Waste Water Treatment

Introduction

          Wastewater treatment is any process, operation or combination of process and operations that makes the wastewater less dangerous and repulsive to man. It is a combination of physical, chemical and biological process. There are three types of operation and process:

1. Physical unit operation: It is an operation in which application of physical forces predominate. They consist of screening, fluctuation, sedimentation and floatation. 
2. Chemical unit process: It is a process in which removal of contaminants are brought about by chemical activity. Eg: Chemical precipitation, adsorption, ion exchange, electro dialysis.  
3. Biological unit process: It is a process in which removal of contaminants is brought about by biological activity. Eg: activated sludge process, trickling filtration.

Conventional Wastewater Treatment process:

            It consists of a combination of physical, chemical and biological process and operations to remove solids and organic matter. General terms used to describe different degree of wastewater treatment are preliminary, primary, secondary and tertiary treatment.

PRIMARY TREATMENT

Sedimentation: It is a physical water treatment process used to settle out suspended solids in water under influence of gravity. It is after used as a primary stage in modern wastewater treatment plant, reducing the content of suspended solids as well as the pollutants embedded in the suspended solids.

           It is an operation by which the suspended particles in liquid settle by gravity to the bottom of the tank, from which they may be removed as a sludge. After some time, lighter solids and oil floats on the surface of liquid and can be separated as sludge.

Coagulation
and Flocculation:  It is a chemical water treatment technique typically applied prior to sedimentation and filtration to enhance the ability of a treatment process to remove particles.

           Coagulation is a process where water are treated with reagents to form coagulates.
Coagulants with charge opposite to that of suspended solids are added to the mass to trap particles thus forming a mass charge enough to settle or be trapped in the filter.

          The substances that are used as coagulants. The most common ones are: (Al₂(SO₄)₃.H₂O), ferrous sulphate, ferric chloride.

Flocculation:     It is gentle stirring or agitation to encourage the particles thus formed to agglomerate into masses large enough to settle are be filtered from the solution.

Equalization:    

          Flow equalization is a method used to overcome the operational problems and flow rate variations to improve the processes.

          Its objective is by providing storage to hold water when it is arriving too rapidly and to supply when it is arriving less rapidly.

The main function of equalization is to act as buffer. To collect the incoming raw effluent that comes at widely fluctuating rates and maintain a steady (average) flow rate.

Neutralisation:

Industrial waste often contains acidic or alkaline components which require neutralisation before discharge or treatment.

• For waters entering biological treatment processes the “pH” should be maintained between “6.5 – 9” for optimum growth of the micro-organisms.
• Acidic waste are commonly neutralised with alkaline streams, ammonia, caustic soda / soda ash.
• Lime is the most widely used alkaline material for neutralisation of acid wastes.
• Alkaline wastes require treatment with acidic stream, Sulphuric acid or Hydrochloride acid.

Ion Exchange

                   H₂O              H⁺ + OH^–

                   NaCl             Na⁺  +  Cl^–

Ion exchange is a treatment method one or more undesirable ionic contaminants are removed from water by exchange with another or less objectionable ionic substance.

          Both the contaminants of the exchanged substance must be dissolved and have the same type of electrical charge (+ve or –ve). This is use on exchange aims to reduce “Calcium and Magnesium” content in water softness.

Adsorption

          The process can take place in any of the following system:          Adsorption is a wastewater pacification for removing a wide range compound from industrial wastewater. Adsorption works on the principle of adhesion the process of adhesion involves separation of a substance from one phase accompanied by its accumulation or concentration at the surface of another.

• Liquid-Gas,
• Liquid-Liquid,
• Solid-Liquid,
• Solid-Gas.

The adsorbing phase is the ‘adsorbent’ and the material concentrated or adsorbed at the surface of adsorbing phase is the ‘adsorbent’.

•  Activated carbon was the first widely used adsorbent.
• Activated carbon is an excellent adsorbent because it has a strong affinity for binding organic substances even at low concentration.
• It is considered to be a refining treatment, and such is applied at the end of the most common treatment systems especially after a biological treatment.

Electro-dialysis

          Electro-dialysis water treatment process is one of the water treatment methods that employ reverse osmosis membranes which are semi-permeable to ions based on their charge, under the influence of electric field to reduce the ionic content of water.

    Electro-dialysis involves passing water between two plates with opposite electrical charges. The metals in the water are attracted to the plate with negative charge (cathode) while the charged non-metals are attracted to the plate with the positive (+) charge. Both the types of ions can be removed from the plates and discorded.

 Chemical Precipitation

          Chemical precipitation is the most common method for removing dissolved metal from wastewater solution containing toxic metals. To convert the dissolved metals into solid particle, form a precipitation reagent is added to the mixture.

