Incorporation of Al-WTR to a depth of 10 cm decreased SP concentrations in subsurface flow and leachate by 37 and 11%, respectively. However, with incorporation of Al-WTR to a depth of 20 cm, both subsurface flow and leachate SP concentrations were reduced by approximately 90%. The incorporated Al-WTR reduced soil water-extractable P (WEP) by approximately 70%. However, Mehlich-1 P concentrations were not affected by the incorporation of Al-WTR in the soil. Care must be taken to ensure complete incorporation of Al-WTR throughout the P-impacted layer, as Al-WTR is only effective in reducing SP concentrations when it is in contact with the impacted soil. Shoot and root growth of stargrass were not adversely affected by the Al-WTR applied at a rate of 2.5% of soil weight.
Increased WTR rates can largely overcome soluble organics impacts and negate the need for massive soil horizon mixing. Al-WTR can be an effective soil amendment to reduce P loss from manure-impacted soil when the WTR is made to contact soluble P in the soil profile. Soluble P not in direct contact with the WTR is unaffected by WTR and is subject to leaching loss.
The SPSC values increased on plots with WTR even after heavy loads of P were applied and some remained high enough that additional P should be retained. Potential nitrification rates increased with higher WTR application rates, indicating no adverse effects occurred on soil microbial populations because of WTR application. Groundwater SRP concentrations decreased with the addition of WTR by 78% in the surface applied treatments. There were no differences in SRP concentrations among the incorporated treatments which was likely a temporary result of the tilling process. Forage yield, crude protein and neutral detergent fiber were not affected by WTR application. Tissue Al did not increase and tissue calcium and magnesium uptake was not restricted. Tissue P decreased as WTR application rate increased. Tissue P levels were higher in the incorporated treatments than the surface applied treatments. However, tissue levels did not fall below the bahiagrass limiting value (0.15%). Based on this study, the use of WTRs is recommended as an amendment to soils with low P retention capacities that have been heavily impacted with P or will receive high amounts of P e.g., a dairy sprayfield.
Excess nutrient loading to the Great Lakes Basin from agricultural runoff has negatively impacted water quality, resulting in harmful algal blooms. Best management practices, including constructed wetlands and sedimentation basins, can be used to reduce phosphorus losses from agricultural fields. Constructed wetlands are efficient in the removal of particulate phosphorus; however, removal of dissolved phosphorus is limited and requires further treatment to improve surface water quality. Several types of filter media (composed of Ca, Fe, and/or Al) can be used to further reduce the amount of dissolved phosphorus that enters surface water, and a media consisting of low-cost waste residual would be beneficial to adoption. Drinking water treatment residuals (DWTR) that often contain Al could be reused as an adsorbent for dissolved phosphorus. We evaluated the use of modified drinking water treatment residuals for removing dissolved phosphorus from wastewater. DWTR were mixed with binders, made into pellets to create an insoluble media with mechanical strength, and pyrolyzed to create a reactive media pellet. Pellets were evaluated using flow through columns and included experiments to determine the impact of pH (i.e. 6, 8, and 10), retention time (i.e. 1, 5, and 10 min), and field-collected agricultural runoff on dissolved P removal. Cement was found to be the best binding material to create an insoluble pellet with mechanical strength. The P removal capacity of the pellet consisting of the cement binder (1,397 mg P/kg) was within the range of previously evaluated steel slag (120-10,210 mg P/kg), a common reactive media for P removal. The addition of drinking water treatment residual and metals decreased the P removal capacity of the cement binder at pH 6-1 min retention at exhaustion. Increasing retention time increased the P removal capacity of the filter media tested. Wastewater pH has a minimal impact on the P removal capacity of all media except the pyrolyzed DWTR + cement binder media. Evaluated media was negatively impacted by real agricultural runoff with a measured decrease in P removal capacity (43-146 mg/kg decrease) compared to P-spiked distilled water at the same retention time. The pyrolyzed cement pellet was the most cost-effective reactive media, due to an increased P removal capacity. Pyrolyzed DWTR + cement binder would be more costly than the pyrolyzed cement binder alone but could provide a solution for the disposal of DWTR.
Biosolids (appropriately treated sewage sludge) can make an important contribution to sustainable environmental management, through the return of organic material, trace elements, moisture and nutrients to our soils. The Guidelines for Environmental Management: Biosolids Land Application enables this beneficial use of biosolids, by providing a management framework that ensures any chemical and microbiological risks are appropriately managed.
