Dear António,

This subject does not belong to my area but its result influences the future of our environment and basic necessities thus I will follow it with interest.

Thanks Dear Mohammad: In fact in regions of scarce water resources, the need of fresh water for different uses is a priority concern. The level of treatment required for the wastewater (primary, secondary or tertiary) depends of the final restitution or of the characteristics of any possible reuse. The available alternatives, including desalinization, reuse of treated wastewater, possible superficial sources or possible underground aquifers, in terms of costs, sustainability and factors of risk must be thoroughly analyzed. Reuse of treated wastewater is an extremely favorable rational solution environmentally friendly.

I will love to know because it is not my area of interest 

I am more an Environmental Engineer, who looks at the complete process as a function of multi- process. viz- It starts with a Pre- process filter, then Primary, next is Secondadry, then comes the Tertiary and it concludes with Polishing and finishing..

What goes on in each of the 5 processes.. and for how long duration is also indicated in the paper....

The flexibility of this type of treatment is it can handle all sewage..or all septage..or mix of both..  The remediation can also slot in kitchen waste- water on remediation Etc.. and final water after remediation conforming to National standards..   

The use of treatment systems (primary, secondary or tertiary) depends on the quality of treated effluent, the valuation perspectives and especially the socio-economic issues played water resources. Some arid regions typically conducts a full treatment of wastewater that is until their purification. This treatment is usually limited to the secondary level for countries where the water resources is not scarce and when the discharge standards into surface water or reuse do not exceed guideline values. Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process.

The use of treatment systems (primary, secondary or tertiary) depends on the quality of treated effluent, the valuation perspectives and especially the socio-economic issues played water resources. Some arid regions typically conducts a full treatment of wastewater that is until their purification. This treatment is usually limited to the secondary level for countries where the water resources is not scarce and when the discharge standards into surface water or reuse do not exceed guideline values. Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process.

The use of treatment systems (primary, secondary or tertiary) depends on the quality of treated effluent, the valuation perspectives and especially the socio-economic issues played water resources. Some arid regions typically conducts a full treatment of wastewater that is until their purification. This treatment is usually limited to the secondary level for countries where the water resources is not scarce and when the discharge standards into surface water or reuse do not exceed guideline values. Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process.

Dear @Antonio, as far as I am informed about the issue, we are far from delimitation between secondary and tertiary treatment, so, so far. I am speaking about Serbia and my city. Less than 20% are connected to a tertiary treatment of waste water.

I am bringing some fine resources to your attention. Kind regards.

http://www.eea.europa.eu/data-and-maps/indicators/urban-waste-water-treatment/urban-waste-water-treatment-assessment-3

http://www.eib.org/attachments/country/study-on-the-reuse-of-wastewater-en.pdf

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.470.8910&rep=rep1&type=pdf

http://esa.un.org/iys/docs/san_lib_docs/guidelines_on_municipal_wastewater_english.pdf

Dear András and Dear Ljubomir: Thanks for your suitable and pedagogic answers, underlining the importance of the quality of municipal wastewater treatment and for the excellent references. In fact, looking to the existing technical literature in this area, and according to my knowledge, I suppose that this is not an easy question, even for the experts. A great evolution have been observed in the last decades, particularly with respect to activated sludge technologies. For example, removal of nutrients (mainly phosphorous and nitrogen) are sometimes included in the secondary treatment and disinfection may be introduced in association to any type of treatment, primary, secondary or tertiary. Reuse of treated wastewater or final restitution in interior lakes are important issues that may require advanced treatment (tertiary).

Sewage treatment is the process of removing contaminants from wastewater, primarily from household sewage. It includes physical, chemical, and biological processes to remove these contaminants and produce environmentally safe treated wastewater (or treated effluent). A by-product of sewage treatment is usually a semi-solid waste or slurry, called sewage sludge, that has to undergo further treatment before being suitable for disposal or land application.

Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment.

Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface.

Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat.

Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow rejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs,...). Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

https://en.wikipedia.org/wiki/Sewage_treatment

In order to hit the nail right on the head, the sewage & waste water treatment ought to be moved to household scale instead of a centralized sewage system. Each house will be helped by local authority to have self -contained systems that they maintain & take responsibility for. The old system – which is also current in some places- was fine when there had been less population than we do have now. Upon going from home to my university daily, the road passes near a sewage & wastewater treatment plant which annoys every traveler by the bad smells. I wish that  we can make the people  responsible for their waste.

