Operational Problems
The most common problems in the operation of an activated sludge plant are bulking sludge, rising sludge and nocardia foam.
Bulking Sludge
Bulking sludge has
Two principle types of sludge bulking problems have been identified.
Caused by the growth of filamentous organisms
Or organisms that can grow in a filamentous form under adverse conditions.
This is the predominant form of bulking
Caused by bound water
The causes of sludge bulking are related to
1. The pysical and chemical characteristics of the WW
2. Treatment plant design limitations
3. Plant operation
1. WW characteristics that can affect sludge bulking include
fluctuations in flow & strength, pH, temperature, nutrient content
3. Operational causes of filamentous bulking include
low dissolved oxygen in the aeration tank
insufficient nutrients: Especially quantity of nitrogen and
phosphorus important, also absence of trace element cause bulking.
widely varying organic waste loading
low F/M ratio: The F/M ratio should be check to make it is
within normal range.
Low F/M ratio encourage the growth of filamentous organisms
High F/M may result in the presence of small disperse flocs.
Limited dissolved oxygen has been noted more frequently than any other cause. If the problem is due to limited D.O. Aerating equipment should operate at full capacity. At least 2 mg/L of dissolved oxygen in the aeration tank (under normal loading conditions) should be maintained.
One of the kinetic features of filamentous organisms that
Filamentous bacteria such as Beggiatoa & Thiothrix
grow well on hydrogen sulfide & reduced substrates respectively that would be found in septic WW.
When the influent WW contains fermentation products such as volitile fatty acids & reduced sulfur compounds (sulfides and thiosulfate) Thiothrix can proliferate.
Beggiatoa
Beggiatoa, a filamentous
In an emergency situation or while these factors are being investigated chlorine & hydrogen peroxide may be used to provide temporary help.
Chlorination of return sludge has been practiced as a means
of controlling bulking sludge caused by filamentous organisms.
It is ineffective when bulking is due to light floc containing bound water.
Chlorination of return sludge in the range of 2-3 mg/L (in
Occasionally sludge that has a good settling
characteristics will be observed to rise or float to the surface.
The cause of this phenomenon is denitrification.
As nitrogen gas is formed in the sludge layer, much of
it is trapped in the sludge mass & sludge rises or floats.
Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids.
Rising sludge problems may be overcome by
Increase return activated sludge withdrawal rate (so
reduce detention time of the sludge in the clarifier)
Increase the speed of the sludge-collecting
mechanism in the settling tank
Increase sludge wasting rate (consequently
Nocardia Foam
A viscous brown foam that covers the aeration basins &
secondary clarifiers has produced many problems, including safety hazards, odors and changes in effluent quality.
The foam is associated with a slow growing filamentous organism usually Nocardia genus.
Nocardia, Type 1863 Nocardia-like Filamentous Bacteria in Activated
Foam on weirs
foaming in an aeration basin
Probable causes:
Measures for nocardia control include
1. Reducing sludge age (most commonly used)
2. Reducing the air flowrate to lower the depth of foam accumulation
3. Adding a selector compartment to control the growth of filamentous organisms
4. Injecting a bacterial additive
5. Chlorinating the return sludge
6. Spraying chloride solution or sprinkling powered calcium hypochloride directly onto the foam
The selector concept entails the selective growth of
floc-forming organisms at the initial stage of the biological process by providing a high F/M ratio at controlled DO levels.
A selector is a small tank (20 to 60 min contact time) or a
series of tanks in which the incoming wastewater is mixed with return sludge under aerobic, anoxic, and anaerobic conditions.
The Selector Concept
The high substrate concentration in the
selector favors the growth of nonfilamentous
organisms (see Fig. 8-13).
Nonfilamentous forms
Filamentous forms
Selector designs are based on either kinetic or metabolic mechanisms
1. Kinetics-Based Selector:
Selector designs based on biokinetic mechanisms provide for reactor substrate concentrations that result in faster substrate uptake by the floc forming bacteria.
