Effect of dissolved organic matter (DOM) and biofilm on the adsorption capacity of powdered activated carbon in activated sludge

The objective of this study is to clarify the effect of DOM and biofilm on the adsorption capacity of PAC in the aeration tank of PACT process. The adsorption capacity of PAC for 3,5-DCP was significantly decreased by the addition into the aeration tank. The decrease in the adsorption capacity significantly deteriorated the performance of PACT process. The adsorption of DOM with molecular weight ranged from 50,000 to 300,000 Da decreased the adsorption capacity of PAC in the aeration tank for 3,5-DCP. However, DOM with molecular weight less than 50,000 Da and more than 300,000 Da had little effect on the adsorption capacity of PAC in the aeration tank.

Effect of dissolved organic matter (DOM) and biofilm on the adsorption capacity of powdered

activated carbon in activated sludge

* Yoichi Nakano, Tri Widjaja, Tomonori Miyata, Wataru Nishijima, and Mitsumasa Okada

Department of Material Science and Chemical Engineering, Graduate School of Engineering Faculty, Hiroshima University 1-4-1 Kagamiyama, Higashi Hiroshima, 739-8527, Japan

Abstract

The objective of this study is to clarify the effect of DOM and biofilm on the adsorption capacity of PAC in the aeration tank of PACT process The adsorption capacity of PAC for 3,5-DCP was significantly decreased by the addition into the aeration tank The decrease in the adsorption capacity significantly deteriorated the performance of PACT process The adsorption of DOM with molecular weight ranged from 50,000 to 300,000 Da decreased the adsorption capacity of PAC in the aeration tank for 3,5-DCP However, DOM with molecular weight less than 50,000 Da and more than 300,000 Da had little effect on the adsorption capacity of PAC in the aeration tank

1 Introduction

In powdered activated carbon treatment (PACT) processes, powdered activated carbon (PAC) is added into aeration tank in activated sludge processes to remove toxic substances and to maintain stable treatment performance

(De Jonge et al 1991; Xiaojan et al., 1991; Caldeira et al 1999) Many researchers reported that the PACT process

provided better effluent quality for shock loadings of toxic substances than conventional activated sludge processes (De

Wale and Chiang, 1977; Sundstrom et al., 1979; Sublette et al., 1982; De Jonge et al., 1991) Toxic substances loading continuously or temporarily in the PACT process can be removed by adsorption (Sundstrom et al.,1979)

Therefore, it is important to evaluate the adsorption capacity of PAC in aeration tank and substances affecting the adsorption capacity of PAC

The adsorption capacity of PAC is known to decrease in the presence of microorganisms (Xiaojian et al., 1991,

Olmstead and Weber 1991) Moreover, the adsorption capacity of granular activated carbon (GAC) decreased over

operation time in anaerobic expanded-bed (Zhao et al., 1999 and Suidan et al., 1983) The decrease in the adsorption

capacity of PAC in bioreactors such as aeration tank might be caused by the adsorption of the metabolites of microorganisms, other dissolved organic matters (DOM) and biofilm attached onto PAC However, it is not clear that the effect of DOM and biofilm attached onto PAC were decrease in adsorption capacity of PAC in the aeration tank The objective of this study is to clarify the effect of DOM and biofilm on the adsorption capacity of PAC in the aeration tank of PACT process

2 Materials and methods

2.1 Operation of powdered activated carbon treatment (PACT) system

The schematic diagram of the PACT system used in this study is shown in Fig.1 The activated sludge reactor was made of polyacrylate and consists of an aeration tank and a sedimentation tank The two tanks are separated by a partition wall that has an opening at the bottom to ensure effluent from the aeration tank into the sediment tank and the return of activated sludge from the sedimentation tank The aeration and sedimentation tanks had working volumes

of 2.5 L and 0.2 L, respectively The activated sludge was obtained from a municipal wastewater treatment plant in Higashi-Hiroshima, Japan The activated sludge was acclimated by a synthetic wastewater for more than 20 days to have stable operation of the PACT process before the start of shock loading The composition of the synthetic wastewater is listed in Table 1 The concentrated waste water was diluted with tap water at a ratio of 1:20 and fed continuously into the reactor at a flow rate of 7 l d-1 PH was adjusted from 6.5 to 7.5 by 0.01 N-NaOH or 0.01 N-H2SO4 Dissolved oxygen concentration was maintained at around 2 mg l-1 by aeration The reactors were placed