We can learn from Sweden which designed sustainable toilet systems that can be run independently for each house as a contained system. This technology is currently applied in some places in Sweden. Other countries are hoped to follow suit.

Dear Nizar: You are correct, as always, even if the aspects that you mention are a little far from the main question that I would like to discuss here. In fact, Wastewater Treatment Plants (WWTPs) are (should be) normally installed sufficiently far from the center of the urban agglomerates, and the predominant local winds should also be considered in the place selected for the WWTPs. Hydrogen sulfide control have been traditionally a major concern of sewerage systems management since the first combined systems in century XIX. Hydrogen sulfide (H2S) is a gas characterized by an intense bad odor (similar to the smell of rotten eggs) and is a poison gas that, depending of its concentration in atmospheric air and time of human exposition, may even kill. It may result from the anaerobic decomposition of the organic matter in the sewerage systems in the absence of a suitable concentration of dissolved oxygen. Large systems, hot temperatures and pipe flow are critical aspects in the conception, design and operation of the systems. There are well regulated rules all over the world and good engineering practices and procedures for controlling sulfide generation and its effects since a long time ago. Even if “small is beautiful”, my preference is clearly for large regional sanitary sewerage systems working properly, which may allow important economy scales and a better control of the environmental aspects. 

Dear António Manuel, the concepts/definition you presented following your question are well accepted. In summary:

primary treatment: predominates physical process

secondary treatment: predominates biological process

tertiary (or advanced) treatment: other process rather than merely physical or biological, such as membranes, adsorption and chemical process, for instance.

@Kundu,Nizar and Gilson good response 

Thanks Dear Gilson for your good answer conceptual that may contribute to develop the discussion also. The word “predominates” may help. However, I suppose that removal of nutrients is (or may be) mainly a biological process (both in the conventional or classic wastewater treatment and in the more recent technologies as SBR, for example). So how do you explain its inclusion in the tertiary treatment?

My experience from designing WWTPs:

PRIMARY TREATMENT - raw influent is bar screened, then fine grit is removed followed by primary clarification to remove suspended solids before discharge into receiving waters which have enough flow to dilute remaining BOD:TSS (the Combined Sewer Overflow outfall discharge to rivers & oceans used this dilution principle to justify discharge of sewage contaminated waters).  The primary sludge is usually anaerobically digested and the methane gas recovered to power the WWTP.

SECONDARY TREATMENT - in this case after primary clarification above, the primary effluent is biologically treated to remove dissolved organic matter by aeration methods using diffusers or fixed-film reactors to form a biological floc to be settled in secondary clarifiers which recirculates part of it as return activated sludge (RAS) to the aeration tanks, with the excess removed as waste activated sludge (WAS) to be treated in  usually aerobic digesters. The secondary clarifier effluent is then disinfected with usually chlorine and discharged. Depending on the final discharge area, the BOD:TSS quality is usually 20:20 to 30:30 mg/l. If the secondary effluent is to be used for irrigation, then total nitrogen (TN) and total phosphorous (TP) removal is incorporated too.

TERTIARY TREATMENT - For highly sensitive receiving waters or WATER REUSE by the public, the secondary clarifier effluent from above is sent to sand  or combination sand-carbon filters. This ensures an effluent with BOD:TSS:TN:TP not usually exceeding 5:3:3:1 mg/l. The filtered effluent is then high-level disinfected meaning longer detention times and higher residual chlorine so it can be used for irrigation of public areas and food crops. In some place the tertiary treatment also uses artificial wetlands/ponds.

In Florida, many WWTPs do not use primary clarifiers, primary treatment is provided by grit removal and then this influent is biologically treated in usually oxidation ditches, membrane batch reactors, aeration tanks   with nitrification/denitrification because it is tertially treated by filtration and high level disinfection for reuse or discharge into designated wetlands.

Hello António, you are correct again. Maybe the concepts must be reviewed: it is true that new biological treatment alternatives can remove nutrients (mainly nitrogen, but phosphorus at a less extent) and this criteria (nutrient removal) may not be adequate to set the boundaries between secondary and tertiary treatment. I think the nature of the process (biological for secondary treatment) should prevail in the definition.

