Sizing a Containerized MBR STP for Chittagong Projects (2026 Guide)

What "sizing" a containerized MBR STP actually means in Chittagong

Sizing a containerized MBR sewage treatment plant is not a single number from a brochure. It is a chain of calculations: occupancy → per-capita water demand → sewage return factor → peak factor → average daily flow in KLD → equalization volume → aeration volume → membrane area → module count → container footprint → disinfection and reuse envelope. Each link in that chain can shift the answer by 30–50%, so the engineering deliverable is the chain itself, not the headline KLD.

Chittagong breaks a brochure sizing in two ways. First, tropical peak factors in monsoon-fed catchments and camp-style synchronized discharge (morning shift wash, evening shift wash) push peak flow well above the 2.0× average typically used for European mid-rise housing. Second, ambient temperatures approaching the 40 °C module limit and intermittent grid reliability force larger equalization volumes and standby blower capacity than a temperate-climate datasheet implies. Treating "200 KLD" or "100 KLD" as a finished specification is the most common design error on Bangladeshi residential and camp tenders.

For the purposes of this guide, a containerized MBR STP is a skid-mounted membrane bioreactor system housed in one or more ISO 20 ft or 40 ft containers, fully wired, plumbed and tested at the factory so it can be commissioned on arrival. The civil scope changes compared with a built-in-situ concrete STP; the process scope — equalization, aeration, membrane separation, sludge handling — does not. The process benchmark used throughout this article is the published 200 KLD domestic reference design by Kaveri Watermarke / CPG Consultants India (S1, MBR-STP design features, published 2025-09), which gives influent, effluent, equipment and operating parameters that an engineer can scale to any Chittagong flow between roughly 25 KLD and 500 KLD.

Step 1 — Convert population and camp load into a design flow (KLD)

The first defensible number on a Chittagong cover slide is the design flow, derived from a stated occupancy, a stated per-capita allowance, a stated return factor and a stated peak factor. The Bangladesh Department of Environment (DoE) expects all four values to appear on the design basis sheet, not just the resulting KLD figure.

For mid-rise residential housing in Chittagong, the per-capita water consumption sits in the 120–150 Lpcd band once fixture counts and pressure are normalised. The project engineer should request the developer's actual fixture schedule and the local CWASA connection category rather than defaulting to a brochure number. For worker camps, the figure drops because cooking and laundry are usually communal and shower durations are controlled; 90 Lpcd is a defensible starting point, but the camp operator's hot-water generation and shift pattern must be confirmed before freezing it. The sewage return factor is typically 0.8 of potable consumption — the rest is irrigation, evaporative losses, leakages and tanker draw-off. Apply 0.8 universally unless the site has an unusual industrial draw (then request the water balance).

Peak factor logic is the step most often skipped. Residential occupancies in Chittagong typically peak at 2.0–2.5× average because evening cooking, bathing and laundry overlap. Camp occupancies peak at 2.5–3.0× because the morning shift change and the evening shift change are near-synchronous. The equalization tank is sized to absorb this peak so the downstream biology and membranes see a steady 24-hour feed; sizing the plant on peak flow rather than average flow is the second most common error after skipping the peak factor entirely.

Worked example A — 50-unit residential block: 50 apartments × 4.5 persons × 130 Lpcd × 0.8 return factor = 23,400 L/day average. Apply a 2.2× peak factor and round up to the next standard MBR module size: design flow ≈ 50 KLD, with the equalization tank sized for at least 8 hours of average flow (≈ 17 m³) per the Kaveri 200 KLD reference (S1). Worked example B — 250-person workers' camp: 250 workers × 90 Lpcd × 0.8 × 2.7 peak factor = 48,600 L/day average, peaking near 130 KLD. The average is what sizes the biology; the peak is what sizes the equalization tank and the transfer pump duty.

Write the four assumptions — occupancy, Lpcd, return factor, peak factor — directly onto the design basis sheet. They are the line items the DoE and BNBC reviewers will check first, and they are the line items the supplier will quietly change to make their standard skid fit. Do not let them be changed without a written revision.

