advanced water treatment with ozone and slow sand filtration - eWISA

Presented at the WISA 2000 Biennial Conference, Sun City, South Africa, 28 May - 1 June 2000
ADVANCED WATER TREATMENT WITH OZONE AND
SLOW SAND FILTRATION.
G.B. Saayman.
City Council of Pretoria, City Engineers: Water and the Environment, PO Box 1409, Pretoria,
0001.
Abstract
Although slow sand filtration had already been described in ancient times, it is still implemented
in major cities around the world. It is a simple, inexpensive and reliable technique ideally suited
for developing countries. Pilot-scale investigations were conducted at the Daspoort Water Care
Plant in Pretoria to evaluate the performance of slow sand filtration with preozonation in
removing natural organic matter and pathogenic organisms from the secondary effluent from a
biological nutrient removal plant. The results were satisfactory with significant reductions in
natural organic matter and removal of pathogenic organisms.
Introduction.
One way to increase a country’s effective water resource is through deliberate reuse. The reuse
of wastewater is a valuable source of water in an arid country such as South Africa. A certain
degree of indirect reuse of treated effluent is currently taking place. The discharged effluent
flows into dams or reservoirs, from where it is withdrawn to water treatment plants, which
prepares the water for distribution for mainly potable use. Philosophically, one should in future,
consider the option of direct reuse, since the treated effluent quality discharged by waste water
care works actually deteriorates in the dams, due to some algal growth. The present phosphatedischarge
standard may lower the growth rate of algae, but will not prevent their growth.. During
their growth phase, algae release sub-products from their metabolism (sugars and amino acids)
that are very biodegradable (Bonnet, et al., 1992). If these products are not removed it will lead
to post-bacterial proliferation in the distribution system necessitating the use of large amounts
of chlorine, in this instance the chlorine is used for its bactericidal and bacteriostatic effects.
High levels of chlorine in the treated water increase corrosion phenomena and is likely to
produce new compounds, such as trihalomethanes, potentially cancer-forming compounds.
Direct reuse would imply connection of the effluent stream with the intake of a downstream

potable water reclamation plant. This type of scheme would obviously require a higher chemical
and microbiological quality effluent and some upgrading to the downstream treatment plant.
Conventional waste water treatment plant effluent contains unacceptable levels of slowly
biodegradable dissolved organic compounds and pathogenic micro-organisms.
Ozone can serve as a chemical oxidizing agent as well as a disinfectant. It does not produce
halogenated organic compounds, unless bromide ions are present in the water being ozonized.
Ozone is second only to fluorine in oxidative strength. The bacterial kill rate of ozone is about
3 125 times that of chlorine. Ozone de-activates E. Coli, fungus, virus, and faecal coliforms by
different means depending on the organism. In bacteria the reaction of ozone with the double
bonds of fatty acids in the bacterial cell walls results in a change of cell permeability and leakage
of cell content. In viruses, alteration of the protein capsid prevents the virus from being taken
up by susceptible cells. This rupture and disintegration of the cell in a process called lysing takes
seconds. Chlorine diffuses through the cell wall and results in death by attack of enzymes. This
process can take minutes or even hours. Cyst organisms such as the Giardia cyst are the most
resistant to all disinfectants, because of their protective shells. The extent of inactivation or
destruction of microorganisms is related to the product (CT) of the concentration of disinfectant
(C, in mg.l -1 ) times the contact time (T, in minutes).
The United State Environmental Protection Agency (US EPA) promulgated regulations for the
disinfection of drinking water which incorporate the concept of CT values. (US EPA, 1989). For
disinfection with ozone, EPA recommends a maximum CT value of 2,9 mg.l -1 -min at
temperatures below 1 O C decreasing to 0.48 mg.l -1 -min at 25 O C. At these values ozone will
guarantee the inactivation of 99,9 % (3-logs) of Giardia cysts, and simultaneously, the
inactivation to greater than 99,999 % (5-logs) of enteric viruses. Table 1 values are
recommended by the US EPA for obtaining varying logarithms of inactivation of Giardia
lamblia cyst with ozone. When even 0,5 log inactivation of Giardia cysts is obtained with
ozone, more than 5-logs of inactivation of enteric viruses will be obtained, along with total
bacterial kills. Table 1 lists CT values recommended by the US EPA for obtaining varying
logarithms of inactivation of Giardia lamblia cysts at varying temperatures.
TABLE 1
CT VALUES FOR INACTIVATION OF Giardia CYSTS BY OZONE (US EPA 1889)
Logs of Inactivation
Temperature, O C

