The most critical decision in selecting a packaged treatment system is determining the correct capacity. Insufficient capacity leads to continuous quality issues, while excessive capacity results in unnecessary investment and biological degradation at low loads. In this article, we present the calculation of person equivalents (PE), flow forecasts by sector, capacity ranges, sizing steps, and a checklist for investment decisions.
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Frequently Asked Questions
Person equivalent (PE) defines the wastewater load produced by one person per day as a standard unit. European standard: 60 g BOD/person·day, 120 g COD, 70 g TSS, 11 g TN, 2 g TP, 150 L wastewater. Industrial or institutional wastewater loads are converted to PE equivalents according to this standard. It is a fundamental input for design.
It varies by hotel type and star rating: Luxury 5-star hotel 1.5-2 KE/bed (towel frequency, restaurant, SPA), standard 3-4 star 1-1.3 KE/bed, apartment/guesthouse 0.8-1 KE/bed. An additional 15-20% should be added for staff. The restaurant is calculated separately (seating capacity × 0.3-0.5). A balancing tank is critical during seasonal peaks.
Peak factor is the ratio between the moment when the wastewater flow reaches its maximum during the day and the average flow. Hotel season is typically 3-4×, highway rest areas 5-8×, and factory CIP 3-5×. Importance: If the design is not made according to the peak flow, there will be overflow + biological shock during peak moments. Solution: Softening peaks with a balancing tank (HRT according to peak period 12-24 hours).
Formula: Daily flow = KE × per capita water consumption (120-200 L). By sector multiplier method: (1) Determine the number of users (bed/room/person/production), (2) Apply the KE multiplier, (3) Multiply by the water consumption amount, (4) Find the maximum flow with the peak factor. Industrial: use the direct wastewater multiplier per production/process (for example, milk 3 L wastewater/L milk).
It varies according to the selected process. Activated sludge/SBR: HRT 8-24 hours, F/M 0.2-0.4, MLSS 2,500-4,500 mg/L → volume ≈ flow × 0.5-1. MBBR: HRT 4-8 hours → volume ≈ flow × 0.25-0.4. MBR: HRT 4-8 hours + MLSS 8-12 g/L → volume ≈ flow × 0.2-0.3 (most compact). If the wastewater BOİ concentration is high, the volume increases.
In industrial wastewater, load-based calculations are made instead of using the number of people. Step: (1) Find the wastewater flow rate from the production volume (for example, milk 3 L wastewater/L product), (2) Measure the wastewater KOİ concentration, (3) Total KOİ load = flow rate × concentration, (4) KE equivalent = KOİ load (kg/day) / 0.120 kg KOİ/KE·day. Example for a dairy factory: 150 m3 × 4 g/L = 600 kg KOİ → 5,000 KE equivalent.
3 main results: (1) Insufficient capacity: overflow during peak times, biological shock, continuous issues with effluent quality, risk of penalties. (2) Excess capacity: biological degradation at low loads (endogenous respiration, bulking, decreased sludge quality), unnecessary CAPEX. (3) Seasonal errors: insufficient wastewater during off-season → biomass dies → re-adaptation at the beginning of the season takes 4-6 weeks. Solution: professional characterization + sizing study.
