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PERFORMANCE

CHARACTERISTICS OF AN

ACTIVATED SLUDGE UNITCHEN40010/40014 PRACTICAL WORK, SEMESTER 2, 2010

Department of Chemical and Biomolecular Engineering

By Mohamed Nishath Mohamed Nizar

Student Number 283195


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Executive Summary

A lab scale activated sludge unit (10 litre capacity) has been set up to treat a waste stream

that simulates industrial waste. The process operated over a time period of 48 hours

(hydraulic residence time) under a extended aeration system. The effluent stream was

found to have TSS and VSS of 263 and 163 mg/L respectively suggesting that high waterquality was produced from the system. The biological oxygen demand (BOD) of the

effluent stream is lower than that of the influent stream resulting in further evidence that

proves a high extent of organic compound degradation has happened in the treatment

unit. In addition, there is only slight discrepancies between the calculated BOD value by

standard and graphical methods; 0.4% and 4% for the influent and effluent streams

respectively. The standard method is more vigorous in such that it includes the seed

concentration factor, although this was not explicitly used as the ratio of seed to volume

was equal.

The pH of the influent, mixed liquor and effluent were found to be 7.9, 7.2, 7.2. SVI is

calculated to be 12.18 and 9.46 mg/L for both week 1 and week 2 of the experiment. The

calculated SVI value was determined to be below 200 mg/L, hence the system can be

considered to have good settling characteristics. In addition, the characteristic microbes

within the activated sludge have been identified, although no microbes were detected in

the second week of proceedings due to the low food to micro-organism ratio caused by the

extended aeration of the system.

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1.0 INTRODUCTION

The reduction of contaminants from water, including chemical and biological species has

been readily practiced, in order to achieve water quality of increased purity for its desired

end use. As waste water effluents (entering treatment plants) consists of a large

distribution of organic and non organic compounds, the treatment utilising mechanical andchemical treatment is not sufficient enough alone.

Industrially the use of micro-organisms (biological treatment process) has been crucial and

has been employed in activated sludge systems, lagoon filtration systems as well as

aerated lagoons. This is due to specific roles the micro-organisms play, in breaking down

particular compounds in a efficient and predictable manner. Experiments have been

conducted using a bench scale activated sludge unit with a volume of ten litres to treat a

simulated industrial waste. This report will aim to provide a more detailed understanding of

the mechanisms that control the biological activity in order for efficient waste water

treatment.

1.1 Aims of the experiment

To determine the following parameters for a bench-scale complete-mix activated sludge

unit:

I. BOD (influent and effluent streams).

II. Total suspended solids (TSS), volatile suspended solids (VSS) and non-volatile

suspended solids (NVSS) in the mixed liquor and effluent streams.III. pH (influent, effluent and mixed liquor).

IV. Sludge settling characteristics

V. Microbiological characteristics

1.2 Background information on the lab scale activated sludge unit

(Martin 2010)

A bench scale complete-mix activated sludge unit (Vol 10L) has been set up to treat a

simulated industrial waste. To ensure carbon limited growth, inorganic nutrients have been

included at appropriate concentrations. (Appendix A). Influent passes to the unit via a

peristaltic pump. Compressed air is supplied at about 2L/min.

The unit consists of a lidded square tank with an inverted pyramid base. It is divided into a

well mixed zone and a settling zone by means of a sliding baffle placed towards one side

of the tank. Liquid passes from the well mixed zone to the settling zone through a gap

underneath the baffle. Air bubbles are unable to enter the settling zone and the sludge is

able to settle under quiescent conditions.

Effluent removal occurs by overflow from the settling section. As the system gives total

sludge recycle, excess sludge is periodically bled off by removing a portion of the contents

from the aeration tank. The plant is operated as an "extended aeration" system with a

hydraulic residence time of approximately 48h.

