Chapt14_IntroductiontoActivatedsludgetreatment[1]

7/29/2019 Chapt14_IntroductiontoActivatedsludgetreatment[1]

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cmutsvangwa: Wastewater Engineering, Dept. of Civil & Water Engineering, NUST 11/10/2006 14-1

ACTIVATED SLUDGE TREATMENT

IntroductionDuring 1913/1914, Arden and Locket aerated settled sewage over an extendedperiod to produce activated sludge, which when mixed with wastewater andaerated, would bring about nearly complete stabilization in few hours. The activatedsludge consists of flocculent mass of bacteria intimately mixed with thepolysaccharide shells of dead microorganisms. This technology has been applied inthe treatment of wastewater and can be described as dispersed suspended growthsystem comprising of a mass of microorganisms and wastewater. Themicroorganisms are kept in intimate contact with the wastewater by mixing. Themixing and the incoming wastewater constantly supply the organic matter andoxygen for the microorganisms. The mixing apparatus is also responsible forkeeping the suspension aerated. The microorganisms (saprophytic bacteria)

convert organic matter and oxygen to ammonium salts, water, CO2, new bacterialcells and energy. The microorganisms are constantly being washed out of thereactor by the flow of the incoming wastewater and settle as sludge in thesecondary sedimentation tanks. A fraction of this settled sludge (activated) isrecycled back to the reactor to provide enough biomass to achieve BOD removaland the other fraction, which is not recycled (excess), is wasted (Fig. 1).

Sedimentation tank

Bioreactor

(MLSS)

Inflow wastewater

FinalEffluent

Air

Qwaste

Return activated sludge (RAS)

Fig. 1 Schematic illustration of activated sludge process

The unique merit of this process is that it produces an effluent, which complies formost of the times with very stringent requirements of the Water Regulations for

discharge directly into a watercourse. The process requires less land compared toother treatment processes. Furthermore, the process is associated with less odoursand health risks. However the process has the following constrains:

Employs more mechanical and electrical plant making it rather expensive.

Requires more skilled manpower to design, construct, supervise andmaintain.

Chapter 14 Introduction to activated sludge treatment

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Components of Activated Sludge Process

Reactor in which the wastewater to be treated is in contact with theactivated sludge.

Means of transferring oxygen to the reactor

Means of agitation to ensure sufficient contact between activated sludge,wastewater and oxygen

System to separate the activated sludge from the purified wastewater

System to recycle the activated sludge to the reactor.

Methods of aeration

Air DiffusionDust free air is passed through narrow pore diffusers at the bottom of the mixedliquor tanks. The small bubbles produced have a high surface area to volume ratio,which encourages oxygen transfer to the liquid phase as the oxygen bubbles.Common types of diffusers are dome shaped and made of ceramic material and

silica. There are about 178mm in diameter with pore sizes of 150m. Thedisadvantage of diffusers is that they get blocked from time to time and hence needsome form of cleaning. Since the diffusers are at the bottom, it becomes difficult toidentify quickly any blockages.

Surface aeratorsThey can either be vertical or horizontal and they are low head volume centrifugalpumps, which create an outward radial torrential flow of MLSS across the tanksurface (Fig. 2). The turbulence produced increases the surface area of contactbetween the MLSS and air enhancing the uptake of atmospheric oxygen.

Fig. 2 Vertical and horizontal shaft aerators


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Parameters of Design and operation

Mixed liquor suspended solids (MLSS)This constitute the reactor contents and comprises of mass of activated sludge

solids per unit volume of aeration channel and ranges from 1.5 to 5 kg/m3depending on air supply, settleability of sludge in final clarifies, return sludgepumping capacity and size of tanks.

Sludge age (Mean cell residence time)The fraction of the activated sludge solids which is wasted determine the averageamount of time which the microorganisms will spend in the bioreactor

Sludge age =the total solids in the reactor (kg) , daysdivided by the solids wastes (kg/days)

The sludge age varies from 2 days in high rate plants to more than 30 days inextended-aerated plants.

Organic loadingIt is the organic amount (BOD) applied per unit volume of aeration tank and variesfrom 0.4 to 2 kg BOD/day.

