Why use CO2 in Wastewater Treatment Plants?

Waste Water treatment
Manufacturing & Process
12 November 2018

Increasingly, governments are promoting alternatives to sulphuric and hydrochloric acids in wastewater treatment plants, due to the significant health risks associated with these aggressive acids. CO2 proves to be an attractive alternative and has a superior capacity to neutralize wastewater acidity compared to the alternatives. What’s more, CO2 does not entail health risks, it is easier to use, more sustainable and more economical.

“CO2 is proving to be a highly effective tool to monitor wastewater acidity”, explains Rudy Lamond, Growth & Innovation Leader Water Treatment at Air Liquide. “Using CO2 results in lower maintenance costs and, unlike the stronger acids often used it’s not corrosive. CO2 also avoids the release of harmful sulphates and chlorides to the aquatic environment. The CO2 itself is a recycled by-product from – amongst others – AD Biogas plants, meaning that there are no additional CO2 emissions to the atmosphere whatsoever. Also, the risk of unpleasant odour problems from H2S is eliminated.”

Easy to handle

“Because of its natural buffer effect, CO2 will prevent influent pH dropping much below 6, even in case of overdose. And as CO2 is a weak acid, the pH value drops more gradually than with mineral acids, making accurate control a lot easier. It’s also possible to optimally dose the polymers, to maximize their operation. But most importantly, using CO2 is safer, because nobody has to handle any corrosive acids.”

Several designs available

Air Liquide has developed a number of gaseous and liquid CO2 injection systems. Depending on the layout and the way the water purification station operates, the following solutions can be chosen:

1-Gaseous CO2 injection (basin): Porous diffusers gently bubble CO2 into an existing basin; low doses of CO2 for basins with a depth of > 3m.

2-Gaseous CO2 injection (pipe): Ideal for low, stable effluent flow rates.

3-Gaseous CO2 injection (BICONETM): Optimal for stable effluent flow rates or when gas/flow ratio is high.

Injection and mixing systems

“There are several injection and mixing systems available, so there is no need for water treatment plants to modify their systems. The BICONETM static mixer – designed by Air Liquide – is the most commonly used solution. It can be easily installed in existing plants and provides economical and highly efficient gas injection with minimal hydraulic loss. Its high efficiency is based on the counter-flow principle of gas and water, which results in a high gas-liquid interface. Furthermore, he mass transfer is optimized by the conical shape of the mixing vessel.”

“Another commonly used solution is the CS nozzle. It has a calibrated opening, which creates a drop in pressure (Delta p from 0.5 to 1.2 bar), leading to an increase in the flow speed and finer bubbles. CS nozzles are very reliable and are maintenance-free. It’s only a small investment and operating costs are remarkably low.”

Injection of CO2 via a CS nozzle producing fine bubbles.

Alternatively, helicoidal static mixers, Primix mixers, sparges (installed at the bottom of the basin) and PoroxalTM or porous diffusers are available for CO2 injection.

Nexelia for pH Control

Through its Nexelia brand, Air Liquide offers comprehensive gas solutions, application technologies and expert support designed for and adapted to your specific needs. We work closely with you to pre-define practical results, and we commit to delivering them. Nexelia for pH Control is suitable for both municipal and industrial wastewater treatment plants.

Please contact Rudy Lamond for more information.