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2.0 UNDERLYING THEORY2.1 Extended aeration system

The term Extended aeration system describes the process treatment, where the waste is

subjected to higher retention times in an aerated environment. Historically, the use of such

method was due to the unavailability of site operators stemming from high labour costs.The use of this technique is limited to small scale decentralised processing units, where

transport of wastewater to an established treatment facility is not economically feasible

(Tchobanoglous 2003).

The long retention times (approximately 24 hours hydraulic retention time (HRT)) allows

for a significant removal of organic compounds within the waste. Additionally, this

contributes to the age of the sludge (residence time of the sludge), which effectively leads

to endogenous decay. Endogenous decay is when the micro-organisms start feeding on

their own tissue in order to provide energy for sustained growth (Droste 1997), during

periods of food source depletion. In this environment utilising high HRT, the food to micro-

organism ratio (F/M) is considered low and is normally associated with floc forming micro-

organisms displaying low settling behaviour (Cheremisinoff 1995).

On both an industrial and laboratory scale, the clarifier is separated from the aeration

basin using baffles. A cyclical flow regime of the sludge, operates within the basin

effectively recycling the suspended solids in the clarifier. The differences can be see in

table 1 below. In addition a mechanical agitation device in operation can be seen in figure

1.

Table 1 - summarises the differences associated with Lab and industrial scale processes.

Industrial Scale Lab Scale

Method of agitation Mechanical surfaceaeration through agitation

Bubbles formed helpagitate the sludge

Aeration Mechanism Oxygen entrained fromatmosphere

Aeration proceeds from thebottom of the basin

Typical HRT 24-48 hours 48 hours

Temperature Harder to control, althoughdeviations are less due tolarge volumes

Easier to control, althoughdue to volume, canfluctuate more

Figure 1- represents the process of mechanical surface aeration at the western treatment plant.

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2.2 Biological oxygen demand

The analysis of the biological oxygen demand (BOD) is crucial in determining the extent of

organic compound degradation (Tchobanoglous 2003). Wastewater treatment runs

explicitly in trying to reduce the BOD content of the effluent stream; limiting the stress to

the environment caused by oxygen depletion during reintroduction into the ecosystem.

A Commonly used analysis of the BOD, is the BOD7 , which evaluates the oxygen demand

over a seven day period. This is used by Standard Methods for the Examination of Water

and Wastewater developed by American Public Health Association, as well as the

graphical method as developed by Klein and Gibbs.

Dilution is needed on a lab scale, as the dissolved oxygen (DO) content in the waste water

samples are considered to be high in concentration, in comparison to experimentallyavailable samples of influent and effluent streams. Dilution water contains nutrients such

as inorganic materials that are required by the microorganisms for their growth

(Tchobanoglous 2003). Seeding of the effluent stream is required as the influent stream

contains few microorganisms (Eaton 1995).

2.21 Standard method calculation of BOD

For effluent stream:

# # # # # # # # # # # # (1)

Where;

D1= dissolved oxygen of diluted sample immediately after preparation (mg/L)

D2 = dissolved oxygen of diluted sample after 5-day incubation at 20 (mg/L)

P = fraction of wastewater sample volume to total combined volume

For influent stream:

# # # # (2)

Where

B1 = dissolved oxygen of seed control before incubation (mg/L)

B2 = dissolved oxygen of seed control after incubation (mg/L)

f = ratio of seed in diluted sample to seed in control

"

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2.22 Calculation of BOD using graphical methods

The method developed by Klein and Gibbs involves plotting the remaining DO in the

sample after 7 days of incubation at 20 against a sample volume (Klein 1971). The BOD

of the sample can be then determined by using the following relationship and graphical

parameters derived from data:

# # # # # # (3)

Where S = DO concentration of the sample (mg/L)""

" "

2.3 Measuring the Sludge volume index (SVI)

According to literature, SVI is defined as the volume (ml) occupied by 1 g (dry weight) of

sludge after 30 minutes of settling in a 1 litre measuring cylinder (Cheremisinoff1995).Sludge volume index (SVI) is a measure of settling characteristics of the sludge

(Reynolds 1996). The SVI is can be determined by allowing the mixed liquor suspended

solids (MLSS) to settle for 30 minutes, and then by measuring the volume of solids that

have settled using the scale of the measuring cylinder.

