Marine sanitation devices (MSD): Type I, II, and III explained

US law under CFR Title 33 Part 159 recognizes three MSD types for treating or storing sewage on vessels over 19.7 ft with installed toilets. Type I: flow-through treatment device that discharges effluent meeting fecal coliform limits (1000/100mL or less) and no visible floating solids. Legal to discharge outside no-discharge zones. Examples: Raritan Electro Scan (chlorine dosing) and Sea Land systems. Requires 12V/24V power and salt makeup. Type II: same concept but with higher treatment standard (200 fecal coliform/100mL), typically for larger vessels, commercial. Type III: no discharge, a holding tank only. Effluent stored and pumped out at a shore station. Simplest, most common on recreational boats. Rules: (a) In no-discharge zones (much of US coastal, all Great Lakes) you MUST use Type III or lock/plug Type I/II. (b) A Y-valve between toilet and tank must be locked or removed in NDZs to prove no direct overboard. USCG boarding officers check this. (c) All three types require the toilet, hoses, tank, vent, and pumpout fitting to be properly plumbed with USCG-approved sanitation hose (marked "sanitation hose", regular hose permeates and stinks in 6 months). Holding tank vent: 5/8-inch minimum, with a charcoal filter to remove sulfur odors, replaced annually.