ABSTRACT

In 2013, the Dead Zone in the Gulf of Mexico totaled roughly 5,800 miles2 in area. The Dead Zone is so named because levels of oxygen are so low virtually nothing lives there. A major factor in the development of the Dead Zone is nitrate and phosphate in agricultural runoff in the Mississippi River Basin. These nutrients start a chain reaction which results in hypoxia. Attempts to reduce the Dead Zone focus on reducing nitrate and phosphate in runoff by 45%. This includes efforts such as the placement of woodchip bioreactors on the edges of farm fields and treating water with CaO and CaOH found in limestone though the two have never before been used together. This study used laboratory-scale model PVC bioreactor columns to examine the effect of combining the woodchip bioreactor with limestone filters. The data suggests that the addition of limestone filters in bioreactors decreases the amount of nitrate removal while increasing the amount of phosphate removal. However, further study is need to determine if the reduction in nutrient removal is due to the addition of the limestone, a reduced volume of woodchips, lower hydraulic retention time (HRT), or some combination thereof.

The material presented here is based upon work supported by the National Science Foundation under Award No. EEC-0813570 and EEC-1406296. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Effect of Limestone on Nutrient Removal Michelle Delvaux, Dowling Catholic High School & Natasha Hoover, Dept. of Agricultural and Biosystems Engineering, Iowa State University

RESEARCH QUESTION

Does the addition of limestone filters to woodchip bioreactors affect nutrient removal?

REFERENCES Hoover, N. L. (2012). Denitrification woodchip bioreactor two-phase column study: Evaluation of nitrate removal at various

hydraulic retention times and effect of temperature on denitrification rates. Maguire, R.O. et al. 2006. Liming Poultry Manures to Decrease Soluble Phosphorus and Suppress the Bacteria Population.

Environ. Qual. 35:849–857. doi:10.2134/jeq2005.0339. Mississippi River/Gulf of Mexico Watershed Nutrient Task Force. 2008. Gulf Hypoxia Action Plan 2008 for Reducing,

Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico and Improving Water Quality in the Mississippi River Basin. Washington, DC.

National Center for Coastal Ocean Science. 2013. 2013 Gulf of Mexico Dead Zone Size Above Average But Not Largest. Coastalscience.noaa.gov.

Schipper, L.A., et al., Denitrifying bioreactors—An approach for reducing nitrate loads to receiving waters. Ecol. Eng. (2010), doi:10.1016/j.ecoleng.2010.04.008

RESULTS

BACKGROUND

•  2013: the Dead Zone in Gulf of Mexico = 5,800 miles2 •  Named Dead Zone because low oxygen content inhibits organismal growth. •  Low O2 is the result of a chain reaction initiated by large amounts of NO3 and PO4 runoff in the MRB. •  2008 Hypoxia Action Plan called for a reduction of NO3 and PO4 runoff by 45%. •  Woodchip bioreactors can be used to remove NO3 while limestone can be used to remove PO4 though the two have always been used separately. •  This study investigated combining woodchip bioreactors and limestone to remove both NO3 and PO4.

METHODS Experimental Setup: Three laboratory-scale PVC columns were constructed to evaluate nutrient removal. One column was

packed with woodchips while the other two were packed with limestone filters and woodchips. The limestone filter in the second column was located at the influent end of the column while the filter was located at the effluent end of the third column. Influent nutrient solution containing nitrate and phosphate was pumped from a 120L container into the columns using Master Flex C/L Pumps and rubber tubing. Solution was pumped into the bottom of the columns to better regulate hydraulic retention time with a target HRT of 4 hours. The effluent solution was collected in three 20 L containers (one per column).

Sample Collection: Samples were collected in 100mL plastic bottles on a daily basis. One sample was collected from each effluent container daily as well as an influent solution sample when possible. Date and time of sample collection was recorded as well as the mass of the effluent solution before disposal.

Sample Analysis: Nitrate Content was analyzed using the AQ2 Method No: EPA-114-A Rev. 7 with a detection range of 0.25 – 15 mg N/L Phosphate Content was analyzed using the AQ2 Method No: EPA-118-A Rev. 5 with a detection range of 0.01 – 1.0 mg P/L

ACKNOWLEDGEMENT Thanks to the National Science Foundation, the Iowa Soybean Association, Michelle Soupir, Leigh Ann Long, members of the Water Quality Lab in the Agricultural and Biosystems Engineering Department at Iowa State University, and the Iowa State University Research Experience for Teachers Program.

DISCUSSION This was a preliminary study to examine the effects of adding limestone to small scale woodchip bioreactors. The data suggests that the addition of limestone slightly decreases the amount of nitrate removal but increases the amount of phosphate removal; though the location of the filter may be of no importance. The amount of nitrate removal may have decreased as a result of the addition of the limestone, the reduction in the volume of woodchips, or a combination of both. This could be examined by re-testing the PVC bioreactor columns with the same volume of woodchips in each column and then again with the addition of limestone to the initial woodchip volume. Additionally, there were some issues with controlling the HRT due to the variability in flow rate through the water pumps which could have contributed to the differences in amount of removal between columns and from day to day. A more stable and similar HRT would provide more conclusive results regarding the amounts and rates of removal. Before this practice can be implemented in the field more research is needed. This may include studies such as examination of flow through the bioreactor columns using tracers, effect of a longer HRT, longer sampling period, and effect of various amounts of limestone on the nutrient removal process. Additionally, the effect of combining this practice with other nitrate and phosphate reduction methods, such as wetland filtration, may be examined to see if nitrate and phosphate removal goals can be reached.

Image Source: Agriculture’s Clean Water Alliance

Image Source: Natasha Hoover, Michelle Delvaux

Image S

ource: EPA

.gov

Red line indicates the

average influent concentration of

Phosphate Table 1. Comparison of Nutrient Removal Loads Over Two Week Time Period. The Woodchip column showed the most nitrate removal and highest nitrate removal rate. The Limestone Influent column showed the most phosphate removal and highest phosphate removal rate. The Woodchip column treated the least amount of solution indicating a higher nitrate removal percentage and lower phosphate removal percentage. The Limestone Influent column treated the greatest amount of solution indicating a lower nitrate removal percentage and higher phosphate removal percentage. The Woodchip column had the greatest HRT while Limestone Influent had the lowest HRT; the Limestone Effluent column was the closest to the target HRT.

Figure 1: Effluent Nitrate Concentration Over Two Weeks. The Woodchip column showed a greater amount of nitrate removal than the other two columns which had similar levels of nitrate removal. On some days the amount of nitrate removal in the Woodchip column was not much greater than the others.

Figure 2: Effluent Phosphate Concentration Over Two Weeks. The Limestone Influent column showed the greatest amount of phosphate removal while the woodchip column showed the least amount of phosphate removal and the Limestone Effluent column was in between.

Nutrient Removal Loads   Column   Solution

Treated (L)  Average

HRT (hrs)  Total Nitrate

Removal (mg)  Total Phosphate Removal (mg)  

Nitrate Removal (mg/L)  

Phosphate Removal (mg/L)  

Woodchips   152.86   4.73   1057.67   5.12   6.92   0.03  

Limestone Influent   172.82   3.75   830.63   8.84   4.80   0.05  

Limestone Effluent   161.59   4.01   835.38   7.18   5.17   0.04  

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