FAQ’s about Mechanical Bar Screen.

Where are Bar Screens used?
The mechanical bar screen is used in most all municipal sewage treatment plants. 




What are Municipal Sewage Treatment Plants?
Every city, town, village or place of human habitation generates sewage. It is necessary to treat this water before it is let out into the environment where it can pollute the water in rivers, lakes and oceans and underground aquifers from the harmful effects of pollution caused by untreated sewage. 
Sewage from houses or commercial buildings are collected and pumped to sewage treatment plants (STP) where it is treated till water that can be safely released back into the environment without any harmful effects that sewage normally creates.
The collected solids are sent to solid waste management centers for disposal.



What is BOD?
BOD is short for Biological Oxygen Demand. It refers to the amount of dissolved oxygen consumed by living organisms such as bacteria, algae, plants, insect and living animal life at a given temperature over a given time.



What is COD?
COD is short for Chemical Oxygen Demand. It refers to the amount of oxygen required by reactive chemicals to breakdown to a stable stage at a given temperature over a given time.
For Example, a piece of steel when immersed into water will use up dissolved oxygen available in the water to convert into rust or Iron Oxide. If the dissolved oxygen is replenished the piece of steel will continue to rust until all the iron in the steel is converted into FeO3 (Iron oxide saturated with oxygen).



How does untreated sewage affect water bodies?
Sewage constitutes industrial and domestic waste. Every natural water body has its equilibrium established in nature due to naturally occurring BOD and COD.
Whenever untreated sewage which has its own BOD and COD is dumped into a natural water body, its equilibrium gets destabilized as more often than not the new BOD/COD requirements of the polluted water body cannot be met by the water bodies existing ecosystem.
When BOD and COD are higher than what is naturally replenished, most living matter in the water dies. Anaerobic bacterial takes over the water body begins to saturate with harmful dissolved gases such as CO2 and Hydrogen Sulphide.
Soon the water in the polluted body begins to emit foul smell does not support useful lifeforms and turns black and slimy.



How do you measure the capacity of a treatment plant?
In India Smaller treatment plants are measured in KLD - Kilo Litres per Day which is essentially 1M3 of Sewage treated per day.
Larger treatments are measured in MLD - Million Litres per Day or 1000M3 of sewage treated per day.




What are the stages of sewage treatment in a Standard STP?
STP's are broadly divided into 3 stages. 
Primary Treatment - Where all suspended solids such as grits, paper, cloth, vegetative matter, plastic waste and other organic and inorganic suspended solids, are treated and removed from the influent.
Secondary Treatment - Where dissolved matter is removed from the influent with the aid of biological agents such as Bacteria and Enzymes.
Tertiary Treatment - Where the solids generated by secondary treatment are ejected from the stream by Filtration, Sedimentation, Evaporation or any other means. 


In which stage of treatment is the Bar Screen used?
In Primary Treatment. Suspended Solids are extracted in the primary treatment Phase over several stages. With the first stage dealing with the largest particles and progressively reducing in subsequent stages until all the suspended solids are removed. 
The Bar Screen is the very first stage in Primary Treatment. It usually deals with particles that are larger than 10mm. They are usually placed in a chamber called screening chamber where all the influent entering the STP flows through.  



How does a Bar Screen Work?
A Bar Screen is primary a physical barrier that is places across the flowing effluents. It is a row or parallel bars where the gap between adjacent bars determines the size of suspended particles captured. All liquids and particles smaller than the gap pass through. 
The accumulated particles are cleared physically by various methods both manual and automatic.




Why opt for a Mechanical Bar Screens?
Most treatment plants operate nonstop with effluent flowing through the plant all the time. A bar screen accumulates captured suspended solids very quickly needing frequent cleaning. If manual methods of cleaning are opted, operators are required to monitor and clean the screens whenever required. This occupation can be treated as Manual Scavenging in the eyes of the law. The Supreme Court of India has banned the use of manual scavenging.
Mechanical Bar Screens completely eliminates the need for an operator thereby providing significant operation cost reduction.



Does a Mechanical Bar Screen require laborers to operate it?
No. The Mechanical Bar Screen is fully automatic and does not require laborers to operate it. Laborers are only needed for preventive maintenance such as lubrication of certain bearing parts.

 

How much power does a Mechanical Bar Screen need to operate?
Usually 1.55 Kilowatts of power is required for operating the bar screen mechanism. However bar screens require only intermittent operation further reducing energy costs. 

 

Does the Mechanical Bar Screen have any Consumable Parts?
No. Other than essential spares that may wear out due to normal usage, no consumables are required for the operation of a Mechanical Bar Screen.

 

What is the Normal Lifespan of a Bar Screen?
With proper maintenance, a bar screen operates for at least 30,000 hours. Under normal usage conditions, since a bar screen is controlled by a level sensor which turns on and off the machine as required, A bar screen can last around 30 years.

 

Are there any cost savings by opting for a Mechanical Bar Screen vs. Manually Operated Screens?
Manually operated screens require one Laborer to be stationed at the post round the clock. When you compute the wages for this person vs. increased capital investment + energy costs, you will find significant cost savings of around 40-70% over a 10 year period.