Yeah, that 3" pipe is pretty big but the fan pushes a lot more air than the Nature Head does. I found this answer from them for not using some kind of composting agent e.g. coconut coir, peat moss, saw dust etc. which sounds reasonable.
"The Separett system holds the solid waste, separated from the liquid waste, within a compostable liner bag in the holding area. The fan is drawing air over the material serving to vent any odor and aid in drying the material. No other material is added as that would impede drying. No composting toilet on the market brings the material to a safe state within the toilet, as it take a min of 6 months for human pathogens to become benign aerobically. Therefore our feeling is why add additional material in the toilet and make it a mess to empty - when the material needs additional composting upon removal. So - upon removal from the Villa in the compostable bag, the waste goes to a compost area, approved disposal area (as in an RV park) or in an incinerator for disposal. With composting, the bag goes into the compost area in the tied off bag, the bag will compost down exposing the contents for composting with other kitchen and yard materials. Our system allows options and less volume for the method of disposal over other systems. More info on our company site at SeparettStore dot com. see less
By Separett-USA SELLER on January 7, 2019"