Municipal effluent has an increase of 150 to 380 mg/L of TDS over the source water. In nonoxidized waste streams, approximately 40% of the dissolved solids will consist of volatile dissolved solids that will be removed in the treatment process or will degrade in the soil. The initial dissolved solids plus 40% of the incremental increase are fixed dissolved solids (FDS) or salts. Plant macronutrients, such as nitrogen, phosphorous, and potassium, and minerals, such as calcium and magnesium, are part of the FDS and are partially removed in land application systems that incorporate growing and harvesting of crops. The remaining inorganic dissolved solids are either leached from the soil profile or precipitate out into non-soluble forms. When inorganic dissolved solids accumulate in the soil, they increase the osmotic stress in plants, resulting in reduced yields or failed germination.
The recommended maximum TDS concentrations for reclaimed water are 500 to 2000 mg/L (USEPA, 2005). At 1000 mg/L of FDS, 32 in. of reclaimed water is equivalent to 7200 lb/ac of salts. A chemical 15-15-15 fertilizer (15% N, 15% P, 15% K) applied at 300 units of N will also apply 2000 pounds of fertilizer salts. It should be recognized that a significant fertilizer salt load is often avoided by reusing water with nutrient value. Salt removal by plants is estimated using the ash content of the harvested crop and can be calculated with similarly as nutrient uptake. Ash content is approximately 10% of the dry weight. Often, salts in excess of crop uptake are applied and leaching of salts is required to limit salt build-up in the root zone.
The leaching requirement is the ratio of the depth of deep percolation to the depth of the applied water. The same ratio exists between the concentration of the conservative salts applied and the concentration of conservative salts in the percolate. The EC of water can reliably indicate the salt concentration when little or no dissolved organics are present. A simple form of this relationship is presented in Equation 8.22; the equation is only valid when weathering and precipitation of salts are insignificant (Hoffman, 1996):
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