Automatic Composting Litter Boxes

Here is my current vision for the automatic composting litter box units. This could be a great design or a terrible one. At this point I can’t tell. If anyone has better ideas, please share them. Watching the design evolve is part of the fun.

I will need the advice of experts in many areas including welding, sheet steel manipulation (such as steel drum barbecue fabrication), bidirectional DC motor drives, long-term compost toilets, compost tea brewing, compost chemistry, organic clumping litter recipes, assembly-line machine design, feline behavior, simple digital control systems and of course parts and materials cannibalization. I’m sure that there are many other areas which I’m overlooking.

Each litter box unit will be composed of three sections stacked vertically, contained within a simple metal framework (probably angle iron), and will likely be somewhere between six and ten feet tall. Multiple units will be installed in large litter closets.

The units will be accessible to cats via tunnels covered with a metal floor grate similar to John & Karen Flynn’s patented Litter Welcome Mat to prevent tracking of litter. I currently don’t know if this metal grating material even exists, so plastic may have to be used, although I would really like to avoid using plastics when possible. A grounded metal grate might also help to passively neutralize clinging, statically-charged, litter particles. However, this might not be very effective, or even an issue, depending on the litter used.

  1. The top section will be a large organic-litter hopper with a small auger at the bottom driven by a DC motor for dispensing the new litter (unless there is a better/simpler way).
  2. The middle section will be a cylindrical, rotating litter drum which operates very similar to the Litter Robot. It will be hand-constructed out of common materials (perhaps a 55 gallon drum on the outside and part of a 30 gallon drum on the inside – see diagram). The outer drum could be held in place by rubber wheels, one or more being powered by a bidirectional DC motor. A stationary LED light plate can be used to provide light and cover the long, narrow waist port at the same time, thus eliminating the need for a complicated flap. An optical sensor and four reflective stickers can be used to identify the drums four main positions.
    • Dump Position
    • Refill Position
    • Litter Leveling
    • Normal Use
  3. The bottom section will be very similar to the composting chamber of an odor-free, enclosed, long-term, composting toilet (see Clivus Multrum and Compostera) The starter bed could be wood shavings, and the organic litter might also act as an aerobic bulking-material. Depending upon the moisture retention qualities of the chosen litter, a moistening system may be necessary which could utilize aerated greywater. The chamber will of course contain compost earthworms (Eisenia Foetida). If helpful, microbe-rich, aerobic compost-tea inoculates could be periodically injected to further reduce pathogens and speed up the composting process. (I will need a lot of advice in this area.)

My goal is that each unit will operate for at least three months without any upkeep, and at least three years before any compost is removed.

Drum Failure Possibilities:

To prevent problems during a drum-rotation malfunction, a digital timer will run whenever the drum rotates. If the time required to reach any one of the drum’s four positions exceeds the maximum allotted, the drum stops and an error condition is transmitted.

Hopper Failure Possibilities:

How can the system detect if the litter hopper becomes empty or the auger fails to turn? I currently haven’t given this much thought. Anyone got any simple ideas? The hopper should never become empty if volunteers are keeping up with regularly scheduled maintenance.

Litter Refill (Option 1): Overfill

After a programmed number of uses, the system will initiate a refill cycle. Because the amount of litter used will fluctuate within a certain range, more than enough new litter will be added and, as the drum rotates, the excess will automatically drop down the waste chute and possibly be used as a bulking material in the compost chamber. Or perhaps the excess litter could instead be redirected by a flap into an overfill bin, which would get manually dumped back into the hopper during a hopper refill. By manually collecting and analyzing overfill data over time, the efficiency of the filling cycle could be optimized.

Failure Possibilities: If the overfill redirecting flap should ever malfunction, waste could pile up in the overfill bin.

Litter Refill (Option 2): Optical Sensors

Optical sensors like those found on automatic garage doors could be placed at the low litter level. If the sensors are triggered after the completion of a dump and leveling cycle, a refill cycle is initiated.

