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2.14 CHAPTER TWO

than by settling. Using GAC media preceded by ozonation provides the capability of ad-
ditional organics reduction by biologically activating the media. Ozonation may also re-
duce or convert many organics to enhance disinfection by-product reduction and to re-
duce chlorine demand. The multiple advantages of the options shown in these figures will
be discussed in greater detail later in this text.
Supplemental and additional treatment options that may be considered for normally
single-pass filters are discussed below. Posttreatment and its options are the same as those
shown in Figure 2.1.
Slow Sand Filter. Principal solids removal occurs in the top surface layer schmutzdecke,
which is also biologically activated. Typical TOC reduction can range from 10% to greater
than 20%. An intermediate GAC sandwich layer may be installed in the deep sand me-
dia to create an additional host layer. With preoxidation, using ozone, chlorine dioxide,
or potassium permanganate, biological removal of organics will be enhanced in both the
top and the sandwich layer.
Diatomaceous Earth Filters. This filter has superior cyst removal capabilities. How-
ever, there is no way to promote organics removal in this filter alone; a biologically ac-
tivated GAC column would have to be added at considerable cost. Use of this filter might,
therefore, be limited to waters of low organic content and low turbidity such as in many
well supplies.
Membrane Filtration. Application of this type of filtration is developing. Little stan-
dardization has been established. Except for grading the pore size of membrane material
from the finest for reverse osmosis to the coarsest for ultrafiltration and microfiltration,
details are proprietary. Each manufacturer has individual arrangement details. For the most
part, the most flexible use is with immersed-type ultrafiltration where coagulants may
more readily be added to improve solids and organics removal. Preoxidation may also be
employed to further enhance organics removal as well as promote the removal of iron and
manganese. Note that the finer membrane types such as nanofiltration and reverse osmo-
sis will directly remove these constituents, but at greater cost and with greater loss in the
waste stream.
Additional-Stage Filtration. Mentioned before, existing filtration installations may not
be able to meet the new requirements. In some instances it may be more practical than
replacement to provide an additional stage of filtration to meet lower-turbidity standards.
For instance, membrane filters could be added at the end of the train to further reduce tur-
bidity and to remove other contaminants. With prior solids removal, membrane treatment
becomes more efficient and waste of water may be eliminated by recycling to the origi-
nal plant inlet. Closed-vessel membranes, however, may not be placed downstream of DE
filters. The very fine siliceous DE particles in the effluent could damage and clog the
membranes. For low-capacity DE filter units, it may be possible to add cartridge filters
to further reduce turbidity. Membrane filtration can readily accommodate the effluent from
all other granular media filters including the slow sand type.
While not exactly second-stage placement, immersed-type membrane units may be in-
stalled in existing rectangular settling basins of conventional filter plants while other pre-
treatment and posttreatment facilities may be kept in service.

Less Common Treatment
As mentioned, the above review of optional treatment trains included those more com-
mon or those more universal. Many systems must address the reduction or removal of