Houston Flow Profile: Why Camps and Subdivisions Need Special Sizing
Houston's clayey Gulf Coast soils and shallow water tables require designers to add a 10–15% infiltration and inflow (I&I) buffer to computed sanitary flow before sizing any on-site sewage facility (OSSF). Houston municipal supply also carries roughly 50–250 mg/L chloride, which an MBR biomass tolerates, but designers must verify this during pre-design. Camp occupancy swings 2–4x between midweek dorm nights and full weekend events, while residential multifamily blocks swing 1.5–2x between morning and evening peaks. TCEQ's typical peak hourly factor for OSSF design is 4x average hourly flow over a 6-hour window, which is why design flow differs from average daily flow. A sizing calculation that ignores these three drivers risks undersizing the unit or failing the reuse permit. Following these flow profiles, the next step is to calculate the specific average and peak daily flows.
Step 1: Build the Average and Peak Daily Flow
Residential baseline demand is 50 gallons per capita per day (gpcpd), multiplied by unit count and 2.6 average occupants per Houston household. Camp baseline starts at 50 gpcpd for dormitories, plus 25 gpcpd for on-site laundry, plus 10–15 gpcpd for centralized meal service, yielding 75–100 gpcpd in practice. Peak daily flow is 1.5–2.0× average daily flow per TCEQ Chapter 285 OSSF design rules, and this multiplier must be defensible in your design basis memo.
| Parameter | Residential (50-unit block) | Camp (200-person) |
|---|---|---|
| Population | 130 residents (50 units × 2.6) | 200 occupants |
| Per-capita demand | 50 gpcpd (S3 Pure Aqua) | 80 gpcpd (50 base + 25 laundry + 5 meal) |
| Average daily flow | 6,500 gpd | 16,000 gpd |
| Peak factor (TCEQ Ch. 285) | 1.6× | 1.7× |
| Peak daily flow (design flow) | 10,400 gpd (≈39 m³/day) | 27,200 gpd (≈103 m³/day) |
| Houston I&I buffer (10–15%) | +1,000–1,500 gpd | +2,700–4,100 gpd |
Worked example 1 — 200-person camp: 200 × 80 gpcpd = 16,000 gpd average. Apply 1.7× peak factor per TCEQ Chapter 285 to get 27,200 gpd peak, which is the design flow for the MBR vendor. Worked example 2 — 50-townhome residential cluster: 130 residents × 50 gpcpd = 6,500 gpd average, then ×1.6 = 10,400 gpd peak. Both figures serve as the anchor for the sizing workflow. For a parallel Dallas case, the Dallas containerized MBR sizing playbook applies the same math using North Texas climate and inflow assumptions. Once these flows are established, you must define the operational parameters for the MBR process.
Step 2: Select MBR Design Parameters for Houston Conditions
Hydraulic retention time (HRT) of 6–8 hours handles municipal-strength waste at 20°C, and Houston winter lows rarely justify cold-water design below 15°C. Mixed liquor suspended solids (MLSS) operates in the 8,000–12,000 mg/L range, which is higher than conventional activated sludge because the membranes, rather than a clarifier, handle solid-liquid separation. Sustainable flux for PVDF hollow-fiber submerged modules sits in the 10–15 L/m²·h (LMH) window, which is the operating band to specify in the RFQ.
| Parameter | Design range | Notes |
|---|---|---|
| HRT | 6–8 h | Municipal-strength waste at ~20°C (S5 process data) |
| MLSS | 8,000–12,000 mg/L | Higher than CAS; membranes replace clarifier (S5) |
| Membrane flux (PVDF HF) | 10–15 LMH | Submerged hollow-fiber operating window (S5, Mordor 2024) |
| Sludge retention time (SRT) | 20–40 days | Suppresses filamentous bulking (S2 Dynatec) |
| Membrane pore size | 0.04 µm nominal UF | S3 Pure Aqua SMM spec |
Membrane area sizing follows A (m²) = Q (m³/day) × 1000 / (24 × flux). For the 103 m³/day camp at 12 LMH: A = 103 × 1000 / (24 × 12) = 358 m² of PVDF hollow fiber. For the 39 m³/day residential case at 12 LMH: A = 39 × 1000 / (24 × 12) = 135 m². Designers seeking a more compact tank envelope can swap in a flat-sheet module such as the DF series flat-sheet membrane module at a modest energy penalty (flat-sheet aeration is 33–37% more energy-intensive than hollow fiber). When flux or TMP starts drifting, the UF/MBR membrane fouling troubleshooting field guide details the CIP recovery steps. With the process parameters defined, you can determine the physical footprint required.