in a room controlled at 20±0.5 oC MLSS was maintained at about 3,000 mg l-1 for the control reactor without PAC The other one, the PACT reactor was added with coal-based PAC (Mitsubishi chemical: Diahope 008N) at a concentration of 1,500 mg l-1 to aeration tank MLSS in the aeration tank were maintained at 4,500 mg l-1 (Biomass = 3,000 mg l-1, PAC = 1,500 mg l-1) Solid retention time (SRT) was around 15 days The amount of PAC removed from the aeration tank by sampling and excess sludge removal was supplied using new PAC to keep PAC concentration

at 1,500 mg/l PAC was sieved in order to obtain particles with diameter ranging from 53 to 75 µm

Table １ Composition of the synthetic wastewater after dilution *

Component Concentration (mg l-1) Polypepton 446

TOC 200

*Nishijima W et al., 1993

Figure 1 Schematic of experiment apparatus

P

The activated sludge reactor

P Air compressor

Effluent

Concentrated

synthetic wastewater

Influent

Influent control unit

Pure water

Aeration part

Sedimentation part

Figure 1 Schematic of experiment apparatus

P

The activated sludge reactor

P Air compressor

Effluent

Concentrated

synthetic wastewater

Influent

Influent control unit

Pure water

Aeration part

Sedimentation part

2.2 Adsorption capacity

3,5-Dichlorophenol (3,5-DCP) was chosen as a toxic substance (Broecker and Zhan, 1977) The adsorption capacity of PAC for 3,5-DCP in activated sludge was determined Activated sludge with PAC was separated from supernatant by centrifugation (8,000 rpm, 20 minutes) From 100 to 200 mg of the sludge were added into 100 ml of 3,5-DCP solution with different concentrations and the mixture was shaken at 120 rpm at 20 ±0.5 oC After equilibrium of 3,5-DCP with PAC, the amount of 3,5-DCP adsorbed on PAC was determined The adsorption capacity

of the sludge without PAC was also evaluated using the sludge in the control reactor

PAC was placed into a bag of dialysis membrane with the molecular weight cut-off of 50,000, 300,000 and 2,000,000 doltons (Da) The bag was submerged in effluent from sedimentation tank for more than 20 days to clarify the effect of DOM on the adsorption capacity of PAC

2.3 Shock-loading experiments

Shock-loading experiments were carried out after the acclimation of activated sludge for more than 20 days with the synthetic wastewater The synthetic wastewater with 50 mg l-1 of 3,5-DCP was fed into each reactor at a flow rate

of 7 l d-1 for 24 h as a shock loading Effluent was sampled periodically from each reactor for about 7 days

The amount of 3,5 DCP adsorbed by PAC in each reactor was estimated from the difference between the amount of 3,5-DCP in influent and the amount of 3,5-DCP discharged and biodegraded The amount of Cl- in effluent was estimated as the amount of 3,5 DCP biodegradation The amount of 3,5-DCP adsorbed by activated sludge was also estimated by the difference between the amount of 3,5-DCP in influent and the amount of 3,5-DCP discharged and biodegraded in the control reactor

2.4 Development of biofilm on PAC

A synthetic medium with glucose (glucose medium) was used to develop biofilm on PAC The glucose medium was composed by 2,500 mg l-1 of glucose (1,000 mg C l-1) and the some inorganic constituents as the synthetic waste water (Table 1) but has 5 times higher concentration Five ml of the activated sludge from the control reactor was suspended into 495 ml of the glucose medium in an Erlenmyer flask as an inoculum Then, 5 g of PAC was added into the flask and shaken at 120 rpm and 20±0.5 oC After the depletion of DOC in the medium, glucose was added to

recover the initial glucose concentration of 1000mg l-1

The incubation was defined as incubation from addition of glucose until DOC concentration in the medium was zero For biofilm developed on PAC, the incubation was repeated 4 times PAC was washed by of 1 N NaOH and 0.5 % NaOCl solution to estimate the amount of biofilm developed on PAC The washing solution was separated from PAC by centrifugation (8,000 rpm, 20 minutes) to determine The amount of organic carbon in washing solution was determined as the amount of biofilm developed on PAC The adsorption capacity of the separated PAC from washing solution for 3,5-DCP was determined for each incubation step