While filamentous bacteria are more efficient for substrate utilization at low substrate concentrations, the floc forming bacteria have higher growth rates at high soluble substrate concentrations.
The kinetics-based selector designs are
2. Metabolic-Based Selector:
With biological nutrient removal processes, improved sludge settling characteristics, and, in many cases, minimal filamentous bacteria growth has been observed.
The anoxic or anaerobic metabolic conditions used in these processes favor growth of the floc forming bacteria.
Similarly, the filamentous bacteria do not store polyphosphates and thus cannot consume acetate in the anaerobic contact zone in biological phosphorus removal designs, giving an advantage for substrate uptake and growth to the phosphorus-storing bacteria.
ACTIVATED SLUDGE
MODIFICATIONS
1. Conventional Activated Sludge Treatment
A typical flow pattern is
As the tank geometry is long and narrow the mixing regime approaches plug-flow.
Operating experience soon revealed a number of problems with this design.
For example as the biomass was recycled back to the head of
the aeration tank and there mixed with incoming WW it was observed that the oxygen requirements at this point often exceeded the capability of the aeration system.
It was also found that such a flow arrangement increased the probability of process failure due to shock loads of toxic or high-strength waste because these loads were controlled at the enterance.
Because of these deficiencies numerous modifications have
2. Tapered Aeration
This modification is identical to the conventional process. The basic difference between the two process is in the
diffuser arrangement.
In tapered aeration diffusers are spaced so that more air is supplied at the head of the tank, where the oxygen demand is greatest, and is then decreased.
It is more economical than conventional.
effluent influent
In this modification return sludge is mixed with a portion of the
WW and enters the head of the aeration tank.
WW is also fed into the tank at different points along its
length.
Advantages are
3. Step Aeration
Aeration tank
Return sludge
a) Better equalization of waste load b) Lower peak O2 demand
This process is an application of the flow regime of a continuos flow stirred tank reactor (CSTR).
The organic load on the aeration tank and the oxygen demand are uniform throughout the tank length.
Because of the rapid blending of feed and tank contents, this process is highly resistant to shock loads.
5. Extended Aeration
Extended aeration plants are generally small (applicable to flow less than 1 MGD) because of the large aeration tanks volumes required.
Since it operates in endogenous respiration phase of the growth curve which requires low organic loading & long aeration time.
Theoretically the extended air process is designed such that all substrate removed is channeled into energy
metabolism & oxidized so that no excess biomass is produced & sludge handling is eliminated.
Lag Log Declining Endogenous
High Rate
Conventional Step Aeration
Contact Stabilization Complete Mix
Contact stabilization uses two separate tanks for the treatment of the WW and the stabilization of the activated sludge
6. Contact Stabilization
Contact tank (adsroption)
Complete mix
Complete mix
Stabilization tank
The first tank provides contact between the biomass and the WW for a short period of time 20-40 min.
In the second aeration tank, the organic material which is adsorbed on the biomass surface is metabolized or “stabilized” (retention time 4-8 hr)
The flow scheme for sludge reaeration is similar to the
contact stabilization. But in this case it is assumed that all
substrate entering the reaeration tank is removed.
Thus no substrate will be present in the recycle from the reaeration tank to the aeration tank.
7. Sludge Reaeration
Aeration tank
Complete mix
Complete mix
For this particular process modification, a low MLSS
concentrations are combined with high volumetric BOD loadings.
This system is characterized by short HRT, high sludge recycle ratio, high F/M loading.
The subtrate removal efficiency is low (typically 60-75 %) mainly because the plant effluent generally contains a high solids concentration. (This high solids concentration is a result of the physiological state of the organisms in the aeration tank.)
Therefore, the high rate process can not be used where a
Lag Log Declining Endogenous
High Rate
Conventional Step Aeration
Contact Stabilization Complete Mix
High quality oxygen is used in stead of air in the activated sludge process.
