LABORATORY PROJECT:

IMPACT OF VARIABLE LOADING ON A FIXED FILM REACTOR

VERSUS A SUSPENDED GROWTH REACTOR

DUE: MAY 6, 2008

PROFESSOR: DR. BELINDA MCSWAIN-STURM

TA: ERIN BELLASSAI

CE773 1 LAB PROJECT

1.0 INTRODUCTION

1.1 BACKGROUND

Suspended growth activated sludge wastewater treatment plants are commonly

used to treat municipal and industrial wastewater. Since the introduction of the

activated sludge process, the development of wastewater treatment has been

generally held back by tradition (Odegaard, 2000). As regulators continue to

strengthen effluent water quality standards, design criteria for suspended growth

activated sludge wastewater treatment plants must be updated accordingly.

Numerous options, including more contemporary technologies, are available to

wastewater treatment entities to improve effluent water quality. Recently, biofilm

treatment processes have become increasingly popular compared to traditional

activated sludge processes. One of the main reasons for the increased interest in

biofilm, according to Odegaard et al. (1994), is that the treatment plant can be

made much more compact, requiring less space. Many types of biofilm processes

exist today, such as trickling filters, rotating biological contactors fixed media

submerged biofilters, etc (Odegaard, 2006). Fixed film media is currently being

incorporated into designs for constructing new and upgrading existing suspended

growth activated sludge wastewater treatment plants to enhance treatment. The

main targets of applying the fixed film media process are increased hydraulic

capacity and increased COD/nutrients removal from the effluent (Azimi et al.,

2007).

1.2 PURPOSE

Industrial and municipal wastewater treatment plants experience variations in

loading throughout any given time frame. Analysis of the impact of variable

loading of a bench scale fixed film activated sludge reactor versus a bench scale

conventional suspended growth reactor is the purpose of this laboratory. Given

the increased surface area for microbial growth, we postulate that the fixed film

activated sludge reactors will achieve more consistent COD removal results under

variable load than the conventional suspended growth reactors.

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2.0 METHODS

Reagents included a premixed wastewater solution and activated sludge biomass

provided by the teacher’s assistant. Approximately 0.7 liters of activated sludge

biomass and 1.3 liters of premixed wastewater solution were added to four 2-liter

chemostats (R6, R7, R8, and R9). Two of the 2-liter chemostats (R8 & R9) were

filled with 0.8 liters of Siemens AGAR type fixed film media; a fill fraction of

40%, which equates to approximately 0.665 m2 of protected surface area for fixed

film growth. Chemostat R8 & R9 model a “Integrated Fixed Film Activated

Sludge (IFAS)” and R6 & R7 model typical activated sludge suspended growth in

a completely mixed reactor. Aeration and mixing was provided by compressed

air at the lab bench and submersible aerators. Figure No. 1 and Figure No. 2 are

photographs of the setup for the chemostats.

Figure 1 – R6 & R7

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Figure 2 – R8 & R9

The chemostats began operating on April 15, 2008 at a hydraulic retention time

(HRT) of 12 hours. Chemostats R6 & R7 were fed from a 20 liter carboy and

chemostats R8 & R9 were fed from a second 20 liter carboy. Each carboy

contained premixed wastewater solution and was fed to associated chemostats via

peristaltic pumps. The pumping rate was established at 2.8 mL/min to maintain a

12-hour HRT. During the first 5-day period the carboys were refilled daily with

premixed wastewater solution with a COD concentration of 500 mg/L. The

purpose for allowing the reactors to operate for five days prior to taking readings

was to allow the biofilm on the media and the suspended growth to reach steady

state. Further, any growth on the wall of Chemostats R6 & R7 was scraped daily.

Following the initial five-day period for establishment of steady state suspended

growth and biofilm on the media, the feed concentration of the premixed

wastewater was increased by 500 mg/L every two days when the carboys were

refilled until finally the influent COD was established at 2500 mg/L. Due to odor

and foaming, the experiment was cut short by one day, but data was still collected

for the 2500 mg/L dose. Figure No. 3 is a photograph of the foaming chemostat.

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Figure No. 3 – Foaming Chemostat

After each increase in chemical oxygen demand concentration the chemostats

were analyzed for influent and effluent chemical oxygen demand (COD), mixed

liquor suspended solids (MLSS), total biomass, dissolved oxygen (D.O.), oxygen

uptake rate (OUR) and pH. D.O., OUR, and pH were measured with probes and

recorded by each laboratory group. The OUR data was not comprehendible and

was excluded from the report; a faulty probe is suspected. Samples to be

analyzed for COD and MLSS were collected by each group and delivered to the

classroom Teacher’s Assistant (TA). COD and MLSS were measured by

Standard Method No. 5220 D Closed Reflux, Cholorimetric Method and Standard

Method No. 2540 D Total Suspended Solids Dried at 103-105 oC respectively;

these Standard Methods were referenced from the Standard Methods for the

Examination of Water and Wastewater and were performed by the TA.

The total biomass for the fixed film media was calculated by adding the

suspended MLSS solids to the solids on 5 media removed from each reactor. The

media were weighed before and after heat drying to capture the weight of the

CE773 5 LAB PROJECT

fixed biomass. The five media removed from each reactor were replaced with

five new media.

