Removal of Sodium Dodecyl Sulfate in an Intermittent Cycle Extended Aeration System
The objective of this study was to investigate the
removal of an anionic surfactant from wastewater in ICEAS. The surfactant
considered here was anionic SDS being widely used in the household and
industrial detergents. Basic wastewater COD was 260 mg L-1
and SDS surfactant added in range 20 to 400 mg L -1. The effect
of the inlet SDS concentration and reaction time on COD and SDS removal
was investigated. The results from this study indicated that the aeration
time of 2 h was sufficient for removal of SDS ranged 20 to 400 mg L-1.
Obtained data showed low effluent SDS concentrations of 0.3 to 5 mg L-1
and removal of SDS was more than 98%. These results revealed that biological
treatment using ICEAS process is capable to treating wastewaters containing
high concentration of SDS surfactant.
Surfactants are organic compounds that reduce surface
tension in water and other liquids (Kowalska et al., 2004). In
the domestic wastewater produced by the households, surfactants invariably
exist in significant amounts due to detergents used for all kinds of washings.
Surfactants have also been widely used in textiles, fibers, food, paints,
polymers, cosmetics, pharmaceuticals, mining, oil recovery and pulp and
paper (Sheng et al., 1999). These applications of the surfactant,
increasing its discharge in the wastewater, produce foam and enter into
the underground water resources and constituting an ecological risk for
aquatic organisms (Nasiruddin and Uzva, 2005). They contain both strong
hydrophobic and hydrophilic moieties. According to the charge of their
hydrophilic moiety, surfactants can be classified into four categories:
anionic, non-ionic, cationic and amphoteric (Mozia et al., 2005).
Anionic surfactants are one of the most frequently employed surfactants
and constitute approximately two-third of these surfactants. Cationic
surfactants constitute less than 10% of the ionic surfactants and rest
is anionic surfactant. Thus Anionic Surfactants (AS) are the major class
of surfactants used in detergent formulations. The predominant class of
anionic surfactant is linear Alkylbenzene sulfonate and linear alkyl sulfate
(Liwarska and Bizukojc, 2006). As a result, their fate in the environment
has been widely studied. An example of linear alkyl sulfate is Sodium
Dodecyl Sulfate (SDS), which is a representative of AS (Adac et al.,
2005). The common procedures for surfactant removal from the water and
wastewater include processes such as chemical precipitation (Aboulhassan
et al., 2006), photocatalytic degradation (Mozia et al.,
2005), oxidation (Sheng et al., 1999; Adams and Daigger, 1999),
adsorption (Nasiruddin and Uzva, 2005), membrane technology, various biological
methods (Langford et al., 2004) etc. Recently, Biodegradation of
anionic surfactants in wastewater treatment processes has been the subject
of the substantial research. The activated sludge process is an aerobic
biological stage in wastewater treatment that oxidizes organic matter
to carbon dioxide and water, generating new biomass (Langford et al.,
2004). Intermittent Cycle Extended Aeration System (ICEAS) is type of
activated sludge process that influent wastewater is fed continuously
through the cycles of react, settling and decant (Metcalf and Eddy, 2003).
The single reactor in this system has the functions of bio-oxidation,
nitrification, denitrification, phosphorus removal, settlement and sludge
stabilization (Jing et al., 1999). The aim of the present study
was to investigate the performance of ICEAS process in removal SDS of
MATERIALS AND METHODS
Inoculum and wastewater composition: Synthetic sewage was used to simulate wastewater
and fed continuously at a constant flow rate of 0.5 L h-1.
To simulate wastewater, a base feed of glucose was used. Ammonia and
||Synthetic wastewater characteristics
phosphate sources were di-ammonium hydrogen phosphate (NH4)2HPO4
and Ammonium chloride (NH4Cl). The COD:N:P ratio was 100:5:1.
The activated sludge taken from the aerated chamber at wastewater treatment
plant in Ecbatan, Tehran was used as an inoculum. Surfactant studied was
sodium dodecyl sulfate in range 20 to 400 mg L-1. Synthetic
wastewater composition (Table 1).
Experimental set up and procedure: This study was carried out
at the Tarbiat Modares University, Tehran, Iran. The investigated reactor
in this study is shown in Fig. 1. The reactor was a 20x20x30
cm (HxWxL) polexyglass tank which was equipped with diffuser air. Wastewater
was feed using a dosing pump continuously through openings at the bottom
of the baffle wall and into the main react zone. After aeration and settling,
separated liquid is removed by an automated, time-controlled decant mechanism.
Solids Retention Time (SRT) was 30 days. It was operated with a dissolved
oxygen content of 3-5 mg L-1 during aeration cycle and 0.25
mg L-1 during settling cycle. Each experiment was conducted
for between 5-7 days until a steady state condition was achieved. The
effluent was analyzed every 12-24 h. Duration of first run was 3 h include
2 h reaction, 45 min settling and 15 min decanting. In run 2, reaction
time was 3 h.
