Wash Water Treatment and Disposal

By Rick Da Tech


Many of us have been disposing of our wash water by either pouring it out on the ground or down the sewer. The "Man" is likely to have a cow if he finds out you are doing that. If any gets into a stream or goes down a storm drain, then you are in violation of the clean water act. If you pour it down the drain on a city sewer, you are probably overloading the system and causing the wastewater treatment plant to get fined by the federal government for discharging untreated sewage.

The wash water contains both soap and biodiesel. The soaps are heavy in carbons, which demand large amounts of oxygen to biodegrade. The term to describe this is Biochemical Oxygen Demand (BOD). Typically, the wash water from the first wash usually kills your lawn. If you use NaOH, the second and third washes can also kill the lawn. One way to reduce the BOD is to let the wash water settle in a drum for a week or so. Over time, some of the biodiesel bound up in the soaps rises to the top of the drum.

Neutralizing Wash Water

The most effective way to treat wash water is to neutralize with acid. Piedmont Biofuels posted a White Paper by Greg Austic and Simon Lobdell on the subject. Their answer was to flash off the methanol before washing, then to add 32%HCl to wash water. Doing so lowered the pH below 3.5, breaking up the soap. Once the soap released the FFA, a grease trap separated the water from the FFA. Finally, NaOH was added to the wash water to bring the pH back up to 7.

If you use KOH as a catalyst and Sulfuric Acid to neutralize the water, then the salts that precipitate out are potassium sulfate. After skimming off the biodiesel, oil, FFA layer (or running through a grease trap), the remaining water can be applied as a 0-0-50 fertilizer as needed. Be sure you do not apply the water too fast and pollute streams with the runoff.

ION Exchange

There is a process called ION Exchange that can also be used to reduce the BOD. ION Exchange is the process of substituting one ion with another. In this case, you are substituting either the Sodium Ion or the Potassium Ion in the soap with a Magnesium Ion to form soap that does not dissolve in either water or biodiesel. Do this by putting your wash water in a drum and dropping an aquarium air bubbler in to stir the water. Then add Epsom Salt and continue to bubble overnight. Any biodiesel in the water rises to the top. A layer of soap forms just below the biodiesel that can be scooped out and dried. The dried soap is non-toxic and safe to put in your trash. The remaining water contains either Sodium Sulfate or Potassium Sulfate depending on the catalyst you used originally. Potassium Sulfate is a 0-0-50 fertilizer often used in vegetable and flower gardens. It promotes the growth of flowers, fruits, and seeds.

Testing Wash Water

Steve Fugate (TDISteve) posted the following on infopop. It is testing he had performed on wash water to see how ION exchange treatment affects the Waste Water Treatment Facilities' ability to accept the wash water.

    Testing performed on wash water was performed on three samples.

  • Sample #1 had the methanol recovered before washing
  • Sample #2 had the methanol recovered before washing and was treated with 10g/L of MgSO3 ION Exchange Treatment
  • Sample #3 had no methanol recover and no ION exchange treatment


Sample  #1  #2  #3 
 pH, s.u.  9.5 8.1  0.2 

 COD, mg/L
(Chemical Oxygen Demand)

 44,500 18,450  133,000 

 CBOD, mg/L
(Carbonaceous Biochemical Oxygen Demand)

24,800  10,100  81,300 

(Total Solids)

0.9 1.0 4.0

TVS, %
(Total Volatile Solids)

100.0 20.0 88.9


Related Links

Biodiesel Wastewater - Infopop thread - 2007

Grease Trap - Infopop thread - 2007

Treatment and Disposal of Biodiesel Wash Water  Piedmont Biofuels White Paper