Hydrated lime or caustic soda: which should you use for pH correction?

In US mine-drainage cost data, hydrated lime is the cheaper chemical for large acid loads and larger plants. Caustic soda is simpler to dose, and it is common on small flows.

Source: US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002

A lime plant costs more to build and run than a caustic soda or soda ash system. Its cost per ton of alkalinity is low, though, and on large acid loads that makes lime the most economical choice (OSMRE). Skousen (2002), citing 1999 US estimates by Skousen et al. (2000), puts caustic soda as the costliest reagent per mole of acid-neutralising capacity, with soda ash next. OSMRE says hydrated lime becomes an option above about 250 US gallons a minute (1.36 MLD) and is very common above 500 US gallons a minute (2.73 MLD). Below 250 US gallons a minute, plants usually treat with caustic soda, soda ash, ready-made lime slurry or passive treatment. The exception is a small flow that is extremely acidic: there, hydrated lime can be an option. OSMRE gives these figures for perspective, not as guidelines. Neither source uses Indian prices.

Source: US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002

Caustic soda is easier to run. It is very soluble and raises pH quickly (Skousen), and it is pumped as a solution (CPHEEO). Lime slurry can cause excess turbidity and needs a lot of operation and maintenance work (US EPA). Neither chemical adds carbonate (EPA lists no DIC increase for either). Both only convert the carbon dioxide already in the water into carbonate (Water New Zealand). So in low-alkalinity water the pH rises quickly, and with either chemical pH control is difficult in poorly buffered water (EPA).

Source: American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; US Environmental Protection Agency, 2016; Water New Zealand (conference paper), 2014

CPHEEO lists all four for pH adjustment and softening. In its list, hydrated lime is 80–90% Ca(OH)2 powder, 'not very soluble'. Quicklime is 40–90% CaO as pebbles, lumps or powder, and is normally slaked first. Caustic soda comes as 96–99% flakes or a 10–40% solution, marked 'dangerous'. Dense soda ash is 98–99%, fed dry or as a 1–10% solution.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Source: National Lime Association (trade association), 2007; National Lime Association (trade association), 2000; US Environmental Protection Agency, 2016; US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; ILO / WHO International Chemical Safety Cards, 2010; Bureau of Indian Standards, 2023; ILO / WHO International Chemical Safety Cards, 1997; US CDC / NIOSH, 30 Oct 2019; ILO / WHO International Chemical Safety Cards, 2024; ILO / WHO International Chemical Safety Cards, 2009; ILO / WHO International Chemical Safety Cards, 2005

First divide the table figure by purity. For 50% caustic lye that is 0.816 ÷ 0.50 = 1.63 kg per kg of sulphuric acid. For hydrated lime at 86% available lime (the minimum for the water-treatment Grade C of IS 1540 (Part 2):1990) it is 0.755 ÷ 0.86 = 0.88 kg. Then divide by reaction efficiency. For mine drainage, Skousen estimates 90% for both limes, 60% for soda ash and 100% for caustic soda. OSMRE warns that lime dosed without mechanical mixing may be only half dissolved.

Source: National Lime Association (trade association), 2007; Bureau of Indian Standards, 1990; American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002; US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022

Example: 50 kg/day of sulphuric acid needs about 50 × 0.755 ÷ 0.86 ÷ 0.90 = 48.8 kg/day of 86% hydrated lime. With 50% caustic lye it needs 50 × 0.816 ÷ 0.50 = 81.6 kg/day. The real need can differ. OSMRE recommends a cold acidity titration of the actual water, using the same chemical the plant will dose.

Source: National Lime Association (trade association), 2007; Bureau of Indian Standards, 1990; American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002; US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022

Alum uses up alkalinity. CPHEEO works out 300 parts of alkalinity as CaCO3 per 666 parts of filter alum, which is 0.45 mg/L per mg/L of alum. TDEC's operator course (Tennessee) uses 0.5 mg/L. It also wants about 10 mg/L of alkalinity as CaCO3 left over, so that leftover coagulant cannot react later and form new floc ('refloc').

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022

Example: 30 mg/L of alum uses 15 mg/L of alkalinity. TDEC's conversion factors are the same as Skousen's. With them, putting that alkalinity back takes 15 × 0.74 = 11.1 mg/L of pure Ca(OH)2, 15 × 0.80 = 12.0 mg/L of NaOH or 15 × 1.06 = 15.9 mg/L of soda ash. At 10 MLD, use kg/day = mg/L × MLD. That gives 111 kg/day of pure hydrated lime, or 111 ÷ 0.86 = 129 kg/day at 86%.

Source: American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002; Bureau of Indian Standards, 1990

CPHEEO doses this lime before the coagulant or with it. Its design example for a 920 m3/h (22.1 MLD) plant uses 25 mg/L, or 23 kg/h (552 kg/day), made up at 10%. Coloured water coagulated at low pH is different: there, CPHEEO corrects pH with lime after colour removal. Poly aluminium chloride was developed to need less pH and alkalinity adjustment than alum (West Virginia). See PAC vs alum.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; West Virginia Department of Health and Human Resources, 2010

CPHEEO says hydrated lime is usually added to make up alkalinity after alum, and that soda ash may be used when an increase in hardness is to be avoided. Soda ash also adds carbonate. It gives 0.94 mg/L of alkalinity as CaCO3 per mg/L, a moderate pH rise and more buffer capacity than hydroxides (US EPA).

