Location: Water Management and Conservation Research
2021 Annual Report
Objectives
The long-term objective of this project is to provide science based data to ensure that treated municipal wastewater used for irrigation poses minimal threat to people and the environment. Specifically, during the next five years the project will focus on the following objectives.
Objective 1: Determine the processes that govern the environmental fate and transport of emerging contaminants and other constituents found in treated wastewater used for irrigation to provide a research basis for potential regulation of these constituents.
Objective 2: Develop and optimize low input treatment systems to reduce emerging
contaminants and nutrients found in degraded waters to increase water resources used for food production.
Approach
Objective 1 is a combination of monitoring of treated wastewater effluent for emerging contaminants (ECs) and research to investigate the potential for organic sorbents to sequester emerging contaminants in the environment. The mass of pharmaceuticals taken up by crops irrigated with treated municipal wastewater depends on the concentration of the pharmaceutical at the soil-root interface and the volume of water needed to meet plant metabolic needs. The concentration of pharmaceuticals at the root is determined by initial concentration applied and soil processes that remove the pharmaceutical from the soil solution.
Evaluating Temporal Patterns of ECs: Pharmaceutical concentration in sewage effluent will be measured on three different time scales from five different regions of the country (arid, semi-arid, humid continental, humid sub-tropical, tropical) to characterize the concentration of ECs found in reclaimed water. One sewage treatment plant from each region will be chosen for sampling. Treatment plants of similar size with similar treatment trains will be selected and sampling will consist of four high intensity sample periods lasting one week each in winter, spring, summer, and fall. Samples will be time averaged composite samples with equal aliquots collected every 30 minutes.
Evaluating the potential for organic residues to remove carbamazepine from irrigation water: Previous research has shown that organic materials can act as sorbents to remove trace organics, however, most of this research is limited to pesticides and industrial pollutants. The sequestration of these compounds by organics has typically been measured on systems where the contaminant is present at part per million levels (ppm), while ECs are typically found at part per billion (ppb) levels or less in irrigation water. It is hypothesized that sub ppm levels of ECs found in irrigation water can be effectively and economically removed from the water through the use of sorbents derived from waste products. Raw waste products to be tested will include post-harvest plant residues, biochars derived from plant residues, and organic wastes. Effective removal will be governed by overall sorbate characteristics, which include sorption kinetics, total sorption potential, and effective sorbent life span.
Objective 2 is a laboratory scale design and engineering endeavor to develop viable treatment practices to remove EC’s from irrigation water prior to plant uptake. Candidate sorbents will be evaluated for EC removal efficacy from irrigation water. It is hypothesized that through proper placement and treatment of organic plant residues the soil solution concentration of ECs can be reduced. Candidate sorbents will be evaluated in both media filters and as soil amendments concentrated where water application occurs to evaluate EC removal potential. Evaluation of field treatment options will use three different removal options: 1) Use of organic amendments as filter media; 2) Use of organic amendments to increase overall soil sorptive capacity; and, 3) Selective placement of organic amendments to intercept irrigation water prior to soil application.
Progress Report
Research related to completing Objective 1 was very limited due to COVID-19 related travel restrictions. Water samples that were collected in fiscal year (FY) 20 were analyzed for target pharmaceuticals and the results tabulated. In addition, samples from a wastewater treatment plant tracer experiments that were conducted in FY20 were analyzed. The tracer experiment consisted of a series of discrete fluorobenzoic acids tracers being added continuously to the inflow of a wastewater treatment plant over a seven day period. Effluent samples were collected and analyzed for the tracers. Preliminary results indicate that retention time within the treatment facility is related to the influent flow rate and that the relationship between flow and retention time is not linear indicating short circuiting of the hydraulic flow paths.
In support of Objective 2, a series of experiments were completed to optimize pharmaceutical removal from water by biochar. Pretreatment of biochar with acid was found to increase overall surface area and increase overall sorption potential by up to ten times. It was found that the wetting and drying cycles associated with irrigation resulted in increased sorption of pharmaceuticals. Experiments showed that sorption efficiency was reduced from 100% removal to less than 20% removal after eight hours. However, after 18 hours with no flow, removal efficiency returned to greater than 95% when flow was resumed. This indicates that biochar could be used to remove pharmaceuticals from treated municipal wastewater used for irrigation at the point of use.
Accomplishments
1. Occurrence and distribution of antibiotics and corresponding antibiotic resistance genes in different soil types irrigated with treated wastewater. Up to 90% of antibiotics used for therapeutic treatment can be eliminated and caried through the sewar treatment system in unchanged form. When wastewater is used for irrigation, it has been hypothesized, that the presence of antibiotics leads to an increase in the development of antibiotic resistance. ARS researchers in Maricopa, Arizona, quantified the development of antibiotic resistance due to antibiotic challenges. Results indicate that there is no direct link between the concentration of antibiotics and the development of antibiotic resistance indicating that growers who use reclaimed water for irrigation are not increasing the prevalence of antibiotic resistance.
2. Low concentrations of copper, ion (Cu2+) in synthetic nutrient containing wastewater inhibit magnesium carbonate (MgCO3)-to-struvite transformation. Struvite synthesis can simultaneously remove both nitrogen and phosphorous from wastewater streams for use as a fertilizer. The precipitation of struvite from wastewater occurs in the presence of many other contaminants that have the potential to limit crystal formation and prevent nutrient removal. Zinc (Zn) and copper are common ions present in wastewater that may affect the production of struvite. ARS researchers in Maricopa, Arizona, found that concentrations of Zn up to 100 milligrams/Liter have no adverse effects on struvite production, but copper concentrations as low as 5 mg/L can prevent struvite precipitation. In addition to interfering with struvite formation, copper was also shown to reduce soil microbial activity consistent with copper toxicity. These results will help provide recommendations for potential pretreatment for wastewater containing coper prior to struvite production.
3. Mesoporous Fe-doped Magnesium oxide (MgO) nanoparticles as a heterogeneous photo-fenton-like catalyst for degradation of pharmaceuticals in wastewater. Trace organic compounds, like pharmaceuticals, are ubiquitous in treated municipal wastewater and current sewage treatment processes are not designed to remove low level concentrations of these compounds. A new porous material made from magnesium oxide and 5% iron has been shown to be a good catalyst for removing salicylic acid in the presence of ultraviolet (UV) light and hydrogen peroxide. ARS researchers in Maricopa, Arizona, found the catalyst was able to remove all salicylic acid from solution within 15 minutes and the catalyst was found to maintain reactivity for more than five reaction cycles. Additionally, degradation of salicylic acid resulted in no unexpected byproducts and the catalyst was conserved. The new catalyst has the potential for use in current wastewater treatment plants to remove trace organics prior to environmental discharge.
