Waste water can contain particles of different sizes. Therefore, different treatment methods are needed for the recycled water safe for consumption, disposal and in accordance with stringent regulatory standards. Particle size can also determine the treatment efficiency.
Separation by floatation, filtration or under gravitational influence is usually very useful for removing larger and visible contaminating particles. However, for colloidal particles which can easily filter through and have similar electrostatic surface charges which impedes their coagulation, physico chemical treatment methods are more effective. Physico chemical treatment involves using chemicals which can modify physical state of colloidal particles which helps in making them more stable and coagulable for further treatment or filtration purposes. These treatment methods have been used for over a century in conjunction with biological treatment methods. These methods have been successfully used for industrial water treatment, conditioning of waste water sludge as part of pre-treatment with increasingly higher efficiency and decreasing costs. Physico chemical treatment can have considerable influence on biodegradation potential of organic material in the waste water.
Physico chemical treatment involves a set of processes which may be consecutively performed in a single unit or in separate units.These processes are coagulation, flocculation and sedimentation. Overall performance of physico chemical treatment process can depend on several aspects and physico chemical properties of waste water. Therefore, laboratory tests are routinely conducted to determine properties based on which the overall treatment process can be customized in terms of times, types and dosage of chemicals etc.
Coagulation process is integral to neutralizing the charges (usually negative) on the colloid surfaces by addition of coagulant while rapidly mixing of waste water. Ferric chloride, aluminium polychloride, ferric sulphate, aluminium sulphate, iron polychloride and sodium aluminate are some of the commonly used coagulants which are applied in varying doses depending on quality of waste water – for example, domestic, agricultural or industrial. Coagulation and rapid mixing with short contact times helps in destabilizing the colloid surfaces so they can clump together and thus conditioning them for next step of flocculation.
Flocculation involves grouping of previously formed flocs or clumps during slow mixing process. This helps in flocs gaining larger volumes and density which will eventually prepare them for sedimentation. Chemicals called flocculants are used to facilitate this process wherein flocs come together and increase in densities and volumes. Organic flocculants are derivatives of algins (seaweed based), starch or cellulose (plant based) and are usually less effective. On the other hand, synthetic flocculants such as polyelectrolytes are highly effective. They are water soluble synthetic macromolecules which may have charges on them. Due to their high efficiency, lower doses are more effective and excessive dosage may be detrimental to the whole process and at the same time increase the costs. During flocculation stage, a gradient is applied so that the next stage of sedimentation can take place. Slow mixing to facilitate longer contact times can happen naturally due to inherent Brownian movement in water (perikinetic flocculation) or by forced agitation of waste water (orthokinetic flocculation).
The last stage of sedimentation is critical for separating liquid from eventual solids that are formed as a result of previous stages of coagulation and flocculation. Settlers with different rates – low, medium or high rates are used depending on condition of waste water at this stage.
Depending on types of contaminants, additionally chlorine, hydrogen peroxide and ozone can be used for waste water treatment – for removing harmful biological agents as well as toxic chemicals. Chlorine can be especially useful in oxidizing the toxic cyanide compounds (for example, from mining and metal industries) and convert them to carbon dioxide and nitrogen. On the other hand, ozone and hydrogen peroxide can be used for effectively degrading organic chemicals present in waste water.
Eventually, total suspended solids, biological oxygen demand (BOD), chemical oxygen demand (COD), nutrients such as nitrogen and phosphorus, heavy metals, and even biological agents such as helminthis eggs, virus and protozoa can be successfully removed from waste water in varying percentages using physico chemical treatment.