1. Introduction
It is generally admitted that the tertiary treatment attached
sometimes in the conventional stations (chlorinetion, ultraviolet, ozone
...) is very costly or have a questionable efficiency. Only the
extensive processes seem to be capable to disinfect waters used in
appropriate economic conditions [1]. Infiltration percolation is an
extensive treatment process aimed at eliminating organic pollution,
oxidizing ammonium and removing pathogens [2]. It has been increasingly
used for the treatment of primary or secondary wastewater effluents
because of its low energy and requirements maintenance [1]. It consists
in the intermittent application of sewage on buried sand filters or
permeable native soils. The infiltrated water percolates through
unsaturated porous medium. The treated water is collected by a drainage
system or percolates down to the underlying aquifer [3].
Research and field experiments have shown that intermittent
filtration can provide high removal efficiency of bacteria if properly
designed and operated [4]. Therefore secondary treated effluents can be
used for unrestricted agricultural irrigation [5], irrigation of public
parks, lawn sand golf courses [6] and ground water recharge [7].
Research works undertook by Institute of Arid lands in Gabes and
National School of Engineers of Sfax achieved on pilots of laboratory
permitted to show that infiltration percolation is one of the rare
extensive techniques capable to function for a long time in Tunisia
sub-Sahara; while satisfying the physico-chemical, bacteriological and
parasitological purification objectives [8,9]. These studies permitted
to elaborate the first applications under soil and climatic context of
the Tunisian south.
The first aim of this work was to determine the performances of the
Dissa infiltration percolation basin in the oxidation of organic matter
and nitrogen. The second aim was to contribute to better describe the
oxidation mechanisms in presence of wild plant to improve the technique.
Assessing disinfection performances of the filtering bed in presence and
in absence of vegetation was a third objective.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
2. Materials and Methods
Experiments were investigated in 100 [m.sup.2] sand filter located
at Dissa agriculture area in the north of Gabes City (south-east of
Tunisia). The sand filter is a trapezoidal basin with 2 m in height and
filled with 30 cm of coarse gravel and 1.5 m of sand. The mean grain
size of sand ([d.sub.50]) is 0.26 mm and the uniformity coefficient of
the particle-size distribution ([d.sub.60]/[d.sub.10]) is 1.93 (Figure
1). A polyethylene pipe was used drilled with 0.5 cm holes to drain
filtered water out. The experimental arrangement is depicted in Figures
2 and 3. We carried out infiltration trials using secondary wastewater
effluents produced by Gabes City treatment plant which employs activated
sludge treatment with a capacity of 17,000 [m.sup.3]/d. In previous work
[10], the optimum hydraulic loading rate to improve secondary wastewater
quality by intermittent sand filter for a single application was
evaluated at 27 cm/d. In this study the operation mode conditions
rotated through a 7-day cycle consisting a 4-day flooding period during
which the basin received a volume of 27 [m.sup.3]/d of secondary
wastewater effluents and a 3-day drying period during which the basin is
allowed to dry out thoroughly. Wetting/drying operation serves several
purposes, including maintenance of aerobic conditions in the upper soil
layers [11]. Performances of the sand filter have been investigated
during a period of 8 months. Secondary effluents and filtered water were
analysed for chemical oxygen demand (COD), Phosphor (P[O.sup.3-.sub.4]),
Kjeldahl nitrogen (NTK), ammonium (N-[NH.sub.4]), nitrates
(N-[NO.sub.3]), Pseudomonas aeriginosa (Pa), faecal coliforms (FC) and
streptococci (FS), pH and electric conductivity were measured. In order
to evaluate the effects of wild plant on oxidation performances of
infiltration percolation, two experimental periods were investigated.
[FIGURE 3 OMITTED]
3. Results and Discussions
3.1. Purification Performance Studies in Presence of Vegetation
During the period of experimentation, the incidental exhaustion of
COD is valued to 60%. On the other hand this output was of 80% in wintry
period in absence of vegetation on the surface of basin. However, the
oxygenation of the porous media remained insufficient and the exhaustion
of the COD is only 48% in the center and 58% in the periphery at 50 cm
depth, the total output of the basin is about 60%. Meftah [12] gets some
results meaningfully better when the beach of infiltration was free.
