12
and the flow passed through the unit declines. Each application has its optimum interval for
backwashing. In some cases, an extended backwash interval produces optimum performance and in
others, and extremely short backwash interval is best. In broad terms, short backwash intervals (<6
hours) are associated with heavy loads. Best performance for lightly loaded applications is usually
associated with extended backwash intervals (>12 hours).
In recirculating aquaculture or wastewater clarifier applications where the HPPG is used solely as solids
capture device reducing suspended solids levels, a high backwash frequency generally produces the
greatest mass removal rate. In these applications, the targeted particle size range is usually of the order
of >50 microns. Organics in the water will create a sticky surface that tends to stick particles together on
the bead surface. Internal settling after a backwash is rapid, and backflush frequencies can be short
(<hour) without adversely filter performance.
A good starting point for the backwash interval in a
recirculating clarifier application is 3 hours.
If
a decline in flow through the filter (or an increase in hull
pressure) is noticeable, increase the backwash pump airflow for more frequent backwashes.
If the application is focused on water clarity for display aquaria or zoo applications, then the HPPG
should be used as a clarifier focusing on small suspended particles. A clean bed of standard sized beads
has relatively poor single pass removal efficiency (20%) for particles below 20 microns. Single pass
capture of these particles is dramatically improved (>40%) once the bed begins to fill with biological or
mineral solids. Excessive backwashing should be avoided in clarifier applications. Lightly loaded HPPG
applications with a focus on water clarity (reduced turbidity) are generally associated with extended
backwash intervals, perhaps, twice a week.
In lightly loaded application seeking high water clarity,
start with a backwash interval of once a day.
Increase the backwash frequency (turn up the air) if the
flow through the filter declines significantly as this is a sign solids are not being backwashed enough for
your application. For single pass applications, the best water clarity is always obtained with reduced
flow and high pressure drop across the bead bed.
In recirculating applications, the benefits of increasing single pass efficiency by flow reduction are offset
by the reduction of number of filtration passes. The optimum in a recirculating application is normally
found at an interval that high pressure drop across the bed and high flows, a zone of moderate pressure
loss across the bed.
Clarifier applications are relatively insensitive to backwash interval, HPPG biological function can be
dramatically influenced by the backwash frequency.
Here backwashing influences several factors
(Table 6). Optimization of backwash frequency is application specific and your HPPG allowing process
optimization under both aerobic and anaerobic conditions across a wide range of loadings and targeted
substrates.
Table 6. Backwashing interval impacts biofiltration through several factors.
Factor
Importance
Comment
Biofilm thickness
Controls the mass of bacteria working
