Essay - Wastewater Engineering the Treatment of Wastewater Is Divided into Three...

Wastewater Engineering

The treatment of wastewater is divided into three main categories: primary treatment, secondary ***** and tertiary treatment, where each stage contains several steps.

The primary treatment stage involves the removal of physical contamination which includes grit, grease and solid particles. The first stage in this is a screening process, which filters our large particles. The next stage is grit *****, this is a settling stage ***** grit is allowed to settle out of the water under low flow conditions (Manahan 226). The final stage of ***** primary ***** is grease removal, which is achieved by allowing the ***** ***** settle, with the ***** materials floating on top. This top layer is *****n scraped off.

***** secondary treatment ***** removes BOD, ***** is defined as "biochemical oxygen demand for d*****solved oxygen by microorganism-mediated degradation of the organic matter" (***** 227). ***** reduction ***** BOD is ***** by adding extra oxygen which allows the microorganisms to degrade ***** ***** matter present.

One method of achieving this involves using a trickling filter. In ***** process the water is sprayed over a solid surface that contains bacteria. This spraying allows ***** the water ***** be in contact with air, ***** provides ***** oxygen required to *****sist with the action ***** the bacteria.

A second method available involves a rot*****ting biological reactor. In this reactor, a disc is rotated so that each half is either emerged in water or exposed to the air. Bacteria on the disk then have access to the ***** and also ***** the water, enc*****aging *****l growth and reduction of BOD.

The most common method of BOD ***** is known as ***** activated sludge *****. In this process wastes from primary treatment are pumped into an aeration tank. The microorganisms in the aeration tank then convert organic material ***** the wastewater to microbial biomass and carbon dioxide, while also converting organic phosphates to orthophosphates ***** organic nitrogen ***** ammonium ions or nitrate (Manahan 227). This biomass remains in the aeration tank "where it is continuously stirred and aerated, resulting in the formation of small aggregates ***** suspended colloidal organic ***** called floc" (A Dictionary of Biology). ***** mass of organisms is ***** ***** sludge. Part of th***** activated sludge is ***** removed while the remaining part is returned to the aeration tank. Th***** is an effective way of reducing BOD since "the combination of a high concentr*****tion of 'hungry' cells in the return sludge and a rich food source ***** the influent sewage provides optimum conditions for ***** rapid degradation of ***** matter" (Manahan 228).

One of the issues that arises from the activated sludge process, is ***** need ***** dispose of excess sludge. ********** is achieved ***** thickening the sludge in a clarifier and by the use of gravity belt thickeners which separate liquids from solids ***** a porous belt (H*****lttunen). Due to the ***** ***** phosphorous content ***** the thickened *****, it can then be used as a fertilizer (Manahan 229).

Another ***** of disposing of sludge