but requires substantially more land — a commercial-scale drainage field can occupy 500–2,000m².
Typical commercial costs
| Scale | Typical installed cost |
|---|
| Small rural pub, 10–15 PE | £15,000–£25,000 |
| Medium hotel / holiday park, 20–40 PE | £25,000–£50,000 |
| Large site, 40–100+ PE | £50,000–£120,000+ |
Figures are indicative and exclude groundworks in especially difficult ground, long effluent runs, pumping stations and permit fees.
What a commercial survey covers
A commercial site survey is more involved than domestic. It covers PE calculation against actual trading patterns, peak-load modelling, discharge route viability, permit strategy, access for future maintenance tankers, and an operational-cost projection (power, servicing, sampling, emptying). Typical fee £400–£1,200 depending on site complexity.
