AAA - Combined System

8/12/2019 AAA - Combined System

1/19

Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt 845

UPGRADING OF SUSPENDED GROWTH PROCESS

USING SUBMERGED MEDIA

Diaa El Monayeri

1

, Salah Bayomi

2

, Hesham El Karamany

3

, and Ibrahim Hendy

4

1Prof.,

2Assoc Prof.,

3Assistant Prof.,

4Eng.

Environmental Engineering Department, Zagazig University, El-Zagazig, Egypt

ABSTRACT

Combined suspended / attached growth (CSAG) systems have been emerging recentlyto increase the efficiency of the existing systems. It takes its importance from high

biomass concentrations that can be achieved within the system. The present studyinvestigates the enhancement (if any) of conventional activated sludge (AS) processperformance using attached media submerged in the aeration tank. The main purposeof this work is to find out the optimum combination of attached growth and suspendedgrowth systems that achieve the best removal ratios (RR) of BOD and COD. Twomodels have been set up and run treating primary treated domestic wastewater. One ofthese two models has been run as an aeration tank without any submerged media. Thismodel has been utilized as a reference for the other model that runs as a combinedsuspended attached growth process. These two models worked parallel with the sameinfluent wastewater and the same experimental conditions to investigate the effect of

the added media volume in the aeration tank, the hydraulic retention time (HRT), andthe sludge retention time (SRT), on the removing BOD5, and COD. Ratios ofsubmerged media volume to reactor volume ranges from 10 % to 60% have beentested. It was found that 30 % was the optimum ratio of media submerged in thereactor. The obtained results indicated that old and over loaded plants can be improvedusing attached media to carry the extra hydraulic and organic loading rates.

Keywords: Combined suspended/attached growth, activated sludge, BOD5, COD,overloaded plants, upgrading.

INTRODUCTION

Aerobic biological systems for wastewater treatment are based on either suspended orattached growth. Biological processes for wastewater treatment such as trickling filtersand activated sludge plants have been used since the late 19

thand early 20

thcenturies

and are well established (Grady, 1983). Recently, the combination of the two types ofgrowth in one system (CSAG) has been found to be advantageous for improvement ofthe efficiency and/or capacity of existing treatment plants. Incorporating biofilmwithin the activated sludge process is one of the most commonly adopted

configurations of the hybrid (CSAG) reactors (Fouad; 2004). In this system the biofilmis grown on a fixed or movable carrier in the biological reactor. As a result of this


2/19

Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt846

combination, the substrate removal efficiency is significant even at low temperature.Further more the system stability and the sludge properties are improved, (Fouad,2004). These systems have been developed as high-rate COD removal processes aswell as an economic means for upgrading COD removal plants to nutrient removal(Yuan et al., 2001).

Although the system has been incorporated in many areas, evaluation of itsperformance based on kinetic principles has not yet been elucidated. The system ismore complex compared to the biofilm or the pure suspended growth reactor. Analysisof the system is difficult due to the need for biofilm analysis, differentiation betweenthe suspended and the attached growth behavior, and the complexity of the combinedsystem. At present, hybrid (CSAG) reactors are designed based on a recommendedratio of the biofilm carrier to the reactor associated with desired removal efficiency.This ratio is obtained from field experience or experimental results, (Gebara, 1999)

Higher BOD removal in a CSAG system is associated with the presence of two typesof microorganisms in the reactor, the fixed and the suspended. Thus moremicroorganisms require more food leading to more BOD removal even at high organicloading rates, (Gebara, 1999)

Biomass concentration in biological reactors can be increased by a variety ofimmobilization techniques. Passive immobilization can be achieved by providing solidsurfaces in aeration tanks to facilitate the natural process of microbial attachment. Thisapplication of support materials to the activated sludge process combines theadvantages of attached-growth systems and suspended-growth systems. By means ofbiomass carriers, it is possible to obtain a twofold increase in biomass concentration inthe aeration tanks compared to that in the conventional activated sludge process. Theincreased concentration of immobilized biomass would reduce the process dependenceof secondary clarification and lead to reduce volume of the reactor, increasedtreatment system stability and improved performance. In other words, a CSAG systemcould almost double the treatment capacity of an activated sludge plant withoutincreasing its physical tankage, (Wang, 2000)

