17 DEC 2003
problem but flow interruption for backwashing or filter media replacement may need to
be considered. If the system cannot be periodically shut down, then redundancy or
other provisions for continuous operation need to be considered.
LEVEL OF FILTRATION. The higher the level of filtration, the finer the parti-
cle that is removed, and the greater the cost. Pressure filters and gravity filters provide a
higher level of treatment than traveling bridge and continuous backwash systems but
they cost more. If the waste stream does not need to be clarified to the extent provided
by a more expensive filter system, then it might not be necessary to incur that expense.
Similarly, cartridge filters come in a wide range of removal ratings. However,
the higher the removal rating, the greater is the cost and, often, the media need more
frequent replacement. If permit conditions or downstream treatment system require-
ments only call for particles larger than 5 microns to be removed, then there is no need
to install a 0.5-micron rated filter element.
Finally, the design professional needs to consider the downstream treatment
cost. Greater levels of filtration may provide cost savings in subsequent treatment sys-
tems. For example, carbon adsorption can be used on a waste stream with a sus-
pended solids content of up to 50 mg/L. However, this does not mean that such a high
level of solids will be good for an adsorption unit. Even though the carbon adsorption will
work at that level, additional filtration will remove particles that may clog the carbon
pores and lead to more frequent media replacement and additional cost. Whenever fil-
tration is being provided to benefit a downstream treatment component, the manufac-
turer of that component should be consulted to determine the optimum level of filtration
for the waste stream going to that equipment.
ALLOWABLE HEAD LOSS THROUGH A SYSTEM. Different filtration sys-
tems require different levels of pressure to function properly. Similarly, different waste
stream treatment systems may be designed for different hydraulic gradients.
Groundwater being extracted from a well may already be under pressure, and
that pressure can be used to process wastewater through the entire treatment system
without additional pumping.
Each component of the treatment process, including the filtration system, will
have certain inlet requirements as well as head loss through the system. Auxiliary
pumping may be required to maintain these design levels. On the other hand, it may be
better for a waste stream to flow by gravity through the entire treatment process. In this
case a low head gravity filtration system may be required.
PROCESS CONTROLS. As with any equipment, the greater the number of
automated features there are, the higher the cost. Conversely, automated controls are
not luxuries but are designed to save manpower and the cost of constant supervision.
The cost of one must be weighed against the other.