Wastewater and Sewage Treatment

November 30, 2019

The domestic sewage or liquid industrial waste that cannot be discarded in untreated form into water sources like lakes or streams due to public health, economic, environmental and aesthetic considerations is called wastewater.
The treatment of wastewater relies on industrial scale use of microorganisms for bio-conversion.
Wastewater enters a treatment plant and following the treatment, the effluent water that is treated waste water discharged from the wastewater treatment facility is suitable for release into surface waters such as lakes and streams.
It can be released to drinking water purification facilities.

Wastewater and Sewage

Wastewater from various sources like domestic sewage and industrial sources cannot be discarded in untreated form.
Sewage is liquid effluent contaminated with human or animal excreta.
Wastewater commonly contains potentially harmful inorganic and organic compounds as well as pathogenic microorganisms.
Waste water treatment can use physical, chemical and biological processes to remove or to neutralize contaminants.
On average, 100-200 gallons of water is produced every day for washing, cooking, drinking and sanitation in USA.
Wastewater collected from these different activities must be treated to make it free from contaminants before it can be released into surface waters.
About 16000 publicly owned treatment works (POTWs) operate in USA that is fairly small; treating 1 million gallons or less of wastewater each day.
Collectively, these all plants can treat about 32 billion gallons of waste water daily.
These plants can handle both domestic and industrial wastes.
Domestic waste water is made up of sewage, “gray water” (the water that results from washing, bathing and cooking) and wastewater from food processing.
Industrial waste water includes discharged from the petrochemical, pesticide, food and dairy, plastics, pharmaceuticals and metallurgical industries.
Water from this source may contain toxic substances for which some plants required by The United States Environmental Protection Agency (EPA) to pretreat those toxins or heavily contaminated discharges before they enter POTWs.
Mechanical processes are used in pretreatment in which large debris is removed.
Some wastewater are pretreated biologically or chemically to remove high toxic substances like cyanide, heavy metals such as arsenic, lead and mercury or organic materials such as acrylamide, atrazine (a herbicide) and benzene.
These substances are converted to less toxic formed by treatment with chemicals or microorganisms capable of neutralizing, oxidizing, precipitating, or volatilizing these wastes.
The water after treatment can then be released to POTW.

Wastewater Treatment and Biochemical Oxygen Demand

The aim of wastewater treatment facility is to minimize organic and inorganic materials in wastewater to certain level that no longer supports microbial growth and also to eliminate other potentially toxic materials.
The efficiency of treatment is expressed in terms of a reduction in the biochemical oxygen demand (BOD).
It is the relative amount of dissolved oxygen consumed by microorganisms to completely oxidize all organic and inorganic materials in a water sample.
A high BOD indicates high organic and inorganic materials in the wastewater.
200 BOD units is a typical value for domestic wastewater including sewage.
For industrial waste water, the BOD values vary according to the sources of its production.
From sources such as dairy plants the values can be as high as 1500 BOD units.
An efficient wastewater treatment facility can reduce this level to less than 5 BOD units in the final treated water.
Wastewater facilities are designed to treat both low BOD sewage and high- BOD industrial wastes.
The various independent physical and biological processes are involved in treatment which is a multistep operation.
Primary, secondary and sometimes tertiary treatments are employed to reduce biological and chemical contamination in the wastewater and each level of treatment employs more complex technologies.

Processes involved in Wastewater Treatment

A) Primary Wastewater Treatment

It uses only physical separation methods which separate solid and particulate organic and inorganic materials from wastewater.
Wastewater that enters the treatment plant is passed through a series of grates and screens that remove large objects.
The effluent is left for several hours to allow solids to settle to the bottom of the separation reservoir.
Municipalities that provide only primary treatment discharge extremely polluted water with high BOD into adjacent waterways.
It is because high levels of soluble and suspended organic matter and other nutrients remain in water following the primary treatment.
These nutrients thus can trigger the undesirable microbial growth, further reducing water quality.
Therefore, most treatment plants employ secondary and even tertiary treatments that reduce the organic content of the wastewater before released to natural waterways.
Secondary treatment processes use both aerobic and anaerobic microbial digestion to further reduce organic nutrients in wastewater.

