The residual concentration each is designed to leave in the discharge. Class I is the tighter of the two and is the one specified where the outfall reaches a surface watercourse or a sensitive environment.

Class II permits a higher residual and suits discharge to foul sewer, or situations where a downstream treatment stage will follow.

The class is not a choice made on site. It comes from the discharge consent and the receiving environment, and fitting the lower class where the higher is required is a compliance failure that surfaces at the first sample rather than at installation.

It gives fine oil droplets somewhere to meet. Small droplets rise too slowly to separate by gravity alone within the residence time available, so they would otherwise pass straight through.

A coalescer presents a large surface area of oleophilic material; droplets adhere to it, merge into larger ones, and those rise fast enough to reach the surface layer and be retained.

It is also the part that fouls. A coalescer blinded with silt stops working long before anything visible changes at the surface, which is why the cleaning schedule for the coalescer is separate from the emptying schedule for the oil.

Where the risk is chronic low-level contamination from parked and passing vehicles rather than a credible large spill. A bypass unit treats the first part of a rainfall event, which carries most of the accumulated hydrocarbons, and passes the rest of a heavy storm untreated.

That is a reasonable trade on a car park, because the dilution during a downpour is high and the contaminant load has already been captured.

It is not acceptable where fuel is handled or stored. A fuelling area, a tanker stand or a workshop needs full retention, because a spill can happen at any point in a storm and the whole flow must be treated.

Because silt fills the volume the separator needs and buries the equipment inside it. Runoff from a yard carries grit and sediment continuously, and it settles in the same chamber the oil is separating in.

As the silt bed grows the effective residence time shortens, so separation degrades gradually and without any obvious symptom at the outlet until it has gone quite far.

Larger installations use a separate silt trap upstream so the sediment is caught where it can be removed easily, which keeps the separator itself on a longer service interval and protects the coalescer.

It shuts the outlet when the oil layer reaches the retention capacity, so that captured hydrocarbon cannot be pushed through into the discharge by the next inflow.

The mechanism is usually a float ballasted to sit at the water and oil interface. As oil accumulates the float descends with the interface and eventually seals the outlet.

Once it operates the separator stops discharging altogether, which is deliberate: backing up is treated as preferable to releasing oil. It also means a closure event needs a prompt response, since the drainage it serves is now out of action.

On most regulated installations yes, and it is often a condition of the consent. The alarm monitors the oil layer depth and the silt level, and warns before the closure device operates rather than after.

Without one, the first indication of a full separator is usually surface water backing up in the yard, by which time the response is an emergency rather than a scheduled visit.

Modern units report remotely, which matters on unstaffed sites. A local sounder in a buried chamber on an empty depot is not a warning anyone will act on.

From the drained area and the rainfall intensity for the design storm, which gives the flow the unit must treat, then adjusted by class and by whether it is full retention or bypass.

The oil storage volume is a separate calculation, set by the credible spill on that site: a tanker delivery, a vehicle fuel tank, or accumulated drips, depending on what the area is used for.

Both figures matter independently. A unit sized correctly for flow but with too little oil storage closes frequently, and one with ample storage but too little flow capacity simply passes untreated water during rain.