• Filtration can then be used to remove the particles from the mixture.

Eg:

(a) Softening of water with milk or lime (removed
by “Ca” or “Mg”).

(b) Removal of phosphorous from wastewater using
Iron Chloride.

(c) Removal of heavy metals as Hydroxides.

SECONDARY
TREATMENT

 Aerobic Treatment

          Wastewater treatment process is governed by aerobic organisms which need oxygen for the breaking process. Aerobic wastewater treatment tanks are constantly supplied with oxygen. It has been done by circulating air through the tanks.
For effective functioning of aerobic organism’s sufficient amounts of oxygen should be present in the aerobic tanks at all times. Therefore, aeration is properly maintained throughout aerobic treatment.

The abovementioned figure is “bacteria” which intakes:

• Organic matter / Contaminants
• Oxygen
• Nutrients 

and it releases:

• Water
• Carbon Di Oxide
• Excess Cell Mass
  

Aerobic Treatment Principle Anaerobic Treatment

          Anaerobic wastewater treatment is a biological treatment process where organisms, especially bacteria, break down organic material in the wastewater in an oxygen absent environment. Anaerobic digestion is well known anaerobic wastewater treatment process. The degradation of organic material is done anaerobically. For the effective anaerobic digestion of organic material, the entry of air into anaerobic tanks is prevented.

During anaerobic digestion, Methane and Carbon dioxide are produced. Methane is a biogas. Hence, anaerobic digestion process can be used to produce biogas which can be utilized as electricity. Its intakes organic contaminants and nutrients and releases carbon dioxide, methane and excess cell mass.

Biological
Treatment

Activated Sludge

          Microorganisms such as bacteria and protozoa can use the small particles and dissolved organic matter, not removed in primary treatment, as food. Secondary or biological treatment is performed in a tank containing a “soup” of starved microbes called activated sludge. Like us, these microbes require air to live (they are aerobic organisms) and thus air is pumped into the tank. Microorganisms in this aeration tank use the dissolved and particulate organic matter as food, producing more microorganisms which can be collected and separated from the water in the next step.

          It then remains to separate out the microorganisms (activated sludge) so that just clean water is left. This is done in a secondary clarifier which operates in the same manner. Some of the solids collected in the secondary clarifier (return activated sludge) are sent back to the aeration tank to treat more wastewater and the excess (waste activated sludge) is pumped to another location in the pant for further treatment. The clean water that flows out the top of the clarifier is sent along for disinfection.

Trickling Filters

Trickling filters are conventional aerobic biological wastewater treatment units, such as active sludge systems or rotating biological contactors which are used to remove organic matter from wastewater. The TF is an aerobic treatment system that utilizes microorganisms attached to a medium to remove organic matter from wastewater. This type of system is common to a number of technologies such as rotating biological contactors and packed bed reactors (Biotowers).

TFs enable organic material in the wastewater to be adsorbed by a population of microorganisms (aerobic, anaerobic, and facultative bacteria; fungi; algae; and protozoa) attached to the medium as a biological film or slime layer (approximately 0.1 to 0.2 mm thick). As the wastewater flows over the medium, microorganisms already in the water gradually attach themselves to the rock, slag, or plastic surface and form a film. The organic material is then degraded by the aerobic microorganisms in the outer part of the slime layer.

As the layer thickens through microbial growth, oxygen cannot penetrate the medium face, and anaerobic organisms develop. As the biological film continues to grow, the microorganisms near the surface lose their ability to cling to the medium, and a portion of the slime layer falls off the filter. This process is known as sloughing. The sloughed solids are picked up by the underdrain system and transported to a clarifier for removal from the wastewater.

Adequate air flow is important to ensure sufficient treatment performance and prevent odors. To evenly distribute the water on the filter, a “rotary sprinkler/distributor” is most often used. The primary factors that must be considered in the design of trickling filters include:

• the type of filter media to be used.
• the spraying system, and
• the configuration of the under-drain system.

TERTIARY TREATMENT

The final step before releasing the treated wastewater to the environment is to kill microorganisms that might be healthy hazards. This process, called disinfection.

Disinfection

          The process of killing or inactive of pathogens or other microorganisms is known as disinfection. This is done by following ways:

1. U.V treatment:    Exposing water to UV rays will kill the germs and microorganisms in water.
2. Ozonisation:      Ozone is passed into water will kill the germs and microorganisms. It produces powerful disinfectant.
3. By Passing Chlorine:  Chlorine gas is passed to the water it Produce hypochlorous acid, which act as disinfection. Liquid Chlorine is a powerful disinfection.