Chemical Engineering @ChemengFiles.Com

The processes used in municipal wastewater treatment plants are often classified as part of primary, secondary or tertiary. Primary Treatment: Wastewater entering a treatment plant contain materials that could clog or damage pumps and machinery. These materials are eliminated through trellis or vertical bars, and burned or buried after being collected manually or mechanically. The wastewater then passes through a crusher, where the leaves and other organic materials are crushed to facilitate further processing and disposal.

House of sand
In the past, were used deposition tanks, long and narrow, in the form of channels, to remove inorganic or mineral matter such as sand, silt and gravel. These cameras were designed so as to allow the inorganic particles of 0.2 mm or more be deposited into the fund, while the smallest particles and most organic solids that remain in suspension continued their journey. Today, most cameras used are the spiral-flow aerated with hopper bottom, or clarifiers, mechanical arms bearing makers scrape. Mineral residue is removed and poured into landfills. The accumulation of these wastes can range from 0.08 to 0.23 m3 per 3.8 million liters of wastewater.

Sedimentation
After removing the solid mineral fraction, the water passes to a settling tank where organic materials are deposited, which are collected for disposal. The process of sedimentation can be reduced from 20 to 40% of BOD5 and 40 to 60% solids in suspension.

The sedimentation rate increases in some industrial treatment plants incorporating processes called chemical coagulation and flocculation in thickener. Coagulation is a process that involves adding chemicals like aluminum sulfate, ferric chloride, or polyelectrolytes to the wastewater, that alters the surface characteristics of suspended solids so that they adhere to each other and precipitate. Flocculation causes the agglutination of suspended solids. Both processes remove more than 80% of suspended solids.

Flotation
An alternative to sedimentation is used in some wastewater treatment is the flotation, which will force the air into them, at pressures of between 1.75 and 3.5 kg per cm2. The wastewater, supersaturated with air, then discharges into an open tank. In it, the rise of air bubbles causes the suspended solids rise to the surface, where they are withdrawn. Flotation can remove more than 75% of suspended solids.

Digestion
Digestion is a microbiological process that converts sludge, organic compound, methane, carbon dioxide and harmless material similar to humus. Reactions occur in a closed tank or digester, and are anaerobic, ie occurring in the absence of oxygen. The conversion occurs through a series of reactions. First, the solid matter is made soluble by the action of enzymes. The resulting substance is fermented by the action of a group of bacteria that produce acids, which reduce it to simple organic acids such as acetic acid. Then the organic acids are converted into methane and carbon dioxide by bacteria. Thickened sludge is added to the digester and heated as often as possible, where it remains between 10 and 30 days until it decomposes. Digestion reduces organic matter content between 45 and 60 per cent.

Desiccation
The digested sludge is spread over sand beds to dry air. Absorption through the sand and evaporation are the main processes responsible for drying. Air drying requires dry, relatively warm climate for maximum efficiency, and some treatment plants have a greenhouse structure to protect the sand beds. The dried sludge is used primarily as a soil conditioner, sometimes is used as fertilizer because it contains 2% nitrogen and 1% phosphorus.

Secondary treatment: Once removed from a 40 to 60% of suspended solids and reduced from 20 to 40% of BOD5 by physical means in the primary treatment, secondary treatment reduces the amount of organic matter in water. Usually, the employees are aerobic microbial processes, ie active microorganisms in the presence of dissolved oxygen. Secondary treatment is, in fact, use and accelerate natural processes of waste disposal. In the presence of oxygen, aerobic bacteria convert organic matter into stable forms, such as carbon dioxide, water, nitrates and phosphates and other organic materials. The production of new organic matter is an indirect result of biological treatment processes, and must be removed before discharging the water in the channel receiver.

There are several alternative processes for secondary treatment, including trickling filter, activated sludge and lagoons.

Trickle filter
In this process, a wastewater stream is distributed intermittently over a bed or column of a porous medium coated with a gelatinous film of microorganisms that act as agents of destruction. The organic matter in the wastewater stream is absorbed by the microbial film and converted into carbon dioxide and water. The process of drip, when preceded bysedimentation, can reduce about 85% of BOD5.

Activated sludge
It is an aerobic process in which gelatinous sludge particles are suspended in an aeration tank and receiving oxygen. Activated sludge particles, called floc, are composed of millions of actively growing bacteria bound together by a gelatinous substance. The floc absorbs organic matter and converts it into products aerobics. The reduction of BOD 5 fluctuates between 60 and 85 per cent.

An important companion in any plant using activated sludge or a trickling filter is the secondary clarifier, which removes bacteria from the water before discharge.