7 Del. Admin. Code § 7103-21.0

Current through Register Vol. 28, No. 4, October 1, 2024

Section 7103-21.0 - Above-Ground Removal

21.1 Crop uptake of applied nutrients and subsequent removal by harvest is one of the main mechanisms of above-ground removal. This is the main way that N is removed from the site as long as N application rates are not much in excess of normal crop fertilization rates. If the soil on the site has a high P fixing capacity, crop removal may not be the most important method of P removal.

21.2 Removal of nutrients is a function of crop yield and nutrient composition. Consequently, any environmental factor that reduces yield (unbalanced fertility, low pH disease, etc.) will also reduce nutrient removal from the site. It must be recognized that as a crop is supplied with increasing amounts of a nutrient, the crop will use that nutrient with decreasing efficiency. Thus, although a crop may take up 400 lb Ncre per year,

21.3 this uptake may have been attainable only when higher amounts were applied. Tables of crop nutrient uptake seldom contain information on quantity of nutrient applied, length of growing season (e.g. Delaware vs. Florida), whether the crop was irrigated, etc. The designer must consider this information when preparing nutrient balances and vegetation management schemes.

21.4 For design purposes loading rates that supply available nutrients at a rate that has been shown or that can be logically estimated to pose no pollution hazard should be used. Current fertilizer recommendations for crops in Delaware (11) are probably a good baseline for setting wastewater-applied nutrient uptake rates for most nutrients. Adjustments in rates can then be made based on ammonia volatilization losses, increased denitrification potential, P fixing potential of the particular soil, increased crop uptake due to irrigation and local experiences with similar soils (and similar management, if possible). Adjustments in yield, crop uptake and fertilization for irrigation should be made using data for the same crop and similar soils and climate if available. Also, N applications (and hydraulic loading) may have to be adjusted because of excess nitrate in drainage water. Nutrient uptake rates to trees are less well established than for forage and row crops.

21.5 Generally, much of the N in wastewater is in the NH4+ or NO3- form which is available to plants. Also, it is likely that most of the organic N in wastewater will become available during the growing season when soil organisms are active. However, if the wastewater is high in organic N and comes from a source that indicates the organic N will be slow to mineralize, then a laboratory incubation of the soil-wastewater mixture may be needed to determine N availability.

21.6 Phosphorus availability is more difficult to determine because of its fixation by soil. However, if incubations are conducted with soil from the proposed site, P as well as N availability could be estimated.

21.7 Gaseous loss of waste constituents is another mechanism of above-ground removal. Evapotranspiration (ET) of applied wastewater is determined by climate and soil moisture content. Data on potential ET (PET)1 are available for Delaware and can be used to calculate water loss by this mechanism. 21.8 Release of carbon dioxide (CO2) by microorganisms decomposing organic matter in wastewater is the mechanism for carbon removal from the site. Consequently, the soil must be kept aerobic to facilitate decomposition and to prevent odors and sealing that occur when the soil is anaerobic. Assimilative capacity of soil for organic matter can be estimated using the oxygen (O2) diffusion rate in soils as a function

21.8 1PET is the rate of ET when the site is completely vegetated and soil moisture does not limit ET. This is generally the case with land treatment systems for wastewater since irrigation keeps soil moisture high. of soil moisture content. Organic loading rate can be critical if high organic matter wastewater is irrigated because O2 supply is decreased not only by the increase in soil moisture (thereby reducing O2 diffusion rate) but also by the high biological and chemical O2 demand of the wastewater.

21.9 Certain industrial wastes contain organic compounds that may be resistant to decomposition and/or toxic to vegetation. Potential toxicity and rate of decomposition of these compounds must be determined from the literature or from actual tests so the site assimilative capacity can be determined.

21.10 Significant quantities of NH3 may be lost from the site by volatilization. If wastewater contains appreciable quantities of NH4+, then NH3 may volatilize during irrigation depending on wastewater pH, temperature, wind speed and droplet size. The EPA design manual (7) states that losses can be up to 10 percent of applied N if wastewater pH is 7.0 or above.

21.11 Gaseous N losses also occur when NO3- is reduced to N2 or oxides of N via microbial activity (denitrification). Several conditions must be met for denitrification to occur: anaerobic conditions, available carbon as an energy source for the organisms and a source of NO3-. Although land treatment systems must remain aerobic to function in the soil, denitrification can occur if carbon is available and NO3- diffuses into the site. Denitrification losses are difficult to measure under field conditions, but there is some evidence from research that wastewater irrigation increases denitrification rates. Denitrification can be affected by frequency of application, hydraulic loading, nitrification, and the inherent denitrification potential of the soil at the site. Denitrification losses are generally reported to vary between 10 and 35 percent of the applied available N. Most soils suitable for wastewater application have denitrification potentials less than 100 lbcre per year.

21.12 The N assimilative capacity of the site is the sum of the fertilizer N recommendation (under irrigated conditions) and the denitrification potential (and any waste or management-induced denitrification if it can be shown to be appreciable), with adjustments for ammonia volatilization losses at application.For general guidelines in design, the EPA design manual (7) recommends that the sum of volatilization losses and denitrification be assumed to be in the range of 15 to 25 percent of the applied N. Certain conditions may exist which denitrification is appreciable in areas adjacent to the irrigated site that receive drainage water. Recent research has shown that nitrate in drainage water from well-drained areas will be denitrified if it moves through adjacent poorly drained areas (e.g. marsh, swamps, base of slopes, etc.).This process may be designed into the N balance for the land treatment system if the necessary conditions exist.

7 Del. Admin. Code § 7103-21.0