Hydrated Lime in Water and Effluent Treatment: Dosing, Purity and Storage

Hydrated lime — calcium hydroxide, Ca(OH)₂ — is the alkali most water and effluent plants reach for. It is cheap per unit of alkalinity, it is a dry, stable powder that can be dosed from the bag or made into a slurry, and it does several jobs at once: it raises pH, it precipitates hardness and metals, it helps coagulation, and in effluent plants it neutralises acid and settles out sludge. This article covers what a treatment plant needs from the material and how to keep it in that condition on site.

It is general guidance. Dose rates, target pH and sludge handling are plant matters, worked out in jar tests and on the plant's own water.

What hydrated lime does in a treatment plant

pH correction and stabilisation

Raising the pH of a treated water to a stable, non-corrosive value before it enters distribution; or holding a process stream at the pH a downstream step needs. Lime is dosed as a slurry (milk of lime) or, in small plants, as a dry powder through a feeder.

Lime softening

Calcium hydroxide reacts with the bicarbonate hardness in a hard water to precipitate calcium carbonate, which settles and is removed. With additional treatment, magnesium hardness can be removed as well. Softening plants are large consumers of lime and are sensitive to its purity, because every impurity ends up in the sludge.

Coagulation support

Lime raises pH into the range where aluminium or iron coagulants work and supplies the alkalinity the coagulant consumes. In some waters it is the alkalinity source rather than a coagulant in its own right.

Acid effluent neutralisation and metals removal

Industrial effluents — pickling liquors, process waters, mine water — are neutralised with lime, and at the right pH dissolved metals precipitate as hydroxides and are settled out. Lime is favoured here because it is inexpensive at the volumes involved and because the sludge it makes dewaters reasonably.

Sludge conditioning

Lime added to sewage or industrial sludge raises pH, stabilises the sludge and helps dewatering. Quick lime is sometimes used instead, for the drying effect of its heat.

What to specify

• Available lime (as Ca(OH)₂) — the reactive fraction, which is what the dosing calculation assumes. The Indian specification for hydrated lime for chemical industries is IS 1540 (Part 2); the test methods are in IS 1514.
• Fineness — the percentage passing a stated sieve. A fine, well-classified hydrate makes a stable slurry and dissolves quickly; a coarse one settles in the tank and blocks dosing lines.
• Grit and insolubles — the unburnt, unhydrated and siliceous matter that will not react and will wear pumps, valves and nozzles. A maximum on the specification.
• Free moisture — a small figure is normal; a rising one means the bag has been open or wet.
• Carbonate (loss on ignition) — the measure of how much of the hydrate has already turned back into carbonate in storage. Low on a fresh lot; the trend across certificates tells you about storage.
• Heavy metals, for drinking-water duty — specified against the plant's own requirements and reported on request.

Deshwali Minerals states the parameters for its hydrated lime on the product page, and the lot certificate reports each one.

Quick lime or hydrated lime for the plant?

A large plant with a slaker can buy quick lime and make its own hydrate, paying less per unit of alkalinity and not transporting the water of hydration. A smaller plant, or one without the operators and equipment a slaker needs, buys hydrated lime and doses it directly. The quick lime vs hydrated lime article sets out the choice.

Making and keeping milk of lime

A lime slurry is made by mixing hydrated lime into water with agitation, to a concentration the dosing system is designed for. The slurry settles if it stops moving and scales pipes and nozzles where it dries, so slurry tanks are kept agitated, lines are flushed, and the slurry is made fresh rather than stored for long. A fine, low-grit hydrate makes all of this easier.

Storage: the enemy is carbon dioxide

Hydrated lime does not react dangerously with water the way quick lime does, but it reacts with the carbon dioxide in air, turning back into calcium carbonate that will not do any of the jobs above. The defence is simple: sealed bags, opened only as needed; dry, covered storage; stock rotation so the oldest bag is used first; and a silo or hopper that is not left open. The packing, storage and transport article covers the rest.

Sources

• Bureau of Indian Standards, IS 1540 (Part 2):1990 — Quicklime and hydrated lime for chemical industries, Part 2 hydrated lime: BIS standards portal
• Bureau of Indian Standards, IS 1514:1990 — Methods of sampling and test for quicklime and hydrated lime: BIS standards portal
• Deshwali Minerals — lime in water treatment and the hydrated lime specification