MLSS is regarded as a mixture containing influent waste water and the recycled sludge.

Understanding settling sludge characteristics is important for efficient process control to

achieve a clear effluent stream exiting the treatment process. A SVI increases, the relative

level of MLSS compaction decreases, resulting in an increase of BOD within the system

and effects of sludge bulking.

This is seen as counterproductive towards the treatment process, hence the SVI should be

maintained below 200 mg/L for good settling behaviour. The SVI can be calculated using

the relationship in equation (4).

# # # # ## # # # # (4)

Where"V = volume of solids after 30 minutes of settling (mL/L)

MLSS = mixed liquor suspended solids (mg/L)

2.4 Calculating the total volatile and non-volatile suspended solids (TSS)

The measure of solid concentration in waste water is referred to as total suspended solids

(TSS) (Tchobanoglous 2003). Using filtration paper, the sample filter cake consisting of the

wastewater solids is incubated at at temperature of 103. Any remaining solids on the

filter paper is regarded as TSS.

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Similarly, volatile suspended solid (VSS) is the amount of solids that can be volatilised by

heating TSS at 550 (Eaton 1995). After incubating at temperatures of 550, most

volatile components of the wastewater is expelled, hence any remaining solids is referred

non-volatile suspended solid (NVSS) (Tchobanoglous 2003). The following equation is

used to determine the total suspended solids within a sample.

# # # # # # # # (5)

Where M103 = mass of solid after heating at 103 (mg)

Vs = volume of sample (L)

Likewise, the VSS and NVSS can be calculated by the following equations:

# # # #

(6)

# # # # # # # #

#

NVSS = TSS VSS # # # # (7)

Where M550= mass of reside after heating at 550 (mg)

According to literature, effluent streams contain less TSS and VSS in comparison to

influent and mixed liquor streams, due to consumption of organic material by the micro-

organisms present in the treatment process. Additionally, VSS is an approximate measure

of the organic material within the sample (representing process conditions); hence the

degree of microbial activity can be quantified to a certain level of accuracy.

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3.0 METHODOLOGY AND APPARATUS3.1 Experimental Method

Please refer to

1.# BIOENVIRONMENTAL ENGINEERING GUIDE TO PRACTICAL# WORK,PERFORMANCE CHARACTERISTICS OF AN ACTIVATED SLUDGE

# UNIT SEMESTER 2, 2010, DEPARTMENT OF CHEMICAL ENGINEERING

2. Standard Methods For The Examination Of Water And Wastewater, Developed By

American Public Health Association.

3.2 Schematic diagram of experimental setup

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Legend

Compressed Air In

BaffleInfluent Stream

ZONE

1

ZON

E2

Activated sludge unit

Effluent Stream

Pump

ZONE2

Well mixed zone

Settling Zone

Process Lines

Baffle

ZONE1

Figure 2- represents the schematic diagram of the activated sludge process


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3.3 Equipment list of the Activated sludge unit

- Buchner funnel

- 100ml measuring cylinder

- DO 200 metre

- pH metre

- Peristaltic pump (pump up oxygen/air into the aeration tank)

- 300ml bottles

- Standard filter paper

- Microscope

- Erlenmeyer flask

- Beakers

- Vacuum pump (connected to Buchner funnel)

3.4 Process description of the Activated sludge unit

The laboratory scale activated sludge unit (see figure 2 ), contains 10 litres of simulated

industrial waste (please refer to Appendix A). A pump (peristaltic) is used to transfer the

influent stream to the activated sludge unit. The effluent stream discharges at the same

rate as the entering influent by utilising a overflow mechanism. This technique allows for a

clear effluent stream to be obtained. Compressed air is fed at a flow rate of 2 litres/min

through the bottom of the unit to aerate the sludge.

