Filaments and Bacteria...
Volume - 4...

Filamentous Bulking...

Introduction...

The start of any problem solving has to involve microscopic examination of the activated sludge. This reveals whether the problem is, or is not, caused by filaments. If caused by filaments, identification of the causative filament(s) yields a direction or approach to take for a remedy. As shown in Table 1, the causes for filament growth include low oxygen concentration, low F/M, septicity, nutrient deficiency, low pH and high grease and oil. Control methods, based on the specific type(s) of filaments causing the problem, follow.

Filament Types as Indicators of Conditions Causing Activated Sludge Bulking...

Causative Condition (1)	Filament Types
Low Dissolved Oxygen (for the applied organic loading)	S. natans, type 1701 and H. hydrossis
Low Organic Loading Rate (low F/M)	M.parvicella, Nocardia spp., and types 0041, 0675, 1851 and 0803
Septic Wastes / Sulfides (high organic acids)	Thiothrix I and II, Beggiatoa spp., N. limicola II*, and types 021N, 0092*, 0914*, 0581*, 0961* and 0411
Nutrient Deficiency - N and/or P (industrial wastes only)	Thiothrix I and II and type 021N and N. limicola III
Low pH (< pH 6.0)	Fungi
High Grease / Oil	Nocardia spp., M. parvicella and type 1863
(1) Note that some filaments occur at several conditions.
* These filaments occur at lower F/M at septic conditions.

Low Dissolved Oxygen Concentration...

Low aeration basin dissolved oxygen (DO) concentration for the applied organic loading (F/M) leads to filamentous bulking by several filaments (see Table 1). The required aeration basin DO concentration to prevent "low DO bulking" is not a constant, rather, is a function of the F/M rate. Simply, higher bulk DO is required to prevent the growth of these filaments as the F/M increases, due to faster oxygen use within the floc at higher F/M, oxygen depletion inside the floc, and the need to maintain aerobic conditions in the interior of the floc. A higher bulk DO concentration increases the diffusion of oxygen into the floc interiors. In general, a bulk DO concentration of 2.0 mg/L is recommended for F/M values up to 0.5, typical of most domestic waste plants. This DO concentration should be maintained at the point of greatest oxygen demand in the system, for example, at the headend of a plug-flow system (not the backend). Some industrial waste systems and high rate domestic plants operated at higher F/M may need higher DO values than 2.0 mg/L due to oxygen diffusion limitations. Type 1701 bulking has occurred in an oxygen activated sludge plant operated at high F/M at a DO concentration of 12-14 mg/L, which was cured by raising the DO to 20 mg/L. As a rule, always trust the microscopic observation of low DO filaments to indicate oxygen limitation rather than the aeration basin DO values. Control of low DO bulking is by raising the aeration basin DO concentration and by raising the aeration basin MLSS concentration (decreasing the F/M). Note that this action is opposite to what intuition directs: to reduce the MLSS concentration, since less biomass needs less oxygen (wrong! -the F/M is increased at lower MLSS concentration and oxygen needs increase). Filamentous bulking is common in completely-mixed, lower F/M systems. Here, a number of filaments can cause bulking (see Table 1) because they grow better than most activated sludge floc-forming bacteria at low aeration basin BOD concentration. Intermittently-fed and plug-flow systems are more resistant to this type of bulking. Control of low F/M bulking is by reducing the aeration basin MLSS concentration and increasing the F/M (manipulating the "M" component). Lowering the MLSS concentration may not be suitable for many plants as this may cause the loss of nitrification and increase waste sludge production. Any change in operation that effectively increases the substrate concentration available to the activated sludge and introduces batch or plug-flow characteristics to the aeration basin, even on a short term basis, will help control low F/M bulking. These include: compartmentalization of aeration basins; fed-batch operation; intermittent feeding of wastes; and use of a selector. These latter methods do not reduce the MLSS concentration in the system. A selector is a mixing basin or channel where RAS and influent wastes mix prior to the aeration basin. Selector design is empirical at this time. Successful examples involve a 15-30 minute contact time of the RAS and influent waste; are aerated; and achieve at 70-80% removal of soluble BOD₅ through the selector. Several newer designs are either operated anoxic (no free oxygen but nitrate present) or anaerobic, however, these are too new to state their general usefulness. A selector can be too large or too small in size to properly function. The goal is to provide a short term, high substrate condition which favors certain floc-formers but which discourages filaments. These floc-formers appear to rapidly store BOD as cellular storage products in the selector, which they use later for growth in the main aeration basin (they pack their own "lunch bags" in the selector). If the selector is too large, the substrate concentration achieved may not be high enough to encourage these special floc-formers and discourage filaments. If too small, insufficient time may be available for substrate uptake and storage. Also, a selector that is too small may cause the floc-formers to shunt carbonaceous substrate to exocellular polymer which can increase the SVI of the sludge ("slime bulking") and pose problems in waste sludge dewatering. The best approach is to try several selector sizes, using a larger basin or channel with movable baffles or exit gates. Use of a variable wastewater bypass around the selector can achieve the same objective and allow the operator some control over the selector. Selectors are specific tools to combat low F/M filaments and are not needed by all plants. Inappropriate selector use may made the problem worse (for example, where bulking is caused by low DO, nutrient deficiency or waste septicity).

