Organisms much larger than the very small unicellular types discussed previously can also form deposits in industrial systems, especially in once-through systems using surface waters. These deposits are termed macrofouling to distinguish them from those formed by the accumulation of large numbers of microorganisms.
The costs associated with macrofouling are estimated to be billions of dollars per year. Fouling by Corbicula fluminea (better known as the Asiatic clam) has reduced some electric utility stations' availability to below fifty percent of normal. In addition to reduced availability, macrofouling costs include plugged lines, screens and traps, reduced water flow, under-deposit corrosion, impaired heat transfer leading to higher turbine backpressure, damage to pump impellers and high costs for manual cleaning during outages.
Macrofouling problems have historically been more common in marine and estuarine environments, where numerous species have been identified as potential problems. This list includes bryozoans, hydroids, oysters, polychaete worms, clams, starfish, mollusks such as Mytilus edulis (the edible blue mussel), and barnacles such as Balanus crenatus.
Of these, the shell-forming organisms such as barnacles and mussels most frequently create problems. Both of these types of organisms release free-swimming larvae which are quite small. The larvae then settle on industrial plant piping or in condenser water boxes where they grow to maturity. Barnacle larvae can attach even in areas with water velocities up to 8-9 feet per second. The blue mussel prefers stable, low-flow environments. In both species, the shell of the adult presents the main difficulty — the barnacle shell will remain attached even after the death of the organism, while the shells of dead mussels can break loose to plug condenser tubes and other small openings.
In general, fresh water environments have a lower density of potential macrofouling organisms than marine areas. Of freshwater macrofouling organisms, two are of the greatest interest.