Nitrogen is usually the limiting design parameter for slow-rate land treatment of wastewater, and the criteria and procedures for nitrogen are presented in Chapter 8. Nitrogen can also limit the annual application rate for many sludge systems, as described in Chapter 9. The removal pathways for both types of systems are similar, and include plant uptake, ammonia volatilization, and nitrification/deni-trification. Ammonium ions can be adsorbed onto soil particles, thus providing a temporary control; soil microorganisms then nitrify this ammonium, restoring the original adsorptive capacity. Nitrate, on the other hand, will not be chemically retained by the soil system. Nitrate removal by plant uptake or denitrification can occur only during the hydraulic residence time of the carrier water in the soil profile. The overall capability for nitrogen removal will be improved if the applied nitrogen is ammonia or other less well-oxidized forms. Nitrification and denitri-fication are the major factors for nitrogen removal in rapid-infiltration systems, and crop uptake is a major method for both slow rate and overland flow systems. Volatilization and denitrification also occur with the latter two types of system and may account for from 10 to over 50% of the applied nitrogen, depending on waste characteristics and application methods, as described in Chapter 8. Design procedures based on nitrogen uptake of agricultural and forest vegetation can be found in Chapter 8.
Phosphorus has no known health significance but is the wastewater constituent that is most often associated with eutrophication of surface waters. Phosphorus in wastewater can occur as polyphosphates, orthophosphates (which can originate from a number of sources), and organic phosphorus, which is more commonly found in industrial discharges. The potential removal pathways in natural treatment systems include vegetation uptake, other biological processes, adsorption, and precipitation. The vegetative uptake can be significant in the slow-rate and overland flow land treatment processes when harvest and removal are routinely practiced. In these cases, the harvested vegetation might account for 20 to 30% of the applied phosphorus. The vegetation typically used in wetland systems is not considered a significant factor for phosphorus removal, even if harvesting is practiced. If the plants are not harvested, their decomposition releases phosphorus back to the water in the system. Phosphorus removal by water hyacinths and other aquatic plants is limited to plant needs and will not exceed 50 to 70% of the phosphorus present in the wastewater, even with careful management and regular harvests.
Adsorption and precipitation reactions are the major pathways for phosphorus removal when wastewater has the opportunity for contact with a significant volume of soil. This is always the case with slow rate and rapid infiltration systems, as well as some wetland systems where infiltration and lateral flow through the subsoil are possible. The possibilities for contact between the waste-water and the soil are more limited with the overland flow process, as relatively impermeable soils are used.
The soil reactions involve clay, oxides of iron and aluminum, calcium compounds present, and the soil pH. Finer-textured soils tend to have the greatest potential for phosphorus sorption due to the higher clay content but also to the increased hydraulic residence time. Coarse-textured, acidic, or organic soils have the lowest capacity for phosphorus. Peat soils are both acidic and organic, but some have a significant sorption potential due to the presence of iron and aluminum.
A laboratory-scale adsorption test can estimate the amount of phosphorus that a soil can remove during short application periods. Actual phosphorus retention in the field will be at least two to five times the value obtained during a typical 5-day adsorption test. The sorption potential of a given soil layer will eventually be exhausted, but until that occurs the removal of phosphorus will be almost complete. It has been estimated that a 30-cm depth of soil in a typical slow-rate system might become saturated with phosphorus every 10 years. The phosphorus concentrations in the percolate from slow-rate systems usually approach background levels for the native groundwater within 2 m of travel in the soil. The coarser textured soils utilized for rapid infiltration might require an order-of-magnitude greater travel distance.
Phosphorus is not usually a critical issue for groundwater quality; however, when the groundwater emerges in a nearby surface stream or pond, eutrophication concerns may arise. Equation 3.29 can be used to estimate the phosphorus concentration at any point on the infiltration/percolation, groundwater flow path. The equation was originally developed from rapid infiltration system responses, so it provides a very conservative basis for all soil systems (USEPA, 1981):
Px = Total P at a distance x on the flow path (mg/L).
P0 = Total P in applied wastewater (mg/L).
kp = 0.048 at pH 7 (d-1) (pH 7 gives the lowest value).
W = Saturated soil water content; assume 0.4.
Kx = Hydraulic conductivity of soil in direction x (ft/d; m/d); thus, Kv =
vertical and Kh = horizontal.
G = Hydraulic gradient for flow system:
= 1 for vertical flow.
= AH/AL for lateral flow.
The equation is solved in two steps: first for the vertical flow component, from the soil surface to the subsurface flow barrier (if one exists), and then for the lateral flow to the adjacent surface water. The calculations are based on assumed saturated conditions, so the lowest possible detention time will result. The actual vertical flow in most cases will be unsaturated, so the actual detention time will be much longer than is calculated with this procedure. If the equation predicts acceptable removal, we have some assurance that the site should perform reliably and detailed tests should not be necessary for preliminary work. Detailed tests should be conducted for final design of large-scale projects.
As a wastewater constituent, potassium usually has no health or environmental effects. It is, however, an essential nutrient for vegetative growth, and it is not typically present in wastewaters in the optimum combination with nitrogen and phosphorus. If a land or aquatic treatment system depends on vegetation for nitrogen removal, it may be necessary to add supplemental potassium to maintain plant uptake of nitrogen at the optimum level. Equation 3.30 can be used to estimate the supplemental potassium that may be required for aquatic systems and for land systems where the soils have a low level of natural potassium:
K = Annual supplemental potassium needed (kg/ha). U = Estimated annual nitrogen uptake of vegetation (kg/ha). Kww = Amount of potassium in the applied wastewater (kg/ha).
Most plants also require magnesium, calcium, and sulfur and, depending on soil characteristics, there may be deficiencies in some locations. Iron, manganese, zinc, boron, copper, molybdenum, and sodium are other micronutrients that are important for vegetative growth. Generally, wastewater contains a sufficient amount of these elements, and in some cases the excess can lead to phytotoxicity problems. Some high-rate hyacinth systems may require supplemental iron to maintain vigorous plant growth.
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