Support material can be placed directly into the activated sludge tank and may beeither firmly arranged in the tank (stationary) or maintained in free motion togetherwith the activated sludge (mobile) depending on their sizes, (Wang, 2000).Macrocarriers such as modular plastic media and synthetic fiber media are fixed in theaeration basin. These macrocarriers can easily be retained in the system without theneed for the secondary clarifiers; however, they occupy a large volume of the reactor.Microcarriers such as plastic foam or other porous materials (less than 1 mm in size)are always suspended in activated sludge tanks and provide a large surface area for theimmobilization of biomass, without reducing the effective volume of the tanks. Thesecarriers are easy to handle; however, due to their size, additional measure of separationand recycling systems are needed to retain them in the aeration tanks, (Wang, 2000)


3/19


MATERIALS AND METHODOLOGY

Primary treated wastewater from El Aslogy Wastewater Treatment Plant, Zagazig,Egypt, was used as an influent of wastewater in this study. The primary treatmentworks in the mentioned facility includes mechanical cleaning screen, aerated gritremoval chamber and primary sedimentation tank. The influent wastewater wascharacterized by measuring such parameters as BOD5, COD, pH, total suspendedsolids (TSS), total solids (TS), total volatile solids (TVS), total fixed solids (TFS),temperature, and ammonia nitrogen. The results of these measurements in influentwastewater are shown in table (1).

Table (1): Influent wastewater characteristics

Item Range Item Range

BOD5 (ppm) 140-300 TS (ppm) 664-2940COD (ppm) 323-461 TVS (ppm) 180-2320pH 6.5-7.5 TFS (ppm) 82-792Temp. C 16.5-27 Ammonia (ppm) 21.1-32.4TSS (ppm) 53.5-234.7

MODEL

The primary treated wastewater was pumped to the model shown in fig. (1) using two

pumps work alternatively. The two pumps discharged wastewater in a head tankthrough a strainer of 2 mm openings to prevent inert materials and solids fromentrance to the aeration tank. The head tank was 2-m3 volume tank. Head tank wascleaned after each run of experiment work to remove accumulated solids and reducegrowth of algae. The head tank worked also as another sedimentation tank and this isthe reason of low values of TSS in influent wastewater.

Two models have the same design; one of them was conventional AS system and theother was a CSAG reactor. The two models worked parallel at the same time with thesame raw wastewater and the same hydraulic conditions. The effluent watercharacteristics from the two systems were compared to evaluate the effectiveness ofthe proposed system (CSAG system) in removing BOD5 and COD. Each modelconsisted of a complete AS process i.e. aeration tank and settling tank as shown infigure (1).


4/19

Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Eg848

sludge withdrawal pipe

Plan

Effluent channel of primary sedimentation tank

submerged

pump

1m

Sec A-A

Influent

1m 0.25m

1m

0.7

5m

0.8m

0.8m

0.7

5m

0.25m

0.7

5m

0.25m

sludge withdrawal pipe

1m 0.8m

1

43

25

air supply

air supply

7

Influent

DiscriNO

1 Constant

Activate2

Combined susp

growth reactor3

Fine scre4

5

8

1

2m

1m

1m

Over flow pipe

1m

Influent

Figure (1) Schematic sketch of the pilot plant model.