Therefore, vegetation affects negatively the oxidization of the carbon
pollution. One also notices that the thickness of the filterable massif
doesn't play a role in the elimination of the organic matter beyond
thickness of 50 cm. The potentialities of the infiltration basin seem to
be raised for the elimination of phosphates. The contents in the
filtrate are always weak (Figure 4). In treated water the output is from
90%; at 50 cm depth the elimination also remains very high with an
output of 83% in center and periphery of the basin. The phosphor is
therefore very easily eliminated from wastewater since the first 50
centimeters in sand filter despite the presence of vegetation. According
to Burkitt et al. [13], these outputs raised of the removal of the
phosphor could be explained by two parameters: the sand filter keeps the
different shapes of phosphor by adsorption, and by precipitation.
The follow-up of the organic nitrogen and ammonium shows good
performances process in the removal of nitrogenous pollution. The
outputs reach values near from 100% (Figure 5). Waters applied on the
filter are characterized respectively by middle concentrations in
Kjeldahl nitrogen and ammonium equal to 24 mg/l and 19 mg/l
respectively. In the filtered water, the middle vestigial contents are
0.64 mg/l and 0.15 mg/l respecttively. Nitrogen under its oxidized
shapes is eliminated at 98% in the sand filter. At 50 cm depth, the
elimination of the total nitrogen is high in the center of the basin
than in the periphery; the vestigial contents are of 1.5 mg/l and 2.3
mg/l respectively corresponding to outputs of 93 and 90% respectively.
To this same depth the elimination of N-[NH.sub.4] is very weak, 0.95
mg/l in the treated water and the middle contents in the center and in
the periphery are 0.45 mg/l. the wild plant on sand filter doesn't
affect the removal efficiency of nitrogen. It is in agreement with the
works of Brissaud et al. [14] that notes essential of the nitrification
happens in the first 50 centimeters of a column of 1.5 m of the fine
sand that treats a hydraulic load of 75 cm/day of septic sewage.
The elimination of the bacterial pollution is varying from 0.1 to
2.5 Ulog. The bacterial purification remained weak after 50 cm
filtration. Indeed, to 50 cm depth, the quality of the filtrate is
slightly better than influent with still tendency to higher removal in
the center than in periphery of the basin (Figure 6). The exhaustion of
fecal Coliforms CF in this depth doesn't exceed 0.75 Ulog. To the
basis of the sand filter, removal of CF can reach two logarithmic units.
The contents of fecal Streptococci are near applied wastewater content.
It is necessary to reach 1.5 m of depth so that the fecal Streptococci
exhaustion stabilizes during the whole tentative period and reaches an
average of 2 Ulog. Some comparable results are found by bali et al. [9]
on the same Basin. Wild plant affects negatively the removal efficiency
bacteria especially in the superficial depth because the preferential
percolation of wastewater with the developed roots of plants.
3.2. Purification Performance Studies in Different Depths of Sand
Filter
During this second experimentation period, the vegetal cover is
completely eliminated from the filter; COD of applied secondary sewage
has middle value of 170 mg [O.sub.2]/l. The removal of the carbon
pollution is satisfactory in winter with a global output of 80% after
1.5 m filtration. 100 cm of filtration permit to eliminate 78% of the
COD in center of the basin and 69% in its periphery, with vestigial
contents of the order of 38 mg[O.sub.2]/l and 53 mg [O.sub.2]/l
respectively. After 50 cm of filtration, the elimination of the carbon
pollution is only 52% in center and 45% in periphery, the vestigial COD
being in order to 83 mg [O.sub.2]/l and 94 mg [O.sub.2]/l respectively.
