Definition:
Nitrate contamination is one
of the major problems in groundwater, which is increasingly becoming a threat
to groundwater supplies. Nitrogen in groundwater also results from human
excreta, ground garbage and industrial effluents, particularly from food
processing plants. In order to remove nitrate from a water source, such as
ground water the biological denitrification technology can be applied for which
a carbon and energy source is needed. The aim of this study was to construct an
easy to operate reactor to treat the NO3 contaminated (ground) water to such
qualities that it can be used as drinking water for cattle.
To achieve this objective it
was investigated whether saw dust as an alternative carbon source for the
bacterial denitrification, could be used. For this purpose a low maintenance
reactor, which was inoculated with anaerobic digester sludge, was fed with
nitrate rich (100-200 mgN/L) water. A 100% nitrate removal was observed when
the feed water contained 200 mg/• NO3-N at a Hydraulic Retention Time (HRT) of
1 day, operating a stable reactor system. It was shown that similar nitrate removal
rates were obtained when the reactor was operated at room or mesophilic
temperatures
Denitrification is a process
in which the oxidised nitrogen substances, i.e. nitrates and nitrites are
reduced to nitrogen gas, such as N2O and N2, when a proton donor (energy
source) is available. In most biological denitrification systems, the nitrate
polluted waste water (e.g.domestic sewage) contains sufficient carbon (organic
matter) to provide the energy source for the conversion of nitrate to nitrogen
gas by the denitrifying bacteria. To treat groundwater, in which the nitrate
contents may be as high as 100 mg NO3-N/ℓ with low dissolved carbon content, an
additional proton acceptor is required. Generally, methanol is used as well as
a more complex product such as molasses. In order to promote nitrate removal in
remote areas, it is not advisable to use complex mechanical, maintenance
requiring equipment .
Robertson as well as Robertson
and Cherry and Blowes demnstrated passive in situ nitrate removal methods that
are mechanically simple and do not require significant additional maintenance.
They showed the use of nitrate reactive permeable sub surface barriers to
passively remove nitrate from septic system effluents. These barriers were
installed as layers downstream of conventional septic systems infiltration beds
and as vertical walls intercepting horizontally flowing septic system plumes.
The barriers contained waste cellulose solids, such as sawdust and leaf
compost, which provided the carbon source for heterotrophic denitrification .
It is envisaged that in South Africa a market exists for an easy to operate
nitrate removal system, which can effectively reduce the nitrate concentration
of the ground water to levels acceptable for the drinking water of cattle and other
farm animals.
Feed
Water:
Artificial nitrate rich water
was used as feed water for the continuous operated reactor, which contained
initially 100 mg/ℓ nitrate (NO3-N), thereafter 200 mg/ℓ. During the first
period of the study (till day 109) a weak NaHCO3 solution was added to the feed
water in order to maintain the reactor pH between 7 and 8. Once the
denitrification process was in operation, the addition of the NaHCO3 solution
was omitted. Reactor(s) During the first part of the experiment, one reactor was
operated consisting of a Perspex reactor (R1) of which the volume was 2ℓ.
The feed water entered the
reactor at the bottom, while the effluent was discharged at the top of the
reactor. The HRT was kept at 24 h during the first 109 days of the experiment.
This reactor was operated at 30 °C. After 110 days of the experiment, a second
reactor (R2) was introduced. This reactor was operated under the same
conditions as the first reactor, however the second reactor was operated at
room temperature. Reactor R1 was inoculated with micro organisms, obtained from
the anaerobic digester at the Daspoort Sewage Plant, Pretoria, South Africa.
Reactor R2 was inoculated with half of the biomass from Reactor R1.
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