Retention time: why large tank volume does not yet mean treatment

Treatment plants are often chosen by a single figure: cubic metres per day or the number of residents. But wastewater is not treated instantly: settling, flotation and bacteria need time, and that time depends on more than tank volume. Below is how to estimate it and why, at an operating plant, it almost always differs from the design value.

What retention time is and how it is calculated

Hydraulic retention time (HRT) is the average time wastewater spends in a given treatment zone: an equalisation tank, an aeration tank, a clarifier or the whole plant. It is calculated simply: the working volume of the zone is divided by the flow. A tank with a working volume of 10 m³ at an inflow of 5 m³/day gives an average of two days of residence.

The word "average" is what matters here. The calculation assumes that all the water passes evenly through the whole volume. In a real tank part of the flow takes a short path from inlet to outlet, and part of the volume stands almost still. So two tanks of the same volume at the same flow can behave quite differently.

It is important not to confuse water retention time with sludge age, the time the biomass spends in the system. At a plant with sludge return, bacteria stay in the structure much longer than the water, and these are two different parameters. Retention time answers whether the wastewater has time to be treated. Sludge age answers whether the bacteria that do the treatment have time to grow.

When there is not enough time

A short residence time means the wastewater passes through the plant in transit. Organic matter has no time to oxidise, ammonium has no time to turn into nitrate, flocs have no time to settle or float. Outwardly this shows as a turbid effluent and sludge carry-over.

Most often time runs short not on average but at the peak. A calculation based on daily flow assumes a steady inflow, while a real site discharges water unevenly. A hotel has morning and evening peaks, a restaurant a surge after the kitchen closes, a food plant a wash-down at the end of the shift. During peak hours the flow can be several times the average, and residence time in every zone shrinks by the same factor for those hours.

That is why, for sites with surge discharges, the equalisation tank comes first: a tank that receives wastewater as the site releases it and passes it on as a steady flow. For small sites its volume is usually sized for several hours of peak inflow. Without mixing and aeration such a tank itself becomes a source of odour, which is the subject of the next section.

When there is too much time

The opposite mistake is to assume that more volume means more reliability. Time works for treatment only under the right conditions in the zone. Wastewater that stands for days in a tank without oxygen, without mixing and without sludge removal is not treated; it goes septic.

Under these conditions bacteria, left without oxygen and nitrate, switch to sulphate and release hydrogen sulphide. The tank starts to smell, especially in hot weather, after a peak load and when hatches are opened. A grease crust grows on the surface and black sediment on the bottom. A typical picture of an old system is several concrete tanks in a row, a large total volume and yet weak treatment, because all that volume works as an anaerobic settling tank.

An anaerobic zone can be a useful part of a scheme, but only as a separate stage with a clear task and limited time, followed by aerobic treatment. An ordinary septic tank provides settling and partial decomposition, but there is no full biological treatment in it, however many days the wastewater stays there. How such a tank is turned into a managed plant is covered in Septic tank retrofit.

Actual volume versus nameplate volume

Even if the volume-based calculation looked right, at an operating plant residence time decreases over time. There are three reasons.

Sediment. A 10 m³ tank in which 3–4 m³ of old sludge has accumulated works as a 6–7 m³ tank. Wastewater passes through it faster than it seems, and the sludge itself becomes a source of odour. The problem is twofold: the working volume has shrunk, and the pollution inside the system has grown.

Short-circuiting. Inlet and outlet on the same side, no baffles, silted corners: the water finds a direct path, and part of the volume stops taking part in treatment. This is checked by observation: how turbidity at the outlet changes after a surge, where a crust stands on the surface and where the water moves.

Load growth. The site has added rooms, opened a kitchen or a laundry, and the flow has grown. The tanks remain the same, and residence time has shortened in proportion.

So in diagnostics the actual working volume of each tank, allowing for sludge and crust, is measured first, and only then is the residence time compared with what the process requires.

How much time each stage needs

There is no single figure for the whole plant. Each stage has its own task and needs its own time. The ranges below are general engineering guide values for domestic and similar wastewater. The exact value depends on wastewater composition, temperature and the design of the structure.

In SBR reactors (sequencing batch reactors, with aeration, settling and decanting in one tank in a cycle) time is set not by volume but by the length of the cycle phases. If the settling phase is shortened, sludge leaves with the decant. If aeration is shortened, organic matter has no time to oxidise. An example of such a plant is the PneumoSBR development. A long time in the clarifier is also risky: if it contains nitrate, the settled sludge starts to rise, as covered in Sludge rising in the secondary clarifier.

What next

If you are designing treatment or your plant is not coping, start with data rather than with choosing equipment. Record the daily flow and the flow at peak hours, the make-up of the site (kitchen, laundry, pool), the dimensions and working level of each tank, the sludge layer height, and the inlet and outlet position in each of them. From these data the residence time in each zone takes a few minutes to calculate, and it becomes clear at once where it runs short and where wastewater stagnates.

If there is enough volume but it is used poorly, existing tanks can often be moved to a managed mode with aeration, circulation and sludge return; this approach is described in the Retrofit Module development, and separate control of air supply and mixing in the SmartAeration development. Equipment for individual stages, such as flotation units, lamella clarifiers and sludge dewatering, is collected in the industrial wastewater treatment section.