Step 3: Match the Design Flow to 20ft vs 40ft Container Configurations
A single 20ft HC containerized MBR typically serves up to about 25 m³/day (≈6,600 gpd), while a single 40ft HC unit serves up to about 100 m³/day (≈26,400 gpd). For the 200-person camp at 103 m³/day peak, the appropriate configuration is two 40ft HC skids in parallel—providing redundancy and a clean phase-2 path. For the 50-unit residential cluster at 39 m³/day peak, a single 40ft HC skid covers the load with 60%+ headroom, allowing for future expansion without re-engineering the pad.
| Configuration | Capacity envelope | Best fit (Houston) | Notes |
|---|---|---|---|
| Single 20ft HC | ≤ 25 m³/day (≈6,600 gpd) | Smallest lodges, <40-person camps, single restrooms | Limited redundancy |
| Single 40ft HC | ≤ 100 m³/day (≈26,400 gpd) | 50-unit residential (39 m³/day), small camp dorms | 60% headroom for phased growth (S1) |
| Two 40ft HC (parallel) | ≤ 200 m³/day | 200-person camp (103 m³/day), mixed-use with reuse | One duty / one standby standard configuration |
| Skid (non-containerized) add-on | Variable | Non-standard flows, mobile / trailer-mounted deploys | See integrated MBR containerized skid (10–2,000 m³/day) |
Selection rule: if peak flow ≤ 25 m³/day, buy a 20ft HC; if 25–100 m³/day, buy one 40ft HC; if 100–200 m³/day, specify two 40ft HC skids with parallel piping and shared SCADA. A broader containerized wastewater treatment engineering reference covers the full OEM options if your flow falls outside this band. Once the hardware is chosen, you must address the effluent quality required for your discharge permit.
Step 4: Confirm Effluent Targets and Reuse End-Use
Default containerized MBR permeate runs BOD under 5 mg/L, TSS near zero, and provides 4–6 log virus and bacteria reduction. This meets TCEQ Type I effluent standards, though a disinfection step (UV or ClO₂) is required to push fecal coliform under 20 CFU/100 mL for surface discharge or Type I reuse. If the Houston project targets a Texas Land Application Permit for irrigation, plan for additional nutrient polishing—a containerized MBR supports denitrification with a dedicated anoxic zone, plus chemical phosphorus removal via metal-salt dosing. NSF/ANSI 350-2022 is the de facto product certification for residential reuse in North America and is a recommended line item for any RFQ. After defining the effluent requirements, you can finalize the budget and procurement strategy.
Step 5: Capex, Footprint, and Where to Hand Off to a Vendor
Reference capex for small package plants runs USD 500–2,500 per GPD of capacity, with MBR systems carrying a 20–40% premium over conventional activated sludge. Applied to the camp example: 27,200 gpd × USD 1,500–2,000/GPD = USD 41K–54K base envelope, with the MBR premium pushing the realistic estimate toward USD 50K–70K before site work. The residential example at 10,400 gpd × USD 1,200–1,800/GPD falls in the USD 12K–19K range before the premium. Container footprint is the primary cost advantage on tight Houston sites: an ISO container skid requires only 6–8 m of pad length per unit compared to months of civil work for a comparable CAS plant.
Hand the vendor an RFQ with these line items: container count and size, electrical spec (460V/3Ph/60Hz), membrane type and pore size (PVDF hollow fiber, 0.04 µm), remote SCADA, NSF/ANSI 350-2022 certification, and a design temperature of 20°C. For projects outside the 20ft/40ft envelope, request the integrated MBR containerized skid (10–2,000 m³/day) or the DF series flat-sheet membrane module as starting points, and pin the membrane area from Step 2 as the design verification line.
Frequently Asked Questions
How many gpd per person should I use to size a containerized MBR for a Houston residence?
Use 50 gpcpd for residential baseline and multiply by 2.6 average occupants per Houston household per U.S. Census ACS norms. Apply a TCEQ Chapter 285 peak factor of 1.5–2.0× to get design flow, then add a 10–15% I&I buffer for Houston's high groundwater table.
What TCEQ design flow applies to a camp sewage treatment plant in Texas?
For TCEQ OSSF Chapter 285, design flow is 1.5–2.0× average daily flow with a 4× peak hourly factor over a 6-hour window. For a 200-person camp at 80 gpcpd (50 base + 25 laundry + 5 meal service), peak design flow lands near 27,200 gpd (≈103 m³/day).
What membrane flux should I specify for a PVDF hollow-fiber MBR in Houston?
Specify 10–15 LMH as the sustainable operating window for PVDF submerged hollow-fiber modules. At