This system is based on the principle that the rate of transfer of oxygen is higher for pure oxygen than for air.
This results in higher availability of dissolved oxygen
leading to improved treatment and reduced production of sludge.
The oxidation ditch consists of a
ring or oval channel and is equipped with one or more
rotating rotors for WW aeration.
Screened WW enters the ditch, is
aerated and circulates at about 0.25-0.35 m/s.
Typically operation an extended aeration mode with long
detention times.
11. Orbal Process
The orbal process is a variation of the oxidation ditch.
And uses a concentric channels with the same structure.
Wastewater enters the larger outer channel and flows toward the center through at least two more channels before entering an internal clarifier or a
This is a fill and draw type reactor
system.
In SBR operation the process are
carried out sequencially in the same tank.
SBR systems have five steps in
common.
5. Idle 1. Fill
2. React
3. Settle(sedimentation clarification
13. Biolac Process
Biolac® is a process that combines long solids retention times with submerged aeration in earthen basins.
A major advantage of the Biolac® system is its low installed cost.
DESIGN EXAMPLE
Design a complete-mix activated sludge system Given: Avarage design flow: 0.32 m3/s (6.3 Mgal/d)
Peak design flow: 0.8 m3/s (21.9 Mgal/d)
Raw WW BOD5 : 240 mg/L Raw WW TSS : 280 mg/L Effluent BOD5 20 mg/L Effluent TSS 24 mg/L WW Temperature : 20 oC
Operational parameters & biokinetic coefficients:
θc = 10 d, MLVSS=2400 mg/L (can be 3600 mg/L), VSS/TSS =0.8 TSS conc. in RAS =9300 mg/L, Y = 0.5 mg VSS/mg BOD5, kd= 0.06 /d BOD5/ultimate BODu = 0.67
Assume: 1) BOD5 and TSS removal in primary clarifiers are 33 & 67 % respectively.
2) Specific gravity of the primary sludge is 1.05 and the sludge has 4.4 % of solids content
3) Oxygen consumption is 1.42 mg per mg of cell oxidized.
Note:
Secondary clarifier design
Pilot plant study for settling tank
MLSS (mg/L) 1200 1800 2400 3300 4000 5500 6800 8100 Initial settling
velocity m/h
4.1 3.1 2.1 1.2 0.77 0.26 0.13 0.06
Plot MLSS settling curve on log-log paper.
İ
X MLSS (mg/L) V1 initial settling Velocity, m/h (X*V1) Solid flux kg/(m2.h)
1000 4.2 4.2
1500 3.7 5.55
2000 2.8 5.6
2500 2.0 5.
3000 1.5 4.5
4000 0.76 3.04
5000 0.41 2.04
6000 0.22 1.32
7000 0.105 0.74
8000 0.062 0.5
9000 0.033 0.3
2 4
Solids flux (kg/m2.h)
Solids conc. in return sludge mg/L
2000 4000 6000 8000 10000
Determine limiting solids flux for an underflow concentration of 9300 mg/L (desired underflow)
Design flow to secondary clarifiers, Q
Q = avarage design flow + return sludge flow – MLSS wasted = 27563 + 12942 – 283 m3/d
= 40222 m3/d
Use two clarifiers each one w/ flow of 20200 m3/d
Area of clarifier
SF
X
Q
A
A: area of the secondary clarifier m2 Q : influent flow of the clarifier m3/h X: MLSS concentration kg/m3
SF: limiting solids flux kg/m2.h
For each clarifier 2
2 3 3 1942 . / 3 . 1 * ) / 1000 * 1 / 24 ( ) 8 . 0 / / 2400 ( / 20200 m h m kg kg g d h m g d m
A
A= π.r2 =1942 r ≈ 25 m
Determine recycle ratio required to maintain MLSS conc at 3000 mg/L (Q+Qr) 3000 = Q X + QrXu Q (3000-X) = Qr (Xu-3000)