2.5 Analytical methods

The concentration of 3,5-DCP was determined by High Performance Liquid Chromatography (JASCO HPLC) with

UV detector at wavelength of 280 nm mobile phase with 0.1% phosphoric acid-water/acetonitrile in a 40/60 ratio, and

flow rate of 0.8 ml/min (Barbeni et al., 1987) MLSS was determined following to the Standard Methods for

Examination of Water and Wastewater (APHA, 1989) Chloride ion (Cl-) concentration was determined by ion-chromatography (Dionex DX-500) DOC concentration was determined by TOC analyzer (Shimadzu TOC-500)

3 Results and Discussion

3.1 Adsorption capacity of PAC in aeration tank

The adsorption isotherms of new PAC and PAC in the aeration tank for 3,5-DCP are shown in Figure 2 Although the adsorption capacity of the PAC in the aeration tank include that of activated sludge, the biosorption of 3,5-DCP by

activated sludge was negligible The K values of new PAC and PAC in the aeration tank were 270.6 mg g-1 and 78.4

mg g-1, respectively The K value of PAC in the aeration tank was 29% of new PAC The adsorption capacity of the

PAC in the aeration tank significantly decreased compared to the new one

Figure 3 shows 3,5-DCP concentrations in the PACT and the control reactors during and after the shock loading of 3,5-DCP The peak concentration in the PACT and the control reactors were 6.0and 45.3 mg l-1, respectively Better effluent quality was obtained in the PACT reactor than the control reactor The adsorption capacity of new PAC at the

equilibrium concentration of 1.0 mg l-1 is around 270 mg g-1 The amount of 3,5-DCP supplied for 1g on PAC during the shock loading was 233mg g-1 Therefore, the peak concentration in effluent should be less than 1.0 mg l-1, it PAC

in the aeration tank has the same adsorption capacity as new PAC The decrease in the adsorption capacity of PAC

in the aeration tank significantly deteriorated in the performance of PACT process

10 100 1000

Equilibrium Concentration (mg l-1)

-1 )

New PAC PAC in the reactor

○ New PAC

△ PAC in the aeration tank

Figure.2 3.5-DCP concentration during and after the shock loading of 3,5-DCP

onto PACT and control reactors

Figure 3 3,5-DCP concentration during and after the shock loading of

3,5-DCP onto PACT and control reactors

Shock loading

0 10 20 30 40 50

Time (hours)

（mg

Control reactor (without PAC) PACT reactor

10 100 1000

Equlibrium concentration of 3,5-DCP（mg l-1)

-1 )

New PAC PAC in the reactor PAC in the 300,000 Da dialysis membrane PAC in the 2,000,000 Da dialysis membrane PAC in the 50,000 Da dialysis membrane

3.2 Effects of dissolved organic matter (DOM) on adsorption capacity of PAC

DOC concentrations in the effluent of the control rector ranged from 9 to 17 mg l-1 Molecular weight distribution of DOC were 54%, 11-18%, 15-30% and 4% DOC for 50,000 Da or less, 50,000-300,000 Da and 300,000-2,000,000Da and 2,000,000 Da or more, respectively Figure 4 shows the adsorption isotherms of PAC in the dialysis membranes with molecular weight cut-off for 3,5-DCP The adsorption capacity of PAC in the dialysis membrane with the molecular weight cut-off of 50,000 Da was very similar to that of new PAC indicating that the DOM had little effects on the adsorption capacity of PAC for 3,5-DCP DOM with molecular weight less than 50,000 Da occupied 54% to total DOC On the other hand, the adsorption isotherms of PAC in the dialysis membrane with molecular weight cut-off of 300,000 and 2,000,000 Da were almost the same and their adsorption capacities were lower than that of new PAC These results show that the adsorption of DOM with molecular weight ranging from 50,000 to 300,000 Da decreased the adsorption capacity of PAC in the aeration tank for 3,5-DCP and DOM higher than 300,000 Da did not decrease on the

