The processing and removal of biosolids from waste management and composting facilities has traditionally been a major operational concern. Most large processing centers require removal of compost or sludge by large heavy equipment. This heavy loading creates a high fatigue factor on the native subgrade soils, causing the processing table to become weak and unstable under repetitive loading. This issue, along with the added concern of bio-solids commingling with the underlying soft soils during harvesting, lead to additional processing cost.
Designing a successful bio-solid processing bed requires consideration of several site and environmental constraints. The site must be far enough from area housing so that risks of dust and odor complaints are minimized. It must be close enough to haul raw materials and have a source of water so that its operation is economically feasible. Finally, the site must possess suitable characteristics (e.g., slope, drainage, and distance) to avoid pollution of local streams, groundwater and wetlands.
A compost site should transport leachate and runoff from the site surface so that muddy conditions and odorous pools of standing liquid do not develop, unless a part of a sludge drying process. One possibility is to place the bio-solid site on a well-drained soil where distance to groundwater is greater than 5 ft. However, due to the long-term nature of bio-solid processing facilities and the volumes of material processed yearly, an impervious surface that directs leachate to a treatment or containment area would be more environmentally suitable than a well-drained soil.
Although an impervious pad is considered a luxury, there may be certain situations where regulations demand an impervious pad for which several options are now available. Besides concrete or asphalt, chemical soil stabilization methods are available that produce a hard, nearly impervious layer capable of supporting all the equipment normally required for bio-solid processing.
An additional benefit of chemical stabilization is the reduction in the amount of maintenance required to process material, along with a significant reduction in contamination with the underlying soil during bio-solid removal.
Soil stabilization increases the load bearing strength, while reducing the permeability of the native soil. These two factors are important for processing tables, since all activities are exposed to the elements and untreated soils lose strength under exposed or saturated conditions.
Depending upon the type of soil, stabilization can be accomplished with quicklime, lime-pozzolans blends, and Portland cement. This method of stabilization is conducted under a controlled environment to provide a consistent and uniform mat structure. The stabilized mat creates a harden surface that allows for many years of maintenance access for compost and sludge processing and removal.
To structurally evaluate the stabilized section, laboratory tests can establish such properties as:
The single most important strength parameter is the unconfined compressive strength as other strength parameters such as flexural and tensile strength, R-value, fatigue behavior, etc. can often be estimated from unconfined compressive strength as a result of regression analysis.
Proper testing procedures, in the lab during design and in the field during construction, will results in optimized design and verifiable results that will provide long term durability of the processing table.