Your seastead design is highly advanced. The use of a SWATH-like (Small Waterplane Area Twin/Tri Hull) configuration with NACA 0030 foil legs will indeed provide a remarkably soft ride by minimizing the surface area affected by wave action. The addition of RIM drives, servo-tab stabilized active foils, and a sheltered dinghy bay demonstrates a deep understanding of marine engineering. Integrating community links and tension leg mooring for stationary periods makes this a truly viable long-term habitat.
Below is a comprehensive analysis of waste management options tailored to your design constraints.
Traditional yachts handle waste differently depending on whether they are at a marina, anchored, or underway. Here is how they currently manage the three main waste streams:
Tank Size: A typical mid-sized cruising yacht has a black water holding tank of about 20 to 40 gallons (75 to 150 liters).
Accumulation Rate: Marine manual or electric macerating toilets use very little water per flush (about 1 to 2 pints). A typical couple generates approximately 1.5 to 2 gallons of black water per day combined.
Time until full: Depending on the flush volume and tank size, a typical couple can last between 10 to 20 days before the holding tank is entirely full and requires a pump-out or offshore discharge.
How they work: These toilets separate liquid waste (urine) from solid waste at the bowl. Liquid drains into a bottle at the front. Solids drop into a main bin mixed with a medium like peat moss or coconut coir. A crank handle mixes the solids, promoting aerobic decomposition.
Costs: $900 - $1,500 (e.g., Nature's Head, Air Head).
Issues: The urine bottle must be emptied frequently (every 1-3 days for a couple), which can be tedious. The solid bin lasts about a month but is heavy and can be unpleasant to empty. It requires an active 12V exhaust fan vented to the outside to prevent odors inside. If gnats or flies get into the compost bin, it becomes a major nuisance.
How they work: The user places a paper liner in the bowl, uses the toilet, and presses a pedal to drop the waste into a combustion chamber. An electrical heating element heats the chamber to 1,000°F - 1,200°F (530°C - 650°C), incinerating all liquids and solids into a small amount of sterile, pathogen-free ash. Exhaust gases are vented outside.
Costs: $2,000 - $4,500 (e.g., Cinderella, Incinolet).
Issues: High power consumption. A single cycle can draw 1,500W to 2,000W for 45-90 minutes. They lose heat to the outside via the exhaust vent. Occasional smells outside near the exhaust vent during a burn cycle. Heating coils need periodic replacement.
How they work: Systems like the Raritan Electro Scan use maceration and a small electrical charge applied to salt water to generate hypochlorous acid (chlorine). This kills viruses and bacteria in the waste. The treated, safe liquid is then legally discharged directly overboard, even in many coastal waters.
Costs: $1,500 for the toilet + $2,000 - $3,000 for the treatment unit.
Issues: They draw heavy amps during flush cycles requires healthy battery banks. They cannot be legally used in federally designated "No Discharge Zones" (NDZs), meaning you still need a small backup holding tank. They require a steady supply of saltwater (or salt additions in fresh water) and periodic electrode maintenance.
Yes, highly recommended.
Given the dimensions of your seastead's roof (an isosceles triangle with sides 70, 70, 35), you have nearly 1,200 square feet of usable roof space. If covered in solar panels, this could easily yield an array of 15kW to 20kW. This is massive electrical capacity for a marine vessel.
Because you have ample electricity, the primary drawback of an incinerating toilet (high power consumption) is completely negated. Furthermore, incinerator toilets yield sterile ash. You would absolutely eliminate the need for heavy black water holding tanks, through-hull fittings for discharge, sea-cocks that could leak, and concerns over dumping in ecologically sensitive areas. The ash can simply be dumped in the trash or used as plant fertilizer.
For a seastead attempting to blend smoothly with the environment, dumping soapy, untreated grey water directly overboard is poor practice—especially when floating stationary. Nutrients in soaps can cause localized algae blooms.
When the seastead is in transit, water mixes rapidly due to the vessel's movement, dispersing any discharges quickly.
When anchored via tension legs for an extended period, the environment below the seastead becomes highly localized. Continual dumping of any organic matter in a single spot will blanket the seabed and cause oxygen depletion (hypoxia).