This test shows that the superficial layers of filter play an
essential role in the removal of the COD; therefore about 50% of the
organic load is eliminated already after 50 cm of filtration depth. It
is especially important than the carbon pollution constitutes the
responsible factor for the development of biomass. This biomass develops
itself therefore less in superficial layers depth. It has important
consequences in the management and the durability of the system. A depth
of 100 cm appears sufficient for the elimination of the organic matter.
The middle vestigial COD in this depth being the order of 46 mg
[O.sub.2]/l. Bali and al., [9] show also that the devices of
infiltration are very effective for the elimination of the organic
matter, whose big part makes itself on the superficial layers of the
filter.
The middle concentration in orthophosphates observed during this
period is about 71 mg/l in the influent. In effluent it is about 24.5
mg/l what corresponds to removing of 65%. The elimination of the
phosphor is relatively variable for the same depth, it is more higher in
the periphery than in center of the basin. The outputs reach 67% to 50
cm and 75% to 100 cm depth; therefore the global output pass extensively
out of the basin and it is about 65%. In the center of basin, these
outputs remain lower on the other hand always to the output of treatment
system and are from 49% after 50 cm filtration and 55% after 100 cm
depth with middle vestigial contents of 36 mg/l and 31 mg/l respectively
(Figure 7). According to Meftah [12], the performances of the systems of
infiltration percolation in the elimination of the orthophosphates
depend on electro-chemical conditions of middle and the mineralogical
composition of the sand filter.
The retention of the Kjeldahl nitrogen is highly effecttive since
the first week of drying. 96% of this nitrogen is kept in the sand
filter (Figure 8). On the other hand, we didn't note sensitive
variations of the vestigial content of the percolate in total nitrogen
in different depths. In 50 cm depth, the elimination of total nitrogen
is already raised with an output of 82% in the center and 78% in
periphery and the contents are 1.8 and 2.2 mg/l respectively. After 100
cm filtration, the total nitrogen has been eliminated at the rate of 93%
in the center and 87% in the periphery respectively with middle
vestigial contents of 0.7 mg/l and 1.4 mg/l. The nitrification in 100 cm
is globally similar to 150 cm. The concentration of the filtrated water
in nitrates is about 15 mg/l whereas it reaches 30 mg/l in 50 cm depth.
The concentrations raised to 50 cm depth explain themselves by a washing
of the nitrates accumulated in sand filter at the time of previous
drainage period, and also it may come from oxidization of ammonium
adsorbed during the previous alimentation. These results confirm the
works of [15] that prove a big part of nitrogen is fixed in the first
centimeters of sand filter.
The removal of the fecal Coliform and the fecal streptococci is 2.2
Ulog in treated water. The microbial quality of the filtrated water
collected to 50 cm depth in the center and in periphery of the basin is
appreciably identical to the influent of wastewaters applied on the
filter. The residence times of this wastewater in sand filter are short,
in the score of minutes that follow the application of the volume of
wastewater; the porous media is saturated by very loaded water in
organic matter. These conditions are not indeed favorable to elimination
of bacteria. After 100 cm filtration exhaustion is about 1.4 Ulog.
Therefore the last 50 cm of sand filter play an essential role in the
sanitary quality of purified water. The bacteriological criteria's
of no restraining reuse are satisfied extensively. Some comparable
results are found by [9] at the time of a survey achieved on a same
basin. These authors show that the removal of Coliform is bound in the
real residence time in sand filter and the temperature of water.
Compared to the other germs of fecal contamination, the exhaustion of
the Pseudomonas Aeriginosa (pathogen) is raised in the beginning of the
test. Therefore after 15 days of drying, we note a tendency to the
stability of the concentration of the Pseudomonas Aeriginosa in treated
water in different depths (Figure 9). We note, as for the other germs,
an increase of removal with the depth of the sand filter. It seems that
the logarithm of the concentration of the bacteria decreases in a linear
way with the depth equally confirmed by works of Bancole [15]. According
to Bali et al. [9], it is the height of the unsaturated sand filter that
would influence the elimination of the microorganisms [16]. This fact
would be due to the increase in water residence time in sand filter with
the amplification of filtration height.