5/19


Aeration tank was constructed from steel. It was painted using epoxy, and haddimensions of 1.0 m length, 0.75 m width, and 1.0 m depth. The depth of the tankdivided to 0.75 m of water and 0.25 m free board. The total volume of tank was0.5625 m

3. The tank was supplied with a diffused air via a system consists of a

network of 12.5 mm diameter plastic pipes placed at the bottom of the tank and have

nozzles of 6 mm diameter for aeration, nozzles were distributed at the whole surfacearea of the tank. The supplied air was delivered from air blowers station at Al AslogyWastewater Treatment Plant with rate of 3.53 m3/hr for the two reactors. The suppliedair quantity was sufficient to maintain dissolved oxygen concentration more than2 mg/l.

DO concentration was measured each run to confirm this value. While a DO of at least1 mg/l appears to be a requirement to prevent oxygen from becoming the limitingnutrient in nitrification (Metcalf and Eddy, 1991), 2 mg/l of dissolved oxygen istypically considered the cutoff value to fully ensure nitrification is not oxygen limited

(Wild et al., 1971). Table (2) shows the design parameters of aeration tank. Roundedshape gravel with size of 4-5 cm was extracted. From previously sieved gravel, andthen is cleaned with tab water to remove any dust or inert materials attached to it.Porosity of gravel was calculated with volumetric method and it was about 40.6 %.Thevolume of media submerged in aeration tank varied from10 % to 60 % using six steelboxes of a volume approximately equal 0.1 of the total aeration tank

Table (2): Actual design parameters of the aeration tank for the present study

Parameter Range

Discharge 0.056- 0.28 m3/hHRT 2- 10 hrSRT 0- 15 daysDissolved oxygen 2 5 ppmAir quantity 6-30 m3air /m3water

Finally a square tank with dimensions of 0.8 m by 0.8 m with depth of 0.75 m andtotal volume of 0.28 m3 has been utilized as a final clarifier. HRTs in the clarifier

varied from 1 to 5 hr; with surface loading rate (SLR) ranges from 2.1 to 10.5 m/d.Settled sludge in the bottom of tank was drawn manually through a control valve fourtimes a day to return it to the reactor. The settling tank was made of Plexiglas; andcovered by a black plastic cover (2 mm thickness) to reduce growth of algae.

EXPERIMENTAL WORK

The BOD5, COD, TSS, Dissolved oxygen, temperature and PH were measured forinfluent and effluent samples. Sampling points were taken at aeration tank inlet for raw

wastewater, and sedimentation tanks outlets for treated wastewater. All measurementswere analyzed in Environmental Engineering Department Laboratory, Faculty of


6/19


Engineering, Zagazig University, Egypt. All measurements were analyzed inaccordance with The Standards Methods for the Examination of Water andWastewater, 16thEdition, 1985.

The two models worked with the same influent wastewater and the same atmospheric

conditions to investigate the effect of volume of media, hydraulic retention time(HRT), and sludge retention time (SRT), on CSAGs process performance in removingBOD5, and COD. To investigate the effect of ratio of volume of media to reactorvolume, and hydraulic retention time, six runs were conducted, utilizing differentpercentage of aeration tank volume occupied by submerged media. For each volume ofmedia different flow rates were applied to the AS reactor and the combined reactorwith the same values at the same time , to compare the effeiciency of each type oftreatment. In this paper the main conventional pollutants BOD5and COD was takeninto consideration. Table (3) shows runs of the experimental work.

Due to presence of the submerged media in the combined reactor, the effective reactorvolume was reduced by volume occupied by media. So, feeding the same dischargeyields retention time in AS reactor longer than that in CSAG reactor. Table (3)illustrate hydraulic retention time and rate of discharge for all runs of the experimentalprogram.


