(GREEN SOLUTIONS)

EFFECT OF SUBSTRATE AND HRTON THE PERFORMANCE OF

ACTIVATED SLUDGE PROCESSBY

K.L.B.DEEPIKA & E.MADHU SRUTHI

madhu_sruthi4@yahoo.com

(BAPATLA ENGINEERING COLLEGE(AUTONOMOUS),BAPATLA)

ABSTRACT

The effect of substrate and Hydraulic Retention Time (HRT) on

the performance of Activated Sludge Process (ASP) is presented in this paper. Two

types of synthetic wastewaters containing glucose and sucrose as the sole carbon

source, which had a COD of 2667 and 2807 mg/l respectively were used for the study.

The aeration tank of ASP was operated at decreasing HRT in steps (12h, 8h, 4h and

2h) first with synthetic glucose medium and then with sucrose medium. For the

glucose medium the ASP could give a COD removal greater than 90% till a HRT of

4h and 75% COD removal for 2h HRT.Whereas for the sucrose medium the COD

removal was in the range of 85-90% till a HRT of 4h and 58% for 2h HRT.The

Volatile Suspended Solids (VSS) in the aerator were significantly higher when

sucrose medium than when are mostly glucose medium was used. The morphological

studies show that the microorganisms are mostly gram negative and rod shaped with

variable size

INTRODUCTION

Biological wastewater treatment is gaining popularity because of its high

potential for waste stabilization with minimal impact on the environment and ecology.

The Activated Sludge Process (ASP) is the most widely used and the most effective

secondary treatment method although it suffers from disadvantages like high sludge

production rates, high aeration costs etc.... The activated sludge contains a consortium

of biological organisms, which usually include bacteria, protozoa and rotifers.

Bacteria form the major microbial community in activated sludge (Shijin, 2004).

The word "activated" refers to the activation of the sludge (i.e.,

microorganisms) by the addition of oxygen to the wastewater. The liquid (i.e.

wastewater and micro organisms) in the aeration tank is referred to as “mixed-liquor.”

There have been several modifications to the activated sludge process. The most

common ones are conventional, step aeration and continuous flow stirred-tank

reactors (Metcalf and Eddy, 2003). The process consists of an aeration tank and a

sedimentation tank (clarifier). The basic process diagram is shown in Fig 1.The

aeration tank is aerated by a sub-surface or surface aerator system. There are several

types of aerators, all of which try to supply adequate dissolved oxygen to the water for

the microorganisms to thrive. The wastewater flows through the tank while the

microorganisms are consuming the unwanted organic matter from the wastewater.

The liquid is then transferred to a clarifier where the microorganisms settle to the

bottom. The supernatant of the clarifier is discharged to an ocean or sometimes it is

further treated by advanced treatment

processes.

Fig 1. Activated Sludge Process.

MATERIALS AND METHODS

Two synthetic effluents containing glucose and sucrose as the sole carbon

source were used for the study. They had a COD of 2667 and 2807 mg/l respectively.

For experimental purposes, a stock solution of 2.5% concentration media was

prepared. It was then diluted according to the requirement. The reactor and clarifier

volumes were 3.4 litres and 4.5 litres respectively. Submerged aeration has been

provided in the aerator. The medium was inoculated with acclimated aerobic culture.

The reactor was operated at room temperature. All the connections were made with

leak proof silicon tubes. The liquid is fed to the reactor by using a peristaltic pump,

which was calibrated before the start-up of the experiment. The analytical methods

used were COD estimation and Volatile Suspended Solids (VSS) as per the Standard

methods (APHA 1998). Fig 2 shows the whole experimental set-up. Details of the

experimental set-up are given in Tables 1 and 2.

