Project Brief
A 60-bed hospital dialysis centre in West Africa needed to treat 30 m³/day of hospital effluent discharged over an 8-hour operating window — a 3.75 m³/h peak against a 1.25 m³/h daily average. This case documents the MBR-based design engineered to meet a guaranteed discharge of BOD₅ ≤ 40 mg/L, COD ≤ 100 mg/L, TSS ≤ 50 mg/L, TN ≤ 30 mg/L and TP ≤ 10 mg/L. Two constraints shaped every decision that followed: a 3:1 peak-to-average flow ratio, and up to 5 mg/L of residual chlorine arriving at the biological stage.
| Parameter | Value |
|---|---|
| Facility type | Hospital dialysis centre |
| Region | West Africa |
| Beds | 60 |
| Design flow | 30 m³/day |
| Discharge window | 8 h/day |
| Peak flow | 3.75 m³/h |
| Effluent character | Hospital wastewater, domestic-equivalent strength |
Influent Design Basis
The following influent envelope was supplied by the client as the contractual design basis. Ranges — not single values — are what a treatment plant must actually be sized against.
| Parameter | Design range | Sizing value used |
|---|---|---|
| BOD₅ | 150–300 mg/L | 300 mg/L |
| COD | 300–600 mg/L | 600 mg/L |
| TSS | 50–150 mg/L | 150 mg/L |
| Total nitrogen | 20–60 mg/L | 60 mg/L |
| Ammonia nitrogen (NH₄⁺) | 10–40 mg/L | 40 mg/L |
| Total phosphorus | 5–15 mg/L | 15 mg/L |
| Conductivity | 1,000–3,500 µS/cm | 3,500 µS/cm |
| Residual chlorine | 0–5 mg/L | 5 mg/L |
Source: client-supplied design basis. Every unit process below is sized against the upper bound, not the midpoint.
Guaranteed Effluent Quality
| Parameter | Guarantee | Required removal (worst-case influent) |
|---|---|---|
| BOD₅ | ≤ 40 mg/L | 87% |
| COD | ≤ 100 mg/L | 83% |
| TSS | ≤ 50 mg/L | 67% |
| Total nitrogen | ≤ 30 mg/L | 50% |
| Total phosphorus | ≤ 10 mg/L | 33% |
Source: client-specified treatment guarantee. Removal percentages are calculated against the upper bound of each influent range.
The Two Constraints That Drove the Design
1. A 3:1 peak-to-average ratio makes equalisation non-negotiable
The facility discharges its entire daily load in 8 hours, then goes quiet for 16. Sizing the biological stage for the 3.75 m³/h peak would mean paying for three times the aeration and membrane capacity actually needed — and then starving the biomass overnight.
The design instead buffers the flow. During the 8-hour discharge window, 30 m³ arrives while the plant treats 8 h × 1.25 m³/h = 10 m³. The equalisation tank must therefore hold the 20 m³ difference; with 20% freeboard the specified working volume is 24 m³. Downstream, every process runs at a steady 1.25 m³/h, 24 hours a day.
Source: HydroPure design calculation from the client's stated flow profile.
2. Residual chlorine will kill the biology before it treats anything
This is the constraint most often missed on hospital projects. Wards and dialysis units disinfect aggressively, and up to 5 mg/L of free chlorine reaches the drain. Nitrifying bacteria — the organisms responsible for ammonia removal — are inhibited well below that level, so an untreated chlorine slug can stall nitrification for days.
The design places a dechlorination step in the equalisation tank, ahead of any biological process, dosing sodium bisulphite (NaHSO₃). On a stoichiometric basis of 1.46 mg NaHSO₃ per mg Cl₂, the worst-case load of 5 mg/L × 30 m³/day = 150 g Cl₂/day requires roughly 220 g/day of NaHSO₃, dosed on ORP feedback so consumption tracks actual chlorine rather than the worst case.
Source: HydroPure design calculation, stoichiometric basis.
Process Train
Screening → equalisation with dechlorination → anoxic tank → aerobic tank → MBR membrane tank → ClO₂ disinfection → discharge. Sludge is aerobically digested and periodically hauled off site.
