alkaline solution (caustic solution)

An alkaline solution is any aqueous liquid with a pH greater than 7, formed by dissolving a base — most commonly sodium hydroxide (NaOH, caustic soda), potassium hydroxide (KOH), or sodium carbonate (Na₂CO₃, soda ash) — in water. The hydroxide ion (OH⁻) released in solution neutralises acidic species and dissolves acid gases through acid–base chemistry. Concentration is usually expressed as weight percent of the dissolved hydroxide.

In biogas plants, alkaline solutions are the working fluid in wet scrubbers used for hydrogen sulfide removal from raw biogas. The scrubbing reaction is H₂S + 2 NaOH → Na₂S + 2 H₂O, with sodium sulfide as the soluble end product. Operating concentrations are typically 5–15% NaOH for gas streams with H₂S above 2,000 ppm. Solution pH is monitored continuously, and fresh caustic is dosed to maintain pH above 9 — below this, scrubbing efficiency drops sharply because dissolved sulfide begins to re-volatilise. The same chemistry handles CO₂ in some upgrading systems, though CO₂ scrubbing consumes far more caustic per Nm³ and is rarely the preferred approach for high-purity biomethane production.

The principal trade-offs are operating cost and spent-liquor disposal. Caustic soda costs ₹50–70 per kg in India, and a 5 TPD CBG plant with 3,000 ppm raw H₂S can consume 30–80 kg of NaOH per day. Spent scrubbing liquor — rich in sulfides — is classified as hazardous waste under the Hazardous and Other Wastes Management Rules, 2016 and must be treated before discharge, typically by oxidation to sulfate or by sale to fertiliser blenders. Safety considerations are significant: alkaline solutions cause severe skin and eye burns, and operators must use full chemical PPE during makeup and sampling. Many SATAT plants prefer dry iron-oxide H₂S removal precisely to avoid the spent-caustic handling burden.