7/19


Table (3) Hydraulic retention time and discharge for each run of experimental program

Retention time (hr)SetNo

Volume of media(% From AT

volume)

Surfacearea of

media (m2)

RunNo

Discharge(M3/hr)

Activated Combined1 0.281 2 1.3

2 0.141 4 2.6

3 0.094 6 3.9

4 0.070 8 5.2

1 60 27.78

5 0.056 10 6.5

6 0.281 2 1.4

7 0.141 4 2.8

8 0.094 6 4.2

9 0.070 8 5.7

2 50 23.15

10 0.056 10 7.1

11 0.281 2 1.5

12 0.141 4 3.1

13 0.094 6 4.6

14 0.070 8 6.1

3 40 18.52

15 0.056 10 7.7

16 0.281 2 1.6

17 0.141 4 3.3

18 0.094 6 4.9

19 0.070 8 6.6

4 30 13.89

20 0.056 10 8.2

21 0.281 2 1.8

22 0.141 4 3.5

23 0.094 6 5.3

24 0.070 8 7.1

5 20 9.26

25 0.056 10 8.8

26 0.281 2 1.9

27 0.141 4 3.8

28 0.094 6 5.6

29 0.070 8 7.5

6 10 4.63

30 0.056 10 9.4

RESULTS AND DISCUSSION

Results of all sets are illustrated in the following figures, in which values of BOD5andCOD removal ratios (RR) for both reactors are plotted.

For the first set, volume of media to the reactor volume was 60%, figures (2, 3), resultsshowed that the mean of RR for AS system is higher than that of CSAG system in both


8/19


BOD5and COD, this may be due to the large volume of inert media that reduced theeffective volume of reactor, reduced HRT and reduced the contact time between airand water. CSAG system seemed to be submerged filter as the attached media isnearly fill all the reactor volume. RR of BOD5 and COD increased with the depressionof flow discharge for the two systems.

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Retention time(hr)

RR (%)

COD AS

COD CSAGBOD5 AS

BOD5 CSAG

Figure (2) (Set no 1) removal ratio of BOD5 and COD for AS system and CSAG system

volume of media = 60%.

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0560.070.0940.1410.281

Discharge(m3/hr)

RR %

COD A S

COD CSAG

BOD A S

BOD CSAG




9/19


When the volume of media reduced by removing one box of media (set no 2) theeffective volume of reactor increased and HRT increased and contact time between airand water increased also. As a result the average RR of the CSAG system increased tobe nearly equal to that of AS in BOD5 removal, but RR of COD in CSAG system wasmore than that of AS system. This may be due to higher biomass concentrations in

fixed-film processes result in higher sorption rates, figures (4, 5). It is noticed that thedifference in COD removal between the two systems increased when the flow rateincreased. This result illustrates that the CSAG system has the ability to carry highorganic loads more than AS system.

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Retention time (hr)

RR %

COD AS

COD CSAG

BOD5 AS

BOD5 CSAG

Figure (4) (Set no 2) removal ratio of BOD5 and COD for AS system and CSAG systemvolume of media = 50%.

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0560.070.0940.1410.281

Discharge(m3/hr)

RR %

COD A SCOD CSAG

BOD A S

BOD CSAG




10/19


By removing another box of media to reach to 4 boxes (40%) the third set gave thesame results as the second set with a little increase of CSAG system performance morethan AS, they still nearly equal in BOD5 removal and the CSAG system had averageRR more than AS in COD removal. These results showed that the trend of reducingvolume of media in the reactor increases the RR performance, figures (6, 7).

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Retention time (hr)

RR %.

COD AS

COD CSAG

BOD5 AS

BOD5 CSAG


volume of media = 40%

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0560.070.0940.1410.281

Discharge(m3/hr)

RR %

COD A S

COD CSAG

BOD A S

BOD CSAG



A further decrease of volume of media was made by removing the third box of media

to reach 30% volume of media (set no 4), it was noticed that the removal ratio of bothBOD5and COD was increased in CSAG system more than AS system with observed


11/19


difference. BOD5average RR was increased by about 3% and COD average removalratio was increased by 8 %. These values means that 30% is better than highervolumes of media (60%, 50%, 40%) and the same trend of reducing volume of mediaincreasing RR still continued. Figs (8, 9) showed that the difference in RR for bothBOD5 and COD between the two systems increased with the depression of flow

discharge. The difference in RR was about 20 % for COD at Q = 0.281 m3/hr. thehighest RR for AS system was 81.7 % for COD it was reached at Q = 0.056 m3/hr. thisRR could be achieved by the CSAG system at Q = 0.09 m3/hr. This means that theCSAG system can achieve the same RR of AS system when carry a discharge equal1.6 of the discharge of AS system.

