Trailer-Based Modular Plumbing System Plan

I was tasked recently to design an entirely off-grid plumbing system for an intentional community that housed 10 individuals. This project was only partially constructed but I thought some of the research done was interesting. Here it is presented with light editing, hopefully it is useful to someone.
Objective Statement

Currently we have immediate needs of a functional, self-contained, plumbing and water management system to support the project. This includes vendor provided grey and white water tanks, a kitchen support assembly, and a single bathroom assembly, and the following design decisions reflect this use case. At it’s very core, the proposed systems are the scaled-up recreation of boat plumbing systems. We are placing heavy emphasis on point-of-service processing, such as debris grating, in order to decrease our dependence on process filtering at the holding system.

Specification :

Design and implement a plumbing system that is housed entirely on mobile platforms. System needs to support the lives of 6-10 users. This includes a fully functioning bathroom and a kitchen. The holding tank module needs to be close enough to the entry of the space for servicing by our ‘pump out’ service, until such time as we find a way to do our own. Freshwater delivery, herein referred to as ‘whitewater’ is starting at 300 gallons per week. Do not mix septic and grey wastewater systems. Attempt to provide at least 5 minutes of non-shower use of the system ‘in reserve’ before a pump initiates. Dampen the noise impact of the plumbing system to a reasonable degree. Expect to purchase a handicap-capable portapotty shell in the near future and convert it to a second bathroom. Maintain a modular design approach and implement no techniques. Collate every component datasheet and create an installation protocol that conforms with documented requirements, while also creating a maintenance schedule. This document will serve as part of this effort.

Phases -

Due to budget constraints this project is being broken up into phases.

Phase 1 - Freshwater system active and circulating. Hot water is provided at point of service. Grey water is collected in a holding tank. Kitchen and Bathroom zone serviced.

Phase 2 - Recycling shower system installed. Second shower installed. Hot water low-flow solution implemented. Quick connect system installed.

Phase 3 - Black water holding tank system installed. Toilets activated, maceration system installed. Portapotty service cancelled.

Phase 4 - Placement of modules on trailers. Extension collection basins and pumps installed.

Project Milestones -
Finalize Feature Specification / First Draft Schematic
Populate BoM Feature Matrix / Mathsman Feedback / Final Draft Schematic
Finalize BoM / Assemble Installation Instructions / Final Schematic / Order


CSV System Description -
Currently I am building a csv file that describes elevations, diameters, lengths, closures, and materials for each length. We also need to describe the number of elbows and T’s (straight and bend/branch). This is being fed to a MathCAD dude for optimization. Once this is done we will be populating a worksheet that describes what diameters, materials, and locations for sourcing components against.

This description also includes an outlets and inlets description; describing pressure, diameter, gpm flow ranges, mins and maxes.

These descriptions and the maths will give us hard criteria for component features needed, which will give us guidance for filling the BOM.



Systems
Holding Tank System -
Holding tanks should be leveled on a platform in a space against the 'back wall/fence' allowing plumbing to stay concentrated against the back of the space. Tanks require vendor access and must be within 15 or so feet from the pull-in driveway; providing the rationale behind placement in the gravel to the left of the pull-in. Secondary sites for tanks include the right side of the pull-in behind the current 'kitchen bus'; which would limit future relocation of vehicles in the front end of the lot. Another more esoteric option is to create a vendor pump out station outside of the fence behind a lock box with access conduit running to the inside of the IBC's located just inside the fence. This installation would require breaking the visibility and sound barrier provided by the street-side bus-wall, and was rejected.
Holding tanks should be blocked for visibility if possible, with a simple enclosure being a likely option. Power service for pumps and sensors to the holding tank assembly is categorized as Critical, and a lack of power should indicate fault.

Freshwater tank is provided by the vendor and is filled 300 gallons once a week on a monthly service contract.
Septic IBC should be ventilated, nontransparent, and can be the smallest of the holding tanks. Following the UN refugee program guideline for amount of septic produced by a human and targeting a 10 person population onsite eventually, an absolute maximum of 336 gallons a week can be anticipated. This dramatically decreases with the inclusion of water saving design decision such as 'flushless' urinals.
IBC containers fed by slightly buried sump pump basins. One for black and one for grey water each. Each basin has a sump pump or ejector pump inside said basin that is activated by float switch. These pumps are disabled via overfill sensors in the IBC tanks to ensure no overfilling, also indicating fault. The septic basin should be as small as possible and serves primarily as a lift pump into the IBC from the horizontal run from feed line, and should be sourced as a vented sealed unit. Grey water basin serves as a collection point for disparate grey water sources as well as lifting pump.
Black water IBC should be installed with through-hole flushing spray nozzles allowing filtered greywater to be pumped through the nozzles cleaning the inside of the tank immediately post-servicing. This can be a manual or automatic feature. Nozzle can be rotating or static type mounted high in the tank, and shouldn't be operable without ventilation or pressure relief to avoid backpressure.
IBC containers should be fitted with specialty lids that allow sealed connection to liftpump intake pipe and a sealed accessible service hatch for contractor pumpout.
An optional faultstate response to greywater being full and blackwater having space is to send greywater to blackwater holding via spray system until greywater levels are lowered out of fault state.
Kitchen -
Sink is plumbed very normally to sinks via freshwater system. Insulated hot water lines deliver hot water from the hot system. Greywater from the kitchen is particulate filtered at the up-to-5 drains and gravity feeds downward towards the bathroom greywater collection system or is pumped from the sink to the ejector pump basin at the greywater holding tank.
Drainage could be assisted by including a waste processor/ garbage disposal as well, but sluicing and removing particulate still needs to happen.
Bathroom System Summary -
Shower -
Shower is outfitted with recycling shower system and low flow showerhead. Inside shower is a control switch of valve that initiates recycling shower. When activated water drained from the shower, and ONLY from the shower, is accumulated in an expansion tank via recycling shower pump. Hand valve control shuts off freshwater feeds and grey water outlet. A pressure regulated valve allows fresh hot water to be added to the system as required to keep the pressure up, which is pressurized by a small pump. A check valve between the pump and pressure valve ensures fresh hot water doesn't drain into greywater. A check valve after the toilet reserve tank ensures other greywater doesn't drain into recycling system. (refining this based on the design of the showerloop open source design).
Sink - Standard sink is plumbed already and just needs a low flow aerator. Connects to ejection system on the wastewater side of the check valve.

