Physico-chemical Water Treatment Processes

Wastewater contains particles of different sizes which can be classified as dissolved (< 0.08 μm), colloidal (0.08 - 1 µm), supracolloidal (> 100 - 100 mm) and settleable (> 100 µm). 

The type of treatment selected depends on the size of particles present in the wastewater.  In practice, treatment efficiency also depends on particle size. 

Solids of the size that are visible to the naked eye can be separated either by settling under the influence of gravity or by flotation (Dissolved Air Flotation), depending on the relative densities of solids and water. They may also be easily separated by filtration. However, very fine particles of a colloidal nature (called colloids, size < 1 µm) which have high stability are significant pollutants. The reason for this stability is that these particles have electrostatic surface charges of the same sign (usually negative). This means that repulsive forces are created between them, preventing their aggregation and subsequent settling. It has therefore proved impossible to separate them by settling or flotation. It is not possible to separate these solids by filtration because they pass through any filter. However, separation by physico-chemical treatments is possible.

Physico-chemical treatment of wastewater focuses primarily on the separation of colloidal particles. This is achieved through the addition of chemicals (called coagulants and flocculants). These change the physical state of the colloids allowing them to remain in an indefinitely stable form and therefore form into particles or flocs with settling properties.

Stages of the Physico-Chemical Process

The physico-chemical process consists of coagulation, flocculation and sedimentation stages. However there may be configurations where all stages are carried out in the same unit.

Coagulation (or rapid mixing)

Derived from the Latin coagulare meaning driving together, coagulation refers to destabilization or neutralization of the negative charges contained in the wastewater by the addition of a coagulant applied during rapid mixing (which can vary from 250 - 1500 s^-1) and a very short contact time (times ranging between 5 - 60 s). The quantity of coagulant applied during coagulation depends on the quality of water (domestic or industrial). In the case of domestic water, commonly used doses are < 50 mg/L, while for industrial water the dose is very variable.

The most commonly used coagulants are ferric chloride, ferric sulfate, aluminum sulfate, aluminum polychloride , sodium aluminate mixtures of organic and inorganic compounds, lime and the more recently studied application of iron polychloride.

Flocculation (or slow mixing)

This is derived from Latin floculare, referring to the formation of flocs and bridges. In this stage, previously formed flocs group together, increasing in volume and density, allowing them to be sedimented. This is achieved by applying a gradient (10 to 100 s^-1) and a contact time varying between 15 min and 3 h.

- By the movement of particles themselves (Brownian motion). In this case flocculation is referred to as per kinetic or natural convection. 

- By the movement of the fluid containing the particles, inducing their movement. This is achieved by agitating the mixture. This mechanism is referred to as orthokinetic or forced convection flocculation. 

During the flocculation stage, chemicals referred to as flocculants are applied (assisted flocculation). These products allow flocs to come together and adhere, increasing their size and density. Flocculants can be classified by their nature (mineral or organic), their origin (synthetic or natural) or their electric charge (anionic, cationic or non-ionic).

Organic flocculants of natural origin are derived from natural products such as alginates (seaweed extract), starches (plant grain extracts) and cellulose derivatives. Their effectiveness is relatively low.

Those of synthetic origin are long chain macromolecules, soluble in water, formed by the association of simple synthetic monomers, some of which have electric charges or ionisable groups. For these reasons they are referred to as polyelectrolyte. These products are highly efficient and recommended concentrations are 0.05% -0.1% for solid products, 0.1% - 0.2% for liquid dispersion and 0.5% - 1.0% for liquids in solution. Applied in excess they may harm the flocculation process.

Sedimentation or clarification

This is the stage of floc removal by solid - liquid separation. For this, low, medium and high rate settlers are commonly used. The rate is determined by the speed at which water and sludge are produced by the system.

Other popular technologies to separate solids from liquids are DAF clarifiers, lamella settlers and more.

Determination of Design and Operating Conditions Using "Jar Tests"

There are many aspects (physico-chemical properties of the wastewater of interest) that affect the performance of physico-chemical treatment. These can be determined by traditional laboratory jar tests. The system consists of six jars of the same size to which varying doses of coagulant are added at the same time. The system implements a rapid mixing sequence for a predetermined time followed by slow mixing for a set time and finally a settling sequence. After this, the supernatant is drained. Jar tests give a good approximation of the actual treatment process and the rapid mixing, slow mixing and sedimentation conditions of a real plant.

At the beginning, middle and end of the treatment tests, it is necessary to evaluate the efficiency of the process. This is achieved by measuring traditional parameters such as TSS, COD, pH, conductivity, turbidity, alkalinity, BOD, nutrients (N and P), thus establishing the efficiency of the system. Other parameters of greater accuracy such as particle size distribution, zeta potential and/or electrophoretic mobility may also be used.

Applications

Physico-chemical treatment may constitute a single stage in the wastewater treatment process or be added as an additional treatment process during pre-treatment (to improve the biodegradation of wastewater in the biological process and secondary treatment (such as polishing).

Physico-chemical processes have been implemented for over 100 years. However in 1930, these processes were replaced by biological processes due to the high costs incurred by the treatment of large quantities of sludge. Recently, they have been reintroduced for various purposes: the elimination of phosphorus for effluent being discharged to the sea, obtaining average quality effluent at lower cost than conventional treatments and for water used for agricultural irrigation, for potabilization, industrial water treatment conditioning of sludge (primary and/or secondary). The resurgence of these processes is also due to increased recognition that the cost of treatment should be consistent with the desired efficiency, as progress in the synthesis of flocculation polymers with high efficiencies has been achieved at a lower cost.

Using this type of process it is feasible to remove 80 to 90% of total suspended solids (TSS), 40 to 70% of BOD₅, 30 to 40% of COD and 17 to 100% of nutrients (N and P), depending on the dose and type of coagulant used. Heavy metals may also be removed by these processes, but the removal efficiency depends on the metal type and concentration. Recently, these processes have been used to remove pathogens such as helminth eggs and have proven to be capable of removing up to 2 log concentration. In addition they are very efficient when used to remove bacteria (0-1 log unit), viruses and protozoa (1-3 log units in each case). Current studies are focusing on their use for the removal of emerging contaminants.

Example, wastewater generated by printing with water based Flexographic ink