Two distinct zones are created by means of a sliding baffle. The baffle mechanism

operates by not allowing any compressed air to reach Zone 2; hence promoting settling.

Zone 1 (highlighted green in figure 2) represents the well mixed zone within the system.

Zone 2 (highlighted yellow in figure 2), represents the settling zone within the system.

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4.0 RESULTS

4.10 Standard Method of evaluating BOD of influent and effluent streamsTable 2- represents the BOD concentrations of the influent and effluent streams in mg/l using the

standard method.

Dilution % Influent stream (BOD mg/L) Effluent stream (BOD mg/L)

0 -

0.05 203.9

0.1 165.9 101.9

0.2 268.9

0.5 224.0 153.2

1 215.8 146.2

2 183.8

5 134.0

Average 215.7 143.82

4.11 Graphical Method of evaluating BOD of influent and effluent streamsTable 3- represents the BOD concentrations of the influent and effluent streams in mg/l using the

graphical method.

Stream BOD (mg/l)

Influent 216.5

Effluent 138.8

4.2 Total suspended solids (TSS), volatile suspended solids (VSS) and non-volatile

suspended solids (NVSS) in the mixed liquor and effluent streams.

Table 4- represents the TSS, VSS and NVSS for of the influent, effluent, and mixed liquor streams

in mg/l.

Stream TSS (mg/l) VSS (mg/l) NVSS

Effluent 263.3 163.3 100

Mixed Liquor 2052.7 1952.7 100

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4.3 pH (influent, effluent and mixed liquor)Table 5- represents the measured pH of each stream.

Stream PH

Effluent 7.2

Mixed Liquor 7.2

Influent 7.9

4.4 Sludge settling characteristics

Table 6- represents the settling characteristics for each week in mg/l.

SVI (mg/l)

Week 1 12.18

Week 2 9.26

4.5 Microbiological characteristics

Table 7- represents the microbial activity seen through the microscope in week 1.

Microbiology(Week 1)

Description

Protozoa Sessile stalked ciliate protozoa in mixed liquorVorticella sp

Large numbers in activated sludge, not branched with an inverted bellattached to stalked containing a contractile myoneme.

Macro nucleus in shape of C

Metazoa BFX 125 A rotifer with distinct pseudosegmentation of the body into ahead, trunk and foot.

Aspidisca costata Free swimming and crawling ciliates

Commonly found in high numbers ini activated sludge. Small, oval andflattened ventral surface. Dorsal surface convex and conspicuouslyridged.7 frontal and 5 anal cirri.

Epistylis rotans Sessile stalked ciliate protozoaStalk striated longitudinally and segmented peristore constricted belowfringe

Trachelophyllum

pusillum

Small elongate, flattened, flexible ciliate. Anterior end constricted to form

a neck. Two round macro nuclei. Single terminal contractile vacuole.

* Microbial characteristics for week 2-no micro-organisms were found the following week

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5.0 DISCUSSION OF RESULTS

5.1 Standard Method of evaluating BOD of influent and effluent streams (please

refer to section 4.10 and section 4.11)

From table 2 and 3 in section 4.10, 4.11 one can see the biological oxygen demand

decreasing from the influent stream to the effluent stream which is in accordance with

literature. On the other hand, the relationship between the BOD and increasing dilution

percentage is not well established when analysing the recorded results.

The BOD is at a lower concentration in the effluent stream when compared to the influent

stream, this suggests that the microbes are comparatively active and have to some extent

degraded the organic compounds which are present.

As the dilution water contains nutrients such as inorganic materials that are needed for the

growth, a higher dilution percentage will result in less nutrients available for the microbes.

Therefore, lower BOD values are expected as the population of bacteria is reduced due to

insufficient nutrient content.

