Air conditioning prior to biofiltration
120.000 m³/h -
www.indaver.be
|
Biofiltration unit
in a waste
water treatment plant |
Humidification unit for biofilter
(installed upstream of gas flow) |
BiofilterA biofilter is one of
the most economic and simplest odour control systems
available. For a good functioning biofilter however, several
conditions are to be fullfilled. Operating principle
of the biofilter
For biofilter applications, the
filling material or biomass is composed of organic material
such as bark chips, compost, peat, heath, coco, root wood,
expanded clay granules, etc. in a well defined mixing ratio,
or in several layers.
This natural "filter bed" or biomass can be contained in an
open or closed biofilter and is surrounded by a thin water
film. Part of the pollutants in the emission flow directed
through the biomass will already dissolve in this water
film; part of the pollutants is retained by the biomass
particles and the remaining pollutants will be further
decomposed by the microorganisms present in the biomass.
These microorganisms will collaborate to decompose the
remaining pollutants out of the emission flow. The microorganisms
partly feed themselves with the remaining pollutants, partly
with the biomass itself. Water and oxygen are of vital
importance for the microorganisms; both these substances
are, in normal operating circumstances, present in the biomass.
The residual of these decomposition processes are CO2, H2O, sulfate,
nitrate and similar. As the biomass gets saturated with
pollutants and residuals and gets crumbled by the action of
the microorganisms, it is to be replaced periodically. The frequence
of these biomass' replacements is depending on the emission flows
offered and the charges present in the waste gasses.
Not all emission flows can be treated in a biofilter.
The pollutants present in the emission flows will determine
wich bacterial groups will survive and/or be predominant in
the biomass (at the same time, under the influence of
temperature and moisture content, a certain microclimate and
balance will develop in the biomass).
Every change in these factors can severely destabilize
the efficiency of the biofilter.
Important parameters for an efficient biofilter
-
Constant moisture content
: Preservation of a optimal
moisture content in the biomass is an essential
criterion for a good functioning biofilter. The biomass
should be neither too dry, neither too wet. Too much
humidity can make the biomass clot together, the air
resistance will increase, the oxygen level will
decrease, the filter itself will become a source of odor
nuisance.
Therefore, it is of utmost importance to prehumidify the
emission flow, prior to direct it through the biofilter.
This prehumidification unit has to be perfectly adjusted
in order to create the correct moisture content within
the biofilter.
-
Constant air supply (emission
flow and oxygen level)
:
There should be a relatively constant emission flow.
When the emissions can only be produced at a rate of 7
to 8 hours/day during 5 days/week, with weekend breaks
and short and longer holiday stops, a biofilter will not
be a good solution. The microbial growth cannot survive without permanent
oxygen supply. The emission flow should also be free of
dust and grease.
-
Constant temperature (ambient
temperature)
: for a good development of the microbial
growth a temperature of 15 to 20 ° to max 30 °C is
ideal. Lower temperatures will slow the process and
higher temperatures will cause of proliferation of
bacteria, that will destabilize the entire biomass.
-
Waste load as constant as
possible
:
A lower pollutant load isn't a problem as such, and will
only transcribe into a higher elimination result (only
on condition that the other parameters remain at a
constant level).
At a high or peak pollutant load, the biomass will clot
together at an accelerated speed, because of the sudden
increase in microbial growth.
-
No high concentrations of
toxic and/or acidifying components
:
Very toxic substances will kill the bacteria. Too high
concentrations of sulphurous, chlorous and nitrogenous
organic components as well as ammonia will eventually
acidify the biomass and decrease the its efficiency. This can partly be prevented by adding certain
additives to the biomass, by replacing the biomass more
frequently or by installing an upstream chemical gas
scrubber.
-
Custom-made biomass
: The biomass should be entirely adapted to the emission
flow offered. The biomass shouldn't be too fine nor too
coarse, the air should be evenly distributed over the
entire biofilter. The biomass should be coarse enough to
allow a sufficiently large exchange surface, but should
also contain enough organic material for a satisfactory
microbial growth and a good living environment for the
bacteria. Additives such as lime and dolomite can
prevent a premature acidification. The biomass is
preferrably installed on a grid, to improve the air
distribution.
For special applications, it might be appropriate to
used an inoculated biomass (inoculation with specific
bacteria depinding on the kind of application). In all
cases, the biomass should be periodically replaced, in
function of the acidification and the salification of
the biomass and the pressure drop over the biofilter
(because of the compaction of the biomass).
The lifetime of the biomass is significantly varying and
is to be set between 6 months and 5 years.
Advantages of the biofilter
Disadvantages of the biofilter
-
Takes a relatively large surface
-
Biomass needs to be replaced
periodically
-
Exact moisture content and pH need to
be monitored accurately, a continuous aeration is vital
Most important application areas
-
Wastewater treatment plants
-
Agro-industry (composting, food industry, meat processing and
fish processing industry)
|