Tanker washout wastewater is from tanker cleaning facilities where an extremely wide variety of transported liquid products are washed from the tankers in preparation for the tanker receiving the next load. Tanker cleaning falls into several categories; Food Grade, Chemical Grade, and Specialty Grade. Tankers carry everything from corn syrup to paint pigment.
Tanker cleaning wastewater is difficult to treat mostly because the wastewater strength and composition regularly changes depending on the mix of tankers being washed that day. Add to that the detergents, sanitizers and in the case of chemical grade, various solvents are often used.
The wastewater typically contains an emulsion that must be broken, followed by precipitation, coagulation, flocculation and flotation separation. A mixed/aerated equalization tank ahead of the wastewater treatment system is necessary to store and homogenize the wastewater which will smooth out the operation of the system.
The typical method to treat Tanker Cleaning Washout wastewater is as follows:
Stage 1 Emulsion Cracking/Acidification:
pH is lowered to ~3.5 with the pH controller using acid to break any emulsion.
Stage 2 Precipitation and Coagulation/pH Adjust:
pH is raised to 8.5 with the pH controller using caustic while adding a coagulant such as alum or a PAC. A “pin floc” is developed indicating the emulsion and the suspended solids are precipitated.
Stage 3 - Flash Mix:
The wastewater with it’s precipitated pin floc is introduced to the flash mix zone where a polymer flocculent is added. This stage maximizes flocculent dispersion throughout the coagulated wastewater.
Stage 4 - Flocculation:
The wastewater is now introduced to the slow mix zone to agglomerate the floc into larger particles suitable to be enmeshed with the air bubbles..
Clarifier, Dissolved Air Flotation (DAF):
The flocculated wastewater is introduced into the DAF inlet where the particles are comingled with a pressurized dissolved fine bubble recycle stream. The floc particles attach to the bubbles and float to the surface where they are mechanically skimmed into the float scum sludge chamber. The clarified treated water then exits the end of the DAF and flows downstream to sewer or further treatment if necessary. The DAF system bubbles come from a Recycle Air Dissolving system that takes a portion of treated effluent, pressurizes it and introduces air to be dissolved. The dissolved air comes out of solution and forms a fine bubble stream when the pressure is released at the DAF entrance in the presence of floc wastewater.
DAF Sludge Handling:
The resulting DAF waste scum/sludge is removed from the DAF automatically as the scum accumulates and is pumped to the sludge holding tank where it further thickens and accumulates a batch for disposal or processing in a filter press. The sludge is mixed and conditioned with a filter aid such as DE to improve porosity and filterability which will improve cake dryness and prevent premature blinding of the filter cloths.
The thickened DAF scum/sludge is allowed to accumulate sufficiently to provide a full batch for the Filter Press. The filter press is pumped with the sludge until it is full. The filter press is then emptied of the “cake” which is a semi solid of approximately 20-35 % solids. Sludge cake is high in various contaminants and should be disposed of according to environmental regulations.
Tanker Cleaning Facilities. Contract and captive facilities. Over the road and rail car.