Microorganisms tend to concentrate at the gas-liquid surface of a bubble due to their surface
active properties and are therefore exposed to higher levels of ozone than the residual ozone
found in the bulk of the water. The degree of microorganism inactivation is greatly improved
for a system containing both ozone bubbles and an ozone residual compared to a system having
only an ozone residual.
Although there are a few organic compounds which are rapidly oxidized to destruction (i.e.
formic acid, phenol), the greater majority are only partially oxidized, even by as strong an
oxidizing agent as ozone, in an aqueous solution. One of the advantages of partial oxidation of
organic compounds in water is that the compounds become more polar than they were originally.
Polar groupings such as carbonyl (>C=O), carboxyl (-COOH), and hydroxyl (-OH) groups are
formed. In the presence of polyvalent cations (calcium, magnesium, iron, aluminium,
manganese), these polar groupings combines with the cations producing insoluble complexes
which can be removed by filtration. This process called “micro flocculation”, the flocculation
of soluble micro pollutants, has been described by Maier (1984).
According to Grady, et al (1984) ozonation of activated sludge effluent resulted in an increase
in low molecular weight material at the expense of larger molecular molecules. The reduction
in molecular size and increased polarity is believed to make the molecules more readily
assimilable for bacterial catabolism (Singer, 1988).
In the water industry there is a misconception that slow sand filtration is an old-fashioned
technology that has been rendered obsolete by advances in high-rate filtration techniques. Slow
sand filtration is not outdated and is at least as efficient if not better than physico-chemical
processes (Montiel, et al, 1989). A conventional slow sand filter requires no pretreatment. slow
sand filters can be successfully implemented where a lack of funding and expertise in the
development, management and operation of complex projects exist.
A number of researchers have investigated the application of slow sand filtration in the tertiary
treatment of sewage effluent. A laboratory-scale slow sand filtration unit operated by Ellis
(1987) consistently removed 90 % of the suspended solids, more than 65 % of the remaining
BOD and over 95 % of the coliform organisms from settled biofilter effluent. The length of
operational runs averaged 20 days at 3.5 m 3 .m -2 .d -1 and 13 days at 7.0 m 3 .m -2 .d -1 using a 0.6 mm
effective sand size filter. Very good removal of total coliform bacteria and turbidity were found
by Farooq and Al-Yousef (1993). Removal of biological oxygen demand (BOD), (79-92 %),
chemical oxygen demand (40-60 %), standard plate count (88-93 %), nitrate (17-30 %),
phosphate (8.3-8.4 %), and sulphate (5-10 %) at various sand depths and two different sizes of
sand were observed by Farooq et al (1994). If filters are properly designed, constructed,
operated, and maintained, they can be effective in removing Giardia cyst from a contaminated
water source over a wide range of operating conditions. More research is necessary to determine
the effectiveness of slow sand filters in removing Cryptosporidium oocysts (Fogel et al).
However, according to Timms et al (1995) slow sand filtration is a highly efficient means of
removing Cryptosporidium oocyst from water. A reduction of better than 99.997 % at filtration
rates of 9.6 m 3 .m -2 .d -1 was achieved. The oocysts were retained in the upper layers of the filter
bed and showed no sign of moving to lower levels. This characteristic of the slow sand filtration
process makes it well suited for the protection of drinking water from Cryptosporidium. Poynter
and Slade (1977) investigated the removal enteroviruses using attenuated polio virus type 1,
naturally occurring bacteria and bacteriophage T7 from contaminated water. The slow sand

