978 PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS
Lynam et al. (1969) reported results of detailed tests con-
ducted on filtration of secondary effluent from an activated
sludge plant of the Chicago Sanitary District. They used a
filter bed of 0.85-mm-effective-size sand in a 280-mm depth
and a filtration rate of 120 mm/min, and analyzed the data in
terms of both hydraulic and SS loadings. Poor correlations
were obtained between effluent quality and hydraulic load-
ing, effluent quality and solids loading, and solid removal
and hydraulic loading. However, an excellent correlation
existed between SS loading and SS removals. It was also
observed that the sand filtration of alum-coagulated solids
was no better than that of uncoagulated solids, and the opti-
mum SS removal was obtained by alum and polymer coagu-
lation in combination with sand filtration.
A review of the retention of pathogenic bacteria in
porous media is presented by Stevik et al. (2004). The
review includes the factor affecting bacteria retention and
the factors that effect elimination of bacteria from porous
media. The authors also suggest priority areas of research
in this field.Multimedia Filtration The limitation of the single medium
rapid sand filter follows from its behavior as a surface filtra-
tion device. During filter backwashing, the sand is graded
hydraulically, with the finest particles rising to the top of the
bed. As a result, most of the material removed by the filter
is retained at or very near the surface of the bed. When the
secondary effluent contains relatively high solids concen-
trations, the head loss increases very rapidly, and SS clog
the surface in only a few minutes. One approach to increase
the effective filter depth is to use dual-media beds consisting
of a discrete layer of coarse coal placed above a layer of
fine sand.
More recently, the concept of mixed-media filters has
been introduced in order to achieve a filter performance that
very closely approaches an ideal one. In this case a third
layer of a very heavy and fine material, garnet (with specific
gravity of 4.2) or illmenite (with specific gravity of 4.5), is
placed beneath the coal and sand. Conley and Hsiung (1965)
have suggested the optimum design values for these filters.
The selection of media for any filtration application should
be based on the floc characteristics. An example of a typical
dual-media filter is shown in Figure 2.Moving-Bed Filters These types of filters were put on the
market by the Johns-Manville Corporation in the late 1960s.
It is a continuous sand filter in which influent wastewater
passes through the bed and becomes product water. Solids
trapped on the filter face and within the bed move with the
filter media, countercurrent to the liquid. Solids and small
amounts of filter media regularly removed from the filter
face are educted to the filter media tower without stopping
operations. Solids are scrubbed from the media and dis-
charged as a waste sludge, while the washed media is fed
back into the bed.
The filter medium usually used is 0.6- to 0.8-mm sand
with a maximum sand-feed rate of 5 mm/min and maxi-
mum filtration rate of 85 m/day (2100 U.S. gal/day/ft^2 ). Theadvantages claimed for this system are (1) automatic and
continuous operation, (2) that the filter allows much higher
and variable solids loadings than is permissible with a sand
bed, and (3) that through an efficient use of coagulant chemi-
cals, the system has the flexibility to reduce turbidity, phos-
phorus, SS, and BOD to the desired level (Johns-Manville
Corporation, 1972).Membrane Filtration Membrane filtration is being applied
more extensively as membrane materials are becoming more
resistant and affordable. Fane (1996) presents a description of
membrane technology and its possible applications in water
and wastewater treatment. An extensive study on microfiltra-
tion performance of membranes with constant flux for the
treatment of secondary effluent was published by his research
group in 2001 (Parameshwaran et al., 2001). Kentish and
Stevens (2001) present a review of technologies for the recy-
cling and reuse of valuable chemicals from wastewater, par-
ticularly from solvent-extraction processes.
A feasibility study on the use of a physico-chemical
treatment that includes nanofiltration for water reuse from
printing, dyeing, and finishing textile industries was per-
formed by Bes-Pia et al. (2003). In this work jar tests were
conducted for flocculation using commercial polymers fol-
lowed by nanofiltration. Their results show that the combina-
tion reduces COD from 700 to 100 mg/l. Another treatment
approach by the same authors (2004) uses ozonation as a
pretreatment for a biological reactor with nanofiltration as
a final step. A combined approach is presented by Wyffels
et al. (2003). In this case a membrane-assisted bioreactor
for the treatment of ammonium-rich wastewater was used,
showing this to be a reliable technology for these effluents.
Galambos et al. (2004) studied the use of nanofiltration
and reverse osmosis for the treatment of two different waste-
waters. For their particular case the use of reverse osmosis
was more convenient due to the high quality of the effluent,
but the permeate of the nanofiltration can only be released into
a sewer line or would have to be treated, resulting in an eco-
nomic compromise. A comparison between a membrane bio-
reactor and hybrid conventional wastewater-treatment systems
at the pilot-plant level is presented by Yoon et al. (2004).
The removal of volatile organic compounds (VOCs)
using a stripper-membrane system was studied by Roizard
et al. (2004). Their results show that this hybrid system can
be used for the removal of toluene or chloromethane with a
global efficiency of about 85%.
Vildiz et al. (2005) investigated the use of a coupled jet
loop reactor and a membrane for the treatment of high-
organic-matter-content wastewater. The main function of the
membrane is the filtration of the effluent and the recycle of
the biomass to the reactor. One advantage of the system is its
reduced size as compared with traditional treatment systems,
as well as a better-quality effluent.
A comprehensive review on the use of nanofiltration
membranes in water and wastewater use, fouling of these
membranes, mechanisms of separation, modeling, and the
use of atomic force microscopy for the study of surface mor-
phology is presented by Hilal et al. (2004). The future ofC016_005_r03.indd 978 11/22/2005 11:25:17 AM