It uses a bed of sacrificial media, typically limestone or marble chips, that reacts with acidic effluent as it passes through. The reaction consumes the media and raises the pH toward neutral.

Nothing is dosed and nothing is controlled: the chemistry regulates itself, because the reaction only proceeds while the water is acidic. That makes a passive tank attractive for small, intermittent discharges like a teaching laboratory.

The limitation is capacity and speed. The media is consumed and must be topped up, and a strongly acidic slug arriving faster than the bed can react will pass through only partly corrected.

When the discharge is alkaline as well as acidic, when the load is large or continuous, or when the consent requires the outlet pH to be demonstrated rather than assumed.

A dosing system measures pH continuously and injects acid or alkali to bring it into band, so it can correct in both directions and produce a record. A limestone bed can only raise pH.

It is a plant item rather than a tank: pumps, reagent storage, probes that need calibration and a controller. That maintenance burden is the reason passive tanks are preferred wherever the duty allows.

Reducing concentration to a level the receiving system tolerates, for substances where the problem is strength rather than chemistry. A dilution tank holds a volume of water into which the discharge is released and mixed.

It is used where a consent sets a concentration limit that the process stream exceeds only because it is small and strong, and where diluting is a legitimate remedy rather than an evasion.

Where a substance is prohibited rather than limited, dilution achieves nothing. The same mass reaches the sewer, and consents are increasingly written on load for exactly that reason.

Because the tank is deliberately holding aggressive liquid. A polypropylene or polyethylene vessel resists acids and alkalis that would attack concrete, mild steel and many stainless grades.

Fittings and pipework matter as much as the vessel. A chemically resistant tank connected with an unsuitable seal or a metal fitting fails at that fitting, usually where it cannot be seen.

The chemistry of the specific discharge decides it. Hydrofluoric acid, strong oxidisers and hot solvent streams each rule out materials that are perfectly sound for general laboratory waste.

Long enough for the reaction to finish before the water reaches the outlet, which for a media bed depends on flow rate, bed depth and how far from neutral the influent is.

The design case is not the average flow but the worst realistic slug: a single vessel emptied down a sink is a short, concentrated pulse, and it is that pulse the tank has to cope with.

Where flows are unpredictable, a buffer volume ahead of the reaction stage evens out the peaks, which produces better correction than simply enlarging the media bed.

Through an access cover, by removing the spent bed and refilling with new chips. The spent material is largely reacted mineral plus whatever it has trapped, and its disposal route depends on what has passed through the tank.

Consumption rate follows the acid load, so a laboratory that changes what it does changes how often the bed needs attention, independently of the passage of time.

Monitoring the outlet pH is the reliable indicator. A bed nearing exhaustion shows as outlet pH drifting away from neutral during discharges, well before it stops working altogether.

Essentially no, and this is the distinction that matters when specifying one. It adjusts pH; it does not remove metals, solvents, oils or solids.

A discharge that is both acidic and contaminated needs the contaminant addressed separately, and the order of operations matters, since pH correction can precipitate dissolved metals that then have to be captured.

Treating a neutralisation tank as general-purpose treatment is a common and expensive misunderstanding. It sits alongside separators and filtration rather than replacing them.