Intensive reported to be extremely low, due to slow

Intensive zero water exchange shrimp grow outs are
specialised and highly dynamic aquauclture production systems where
bioremediation of NH4+ – N and NO2– N are the vital processes for  successful culmination of the culture. NH4+
– N originates from  animal excreta, uneaten feed and decomposing
organic matter generated from phyto and zooplankton. Unionized ammonia (NH3)
is the  toxic species and its percentage
depends upon the variation of pH and tempreature; at high pH and tempreature
the NH3 concentrations shoots up. In nature nitrifying
bacteria bring forth the oxidation of ammonia to nitrite and to comapartively
innocuous nitrate, the process termed 
nitrification. In biological ammonia removal systems, nitrifying
activity of suspended bacteria has been reported to be extremely low, due to
slow growth rate (Bower and Turner 1981; Furukwa et al. 1993). With out the
addition of nitrifiers as start up culture, 2-3 months are needed to establish
nitrification in marine systems (Manthe and Malone 1987) and 2-3 weeks in fresh
water (Masser et al. 1999). There is an agreement, among researchers and
between laboratory research and commercial application, on the fact that salt
water systems need much longer start up period. Under such situations,
immobilization techniques have been found useful to overcome the delay in the
initiation of nitrification (Sung Koo et al. 2000). For such applications,
nitrifying bacteria have to be generated in large quantity, and an important
consideration of which is cost – effectiveness. The medium optimized here has
been seawater based and required only addition of the substrate NH4+
– N as NH4Cl and Ca CO3 to maintain optimum pH. The
design consisted of 50 L conical tapering fermentation tank made of fiber
reinforced plastic. An electrically operated stirrer/agitator is used to
accomplish agitation and mix up of the carrier material and nitrifying bacteria
to maintain them in suspension. The fermentation tank has been made opaque and
placed well protected from sunlight, as the visible and UV light rays are
lethal to nitrifying organisms (Johnstone and Jone 1988; Diab and Shilo 1988). The carrier material, wood powder, used for
immobilization was locally available and inexpensive. Initially 50 L seawater
was chlorinated with 300 mg/ L sodium hypochlorite and subsequently aerated to
remove chlorine and supplemented with sodium thiosulphate to ensure its total
removal. The carrier material was sterilized by autoclaving at 15 lbs for 15
min. NBC was drawn from nitrifying bacterial production unit (Kumar et al. 2009).

            Active 4 L NBC (105
cells/mL) and 800 g wood powder were introduced in to 46 L seawater based
medium in the device for immobilization of NBC and maintained in suspension. When
population of nitrifying bacteria gets established under steady state
conditions residual nitrite shall be too low to be detected with progressive
building up of nitrate according to the observations made by Achuthan et al. (2006) during the enrichment of nitrifying
bacterial cultures form shrimp ponds, and by Kumar et al. (2009) during their mass production. It has also been established
that nitrite oxidation to nitrate is more rapid than the preceeding step (Stensel
and Barnad 1992). This was proved to be true in the present study over and
again as nitrite turned out to be below detectable level after seven to nine
days of initiation of immobilization.

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            The time period required for immobilization
of NBC was determined by inoculating  immobilized
NBC into fresh medium and analyzing the TAN removal rates which were
0.27, and 0.35 g/ m2/ day on the 7th, 8th,
9th and 10th day respectively (Manju et al. 2009).

            A simple technique for the
processing of immobilized NBC after harvest with out loss of its nitrifying
potency was developed. Two methods could be evolved, one was drying the wood
powder immobilized with nitrifiers by spreading at room temperature and the other
was drying in a desiccator with out vaccum. NBC processed by both the methods
exhibited significant TAN removal compared to the one processed under vaccum (P


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