| When the process is
operating in the BioDenipho mode, the mixed liquor passes through the
oxidation ditches in series for the majority of the process. However, there
are periods of time, or phases, when the mixed liquor only passes through
one ditch, while the other ditch is essentially isolated from influent. The
specific phases of the BioDenipho process are discussed in detail in the
Process Description and Theory of Operations sections.
As the mixed liquor passes through the
ditches, the process conditions within the ditches are alternated between
oxic and anoxic to accomplish both nitrification and denitrification without
internal recycle pumping. The hydraulic capacity or volume in the oxidation
ditches is large in comparison to the influent flow volume. Therefore, the
concentrations of the pollutants are greatly diluted upon entering the
ditch. This dilution of the influent wastewater helps the oxidation ditch
process resist upsets from shock loadings of organics.
As described, aeration equipment is
turned off during anoxic phases and the activated sludge is kept in
suspension by submerged mixers. However, the mixers are not limited to use
only during anoxic phases. Kruger implements Dissolved Oxygen (DO)
controlled aeration operation. This ensures that only the actual volume of
oxygen required to maintain an oxic environment is supplied. When enough
oxygen is present in the wastewater, the rotors are turned off while
submerged mixers remain on to maintain the mixed liquor in suspension.
DO control minimizes energy consumption
by replacing wasted over-aeration with efficient mixing at significantly
lower horsepower loads. Thus, submerged mixers enhance the economic
operation of the BioDenipho process over conventional processes that rely on
aerators operating for continuous mixing. In addition, by alternating the
mode of operation within each oxidation ditch between oxic and anoxic
environments, typical internal recalculation pumping station and associated
operating costs required for nitrogen removal in a conventional treatment
facility are not used. Finally, by using the influent wastewater as the
source of organic carbon to fuel the denitrification process, no expensive
external carbon feed is required.
The initial sizing of a phased
isolation ditch basically follows "conventional" design procedures.
Nitrification and denitrification volumes are determined based on the
influent wastewater characteristics. A sufficient aerobic solids retention
time is incorporated to ensure carbonaceous BOD removal and complete
nitrification at the lowest anticipated wastewater temperature. Typically, a
safety factor is also applied which results in a relatively conservative
design to accommodate for diurnal and seasonal variations in the rate and
volume of flow.
The design procedure is similar to that
followed when designing a conventional wastewater treatment facility.
However, this is where the similarity with conventional facilities ends.
Once the tanks are constructed for the various stages of a conventional
wastewater treatment facility, the associated oxic and anoxic tank volumes
cannot be readily modified because of physical limitations and hydraulic
constraints. However, the oxic and anoxic treatment volumes of a phased
isolation ditch can easily be modified because of the alternating mode of
operation of phased isolation ditch technology.
The alternating mode of operation
utilized in the BioDenipho process offers a great deal of flexibility that
enables the operator to customize the process to accommodate the specific
characteristics of the wastewater being treated. This flexibility, which may
not be immediately apparent, is provided by means of phase control. By
varying the duration of the phases, one actually varies the oxic and anoxic
volumes supplied for nitrification and denitrification of the wastewater. |
