Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers



Last updated: September 12, 2019

What Does Sludge Mean?

Sludge is a semi-solid slurry and can be produced as sewage sludge from wastewater treatment processes or as a settled suspension obtained from conventional drinking water treatment and numerous other industrial processes. The term is also sometimes used as a generic term for solids separated from suspension in a liquid. Sludge can cause:

  • Tube failures
  • Restricted circulation
  • Reduced system efficiency
  • Compromised boiler system reliability

Industrial wastewater solids are also referred to as sludge, whether generated from biological or physical-chemical processes. Surface water plants also generate sludge made up of solids removed from raw water.


Corrosionpedia Explains Sludge

Sludge is a water-formed sedimentary deposit which may include all suspended solids carried by the water and trace elements in solution in the water. Sludge reduces heat efficiency and may cause scale formation in boilers and heat exchangers. Boiler sludge is a deposit that forms when suspended materials present in the boiler water settle on, or adhere to, hot boiler tubes or other surfaces.

Sludge may be formed from a combination of whatever suspended materials are in the water, including loose corrosion products, insoluble mineral precipitates and oil. A significant proportion of suspended material generated outside the boiler can find its way inside. When the amount of sludge (agglomerated suspended material) exceeds the ability of the boiler water to carry it in suspension, it settles on the boiler surfaces. A film of sludge deposited on a heat transfer surface is commonly called "baked-on sludge." It acts as a heat insulator and provides an environment promoting crevice corrosion.

Baked sludge is very difficult to remove by mechanical means, and boiler compound has no effect on it. The best method is to determine where it comes from, collect it with proper treatment and then blow it out before it cooks.

Generally corrosion and sludge build up is due to the corrosion process and the consequences of the system. The abrasive nature of sludge may lead to premature pump failure and motorized valve failure. Polymeric dispersants or sludge conditioners are added with phosphates to condition the sludge for improved removal from the boiler.


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