How do you size an equalization tank
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How do you size an equalization tank?
An equalization tank exists to absorb the difference between what the sewer sends and what the plant can treat steadily. Sewage flow and strength swing through the day - a morning peak, an industrial slug, a storm dilute - and if those swings reach the biological stage unprotected, the whole plant's performance suffers. The sizing question is therefore not 'how big a box' but 'how much accumulated difference must be stored so the downstream process sees a steady feed'.
Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) supplies equalisation and balancing tanks as part of the treatment train, and sizes them from the actual inflow profile rather than a rule-of-thumb percentage, because the profile is what drives the volume.
1. The Data You Need First
Start with a 24-hour (or longer, for industrial loads) profile of both flow and concentration - ideally BOD or COD - measured at the point the equalisation tank will sit. A single average flow figure is useless for sizing, because equalisation is about the variation around the average, not the average itself. The more variable and shock-loaded the inflow, the larger the required volume, sometimes several times a simple average-based guess.
· Flow Profile: Hourly inflow over at least 24 hours, longer for industrial loads.
· Strength Profile: BOD or COD by hour, not just a single average.
· Variation Matters: The swing around the mean, not the mean, sets volume.
· Shock Loads: Industrial slugs can dominate the required size.
· Avoid Averages: An average alone under-sizes the tank severely.
2. The Volume Calculation
Plot cumulative inflow and cumulative target outflow (the steady downstream design flow) against time. The required equalisation volume is the largest vertical gap between the two curves over the cycle - the point where cumulative inflow most exceeds cumulative outflow. This is the mass-balance method and it directly captures both flow and load equalisation if concentration is included. Add freeboard and any sludge storage below the operating band, and size the coating against the extremes of the envelope rather than the average.
· Cumulative Method: Largest gap between inflow and outflow integrals.
· Flow and Load: Include concentration to equalise strength, not just volume.
· Freeboard: Add margin above the operating band.
· Sludge Zone: Account for any solids accumulating below the band.
· Extremes: Coat for the worst-case envelope, not the average.
3. Mixing and Practical Sizing
An equalisation tank that stores but does not mix becomes a settlement tank, losing the solids it was meant to buffer. Mixing is normally around 5-10 W/m3, enough to keep grit and solids in suspension without creating a vortex. A typical retention is several hours to a day, but the real driver is the gap in the profile, not the retention number. Inlet and outlet should be arranged to avoid short-circuiting, and the coating must tolerate the full incoming chemical envelope, including the occasional high-pH or high-sulphide slug.
· Mixing 5-10 W/m3: Keeps solids suspended; too little settles, too much froths.
· Retention Hours to a Day: A consequence of the profile, not the input.
· Avoid Short-Circuiting: Separate and at different levels for inlet and outlet.
· Full Envelope: Coat for the worst incoming chemistry, not the average.
· Slug Tolerance: Size for the occasional extreme, not the typical hour.
Design parameter | Typical value or range | Why it matters |
Retention | Several hours to 1 day | Result of the profile, not an input |
Mixing intensity | 5-10 W/m3 | Keeps solids suspended, avoids vortex |
Flow profile | Hourly over 24 h+ | Variation sets the volume |
Coating envelope | Full incoming range | Includes high-pH, high-sulphide slugs |
Freeboard | Project-specific margin | Prevents overflow at the peak |
Limitation to check | Short record under-sizes the tank | Use full diurnal/industrial profile |
Input | What it sets | Typical value | If ignored |
Flow profile | Storage volume | Several hours to 1 day | Under-sized buffer |
Strength profile | Load equalisation | BOD/COD by hour | Biological upset |
Mixing intensity | Solids suspension | 5-10 W/m3 | Settling in tank |
Freeboard | Operating margin | Project-specific | Overflow risk |
Coating envelope | Lining selection | Full incoming range | Localised attack |
Engineering Assurance and Project Support
Every tank delivered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) is engineered against AWWA D103-09 and EN 1090 with finite element verification of shell, roof and nozzle loads, fused at 820-930°C under ISO 9001 and ISO 45001 control, holiday tested at 1500 V across one hundred percent of the surface, and assembled with Grade 8.8 bolts and manufacturer-certified sealant. Equalisation tanks are supplied with volume set from the measured inflow and load profile, mixing specified at around 5-10 W/m3 to keep solids suspended, inlet and outlet arranged to avoid short-circuiting, and the coating grade matched to the full incoming chemical envelope with test records delivered at handover.
Sizing an equalisation tank is a mass-balance problem: find the largest gap between cumulative inflow and steady outflow over the daily cycle, add freeboard and mixing, and coat for the full chemical envelope. The retention number is an output, not an input.
Frequently Asked Questions (FAQ)
Is a 24-hour profile always enough?
For municipal sewage a 24-hour profile usually captures the daily cycle, but industrial or seasonal loads need longer records - a week or more, or a full production cycle - because the dominant variation may be weekly or batch-driven rather than diurnal. Using too short a record under-sizes the tank.
Should I equalise flow, load, or both?
Both where possible. Flow equalisation alone smooths volume; load equalisation - including concentration in the mass balance - also smooths the strength reaching the biological stage, which protects the more sensitive and expensive downstream process. The cumulative method handles both if the strength profile is available.
What if the inflow is mostly storm water?
Storm dilution and first-flush concentration both matter. A tank sized only for dry-weather flow will be overwhelmed by a storm surge, while one sized for the storm may be oversized for normal operation. The profile, including storm events, is what resolves the trade-off.
Can the equalisation tank be customized to our inflow?
Yes. Volume and geometry from the actual profile, mixing intensity, inlet and outlet arrangement, coating grade for the full envelope, and the test and documentation package are all configured to your inflow, delivered at handover.
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