3.0 RESULTS

Chart No. 1 shows %COD consumed relative to influent COD concentration. As

originally postulated, the IFAS reactors (R8 & R9) achieved more consistent

results than the conventional suspended growth reactors (R6 & R7).

Chart No. 1 - % COD Consumed vs. Influent COD

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500

Inffluent COD mg/l

% COD Consumed

Legend

6789

Chart No. 2 shows dissolved oxygen (DO) relative to influent COD concentration.

The DO did not vary with the exception of the 1,500 mg/L COD concentration.

The data points associated with the 1,500 mg/L COD concentration are not likely

correct and could potentially be associated with the DO meter being out of

CE773 6 LAB PROJECT

calibration. The DO concentration in R6 & R7 does not correlate well to the other

data for R6 & R7 and the data for R8 & R9 is well above saturation for the 1,500

mg/L COD data point. Further, the %COD removal performance of R6 & R7

would have significantly deteriorated and then rebounded, but Chart No. 1 shows

that they did not. Otherwise, the data was consistently at or above oxygen

saturation.

Chart No. 2 – DO vs. Influent COD

0

5

10

15

20

0 500 1000 1500 2000 2500

Inffluent COD mg/l

DO mg/l

Legend

6789

Chart No. 3 shows MLSS relative to influent COD concentration. The MLSS

measured the suspended solids concentration of the interstitial fluid in R8 & R9

and the suspended biomass in R6 & R7. Visually, the interstitial fluid was

clearer, or less murky, in R8 & R9. R6 & R7 exhibited fluid typical of a

conventional suspended growth reactor. Refer to photos in Methods section.

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Chart No. 3 – MLSS vs. Influent COD

0.00

0.75

1.50

2.25

3.00

0 500 1000 1500 2000 2500

Inffluent COD mg/l

MLSS g/l

Legend

6897

Chart No. 4 shows Total Biomass relative to influent COD concentration. The

total biomass measured the suspended solids concentration of the interstitial fluid

plus the mass of the solids on the fixed media in R8 & R9 and the suspended

biomass in R6 & R7. Visually, the interstitial fluid was clearer, or less murky, in

R8 & R9. R6 & R7 exhibited fluid typical of a conventional suspended growth

reactor. Refer to photos in Methods section.

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Chart No. 4 – Total Biomass vs. Influent COD

0

5

10

15

20

0 500 1000 1500 2000 2500

Inffluent COD mg/l

Total Biomass g/l

Legend

6897

4.0 DISCUSSION

As shown in Chart No. 1, under lower COD concentration R6 & R7 achieved

similar results to R8 & R9 and as the influent COD concentration increased R8 &

R9 performed significantly better. The better performance of R8 & R9 is likely

attributable to the higher biomass concentration sustainable in the IFAS due to

increased surface area for microbial growth; additional information regarding the

quantity and type of microbes on the media is necessary to verify. Visible growth

did occur on the media, but to determine the quantity and type of microbes an

additional study where broad DNA analysis of microbes present on the fixed film

should be performed.

IFAS type reactors typically require more oxygen than conventional suspended

growth reactors. Recent IFAS models and research suggest that dissolved oxygen

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levels are especially critical to nitrification in an IFAS reactor (Takács et al.,

2007), and may be of minimal importance to COD removal (Sriwiriyarat, et al.,

2008). The dissolved oxygen saturation shown in Chart No. 2 confirms that the

microbial activity in R8 & R9 was not limited by oxygen supply. Rather,

microbial activity was limited by HRT and influent COD concentration, which

was the intent of the experiment.

Fixed film media has a propensity to slough older biomass and re-grow new

biomass. One suspected caused of such biomass detachment is nutrient starvation

(Hunt etal., 2004). The older, sloughed biomass in a typical wastewater plant

would be settled in clarifiers and wasted to sludge disposal; the laboratory setup

for this experiment did not allow for wastage of sloughed material off the

biomass. For this reason, chart No. 3 shows significantly more MLSS in R8 & R9.

Chart No. 4 shows that the total biomass in R8 & R9 increased as a response to

additional load. The growth on the media likely responds to increased COD

concentration, thus increasing the total biomass in the reactor further providing

consistent removal of COD. The additional biomass in the IFAS reactors is the

key to the more consistent COD removal. With more biomass, more COD can be

removed as long as enough oxygen is present to support the microbial growth

under sufficiently long HRT’s.

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5.0 CONCLUSION

Prior research has shown that biofilm treatment plants may have more efficient

COD removal than activated sludge plants at increasingly high COD loads

(Odegaard et al., 2000). Andreottola et al. (2000) suggested that minimum values

for specific surface area of the media must be met in order to ensure the IFAS is

competitive with an activated sludge system. This experiment confirmed that

IFAS type reactors (R8 & R9), employing sufficient surface area, achieve more

consistently high COD removal than conventional suspended growth type reactors

(R6 & R7). More efficient removal of COD is documented in the experiment

under conditions of oxygen saturation, as was directly correlated to increasing

total biomass in relationship to increasing influent substrate (COD). The next

step would be to further explore the microbial mechanics of the IFAS by

determining the quantity and variability of microbial populations present on the

media.

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