Table 1: Experimental set-up details of aerator

Shape of the reactorFlat bottomed, vertical

sides

Material used Stainless steel

Size of the reactor 0.2m*0.2m*0.2m

Inlet 2 no

Outlet 2 no

Internal volume 4.4 litres

Liquid volume 3.4 litres

Table 2: Experimental set-up details of clarifier

Shape of the clarifier Conical

Material used Stainless steel

Column diameter0.2m (top);

0.025m(bottom)

Column length 0.25m

Inlet 1 no

Outlet 3 no

Sampling ports 2 no

Internal volume 6.1 litres

Liquid volume4.5litres

Fig:2 ACTIVATED SLUDGE PROCESS WITH CONNECTIONS AND

ACCESSORIES

RESULTS AND DISCUSSION

The reactor was started with synthetic glucose medium in batch mode. After

attaining stability, the reactor was switched to continuous mode. The aeration tank of

ASP was operated at decreasing HRT in steps (12h, 8h, 4h and 2h ) first with

synthetic glucose medium and then with synthetic sucrose medium. The inlet COD

was fixed at 2667 and 2807 mg/l for synthetic glucose and sucrose feeds respectively.

Outlet COD and VSS concentration in the aerator were measured daily. Figs 3 and 4

depict the percentage COD removal and VSS in the aerator at different HRTs for the

glucose and sucrose medium.

■glucose □sucrose

Fig 3: COD removal (%) at different HRTs

■glucose □sucrose

Fig 4: VSS (ppm) in the aerator at different HRTs

Fig 3 shows that the ASP can give a COD removal greater than 90% till a

HRT of 4h and 75% COD removal for 2h HRT. Whereas for the sucrose medium the

COD removal was in the range of 85-90% till a HRT of 4h and 58% for 2h HRT. This

could be due to the carry over of VSS in the outlet or the insufficient HRT of the

substrate in the aerator. It can be seen from Fig 4 that the VSS concentration in the

aerator has increased gradually with the decrease in HRT. Another observation that

can be made is that, the COD removal with sucrose as the carbon source was less than

that of glucose, but the VSS in the aerator for sucrose medium was much higher than

that of glucose. It can be thus understood that the microorganisms efficiently

breakdown glucose when present at a smaller concentration, but find it difficult to

degrade sucrose even when present in higher concentration.

In order to identify and analyze the microbial consortia responsible for the

removal of COD in the synthetic wastewater, samples were taken frequently from the

aeration tank, plated onto nutrient agar plates, and incubated at 370C overnight. The

spread plate method was used in plating at different dilutions from 10^-4 to10^-8.The

different colonies obtained were analyzed using Gram stain and the morphological

characteristics observed. In total, 7 different colonies were observed which have been

described in table 3

Table 3: Morphological characteristics of different colonies

Size Edge Surface

Texture

Elevatio

n

Consist

ency

Optical

Feature

s

Pigmen

t

Gram

Stain

Other

Feature

s

Small Circular Smooth Thin,

Convex

But

yrous Opaque Yellow

Gram

+ve

Rod

shaped,

forming

a

network

Small Project-

ions Rough

Thin,

FlatDry Opaque -

Gram -

ve

Circular

cells,

generall

y single

(nucleus

promine

nt)

Medium Circular Smooth Thin,

Convex

Butyrous Opaque -

Gram -

ve

Cocci,

2-3 cells

together

generall

y

Medium Circular Smooth Thin,

Convex

Butyrou

sOpaque

Light

Pink

Gram -

ve

Rods,

looking

like

myceliu

m, very

long

Large Irregular Smooth Thin,

FlatRubbery Opaque -

Gram -

ve

Rod

shaped,

2-3

joined

linearly

Large Irregular

Rough,

Wrinkle

d

Thick,

Convex Rubbery Opaque -

Gram

+ve

Elongat

ed cells

forming

aggregat

es of

star

shape.

Circular

cells

also

present

Large Circular Smooth Thin,

FlatRubbery

Transluc

ent-

Gram -

ve

Elongat

ed cells,

2-3 cells

together

From the above table it can be observed that a majority of Gram-negative

bacteria are present. Morphologically, the microorganisms are mostly rod shaped

forming networks, very dense at times. The colony size is however variable, varying

from small to large colonies.

CONCLUSIONS

The ASP can give very good COD removal till a HRT of 4h for both glucose

and sucrose mediums. However, for 2h HRT the COD removal was not satisfactory.

The microorganisms efficiently breakdown glucose when present at a smaller

concentration, but find it difficult to degrade sucrose even when present in higher

concentration. The microorganisms are mostly gram negative and rod shaped with

variable size