INTRODUCTION

Sludge bulking has been one of the major

problems affecting biological waste treatment.

There are several problems

regarding solid separation in

activated sludge.

►industrial and municipal activated sludge

processes a nutrient deficiency may occur.

►The nutrients that are usually deficient in these

processes are either nitrogen or phosphorus.

results in the production of nutrient deficient floc

particles, loss of settleability, and possibly a billowy

white or greasy gray foam on the surface of the

aeration tank.

►NUTRIENT DEFICIENCY - the bacteria within the

floc particles remove soluble BOD from the

wastewater

INTRODUCTION

►NITROGEN OR PHOSPHOROUS is deficient the

soluble BOD is not degraded but it is stored within

the floc particles as an exocellular polymer-like

material. This slimy material interferes with settling

and may cause foam upon aeration.

►BULKING is a problem consisting of slow settling and

poor compaction of solids in the clarifier of the

activated sludge system.

INTRODUCTION

Filamentous bulking is usually caused by the excessive

growth of filamentous microorganisms.

Bulking is caused by the overgrowth of filamentous

bacteria in activated sludge.

Name of Problem Cause of Problem Effect of Problem

Dispersed growth Microorganisms do not form flocs but are dispersed, forming only small clumps or single cells

Turbid effluent. No zone settling of sludge

Sime (jelly) Viscous bulking Microorganisms are present in large amounts of extracellulr slime

Reduced settling and

compaction rates. Virtually no solids separation, in severe cases in overflow of sludge blanket from secondary clarifier.

Pin floc (or pinpoint floc)

Small, compact, weak, roughly spherical flocs are formed, the larger of which settle rapidly. Smaller aggregates settle slowly.

Low sludge volume index (SVI) and a cloudy, turbid effluent.

Bulking

Filamentous organisms extend from flocs into the bulk solution and interfere with compaction and settling of activated sludge.

High SVI; very clear supernatant.

Rising sludge

Denitrification in secondary clarifer releases poorly soluble N2

gas, which attaches to activated sludge flocs and floats them to the secondary clarifier surface.

A scum of activated sludge forms

on the surface of the secondary clarifier.

Foaming/scum formation

Caused by (1) nondegradable surfactants and (2) the presence of Nocardia and sometimes (3)

the presence of Microthrix parvicella

Foams float large amounts activated sludge solids to the surface of treatment units. Foam accumulates and putrefies. Solids

can overflow into secondary effluent or overflow onto walkways.

Relationship between floc-forming and filamentous

bacteria, three types of flocs are observed in activated

sludge:

INTRODUCTION

▼Normal flocs: A balance between floc-forming and

filamentous bacteria results in strong flocs that keep

their integrity in the aeration basin and settle well in the

sedimentation tank.

▼Pin-point flocs: In these flocs, filamentous bacteria

are absent or occur in low numbers. This results in small

flocs that do not settle well. The secondary effluent is

turbid despite the low SVI.

Normal flocs

INTRODUCTION

▼Filamentous bulking: Filamentous bulking is caused

by the predominance of filamentous organisms. The

filaments interfere with sludge settling and

compaction.

Filamentous bacteria have a higher surface-to-volume

ratio than that of their floc-forming counterparts, which

helps them survive under low oxygen concentration

and low nutrient conditions.

Filamentous bacteria are able to predominate under

low dissolved oxygen, low F/M, low nutrient conditions

or high sulfide levels.

Filamentous bulking

TYPES OF FILAMENTOUS MICROORGANISMS

20 to 30 types of filamentous microorganisms are

known to be involved in activated sludge bulking

A survey of bulking activated sludge plants in the U.S.

has revealed that approximately 15 major types of

filamentous microorganisms are responsible for

bulking, one of the most predominant being

Nocardia, which is responsible for foaming.

FACTORS CAUSING FILAMENTOUS BULKING

High carbohydrate wastes appear to be conducive

to sludge bulking.

Low substrate concentration (low F/M ratio) appears

to be the most prevalent cause of filamentous

bulking.

╬At low substrate concentration filamentous

microorganisms have a higher substrate removal rate than that of floc-formers, which prevail at high

substrate concentrations.

completely mixed systems, increasing sludge

loading leads to a decrease of SVI and thus to a

decrease of filamentous microorganisms.

below 6.0PH may favor the growth of fungi and

cause filamentous bulking.

The growth of certain filamentous bacteria is

favored by relatively low dissolved oxygen levels

in the aeration tank.

A substrate overload in the tank may induce

oxygen deficiency.

Deficiences in nitrogen, phosphorus, iron or trace

elements may cause bulking.

Increased temperature supports the growth of

filamentous bacteria associated with low dissolved

oxygen concentrations.

Microthrix parvicella to the dominant

filamentous microorganism during the winter

season.

Hypothesis on filamentous bulking is based on the

ability of filamentous bacteria to denitrify nitrate to

only nitrite with no accumulation of toxic nitric

oxide by the cells. This gives a competitive

advantage over floc-forming bacteria.

Control of Sludge Bulking

Bulking control by chlorination

chlorination was proposed over 50 years

ago and this practice is probably the most

widely used cost-effective and short-term

method for controlling filamentous

bacteria.

Chlorine concentration should be 10-20

mg/L (concentrations greater than 20

mg/L may cause deflocculation and

formation of pin-point flocs).

Bulking control by Hydrogen peroxide

added tot he RAS at concentrations of 100-

200 mg/L

excessive levels of hydrogen peroxide

can be deleterious to floc-forming

bacteria

hydrogen peroxide may also act as a

source of oxygen in the aeration tank.

Ozone was also proposed for curing

filamentous bulking.

SYNTHETIC ORGANIC

POLYMERS, LIME, AND IRON

SALTS

added to the mixed liquor to improve

bridging between the flocs and thus

promote sludge settling

the addition of lime and iron salts

increases the solids load, and the use of

polymers is costly

coagulants leads to an immediate

improvement in sedimentation