Step 2 — Set the influent and effluent envelope before picking modules

The next defensible step is locking the influent and effluent envelopes before any module count is discussed. If the envelope cannot be met by the membrane type proposed, no amount of container engineering will rescue the design.

Domestic influent parameters for Chittagong residential and camp projects can be set against the Kaveri 200 KLD reference (S1): pH 7.5–8.5, TSS 300 mg/L, BOD₅ 400 mg/L, COD 500 mg/L, over an 18-hour inflow design window. The 18-hour inflow window is the trigger for the equalization tank — if the design window is shorter, the equalization volume must grow accordingly. Treated water quality the MBR must hit, again from S1: pH 7.0–8.0, TSS effectively nil, BOD₅ < 5 mg/L, COD < 20 mg/L, turbidity < 2 mg/L, colour < 5 Hazen, unobjectionable odour. Anything weaker than this on the effluent side will fail either Bangladesh DoE Schedule of Standards or a typical hotel/compound reuse contract for gardening and toilet flushing.

Each effluent parameter implies a specific design feature. BOD₅ below 5 mg/L demands a complete biological stage followed by a 0.1 µm membrane barrier — not a 0.2 or 0.4 µm UF membrane marketed as "MBR-equivalent". Turbidity below 2 NTU requires routine membrane integrity testing (pressure-hold or bubble-point) as part of the operating contract, not just an annual check. A COD below 20 mg/L forces the designer to confirm that MLSS is held in the 3,000–6,000 mg/L range and that the aeration tank is sized for genuine BOD removal rather than partial nitrification.

Chittagong-specific risk overlays must be added on top of the temperate baseline. Summer ambient temperatures approaching the 40 °C module maximum specified in S1 reduce oxygen transfer efficiency in fine-bubble diffusers and force blower upsizing or mixed-liquor temperature control. Monsoon infiltration through defective laterals dilutes BOD/COD but spikes flow by 20–40% in May–September, which means the equalization tank and bar screen chamber must be sized for the wet-weather peak, not the dry-weather average. Confirm DoE consent conditions and any local CWASA or Chittagong City Corporation (CCC) sewer admissibility limits before freezing the envelope; a design that meets a generic BOD<5 spec but exceeds the local sewer discharge limit on TSS or oil/grease will still be rejected at commissioning.

Step 3 — Size the unit processes inside the container

With the KLD and the envelope fixed, the next chain link is the unit processes that live inside the container. The Kaveri 200 KLD reference (S1) gives a complete process train: coarse bar screen (8/6 mm openings) → oil and grease separator → collection/equalization tank with coarse-bubble mixing at 8 hr HRT → fine screen (2 mm, semi-automatic) → aeration tank with fine-bubble diffusers → MBR tank with air-scour cassettes → sludge holding tank with coarse diffuser → filter press with screw pump → UV or chlorination → treated water sump. The engineer should expect any quotation to deviate from this train only with a written reason.

Aeration tank sizing is driven by two numbers from S1: MLSS in the 3,000–6,000 mg/L range, and oxygen demand of 2 kg O₂ per kg of BOD removed. A 50 KLD plant treating 400 mg/L BOD₅ down to <5 mg/L must deliver roughly 40 kg O₂/day in the biological stage; a 200 KLD plant must deliver roughly 160 kg O₂/day. The blower specification in S1 is one working + one standby twin-lobe blower at 1,600 RPM, IP 55 enclosure, Class F insulation, supplying the equalization tank, the aeration tank, the MBR air-scour grid, and the sludge holding tank. A second, separate blower is sized for the MBR air-scour grid — never share this duty with the process aeration blower, or membrane fouling will accelerate.

