Bio Tower
Aerobic BOD Removal
It
is well known that different types of plastic media give varying waste
water treatment efficiencies for both BOD and ammoniacal nitrogen removal
applications. The prime factors affecting specific
Media performance are :-
 |
Media Configuration |
|
Liquid Distribution |
|
Irrigation Rates |
|
Liquid Retention Time |
|
Oxygen Transfer and Sludge Production |
|
Surface Area Utilisation |
|
Media Volume Requirement |
The
above factors will primarily determine the capital and operating costs
of an effluent treatment Biofilter and are discussed here.
Media Configuration
Media
performance is very much dependent upon its configuration. A high effective
surface area, plus good ventilation and liquid distribution must be
provided, and the combination of there factors applies to both carbonaceous
removal and nitrification. As removal efficiency depends highly upon
the rate of diffusion of oxygen and organic pollutant, each element
of media surface must be available at the same time and rate to achieve
optimum efficiency from a given media volume, by exhibiting excellent
flow of effluent and oxygen in all directions.
The random orientation of Etapak Biofilter Media ensures that all media
surface is available at the same time for contact between substrate and
biomass, thus promoting a high removal rate.
Liquid Distribution
Vertical tube and sheet media depend completely on initial liquid distribution
quality at the top of the packed bed. Once liquid has entered a tube it
cannot wet other tubes. A tube or plane which receives no liquid will remain
dry throughout its length,hence the surface area in that tube or plane
is lost
Cross flow sheet media can distribute liquid in one direction and their
surface area utilisation tends to be, but is not necessarily, better than
vertical tube or sheet media.
Etapak random media on the other hand can distribute liquid in all directions.
Consequently, its performance is better than vertical and cross flow sheet
media.
Irrigation Rates
As a direct consequence of the mabihts of sheet media to redistribute liquid
high irrigation rates are required to wet the available surface area a
typical minimum irrigation rate for sheet media is 0.70 gpm/ft2 (1.7 m3/m2.h).
Etapak random media has been successfully used with much lower irrigation
rates. As a general rule, the minimum irrigation rate for Etapak media
is 0.2 gpm/ ft2 (0.5 m3/m2.h).
The energy required for pumping liquid depends upon the volume flow and
height the liquid has to be raised. As this is approximately 80% of the
total energy requirement of a Biofilter, Etapak random media will give
significantly tower operating costs.
Liquid Retention Time
Etapak random media in a packed bed provides a uniform surface area in
three dimensions. This results in greater liquid retention time compared
to modular sheet media. Since the contact time between biomass and substrate
is related to the liquid residence time. a greater removal rate per unit
volume (i.e., media surface media) is shown by Etapak compared to other
media.
Oxygen Transfer and Sludge Production
In all aerobic biological waste water treatment systems contact between
biomass and substrate has to be in the presence of oxygen. Thus media oxygen
transfer efficiency influences performance, in both BOD and ammoniacal
nitrogen removal terms. Operation in Etapak Biofilters is at near oxygen
saturation levels.
Nitrification oxygen demand is more than four times the biochemical oxygen
demand. However, ammoniacal nitrogen is not absorbed into the biological
floc for later oxidation in contrast to BOD. Therefore, the ammonia must
be oxidised during the period it is in the treatment process.
Consequently, sufficient oxygen must be provided to handle the load imposed
at all times. The above factors give high nitrogen removal rates in Etapak
Biofilters.
A further oxygen demand is imposed by endogenous respiration, where approximately
1 kg 02 is required to stabilise 1 lb. of sludge. Etapak Biofilter sludge
production is approximately 50% of that for modular sheet media. This further
indicates the excellent oxygen transfer properties of Etapak random media.
In addition, sludge treatment and disposal costs are reduced.
Surface Area Utilisation
Media performance also depends upon how effectively the available surface
area is used. Independent analysis have shown that the surface area utilisation
of random media is greater than modular sheet media.
Media Volume Requirement
At the end of the day, it is impossible to consider the effects of factors
outlined in sections 2.0 to 7.0 without appreciating the implication in
terms of required media volume. The total installed capital cost is to
a great extent dependent upon the size of plant required (i.e., media volume).
The media volume is a function of the permissible organic loading for a
given BOD removal efficiency. As a result of the better performance of
Etapak per unit volume of media, it is possible to specify reduced media
volume compared with those required for sheet media of similar specific
surface area to achieve the same performance.
Figure 1.0 shows design published curves for treatment of domestic waste
water, obtained from operating plants, using Etapak 120 and sheet media.
The surface area of both media is 100 m2/m3. It is very apparent that the
performance of the cross flow media is consistently less than Etapak 120.
In order to illustrate the advantages of Etapak media over sheet media
quantitatively consider the following example to treat a municipal waste
water Average Dry Weather Flow 10,000 m3/d
Waste Water Influent after | BOD 200 mg/l Primary Settlement | SS 100 mg/l
Waste Water Temperature 20°C
Table 1.0 gives the media volumes required for each type of media for a
range of removal efficiencies, the volumes calculated from the published
design curves given in Figure 1.0. Inspection of table 1.0 clearly shows
that on average, the Etapak media volume is only 54.6 percent of the cross
flow media volume.
Hence the capital costs of a Biofilter employing media are considerably
lower than a Biofilter using sheet media.
Although the example given above concerns BOD removal, the factors outlined
in Section 2.0 - 8.0 will also determine nitrification performance i.e.,
the media volume required for a specific nitrogen removal duty will also
be considerably lower for Etapak compared to cross flow media.
Conclusions
Specific media performance is dependant upon a number of factors :-
|
