Laundry Wastewater Treatment

Uniforms worn by manufacturing personnel, hospital employees, and meat packers; shop towels used by service stations, printing companies, and bakers; and floor mats from offices, stores and schools are all items that are washed on a daily basis by industrial laundries.

The wastewater generated from industrial washers contains sand and grit, lint, free and emulsified oil and grease, heavy metals and volatile organic compounds (VOC’s).

The treatment of these waste streams involves multiple stages. Lint removal is accomplished through screening, grit removal requires sedimentation, oils and greases are de-emulsified, coagulated and flocculated through chemical conditioning, solubilized metals are precipitated by pH adjustment, and VOC’s  can be removed with activated carbon treatment.

The typical method to treat Oil Recycling Facility wastewater is as follows:

Stage 1 Emulsion Cracking/pH Adjustment:
pH is lowered (or raised) to ~3.5 with the pH controller using acid (or caustic).  A coagulant de-emulsifier is added to break any emulsion.

Stage 2 pH Adjustment/Precipitation and Coagulation:
pH is adjusted to ~8.5 using caustic and a coagulant is added such as Alum or PAC to cause further de-emulsification and precipitation of the solids. A “pin floc” is developed indicating the emulsion and the suspended solids are precipitated.

Stage 2  - 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 3 - 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 floc 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.

Sludge Dewatering:
The thickened DAF scum/sludge is allowed to accumulate sufficiently to provide a full batch for the Filter Press.  First a precoat slurry is circulated thru the filter press to coat the cloths with DE to prevent blinding of the cloth and aid in easy cake removal.  The filter press is then pumped with sludge until it is full.  The press is emptied of the “cake” which is a semi solid of approximately 20-35 % solids.  Sludge cake is high in fats and greases and solids and should be disposed of according to environmental regulations.

NOTE:
These operations can be performed with screening, microfiltration, dissolved air flotation and/or clarification.  All of these treatments produce a thickened slurry or sludge that may require additional conditioning before being processed with a filter press.  Precoating of the filter cloth may be required to reduce cloth blinding and cleaning problems.