filters were found to be highly effective in removing viruses. In comparison bacteria were less
and bacteriophages were more efficiently removed than the polio virus.
Gould et al (1984) investigated the use of ozone and slow sand filtration in removal of humic
colour from water. Their results show that approximately 10 % of the total organic carbon
(TOC) was removed by slow sand filtration. With pre-ozonation TOC removal increases to
25%.
The objective of this study is demonstrate on pilot scale the application of slow sand filtration
with pre-ozonation as a means of tertiary treatment of secondary effluent from an activatedsludge
plant to meet the requirements for the direct production of save drinking water.
Material and methods,
This study was undertaken using two pilot-scale slow
sand filters at the Daspoort Water Care Plant in
Pretoria where a high quality effluent from a effective
biological nutrient removal was available.
Ozonation.
Ozone was generated on site using an Ozonia Ozat
type CFS-1 ozone generator with a nominal production
65 g O 3.h -1 at a nominal concentration of 6 % (w/w)
using pure oxygen. The ozone transfer efficiency of
counter flow bubble column contactor is better than a
packed column system (Gould et al, 1984). Therefore,
use was made of a counter flow bubble column
contactor, 7 m high and 300 mm in diameter. The
effective water depth was 6 m (Figure 1). A painted
steel pipe was used. The feed water was introduced at
the top of the column, while the ozone was let in at the
bottom. The ozone was evenly dispersed through a
fine bubble diffuser. Initially a diffuser made from
compressed polyethylene balls was used. This diffuser
was not ozone-resistant and was replaced by a
Figure 1 Counter flow bubble
column contactor.
Carborundum diffuser. Ozone that was not utilized in the column was wasted into the
atmosphere. To determine the ozone transfer efficiency the ozone concentration in the inlet of
the counter flow bubble column and the gases escaping from the top of the column was
measured.
Slow sand filters.
The pilot plant, shown diagrammatically in Figure 2, consisted of two slow sand filters operating
in parallel. The diameter of the circular spiral wound polyethylene filters were 2.5 m and the total
height 2.8 m .This provided a filter area of 4.91 m 2 per filter. At a filtration rate of 7 l.min >1 ,
the filter loading rate was ca 2 m 3 .m -2 .d -1 or 2 m.d -1 .

Figure 2 Slow sand filter.
The filter beds consisted of a 1 000 mm layer of fine sand with an effective size of 0.35 mm and
a uniformity coefficient of 2.5. Underneath the sand was a 300 mm layer of gravel (5 - 13 mm).
The water depth above the sand was controlled at 1 200 mm by a float valve fitted to the outlet
of the filter. The filtration rate was adjusted by means of a gate valve on the outlet of the float
valve basin. An overflow in the inlet to the filter ensured the level of the water and thus the
loading rate of the filter. Head losses were measured with manometers at 300 mm, 600 mm, and
in the gravel below the fine sand layer. The was no under drain system, the effluent were drawn
off from the centre of the filter in the gravel layer.
Sampling
Two grab samples per week of the raw feed, ozonated feed, and filtrate were collected and
analysed. During the initial stages of the experiment it was argued that because the loading rate
of the filters was 2 m.d -1 effluent sample must be collected 24 hours after the influent sample in
order to compare the results. However, no significant differences were observed in the removal
rates calculated on samples collected on the same day and those collected 24 hours apart.
Analytical methods
Standard analytical techniques were applied to determination of chemical parameters and
bacterial count in the samples.
Use was made of an Aquadoc Analyzer to measure the organic carbon content of the water. The
analyser utilizes the persulphate-ultraviolet oxidation method, which is a rapid and precise
method for the measurement of trace levels of dissolved organic carbon in water. Before
measuring the DOC the samples are filtered through 0.45 µm filters.
Experimental Design

For the first day of every filter run the secondary effluent from the biological nutrient removal
plant was let into the filter without ozone pretreatment. This was done to ensure the
establishment of an active layer on bacteria on the sand media surface.
Experiment 1
Water was extracted from a holding pond with hydraulic retention time of 12 hours. In this pond
further sedimentation takes place resulting in water with low suspended solids. The bottom of
this pond must be scoured on a regular basis.
Filter 1 received water ozonated at a rate of 1.75 mg O 3 / mg DOC. The other was used as a
control and received untreated water. Due to problems in the commissioning of the control filter
(Filter 2) filtration started twenty days later than the filter receiving ozonated water.
Experiment 2
This run was a repeat of the first run. However, the water was extracted directly after the
secondary settling tank. The suspended biomass in the secondary effluent blocked the filter
receiving untreated water (Filter 2) within nine days and the filter receiving ozonated (Filter 2)
water after thirteen days. The experiment was terminated after sixteen days. To overcome this
problem a coarse sand filter was installed. A normal swimming pool filter was used.. This filter
was back washed once a day.
Experiment 3
In experiment 3 the feed to Filter 1 was initially ozonated at a rate of 1.82 mg O 3 / mg DOC.
This dosage was decreased to 1.46 mg O 3 / mg DOC on day 25. Filter 2 received untreated
water. Removal attributable to ozonation, before filtration, was also measured.
Experiment 4
Both filters received water ozonated at a rate of 0.75 mg O 3 / mg DOC. Filter 1 was protected
against sunlight to prevent the growth of algae.
Experiment 5
Both filters received ozonated water. Filter 1 was protected against sunlight to prevent the
growth of algae. The water in the ozone contact column was recycled through a germicidal
ultraviolet light unit in order to enhance the oxidation potential of the ozone. During the first
twenty five days a ozone dosage of 1.04 mg O 3 / mg DOC was maintained. The dosage was
increased to 2.48 mg O 3 / mg DOC for the remainder of the filter run.
Experiment 6
Both filters received water ozonated at a rate of 1.08 mg O 3 / mg DOC. Both filters were
exposed to sunlight to allow the growth of algae. The filtration rates were 4 m 3 .m -2 .d -1 and
2 m 3 .m -2 .d -1 through Filter 1 and Filter 2 respectively.

