Since
1987 Stip, now Isco-Stip, have manufactured the Biox
- a genuine on-line BOD analyser.
Developed out of a German government sponsored R&D
project to simulate BOD within a river bed, the Biox
series of cabinet analysers are probably still the only
on-line BOD that incorporates key elements such as :
Adapted,
acclimatised and immobilised mixed species biomass
that is analogous to the seed within the BOD-5 test
Patented
feedback dilution system that ensures a constant low
loading rate - ensuring a direct linear realtionship
between BOD and oxygen consumption
Biomass
within controlled conditions
- the secret of the on-line BOD technique
Adapted
to effluent / sample stream
biomass
to substrate ratio must be constant and high
constant
quantity of bio enzyme
constant
supply of substrate
Unhindered
diffusion of substrate to the enzyme
Computer
controlled feedback dilution using an adapted, acclimatised
biomass ensures a relevant and rapid measure of BOD
Additional
Advantages of Dilution
Reaction
kept at optimal response rate - biomass starved
Ensures
direct relationship between DO and BOD
Reactor
kept “clean”
Biomass
buffered from “shocks”
Improves
correlation with BOD-5 dilution
Biox-1010-
schematic
The
wastewater sample concentration in the bio-reactor is
controlled by the wastewater pump and the dilution pump,
so as to maintain the target rate of 3 mg/liter O2 consumption.
Thousands of small plastic rings within the bio-reactor
provide a growth surface for the biomass.
The mixing ratio of wastewater and dilution water is used
to calculate the BOD parameter.
Calibration
against BOD-5
Calibration
against BOD-5 - using a defined step-wise dilution can
help to remove error from the lab test:
Independently tested samples using known concentrations
of glycol were tested by both the on-line BOD analyser
and by laboratory analysis. The BOD analyser follows the
theoretical values very closely - a large fall off with
the lab BOD-5 occurs with the higher glycol concentrations.
The results highlight the well documented problems of
poor reproducibility with the lab BOD-5 test. Also the
ability of an analyser that inherently reduces such error
by incorporating an acclimatised biomass is clearly demonstrated.
List
of Articles : BOD Monitoring (organic monitoring)
On-line
BOD for process control
"We
also review the analyzer data on a monthly basis and compare the
results with routine laboratory BOD5 data for the same time period.
We have found there to be an excellent correlation between the lab
and the analyzer results."