Toilet -
Low flow rear-drain toilet is installed parallel to rear-mounted macerator outside of bathroom on stand. Urinal is also piped to macerator unit via 2in port. Other ports are sealed. Outside mounted greywater reserve tank/expansion tank used in recirculating shower is used to feed into toilet flush reserve, falling back to freshwater in the toilet reserve tank as needed. When recycling shower control valve is in the 'off' position the recycling system gravity drains into the toilet water reserve tank. Toilet bowl is always filled with water from the reserve tank with no direct freshwater inlet. Reserve tank has a float sensor for indicating when full, closing the reserve tank inlet via solenoid and forcing further greywater into the ejection pump system via shower drain. Float sensor also indicates when levels hit 'low storage' and open a solenoid valve to release enough freshwater into the reserve tank for 'a few' flushes.

Optional Urinal changes some but not much of this calculus and is tapped into the reserve tank just like the toilet if it flushes. Is connected to the macerating pump elsewise.

Macerating Pump-

Initial maceration unit was planned to be a SaniFlo unit. Somewhat exhaustive research into how this unit functions has opened up the possibility of recreating the unit at a fraction of the cost. Home-made maceration units can be found on youtube and in RV forums, being gravity fed by a black water system and macerated by a garbage disposal; which has a BoM of around $100. This is an interesting system but ignores a few problems that are somewhat easily rectified. First and foremost the grinding unit in a garbage disposal is NOT a pump and any forward head pressure created by the unit is a combination of incidental pressure from the centrifuge and the reverse pressure from the gravity feed of the black water tank. One could easily attach a collection vessel to the outlet of said garbage disposal that includes a sump pump, giving meaningful pressure to the output in a sealed unit. Another problem with the ‘garbage disposal’ unit is that it is always on, which can ALSO be solved by having a secondary vassal with a sensor that indicates fluid level to initiate grinding pump.

Lets look at an example unit, shall we. The SaniPro:
The macerator pump is actually an impeller with a slightly extended motor shaft, creating a single unit that grinds AND pumps after attaching standard blender-style stainless cutting blades to the shaft. The impeller picks up from the basin and ejects to the outlet. The pressure vessel (#14) is a sealed unit that provides the on/off switch mechanism, hidden behind a membrane. Impeller-driven pumping action ejects materials from the bottom of the motor out via #8. The cutter is only run when the impeller is active, meaning the colander tube holds solids until enough liquid is collected in the larger basin to trigger the switch.

So, I propose purchasing an impeller pump with an extended shaft, attaching some grinding stainless steel blades via a likely woodruff cut shaft. I propose placing it in a basin with around 2 gallons of capacity, installing a float switch triggered at 1.2 gallons and air vent. Additional inlets into the system can be fed into the chopping colander or, if filtered for sediment to the impeller spec, directly into the tank.


Freshwater System -
Freshwater is pumped out of the holding tank by a pump that pressurizes the entire freshwater system to 45psi after pulling fluid through a particulate filter. Water pressure is normalized via accumulator tank creating at least a few gallons of use without initiating the freshwater pump as well as removing water system choppyness. Immediately after the accumulator a check valve is installed after a 50psi pressure relief valve and an optional garden hose attachment. The pressurized line then splits into cold and hot water lines. The hot water line runs to the fresh hot water system. The cold water system runs to a manifold and is split to separate service zones, in this case being the kitchen and bathroom. This manifold should have at least one if not a few sealed ports for later expansion, and at least one garden house connection.
The shower recycling system is kept pressurized to 45psi via freshwater intake where required to maintain pressure.
Hot Water System-
Hot water system is pressurized by the freshwater system. Hot water system starts at a check valve to ensure hot water doesn't infiltrate regular freshwater system. Instant-hot water heater is propane fired. A possible solar thermal heater loop connection is left as two sealed connections in the line between the water heater and the hot water check valve with a shutoff valve between the two connectors, making this valve a temporary hot water shutoff and an eventual solar heater bypass valve. A hot water manifold provides distribution to the shower, shower recycling system, sink, and kitchen via insulated line. This position can be reversed with the hot water tank installed at the kitchen and insulated line run to the bathroom; or the kitchen can have it's own water heater if desired (this is not an idle thought, it would be preferable for the kitchen hot water to be hotter than the rest of the system). Hot water should be set to 110 degrees, allowing comfortable hot water to be available using only hot water valves. Because of the low flow of all fixtures care must be taken that enough flow is happening to initiate the hot water heater. Hot water heater temperature can be increased linearly based on the flowrate of the lowest fixture in our system. (eg: if the water heater has a .5 gpm flowrate and a .6gpm faucet is opened as 'only hot' the heater will engage, but if the valve is opened for half cold half hot only .3gpm would flow through the water heater and would NOT initiate)

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