B) Secondary Wastewater Treatment

a) Anoxic treatment (anaerobic method)

This treatment involves a series of digestive and fermentative reactions carried out by various prokaryotes under anoxic conditions.
It is typically used to treat wastewater that contains large quantities of insoluble organic matter (and therefore having a very high BOD) such as fiber and cellulose waste from food and dairy plants.
This process is itself carried out in large enclosed tanks called sludge digesters or bioreactors.
The process requires the collective activities of many different types of prokaryotes.
At first, anaerobes use polysaccharides, proteases and lipases to digest macromolecular and suspended waste into soluble components.
These soluble components are then fermented to yield a mixture of fatty acids, H₂ and CO₂.
The fatty acids are then fermented by syntrophic bacteria to acetate, CO₂ and H₂.
These products are then used as substrates by methanogenic Archaea fermenting acetate to produce methane and carbon dioxide.

These are the major products of anoxic sewage treatment.
The methane thus produced is burned off or used as fuel to heat and power the wastewater treatment plant.

b) Aerobic treatment

This method uses digestive reactions carried out by microorganisms under aerobic conditions.
It treats wastewater containing low levels of organic materials.
Generally, wastewater from residential sources is treated efficiently only by aerobic treatment.
Several kinds of aerobic decomposition processes are used for treatment where trickling filter and activated sludge methods are the most common.

i) Trickling filter

It is a bed of crushed rocks about 2m thick.
Wastewater is sprayed on the top which slowly passes through the bed.
The organic materials get absorbed to the rocks and microorganisms grow on their large exposed surface.
The complete mineralization of organic matter to carbon dioxide, ammonia, nitrate, sulfate, and phosphate takes place after the extensive microbial film is formed on the rocks.

ii) Activated sludge method

It is the most common aerobic treatment methods.
The wastewater to be treated is mixed and aerated in large tanks where slime forming bacteria including Zoogloea ramigera among others grow and form flocs (aggregated mass).
Protists, small animals, filamentous bacteria and fungi attaches to the flocs which is then followed by oxidation as with the trickling filter.
The aerated effluent containing the flocs is then pumped into a holding tank or clarifier where the flocs settle.
Some of the floc material (activated sludge) is then returned to the aerator as inoculum for new wastewater.
Then, the rest is pumped to the anoxic sludge digester or is removed, dried and burned or used as fertilizer.
Wastewater normally stays in an activated sludge tank for 5-10 hours which is too short for complete oxidation of all organic matter.
However, most of the organic matter is adsorbed to the floc during this time and incorporated by the microbial cells.
The BOD of the liquid effluent is considerably reduced up-to 95% when compared to the incoming wastewater as most of the material with high BOD is now in the settled flocs.
The flocs can then be transferred to the anoxic sludge digestor for conversion to CO₂ and CH₄.
Most treatment plants undergo chlorination after secondary treatment to further reduce the possibility of biological contamination.
Now, the treated effluent can be discharged into streams or lakes.
UV radiation as disinfectant and O₃ as a strong oxidizing agent that is an effective bactericide and Viricide are also used to treat effluent water at over 40 plants in the eastern United States.

C) Tertiary Wastewater Treatment

This treatment is any physicochemical or biological process that employs bioreactors, precipitation, filtration, or chlorination procedures.
These procedures are similar to those employed for drinking water purification.
There is sharp reduction in the levels of inorganic nutrients especially phosphate, nitrite, and nitrate, from the final effluent.

Wastewater after this treatment especially contains no nutrients and cannot support extensive microbial growth.
So, this is the most complete method of treating sewage but has not been widely adopted due to the costs associated with such complete nutrient removal.

References:

i) https://www.safewater.org/fact-sheets-1/2017/1/23/wastewater-treatment

ii) https://www.fao.org/3/t0551e/t0551e05.htm

iii) https://environment.des.qld.gov.au/management/water/pollution/wastewater

iv) https://arviatechnology.com/blog/2018/12/06/what-are-the-three-stages-of-wastewater-treatment/

v) https://aosts.com/how-does-activated-sludge-wastewater-treatment-work/

vi) https://www.samcotech.com/what-are-aerobic-wastewater-treatment-systems-and-how-do-they-work/