What is the Metabolic Selection ?...
By "Hasan Sarptas" from our Department...
Bulking Sludge Problem in Activated Sludge Process...
The principle objectives of the biological treatment of wastewater are to coagulate and remove the nonsettleable colloidal solids and to stabilize the organic matter. It is important to note that, unless the cell tissue produced from the organic matter is removed from the treated wastewater, complete treatment has not been accomplished because the cell tissue, which itself is organic, will be measured as BOD in the effluent. If the cell tissue is not removed, the only treatment that has been achieved is that associated with the bacterial conversion of a portion of the organic matter originally present to various gases end products. From this point of view, separation and removal of the mixed-liquor suspended solids from the treated wastewater is a critical operation affecting overall activated sludge efficiency.

Formation of bulking sludge in the aeration tank of activated sludge process is one of the most important problems decreasing the overall performance of activated sludge process. Bulking is the term applied to a condition in which an overabundance of filamentous organisms is present in the mixed liquor in the activated sludge process. A proliferation of filamentous organisms in the mixed liquor causes the biological flocs to be bulky and loosely packed and results in poorly settling sludge, commonly termed “bulking sludge”. The bulky flocs do not settle well and are often carried over in great quantities in the effluent from the sedimentation tank.
Types and Principal Reasons of Bulking Sludge...
Two principle types of sludge-bulking problems have been identified. One is 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. The other is caused by bound water, in which the bacterial cells composing the floc swell through the addition of water to the extent that their density is reduced and they will not settle. The causes of sludge bulking that are most commonly cited in the literature are related to ; ( a ) The physical and chemical characteristics of the wastewater, ( b ) Treatment plant design limitations and ( c ) Plant operation.
Conditions Favoring the Growth of Filamentous Organisms...
It has been found that the relative abundance of filamentous and nonfilamentous (floc-forming) organisms is related to their relative growth rates when exposed to the varying concentration of substrate. Nonfilamentous floc-formers have a high mu_max but a low affinity for the substrate (high K_s), whereas the filamentous forms have a low mu_max but a high affinity for the substrate (low K_s). Thus, the low substrate concentrations found in the complete-mix reactor favor the growth of filamentous microorganisms.
Control of Filamentous Organisms / Bulking Sludge...
Control of filamentous organisms has been accomplished in a number of ways including the addition of chlorine or hydrogen peroxide to the return waste-activated sludge, alteration of the dissolved oxygen concentration in the aeration tank, alteration of the points of waste addition to the aeration tank to increase the F/M ratio, the addition of major nutrients, the addition of trace nutrients and growth factors, and more recently the use of selectors. Selector application for the control of filamentous organisms is discussed following sections.
Selector : Definition & Principles...
The single-stage complete-mix system in particular tends to promote the growth of filamentous organisms because of the low-substrate levels uniformly present in the reactor. The use of selector, in which the primary effluent and return activated sludge are combined, increases the F/M ratio or substrate level in the reactor. Selector, which can be run as aerobic, anoxic or anaerobic conditions, is either a separate compartment (a small volume-tank prior to the main aeration unit) or the initial contact zone of the biological reactor of the activated sludge process.
Selection Mechanisms...
1. Kinetic Selection : In kinetic selection mechanism, the environmental conditions in the selector are controlled to select for a specific type of organism (nonfilamentous & floc-formers) to cope with bulking sludge problem in activated sludge process. A substrate gradient is created which selects for floc-forming bacteria in two ways. Firstly, the difference in growth rate between floc-forming bacteria and filamentous bacteria is used. At high substrate concentrations, floc-forming bacteria have a higher utilization rate than filamentous bacteria, and since a high initial concentration of substrate is created in the selector, the floc-forming bacteria are favored. Secondly, the higher ability of the floc-forming bacteria (compared to filamentous bacteria) to accumulate substrate is used. When the configuration of the activated sludge process is made in this way, the sludge adjusts to the situation and develops the ability to rapidly take up substrate and store it internally for later use. The floc-forming bacteria can more readily adjust to these circumstances than the filamentous organisms, and thereby the floc-forming bacteria are selected.

