TEG offers alternative to windrow composting
Dr Alan Heyworth of TEG Environmental argues for an alternative to windrow composting, which he claims will result in a more consistent end-product.
TEG’s continuous-flow composting Silo-Cage has been designed to overcome the following problems encountered with windrow composting.
The TEG Silo-Cage consists of a bank of 4-20 large silos, with walls made out of stainless-steel mesh.
To maximise aeration, the silos are raised above the floor and have air-spaces between them. This ensures that the compost is never more than 60cm from the air, and aerobic conditions are therefore maintained without turning.
An auger is used to unload the cages, by sliding between the bottom of the silos and the concrete floor.
The auger pulls the composted material from the bottom of each silo in turn, and delivers it to a side-conveyor, which moves it to a bunker.
As material is removed from the bottom, the upper surface of the compost sinks, and fresh material can be added. This rests on the already active, hot compost and quickly warms and becomes active.
The material to be composted must usually be mixed with wood chips, shredded green waste or poultry litter, to achieve the correct water content (55%-60%) and structure.
The moisture-content of the input material is a highly critical factor. With a sludge cake containing 78% water, for instance, an additive-to-cake ratio of >3:1 may be needed, whereas with a cake moisture-content of 70%, the ratio may be as low as 1:3.
Fortunately, unlike windrows outdoors, the process can be kept sufficiently dry to prevent the generation of any run-off.
A hopper is used to add the material to the top of the silos. The hopper is filled at ground level, raised and then travels automatically along the the row of silos.
The material is sprinkled onto the surface of the compost, so ensuring the friable open structure needed for aerobic composting.
The depth of material which can be added per day to the top of a silo is determined principally by its compressibility and water content.
Primary and digested sewage sludge cakes of the same water-content will behave very similarly when processed using this system.
As the silos are kept full of rapidly composting material, throughput is likely to be much faster than with windrow composting, around 8-10 times faster for a given floor area.
The progress of composting is monitored by temperature sensors suspended in each silo.
For many materials one or two sensors per silo is sufficient, but where temperatures are critical for pathogen-kill (eg sewage sludge) more sensors may be needed to ensure that all parts of the compost have reached the required time- and-temperature criteria.
If there is a need for high temperature to be maintained on the surface layers, insulating panels can be fixed to the mesh walls. Temperatures in the silos typically range from 65°C-75°C at the top, to 50°C-55°C at the bottom. As the compost descends through the silo, during say, 14 days, it will pass through temperature zones of >65°C for 4 days, 55°C-65°C for 7 days and 50°C-55°C for 3 days. These conditions meet government regulations and are more than adequate for an acceptable pathogen-kill.
Residence time is between 12-21 days, depending on the type of material.
Primary and digested sewage sludge cakes of the same water-content behave very similarly.
Recent tests of the system to compost sewage sludge cake have shown pathogen reduction to undetectable levels.
After removal from the bottom of the silo, the compost is allowed to cool and mature in a stockpile for 2-3 weeks.
The end-product is a dry, friable material, suitable for use as a high-quality organic fertiliser, with a typical analysis of N: 3.5%, P: 1.8%, K: 2.3%.
Growing-trials have produced results equal to or exceeding those from established brands of composted organic fertiliser, and much superior to the usual composted MSW soil-improvers.
The energy cost for TEG Silo-Cage is approximately £1/silo/week. One man can easily operate a battery of 20 silos, with a throughput of up to 20,000m3/yr.