Evaluating the impact of a wood-chip bioreactor on phosphorus concentrations

Amanda M Brock, Purdue University

Abstract

Loss of nitrogen (N) and phosphorus (P) into rivers through subsurface tile drains causes eutrophication, which is a concern for aquatic ecosystems. Denitrifying bioreactors are shown to effectively reduce the losses of nitrate (NO3), however, little is known of their effects on P losses. A bioreactor at Throckmorton Purdue Agricultural Center (TPAC), located south of Lafayette, Indiana, has been shown to reduce NO3 concentrations of the effluent to approximately 0 mg N/L. However, an increase in the concentration of soluble reactive phosphorus (SRP) was observed in the bioreactor effluent during the monitoring period. To evaluate the effects of a bioreactor on P losses, three lab-scale anaerobic water columns were constructed using the same wood-chips as those in the TPAC bioreactor. Results from the water column tests showed an increase in the SRP concentrations for all columns with varying rates of increase. The highest concentration was over 7 mg P/L. Fungal growth discovered on the wood-chips led to two subsequent tests to determine the effect of the growth on SRP losses. An aerobic degradation test was done using wood-chips from the water columns and a nitrate solution on a shaker table. Under aerobic conditions, SRP concentrations increased to a maximum of 1.2 mg P/L, after which they leveled off and remained around 0.07 mg P/L. An anaerobic test was conducted using three replicates of non-sterilized wood-chips and three replicates of sterilized wood-chips. Results indicated that the sterilized wood-chips had slightly higher SRP concentration outputs than the non-sterilized wood-chips. However, the SRP concentrations between the two types of wood-chips did not vary significantly. Bauxite residue pressed into disks and sintered was tested as a potential remediation for reducing SRP loss from the bioreactor effluent. Bauxite disks of varying pH and varying salt content were used for bench-top SRP sorption tests. The results indicated that the bauxite disks could reduce SRP concentrations by 70-100%. A one-hour P-test was conducted on one bauxite disk with 3% salt. The 3% salt disk was observed to reduce SRP concentrations by roughly 25%. A lab-scale version of a bioreactor outflow box was constructed to test the performance of the bauxite disks under scaled field operating conditions. Two bauxite disks with different percentages of salt were individually placed in the box and samples were taken from the effluent. For both disks, there was no noticeable change in the SRP concentration of the effluent. Because the observed influent of the bioreactor lab test has an SRP concentration of 0 mg P/L, it is hypothesized that the P concentration seen in the effluent originated from the wood-chips as they decomposed. This study indicates that the solid bauxite disks are capable of absorbing P from the overlying water column, but their performance decreased considerably under flowing water conditions.

Degree

M.S.A.B.E.

Advisors

Chaubey, Purdue University.

Subject Area

Agricultural engineering

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