Effluent

Sewage is what arrives at the works, effluent is what leaves a treatment stage, and sludge is the solids taken out in between. The three get used as though they were interchangeable. They are not, and the pump changes at each step.

Raw sewage is the foul flow arriving at a station or a works, gross solids and rag included. Effluent is what leaves a process or a treatment stage once those gross solids have been screened, settled or digested out of it. Sludge is the other half of that separation: the solids concentrated into a thick medium, measured as dry solids content (% DS).

Effluent is the middle one, and the loosest. Final effluent leaving a works is treated water, and a centrifugal pump is the efficient choice for transferring or boosting it. Untreated industrial effluent can carry more fibre and abrasive than the raw sewage two miles up the sewer. The word tells you where the liquid came from. It does not tell you what is in it.

Sludge is where the centrifugal stops. The trade rule of thumb is roughly 3–4% solids; above that, move to positive displacement, and rising viscosity pushes the same way. A progressing cavity pump holds its flow to within a few per cent as the pressure moves, which a centrifugal does not. If someone hands you a duty labelled effluent and the sample is thick enough to hold a spoon, you have been handed sludge.

In the UK, trade effluent is defined in statute. The Water Industry Act 1991 calls it any liquid, with or without particles of matter in suspension, wholly or partly produced in the course of any trade or industry carried on at trade premises. Domestic sewage is excluded. An occupier may discharge it to a public sewer only with the sewerage undertaker's consent, and discharging without one is an offence.

That is worth knowing before you size a pump, for two reasons. The consent normally caps the discharge rate, and a capped rate is a duty point, so read it before anyone specifies flow. And the consent describes what you are allowed to put down the sewer, which is a decent second opinion on what is actually in the medium. Sites are often clearer about their consent conditions than about their pump duty.

On wastewater, three causes account for most of what we get called about, and they arrive in this order.

Rag and solids build-up. Wipes and fibre bind up the wet end until the pump is working against itself. This one catches effluent duties particularly hard, because effluent is supposed to have had its gross solids removed. The pump gets selected for the medium on the drawing. A screen that blinds, a bypass that opens in storm flow, or a process upstream that changes, and the pump is passing rag it was never chosen to pass.

Grit. Grit reaches the pumps when the grit removal on the works is not doing its job. It abrades the wet end, wear follows, and the pump fails early. The pump is rarely the thing at fault, and replacing it with the same pump buys you the same wear life again.

Dry running. Usually because the suction conditions were never right. On a centrifugal you lose the mechanical seal, because a mechanical seal needs liquid to lubricate it. There is a fourth cause with no design fault in it at all: somebody shuts a valve, forgets it is shut, and runs the pump. Same outcome, and no amount of specification prevents it.

The enquiry we decline most often sits on top of the first of those. A pump keeps clogging and we are asked to supply a better solids-handling pump. If what is fitted is already a good solids-handling pump, a like-for-like swap fails the same way in the same place. What that site needs is rag removal upstream, which means a macerator or chopper pump. That is a system change nobody has asked us to sell, and we would rather say so than quote the swap.

Select the type against the medium, then size the unit against the duty point.

For effluent carrying solids, a submersible pump with a large free passage, sitting in the sump, is the default and for most wet wells it is the right answer. The impeller is the decision inside that decision. A single-channel impeller carries the flow down one cast passage, so the medium is worked by the vane and the pump holds a real duty point. A free-flow or vortex impeller sits recessed out of the flow and drives the medium by the swirl ahead of it, so very little of what you pump ever touches it. That is why it survives fibre a channel impeller would wrap, and why it is the less efficient pump. You pay for the tolerance every hour the machine runs.

The passage sizes are published, so the trade can be read off a catalogue rather than argued about. On Calpeda's GM 50 range, the single-channel GMC passes 45 mm solids and the free-flow GMV takes 50 mm.

For final effluent and treated water, go centrifugal. There is nothing to gain by paying for solids handling the medium will never test. Materials follow the medium rather than the sector: coated cast iron suits most sewage and screened effluent, gritty duties want hardened or replaceable wear parts, and stainless earns its cost only where the medium is genuinely corrosive.

The sewage and effluent pumps we supply cover both ends of that, and the stage-by-stage view of a works is in our water and wastewater sector page. Send us the flow, the head, what is in the medium and how it gets there, and we will tell you what the duty needs.