Failure Possibilities: If either sensor becomes coated with waste or the sensor wires break from rotation fatigue, a refill cycle will never initiate and the litter box will eventually become empty, creating a potentially messy situation. To prevent this, the tracking computer could send a notification if a litter box does not refill after a specified number of uses.

Internal Environment

While one litter drum is executing a cleaning cycle, A cat inside a nearby drum may become frightened by the sound. Noise minimization is an important design consideration for both the litter equipment and the litter-closet rooms. Cats won’t use a litter box if they are uncomfortable inside it. This is just one of the reasons experimental data is necessary.

Drum Fabrication:

Photo 1: Here is a typical 55 gallon steel drum.

Photo 2: The top has been removed, and the bottom and waste port have been cut away.

Photo 3: The white lines indicate where the front and back walls will be placed. The wheels shown are just examples, and are not accurately positioned.

Long-Term Composting Container:

This image shows two possible long-term compost chamber designs. I don’t know how to determine which of these, or some combination of both, would be easiest to build out of easy-to-find items.

The composting chamber and its operation must comply with the National Sanitation Foundation’s Standard 41. If it is designed properly to handle whatever homemade organic-litter is finally chosen, then I believe that waiting at least three years before removing compost would more than comply with the standard.


This design is very preliminary. There are many questions that need to be answered and design specifics to work out. Here are just a few. Please feel free to suggest others.

 

Questions:

  • Will the two corrugated reinforcing-rings on a 55 gallon drum be more of a problem than they are useful (especially during fabrication)?
  • What other common/easy-to-find items could be used in place of steel drums?
  • Why is the Litter Robot spherical? Does it have something to do with keeping the litter leveled?
  • Why does the Litter Robot wait seven minutes before sifting the litter? Do urine clumps need time to gain sufficient structural integrity with certain litters?
  • What makes organic litter form clumps? Is there some kind of powder that gets added to the organic matter?
  • How can we make our own innocuous, organic, clumping litter? (donated wheat husks, etc.)
  • Are insect infestations of organic litter a potential problem that should be addressed?

Possible Problems:

  • Spraying or eliminating on the drum surface – missing the litter entirely
  • Urine clumps sticking to the drum surface – May only be a problem with clay litter
  • Diarrhea that does not solidify or absorb and clump – could leak through and/or clog the screen

Design Issues:

  • Design a simple digital-scale/trap-door in the waste chute to weigh the used litter and then release it into the composter
  • Determine the best/simplest way to keep the drum in place (Roller Blade Wheels Maybe?)
  • Design hopper augers out of common materials somehow, or glean then from somewhere if possible (otherwise they may have to be purchased)
  • Determine the horsepower/torque requirements of each DC motor and where to glean them
  • Design a simple digital-control-system (Perhaps out of an EEPROM and digital timer chip or small gleanable micro-controller from somewhere)
  • Design both drum walls (the tunnel end and the service end which should contain a door or removable hatch)
  • Determine how to install web cameras into the drum ends such that the lenses stay clean and the wires don’t fatigue from drum rotation
  • Determine an appropriate container for the compost section, and determine how to build the internal structures (The container material must comply with Standard 41. Does anyone have a copy of the standard?)

6 Responses to Automatic Composting Litter Boxes

  1. Bob says:

    Use plastic 55gal drum, saves the high price welding, and less sticking of wet clumps . – WIFI WEB CAM , no wires to get broke .

    • Bob says:

      Also design like a small clothes dryer, one belt drive, one motor, one pulley, pull-out tray at bottom, put a entry door where you would clothes in a dryer. Controls and senders on top just like on clothes dryer.

  2. WinterDarkmoon says:

    How safe is this for a cat of any size? That hole looks large, and cats love to explore. You need a system for a.) Keeping the drum from spinning with a cat inside, even during malfunction events, and b.) A way to ensure that kitty isn’t exploring the holding tank…

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