A few anomalous results can be seen, although this could be due to the multiple sources

of error present with the experimental technique and equipment available. For example,

the changes in BOD are relatively small (mg/l), hence any deviation from aseptic

techniques will have some significance towards the final reading. In addition, since the

data was collected by a large group of people who practice varying degrees of laboratoryskills, it is inevitable for some discrepancies to rise.

When processing the raw data, two methods were applicable. The standard method and

the graphical method developed by Klein and Gibbs. The results obtained from both

methods are similar. For the influent stream, the values differ by only 0.4%, and for the

effluent stream, the BOD differs by 3.5%. Hence, the different methods compliment each

other which proves the data processing stage was conducted well. The slight

discrepancies between the two methods arises from the amount of data points plotted,

which affects the trend line equation and the overall BOD concentration calculated.

5.2 Total suspended solids (TSS), volatile suspended solids (VSS) and non-volatile

suspended solids (NVSS) in the mixed liquor and effluent streams.

Literature values (Klein 1971) suggest the VSS to TSS ratio is in the range of 0.75-0.80.

From the results displayed in table 4 (section 4.2), the VSS to TSS ratio is 0.62 for the

effluent stream, and 0.95 for the mixed liquor stream (MLS).

The TSS, VSS and NVSS are all comparatively larger for the MLS. This is understandable

as the MLS compromises predominantly of organic matter in comparison to the effluent

stream. The effluent stream should be show lower in TSS,VSS and NVSS if the microbial

activity is well established and consuming organic matter. This is the case as shown by the

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results achieved. In addition, the literature values state that for extended aeration cases,

the TSS ranges in between 3000 and 6000 mg/l.

The determined TSS for this case was determined to be 2052.7 mg/L. The value is

significantly lower, which could potentially indicate that the microbial activity has been

efficient and has already started to consume the organic matter within the MLS but has not

had much time to reproduce, or deviations exist from extended aeration conditions.

According to (Eaton 1995), VSS is directly proportional to the biomass; hence can be used

as a indicator to distinguish the condition of the microbial population. A larger VSS value,

represents a healthier microorganism population, as they are able to consume organic

compounds,effectively producing more biomass. In addition, if a large VSS is detected in

the effluent stream, the performance of the activated sludge can be considered not to be in

working order, as the microbes are failing to digest the organic matter to the the

appropriate specification. Temperature fluctuations within the laboratory due to changes inexternal weather conditions, might have an impact on the microbial activity as they are

temperature dependent, i.e the microbes are converting less organics due to temperature

changes. In relation to the experiment conducted, the VSS in the effluent stream registers

a decrease when compared to the MLS, hence this is in accordance to what we should

expect.

The low value of TSS indicates a high water quality. Class A water requires TSS amount to

be below 5 g/L. As the TSS of the effluent stream is 0.0263 g/L, it indicates that the

laboratory set up of the activated sludge is working in order, as it is purifying the influent

stream consisting of the mock up industrial waste. However this might not be a significantas It appears to be, as the process volumes being dealt with are very small in the lab,

where microbe populations can establish relatively quickly. Industrial treatment plants are

more harder to control due to the large process volume and large microbe activity.

It is important to note that the NVSS for both streams are equal, as it shows there is a

change in VSS but not NVSS; probably due to the specificity of the microorganism within

the activated sludge. I.e the microbes cannot digest the NVSS.

5.3 pH (please refer to section 4.3, table 5)

The pH(s) for all the three streams are close to being neutral, pH 7. The permissible pH

range for treated effluents varies from 6.4 to 8.5 (Klein 1971). Since the pH of the effluent

is 7.24, the performance of the bench scale activated sludge unit is considered good.

5.4 Sludge volume index (please refer to section 4.4, table 6)

Sludge volume index recorded for both week 1 and 2 were slightly different. They differed

by 24%, with the second week results recording a percentage decrease. This is consistent

with the theory, as over time the bio organisms are degrading the organic compoundsresulting in a increase in settling rate. Additionally the the first week experiment was

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conducted over a duration of 1 hour, while the 2nd week experiment was conducted within

30 minutes providing more supporting evidence of the productivity of the microbes.

