Most of surface waters and some of ground waters have particles too small to be removed by conventional settling.
These particles can take up to months to settle naturally due to their size and mainly because of electrostatic forces.
These forces are the product of electric charges that keep particles suspended because they repel one to another.
These solids are known as suspended, colloidal or dissolved solids, depending on their origin and chemistry. They usually have negative charge which creates the repulsive force that keeps them suspended. This force is known as Zeta Potential.
Suspended particles can be biological organisms, bacteria, viruses, organic or inorganic charges. Altogether they
give water a murky appearance, technically known as turbidity. This quality parameters is questionable by the
consumers and besides these particles usually go hand in hand with pathogenic biological agents and therefore
should be removed.
To remove turbidity water treatment plants usually use chemicals that accelerate its removal. These chemicals
neutralise Zeta potential and allow particles to attach to themselves and settle. This is the traditional water treatment
and includes the following phases:
Settling or Decanting
At TAERSA we design, build and operate water purification systems choosing the more suitable technologies according to each case.
Coagulation and Flocculation
This system comprises the addition and quick mixture of chemicals that promotes the destabilisation of the negative charge of the particles and colloidal material.
Chemicals commonly used at water treatment plants are: aluminium sulphate, ferric chloride, ferric sulphate, PAC, and sodium aluminate.
Both aluminium salts and iron salts react with water natural alkalinity creating aluminium hydroxide or iron hydroxide particles named flocs. Coagulants introduce great amounts of positive charges into the water, which electrically neutralises the negative loaded particles such as the ones giving colour and turbidity. As these neutralised particles “collide” in the mixing area they start getting size. This coagulation process is ruled by several aspects such as water temperature, G values, pH, alkalinity and turbidity.
In most of the treatment plants, the coagulant is not enough to achieve a quick settling and thus splitting of turbidity so it is usually supplemented by adding several products known as coagulation adjuvants.
These products are used to improve coagulation process and to create heavier flocs, which are easier to settle. The most common are polyelectrolytes (polymers), composed of great molecular weight with ionic charge. Depending on the charge they are classified as cationic, anionic or non-ionic.
After coagulation, the recently formed floc is taken to flocculation phase where a gentle mixture will allow it to increase its size. This process is mainly controlled by detention time and G value. As particles grow in size they are more fragile and that is why flocculators provide progressive mixture.
Flocculation units are created to keep HRT between 15 and 45 minutes.
Settling or Decanting
This process depends on gravity to remove or split solids from water. Decanters for this process can be rectangular, square or round. In traditional water treatment plants this process goes after coagulation and of flocculation previous
to filtering. These units are designed for water to flow in a laminate way minimising turbulence at the inlet and outlet of the unit.
The settled material known as sludge or settling waste should be purged from the system and treated in the proper way.
Settling units usually have a bottom scraper/arm that takes the sludge to a hopper and weirs to drain “clarified” water and take it to the next phase of the process. These units have four areas: each one has a specific function. Inlet area reduces the incoming water speed and splits it uniformly; settling area is where water stills, a step needed for settling of particles; effluent area which is a transition between the settling and the outlet of the unit, and finally “sludge” area where all the solids and particles settled are detached from the liquid flow for after treatment.
Over time many alternatives for traditional decanters have been developed, with the target of generating more efficient units, with less occupied area and to a lesser extent to reduce response times for the operational responses to be quicker in front of problems (high rate decanters). Among them we can mention:
Settlers with plates.
Settlers with Lamella plates.
Flotation units for solids detachment.
Other patented processes such as: Superpulsators, Actiflo, etc.
This way, settlers are an important “barrier” against the passing of solids and particles to later phases (commonly: filtering).
If designed and operated in the proper way, clarifiers remove large amounts of solids, reaching 90% or more in terms of turbidity removal.
This process is the forth in sequence of a conventional water treatment and it is used for holding the particles material that leaves the previous process. This particle material includes those substances present in water such as clay, sand, bacteria, virus, organic charge as well as those generated in the previous treatments: flocs.
TAERSA uses Tetra LP Block dual underdrains to gathering of filtered water and filters backwash. Mainly because of their low operational and maintenance need, less civil works investment, less water use for backwash and longer lifetime.
In a water treatment plant different filtration types can be found. Most common type is granular media filtering, such as sand filters.
There exist many classifications for this process: by type of filter, by operating mode, by filtered material among others.
This way they can be classified according to:
Slow sand filters: they are open filters; they operate at a low filtration rate and do not accept coagulation treated water. Filters that use a smaller sand size than the ones called “high rate”, operate with low filtration rates.
Gravity allows water to flow downward through the media, resulting in a media with its first layer saturated with solids as the filtering cycle goes. The run of this kind of filters can take between 1 and 6 months. When headloss in the filter is excessive, the filter must be cleaned, drained and the first layer of the media should be removed (first inches). These filters have regained interest due to their technological simplicity and easy operation since they do not require knowledge about coagulation or flocculating processes. Correctly operated, these types of filters can reach turbidity values of 0.5 NTU.
High Rate Filters: sand size of this filters are bigger than the previous ones. If you add to this feature the higher filtration rates suitable for these cases, they create “short” filtering runs that go from hours to days. Unlike other filters, high rate filters use all the depth of the media to catch solids. Therefore, they cannot be cleaned by “scratching” the surface. In these cases the cleaning of the media is achieved by introducing a water or air/water flow in counterflow at a rate higher than filtering rate, in order to give fluidity to the media bed and free particles holded in it. These particles are driven to a special current for post treatment. Inside this kind of filter there are units that work by gravity and are opened to the atmosphere and others that are closed and they are called: pressure filters.
Gravity filters: they are usually designed with single media bed, double media bed or triple media bed in some cases. First of these filters were fine sand media beds, that in time were replaced by the double media bed ones. Double media beds allow a deeper penetration in the particle media bed and thus create longer runs and higher filtering rates. Other characteristic of this type of filter is that the operator can see the filtering media bed and its backwash at any moment. It is common that these filters reach turbidity levels lower than 0.1 NTU during most of their runs.
Pressure filters: these are closed filters (not opened to the atmosphere) and they don’t allow the operator to trust in pumping flow to the filter without the headloss as it happens with gravity filters. They operate at pressure specified by design; they occupy less space and the requirements of civil works are lower because they are made entirely of carbon steel or stainless steel. On the disadvantages side, the operator cannot see the filtering media bed during the career or backwash. The operation of this type of filter is similar to the gravity filters: water should be pre-treated, the size of the material forming the media bed is akin, as well as its depths. Generally, pressure filters are widely used in industries and water care in swimming pools.
Diatomaceous earth filters: they are known as pre-coat filters. They are an acceptable technology for filtering in water purification systems because previous coagulation is not required. They are systems where the operator’s mechanical abilities are much important than the “chemical” ones. They have reached high efficiency in Giardia and Cryptosporidium removal, using this type of filter.
Regardless of the amount of solids and turbidity, the main parameters for which the filters were designed, they aid to the reduction of pathogens or microorganisms harmful to human health, which combined with disinfection they have significantly reduced water-borne diseases for decades.