For wastewater treatment biologically, need to understand type of wastewater. Depends on waster type biological organism can be used for treatment.

In a well-maintained aeration tank, the bacteria are concentrated in the flocculent material of the activated sludge, although some always occur free in the wastewater. The flocs are formed from aggregates of non-living organic polymers

that are probably secreted by bacteria. They have an open porous structure, and are sufficiently robust to withstand the shear forces created by water movement, during aeration of the tanks. They vary in size from less than 10 µm up to 1mm (1000 µm). The bacteria are adsorbed on to the internal and external surfaces of the floc, and a medium sized floc may harbour several million bacteria. Immediately after the wastewater enters the aeration tank, the fine particulates, colloidal particles and large molecules, become entangled with, and adsorbed to, the floc material. The bacteria of disaggregated flocs may continue to grow when the oxygen concentration of the mixed liquor is only 0.6 mg O2/l, whereas to ensure this concentration on the inside of a large floc, a mixed liquor oxygen concentration of 1.2 – 2.0 mg O2/l may be required. In a well-operated plant, most of the bacterial biomass is associated with the activated sludge floc. By filtering and drying a sample of the suspended solids, and then weighing the dried residue, a measure of the biomass may be obtained. It is referred to as the Mixed Liquor Suspended Solids.

Suspended growth processes:

The microorganisms responsible for treatment are maintained suspension

by appropriate mixing methods.

Attached – growth processes :

The microorganisms are attached to some inert medium, such as rock, slag or specially designed ceramic or plastic materials.

Both processes have its own adv and disadv. Like suspended gr. it requires larger area, no small equipments involved, monitoring is tough etc. whereas in attached system small equipments r involved, no larger area necessary, maintainence is easier but labour intensive, cleaning systems available r mutiple n so on....Pretreatment processes, Type of filter media, Recirculation rate , Hydraulic and organic loadings applied to the filter, Underdrainage and ventilation systems play an imp role here....

refer:

http://www.chemistry.sc.chula.ac.th/course_info/2302547/biological.pdf

It depends on the type of wastewater you wish to treat and what you wish to accomplish. Both have advantages and disadvantages, which can be found in any text book on the subject. Suspended growth systems are the most widely used for domestic wastewater treatment and rely on good formation of flocs to settle out nutrients in the form of biomass. Process parameters need to be well controlled to make sure the floc structure is good.

As Pamela states “suspended growth systems are the most widely used for domestic wastewater treatment”. This is mainly because it is more cost effective for large scale treatment. Suspended growth (activated sludge) takes skill and continuous control to maintain good sludge structure and thus efficiency.

Attached growth systems are used mainly for compact systems where space is limited or for small scale treatment as they are easier to keep stable.

Attached growth (biofilm) has some advantages in being able to maintain bacteria that cannot survive in suspension, which gives new functionality such as the ability to nitrify at low temperatures (Hallvard Ødegaard) or degradation of micro-pollutants that are otherwise not degraded(Per Falås).

Article Suspended biofilm carrier and activated sludge removal of ac...

Dear Chitta,

If your wastewater has weak character you should choose attached growth process. In suspended growth system, weak wastewaters may cause foaming, bulking problem. Especially nutrients must be adequate in your wastewater for suspended growth system. Please inform us about your wastewater characteristics.

Hello chitta,

As per your wastewater characteristics and scale, you can choose either one and also both one. Many times suspended and attached biomass both can used for treatment.

Attached microorganisms have more resistant against improper conditions. If the system will face undesirable situation such as organic or hydraulic shocks, attached biofilm will be a good choice.

To decide  which one better you needed to know first type of pollutants (recalcitrant or biodegradable) in your wastewater and also the treatment process you will follow (batch or continuous flow). 

Chitta

The two systems differ in the way the biomass is immobalised.However the options are as folows.

Suspended (activated sludge ) is efficient ( BOD removal of > 95 to 98 % ) achievable.

Attached ( Trickling Filters or Bio Towers ) generaly remove 65 to 75 % and very rarely give > 80 %.

Acivated sludge systems can tolerate higher  suspended solids in the effluent ( without affecting performance ) .While  BT/TF will pass the high load SS affecting efficiency.

This is because we recycle the sludge in aeration tank while in TF/BT we recycle clarified effluent

AS is more energy intensive 5-7 KW/m3 as against 3-5 KW/M3 for TF/BT.This is because  in AS energy is required for Aeration & Recycly ( about 30-  50 %).For TF/BT energy is required only for recycle no aeration.

You therefore decide based on effluent inlet characteristics and the outlet quality desired.

If is low strength easy biodegradable go for TF/BT if high strength go for AS. Hope this clarifies your doubts

One more thing TF/BT can not be flushed  easily as  manty times if the media gets blocked, you may  have to remove and replace all the media.For AS it is easy to flush the tank.

If you requiremment is small best would be SBR 

Dr Pimparkar

May be what you can do is just to test these two different methods and see which one will give you the best results. Have your waste water as control and pass it through he two different treatments and analyze it at the end. The one that will give you the best results at the other end will be the best!