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; US Environmental Protection Agency, 2016

By molecular weight, adding 20 mg/L of alkalinity with lime takes 14.8 mg/L of Ca(OH)2 and adds 8.0 mg/L of calcium, which is 20 mg/L of hardness as CaCO3. IS 10500:2012 sets acceptable limits of 75 mg/L for calcium and 200 mg/L as CaCO3 for hardness and alkalinity. Its permissible limits (200 and 600 mg/L) apply only in the absence of an alternate source. The same 20 mg/L from caustic soda or soda ash adds 9.2 mg/L of sodium. WHO sets no health-based value for sodium and puts its average taste threshold at about 200 mg/L. IS 10500 keeps pH at 6.5–8.5 with no relaxation.

Source: Bureau of Indian Standards, 2012; World Health Organization, 2022

In a nitrifying STP, CPHEEO's sewerage manual says nitrification needs bicarbonate alkalinity of about 7 times the ammonia nitrified. If the sewage is short, sodium carbonate or bicarbonate goes in before the aeration tank.

Source: CPHEEO, Ministry of Urban Development, Government of India, 2012

On strong sulphuric acid wastes, lime leaves less TDS but makes gypsum sludge. Caustic soda makes no gypsum sludge but leaves more TDS. The sodium sulphate it forms, about 1.45 kg per kg of acid, is very soluble (ICSC), so it stays dissolved.

Source: ILO / WHO International Chemical Safety Cards, 2009; ILO / WHO International Chemical Safety Cards, 2005; National Lime Association (trade association), 2000

India's general discharge standards require effluent pH of 5.5 to 9.0. That holds whether the effluent goes to surface water, a sewer, irrigation land or the sea. CPHEEO's sewerage manual says neutralisation tanks may have to be RCC lined with acid-resistant tiles or bricks, with the chemicals in acid- or alkali-resistant containers and PVC piping.

Source: Ministry of Environment and Forests / Central Pollution Control Board, Government of India, 19 May 1993; CPHEEO, Ministry of Urban Development, Government of India, 2012

Lime forms 1.755 kg of gypsum (CaSO4·2H2O) per kg of sulphuric acid. Gypsum dissolves to 0.2 g/100 mL, or 2,000 mg/L, at 20 °C (ICSC). Maziarz et al. (2019) give 1,500–2,000 mg/L. Only the excess settles. So, by arithmetic, water with no sulphate to start with forms little gypsum sludge until it has neutralised roughly 2,000 ÷ 1.755 = 1,140 mg/L of acid. Gypsum is also slow to crystallise. The Degrémont (SUEZ) handbook seeds the reactor with over 20 g/L of crystals, so the gypsum does not stay supersaturated and then precipitate later, downstream. The handbook says lime alone typically leaves 1.5–2.5 g/L of sulphate, depending on salinity and excess calcium.

Source: ILO / WHO International Chemical Safety Cards, 2009; Materials (MDPI), 2019; SUEZ (Degrémont Water Handbook, vendor), 2026

The National Lime Association, a lime trade body, says sodium alkalis give the lowest sludge volume and the highest dissolved solids, with gel-like sludge when heavy metals are present. Its own table, though, lists that sludge as high-volume. Soda ash reacts violently with acids and gives off heat and carbon dioxide (ICSC). Add it slowly and expect foaming.

Source: National Lime Association (trade association), 2000; ILO / WHO International Chemical Safety Cards, 2024

Lime dissolves poorly, and even less when hot: 0.165 g per 100 g of saturated solution at 20 °C, and 0.071 g at 100 °C (NLA). Drinking-water plants often make saturated lime water, about 1.6 g/L, in a saturator (SUEZ). Making lime water this way needs large volumes of water and tanks (OSMRE). Otherwise hydrated lime is dosed as milk of lime, which is a suspension.

Source: National Lime Association (trade association), 2007; SUEZ (Degrémont Water Handbook, vendor), 2026; US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022

SUEZ keeps milk of lime at 50–100 g/L. Stronger slurry risks deposits and blockages. Weaker slurry risks carbonation. Never stop the mixer while there is slurry in the tank. Some limes turn to paste above 30% solids and are too stiff to pump at 40% (NLA). Flush lime feed pipes with an acid solution from time to time to dissolve calcium carbonate scale (CPHEEO).

Source: SUEZ (Degrémont Water Handbook, vendor), 2026; National Lime Association (trade association), 2007; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Quicklime is slaked first. In theory it needs 3.1 parts lime to 1 part water, but slakers run at 1 part lime to 3–6 parts water (OSMRE). Slaking releases about 275 kcal per kg of CaO (SUEZ; NLA gives 273). That is enough to heat 3.9 kg of water from 12.8 °C to 82.2 °C if no heat is lost (OSMRE). Slakers run at 77–88 °C. Slaking takes under 1 to 10 minutes, and then the grit is removed.

