Methods of Liquid Waste Treatment

Methods of Liquid Waste Treatment

Sewage Treatment

A high BOD indicates the prescence of excess amounts of organic carbon. Oxygen depletion is a consequence of adding wastes with high BOD values to aquatic ecosystems. The higher the BOD of a source of wastes the higher the polluting power of that waste. BOD's of certain wastes are listed in the table below.

Type of WasteBOD(mg/L)
Domestic Sewage200-600
Slaughterhouse Wastes1000-4000
Cattle Shed Effluents20000
Vegetable Processing 200-5000

There are numerous ways to reduce the BOD of waste before discharging it into the water. Treatment of the wastes is aimed at removing organic material, human pathogens, and toxic chemicals.

Primary sewage treatment involves physical seperation to lower the BOD of the waste. Suspended solids are removed in this step through the use of settling tanks. Primary treatment usually removes from 30% to 40% of the BOD from typical domestic sewage. Secondary treatment uses microbial degradation to reduce the concentration of organic compounds furthur; it involves microbial processes which can be aerobic or anaerobic. The combined use of primary and secondary treatment reduces approximately 80% to 90% of the BOD. However, because secondary treatment involves microorganisms it is extremely sensitive to toxic chemicals. Finally, tertiary treatment uses chemicals to remove inorganic compounds and pathogens.


Oxidation Ponds

Oxidation Ponds are also known as stabilization ponds or lagoons. They are used for simple secondary treatment of sewage effluents. Within an oxidation pond heterotrophic bacteria degrade organic matter in the sewage which results in production of cellular material and minerals. The production of these supports the growth of algae in the oxidation pond. Growth of algal populations allows furthur decomposition of the organic matter by producing oxygen. The production of this oxygen replenishes the oxygen used by the heterotrophic bacteria. Typically oxidation ponds need to be less than 10 feet deep in order to support the algal growth. In addition, the use of oxidation ponds is largely restricted to warmer climate regions because they are strongly influenced by seasonal temperature changes. Oxidation ponds also tend to fill, due to the settling of the bacterial and algal cells formed during the decomposition of the sewage. Overall, oxidation ponds tend to be inefficient and require large holding capacities and long retention times. The degradation is relatively slow and the effluents containing the oxidized products need to be periodically removed from the ponds. An oxidation pond can be seen in the figure below.

Oxidation Pond

Trickling Filter

The trickling filter system is relatively simple and inexpensive. It is an aerobic sewage treatment method in which the sewage is distributed by a revolving sprinkler suspended over a bed of porous material as seen in the Figure below.

Trickling Filter

The sewage slowly moves through the porous bed and the effluent is collected at the bottom. This porous material becomes coated with a dense slimy bacterial growth which provides a home for a heterogeneous microbial community which includes bacteria, fungi, and protozoa as well as other organisms. As the sewage drains through the porous bed, this microbial community absorbs and breaks down dissolved organic nutrients in the sewage; this reduces the BOD. Aeration of the sewage occurs by the movement of air through the porous bed. The sewage may need to be recirculated several times through the filter in order to reduce the BOD sufficiently. One dissadvantage to this system is that an excess amount of nutrients produces an excessive amount of slime on the bed which in turn reduces aeration, leading to the need to renew the porous bed. Cold winter temperatures also reduce the effectiveness of this method in outdoor treatment facilities.



Activated Sludge

Activated Sludge is a widely used aerobic method of sewage treatment. After primary settling, the waste stream is brought to an aeration tank. Air is put in and/or there is mechanical stirring which provides aeration of the waste. Sludge from a previous run is usually reintroduced to the tanks to provide microorganisms. This is why it is called activated sludge. During the period in the aeration tank, large developments of heterotrophic organisms occur. In the activated sludge tank the bacteria occur in free suspension and as aggregates or flocs. Extensive microbial metabolism of organic compunds in the sewage results in the production of new microbial biomass. Most of this biomass becomes associated with flocs that can be removed from suspension by settling. A portion of the settled sewage sludge is recycled and the remainder must be treated by composting or anaerobic digestion. Combined with primary settling, activated sludge reduces the BOD by 85% to 90%. It also drastically reduces the number of intestinal pathogens. An illustration of an aeration basin is shown below.

Aeration Basin


Anaerobic Digestors

Anaerobic digestors are large fermentation tanks which are continuously operated under anaerobic conditions, as seen below.

Anaerobic Sludge Digestor
Anaerobic decomposition could be used for direct treatment of sewage, but it is economically favorable to treat the waste aerobically. Large-scale anaerobic digestors are usually used for processing of the sludge produced by primary and secondary treatments. It is also used for the treatment of industrial effluents which have very high BOD levels. The mechanisms for mechanical mixing, heating, gas collection, sludge addition and removal of stabilized sludge are incorporated into the design of large-scale anaerobic digestors. Anaerobic digestion uses a large variety of
nonmethanogenic, obligately, or facultatively anaerobic bacteria. In the first part of the process, complex organic materials are broken down and in the next step, methane is generated. The final products of anaerobic digestion are approximately 70% methane and 30% carbon dioxide, microbial biomass and a nonbiodegradable residue.


Tertiary Treatment

The treatment processes used to reduce the BOD of sewage waste are secondary treatment processes. Tertiary treatment is any practice beyond secondary treatment and is designed to remove nonbiodegradable organic pollutants and mineral nutrients such as nitrogen and phosphorus salts. For tertiary treatment, activated carbon filters are commonly used.



Disinfection

Disinfection is the final step in the sewage treatment process and is designed to kill enteropathogenic bacteria and viruses that were not eliminated during the previous stages of treatment. Disinfection is commonly done by chlorination with chlorine gas or hypochlorite. Chlorine gas reacts with water to yield hypochlorous and hydrochloric acids which are the actual disinfectants. A disadvantage of using chlorination for disinfection is the formation of disinfection by-products, such as chlorinated hydrocarbons. Chlorinated hydrocarbons are toxic and difficult to mineralize. Trihalomethanes may also be formed such as chloroform and bromoform, which are suspected carcinogens. Ozonation is an alternative to chlorination, which uses ozone as the oxidant. This however, is more expensive. Currently, alternative disinfection processes are being sought.

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Reference: Principles of Microbiology, R.M. Atlas, Mosby-Year, 1995

Created by Gianna Aiezza and Meredith Streeter