4. Conclusions
Oxidation performances of infiltration percolation in Dissa sand
filter were very high. Results confirmed that this technique is
performed as an advanced treatment system for COD and nitrogen. The
experimental study has shown the influence of the filter depth and the
vegetaion on the output purification of the process. On the other hand
the elimination of the orthophosphate depends on electro-chemical
conditions of the middle and not too much the thickness of sand filter.
Analyses have shown significant improvements in effluent quality due to
the presence of vegetal cover on sand filter surface. Water quality
analyses demonstrated that oxidation activity was mainly located in
upper sand layers. Disinfection performances were disappointing because
of heterogeneous infiltration. Maintaining infiltration surface evenness
helps providing uniform infiltration. Infiltration percolation allows
oxidizing and disinfecting secondary wastewater effluents. This
technique is used as a tertiary treatment with the aim of removing
pathogen microorganisms from the effluents of conventional wastewater
treatment plants. It is a low technology method that can be used to
improve water quality for possible reuse in unrestricted irrigations.
doi: 10.4236/jwarp.2011.37058
Received May 7, 2011; revised June 8, 2011; accepted July 4, 2011
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Saifeddine Eturki (1), Hayet Makni (2), Rachid Boukchina (3), Hamed
Ben Dhia (2)
(1) Wastewater Treatment Laboratory, Water Researches and
Technologies Centre, Soliman, Tunisia
(2) Water, Energy and Environment Laboratory, National School of
the Engineers of Sfax, Sfax, Tunisia
(3) Water and Soil Laboratory, Institute of Arid Lands Nahhal,
Gabes, Tunisia
E-mail: turkisaifeddine@yahoo.fr
Figure 4. Concentration of the COD and [PO.sup.3-.sub.4]
in influent, in 50 cm and in the effluent.
Concentration (mg/l) DCO [PO.sup.-.sub.4]
Inf 100 40.4
50C 52 7.2
50P 42.3 6.5
Eff 40 4.4
Note: Table made from bar graph.
Figure 5. Concentrations of the nitrogenous in influent,
In 50 cm and in the effluent.
Concentration (mg/l) NTK N[H.sup.+.sub.4]
Inf 23.8 19.7
50C 1.6 0.95
50P 2.2 0.4
Eff 0.6 0.2
Note: Table made from bar graph.
Figure 6. Contents in CF and SF in the wastewater before,
to 50 cm, and after treatment by infiltration percolation.
Ulog (UFC/100 ml) CF SF
Inf 3.6 3.6
50C 2.8 2.9
50P 3 3
Eff 2.25 2.2
Note: Table made from bar graph.
Figure 7. Concentrations of the COD and [PO.sup.3-.sub.4]
in the influent, in 50 cm, in 100 cm and in the effluent.
Concentration (mg/l) DCO [PO.sup.-.sub.4]
Inf 169.5 71.4
50P 93.5 23.5
50C 83 36.4
100P 53 17.8
100C 38 31.8
Eff 32.5 24.8
Note: Table made from bar graph.
Figure 8. Concentrations of the nitrogenous pollution in
influent, in 50 cm, in 100 cm and in the effluent of
the basin.
Concentration (mg/l) NTH [PO.sup.-.sub.3]
Inf 9.7 4.7
50P 2.1 11.7
50C 1.75 22.5
100P 1.3 11.4
100C 0.7 14.3
Eff 0.4 14.75
Note: Table made from bar graph.
Figure 9. Contents in CF, SF and of Pa in the wastewater
before, to 50 cm, to 100 cm, and after treatment by
infiltration percolation.
Concentration (mg/l) CF SF Pa
Inf 4.5 4.25 4.75
50P 4 3.78 4.07
50C 3.9 3.78 4.04
100P 3.25 3 3.5
100C 3.1 3.2 3.5
Eff 2.4 2.25 2.5
Note: Table made from bar graph.