The Trench Arch Drainage (TAD) system was a spin-off from a 3 year R&D project which looked into sludge-free domestic sewage treatment. Raw effluent from a single WC discharges into a carefully constructed shallow culvert which acts as a vermicomposting filter bed.
The Trench Arch is intended for very low use applications and has been very popular for rural churches wishing to add a WC for occasional use. In 2001 we produced a short design note for the Gloucester DAC which is available to download here. Whilst not up to date the basic principles stand.
A more recent report on the Trench Arch Drainage system by Oxford Archeology for Historic England contains more up to date information and is available to download here.
Whilst the Trench Arch is technically simple, it must be designed by a professional who can take full design responsibility. Where mains drainage is available, this should always be the preferred option even for very low use applications.
Elemental Solutions no longer offer design advice or consultancy on Trench Arch Systems. See the NatSol website for an alternative waterless toilet solution that requires no water connection and so is immune to frozen pipes.