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Retention time (hr)

RR(%)

COD AS

COD CSAG

BOD5 AS

BOD5 CSAG



40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0560.070.0940.1410.281

Discharge(m3/hr)

RR %

COD A S

COD CSAG

BOD A S

BOD CSAG

Figure (9) (Set no 4) removal ratio of BOD5 and COD for AS system and CSAG systemvolume of media = 30%


12/19


With the same trend a further decrease of media to be 20% (two boxes only) and RRswere measured. It was found that the results begin to reduce and the trend of reducingvolume of media increasing the RR was finished at the fourth set with 30% volume ofmedia. RR of BOD5 is reduced in CSAG system to reach values nearly equal that ofAS system, and COD also was reduced but still more than COD RR in AS system by

5%. This value illustrates the effect of high concentration of biomass in the reactor inincreasing COD removal ratio, which may be due to adsorption, Figures (10, 11).

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Retention time (hr)

RR(%)

COD AS

COD CSAG

BOD5 AS

BOD5 CSAG



40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0560.070.0940.1410.281

Discharge(m3/hr)

RR %

COD A S

COD CSAG

BOD A S

BOD CSAG




13/19


In the last run with volume of media =10% (one box of media) results showed RR ofBOD5in CSAG system higher than that of AS by 2%, and COD RR of CSAG systemhigher than that of AS by 3% this result illustrated that CSAG system acted as AS withan enhancement in both BOD5and COD with similar increasing values. This due tothe presence of small volume of media, which increases the total biomass

concentration in the reactor and at the same time the volume of reactor still big enoughto act as, AS system with high HRT result in contact time between air and water,Figures (12, 13).

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Retention time (hr)

RR %

COD AS

COD CSAG

BOD5 AS

BOD5 CSAG



40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0560.070.0940.1410.281

Discharge(m3/hr)

RR %

COD A S

COD CSAG

BOD A S

BOD CSAG




14/19


To compare results of the two systems a comparison ratio was used. This ratio was theratio of (RRs for CSAG system / RRs for AS system). It is a dimensionless valuewhich may be named R, it has the following expression:

AS

CSAG

RR

RRR =

Where:R = comparison ratio.

CSAGRR = RR for BOD5or COD for CSAG system for any set.

ASRR = RR for BOD5or COD for AS system for the same set.

R was a number around 1 it acts as the CSAG system, when R is bigger than 1 thismeans that CSAG system have a better performance than AS system and vice versa.When R plotted versus the datum value which is 1 (for all sets) and acts as the ASsystem, the graph will give a global overview of the study results. Figure (14) shows

values of R for BOD5against the datum value, and figure (15) shows values of R forCOD against the datum value.

0.85

0.90

0.95

1.00

1.05

1.10

1.15

0 10 20 30 40 50 60 70Volume of media/Volume of Aeration tank(%)

R Datum value

Figure (14) values of R for BOD5for all sets

COD

RR

(CSAG)/COD

RR

(AS)


15/19


0.85

0.90

0.95

1.00

1.05

1.10

1.15

0 10 20 30 40 50 60 70

Volume of media/Volume of Aeration tank(%)