Source: US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; SUEZ (Degrémont Water Handbook, vendor), 2026; National Lime Association (trade association), 2007

OSMRE says hydrated lime holds 25% less CaO by weight than quicklime. Put the other way, 1 kg of quicklime does the work of about 1.32 kg of hydrated lime (0.755 ÷ 0.572). Quicklime is also denser and less dusty. But it needs a slaker, operator skill and maintenance (OSMRE), so small plants commonly use hydrated lime, which goes straight into the process (West Virginia).

Source: US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; West Virginia Department of Health and Human Resources, 2010; National Lime Association (trade association), 2007

• ICSC lists caustic soda flakes as UN 1823, Class 8, packing group II, and caustic soda solution as UN 1824, Class 8, packing group II or III. IS 4264:2023 lists packing group II for both.
• Add flakes slowly to a well-stirred tank of water. Never pour water onto caustic soda: the heat can boil and spatter the solution (IS 4264; ICSC).
• Wear goggles, a face shield, rubber gloves, an apron and boots (CPHEEO). Store dry caustic soda where moisture cannot reach it, and liquid caustic soda in covered steel tanks (CPHEEO). Keep caustic soda away from aluminium, tin, lead and zinc. It corrodes them and gives off flammable hydrogen gas (ICSC). Store caustic soda and lime away from strong acids (ICSC).
• 50% caustic lye starts to crystallise at about 12 °C (54 °F) (OxyChem). IS 4264 asks for a way to heat pipelines exposed to low temperatures.
• Quicklime gets very hot with water, hot enough to ignite combustible materials (ICSC). CPHEEO warns that damp lime storage may set nearby flammable material alight. It calls hydrated lime less troublesome than quicklime.
• Never mix dry quicklime with chemicals that contain water of crystallisation, such as alum or ammonium sulphate (CPHEEO). West Virginia's operator course warns that quicklime and alum may explode together. Never use the same conveyor for both.
• Prolonged exposure to lime dust causes dermatitis. Wear face shields and goggles at slakers (CPHEEO). NIOSH recommended exposure limits are 5 mg/m3 for hydrated lime and 2 mg/m3 for quicklime (time-weighted), and a 2 mg/m3 ceiling for caustic soda.
• Lime absorbs CO2 from the air and turns back into calcium carbonate (NIOSH; NLA). Keep bags closed and dry.
• ICSC lists quicklime as UN 1910, Class 8, packing group III. That entry applies mainly to air transport: the IMDG Code (sea, special provision 960) and ADR (road) exempt calcium oxide. ICSC gives no UN class for hydrated lime or soda ash. Follow section 14 of the product's SDS.

Source: ILO / WHO International Chemical Safety Cards, 2010; Bureau of Indian Standards, 2023; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; OxyChem (vendor), 2026; ILO / WHO International Chemical Safety Cards, 1997; West Virginia Department of Health and Human Resources, 2010; US CDC / NIOSH, 2019; US CDC / NIOSH, 30 Oct 2019; National Lime Association (trade association), 2007; ILO / WHO International Chemical Safety Cards, 2024

Compare cost per kg of acid neutralised, or per kg of alkalinity, as OSMRE and Skousen do. Do not compare per kg of product.

Source: US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002

1. Cost per kg of acid = delivered price per kg × kg of pure alkali per kg of acid (from the table) ÷ purity ÷ reaction efficiency (from titration or plant records).
2. Add the slaker or slurry system, mixers and operator time.
3. Add sludge disposal: with lime, up to 1.755 kg of dry gypsum per kg of sulphuric acid, plus the water the sludge holds.
4. Check that the sodium and TDS from caustic soda or soda ash still meet your discharge or drinking-water limits.

Source: National Lime Association (trade association), 2007; American Society of Mining and Reclamation (J. Skousen, West Virginia University), 2002; US Office of Surface Mining Reclamation and Enforcement (OSMRE), 2022; ILO / WHO International Chemical Safety Cards, 2009

Rise Chemical India supplies all four alkalis in this guide but does not make them. Lime powder is hydrated lime of 90–95% purity in 25 kg and 50 kg bags, minimum order 100 kg, and quick lime is 90–95% CaO, as lumps or powder, in 25 kg and 50 kg bags, minimum 250 kg. Caustic soda flakes (NaOH 98–99%, UN1823, Class 8) and dense soda ash come in 25 kg and 50 kg bags with a 250 kg minimum. At its own factory in Bharatpur, Rajasthan, the company makes ferric alum, which a plant can put on the same order as its alkali. The company is ISO 9001:2015 certified (certificate E20251237903). Its office in Kirti Nagar, Delhi handles every enquiry, quote and order (+91 95829 99838). It sells in India and quotes every product for export, per order. The price depends on quantity and destination. Send an enquiry.