The ability of coagulants, which neutralise electric charges and allow fine particles to coalesce into larger ones, has been known for hundreds of years. Modern flocculation, however, only started after long-chain synthetic polymer production came into being after World War II. As more types of polymers became available, particularly polyacrylamides and other derivatives of acrylic acid, clarification of water, especially drinking water, using the new flocculants gained popularity quickly worldwide. It spread into many manufacturing and mineral extraction processes, where fast and efficient separation of suspended solids from water is required.
The products used to achieve flocculation are still often written as either ‘flocculants’ or ‘flocculents’ which can lead to some confusion because ‘flocculent’ is derived from the Latin, floccus, meaning ‘flock of wool’. The modern sense of the word “flocculent” according to various dictionaries is as an adjective meaning ‘like tufts of wool’ describing objects with furry or fuzzy appearances ranging from microscopic views of bacteria to telescopic images of remote spiral galaxies. ‘Flocculant’, however, has now become generally recognised as the correct spelling of the noun and the technology of flocculation falls within the spectrum of Physical Chemistry. Flocculation is synonymous with agglomeration, aggregation, and coagulation or coalescence. To simplify this terminology a bit further, the two terms used most are coagulation and flocculation.
Coagulants and flocculants are formulated to assist in the solids/liquid separation of suspended particles in solution. Such particles are often very small and the suspended stability of such particles is due to both their small size and to the electrical charge between the particles. Conditioning dispersion for the purposes of promoting the removal of suspended particles requires chemical coagulation and/or flocculation.
Coagulants neutralise the repulsive electrical charges, which are typically negative, surrounding particles thereby allowing them to ‘stick together’ creating clumps which are now generally called flocs. There are situations where coagulants can serve the dual purpose of both coagulation and flocculation in that they create flocs that are large enough to settle readily. However, more often than not, flocculants are needed in addition to coagulants for further agglomeration or aggregation of the coagulated particles to form larger flocs which speed up gravitational settling.
Coagulants are either aqueous solutions of inorganic metallic salts such as ferrous and ferric sulphates and chlorides, or aluminium sulphate [alum], or organic amine and amide polymers. These are all 100 percent cationic for rapid neutralisation of the negative charges keeping suspended particles apart. As primary functions, coagulants:
Destabilise colloidal solids into particulate matter, preparatory to flocculation;
Remove colour; and
Break oil/water emulsions.
Accelerate solids settling rate;
Increase solids density in processes such as thickening and floatation;
Facilitate mechanical solids dewatering such as centrifuges, belt presses, screw presses, and general filtration.
Although many polymer chemicals have been studied in research programmes, the different types of suspensions of particles in water needing water/particles separation by coagulation and/or flocculation is endless, with chemical and physical characteristics covering a very wide range. In other words, although types of coagulant/flocculant products can be indicated scientifically, the best products can only be determined by practical experiments which are normally straightforward settling tests done using basic equipment such as a measuring cylinder and a stopwatch. A dose of coagulant/flocculant in liquid form for easy handling is shaken up with the water suspension in a measuring cylinder and the time taken for the flocs formed to settle to a predetermined level near the bottom of the measuring cylinder. Generally, fast settling times mean that large size flocs form rapidly when the flocculant is added and that these flocs will form a compact sludge. Normally, several different types of flocculants are used in these initial stage-one settling tests which narrow down options to a few products for further testing.
For stage two-testing, the same settling test procedures are done but various parameters of the full scale process where water/particle separation is required must be borne in mind, for example variations in temperature, pH, particle size ranges, and concentrations. Another consideration before proceeding with stage two testing is the required clarity of supernatant water and the characteristics of the sludge formed. If a single flocculant does not clarify the supernatant sufficiently then possibly a coagulant should be added prior to the flocculant. Coagulation results in fewer larger particles in the suspension, simplifying and accelerating the action of the flocculant. Complex water/particle suspensions may, in fact, need pre-separation using a suitable coagulant followed by application of two or three different types of flocculants for efficient separation of diverse suspended particles from the water carrier. Figure 1 shows a stable suspension of particles of fine white chalk and some particles of dirt and dust.
|Figure 1. Particles of fine white chalk and dirt in suspension in water.||Figure 2. Separation of settled flocked sludge from clarified supernatant water.|
In Figure 2 it is clear how the flocculation process has produced large flocs which have settled to the bottom leaving behind clear supernatant water which looks brilliantly clear. However, this supernatant water may not be quite as clear as it seems to be at first sight. There may still be large numbers of suspended solids left which are very tiny but could be unacceptable if the clarified water has to be pure enough for example to be used in DVD manufacturing, or, if the water is destined for reverse osmosis where any particles above atomic and molecular dimensions will block up the membranes. In many cases the optimum practical solution is a smaller average floc which includes all particle sizes but settles at a slightly slower rate. A simple optical tool called a “clarity wedge” can be used to determine if water has been sufficiently clarified.
