As can be seen from the illustrations of our previous yellow series digesters, the digester tanks are largely underground structures. However, the tank is a critical piece of civil engineering and is made to our strict instructions. The unit is heavily insulated with virtually everlasting, vermin proof insulation.
It is important to be able to drain water away from the base of the digester to prevent the empty digester tank floating like a boat in the groundwater - this may place limitations on the digester design and its location at a given site and has implications for civils costs. Please consult fully with Practically Green regarding this important aspect of digester design.
Two types of contract are available for the Gold Series of digesters,
1. The client purchases detailed civil engineering drawings and tank building instructions and completes the tank ready for the technical
components which are fitted by Practically Green. The unit is then commissioned by Practically Green.
2. The whole system is designed, built and commissioned by Practically Green.
With either contract it is best for the site to be visited by the design team from Practically Green and the unit designed to make use of all the natural opportunities of the individual site. This can save valuable capital at construction time. Any design fees (excluding expenses) may be removed from the list price of the digester components. Thus as long as the system is purchased it does not cost the client any more than a 'blind' purchase which may have unseen costs. At the same time our engineers can be paid for design work by the client who receives the benefit of this - its great to be fair all round!
The smallest practical tank size is 3 lids. If the tank is 2.5 meters deep then this produces a small unit with a volume of 75 m3.
Each subsequent 5 meter x 2 meter (approximately) lid adds a volume of approximately 25 cubic meters to the tank for a 2.5 meter depth.
The maximum practical size for a single digester has never been ascertained as it is completely modular, however 20 lids on a digester gives a 500 cubic meter digester and two side by side is 1000 m3. Several of these 1000 cubic meter modules could be placed side by side if space is available leaving a tractor lane between each 1000 cubic meter module.
An existing digester system can be expanded by adding further modules if the space required is planned on the site prior to building the system - useful if the treatment plant follows the un predictable growth of a factory.
The position of the inlet and outlet can be changed to suit local requirements but generally end to end working is found to be most satisfactory and we have good data on the mixing characteristics of high aspect ratio digesters which indicate definite bacteriological advantages to high aspect ratio installations.
Practically Green