Study of Waste-Water Treatment Plant

Waste-water relates to output source of water which we get from several activities like bathing, washing, using the toilet, outsource water from industries, rainwater runoff etc. Waste-water treatment is the process of converting the water that is no longer useful for human activities into the bilge water that can be discharged back to environment. Waste water contains all sorts of bacteria, chemicals and toxic elements which are very harmful to discharge directly in the atmosphere therefore the waste-water treatment is been brought into action so as to reduce the contaminants to acceptable level to make water safe to discharge in environment.

There are two forms of waste water treatment plants which depend upon the effluent that is to be treated. The two types of waste water treatment plants are chemical or physical treatment plant & biological treatment plant. Biological waste treatment plants use biological matter and bacteria to break down waste matter whereas physical waste water treatment plants use chemicals and physical process to treat waste water. Biological treatment systems are ideal for treating waste water from households and business premises. Physical waste water treatment are used to treat waste water from industries, manufacturing firms and factories since these firms produce waste water containing toxic chemicals which are hazardous to discharge.

Simple Waste water Treatment Process Diagram.

Procedure of Waste-water Treatment Plant

The following is the step by step process for treating the waste water

Step 1- Waste Water Collection.

This is the first step of any plant to collect the waste water from every part of city. The waste water from buildings, malls, public offices and any other source of waste water is been collected with help of drainage underground pipes at one point and then send to treatment plant.

Drainage Pipes.

Step 2-  Odour Control.

This step plays an important role at treatment plant. The waste water contains various unwanted materials which results in anaerobic or septic conditions. This condition takes place when there is limited flow of oxygen or no dissolved oxygen available for respiration of microbes in waste water. This allows microbes to emerge as an "sulphate reducing bacteria" to thrive. These bacteria use sulphate ion and gives byproduct as hydrogen sulphate (H2S) which smells like rotten egg.

There are many different types of technologies to control odour from waste water collection and treatment systems. The two technologies used are liquid-phase and  vapour-phase technology. The liquid- phase technology is used where both corrosion and odour are concern whereas the vapour-phase technology is used to control odour in air and gas, usually they are used in point-source applications.

Step 3 - Screening.

The next process is of screening where the unwanted floating material is been separated from waste water. The waste water is passed through the screens and floating unwanted materials is been removed. there are three types of screens used  coarse, medium & fine.

1. Coarse screen has larger openings from 75- 150 mm.
2. Medium screens has opening from 20-50mm.
3. Fine screens have openings less than 20mm.

Grit Chamber- The waste water is then passed through grit chamber where the inorganic materials like sand, ash etc are removed. Basically the sedimentation process is been carried out in this part.

Skimming Tanks-  There are chances of having greasy and oily elements in waste water from industries and manufacturing firm outlets. Thus skimming tanks helps to remove this unwanted materials form waste water. The skimming tank is surrounded by baffle walls and the compressed air is been flushed inside from the floor of skimming walls and the rising air bubbles coagulates and solidify these materials and these material are then pushed to side compartment from where these elements are easily removed.

Step 4-  Primary Treatment.

The unwanted materials which are not removed from previous material are removed in this step by application of sedimentation process. The unwanted materials remains in suspended state in flowing water, however when this flowing waste water is stopped and stored in tank called sedimentation tank
then this matter gets settled down due to action of gravity after a certain amount of time. Sometimes the coagulants are used to facilitate the sedimentation process & then this process is called as coagulation- aided sedimentation. 

The process of sedimentation depends upon some factors like size, shape & specific gravity of particles besides viscosity and flow velocity of  waste-water. In normal process of treatment, sedimentation is usually carried out twice-once before the secondary treatment, referred to as primary sedimentation and after this process the secondary sedimentation is brought to action.

Coagulants: These are chemicals usually positively charged & which form insoluble & gelatinous precipitates with colloidal particles which are negatively charged which are present in waste water and thus this particles become denser and due to gravity action they gets settled down at bottom of sedimentation tank. The most common used coagulants in treatment process are alum (aluminium sulphate), iron salts (ferric sulphate, ferric chloride), lime and soda ash (sodium carbonate), sodium silicate & sodium aluminate.

Step 5- Secondary Treatment. 

This process is also known as activated sludge process. The secondary treatment process involves adding seed sludge to waste water to ensure that it is broken down further. Air is first pumped into huge aeration tank which mix the waste water with seed sludge which is basically small amount of sludge, which fuels the growth of bacteria which uses oxygen and other microbes that consumes the remaining organic matter. This process leads to the production of large particles  that settle down in huge tanks. The waste water passes through large tanks for a period of 3-6 hours. 

Some secondary treatment methods include a secondary clarifier to settle out and the biological floc or filter material grown in  the secondary treatment bioreactor. Some alternative methods are activated sludge, aerated lagoon, aerobic granulation, constructed wetland, membrane bioreactor, rotating biological contactor, trickling filter.

• Activated Sludge - It is the process of treating sewage and industrial waste waters using air and biological floc composed of bacteria and protozoa.
• Aerated Lagoon - It is a holding or treatment pond provided with artificial aeration to promote the biological oxidation of waste water. There are many other biological process for waste water treatment for example trickling filters, rotating biological contactors & biofilters.
• Aerobic Granulation - The biological waste water treatment is often accomplished using conventional activated sludge systems. These systems generally require large surface area for treatments and biomass separation units due to generally poor settling properties of the sludge.

Step 6- Tertiary Treatment.

This stage is similar to those used in water treatment plant which clean raw water for drinking purposes. The treatment Stage has the ability to remove up to 99% of the impurities from waster water. This produces effluent water that is close to drinking water quality. Unfortunately this process tends to be costly as it requires special equipments, well trained and highly skilled operators.

Step 7 - Disinfection. 

In this process the waste water treated from primary and secondary treatment contains some harmful elements which are to be removed so the disinfection process is been brought to action. The waste water is kept in tank known as disinfection  tank which contains chlorine and hypo chlorine present in it which helps to remove the unwanted materials and keep water to acceptable limit for a long time span. The waste water is kept in disinfection tank for nearly 30 min. The UV rays method is also applicable but its to costly therefore the chlorination process is more popular.

Step 8 - Sludge Treatment.

The sludge that is collected during primary and secondary treatment requires concentration and thickening to enable further processing. It is put in to thickening tanks that allow it to settle down & later separate from water this requires a time span of around 24 hours. The remaining water is collected and send back to aeration tank for further treatment. The sludge collected and be used in agriculture lands as manure.