adsorption capacity of PAC

Soluble microbial products (SMP) are the major constituents of DOM in effluents from biological treatment

processes (Barker, J D et al.,1999) Heizlar and Chudoba (1986) found that all the polymers with molecular weight

larger than 10,000 Da isolated from the effluent of an activated sludge process contained sugars, amino sugars, uronic acid and amino acids, and concluded that these polymers

Figure 4 The adsorption isotherms of PAC in the dialysis membranes with molecular weight cut-off

○ New PAC

△PAC in the aeration tank

◇PAC in the 50,000 Da dialysis membrane

□PAC in the 300,000 Da dialysis membrane

■PAC in the 2,000,000 Da dialysis membrane

3.3 Effect of biofilm on PAC on adsorption capacity of PAC

Although the adsorption of DOM with molecular weight ranging from 50,000 to 300,000 Da decreased the adsorption capacity of PAC in the aeration tank for 3,5-DCP, the adsorption capacity of PAC in the membrane with molecular weight cut-off of 300,000 Da was significantly higher than that in the aeration tank The decrease in the adsorption capacity of PAC does not totally due to the adsorption of DOM on PAC

According to Sutherland (2001), extracellular polymers substance (EPS) contain essentially of proteins, polysaccharide and smaller amounts of nucleic acid, lipids of humic substances, and has an important role to develop biofilm Blocking pores on PAC by biofilm may also decrease the adsorption capacity of PAC for 3,5-DCP To clarify the effect of biofilm on the adsorption capacity of PAC, PAC covered with biofilm was made and its adsorption capacity was compared to the adsorption capacity of new PAC For biofilm developed on PAC, the incubation was repeated 4 times The amount of organic carbon was indicator as the amount of biofilm developed on PAC Figure

5 shows the amount of organic carbon on PAC after each incubation Figure 6 shows the adsorption isotherm of new PAC, PAC in the aeration tank, and PAC after 1st and 4th incubation The adsorption capacity of PAC decreased with incubation time In other word, the adsorption capacity of PAC increased with increase in the amount of organic carbon on PAC The adsorption capacity of PAC after 4th incubation was almost the same as that in the aeration tank

Figure 5 Change in amount of organic carbon on PAC for every incubation by glucose

-1)

0 2 4 6 8 10

C

AC

Since the adsorption isotherms were obtained after each incubation, when DOC concentration in the medium was less than 10 mg l-1, the adsorption of glucose and/or DOM on PAC would be neglected and most of organic carbon on PAC would be originated from biofilm Therefore, it is concluded that the development of biofilm on PAC results in the decrease in adsorption capacity of PAC

Figure 6 Adsorption isotherm of 3,5-DCP onto new PAC, PAC in the aeration tank, and PAC in the bacterial

cultivations

4 Summary and conclusions

The objective of this study is to clarify the effect of DOM and biofilm on the adsorption capacity of PAC in the aeration tank of PACT process The specific conclusions derived from this study are as follows:

(1) The adsorption capacity of PAC for 3,5-DCP was significantly decreased by the addition into the aeration tank The decrease in the adsorption capacity of PAC in the aeration tank significantly deteriorated the performance of PACT process

(2) The adsorption of DOM with molecular weight ranged from 50,000 to 300,000 Da decreased the adsorption capacity of PAC in the aeration tank for 3,5-DCP However, DOM with molecular weight less than 50,000 Da and more than 300,000 Da had little effect on the adsorption capacity of PAC in the aeration tank

10 100 1000

Equilibrium Concentration of 3,5-DCP（mg l-1)

New PAC PAC in the aeration tank 3rd incubation of glucose on PAC 4th incubation of glucose on PAC

-1 )

(3) The biofilm on PAC also decreased the adsorption capacity of PAC in the aeration tank

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