Biological Nitrogen Removal Using Immobilized Bacteria
401
Thereafter, the feed was changed to settled municipal wastewater and after 1
month of stable operation synthetic wastewater feed was reestablished and the test
was extended to a 1- year period.
4.3
Upgrading Configuration of the Nitrification Reactor
Upgrading configuration of the nitrification system was aimed at by minimizing
heterotrophic bacteria accumulation in the nitrification reactor and enhancing
process efficiency. The drawback of the fluidized-bed system, commonly used for
entrapped biomass, is in the low ability of washing-out particles having the same
settling rate as gel beads. Even though the gel particles are retained, biological
suspended solids are accumulated, which leads to removal of BOD as well as to
suppressing nitrification by competing for dissolved oxygen. Furthermore, gel
beads have a thickness of at least 300 mcm, which creates substrate mass-transfer
limitation problem. In order to avoid the above problems, an innovative technique
of immobilizing bacteria was developed. This technique is based on forming a
thin 50-100-mcm layer of gel on the surface of filament, which is mounted in the
aerated column reactor. This fixed-bed reactor should allow better washout of
heterotrophs, improved substrate mass transfer, and more effective use of gel.
A special device was constructed for the gel-thread preparation. It consisted of
the frame connected to an electrical motor. The biomass suspension was harvested
from the chemostat with nitrifier concentration of about 200 mgVSS L". The
suspension was concentrated by centrifuge. This concentrate was added to the 50
ml of PYA-glycerol solution and mixed thoroughly. The fixed bed was prepared
by immersing O.2-mm-thick synthetic polyester thread into the liquid containing a
mixture of polymer solution with autotrophic bacteria. The obtained gel-covered
thread was mounted on the slowly rotating frame and dried under a ventilator for 1
h. The dry thread was subsequently mounted on the stainless steel frame in the
reactors. Final gel concentration in the reactors was approximately 6%. These
reactors were operated in parallel with those containing gel beads for verification
of the advantages above.
4.4 Feasibility Study of Prolonged Nitrification Interruption
The main goal of this experiment was to determine the ability of nitrifYing
biomass to recover activity after prolonged starvation, which may be needed due
to the seasonal demand of ammonium-rich effluent for agriculture. This
experiment was conducted using three continuous-flow laboratory-scale aerated
reactors (RA' RB, and Rc) containing 10% V dV R of the PV A gel beads with
6 g Lge," of seeded autotrophic biomass. All three reactors were initially fed
with inorganic synthetic wastewater containing ammonium concentration under
nonlimiting nitrification. On reaching constant effluent parameters, this feed to
two of the reactors (RB and Rc) was stopped for 3 months. Reactor RB received
once a week a portion of ammonia (l00mgNH4-NL"), while Reactor Rc did not
401
Thereafter, the feed was changed to settled municipal wastewater and after 1
month of stable operation synthetic wastewater feed was reestablished and the test
was extended to a 1- year period.
4.3
Upgrading Configuration of the Nitrification Reactor
Upgrading configuration of the nitrification system was aimed at by minimizing
heterotrophic bacteria accumulation in the nitrification reactor and enhancing
process efficiency. The drawback of the fluidized-bed system, commonly used for
entrapped biomass, is in the low ability of washing-out particles having the same
settling rate as gel beads. Even though the gel particles are retained, biological
suspended solids are accumulated, which leads to removal of BOD as well as to
suppressing nitrification by competing for dissolved oxygen. Furthermore, gel
beads have a thickness of at least 300 mcm, which creates substrate mass-transfer
limitation problem. In order to avoid the above problems, an innovative technique
of immobilizing bacteria was developed. This technique is based on forming a
thin 50-100-mcm layer of gel on the surface of filament, which is mounted in the
aerated column reactor. This fixed-bed reactor should allow better washout of
heterotrophs, improved substrate mass transfer, and more effective use of gel.
A special device was constructed for the gel-thread preparation. It consisted of
the frame connected to an electrical motor. The biomass suspension was harvested
from the chemostat with nitrifier concentration of about 200 mgVSS L". The
suspension was concentrated by centrifuge. This concentrate was added to the 50
ml of PYA-glycerol solution and mixed thoroughly. The fixed bed was prepared
by immersing O.2-mm-thick synthetic polyester thread into the liquid containing a
mixture of polymer solution with autotrophic bacteria. The obtained gel-covered
thread was mounted on the slowly rotating frame and dried under a ventilator for 1
h. The dry thread was subsequently mounted on the stainless steel frame in the
reactors. Final gel concentration in the reactors was approximately 6%. These
reactors were operated in parallel with those containing gel beads for verification
of the advantages above.
4.4 Feasibility Study of Prolonged Nitrification Interruption
The main goal of this experiment was to determine the ability of nitrifYing
biomass to recover activity after prolonged starvation, which may be needed due
to the seasonal demand of ammonium-rich effluent for agriculture. This
experiment was conducted using three continuous-flow laboratory-scale aerated
reactors (RA' RB, and Rc) containing 10% V dV R of the PV A gel beads with
6 g Lge," of seeded autotrophic biomass. All three reactors were initially fed
with inorganic synthetic wastewater containing ammonium concentration under
nonlimiting nitrification. On reaching constant effluent parameters, this feed to
two of the reactors (RB and Rc) was stopped for 3 months. Reactor RB received
once a week a portion of ammonia (l00mgNH4-NL"), while Reactor Rc did not