ACTIVATED SLUDGE

ACTIVATED SLUDGE• About 100 years ago, engineers noticed that re-

suspension and re-aeration of the settled sludge from secondary clarifiers in a fresh batch of supernatant from primary clarifiers caused the BOD to drop.

• Secondary clarifiers left a purified supernatant and active sludge. The sludge could be recycled again and would become activated with re-aeration with new supernatant from the primary clarifiers.

 • “Activated mass of microbes capable of

aerobically stabilising the organic content of a waste”

• AS is probably the most widely used biological process for the treatment of organic and industrial waste waters.

• The modern , AS process simply creates an agitated environment where the same microbes are able to stabilise the degradable material at a fast rate than before.

• Suspended and colloidal material is removed rapidly from the waste water by adsorption and agglomeration on to the microbial flocs.

The general processes that go on

– Stabilisation – slow breakdown of adsorbed materials– Mineralisation – conversion of nutrients to

substances like carbon dioxide– Assimilation – conversion of nutrients to cell material– Endogenous respiration – microbial mass converted

to new cell material for new cells.

• The flocculent microbial mass is called Activated Sludge although it obviously contains some non-active waste components as well. This material can be separated from the treated waste water effluent by a clarification stage.

Essential features of the activated sludge process

Aeration Stage

SludgeSeparat

ion

InputTreated Effluent

Sludge Recycle (RAS)

Waste Sludge (WAS)

Pre-aeration

stage

Is the AS process just a fermenter?In some respects the AS process is like a

normal bioreactor • With the AS process the microbes tend to

be on their last legs, hence flocculation and use of nutrients only for energy purposes. The fact that they ‘floc’ together enables them to settle out during clarification.

• The AS process seeks to remove material whereas fermentation seeks to create it.

• The [nutrient] in waste water effluent is very low compared with a fermenter. The microbes are practically starving!

• AS has lower land-space requirements.

Aeration of the Activated sludge

Batch closed reactor

Simplest[homogeneous] of mixed liquor suspended solids (mlss)

• mixed liquor to clarifier is the same in an aerator

Plug FlowWorks out as the equivalent to 8 aerator tanks

• F/M (Food to microbe ratio) loading is very high since the residence time is long

Tapered Aeration

Decreasing numbers of aeration inlets to correspond to decreasing F/M ratio

• The possibility of an anoxic zone for de-nitrification

Step Feed

Stepped input of effluent to control F/M ratio and getting steady BOD load

• Good for handling shock loads

Oxidation Ditch

Mechanical surface aerator moves around the circuit keeping the solids in suspension and well aerated. Carousel arrangement lengthens the residence time and allows an anoxic zone to be established somewhere in the circuit.

Sequencing Batch Reactor

Aeration and clarification in one stage

• Hydraulic loading and [organic] loading can be controlled.

• Other systems can suffer from overloading of one or more components which could create undesirable anoxic zones or cause microbial colonies to die out

PHOSTRIP process (1960-70’s)

• Aerobic zone allows carbonaceous oxidation, nitrification and luxury uptake of phosphorus.

• Anoxic zone strips NO3- to N2

(denitrification)• Anaerobic zone (no oxygen, nitrates,

nitrites) causes microbes to release orthophophate into solution This phosphate rich supernatant is extracted and mixed with lime to generate a Calcium phosphate precipitate.

• Clarifier – separates out the phosphate rich AS for recycle.

Stripper

RAS

WAS

Aerobic

Anaerobic

Anoxic

Primary Settlem

ent

AerationTank

Secondary

Settlement

RASLime

ADVANCED ACTIVATED SLUDGE PROCESSES

•Most of the advanced techniques are improvements on the AS process•Increased metabolic rates have been achieved by increasing the partial pressure of oxygen in the gas phase, thereby increasing the rate of oxygen transfer from the gas phase to the liquid phase.

– Run @ elevated pressure including the gas phase. Use an enriched oxygen supply e.g. pure oxygen instead of air, or even using ozone (O3). E-coli and Bacillus subtilis, they die at elevated oxygen levels (>35 g m-3) when at high pressures

DEEP SHAFT

• An intensive plug flow activated sludge process using a 200 metre column filled with waste water effluent.

• High [microbe] are required hence the oxygen transfer requirements are around 10 kg m-3 hr-1.

• Air is injected co-current somewhere down the shaft to promote the flow as well as increase the [oxygen] in solution.

•Towards the surface, the pressure decreases which causes various gases to come out of solution and form bubbles.

•At the surface, ‘de-bubbling’ is encouraged since it is has a low [oxygen] and hence won’t promote further metabolic reactions and reduce the efficiency of the flow in the initial downstream section of the shaft.

•Liquor that isn’t recycled is taken through further degassing stages (under vacuum) to remove micro-bubbles that could prevent efficient clarification.

DEEP SHAFT

UNOXThe UNOX process consists of an enclosed oxygenation channel divided into compartments to create multiple-stage oxygenation steps where the oxygen flow co-current with the liquor. 

VITOX 1Oxygen can be introduced to the high pressure liquor as it passes through a venturi. The oxygenated liquor is then fed through an expansion nozzle. Oxygen utilisation is around 95%.

VITOX 2• Oxygen is injected into

the effluent liquor as it flows upwards. The liquor is then dropped into a bell diffuser.

• The largest bubbles move upwards the smallest bubbles are carried along with the liquid flow and eventually dissolve producing a high[oxygen] liquid which is then deposited into a reaction tank.

Combination methodsFluidised Bed Systems• These are a combination of submerged and

attached growth systems. A biofilm is established on small mineral particulates of about 0.2-3mm in size or fine wire meshes.

• Undue build up of slime is less likely because the particles are constantly moving about reducing the slime layer to ‘optimum’ thickness.

• This system is better for ensuring efficient BOD removal all year round.

• Standard static trickling beds are slower in the winter and may freeze. Fluidised bed systems are fairly steady throughout the year.

Oxygenation

FluidisedBed Reactor

Wastewater Effluent Input

Biomass Separation

Treated Effluent

Excess Sludge

Submerged Biological Aerated Filters

• Combination AS and filtration techniques • A biofilm is attached to a submerged support

medium which can float around in a vessel into which waste water effluent is added.

• Some packing is buoyant.• Other are designed to sink providing a

microbial population in the bottom of the reactor.

• SBAF occasionally requires sparging with excess air flow or backwashing in the event of excess biogrowth.

• Loading rates are around 0.3-2.0 kg BODm-3d-

1. Nitrification has similar loading levels.

The future?

•Enhanced microbes using Recombinant DNA?…

•Combination of Deep shaft with other techniques

•Advance of Supercritical Water oxidation techniques