Table 2 summarize the various experimental conditions.
Table 2 : Summary of experimental conditions.
Experiment
number 1 2 3 4 5 6
Ozonated feed
monitored No No Yes Yes Yes Yes
Filter 1 1.75 mg 1.67 mg
1.82 mg
1.46 mg
Filter 2 Control Control Control
0.75 mg
Covered
0.75 mg
Open
1.04 mg
2.48 mg
+UV Covered
1.04 mg
2.48 mg
+UV Open
1.08 mg
4 m 3 .m -2 .d -1
1.08 mg
2 m 3 .m -2 .d -1
Dosage in mg O 3 per mg DOC (average calculated dosages). Filtration rate 2 m 3 .m -2 .d -1 (exception in
Experiment 6)
Results and Discussion
The methods used for the physical and chemical examination of the samples were the same as
those adopted for the analysis of potable water The details of the analytical results are
summarize in Table 3.

Feed water
TABLE 3
Average Analytical Results under various Experimental Conditions.
Experiment 1 2 3 4 5 6
Colour in Hazen units 9.2 13.8 12.9 10.4 10.6 9.5
Turbidity NTU 3.3 3 1.6 1.3 1.4 2
Nitrate as mg N/l 4.2 5.3 3.3 5.5 6.1 4.9
Ammonia as mg N/l 6.3 4.2 4.4 6.5 5.3 5
o-Phosphate as mg P/l 0.5 0.3 0.8 0.4 1.2 0.2
DOC 11.8 11.6 11.8 12.8 14.3 13.4
Feed water after ozonation
Colour in Hazen units 8.7 6.8 8.4 6.5
Turbidity NTU 1.1 1.0 1.2 1.5
Nitrate as mg N/l
3.6
5 3.7 4.9
Ammonia as mg N/l 4 6.4 5.3 5
o-Phosphate as mg P/l 0.8 0.3 1.2 0.3
DOC 10.9 12.2 13.7 12.1
Filter 1
Colour in Hazen units 2.9 3.5 3.8 4.4 5.3 5.3
Turbidity NTU 0.5 0.3 0.5 0.3 0.3 0.3
Nitrate as mg N/l 5.5 7.1 5 5.9 5.9 6.3
Ammonia as mg N/l 0.4 0.1 0.2 0.3 0.4 0
o-Phosphate as mg P/l 0.9 0.4 0.9 0.5 0.2 0.5
DOC 8.7 9.6 7.7 9.5 10.7 10.5
Filter 2
Colour in Hazen units 5.1 6.2 7.1 5.2 5.1 5
Turbidity 0.6 0.5 0.5 0.4 0.3 0.2
Nitrate as mg N/l 5.9 5.8 4.2 5.5 5.9 6.6
Ammonia as mg N/l 0.6 0.3 0.3 0.3 0.4 0
o-Phosphate as mg P/l 1 0.5 1.01 0.5 0.2 0.5
DOC 10.7 10.8 10.5 9 10.6 9.6

