Formalin is a solution of formaldehyde, methanol, and water. Solutions with formaldehyde concentration greater than or equal to 20% are considered to be lethal hazardous wastes whether used or unused according to MPCA hazardous waste fact sheet #3.35, Regulatory Consensus on Healthcare Issues [PDF 213KB]. Used solutions having a concentration of less than 20% formaldehyde are not considered hazardous waste according to MPCA hazardous waste fact sheet #2.05, The Lethality Characteristic [PDF 130KB]. Unused solutions in which formaldehyde is the sole active ingredient are U-Listed hazardous wastes at all concentrations according to 40 CFR §261.33.
Waste Reduction Tips
While formalin solutions of 10% or less are not technically considered hazardous waste, they are toxic. Best management practices suggest they be incinerated rather than discharged to a sewer. When formalin is discharged to a sewer notification of the local wastewater treatment authority is required. Formalin that is discharged to a sewer should be treated to cross link the formaldehyde and reduce the toxicity. Aldex is an example of a formalin treatment chemical.
For facilities that use five gallons or more of formalin per week, recycling through distillation or filtration is cost effective and is environmentally preferable. For more information, see MnTAP fact sheet Formalin Recovery in Healthcare Labs.