Table 1 - Denitrification efficiency vs. available volume of active biomass in the filter bed Click here to enlarge imageWithin the filter anoxic conditions will prevail, enabling the denitrifying biomass to grow on top of and in between the sand grains. The water flows in upward direction through the filter bed, while NOx-N is converted and suspended solids are retained. The produced N2 gas will migrate through the filter and be released at the top.
As the filter sand is rinsed continuously using an airlift and washer assembly, a steady state is created. The filter sand moves downward slowly, towards the bottom, using an airlift in the centre of the filter. In the airlift the sand is continuously transported to the washer section at the top of the filter and rinsed finally before it is released back onto the top of the sand bed. A continuous flow of wash water is released, containing the excess biomass and the retained suspended solids. The filtrate is discharged via the effluent outlet channel onto surface water.
In order to achieve an optimal filter performance, a process control system was developed to maintain high biological activity within the filter at varying feed conditions. The control system is based upon a continuous adjustment of the sand movement and washing rate, focussing upon a fixed volume of active biomass present in the filter bed.
In Table 1, the net effect of this control system is illustrated in terms of denitrification efficiency. The optimised control system helps obtain a 98% denitrification efficiency, and high efficiencies for suspended solids removal.
Biofilter performance data
Typically, NOx-N feed levels of 15-25 mg/l are reduced to filtrate levels of 0-3 mg/l N. Furthermore, the process control guarantees an overall COD decrease over the biofilter of 10%. Typical filtrate suspended solids levels are below 5 mg/l.
Specific methanol consumption and biomass growth have been determined based upon actual methanol consumption and mass balances over the biofilter plant. Specific methanol consumption is found to be 3 mg CH3OH per mg of NOx-N removed, while biomass growth is 1.3 mg per mg NOx-N removed, equal to 0.2 mg/mg COD consumed.
Authors' Note
J. W. Wouters is the project director for Paques bv, Balk, The Netherlands. P. de Been is the manager of the process area south of the Water Authority of Delfland, The Netherlands. For more information, contact the author at Email: [email protected]