Analysis techniques: A rapid and reliable solvent extraction spectrophotometric
method has been developed for the determination of SDS. Acridine orange
(ACO) (λmax = 467 nm) has the potential for being used
as an ion-pairing agent with SDS. The ion-pair formed between SDS and
ACO is extractable in toluene. Sample solution (10 mL) containing SDS
was poured into a 25 mL separating funnel. ACO (5x10-3 M) and
glacial acetic acid 100 μL each was added. Then 5 mL of toluene was
added to it and shaken for 1 min. The aqueous layer was then discarded
and the toluene layer was used for absorbance measurement at 467 nm (Adac
et al., 2005). Chemical oxygen demand (5220B), total suspended
solids (2540B), volatile suspended solids (2540E), Sludge volatile index
(2710D) Ammonia (4500C) and phosphate (4500C) were measured according
to Standard Methods (1998).
||Flow diagram of the investigated system
RESULTS AND DISCUSSION
For a bench scale reactor treating a synthetic wastewater containing
various concentrations of SDS, removal achieves in ICEAS process configuration
tested. In run 1 (COD = 260 mg L-1), inlet SDS concentration
started of 20, 50, 100, 200 and 400 mg L-1, respectively. Chemical
Oxygen Demand (COD) equivalent for SDS was about 1.8 mg mg-1
SDS. Base initial COD concentration (260 mg L-1) increased
with the increase of the surfactant value to 290, 350, 430, 600 and 900
mg L-1. MLSS in the above mentioned inlet SDS concentrations
at the beginning of aeration cycle were about 2800, 3000, 3500, 4500 and
6000 mg L-1, respectively. Results of SDS removal (runs 1 and
2) (Fig. 2, 3). In run 1, effluent SDS
for the mentioned SDS concentrations (20, 50, 100, 200 and 400 mg L-1)
was 0.3, 0.64, 1.47, 2.5 and 4.57 mg L-1, respectively. In
addition, these values in run 2 for the concentrations of 50, 100, 200
and 400 mg L-1 were 0.57, 1.29, 1.37 and 2.12 mg L-1,
respectively. Table 2 depicts the performance of the ICEAS in terms of
COD in the effluent wastewater. Effluent COD and SDS decreased from the
first run to the second run. Also with the increase of SDS concentration,
effluent COD increased slightly. The Sludge Volume Index (SVI) was determined
as an indicator of sludge settling capacity (Fig. 4).
SVI values in ICEAS decreased with the increase of SDS concentration.
From the results (Fig. 2, 3) it is
observed that removal yield of SDS was more than 98%. These results are
close to Luis' observations (2004), who showed that the removal efficiency
of SDS in ICEAS is more than 95%. Ebrahimi et al. (2006) studied
removal of LAS in municipal wastewater (concentration of 20 mg L-1)
Conventional Active Sludge (CAS) and Fixed bed Active Sludge (FAS) and
removal efficiency of LAS reported 93 and 97%, respectively. In this study,
removal efficiency of COD in conventional active sludge system was 87%
(Ebrahimi et al., 2006). In addition, in a study of SDS biodegradation
by the Acinetobacter isolated from active sludge, removal efficiency
was 96.4% (Hosseini et al., 2007). These results revealed that
the efficiency of our
Removal of SDS as
a function of inlet concentration in Run 1
Removal of SDS as
a function of inlet concentration in Run 2
variation of COD as a function of Inlet SDS and COD concentration
in runs 1 and 2
ICEAS in COD and SDS removal was higher than the other
reported systems. Generally, it is found that ICEAS processes with longer
hydraulic retention time and intermittent aeration usually produce better
effluent with respect to COD, than conventional processes (Bicudo and
Svobada, 1995). IEPA (1999) has set a value of 1.5 mg L-1 for
anionic surfactants to discharge to the surface waters. With the inlet
concentration of SDS 200 mg L-1, effluent SDS concentration
in run 1 was more than IEPA standard
of Sludge Volume Index (SVI) with the SDS concentration
and for achieving to IEPA standard, the aeration cycle time was increased
to 3 h. In these conditions, SDS concentration in the effluent was 1.37
mg L -1. Moreover, in run 2 and SDS concentration of 400 mg
L-1, effluent concentration of the tested pollutant was 2.12
mg L-1. If higher removal efficiency is needed, increase of
the aeration time or tertiary treatment such as adsorption on activated
carbon could be a suitable alternative. From point of view of sludge settling
characteristics, SVI during the entire course of the experiment was within
the generally accepted range of 50-150 mg L-1. SVI values in
the tested ICEAS decreased with the increase of SDS concentration (Fig.
4). It was due to the decrement of activated sludge floc dimensions,
which was a result of saponification (Liwarska and Bizukojc, 2006). One
of disadvantages observed in the ICEAS was accumulation high concentrations
of surfactant during the settling period that caused foam production (100
mg L-1) as soon the aeration was started. This foam adsorbed
after ten minutes by sludge solids. It recommends that eliminate the foaming
problem due to high concentrations of SDS using anti-foaming agents. Totally,
ICEAS process is capable to reduce carbon materials (include surfactant),
phosphate and nitrate in industrial and domestic wastewaters with the
repeated cycles of aeration/settling in single activated sludge (Jing
et al., 1999; Luis, 2004). Therefore, based on the results from
this study, we can utilize this system to biological treatment of surfactant
wastewaters in full-scale plants and in comparison to other biological
systems, chemical processes and advanced oxidation methods; this process
is an effective alternative.
This research was supported by the Department of
Environmental Health Engineering, Tarbiat Modares University, Tehran,
Iran. The authors are grateful to the department academic staff and laboratory
personnel for their cooperation.
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