Nitrogen removal in low pH and heavy metal contaminated mine wastewaters

i
ABSTRACT
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 (NH
4
+
) and nitrate (NO
3
-
) discharge in ground and
surface waters and the release of N
2
, NH
3
, N
2
O, NO and NO
2
as detonation
gases. Both ammonium and nitrate are nutrients for aquatic plants. If over-
discharged, NH
4
+
and NO
3
-
favor the production of algal blooms and contribute
to the eutrophication of receiving waters. Most of metals are toxic and non-
biodegradable 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
ii
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).
Chapter 1 Introduction
2
Mining activity refers to the extraction and enrichment or refinement of metallic
ores, coal and industrial mineral deposits. Proposed mining projects vary
according to the type of metals or materials to be extracted from the earth. The
majority of proposed mining projects involve the extraction of metals such as
copper, nickel, cobalt, gold, silver, lead, zinc, molybdenum, and platinum. The
presence of nitrogen compounds (i.e. ammonium, nitrite and nitrate) in
wastewaters originating from mining activities can have detrimental effects to
the environment. The main source of ammonium and nitrate in mine water
originates from blasting agents such as ammonium nitrate fuel oil explosives
[Forsberg and Åkerlund, 1999]. Other sources of ammonium and nitrate in
mining water are cyanide destruction, transformation of amines in flotation
circuits, pH regulation agents, ammonium sulphate as eluent of uranium from
ion exchange resins, ammonium hydroxide used in uranium precipitation, and
ammonia used as lixiviant to recover copper and nickel in hydrometallurgical
processes [EPA, 2003]. Since the 1950s, one of the most used explosives has
been “Ammonium Nitrate Fuel Oil” (ANFO). ANFO explosives contain a mix
of ammonium nitrate and fuel oil. More water-proof explosives like emulsion
explosives have been developed by companies such as Oy Forcit Ab in Finland.
These explosives are used for mass blasting, which are used outside populated
areas and in tunnel blasting.
One of the most significant impacts of a mining project is its effects on water
quality and availability of water resources within the project area. In particular,
mining can result in:
 production of acid drainage and contaminant leaching from mined
materials (Paragraph 2.3);
 nitrogen contamination of mine wastewater from heap leaching and
blasting;
Chapter 1 Introduction
3
 erosions of soils and mine wastes into surface waters;
 impacts of wet tailings impoundments, waste rock, heap leach and dump
leach facilities.
Nitrogen and metal contamination can seriously affect the quality of
groundwater used as source of drinking water and impact on the ecosystems
influencing active aquatic and terrestrial life. Nitrate is a key water pollutant and
its minimization of discharge into the environment has been addressed by the
EU in directive 91/676/EC (protection of water from nitrate pollution from
agricultural activities). The European Union Water Framework Directive
(2000/60/EC, 23rd October 2000), which is the general legislation for the
protection of Europe’s aquatic environment demands a good state of all aquatic
environments throughout the European Union by 2015. The assessment of the
state of the aquatic environment is based on chemical as well as ecological
conditions of streams and water bodies. Furthermore, in addition to the rules in
Directive 80/68/EEC on the protection of groundwater against pollution caused
by certain dangerous substances, Article 17 of the WFD requires a Groundwater
Daughter Directive on the protection of groundwater against pollution
(2003/0210, COD).
In nature, a two-step process exists for the removal of nitrogen from water
bodies. Ammonium NH
4
+
is first oxidized to nitrite NO
2
-
(Eq. 1.1) and then to
nitrate (Eq. 1.2) NO
3
-
biologically. Nowadays, nitrogen biological removal by
combining nitrification and denitrification in engineered processes is widespread
in municipal and industrial wastewater treatment plants. Ammonium is oxidized
by bacteria of the genera Nitrosomonas and Nitrospira among others:
NH
4
+
+ 1.5 O
2
→ NO
2
-
+ H
2
O + 2 H
+
(1.1)
Chapter 1 Introduction
4
The first step is responsible for the production of acidity. Nitrite is oxidized by
bacteria of the genera Nitrobacter and Nitrospira among others:
NO
2
-
+ 0.5 O
2
→ NO
3
-
(1.2) Biological denitrification proceeds by reduction of nitrate to dinitrogen gas by
facultative, anaerobic, heterotrophic bacteria:
2 NO
3
-
→ 2 NO
2
-
→ 2 NO → N
2
O → N
2
(1.3) Since mine effluents contain very low organic content, an external organic
carbon source has to be added to allow denitrification. In scientific literature,
many simple organic compounds have been used as electron donors for
promoting denitrification. Ethanol has been found to be the most effective in
terms of denitrification rates, reaction completeness and microbial growth
[Christensson et al., 1994; dos Santos et al., 2004]. The reaction between
ethanol and nitrate is expressed by the following equation:
12NO
3
-
+ 5CH
3
CH
2
OH → 6N
2
+ 10CO
2
+ 9H
2
O + 12OH
-
(1.4) If the reaction develops completely, nitrate is totally converted to nitrogen gas
N
2
that releases to the gas phase. Moreover, OH
-
ions are produced neutralizing
the eventual acidic pH of the solution. On the contrary, if nitrate is partially
reduced, nitrite accumulates in solution as intermediate of the reaction:
6NO
3
-
+ CH
3
CH
2
OH → 6NO
2
-
+ 2CO
2
+ 3H
2
O(1.5) Nitrite still represents nitrogen pollution and no alkalinity is produced to
neutralize the initial pH.
The objective of this study was to evaluate the feasibility of biological
nitrification/denitrification for nitrogen removal from simulated mine waters.
Chapter 1 Introduction
5
Effects of low temperature, low pH, heavy metals, hydraulic retention time
(HRT) changes, and different ethanol/nitrate ratios on denitrification were
investigated both within bioreactors and batch assays. Three laboratory-scale
fluidized-bed reactors(FBRs) were used to investigate denitrification at low pH,
both at 7-8 and 22°C. Different feed ethanol/nitrite ratios and HRTs were used.
Nickel toxicity on denitrifying biomass was investigated adding 5 mg/L of Ni
+2
to feed solution of the FBRs. Low feed pH and iron and nickel effect on
denitrifiers were investigate in batch essays as well. Nitrification was studied
and monitored in one FBR and one membrane bioreactor (MBR) in order to
compare the two different bioreactor technologies. Finally, the development of
microbial communities enriched and maintained within the bioreactors was
monitored.