REMEDIATION OF NITRATE-CONTAMINATED SURFACE WATER USING SULFUR & LIMESTONE AUTOTROPHIC DENITRIFICATION
|J.M. Flere and T.C. Zhang, Department of Civil Engineering, University of Nebraska-Lincoln at Omaha Campus, Omaha, NE 68182-0178||
Bioremediation of nitrate-contaminated water is usually limited by insufficient organic carbon if biological heterotrophic denitrification processes are involved. In contrast, sulfur-based autotrophic denitrifiers can use the elemental sulfur as an electron donor, CO2 as carbon source, and nitrate as the electron acceptor to reduce nitrate to nitrogen gas without the supply of external organic carbon. In this study, the feasibility of using a sulfur-limestone autotrophic denitrification (SLAD) process to remediate nitrate-contaminated surface water was investigated.
Bench-scale ponds systems were completely stirred tank reactors (CSTR) with a working volume of 21.4 liters each and a hydraulic retention time (HRT) of 30 days. These CSTRs were open to the atmosphere to simulate aerobic pond systems. The pond systems were set up as follows: (1) CSTR 1 & 2 contained 1.75 in. thick sediment from natural wetlands and ponds, one layer of elemental sulfur and limestone in a 3:1 ratio, and influent (flow rate = 0.49 mL/min) with 30 mg/L of NO3--N; (2) CSTR 3 contained the same material and influent as reactor 1 & 2, but a 250 mL of Thiobacillus denitrificans seed sludge (cultured in our lab) was introduced to the sediment before the influent was started; and (3) CSTR 4 was a control reactor with only sediment and influent without the addition of sulfur/limestone. No organic carbon was added into any CSTRs during the experimental period.
It was found that: (1) the start-up periods of the CSTRs 1 to 3 were one month, while it was very difficult for the CSTR 4 to reach its steady-state; (2) NO3--N were removed by 65%-88% with removal rates of 0.505 to 0.844 mg/L/d in the CSTR 1 to 3; while the CSTR 4 showed no nitrate removal; (3) nitrite was measured at less than 0.7 mg/L in the CSTR 1 to 3; (4) the sulfate production of the CSTRs 1 to 3 was approximately 7.1 mg SO42- produced/mg NO3--N reduced, corresponding to the stoichiometric equation; (5) the measurement of DO along the depth of the reactor indicated that the CSTRs were under aerobic conditions (DO > 6 mg/L); and (6) inoculation of autotrophic denitrifiers (seed sludge) can accelerate the start up of the SLAD process. However, autotrophic denitrifiers found naturally in sediments or soil can easily be accumulated once sulfur and limestone were introduced into the system. The time difference between CSTRs 1 and 2 and CSTR 3 to reach the steady-state condition was only 2 to 3 weeks.
The SLAD process may be a replacement of heterotrophic denitrification in pond systems due to the fact that no organic carbon source is needed in the SLAD process.
Key words: nitrate, autotrophic denitrification, in situ remediation, sulfur, surface water
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