R Datum value

Figure (15) values of R for COD for all sets

From previous results and figures (14, 15) it is clearly noticed that the optimum ratioof the specified media was 30 %, as the mean BOD5 removal ratio of the CSAGsystem was higher than the mean BOD5removal ratio for AS system by 3% (from 80.5% to 83.4%) , with R =83.4 / 80.5 = 1.04. And the mean COD removal ratio of theCSAG system was higher than the mean COD removal ratio for AS system by 8%(from 71.2% to 79.4%), with R = 79.4 / 71.2 = 1.12. This means that the combination

of suspended growth system with attached growth system using attached media in anaerated reactor have better performance than the solo suspended growth in the samereactor. Figures (8, 9) showed also that to reach the maximum value of efficiency forthe existing system, CSAG system could treat a discharge of 1.6 of the existing systemdischarge. This means that old and over loaded plants can be enhanced to carryhydraulic loading rates equal 1.6 of its design rates without additional reactors.

To design a simple empirical model for biological aerated filters (A.T. Mann and T.Stephenson, 1997) used two identical polyvinyl chloride (PVC) reactors. The tworeactors were built 2 m in height and with diameter of 0.2 m. The reactors were run

with settled domestic sewage introduced at the bottom through a centrally placed 5-cmfilter nozzle. Two types were used as submerged media one of them was sunken mediaand the other was floating. Results of the two reactors show that RRs for TCOD were59.2, and 75.3% for sunken media and floating media respectively. For any otherdetails of the models and other parameters refer to (Mann and Stephenson, 1997). It isnoticed that RR for the present study are similar to results of floating media. It shouldbe mentioned that the conditions of the two works are approximately similar. But thebest set of the present study (set no 4) has RRs for TCOD more than that mentionedfor floating media as set no 4 has mean TCOD RR of 79.4%, while the floating mediamodel has mean TCOD RR of 75.3%. This may be due to two reasons 1) the

combination of suspended growth and attached growth systems in the present study. 2)Longer HRT in set no 4 than that of floating media model as the floating media reactor

COD

RR

(CSAG)/COD

RR

(AS)


16/19


was aerated biological filter i.e. the filter is full with media. It should be mentionedalso that the media used in floating model was plastic media with high surface area(1160 m2/m3), and high voids ratio compared with that of the present study, which isgravel. And this leads to the need of trying such a good media in CSAG processes toinvestigate its effect on the process performance, and this is one of the important

recommendations of this study.

Suzuki et al., 1999, developed a simulation model for a combined activated sludge andbiofilm process to remove nitrogen and phosphorous. Two kinds of experiments wereconducted for model development one investigated nitrification efficiency of biofilmsformed on media with different substrate loads. It was consist of aeration tank withvolume of 18.5 L and fluidized media, without activated sludge. Three reactors thatdifferent in influent concentration were operated with four different flow rates 0.0554,0.111, 0.222, and 0.444 m3/d which gives HRT of 8, 4, 2, and 1 hr respectively. Theresults of one of the reactors was mentioned to be 77.9, 77.2, 73.3, and 59% for sCODremoval ratio, these data were for HRT of 8, 4, 2, and 1 respectively, (Suzuki et al.,1999). It should be mentioned that all results were for tertiary treatment as influentCOD, and ammonia nitrogen were about 30 mg/l, and 24 mg/l respectively. So, theresults of COD are approximately similar to the present study however, the two studieswere completely different in conditions.

The other was a pilot plant experiment, in which performance of combined activatedsludge and biofilm process to remove nitrogen and phosphorus process (CAB/NP) wasinvestigated using a pilot plant supplied with settled municipal wastewater, (Suzuki etal., 1999). The influent wastewater has an average COD, and T-N (total nitrogen)values of about 275, and 19 mg/l respectively. These were measured at time withoutrainfall, the values of rainfall conditions were about 120, and 17 mg/l. Average valuesof soluble COD (sCOD) was about 70ppm and 35 ppm for time without rainfall, andwith rainfall respectively. When these water characteristics treated with the pilot plantit gives RRs of sCOD about 71% at without rainfall conditions and 43% at withrainfall conditions, (Suzuki et al., 1999). It in noticed that the values of RR were forsCOD only without taking TCOD into consideration. It is noticed also that thesevalues of RR are small values compared with RRs of the present study results; it maybe due to using big ratio of media 40%.

