Equalisation Basins: The Capacity That Pays for Everything Downstream

Suggested readership: designers who have just watched a downstream unit get sized for a peak that arrives two hours a day, and operators who inherited a plant whose "shock load" is actually just Tuesday. Reading time: ~6 min. Companion tools: Equalisation basin · Pipe & dosing-line sizing

Peak factor 2.5 is a weather report, not a design opinion

Municipal sewage in a tropical wet season does not have a polite diurnal curve. A catchment that is dry by 9 a.m. can deliver 2.5 times the daily average by 5 p.m. — and every unit downstream of the wet well has to be sized for that hour, or fail in it. The standard answer is to design everything for the peak: bigger clarifiers, bigger aeration capacity, bigger pipework, all of it used at capacity for perhaps six hours a day.

The alternative sits between the headworks and the biology: an equalisation basin that stores the peak and releases it at the average. On a 10,000 m³/d plant (417 m³/h average, 1,042 m³/h at a 2.5× peak), holding the six hours of excess gives:

• EQ volume: 3,750 m³ — the shaded area between the inflow peak and the outflow line, (peak factor − 1) × daily flow / 24 × fill duration.
• A 938 m² tank at 4 m depth, mixed at 6 W/m³ — 22.5 kW of mixer power.
• Downstream, every unit is sized for 417 m³/h, not 1,042.

That is the whole economic case in one line: an EQ basin converts a rare peak into a constant flow, and constants are what clarifiers, filters, membranes and dosing pumps actually like. A 3,750 m³ lined basin plus mixers is, almost without exception, a fraction of the capital of the downstream overcapacity it replaces — and it never stops paying, because the load it smooths is not only flow.

What equalisation actually buys you

Flow equalisation is the headline, but the quieter benefit is load equalisation. The same basin damps the concentration swings that arrive with the flow — the morning COD slug, the factory discharge at noon — so the biology sees a load that varies by tens of percent instead of a factor of three. In practice this means:

• Clarifiers stop being the weakest link. A clarifier sized for the average SOR no longer has to swallow the evening peak that lifts the blanket and washes out solids. The SVI episodes that scar plant histories are usually peak-flow episodes.
• Dosing and disinfection stop chasing the flow. Chlorine, carbon and coagulant doses are paced by flow; a smooth flow makes every feed-forward loop accurate and every residual steadier. The instruments (SE10A flowmeters on the EQ outlet and the dosing lines) finally see a number they can pace against.
• The aeration system stops being oversized. Design AOR is set by the peak oxygen demand; shaving the peak lets the blower fleet be sized to the mean plus a modest margin, which is where the energy saving lives.

The basin has its own rules

An EQ basin is a tank with a mixer, and it fails in three specific ways:

1. Stratification and septicity. Warm tropical sewage in a 9-hour basin goes septic from the bottom up unless the tank is mixed well enough to keep solids suspended and turnover happening. The 6 W/m³ rule is not a suggestion — drop to 3 W/m³ and you get a sludge layer, sulphide and an odour complaint, and you will spend more on odour control than the extra mixer power would have cost.
2. The bypass reflex. When the basin "isn't needed", operators bypass it to save pumping energy — and the plant quietly loses its peak protection. The design should make the basin the path of least resistance, and the instrumentation should prove it is working: level (S3000L class) for the fill/empty cycle and flow on the outlet for the smoothing ratio.
3. The mixing energy bill. 22.5 kW continuous is real money. Where the load profile is genuinely predictable, duty-cycling the mixer on level or time (full mixing during fill, reduced during the night trough) captures most of the benefit at a fraction of the energy — but the reduction must never be so deep that solids settle in the corners.

Pipework: where the design budget leaks

Equalisation only helps if the pipework between units actually carries the flow it was sized for — and the classic leak is the line sized on nominal velocity without checking the friction loss. On a 2,000 m³/d dosing or transfer line, a theoretical bore of 171.7 mm at 1 m/s becomes a standard DN200, running at 0.74 m/s average and 1.47 m/s at peak — with a Hazen-Williams friction loss of about 0.12 m per 100 m in clean HDPE (C ≈ 150). Two habits prevent most field surprises:

• Size on the peak, check on the average. A line sized for 1 m/s at average flow will exceed 2 m/s at peak, and velocity is where headloss goes quadratic. Conversely a line oversized "for safety" runs so slowly that solids settle — 0.6–0.7 m/s at average is the working floor for solids-bearing lines.
• Let the ageing of C be a number, not a hope. C = 150 is a brand-new pipe. Ten years of biofilm and scale in a warm climate drops it toward 100–120, and headloss climbs by the same ratio. Design headroom for the end-of-life C, not the first-month C.

For dosing lines specifically — polymer, carbon, chlorine — the governing constraint is usually not velocity but the dosing pump's pressure capability at the required rate, which is why the sizing calculation should pair the line bore with the pump duty rather than treating the pipe as a free choice. And every one of these lines earns its keep with a flowmeter: the EQ outlet (SE10A class), the transfer lines, and the dosing line itself. Flow that is not measured is flow that is assumed, and assumed flow is how plants discover their "design" performance was never actually delivered.

The design sequence that saves the most money

Put the EQ basin decision early — before the clarifiers, the biology and the blowers get their sizes. Every unit sized after the basin is smaller, simpler and more reliable than the same unit sized before it, and the basin's own cost (tank, liner, mixers, one flowmeter) is a fraction of the capacity it buys back. In a tropical climate with a real wet season, skipping equalisation to save a tank is usually the most expensive economy in the whole flowsheet.