Passive Municipal digester.

In many rural areas it is difficult to build a proper high-technology municipal biogas digester. To this end Australian Biomass has designed a very simple digester which is very low maintenance and very low energy. And even creates energy.

The ABM D2 consists of two inground ponds. The first is the biogas digester: the anaerobic pond and the second is the aerobic pond. The first pond is where municipal waste water flows after large objects have been removed and is called the anaerobic pond. Waste water entering this pond first passes  through shredding and / or macerating pumps.  The pond is operated as an anaerobic digester for the natural breakdown of organic wastes. The nominal retention time is designed to be from 30 days to as long as 60 days. There is minimal heating for this type of digester and there is a combination of mechanically operated and gas operated fluid agitation systems to minimize the accumulation of scum and sedimentation. Heavier solids are removed either manually or automatically. Sufficient electricity to operate the facility is from solar, wind and biogas operated stationary plant.  Overflow from the anaerobic pond flows into the aerobic pond where solar and wind powered aerators operate pumps to aerate the spent supernatant.

The residual aerated supernatant flows into the flushing pond where the treated water is used for irrigation of suitable crop(s).  Such as lucerne, hay, sugar-cane, hemp, kenaf, miscanthus, maize, kale, oats, sunflowers or any other crop suited to the climate where the D2 is installed.

Advantages of this system are that one operator is all that is required and need only  attend the site for as little as once or twice a week depending on the size of the installation. Larger installations require a more rigorous attendance but generally not exceeding a few hours a day for a single skilled operator. The ponds are as per the diagram: click the duck. This shows the first two ponds.  The first is the anaerobic pond and is from 6 - 12 meters deep at its deepest rising up to a shallow outlet of 2 meters where the supernatant flows into the aerobic pond. Width is from 6 to 15 meters and length is determined according to volume required.

The anaerobic pond is sized by taking the maximum amount of wastewater that a given population that it serves may dispose of each day and this figure is multiplied by 50. The anaerobic pond is made to this volume. The aerobic pond is sized to be from one half to one quarter this size.

The flushing pond is sized to the daily demand of the crop. The reason a crop is mandatory is that a crop and especially a cultivated crop will be water demanding and so be able to cope with continuous watering.

Crops also are part of the final ‘cleaning' process. Downstream from the crops water quality should be potable. The crop area is sized so! The crop area also includes trees and other suitable plants to take full advantage of any residual nutrients remaining in the waste water irrigation run-off.

Pollution of the surrounding environment is minimized and a useful liquid fertilizer has been produced in the process. Heavy sludge removed from the bottom of the anaerobic pond is an excellent soil conditioner and can be ploughed directly into the soil.

The D2 operates on the similar principle to the thousands of digesters that are attached to farms all over China today.

Pond construction is either as an in-ground concrete lined pond or with a suitable butyl-rubber liner. The gasometer is a PE or butyl-rubber cover for trapping the biogas produced by the anaerobic process. The biogas is used to supply some heating to the process and for the generation of electricity to operate the facility.

A biogas scrubbing facility complete with fuel station for motor vehicles is easily adapted into this installation.