Clap et al., 1994, made a performance comparison between activated sludge and fixedfilm processes for priority pollutants removal. Parallel AS and biological aerated filter(BAF) reactors were used; treating settled raw wastewater from the Niagara Falls,New York treatment plant. Approximately 20% of the influent was attributed todomestic sources, 27% to industrial sources, and the remainder to infiltration andinflow. In addition to the priority pollutants analysis, which is the main purpose of thestudy, conventional pollutant removals were evaluated through out the study. For moredetails refer to Clap et al., 1994. RRs of TCOD were 80.3, 75, 80.9, and 70.4% forlow rate activated sludge (AS-L), high rate activated sludge (AS-H), low rate

biological aerated filter (BAF-L), and high rate biological aerated filter (BAF-H)respectively. For TBOD results were 96.5, 96.5, 94.7, and 89.5% for AS-L, AS-H,


17/19


BAF-L, and BAF-H respectively. Results are in general more than that of the presentstudy. This may be due to the low strength of the influent wastewater, result in highremoval ratio for TBOD and TCOD.

Another study of CSAG systems was the hybrid biological nutrient removal (BNR)

process combines activated sludge with fully and partially submerged RBC inanaerobic, anoxic and aerobic reactors to improve nitrogen, phosphorus and organicmatter removal from municipal wastewater. The process, developed and modified atNational Central University, Taiwan, is referred to as the Taiwan National CentralUniversity Process or the TNCU-I Process, (Ouyang et al., 1999). The TNCU-Iprocess includes anaerobic, anoxic and aerobic activated sludge reactors in series, withtwo stages of fully submerged RBC in each of the first two reactors, and two stages ofpartially submerged RBC in the third reactor, followed by a clarifier, (Ouyang et al.,1999). Results of RR for TCOD were 91.3, 90.7, and 90.3% and TBOD5 RRs were97.1, 96.1, and 96.6% respectively, these RRs were for HRT of 12, 10, and 8 hrrespectively. Results shown previously have high ratios of COD and BOD5comparedwith that of the present study. This is may be due to the following reasons: 1) Highervalues of HRTs compared with the present study as the HRT ranged between 8-12 hrwhile the HRT of the present study ranges between 2-10 hr. 2) Using Syntheticwastewater with high ratio of biodegradable organic matter that can be easily removedwhile the present study using primary treated wastewater from actual operatedwastewater treatment plant. 3) Using three kinds of treatment processes in series(anaerobic, anoxic, and two aerobic reactors) result in high COD, and BOD5RRs.

Using the same reactors with the same conditions but without added RBCs RR forTCOD was 90.3%. The effluent TCOD concentration is not very different from that ofthe TNCU-I process. The degradation of organic carbon in the process without addedRBC is approximately equivalent to that in the TNCU-I process, (Ouyang et al., 1999)

CONCLUSIONS

1. The combination of suspended growth system and attached growth system(CSAG) has higher efficiency than suspended growth system (AS), for volume of

media from 10 to 50% of the reactor volume. More than 50% CSAG has lessefficiency than AS. It is not useful to use submerged media with volumes morethan 50% of reactor volume.

2. When HRT increased RR values were increased for BOD5, COD and ammonianitrogen. RR values were increased with decreasing rate of increasing. It wasgenerally noticed that after HRT = 8 hr RRs (flow rate less than 0.07 m

3/hr) were

approximately constant for AS system. And after flow rate less than 0.094 m3/hrRRs were approximately constant for CSAG system. These results showed thatCSAG system was more effective and had capacity more than AS system.

3. Increasing surface area increase process performance and in opposite inert volume

of media reduced the actual volume of reactor and reduces process performance.


