Coagulation and Flocculation

CHEMICAL ADDITION-> MIXING-> COAGULATION -> FLOCCULATION -> then to Sedimentation or to a Tertiary Filtration Process

The BASICS: The very small size of the particles is the most important factor in the stability of a colloidal suspension. The surface electrical charge on the particles and the 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, I have taken some activated sludge mixed liquor samples from my wastewater treatment plant. Now, the activated sludge wants to form a floc on its own, without chemicals added, but I wish to use the image at the left to illustrate a "colloidal suspension." (Please work with me on this!!!)

MIXING: The first step is to mix the coagulation chemical(s) into the water to be treated. This is usually done with mechanical mixers, hydraulic jetting, diffusers, or blending pumps are to mix the coagulant chemical(s) into the treatment stream as completely and as vigorously as possible. The detention time is not greater than 30 seconds. The chemical feed pumps should be calibrated and checked on a specified schedule. The mixing basins are best located close to the downstream flocculation basins. 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. It is usually best if the 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). Overdosing and underdosing of coagulant(s) results in poor coagulation. Higher water temperature requires less mixing turbulence, as the water is less dense. Turbidity gives us 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 tertiary filtration process to capture them. Low turbidities in the raw water means fewer particles, harder to clump together: recycle solids to provide sufficient collisions or add weighting agent such as clay (bentonite). I once operated a wastewater treatment plant that utilized this solids recycle from the bottom of the clarifiers back to the mixing basins when the incoming wastewater was low in suspended solids. We were removing algae and bacteria from wastewater treatment ponds. The recycle element was beneficial under the low particle condition.

FLOCCULATION:
is largely a physical process where the coagulated clumps are gently moved into contact with each other to form masses as a cloud, or as "a precipitate." This is usually accomplished in basins with vertical turbines or with horizontal paddle wheel types. Please note: the floc is fragile. Utilize a slow, easy, gentle mixing action to build it, but not enough energy to 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. Wastewater: detention times of 15 to 20 minutes for sedimentation.

OPERATION:

Measure and monitor the following water quality parameters: pH, turbidity or suspended solids, temperature, chemical usage, and alkalinity.

INTERACTIONS:

Wastewater sedimentation impacts:
Many of the particles are too small and will not settle properly due to gravitational forces alone. By coagulating them and then flocculating them into larger particles that will settle due to gravitational forces, the sedimentation process becomes more effective. This also means that there will be a marked increase in the volume of sludge produced utilizing coagulation/flocculation upstream. I have been able to get up to 90% suspended solids removal and 60 to 70% BOD removal in primary wastewater sedimentation with upstream coag/floc. treatment.

This is more of a "snake oil science", where a series of experiments are performed (jar tests & field testing), than a true applied 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.)

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