Biosolids can be applied in liquid or dewatered form. The method of application depends on whether the biosolids are in liquid, dewatered cake, composted, or pelletized form; the topography of the site; and the type of vegetation, such as annual field crops, existing forage crops, trees, or pre-planted land.
Liquid Biosolids Application The principal advantage of the application of biosolids in the liquid form is its simplicity. It has the added advantages of not requiring dewatering processes at the sludge processing facilities and that the liquid biosolids can be transferred by pumping. However, large quantities of biosolids have to be transported to the application site.
Liquid biosolids have solids concentrations in the range 1 to 7%. Application methods include simple surface spreading and subsurface incorporation by injection or plowing. For small facilities, tank trucks equipped with rear-mounted spreading manifolds or high-capacity spray nozzles are commonly used. If the application sites are near the treatment plant, spray irrigation with high-pressure pumps may be used, thus eliminating the need for vehicular transportation.
Liquid biosolids can be injected below the soil surface by using tank trucks equipped with injection shanks or incorporated immediately after surface application by using plows. A tractor equipped with liquid biosolids injection lines can also accomplish subsurface application. The liquid to be injected is supplied by a hose connected to the injection device. The tethered hose is dragged along by the tractor. Advantages of subsurface application include minimization of potential odor and vector attraction, reducing ammonia loss due to volatilization, and elimination of surface runoff.
Irrigation methods include sprinkling with large-diameter high-capacity sprinkler guns, and furrow irrigation. Sprinkling is used primarily for application to forested lands. The principal advantage of furrow irrigation is that liquid biosolids can be applied to row crops during the growing season without having the biosolids contact the crops themselves.
Dewatered Biosolids Application Dewatered biosolids application is generally used when the application site is some distance from the treatment plant or when the climatic conditions make it necessary to store the biosolids in a confined space for up to six months. Thirty-two to forty kilometers (20 to 25 miles) is usually the distance where liquid transport becomes more expensive than dewatered biosolids transport (Lue-Hing et al., 1998).
Achieving consistent surface application can be difficult with dewatered cake, although dewatered material has the advantage of allowing higher rates of application because of less moisture addition. The main advantage of dewatered biosolids is that farmers can apply them on their lands with their own conventional manure spreaders. Typical solids concentrations of dewa-
tered biosolids applied to land range from 15 to 35%. They are most commonly spread by tractor-mounted box spreaders or manure spreaders followed by plowing into the soil. Compost and thermally dried biosolids are more easier to spread.
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