Figure 4 Changes in the Nitrate-N
concentrations under various
experimental conditions.
Nitrification and Denitrification
The changes in the nitrate-nitrogen and saline
ammonium-nitrogen are plotted in Figure 3
and Figure 4 respectively.
The influent nitrate-N concentrations range
from 3.3 to 5.5 mg N.l -1 (Table 3), whereas
the effluent concentrations ranged from 4.2 to
7.1 mg N.l -1 . Ellis (1993) and Farooq et al
(1994) reported a nett removal of nitrate-N.
However, from their reports it is not clear
whether the effluent used contained any saline
ammonium-N. The saline ammonium-N
concentration of the effluent use in this
investigation ranged from 4.2 to 6.5 mg N.l -1 .
Virtually complete nitrification of this
ammonium-N was observed (Figure 4). The
total nitrogen (sum of the nitrate-N and ammonium-N) as a percentage of the influent value is
plotted in Figure 5. From this figure it is clear that more than 40 % of the influent nitrogen was
removed in the slow sand filter. The factors commonly favouring nitrification are dissolved
oxygen (DO), pH, and temperature. The influent pH range of 7.04 and 7.83, is in the excellent
range. DO levels were not measured but the water leaving the ozone column will be saturated
with pure oxygen, therefore, the DO levels above the sand layer should be sufficient to promote
nitrification. From this it can be concluded that nitrification is possible in the upper biological
active layer of the filter. Denitrification is
considered to be an anoxic process, occurring
in the absence of oxygen, and requires an
organic or inorganic electron donor. Bacteria
capable of denitrification are both
heterotrophic and autotrophic. As the water
percolates through the biological layer in the
presence of biodegradable organic carbon
compounds the DO is depleted. The DO
levels in the sand will be low. If some of the
DOC measured in the effluent of the filter is
biodegradable denitrification will take place
in the sand layer. This conclusion is
supported by the observations of Farooq et al
(1994) that denitrification decreases with
decreasing filter depth.
Figure 3 Nitrification of saline ammonium-
N under various experimental
conditions.
The extend of denitrification in all the
Experiments was similar as can be see in
Figure 5. The denitrification measured in
Experiment 3 is plotted in Figure 6. The initial dosage in this Experiment was 1.82 mg O 3 /mg
DOC . On day 25 this was lowered to 1.46 mg O 3 /mg DOC.

Figure 5 Denitrification under various
experimental conditions
nitrogen removal can be attributed directly to
ozonation. The average nitrogen removal in
the filters was 40.0% with ozonation and
38.4% without.
The Effect of Slow Sand Filtration on Ortho-phosphate.
Figure 7 The release(+)/uptake(-) of
phosphate under various
experimental conditions.
The difference in the denitrification from
treated and untreated water was rather small,
however, in the filter receiving ozonated
water the denitrification was slightly higher.
From Figure 6 it is also clear that little or no
Figure 6 Total nitrogen ( NH 4-N + NO 3-N)
measured in Experiment 3.
The changes in the ortho-phosphate concentrations in the effluent of the various processes are
illustrated in Figure 7. The values are expressed as a percentage of the incoming phosphate.
The suspended solids in the secondary effluent used in this investigation consists mainly of
activated sludge from a nutrient removal plant rich in phosphate. This phosphate is easily
released under anaerobic and anoxic conditions.
The o-phosphate concentrations in the feed
water ranged from 0.3 to 0.8 mg P.l -1 . The
phosphate concentrations in the filtrate
produced from treated water ranged from 0.4
to 1.1 and in the filtrate produced from
untreated water 0.5 to 1.1 mg P.l -1 . This
supports the conclusion that anoxic conditions
develops in the filter where denitrification and
phosphate release are possible. The average
release of phosphate in the filters ranged from
23.3 % to 129.2 %. The average phosphate
release for all the filter runs was 56.9 % In the
filters receiving water treated with ozone
consistently more phosphate was released.
In Experiments 3 and 4, uptake of phosphate,
and in Experiment 5 and 6, release of
phosphate was measured after treating the

water with ozone. It is, therefore, not possible to came to a valid conclusion on the effect of
ozonation on the phosphate in the water.
Turbidity and Colour
The effect of ozonation and/or filtration is
graphically represented in Figure 8 and Figure
9. Regrettably the lower detection of the
equipment available to measure colour was ca 5
Hazen units, therefore values below 5 are not
reliable. The colour of the incoming water
ranged from 9.2 to 13.8 Hazen units. After
ozonation this was lower to 6.5 to 8.7 Hazen
units. The colour of the filtrate without ozone
pretreatment ranged from 5.1 to 7.1 units. In
the experiments where the water was ozonated
the colour of the filtrate ranged from 2.9 to 5.3
units.
Superficially it appears as if there is relation
between the incoming colour intensity and the
removal efficiency, however, the limitations of
the equipment used in measuring the colour
Figure 9 Percentage of the incoming turbidity
removed under various experimental
conditions.
Figure 8 Percentage of the incoming colour
removed under various
experimental conditions.
make such a statement invalid. To the naked
eye the filtrate from the filters receiving ozone
treated water appears “colourless”. Without
ozone, colour was still visible in the filtrate.
Treating the influent water with ozone improve
colour removal.
The turbidity of incoming water ranged from
0.9 to 4.9 NTU. The turbidity of the filtrate in
al the experiments was less than 1 NTU. The
individual values ranged from 0.2 to 1.1 NTU.
Protecting the filter against sunlight have had
no effect on the turbidity of the filtrate. Slow
sand filters are known to very efficient in the
removal of turbidity, its not surprising that
treating the water with ozone before filtration
did not improve the removal significantly.
Ozonation without filtration removed between
19.7% and 30.2% of the turbidity due to micro-flocculation. Although the centrifugal pump used
in Experiment 5 to recycle the water across the ozone contact column might have caused
shearing of floc, no increase in of the turbidity of the water entering the filters was observed.
Organic Carbon
The usual parameters to assess the organic quality of waste water are biological oxygen demand