2. Metabolic Selection : With the development and introduction of activated sludge modifications for biological nutrient removal, the effect of anoxic and anaerobic conditions on the growth of filamentous microorganisms was also studied. The selection mechanism based on the ability of microorganisms to utilize carbon and energy sources under different cultivation conditions is called as metabolic selection. Metabolic selection can affect the composition of activated sludge biocenoses whenever the cells can obtain energy by means of more than one metabolic process. For metabolic selection, the most important processes are ; ( a ) Nitrate respiration and dissimilative nitrate reduction (denitrification) and ( b ) Enhanced biological phosphorus removal (EBPR). The direct role of nitrification in the competition between floc-formers and filaments can be neglected because of low energy yield in comparison with the metabolism of carbonaceous compounds. In addition, nitrifying bacteria apparently are not involved in floc formation. On the contrary, they need well-established flocs to which they can attach. However, nitrification can affect the composition of activated sludge biocenoses indirectly. The nitrifiers, as slow-growing microbes, require an increased biomass retention time in the system, and nitrification is most effective when the organotrophic bacteria are in the endogenous metabolism phase. These specific conditions favor the growth of starvation resistant microorganisms.
The metabolism of sulphur compounds is important only when there are elevated concentrations in wastewaters. Metabolic selection can be expected in all nutrient removal systems in which the activated sludge is alternately exposed to anaerobic/anoxic and oxic conditions. The alternating exposure of activated sludge microorganisms to different cultivation conditions can also occur in fully oxic activated sludge systems with high substrate concentration gradients. The principle of metabolic selection first formulated for systems with anaerobic zones can be generalized for all nutrient removal activated sludge systems; In nutrient removal activated sludge systems, the microorganisms are equipped with such an enzymatic apparatus enabling them to obtain energy by all possible metabolic mechanisms more efficiently than the other microorganisms will be selected. As the decisive fraction of available substrate is removed under anaerobic and/or anoxic conditions the biocenoses of activated sludges from nutrient removal systems will be dominated by microorganisms utilizing substrates under these conditions. The question of practical importance is which filamentous microorganisms can be selected according to the above principle.
Advantages & Disadvantages of Selectors Operated Under Anoxic Conditions...
The anoxic selector, requiring the presence of nitrate in the water, is often the choice for activated sludge systems that nitrify. In addition to providing effective control of filamentous bacteria, anoxic selectors provide the benefits of reducing process oxygen requirements, since nitrate-nitrogen is used as a terminal electron acceptor for the oxidation of influent biodegradable organic matter, and reducing the consumption of alkalinity during nitrification, as a result of recovery of alkalinity in the anoxic zone. Moreover, anoxic selectors can be quite effective in controlling filament growth because they use both kinetic and metabolic selection mechanisms. Marten & Daigger have reported that despite these benefits, recent experiments indicate that anoxic selectors are not always effective in controlling filamentous sludge bulking. Filamentous organisms, such as Microthrix parvicella and types 0041, 0092, and 0675 are sometimes controlled, but not in all cases. Thus, studies are needed to quantify the selector operating conditions necessary to control these filaments. Unlike the anaerobic/oxic selective mechanism, metabolic selection under anoxic conditions does not necessarily require alternation between anoxic and oxic conditions. For balanced growth, the substrate is taken up and metabolized directly in anoxic conditions. Of course, balanced growth under anoxic conditions has no practical meaning, but was used as an experimental proof of this phenomenon. In actual activated sludge plants, the alternation between anoxic and oxic conditions is unavoidable when nitrification and denitrification are required. In addition, most denitrifiers exhibit accumulation/storage capacity during unbalanced growth under anoxic conditions, so that the subsequent oxic conditions are necessary for restoring this capacity.

An anoxic environment brings about an improvement in sludge settling characteristics probably due to the decrease in longitudinal mixing. It has been reported that during the hydraulic retention time in the anoxic zones of about 25 to 30 minutes in an experimental study about anoxic selector a substantial proportion of the nitrate in the recycled sludge has been utilized as an oxygen source by denitrifying organisms. It is possible that the selection of microbial flora is affected by the denitrification reactions, which occur, or by the adsorption of substrate by organisms in the absence of normal aerobic metabolism. Recent worldwide wastewater treatment practice confirms the conclusions from lab-scale and pilot plant experiments. When filamentous microorganisms from Group S and Group C caused the bulking troubles, the introduction of anoxic cultivation conditions always resulted in very fast and effective improvement of settling properties. It has been also reported that the low SVIs can be achieved in an anoxic zone without substrate concentration gradient and when the anoxic zone is partially aerated the SVIs immediately started to deteriorate.
Advantages & Disadvantages of Selectors Operated Under Anaerobic Conditions...
In the anaerobic selector, internally stored polyphosphate is released. This favours growth of floc-forming microorganisms that can take up and store substrate using polyphosphate hydrolysis for energy generation (e.g. Acinetobacter). Many types of filamentous bacteria are unable to do this and thereby they are not favoured in an anaerobic zone. The metabolic selection of activated sludge microorganisms in alternating anaerobic and oxic conditions is connected with the mechanism of enhanced biological phosphorus removal, especially with the capability of transferring energy chemically bound in polyphosphates and intracellular storage products between anaerobic and oxic conditions. As building blocks for the synthesis of organic storage products are scarce only in municipal wastewaters, the poly-P bacteria should be accompanied by fermentive bacteria, which can generate low-molecular weight compounds from wastewater organics. Both fermentive and poly-P bacteria generally contribute to a better flocculation of activated sludges. Thus, the more available substrate is removed in an anaerobic zone of activated sludge systems; the better should be the activated sludge settling properties. It has been noted that substrate removal under anaerobic conditions has a positive effect on activated sludge settleability. It has been also proved that no substrate gradient is necessary in anaerobic conditions for suppressing the growth of such filamentous microorganisms as Sphaerotilus natans and Type 021N.