Organic loading =BOD of wastewater (kg/m3) x influent flow (m3/d) ,kg/BOD/days

Reactor volume (m3) x reactor solids (kg/m3)

Hydraulic retention timeThe time the wastewater spends in the reactor and it varies from 3 to 48 hours andcan be computed from:

= Reactor volume (m3)Flow (m3/day)

Food: Microorganism ration (F/M)The ration of the food substrate to the microorganisms in the reactor.

F/M = BOD of wastewater (kg/m3) x influent flow (m3/d) , days-1

Reactor volume (m3) x Reactor solids (kg/m3)

= BODxQV x MLSS

The F/M ratio is the only the form of loading over which the operator has control. Itgives a good indication of the state of the plant without reference to other more


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variable parameters. An operator has some control by adjusting the proportions ofsludge wasted and returned. Increasing sludge wastage rate will cause an increasein the F/M ratio. The F/M ratio varies from 0.05 to 1 day-1.

Sludge settleability

The degree of treatment achieved in an aeration process depends directly onsettleability of the activated sludge in the final sedimentation tank. A biological flocthat settles leaves a clear supernatant for discharge and poorly flocculated particlescontribute to suspended solids and BOD in the effluent. The excessive carryover offlocs due to poor settleability is referred to as sludge bulking and is caused by:

Insufficient aeration

Return sludge capacity

Presence of toxic substances

Overloading

High F/M ratio

Fluctuation in flow and strength of wastewater

Short-circuiting

Sludge volume index (SVI)A measure of the settleability of sludge and is the volume in ml occupied by 1 gramof settled suspended solids. A method to determine the SVI is schematicallyillustrated in Fig. 3. MLSS samples are drawn from reactor near the discharge endand filled into a one-liter cylinder. The initial concentration of MLSS is noted. Thesample is allowed to settle for 30 minutes and the volume occupied by the settledsolids is read. An SVI of 50 to 150 ml/g indicates a good settling sludge. The SVI iscalculated as:

SVI = Vx1000 ml/gMLSS

Where V =Volume of settled solids, mlMLSS =Suspended solids in mg/l

Reactor (MLSS)

Return activated sludge, Qr

Qe

Volume ofsettled sludge

Qi Qi+Qr

Fig. 3 Schematic illustration on how to determine the SVI


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By performing a mass balance on Fig. 3, the quantity of return sludge can beestimated as:

1000

V

QQ

Q

r

r

=+

Where; Qr =flow of recirculated activated sludge, m3/dayQ =average flow of wastewater to aeration basin, m3/dayV =volume of settled solids in 1 liter graduated cylinder, ml/l1000 =ml/l

The suspended solids in the recirculated activated sludge:SVI

SS1000000

=

SVI in ml/gram and SS in mg/l

Rising sludgeThis refers to a sludge, which will rise after settling and float to the surface afterrelatively a short settling period. This is due to the denitrification in which thenitrates are converted to gaseous nitrogen. The nitrogen gas is formed in thesludge layer and much of it is trapped in the sludge mass. If enough gas is formed,the sludge becomes buoyant and rises to float at the top. Rising sludge cab bedifferentiated from bulking sludge by noting the presence of small gas bubblesattached to the floating solids.

Rising sludge can be overcome by:

Increasing the return activated sludge withdrawal from the clarifier to reducethe detention time of the sludge in the clarifier. This reduces the sludgedepth thereby reduce the anoxic conditions which are conducive for thedenitrification.

Decreasing the rate of flow of the mixed liquor into the offending clarifier

References

1. Droste L., (1997), Theory and Practice of Water and Wastewater Treatment,J ohn Wiley, Canada

2. Ellis K., (1995), Unpublished Lecture Notes in Wastewater Engineering,

Loughborough University, UK3. Gray N., (1992), Biology of Wastewater, Oxford Science, UK4. Mara D., (1976), Sewage Treatment in Hot Climates, J ohn Wiley, UK5. Mara D., (1997), Design of Waste Stabilisation Ponds in India, Lagoon

Technology, UK6. Metcalf and Eddy, (1991), Wastewater Engineering, Treatment, Disposal and

Reuse, McGraw Hill, US


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Chapt14_IntroductiontoActivatedsludgetreatment[1]