18/19


4. The optimum ratio (the ratio that achieve max RRS of pollutants) of volume of thespecified media which was gravel with size of 4-5 cm to reactor volume was 30%.

5. CSAG has the ability to remove COD more than BOD5with comparison to ASsystem.

6. The obtained results indicated that old and over loaded plants can be improved

using attached media to carry the extra hydraulic and organic loading rates.

RECOMMENDATIONS

From the results of this study, it is suggested that the future research should study thefollowing points.

1- Other types of media especially media with high surface area and highporosity like random plastic media, plastic sheets or nets, or any suitablemedia.

2- Effect of dissolved oxygen concentration on RR efficiency.3- Detailed study for the effect of SRT on RR efficiency.4- Detailed study for Nutrients removal taking into consideration

phosphorous as an important pollutant.5- Study of sludge properties results from CSAG system.6- Air requirements, energy requirements and economics for CSAG system.7- Mathematical modeling for CSAG system8- The effect of adding bacteria to the system

REFERENCES

1. Mann, A.T., and Stephenson, T., Modeling Biological Aerated Filters ForWastewater Treatment. Wat. Res., Vol. 31, No. 10, pp. 2443-2448, 1997.

2. Ouyang, C.F., Chuang, S.H., and Su, J.L., Nitrogen and Phosphorus Removal ina Combined Activated Sludge. RBC Process Proc. Natl. Sci. Counc. ROC(A)Vol. 23, No. 2, pp. 181-204, 1999.

3. Chuang, S.H., Ouyang, C.F., and Wang Y.B., The Kinetic Behaviors ofSimultaneous Phosphorus Release and Denitrification on Sludge for BNRProcesses. J. Chine. Instit. Eng., 19(5), pp. 575- 583, 1996b.

4. Gebara, F., Activated Sludge Biofilm Wastewater Treatment System, Wat. Res.Vol. 33, No. 1, pp. 230-238, 1999.

5. Grady, C.P.L., Modeling of biological fixed films a state-of-the-art review infixed film biological processes for wastewater treatment. (Edited by Wu Y. C.and Smith E.) pp. 75-134. Noyes Data Corp., Park Ridge, NJ, U.S.A., 1983.

6. Clapp, L.W., Talarzyk, M.R., Park, J.K., and Boyle, W.C., PerformanceComparison Between Activated Sludge and Fixed Film Processes for Priority

Pollutant Removals, Water Environ. Res., 66, 153, 1994.


19/19


7. Lawrence, A. and McCarty, P., Unified Basis for Biological Treatment Designand Operation. Journal of the Proceedings of the American Society of CivilEngineers. 96, 757-778, 1970.

8. Metcalf and Eddy, Inc. Wastewater Engineering: Treatment, Disposal, andReuse, Third Ed. McGraw-Hill, Inc., New York, NY, 1991.

9. Fouad, M., and Bhargava, R., A Simplified Model for the Steady-State Biofilm-Activated Sludge Reactor, Journal of Environmental Management 74, 245253,2005.

10.Standard Methods for the Examination of Water and Wastewater, 16thEdition,

APHA, AWWA, WPCF, 1985.

11.Wang, J., Hanchang, S., Yi Q., Wastewater Treatment in a Hybrid BiologicalReactor (HBR): Effect of Organic Loading Rates, Process Biochemistry 36,pp. 297-303, 2000.

12.Wild, H.E.Jr., Sawyer, C.N., and McMahon, T.C., Factors AffectingNitrification Kinetics, Journal WPCF, Vol. 43, 9, pp. 1845-1854, 1971.

13.Suzuki, Y., Takahashi, M., Haesslien,M., and Seyfried, C.F., Development ofSimulation Model for a Combined Activated-Sludge and Biofilm Process toRemove Nitrogen and Phosphorus, Water Environ. Res., Vol. 71, No. 4,pp. 388-397, 1999.

AAA - Combined System

Documents

Transcript of AAA - Combined System