Diffuser choice in S1 is EPDM fine-bubble tubular membrane, 90 mm × 1,000 mm, paired with a coarse-bubble grid in the equalization tank to keep the buffer mixed and prevent septicity. Pump selection rules from the reference are consistent across duties: raw-effluent transfer pumps are horizontal non-clog, IP 68, Class F, sized at 12 m total head and 2,900 RPM continuous rating; one working + one standby is supplied for transfer, MBR suction, MBR backwash, and chemical cleaning. Sludge handling closes the train: a sludge holding tank plus a filter press with screw pump is the standard finishing step — confirm whether the site has slab space for a filter press, or whether a mobile dewatering service will collect sludge on a route.

For the membrane module design logic in this section, the 2026 MBR membrane module design criteria guide covers air-scour rates, cassette geometry and CIP frequency in more depth than this article can.

Step 4 — Pick the right MBR module count and cassette layout

The membrane module specification to design against is taken directly from S1: immersed PVDF fibre, ID 0.8 mm / OD 2 mm, 0.1 µm pore size, 20 m² effective area per module, ABS casing, suction-side operation at 100–300 mmHg, pH 2–10 tolerance, max temperature 40 °C, outside-to-inside flow mode. The design flux is 25 LMH/m², inside a 15–70 LMH/m² operating envelope. Specify at 25 LMH and never at the upper bound without an air-scour guarantee in writing.

Module count calculation: daily permeate volume (m³/day) ÷ (design flux 25 LMH × effective area per module 20 m² × 24 hr) = baseline module count. Round up to the next full cassette, then to the next full container. For a 50 KLD plant that gives roughly 4–5 modules; for 200 KLD, roughly 17 modules in 3–4 cassettes. Air-scour sizing is a dedicated blower (separate from the process aeration blower) sized to deliver the manufacturer's specified N m³/hr per m² of membrane — the published design figure is 0.1 micron PVDF operating at the 25 LMH flux above, with the air-scour blower duty sized by cassette.

Internationally, Kubota cassette-based systems with 5-year membrane warranties are the default commercial choice for packaged MBR plants (per B&P Water Tech and similar packagers, S4) — verify the membrane warranty, not just the plant warranty, because membrane replacement is the dominant lifecycle cost. Flat-sheet modules such as the DF series flat-sheet MBR module tolerate higher solids spikes and are easier to clean in place, which suits camp projects with variable influent quality; hollow-fibre modules such as those in a HydropureWater containerized MBR system give a higher packing density per cassette and lower specific air demand, which suits steady residential loads.

Step 5 — Container footprint, logistics, and on-site integration

A containerized MBR ships as a standard 20 ft or 40 ft ISO skid. Chittagong port handling and the road access to most residential blocks and camps in older wards cap the practical skid size at 40 ft; 20 ft skids are more common for projects below 100 KLD and for sites with tight turning radii. Sludge and treated-water tanks are usually supplied as separate HDPE or FRP modules rather than inside the container, so the site layout has to plan for: the container footprint, the equalization tank (often buried or partially buried), the sludge holding tank, the treated water sump, and the UV / chemical room. For sites where excavation is constrained, an WSZ underground packaged STP can take the equalization and biology underground and free up the container for the membrane and disinfection stages only.

Power and instrument scope is the second on-site question. A 200 KLD reference plant needs blowers, transfer pumps, an MBR suction pump, a backwash pump, a chemical cleaning pump, and a PLC panel (S1) — confirm generator backup sizing against Chittagong's grid reliability, because a 4-hour grid outage during a heatwave will kill an unaerated MBR within hours. The PLC panel must be specified for the local supply voltage and phase (415 V 3-phase is typical) and must log membrane transmembrane pressure (TMP) trends so that cleaning intervals can be planned rather than reactive.

Civil works on the client side are usually under-estimated: incoming sewer invert elevation drives the transfer pump head, the treated-water discharge point must be coordinated with the CCC or CWASA sewer, the sludge storage slab must take a full filter press cake, and the access road must allow periodic membrane cassette replacement (a loaded cassette is not a one-person lift). The operational envelope that should appear in the contract — membrane CIP frequency, spare cassette stockholding, and supplier service response time inside Bangladesh — are the real lifecycle cost drivers, not the headline skid price. An unscheduled membrane replacement 18 months after commissioning, with no local spares, will exceed the original capex delta between two competing bids.

