Nitrogen removal in low pH and heavy metal contaminated mine wastewaters

Wastewaters from mining and mineral processing are generally characterized by low pH and high metal concentrations. Moreover, the use of blasting agents in mining promotes ammonium (NH4+) and nitrate (NO3-) discharge in ground and surface waters and the release of N2, NH3, N2O, NO and NO2)as detonation gases. Both ammonium and nitrate are nutrients for aquatic plants. If overdischarged, NH4+ and NO3- favor the production of algal blooms and contribute to the eutrophication of receiving waters. Most of metals are toxic and nonbiodegradable pollutants which tend to accumulate in the food chain and be absorbed by living organisms, human bodies inclusive. Nowadays, biological nitrification and denitrification are widely used instead of traditional processes.
Both the processes can be performed in several bioreactor configurations such as the fluidized-bed reactor (FBR) and the membrane bioreactor (MBR). FBRs have been observed to be very efficient for acid mine drainage (AMD) remediation, due to the great resistance to inhibitors and the potential of recycling the produced pH-buffered water. The use of MBRs in wastewater treatment has grown widely in recent years. MBR advantages over conventional wastewater treatment processes include small footprint and reactor requirements, high effluent quality, good disinfection capability, higher volumetric loading and less sludge production.
The present work aimed at developing both nitrification and denitrification of simulated low pH- and heavy metal-contaminated mine effluents. Three laboratory-scale glass FBRs were operated for biological denitrification for 539 days under different feed pHs, temperatures, hydraulic retention times (HRTs) and feed nickel concentrations. DFBR1 was operated at 7-8˚C, whereas DFBR2 and DFBR3 at room temperature (22˚C). DFBR3 was used only for biomass enrichment for batch assays. Within batch experiments, denitrification was inhibited at feed pH 3. On the contrary, both DFBRs resulted capable to neutralize a feed pH of 2.5 and maintain denitrification both at 7-8°C and 22°C, when double stoichiometric ethanol/nitrate ratio was provided.
Nickel and iron effects on denitrification were investigated. In batch assays, nickel concentrations of 50 and 100 mg/L decreased denitrification of 18% and 65%, respectively, at pH 7. Feed nickel concentration of 5 mg/l in DFBRs resulted in a pH decrease without affecting nitrate removal efficiency. Low soluble iron concentration (1 mg/L) showed a stimulatory effect on denitrification, increasing nitrate removal and reducing nitrite accumulation.
Both FBR and MBR technologies were used for biological nitrification at 21°C for 236 and 206 days, respectively. Ammonium (100 mg/l) was oxidized to nitrate averagely with yields of 78% in NFBR and 76% in NMBR.
DGGE analyses showed the growth of strong and several microbial communities during the operation of the reactors. Nitrate reducing bacteria (e.g. Dechloromonas denitrificans, Hydrogenophaga caeni and Zoogloea caeni), enriched on ethanol, colonized the support of the DFBRs. NFBR and NMBR efficiently maintained communities of slowly-growing nitrifiers (e.g. Terrimonas lutea).