Chan, T.F.*; Koodie, T.; Sloper, M.J. and Wiggam, R.W.
Black and Veatch, UK
Black and Veatch installed and commissioned continuous blackwash nitrifying sand filters as the
tertiary treatment stage at 10 sewage treatment works. The filters were installed to enhance the
effluent quality of the existing works in order to meet the more stringent ammonia consents. Nitrifying
sand filters have a number of advantages including their compactness and the simplicity of their
operation. This paper has been written to present the key lessons learned and to provide guidance
on achieving process optimisation during commissioning and operation.
Continuous backwash filter, nitrifying sand filter, process commissioning, optimisation, backwash, air
flow, dissolved oxygen profile, nutrient.
Upflow continuous backwash filter technology was first developed in the 1970s for suspended solids
removal. The filter operates in an upflow mode through the filter media, typically sand. The feed
wastewater enters the lower section of the filter. The filter bed is moved continuously downwards,
countercurrent to the upward feed wastewater flow, by means of an air lift created using compressed
air at the base of the unit. The dirty sand/solids mixture is then passed, with added washwasher,
through the sand washer and into a wash box. The flow out of the wash box, and hence the
washwater flow, is controlled by an outlet weir which is always lower than the filtrate outlet. In the
wash box, the denser cleaned sand is separated from the lighter solids and returned to the top of the
filter bed. The solids laden washwater is diverted to the inlet works for treatment.
The key advantages the continuous backwash filter offers include compact installation, continuous
treatment and continuous dirty backwash water production. The process configuration has been
further modified to incorporate tertiary nitrification or denitrification. In a nitrifying sand filter (NSF), air
is added to the filter bed through aerators. The schematic of a typical continuous backwash nitrifying
sand filter is shown in Figure 1. For a denitrifying sand filter, a carbon source is added to the
incoming wastewater. These process configurations have since been widely applied in wastewater
treatment worldwide (Feldthusen, 2004)
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