Agricultural Application

        Over 60% of all land applied biosolids are used on agricultural land.  Application rates are usually determined by the nitrogen demand of the crop  and cumulative metal loadings.  For example, if you had a crop the needed 130 kg N/ha and you were applying a perfectly "clean" sludge that contained 3% N (2% organic and 1% inorganic with 30% mineralization in the soil).  The total application rate would be 130/(0.01 + 0.30 * 0.20) = 1857 kg/ha/year or 1.86 Mg/ha/year.
    On the other hand, if your biosolids had an arsenic (As) content of 6 mg As/kg of dry sludge and your site had a proposed 20 year lifetime, then to meet the 18 kg As/ha cumulative loading limit your yearly As loading would be (18/20) = 0.9 kg As/year.  The corresponding sludge application rate would be (0.9 kg As/year)(1000000 mg/kg)/(6 mg As/kg of dry sludge) = 150000 kg/ha/year or 150 Mg/ha/year.
    Typical application rates in moderate climate would be 15 Mg/ha/year.

Images of common Application techniques.

Control

    According to 40 CFR-257, PSRP's must be accomplished and access must be restricted to sludge applied land.

or PSRPs must be used.
 

Forest Application

Forest soil-plant systems are ideal for biosolids application for several reasons:

  1. Perennial root systems allow for year round application if weather permits
  2. Less traffic decreases the exposure to humans
  3. Covered forest floor protects against erosion of surface metals and phosphorus into near-by surface water

    EPA suggests a one-time loading of 47 Mg/ha can improve growth for up to 5 years with negligible nitrogen leaching.

Example of forest application system.
 

Reclamation

    Marginal land, such as surface mines, mine tailings, quarries, clear forests, landfills, lack nutrients.  Specifically, old mines can have very compacted and acidic soils (pH<3).  These conditions are very difficult and expensive to restore using traditional remediation techniques.  Lime-stabilized, dewatered biosolids have had great success restoring these soils.  The lime increases the pH and the high organic matter content helps restore soil structure.
    Initially, trace metal and nitrate concentrations in percolate can increase, but these drawbacks are short term.  After two years these concentrations usually return to normal.
 
 

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