Tassal Tasmanian Salmon, an Australian salmon farming company, backed away from plans to dump treated wastewater from salmon pens into...
A manufacturer of air craft components generates a free and emulsified oily wastewater that results from their machining, milling, cutting and other operations.
Presently, they are using skimmers and dissolved air flotation to remove oil from the water. To remove their heavy metals, they follow up with flocculation, precipitation and clarification.
Due to the rapidly increasing cost of water, and the possibility of discharge permit violations (due to potential failures in their present treatment system), the company installed a 25 gallon per minute reverse osmosis system (RO).
The RO system separates dissolved salts, heavy metals and organics from water by producing two streams - one high in contaminants and dissolved solids (the concentrate) and the other containing relatively pure water (the permeate). The concentrate is returned to the clarifier for additional treatment, and the permeate is recycled back into the manufacturing process. Most of the highly soluble salts from the clarifier, such as sodium, are removed in the liquid portion of the sludge. However, the operator is prepared to "purge" the system of high TDS (total dissolved solids) in case the concentration gets too high.
Reverse osmosis membranes are very susceptible to fouling by free and mechanically emulsified oils. The oil creates a continuous film on the surface of the membrane that plugs the pores, rendering it inoperable. Under certain flow rates less than 5 ppm of oil can totally blind the membranes. This means the membrane can no longer remove the salts and metals from the wastewater. If the membrane is not overly fouled, it can be cleaned and most of its performance characteristics restored. This results in costly downtime and added expense for cleaners.
As a solution to this problem, the company installed an adsorber vessel filled with organoclay in front of the RO unit. The organoclay removes the oil to the non-detect level (the vessel and quantity of organoclay was sized that way), allowing the RO unit to function unencumbered.
The original RO system required cleaning every two days. Pre-treatment of the water with the organoclay has reduced cleaning frequency to once every 30 days.