(BOD), oxygen absorbed (OA), and chemical oxygen demand (COD). However, if the pollution
level of water is low these parameters are not reliable indicators to use. In such cases dissolved
organic carbon (DOC) is a better indicator of the organic quality. COD in the filtrate ranged from
0 to 12 mg O.l -1 while the DOC range was 7.7 to 10.8 mg C.l -1 .
Initially the effect of ozone without filtration on the water quality was not monitored. Only after
seeing the improvement in the visual appearance of the water, it was decided to determine the
effect on the chemical quality. In Experiment 1 a mishap in the control filter delayed the
commissioning of the filter by twenty days.
The results are, therefore, not directly
comparable and are plotted separately in
Figures 10 and 11.
Figure 10 DOC removed from ozonated
secondary effluent by slow sand
filtration.
In the control filter an active microbial
population was established in twenty days. This
longer filter ripening period in the filter
receiving ozonated secondary effluent may be
attributed to the residual ozone in the feed
water. No time for the decomposition of the
ozone was initially allowed because it was
belief that ozone does not have a residual
effect. At the end of Experiment 1 a holding
tank with a ten minute retention time was
installed.
In this investigation time for filter ripening,
which refers to the period normally allowed for
the microbial development in the
schmutzdecke and underlying filter media, was
not allowed. From Figure 10 it is clear that
filter ripening period was ca twenty-five days.
Figure 11 DOC removed from secondary
effluent by slow sand filtration.
From an analysis of the Figure 10 and 11 one can state that ca 9% of the residual carbon
compounds in this activated sludge plant is biodegradable, and that by treating the effluent with
ozone a further ca 19% is converted into compounds that are biodegradable.
The secondary effluent used in Experiment 1 was withdrawn from a holding pond, the suspended
solids was less than 10 mg.l -1 . Normally the suspended solids in activated sludge is ca 15 mg.l -1 .
Another observation is the sizes of the floc in the water. The floc in the water from the holding
pond was smaller than the floc carried over in the settling tanks. In Experiment 2 water was
abstracted from a canal direct downstream from the secondary settling tanks. The DOC removals
achieved under this conditions are plotted in Figure 12.

The biomass in this water blocked the sand filters within two weeks. The experiment was,
therefore, terminated after sixteen days. It is
evident that for long filter runs the floc must be
small.
Figure 12 Percentage of the incoming DOC
removed in Experiment 2.
In this experiment DOC removal stabilizes
within eight days in the filter receiving water
treated with ozone. In the normal operation of
a slow sand filter the top layer is scrap off been
filter runs. If the previously formed
schmutzdecke is not removed carefully
inoculation of the filter media for the next filter
run will occur. As this was the only the second
run, due to a lack in experience in removing the
top layer, this could have happened. The lower
dosage (1.67 mg O 3 against 1.75 mg O 3/ mg
DOC) did not lower the degree of removal.
To solve the problems of filter blockages resulting in short filter runs a coarse sand filter was
installed to filter the feed water before ozonation.
In Experiment 3 the effect of ozone on the DOC removal before filtration was monitored and the
dosage were lower from 1.82 mg O 3 to 1.46 mg O 3/mg DOC after twenty-five days to test the
sensitivity of DOC removal to the ozone dose. The DOC removal in Experiment 3 is plotted in
Figure13.
In this experiment there was no acclimatization
period. Maximum DOC removal was
observed right away. It is thus clear that the
active population established on the media in
Experiment 2 was not removed in the cleaning
cycle.
In this experiment the average DOC removed
by ozonation without filtration was 7.7%.
Filtration without ozonation removed 11.7%.
From this it can be concluded that for the
period of this experiment at least 4% of the
incoming was biodegradable. Filter 1 removed
on the average 35.9% of the incoming DOC. In
this case 24.2% of the DOC in the secondary
effluent was made biodegradable by ozonation.
Figure 13 Percentage of the incoming DOC
removed in Experiment 3
Lowering the dosage from 1.82 mg O 3 /mg DOC to 1.46 mg O 3 /mg DOC has had a small impact
on the amount of DOC removed. The differences between the amount of DOC removed in the
filter receiving ozone treated water and the filter fed untreated water ranged from 18.2 to 29.8%
at the higher dosage and 17.0 to 29.3% at the lower dosage (one day 42 an abnormal high
difference of 37.5% was measure). From this it is clear that under specific conditions only certain
organic compounds can be oxidized. If enough ozone is dosed to meet this demand the excess