The primary objective in most flocculation processes is to clarify water. However, the sludge produced by settling must be dense enough so that a definite boundary exists between the sludge and the supernatant water. However, the sludge density must not be too high otherwise problems will arise in pumping it out of the settling vessels. Where the primary reason for the flocculation is concentrating suspended particles into a sludge by settling then the characteristics of the sludge must be suitable for further downstream processes such as cyanidation or CIX gold extraction. Another example is when sludge, now called “slimes” in mineral extraction parlance, is fed into further separation vessels where air bubbles are introduced into the bottom of the vessels and rise up to the surface taking with them specific minerals from the “slimes” which become attached to the air bubbles via collector chemicals dosed for this purpose. The unwanted non-mineral particles settle and become the waste or gangue which, once again, must not be too dense because it is pumped away to slimes dams. This is the widely used Flotation Process.
Both coagulants and flocculants build up flocs very quickly when they are properly dosed and evenly mixed into water/particle suspensions. However, because adherence of particles to coagulants and flocculants is through electrical charges rather than chemical bonds, the built-up flocs are not intrinsically strong and will break up if subjected to mechanical shearing forces in turbulent flow. For this reason, commercial and industrial water clarifiers and thickeners are designed to avoid any shear within the settling vessels. However, in the most numerous of all flocculation applications, which is where flocculants are used as ‘filtration aids’, shearing forces in the inlet approach sections to filters can reduce or even negate altogether any benefits of flocculation.
How does flocculation aid filtration?
In the context of HVAC installations there are only a few large once-through water circuits which do not require filtration such as cooling water on power stations and large cooling plants next to oceans or other abundant water sources. All of the other water circuits, excepting some potable hot water supplies, are re-circulating in either closed or open evaporative configurations and are normally equipped with a variety of types of screens and filters. Wide mesh screens installed to protect against solid objects in water flows will not be affected by flocs in circulating water. Finer mesh screens, below 200µ, as well as bowl-type filters which have thin filter meshes or sleeves and are forms of screens, may become blocked up quickly. It is always advisable therefore to have water pressure gauges installed on the inlet and exit pipes of all screens and bowl-type filters and, wherever possible, and incorporate these gauges into Building Management Systems (BMS). Flocculation provides only limited benefits for screens and is mostly used to reduce the suspended solids levels in circulating water over short periods during which screens have to be regularly removed, cleaned and replaced. Automatic self-cleaning screen-type filters are available but are used mainly on larger industrial water circuits due to their high cost.
Bowl-type filters equipped with filtration cartridges having thicknesses from 5mm to 25mm trap smaller particles which build up throughout the cartridge thickness reducing water flow rate. By incorporating most of the small particles into much larger flocs which deposit in relatively loose matted form on the outside of cartridges, flocculants allow thick cartridge filters to run for much longer periods than thin screen types during which thick cartridges collect far greater amounts of suspended solids. For filters using even thicker filtration media such as the common sand and mixed sand/anthracite media filters, treating dirty inlet water suspensions with flocculants provides even better filtration performance for longer periods between backwashing. There are always some small- and medium-sized particles which are not gathered into flocs and these gradually fill up the whole depth of the filter media while larger flocs accumulate loosely on the outside, forming in effect, an additional external layer of media. Flocculating inlet water to filters which have filter media increases both filtration efficiency and mass of suspended solids particles filtered out before each successive backwash cycle. In many instances these benefits allow smaller lower cost filters which also use less backwash water. Applying similar effects to the formation of sheets of paper and cardboard where water is removed from wet layers of feedstock comprising 99 percent water in a matte containing only one percent cellulose fibres by adding flocculent chemicals to the feedstock retains more fine cellulose fibres and also speeds up the rate of water removal. The result is faster production of more tonnes per hour of paper and cardboard.
In addition to water clarifying and sludge conditioning, flocculation has become an important stage in many different types of processes such as:
Breaking emulsions by coalescing droplets;
Forming slurries of clay which can be easily removed in construction projects;
Coagulation of rennet micelles to measure progress of curd formation in cheese production;
Sewage and wastewater treatment; and
Determination of yeast settling in beer brewing which measures degrees of fermentation.
Finally, a note of caution for people using flocculants: flocculants are very large molecules which become ‘soapy’ when they dissolve in water.
Spills: Flocculant spills are extremely slippery and therefore hazardous. Dry flocculant spills should be left dry, swept up and disposed of according to local, state, or federal regulations. For liquid flocculant spills, an absorbent material should be applied, then swept up and discarded.
Do not add water to a flocculant spill.