Methacrolein disposal can be treated in the same way as the acrolein one:
“Acrolein may be subject to explosive self-polymerization. Handle with care. Discharge carefully into water, add excess 10% sodium bisulfite (Na2HSO3) solution (prepared by dissolving sodium metabisulfite Na2S2O5 crystals in water). Dilute product with a lot of water and discharge into an oxidation pond, or transport without dilution to an incineration plant.”
But please, note that methacrolein is more reactive than acrolein. So, work carefully in the safety hood with shut safety screen and take the appropriate safety measures.
1). ACROLEIN-HEALTH AND SAFETY GUIDE, Health and Safety Guide No. 67, IPCS International Programme on Chemical Safety, WHO, Geneva, (1991) http://www.inchem.org/documents/hsg/hsg/hsg067.htm
2). Bisulfite Adducts of Acrolein, J. Org. Chem., (1962), 27(2), 649-651https://pubs.acs.org/doi/pdf/10.1021/jo01049a520