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Municipal Wastes Treatment Plants     

 All municipal sewer treatment plants use bacterial digestion to treat wastewater. There are many methods and devices used to do this, but they all use bacterial digestion to remove organic waste from wastewater.

Raw sewage coming into the treatment plant (the influent) contains:

The job of the treatment plant is to remove as much of the grit, dissolved and suspended solids as possible. When these are removed, the cleaned water (the effluent) is discharged into a lake, river or ocean, or simply allowed to trickle down into the ground.

All municipal sewage treatment plants are regulated by state and Federal agencies, and the performance of every treatment plant is measured by testing the treated effluent as it is discharged from the facility. The most important measurement of water quality is the Biological Oxygen Demand ( BOD ).

BOD is an indirect measurement of the amount of organic matter in the water. This test shows how much oxygen would be required (demanded) by aerobic bacteria if they were to digest all the organic material in the water. This test is based on the fundamental chemical reaction that describes bacterial digestion:

Organic waste + Oxygen ----Bacteria----> Water + Carbon Dioxide

BOD is expressed in parts per million, or "ppm". When the BOD of water is high, it means that the water has a lot of organic matter in it - and that bacteria would demand - or use - a lot oxygen or digest that amount of organic material. Likewise, a low BOD measurement shows that most or all of the organic material has been removed from the water.

As raw sewage influent enters the treatment plant, the first step is to filter out any sand, grit, stones, cigarette butts and other hard, insoluble matter. This is done by a mechanical filter, screen assemblies and settling basins.

The next step in the process is to separate the suspended organic solids from the water. This is done in a device called a primary clarifier. In this clarifier, the suspended solids are settled to the bottom, sometimes with the aid of flocculating agents. After settling out as much of the suspended solids a possible, these solids are pumped out of the bottom of the primary clarifier as sludge. This process removes most of the organic waste from the water. However, the liquid portion that remains behind is still far from clean. It still contains all the dissolved solids, and so further processing of this liquid is required.

After this point, the sludge and the liquid portions of the waste are treated separately. First, we will follow the liquid portion through the remaining treatment processes, until it is clean and ready for discharge. The remaining liquid contains  dissolved organic matter that must be removed by bacterial digestion before the water is actually clean. This is done in another device, often called the oxidation tank (pond). This device is easy to identify by its bubbling and gurgling action, caused by air being forced through it (aeration). This aeration promotes the bacterial digestion process, and within several hours, the bacteria will digest most of the remaining dissolved solids.

When the digestion is deemed to be complete, the liquid waste then goes into a "final" clarifier or finishing pond. Here, the liquid is held motionless, and any remaining solids (including the bacteria) are settled to the bottom, to be pumped off and treated with the sludge. The remaining liquid will be quite clear, as almost all of the organic matter has been removed from it. At this point, the water must meet state and Federal guidelines and be clean enough for discharge.

Often, the water receives on final treatment before it is discharged to a lake, ocean, stream, river, etc. The final treatment is disinfection to kill any and all the bacteria that being discharged into the environment. There are several different methods of disinfection, but chlorination (using chlorine gas ) is the most common disinfection process used in the world today.