Calix - CXL (mc $346m). Of the 5 x major arms of the business 2 of them are involved in water tech.
As described by Calix on their website:-
ACTI-Mag for wastewater treatment. ACTI-Mag is one of the safest, most cost-effective and earth-friendly ways to manage hydrogen sulphide, odour, pH and alkalinity, BOD/COD, phosphate and fats/oils/greases (FOGs) from wastewater. ACTI-Mag can increase the quality and quantity of biogas coming from anaerobic systems, and provide a significant economic boost by turning wastewater into a viable source of renewable energy.
AQUA-Cal+ for pond and lake remediation. AQUA-Cal+ addresses problems associated with lake remediation such as fish kills, odour, bad algae blooms, little circulation and sludge build-ups.
I hold Calix. But I'm mainly interested in their Carbon capture technology. This water treatment space seems like a hotly contested and competitive area.