(171g) Evaluating Water Quality Changes in Wastewater Reuse Using Pilot-Scale Experiments

In FY2007, U.S. EPA began to implement a new science and engineering research program on the Nation's aging water and wastewater infrastructure. The program is aimed to meet the future water needs for the Nation and, in doing so, protect human health and the environment. One program component is to evaluate water (wastewater, stormwater, and graywater) reuse and further develop innovative water reuse applications, establish water quality criteria, and evaluate infrastructure requirements for expanded water reuse in the nation. Currently, rehabilitation and capital improvement of the aged water infrastructure is urgent in order to provide adequate water supply and wastewater management in the face of climate change and socioeconomic development in the 21^st century. This also provides a good opportunity to incorporate water reuse practice into the overall sustainable water resources development.

The pilot-scale experimental study described in this paper was designed to evaluate water quality changes in wastewater reuse applications and, thus, to qualify the water quality treatment requirements for environmentally safe applications. For this purpose, two types of pilot-scale systems (an activated sludge treatment unit and soil columns) were set up at the U.S. EPA Test & Evaluation (T&E) Facility in Cincinnati, Ohio. The activated sludge (AS) treatment unit was used to simulate contaminant behaviors in conventional municipal wastewater treatment systems, while the soil column was constructed to investigate contaminant fate and transport in land applications for wastewater reuse. Aldicarb, a pesticide that is often found in low concentrations in wastewater, was used as the target contaminant for evaluation in the pilot-scale systems. The pilot-scale water reuse investigations were designed to:

1) Determine the extent of aldicarb and macronutrients removal in the AS unit. Chemical oxygen demand (COD) was selected as the macronutrient parameter in the investigation.

2) Evaluate aldicarb and total organic carbon (TOC) transportation in the soil column under simulated land applications of treated wastewater effluent.

3) Assess the mechanism/pathway for aldicarb degradation in the soil columns.

4) Determine the role of microorganisms attached to soil particles of the soil column in aldicarb degradations.

Activated Sludge Treatment Unit

A pilot-scale AS treatment unit, consisting of a 25-L primary clarifier, a 213-L aeration basin, and a 52-L secondary clarifier, was fabricated from Plexiglas^® and set up at the T&E Facility. The system was seeded with AS from the Greater Cincinnati Metropolitan Sewer District (MSD) Wastewater Treatment Plant (WWTP) located adjacent to the T&E Facility.

A synthetic wastewater stream was prepared into which the target contaminant (aldicarb) was introduced for the AS unit study. The synthetic wastewater included 6 organic compounds and 10 nutrient chemicals, and the composition was designed to simulate constituents in raw wastewater at WWTPs. The COD of the synthetic wastewater was approximately 300 mg/l. Synthetic wastewater was fed to the activated sludge system at 0.5 L/min, resulting in hydraulic residence times (HRTs) of approximately 0.84 hrs, 7.09 hrs, and 1.75 hrs in the primary clarifier, aeration basin, and secondary clarifier, respectively.

Prior to conducting the pilot-scale tests, bench-scale studies using 100 µg/l to 1,000 µg/l aldicarb in synthetic wastewater were conducted for 24 hours in 2-L glass beakers to determine the reaction kinetics of aldicarb degradation by AS. Aldicarb analysis was performed using extraction with methylene chloride and analysis by high-pressure liquid chromatography (HPLC) according to EPA Method 8318. The COD samples were digested and analyzed by Hach Method 10067. Little to no degradation of aldicarb was observed during the 24-hour test period in the bench-scale tests. COD reductions between 57% and 81% were observed, depending on the amount of AS used in the bench-scale tests.

Pilot-scale tests were performed by spiking aldicarb into the pilot-scale system at a concentration of 100 µg/l for 56 hours. Confirming the bench-scale tests, little to no degradation of aldicarb was observed during the test period. COD concentrations decreased from an average of 210 mg/l to 54 mg/l, representing a 74% reduction in COD in the pilot-scale AS treatment unit. Therefore, the results from the pilot-scale tests are consistent with those from bench-scale tests.

Soil Columns

Two vertical soil columns, each 3 meters tall and 40.6 centimeters in diameter were fabricated from Sch. 80 PVC pipe and assembled at the T&E Facility for this water reuse application. Sample ports were located at 45.7-centimeter intervals along the height of the column. The column was filled with ?sandy? soil obtained from Preble County, Ohio. A gradation sieve analysis shows that ninety percent of the soil particles were in the range of 75 µm to 4.75 mm. One column was operated in saturated mode with the water flow rate at approximately 350 ml/min at 10 psig. The second column was operated under unsaturated conditions with the water flow rate at approximately 35 ml/min and atmospheric pressure.

Tracer studies were conducted by introducing 5 L and 10 L of a 10 wt.-percent sodium chloride solution into the top of the saturated and unsaturated columns, respectively, to evaluate solute transport in the soil columns. The salt solution eluted from the two columns at 350 ml/min and 35 ml/min, respectively. The conductivity of the column effluents from the saturated and unsaturated columns was monitored over 18 hours and 7 days, respectively. The conductivity in the saturated column began to increase after 4 hours, and the maximum conductivity was measured 6 hours after introducing the salt solution. The majority of the salt solution passed through the saturated column within 12 hours. The conductivity in the unsaturated column began to increase after 25 hours, and the maximum conductivity was measured 49 hours after introducing the salt solution. The majority of the salt solution passed through the unsaturated column within 89 hours. In both columns, the effluent conductivity formed a classic tracer elution curve from which the hydraulic properties of the soil column were determined.

Following the tracer studies, aldicarb studies were conducted on both of the soil columns by spiking 100 µg/l aldicarb into non-chlorinated secondary effluent obtained from the MSD WWTP and introducing the solution into the saturated and unsaturated columns at approximately 350 ml/min and 35 ml/min, respectively. The study lasted 30 days, and samples were collected approximately every 5 days.

During the study, eight quartz coupons were placed inside the saturated soil column to determine the mechanism of microorganism attachment to the soil particles and its potential roles in the contaminant biodegradation. These coupons were located at 2 depths in the column, at the center and wall of the column, and were oriented perpendicular and parallel to the water flow through the column. Atomic force microscopy (AFM) was used to observe the surface morphologies of the quartz coupons and the presence of extracellular polysaccharides (biofilms) before and after the 30-day study period. The changes in the quantity of biofilm formation and microbial growth were also observed to estimate the potential risk of blocking the soil pores in the wastewater systems. The results indicated that the microorganisms and density of the biofilms were closely related to the location (depth in the column) as well as the orientation of the coupons with respect to the flow of the wastewater.

Fourier Transform Infrared (FTIR) Spectroscopy experiments were also conducted to provide information regarding the mechanism of binding/transformation inside the soil column. FTIR spectroscopy provided us fundamental information about 1) whether the chemicals bind with soil particles, 2) how strongly they bind to the soil surface, and 3) if the chemicals undergo transformations.

Future testing of water reuse applications will include additional contaminants (pesticides, pharmaceuticals, EDCs) as well as tertiary treatment processes (chlorination, advanced oxidation, membrane biological reactors) to determine treatment effectiveness and the economics associated with these advanced treatment processes.