Coagulation and Flocculation

CHEMICAL ADDITION-> MIXING-> COAGULATION -> FLOCCULATION -> then to Sedimentation or Filtration process

THE BASICS: The very small size of the particles is the most important factor in the stability of a colloidal suspension. Surface electrical charge of particles and combination with water (state of hydration) are also factors. The floc size is primarily contingent upon the factors of the physical agitation and the chemical intermolecular forces. Ionic strength, pH, temperature, other properties of the water affect the coagulation activities. For examples THAT WILL PHOTOGRAPH WELL (some frustration here, ok, I'm better), I have chosen activated sludge mixed liquor from my wastewater treatment plant.... raw water samples from my surface water treatment plant floc well, but do not show up well in pictures here. In fact, they "turn out terrible!" The image at the left is to illustrate a "colloidal suspension." (Please work with me on this!!!)

MIXING: the first step is mechanical in nature, where we mix the chemical(s) into the water to be treated. Mechanical mixers, hydraulic jetting, diffusers, or blending pumps are used to mix the coagulant chemical(s) into the treatment stream as completely and as vigorously as possible. Detention time is not greater than 30 seconds. The chemical feed pumps should be calibrated and checked on a specified schedule. These mixing basins are best located close to the downstream flocculation basins. A square mix basin or tank is superior to a round mix basin or tank; baffles advantageous: with goal to reduce vortex action about the impeller. Best if chemicals are introduced close to the impeller level with vigorous mixing.

COAGULATION:
After the coagulant chemical(s) is/are mixed into the process stream, mixing/coagulation may occur in the channels/pipes or special chambers as the process stream moves to the flocculation basins. Coagulation: to curdle, congeal- largely a chemical process where the coagulant causes the non-settleable particles to destabilize and clump together. Particles may be classified as colloidal (bacteria, fine silts, virus, color-causing), dissolved (any inorganic or organic (salts, gases, chemicals) or suspended solids (particles of less than 0.01mm in size). The alkalinity of the water affects the formation of the floc; low alkalinity more difficult esp. with alum. Best pH for coagulation is 5 to 7; low alkalinity (buffering) may result in pH change outside of best range. Can add lime to increase alkalinity. Overdosing and Under-dosing of coagulant(s) results in poor coagulation. Higher water temperature requires less mixing turbulence. Turbidity gives an indirect measurement of particles, but does not describe particle density, volume, size, nor the ability of the sedimentation process to settle them nor the filtration process to capture them. Low turbidities in the raw water generally means fewer particles, which is then harder to clump together. You may wish to try and recycle solids from your sedimentation basins to provide sufficient collisions with the low turbidity water, or add a weighting agent such as clay (bentonite).

FLOCCULATION:
After coagulation, we encounter flocculation. Flocculation is largely a "physical or mechanical process" where the coagulated clumps are gently moved into contact with each other to form masses as a cloud, a precipitate. This is usually accomplished in basins with vertical turbines or with horizontal paddle wheel types. The "Floc" is fragile: so take it slow, with a gentle mixing action so build it, not shear it (break it up). Prevent short circuiting; best if floc basins are located very close to downstream sedimentation basins or filters; flow from floc basins to sedimentation basins or filters best if it is very gentle, minimizing turbulence, do not shear floc in transport; flow velocity in floc basins and in downstream channels not less than 0.5 ft/min nor greater than 1.5 ft/min. Detention times are 5 to 20 minutes for direct filtration; and up to 30 minutes or 60 minutes for conventional filtration.

OPERATION:

Measure and monitor: pH, color, turbidity, temperature, chlorine demand, chemical usage, and alkalinity. On the left, the photo is of several floc basins proceeding sedimentation basins at a water treatment facility.

INTERACTIONS:
Very small particles will not settle out in the sedimentation process as they are "neutrally buoyant"; they will usually not be filtered out in the filters UNLESS a coagulation-flocculation process is utilized. If particles are not combined to form a larger floc they may increase the loading on the filters downstream of the sedimentation process, (or may pass through the filters in a direct filtration mode. This will also have the high potential to then affect the disinfection process as organisms can be tied up in the suspended particles, and thereby "protected" from the disinfectant chemical. Jar tests should be run daily (and more frequently when the raw water quality changes); doses recorded, with plant process parameters trended against the dosage. Problems include: slow floc formation (correct with low turbidities recycling of formed particles or adding clay), low water temperature (cold water floc penetrates through the filters), and fragile floc (floc breaks up and goes through the filters: check mixing, flocculation process by varying mixer speeds and correlating against tests).

Coagulation and flocculation is more of a "snake oil science." It is often "more of a feel" than a "precise science." This is best shown where we perform "jar tests." This is where a series of experiments with known chemical dosages are performed (jar tests & field testing) and the results utilized to make chemical feed (dosage) adjustments, mixing and flocculation energy adjustments to the actual water treatment facility process'. Ask any operator who has a broad experience in this area, and they will all point to jar tests working well with one series of chemicals and a raw water, and not with others. I have personally had so-so jar test results with a particular chemical, only to have it excel in the actual treatment plant. Others have looked great in jar tests, only to have poor results in the facility. The key is to bring into your lab for jar testing the key elements of how your facility operates. It takes lots of time, but once the correlation is made, it will prove to be an immense benefit. Never forget that as your raw water source changes in temperature, turbidity, etc, you will have to make adjustments in your dosages. It is a true art more than an applied science at this point!

(Coagulation and flocculation in sludges will be discussed in the solids handling topics.)

Return to Past Operator Notebook List

Return to Wright's Training Homepage