- Fine screening — removes gauze, plastics and dialysis consumables that would otherwise foul membranes.
- Equalisation + dechlorination (24 m³) — flattens the 3:1 peak and neutralises residual chlorine before it reaches the biology.
- Anoxic tank — denitrification driven by internal mixed-liquor recirculation. The influent BOD:TN ratio of roughly 5:1 provides sufficient carbon without a supplemental source, comfortably above the 4:1 minimum denitrification generally requires.
- Aerobic tank — carbon oxidation and nitrification of the 40 mg/L ammonia peak.
- MBR membrane tank — at a design flux of 18 LMH, the 1.25 m³/h continuous throughput needs approximately 70 m² of membrane area. The membrane barrier makes the TSS ≤ 50 mg/L guarantee structural rather than operational.
- ClO₂ disinfection — chlorine dioxide is specified over hypochlorite because it does not form the chlorinated by-products hospital effluent is already prone to.
Source: HydroPure design calculation. Flux of 18 LMH is a conservative operating point for municipal-strength effluent.
Equipment Selection
The plant is delivered as a packaged, containerised unit rather than a civil-works build — a decision driven by the site's limited footprint and the need to commission without a long local construction programme.
- ZS-L Medical Wastewater Treatment Equipment — the process core, purpose-built for healthcare effluent.
- MBR Integrated Wastewater Treatment System — membrane separation stage.
- ZS Chlorine Dioxide Generator — final disinfection.
Why MBR Rather Than a Conventional Process
The effluent guarantee itself (BOD₅ ≤ 40 mg/L, TSS ≤ 50 mg/L) is a secondary-treatment standard that conventional activated sludge can meet on paper. MBR was selected for three reasons specific to this site:
- Footprint — operating at 8,000–10,000 mg/L MLSS instead of 3,000–4,000 mg/L, an MBR needs roughly half the biological tank volume, which matters on a constrained hospital compound.
- Shock tolerance — the membrane retains biomass regardless of sludge settleability, so a chlorine slug or a load spike cannot wash out the process the way a clarifier upset would.
- Operator load — no sludge-blanket management, no return-sludge tuning. Hospital maintenance teams are not wastewater operators.
The full trade-off between membrane and fixed-film processes is covered in our MBR vs MBBR comparison. For the wider regulatory and design context of hospital effluent in the region, see our hospital wastewater treatment engineering guide.
Frequently Asked Questions
How much does a 30 m³/day hospital wastewater treatment plant cost?
Equipment cost for a containerised MBR plant at this scale is driven by membrane area, degree of automation and disinfection type, not by nameplate flow alone. Because freight, installation and local civil works vary widely by destination, we quote against a specific site rather than publishing a figure. Send us your flow profile and effluent standard and we will return a scoped quotation.
Why does residual chlorine matter so much in hospital wastewater?
Because it attacks the process that removes ammonia. Nitrifying bacteria are inhibited at chlorine concentrations far below the 5 mg/L that hospital drains can deliver, and nitrification recovers slowly once lost. Any hospital design that omits dechlorination ahead of the biological stage is carrying an unpriced risk.
Can an MBR handle a hospital that only discharges 8 hours a day?
Yes, provided equalisation is sized correctly. The membranes themselves should never see the raw peak — in this design a 24 m³ equalisation tank converts a 3.75 m³/h peak into a steady 1.25 m³/h feed. Without that buffer, the plant would need triple the membrane area and would still starve the biomass for 16 hours a day.
What footprint does a 30 m³/day MBR need?
At this scale the packaged plant is typically containerised, with the equalisation tank as the largest single component. Elevated MLSS roughly halves the biological volume compared with conventional activated sludge, which is generally the deciding factor on hospital sites where space is already committed.
Is this design specific to West Africa?
The process train is not, but two inputs are climate- and region-sensitive: influent conductivity (1,000–3,500 µS/cm here) and ambient temperature, which affects nitrification rate and therefore aerobic tank sizing. The same configuration transfers to comparable dialysis and small-hospital projects once those two inputs are re-checked.