will be wasted.
Figure 14 Percentage of the incoming
dissolved organic carbon removed
in Experiment 4.
During the first experiments a layer of algae
formed on top of the filters. To minimize the
risk of the development of odours and/or tastes
filter 1 in Experiment 4 was covered with a
tarpaulin. In Experiment 4 both filters received
secondary effluent ozonated at a rate of
0.75 mg O 3 /mg DOC. The DOC removed in
Experiment 4 is plotted in Figure 14 as a
percentage of the incoming DOC.
The average DOC removed in the filters was
25.6% and 25.9%. Preventing the growth of
algae in the filters did not affect the DOC
removal negatively. This result was expected
seeing that algae are autotrophic organisms.
Limiting algae growth extended the filter run
by 43%. The head loss limit set at 350 mm.
was reached on day 71 in filter 2 and on day
104 in filter 2.
In Experiment 5 the influent to both the filters was ozonated. Filter 1 was covered to slow down
or prevent algal growth. A recycling pump was installed to recycle the ozonated water from the
bottom of the contact column through a germicidal ultra-violet light unit to the top. Ozone is
photochemically decomposed by UV radiation forming hydroxyl free radicals (OH > ). In aqueous
solution, these radicals are stronger oxidising agents than molecular ozone. The half-life of
hydroxyl free radicals is in the order of microseconds, but considerable chemical oxidation of
dissolved organic material may be possible. Most of spectral energy of a germicidal UV lamp
is emitted at 254 nm. This wavelength is in the range for optimum destruction of ozone by UV.
The degree of DOC removal observed in this experiment is plotted in Figure 15.
At the dosage of 1.04 mg O 3 /mg DOC the
average removal of DOC in solution after
ozonation was 18.1% (range 11.3 to 23.5%).
In comparison 19.2% of the DOC after
ozonation was removed in Experiment 6
under similar conditions. Increasing the
dosage rate to 2.48 mg O 3 improved the DOC
removal to 24.8% (range 13.1 to 35.6%), an
increase of 37%. Apparently UV light did not
enhance the oxidation process at this higher
dosage level.
In Experiments 3 the dosage of 1.46
mg O 3/mg DOC removal was 40.8% higher
than the removal at the dosage of 0.75 mg
O 3/mg DOC in Experiment 4.
Figure 15 Percentage of the incoming
dissolved organic carbon removed
in Experiment 5

Regrettably the UV unit was not available for further trails at lower dosages.
In Experiment 6 the effect of filtration rate on
DOC removal was investigated. The
secondary effluent to both filters was treated
with ozone at a rate of 1.08 mg O 3 / mg DOC.
The DOC removed in this experiment is
plotted in Figure 16.
The filtration rate through Filter 1 was
4 m.d >1 and through Filter 2, 2 m.d >1 . At the
slower filtration rate 27.9% of the incoming
dissolved organic carbon was removed and at
the faster rate 22.1%. The experiment was
terminated on day 34 because the head loss in
Filter 1 had increased to 472 mm. In Filter 1
the head loss was only 71 mm.
Figure 16 Percentage of the incoming
dissolved organic carbon removed
at various filter loadings.
At the filtration rate of 4 m.d >1 62.5% more
dissolved organic carbon was removed, but the concentration of the residual DOC in the filtrate
was higher. The objective of this investigation was to lower the concentration preferably to the
potable target water quality range of 0 > 5 mg C .l >1 .
At the higher filter loading the length of the filter run was also reduced substantially.
Figure 17 The relationship between ozone dosage in mg O 3
per mg DOC and the %DOC removed.
% DOC removed = 3.38[O 3] + 23.
In Figure 17, the daily measured percentage dissolved carbon removed is plotted against the
ozone dosage. The standard error in the coefficient of the regression line drawn in Figure 17 is