Septicity...

Influent wastewater septicity is usually indicated by odors (H₂S or "rotten egg" smell) and a dark color to the wastewater, caused by precipitated ferric sulfide. Septic wastes contain elevated amounts of sulfides and low molecular weight organic acids (such as acetic and butyric acids), both of which encourage the growth of certain filaments (see Table 1). Observation of these filaments with intracellular sulfur granules is a tip-off of a septicity problem. Septicity is more common in systems in warmer climates and in those with large wastewater collection systems that have lift stations and force mains. Waste septicity can be treated by preaeration (which releases odors), by chemical oxidation (chlorine, hydrogen peroxide or potassium permanganate), by chemical precipitation (ferric chloride), or use of sodium nitrate in the collection system as an "oxygen source". Septicity can originate within plant processes. Common sources of septicity include equalization basins, primary and secondary clarifiers, andèreturn streams from sludge processing. These can be tested for sulfide or organic acid concentration to determine whether they are a significant source of septicity. A sulfide concentration >1-2 mg/L and an organic acid concentration >100 mg/L favors filament growth. Sulfide can be tested for using one of the simple sulfide test kits available commercially (such as by HACH). Organic acids can be tested for using distillation and titration as per Standard Methods (the same test as for digesters).

Low Nutrients...

Nitrogen and phosphorus can be growth limiting if not present in sufficient amounts in influent wastewater, a problem with industrial wastes and not domestic wastes. In general, a BOD5:N:P weight ratio in the wastewater of 100:5:1 is needed for complete BOD removal. Other nutrients such as iron or sulfur have been reported as limiting to activated sludge, but this is not common. Signs of nutrient deficiency include: filamentous bulking (see Table 1); a viscous activated sludge which exhibits significant exopolysaccharide ("slime") when "stained" with India ink; and foam on the aeration basin which contains exopolysaccharide (which has surface active properties). One check for nutrient deficiency is to be sure that at least 1.0 mg/L total inorganic nitrogen (TIN = ammonia + nitrite + nitrate) and 0.5-1.0 mg/L ortho-phosphorus remains in the effluent at all times. In systems treating mixed domestic and industrial wastes, only TIN and ortho-phosphorus should be used to calculate nutrient availability. Organically combined nitrogen and phosphorus (Kjeldahl nitrogen and total phosphorus) may not be hydrolyzed fast enough by the microorganisms in the activated sludge to keep pace with BOD use.

Low pH...

The aeration basin pH should be maintained in the range 6.5 to 8.5. Low pH, <6.5, may cause the growth of fungi and fungal bulking. The aeration basin pH can be adjusted using caustic, lime or magnesium hydroxide.

Summary...

Control methods for filamentous bulking are based on, first, confirmation that the problem is indeed caused by filaments (some are not) and, second, identification of the causative filament(s). This information leads to specific remedies that can be used, appropriate for the filament(s) involved.