Bangladesh-specific compliance and risk checklist for Chittagong

Walk into a DoE or CCC review with this one-page artifact attached to the design memo, and the design is defensible.

• Regulatory. Confirm DoE Environmental Clearance for the project category (residential, camp, or hospitality), plus any conditional wastewater reuse permission if the treated water is going to gardening or cooling rather than river discharge. The reuse permission is a separate application and a common cause of commissioning delay.
• Standards alignment. Anchor the design basis to BNBC (Bangladesh National Building Code) Part 8 for plumbing and sewerage, and to the DoE Schedule of Standards for industrial and domestic effluents. If the project will discharge to a CCC or CWASA sewer, request the local admissibility limits in writing — they are not always the same as the DoE river-discharge limits.
• Local utility coordination. Engage Chittagong WASA (CWASA) and Chittagong City Corporation (CCC) early on sewer connection conditions, septage handling, and any trade effluent acceptance rules if the development has a commercial component.
• Climatic and site risks. Write the monsoon peak factor (May–September), summer ambient temperature approaching 40 °C, and grid reliability into the design basis as explicit assumptions — none of these appear on a generic European MBR datasheet. The Chittagong STP supplier buyer's guide covers how to verify these in supplier submittals.
• Procurement safeguards. Insist on a pre-shipment factory acceptance test (FAT) on the skid, a membrane integrity test certificate, and a documented commissioning protocol that covers MLSS ramp-up and membrane flux ramp-up. Without these, the supplier's warranty is hollow.

Frequently Asked Questions

What KLD figure should I use as a first pass for a 250-person workers' camp in Chittagong?

Use the calculation chain, not a brochure number. For 250 workers at 90 Lpcd × 0.8 return factor, the average daily flow is around 18 m³/day (18 KLD) before peaking. Apply a 2.5–3.0× peak factor for synchronized camp discharge, size the equalization tank on 8 hr HRT per the Kaveri 200 KLD reference (S1), and round the design flow up to the next standard module size — typically 25 KLD for a single 20 ft container. The headline KLD that sizes the skid, the equalization volume, the blower duty, and the membrane count are all different numbers, and the DoE design basis sheet must show all four.

How much does a containerized MBR STP cost per KLD in Bangladesh, and how do I budget it?

No published price for a Chittagong-supplied containerized MBR was available in the research data, so the budget cannot be quoted as a per-KLD figure. Request an itemised quotation that separates the containerized skid, the membrane modules and warranty, the equalization and sludge tanks, the blower and pump set, the PLC panel, and the installation / commissioning scope — then compare like-for-like across at least three suppliers. Ask each supplier for a 5-year lifecycle cost including membrane replacement CIP chemicals and electricity, not just the headline skid price.

How do I choose between a flat-sheet and a hollow-fibre MBR module for a Chittagong project?

Flat-sheet modules tolerate higher solids spikes and are easier to clean in place, which suits camp projects with variable influent quality and intermittent maintenance windows. Hollow-fibre modules give a higher packing density per cassette and lower specific air demand, which suits steady residential loads where footprint and energy use dominate. Verify the membrane warranty separately from the plant warranty, and confirm the membrane material is PVDF at 0.1 µm pore size as specified in the S1 reference — not a generic UF membrane at 0.2 or 0.4 µm.

What is the lead time and shipping risk for a 40 ft containerized MLR STP into Chittagong port?

The research did not include supplier-specific lead times for Chittagong-bound containerized MBR skids, so no figure is quoted here. Request a written lead time broken into engineering approval, procurement, factory fabrication, FAT, sea freight to Chittagong port, customs clearance, inland transport, and commissioning — and ask the supplier to identify the longest pole in that list before signing. For camp and hospitality projects with hard opening dates, build at least 12–16 weeks of buffer between FAT and on-site commissioning, and confirm the inland transport route from the port to the site can take a 40 ft trailer.

Further Reading

• sister sizing guide for Luanda projects