According to literature, the SVI should be kept below 200 mg/L for good settling

characteristics . Since SVI calculated is 12.18 and 9.46 mg/L for both week 1 and week 2

respectively, the sludge in the system has good settling characteristics, resulting in low

BOD level in the effluent stream.

5.5 Microbial characteristics

In the first week, several type of micro-organisms were found to be present within the

sample. This can be seen in table 7, section 4.5. Despite this, no micro-organisms were

present within the 2nd weeks sample. This could be due to conditions rapidly deteriorating

in terms of food to microbe ratio resulting in a dying population, or could result from a

anomalous sample being drawn to be analysed.

6.0 CONCLUSIONThe aims of the experiment were all satisfied, this includes determining the BOD influent

and effluent concentrations, the TTS, VSS and NVSS, determining the pH of the influent,

effluent and MLS, as well as determining sludge settling and microbiological

characteristics.

TSS calculation of the effluent stream shows that high quality (Class A) is produced by the

unit. TSS of the effluent stream is calculated to be 0.0263 g/L while class A water requiresTSS amount to be below 5 g/L. Furthermore, the BOD for the effluent stream is much

lower than that of influent stream, which shows that most of the organic compounds have

been degraded during the treatment process. In addition, the calculated SVI indicates that

settling characteristics of the system is adequate and that the BOD in the effluent stream is

relatively low.

Observation of the organisms through the microscope proved to be useful in identifying the

types of micro-organisms present within the mixed liquor utilised in the treatment of waste

water.

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7.0 ReferencesCheremisinoff, P.N.,Handbook of Water and Wastewater Treatment Technology. 1995, New Jersey:

Marcel Dekker.

Droste, R.L., Theory and Practice of Water and Wastewater Treatment. 1997, New York: JohnWileys & Sons. 578.

Eaton, A.D., L.S. Clesceri, and A.E. Greenberg, Standard Methods for the Examination of Water

and Wastewater. 1995: American Public Health Association.

Klein, R.L. and C.R. Gibbs, Graphical Method for Calculating Biochemical Oxygen Demand.Journal of Water Pollution Control Federation, 1971. 51: p. 2257-2266.

Martin, G., 411-448 Bioenvironmental Engineering. 2010, The University of Melbourne:Melbourne.

Reynolds, T.D. and P.A. Richards, Unit Operations and Processes In Environmental Engineering.

2nd ed. 1996, Boston: PWS Publishing Company.Tchobanoglous, G., F.L. Burton, and H.D. Stensel, Wastewater Engineering: Treatment and Reuse.

3rd ed. 2003, New York: McGraw-Hill.8.0 Nomenclature

Notation Description Unit

B1Dissolved oxygen of seed control before incubation

mg/L

B2Dissolved oxygen of seed control after incubation

mg/L

BOD7 Biological oxygen demand (7 days test period) mg/L

D1Dissolved oxygen of diluted sample immediately after preparation

mg/L

D2 Dissolved oxygen of diluted sample after 5-day incubation at 20oC mg/L

DO Dissolved oxygen mg/L

fRatio of seed in diluted sample to seed in control

-

F/M Food to microorganism ratio -

M103 Mass of solid after heating at 103oC mg/L

M550 Mass of solid after heating at 550oC mg/L

MLSS Mixed liquor suspended solids mg/L

NVSS Non-volatile suspended solids mg/L

P Fraction of wastewater sample volume to total combined volume -

RASRecycled activated sludge

-

S Dissolved oxygen concentration of the sample mg/L

SVI Sludge volume index mg/L

TSS Total suspended solids mg/L

V Volume of solids in 1000mL of mixed liquor after 30 minutes of settling mL/L

Vs Volume of sample L

VSS Volatile suspended solids mg/L

WASWaste activated sludge

-

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