0.96 and in the constant 6.63. In each filter run the ozone dose based on the filtration rate was
kept constant. However, the concentration and nature of the incoming dissolved organic carbon
varies from sample to sample. This explains the poor correlation between DOC removal and
ozone dosage based on DOC concentration. From Figure 17 it is obvious that if the ozone
demand of the incoming water is satisfied the amount of dissolved organic carbon removed is
not a direct function of the ozone dosed. Therefore, the presence of free ozone in the water
leaving the ozone contact column should be used to control the ozone dosage.
The chemical quality of the secondary effluent used in this investigation is such that it will be
aggressive to cement-like material and corrosive to ferrous and other metals because it has a
negative precipitation potential of 9.74 mg CaCO 3.l >1 , at 20 o C. Removal of the organic carbon
increases this negative precipitation potential even further to 14.20 mg CaCO 3.l >1 . The addition
of 6.02 mg Ca(OH) 2.l >1 will stabilise the filtrate.
Microbiological Results
The total coliforms in the influent ranged from 25 000 to >250 000 colony forming units (CFU)
per 100 ml, and the faecal coliforms from 20 000 to >250 000 CFU per 100 ml. Ozonation
followed by filtration reduced the total coliforms count to a range of nil to 100 CFU per 100 ml
(average 5) and the faecal coliforms from nil to 20 CFU per 100 ml (average 1). In the case
where the secondary effluent was filtered without ozone pretreatment the ranges were nil to 14
000 CFU/100 ml for total coliforms and nil to 3 800 CFU/100 ml. The 95 percentile value for
faecal coliforms in the filtrate produced from ozonated secondary effluent was 1.9 that is 2 CFU
per 100 ml. Thus, the filtrate did not meet the requirement set in the Special Standards for
discharging treated waste water into the environment. Further disinfection will, therefore, be
required.
Head loss in the Filters.
The head loss development in all the
filter runs was similar. The head loss
measure in Experiment 4 is plotted in
Figure 18.
From Figure 18 it is clear that the
development of head loss was
extremely small during the initial
period of the filter operation which
later increased exponentially. Most
of the head loss occurred in the top
layer (Schumtzdecke). Therefore, by
removing this layer between filter
runs restores the head loss.
In this experiment Filter 1 was Figure 18 The development of head loss in a slow
covered with a tarpaulin to prevent
sand filter.
algae growth. This led to a slower
head loss development and an increase in the filter run.

Conclusions
The following specific conclusions may be drawn from this investigation:
1. Substantially all the incoming saline ammonium-nitrogen was nitrified in/or on the slow sand
filters in all the filter run. Although an increase in the nitrate-nitrogen concentrations
occurred, 30 to 40% of the incoming total nitrogen was removed by denitrification. The final
nitrate-nitrogen concentration was in the range which is generally well tolerated in potable
water.
2. Under all conditions ortho-phosphate was released during slow sand filtration from the
biomass in the secondary effluent from an biological nutrient removal plant, used in the
investigation In the filters receiving water pre-treated with ozone consistently more phosphate
was released.
3. Ozonation per se has had little impact on the total nitrogen and ortho-phosphate.
4. Slow sand filtration with and without ozone pretreatment reduced turbidity consistently to
below 1 NTU.
5. Slow sand filtration with ozone pretreatment removed the colour in secondary effluent to
levels lower than the detection limits of the equipment normally used for colour
measurement. Without ozone pretreatment the colour in the filtrate was still detectable.
6. Direct filtration of effluent after the secondary settling tanks reduces the filter run drastically.
This can be overcome by using an intermediate maturation pond with a retention time short
enough to prevent the growth of algae or by coarse sand filtration to remove the biomass
carried over in the secondary settling tanks.
7. Slow sand filtration without ozone pretreatment removes approximately 10% of the incoming
dissolved organic carbon, 40% of the DOC removed is biodegradable. Very good treated
water quality was achieved with pre-ozonation followed by slow sand filtration, 20 to 35%
of the incoming dissolved organic carbon was removed.
8. On virgin sand filter ripening took 20 to 25 days. In successive filter runs no allowances for
filter ripening need to be made.
9. Head loss development was extremely small during the initial period of the filter operation.
Later it increased exponentially. Most of the head loss occurred in the top layer. Limiting
the growth of algae by protecting the filters against sunlight, reduces head loss development
and increases the length of the filter runs substantially.
10.Very good removal of total coliforms and faecal coliforms was achieved, however, additional
disinfection will be required to comply with the 0 CFU per 100 ml faecal coliforms
requirement of the Special Standards set for discharging into the environment.
Acknowledgement

The author is grateful to the Department of Water and the Environment, City Council of Pretoria,
for providing the necessary funds to conduct this investigation. Sincere thanks to the staff at the
Daspoort Chemical Laboratory and Water Care Works for their help with the analysis of the
samples and operation of the pilot plants.
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