Floc formation problems and solutions | Getting better flocs biologically

We saw in our last article how bacteria lower COD and BOD (MAR- APR 2011 issue). COD/BOD reduction is achieved by microbial action on organic compounds.  However, complete BOD and COD removal is achieved only after microbial biomass settles down as sludge and is removed from the system as sludge. 

BOD and COD removal from the system after microbial treatment depends on settling characteristics of the formed sludge. The sludge settling in turn depends on efficiency of floc formation. Floc formation is a highly critical step in effluent treatment deciding its success or failure.  Let us try to explore few microbial aspects of floc formation and how microbiology can help us improve the efficiency of our settling tanks.

Flocs could be explained in simple words as aggregates or clusters of microbes which are stuck together. Microbes themselves being very tiny (1-3 microns) do not settle readily in water. Settling needs to be helped by flocculating agents which physically joins them as a single mass to make a floc. These flocculating agents help in clustering these microbes into a single particle improving floc settling.  Some microbes have the natural ability to make these flocculating agents and secrete them in water.  Use of such microbes in the product helps in achieving good flocculating characters naturally.

Some of the microbial genera that are known to enhance floc formation include Bacillus, Achromobacter, Citromonas Aerobacter , Alcaligenes , Flavobacterium Arthrobacter , Zoogloea . Bacterial surfaces are known to harbour protein moieties on their surface.  These protein moieties have COOH groups and OH groups on their surface. COOH groups and OH groups become charged by losing an H ion.  These charged COO- groups and O- groups give a net negative charge to the bacterial surface. If positively charged divalent calcium2+ ions are present in effluent water, they serve in aggregating negatively charged bacteria.

Microbes then further enhance flocculation by production of polymers. Bacterial polymers serve both as floc backbone and a binding network that holds bacteria.  Bacteria majorly produce two types of polymers, starch and Polyhydroxybutrate. Starch (a polymer of sugars) is produced and secreted externally.  This starch envelopes the bacteria as well as neighboring cells holding them together.

Young bacterial cells produce liberally spaced loose mass of starch. Whereas old bacterial cells produce tightly bound dense polymer network of starch leading to a denser and stable floc.

Production of Polyhydroxybutrate (PHB) by some bacteria further strengthens the floc.  PHB serves as a food source for bacteria. It is stored both internally and externally by the bacteria. PHB helps in anchoring bacterial cell flocs together.  Being significantly denser then starch polymers, PHB increases the floc density and leads to faster settling. 

Filamentous microbes too play an important role in floc formation. Such microbes when present in right levels significantly improve floc formation and floc stability.  Filaments serve as backbone to the floc.  Filamentous microbes start to appear in floc at later stages.


Young flocs, which do not have filamentous microbes, appear spherical when seen under the microscope.  While older flocs, having a network filaments throughout them, appear irregular in shape.  Microbial observation is a quick and conclusive method to check floc formation the efficiency of the effluent plant. Generally, floc formation and settling proceeds well under neutral to slightly alkaline conditions.  Acidic conditions may lead to uncontrolled growth of filamentous fungi leading to "bridging of flocs." 


This not only reduces floc setting efficiency but also leads to foaming problems.

So to achieve good flocculation one must ensure presence of starch and PHB secreting bacteria and a few filamentous microbes in the effluent treatment plant. With right conditions and right microbes floc formation is a settled matter.