Cast Iron, 304 or 316L: Choosing a Sewage Pump Material

Our standard sewage pumps are cast iron, and for good reason. When a buyer asks whether stainless steel is worth the extra cost, the honest answer depends on three things: what is in the liquid, what is in the air around the pump, and how long you need the pump to last.

Cast iron: the default for ordinary sewage

Domestic sewage, drainage water and most rainwater are close to neutral, and cast iron handles them well for years. It is heavy, strong and easy to cast into thick, wear-resistant volutes. It is also by far the cheapest option. If your liquid is ordinary wastewater and the pump will sit in a pit, cast iron is usually the right choice.

304 stainless steel: mildly corrosive liquids and clean processes

Choose 304 when the liquid is mildly corrosive, when rust would contaminate the product, or when the site has strict hygiene rules: food processing wastewater, breweries, some light industrial effluent. 304 resists general corrosion much better than cast iron, but it does not cope well with high chloride levels.

316L stainless steel: chlorides, salt and chemicals

316L adds molybdenum, which makes it far more resistant to pitting from chlorides. That is what you need for seawater, brackish water, salty process water and many chemical effluents. It is also the safer choice where the air itself is aggressive. In a recent tender for a power plant in Indonesia, the liquid was only mildly contaminated wastewater, but the site atmosphere carried ammonia and urea dust, so we quoted the pump in 316L with a 316L suction strainer.

How big is the price difference?

It is large. In a recent quotation for a DN150, 7.5 kW cutter pump, the 304 stainless version cost about two and a half times as much as the cast iron version. A mixed solution can be worth considering: a cast iron pump with stainless parts only where corrosion actually happens, such as the suction strainer or the fasteners. Ask us for both prices before you decide.

Quick guide

Liquid or site	Suggested material
Domestic sewage, drainage, rainwater	Cast iron
Food, brewery or light industrial wastewater	304 stainless steel
Seawater, brackish or salty water	316L stainless steel
Chemical effluent	316L, after checking the chemicals with us
Corrosive atmosphere (ammonia, salt spray)	316L, or cast iron with a protective coating

Not sure? Send us a description of the liquid, its temperature and, if you have it, a water analysis. You can also shortlist pump types first with our free pump selector, which lets you filter by material.