Dear Dr. António, Dear All,

Thank you for your correct response. As you know, H2S can be easily captured by an absorption or scrubbing tower to be utilized in e.g. the production of the important chemical, sulfuric acid. I saw the men responsible for that plant & they told me that they captured methane gas adding that (You will be pleased to see a flame coming out which indicates burning this CH4 gas). I told them that I am not pleased unless the gas is collected & then sold. This also apply to other side products. 

This sewage & waste water treatment plant has an input coming from a population of about 200,000. They apply the treatments (you mentioned in the question) in the most old fashioned way. But even as such, they can get a fortune out of trash. In our country, it is unfortunate that old scholars (like me) are not listened to in most cases.

In India, there are 234-Sewage Water Treatment plants (STPs). Most of these were developed under various river action plans (from 1978-79 onwards) and are located in (just 5% of) cities/ towns along the banks of major rivers. In class-I cities, oxidation pond or Activated sludge process is the most commonly employed 3 technology, covering 59.5% of total installed capacity. This is followed by Up-flow Anaerobic Sludge Blanket technology, covering 26% of total installed capacity. Series of Waste Stabilization Ponds technology is also employed in 28% of the plants, though its combined capacity is only 5.6%. A recent World Bank Report came out strongly in favour of stabilization ponds as the most suitable wastewater treatment system in developing countries, where land is often available at reasonable opportunity cost and skilled labour is in short supply.

http://www.ais.unwater.org/ais/pluginfile.php/356/mod_page/content/111/CountryReport_India.pdf

Subhash C. Kundu · 76.25 · 4.7 · Guru Jambheshwar University of Science & Technology

We commend Mr. Subash C Kundu, with appreciation to give a vivid presentation of all processes.

To just mention the processes used in out natural remediation wwt plant .. we follow it in following steps..allowing a HRT- 8 to  10 days..

1. Pre- process,2, Primary 3. Secondary 4. Tertiary and finally 5. Finishing and polishing to get the best quality processed water..

Dear Hajit: Thanks very much for your answers that I much appreciate and for the reference of your Conference Paper from 2012 entitled “Standardisation of Design and Maintenance of DEWATS Plants in India” that, at a first glance, looks very interesting and totally suitable for this discussion too. I would suggest that perhaps you could complement your answers by posting a direct link to your paper here. I also agree with you that the contributions of our colleague Subhash Kundu in this discussion are much appreciated for all, as well as all remaining contributions.

Usually, secondary wastewater treatment deals with the removal of carbonaceous organics as BOD or COD by aerobic and anaerobic biological methods. In tertiary the efluent from biolgical treatment goes through removal of nutrients by nitrification or desnitrification and phosphorous uptake, pesticides by activated carbon, trace metals by ion exchange, disinfection and final filtration. These proceses are very important when water is scarce and wastewater has to be recycled for drinking water.

Dear Luís: Thanks for your answer. In fact, treated wastewater is reused for many purposes since many time ago. For extensive (generalized) water supply, perhaps still only indirectly through the recharge of underground aquifers (as far as I suppose). However, WWTP staff told me that, in a little WWTP by activated sludge, with prolonged aeration, the final effluent treated and stored in a small tank, for purposes of irrigation and washing in the station, connected to a tap in the station, had such quality that it could not be easily distinguished, at first sight, of the potable water of the supply system.

Well, there is still a debate between uses of recovered wastewater, but in Israel I have seen they use part of the treated wastewater as drinking water. Of course, it has to go through a carefull quility control assessment. If this water is not it palatable y safe they include some amedments. In my country Ecuador, we are very fortunate because we have a lot of fresh raw water.

Thanks, Dear Luís: You guessed and answered with wisdom to the question that I did not write explicitly. Many times people do not give the importance to water resources that they should. Often, we only realize and value the natural resources that we have, when we face situations where we have not them.

It is estimated that about 38,254 million litres per day (mld) of wastewater is generated in urban centres comprising Class I cities and Class II towns having population of more than 50,000 (accounting for more than 70 per cent of the total urban population). Th e municipal wastewater treatment capacity developed so far is about 11,787 mld, that is about 31 per cent of wastewater generation in these two classes of urban centres.

http://www.idfc.com/pdf/report/2011/Chp-20-Municipal-Wastewater-Management-In-India.pdf

Thanks, António, for your message. I appreciate it.

Luis Cumbal

First of all I do not see a clear question!.. Then, perhaps it is better to focus on how should we currently interpret these terms, in view of tremendous achievements in wastewater treatment technology and science.

If this is the case, the delimitation must be done on the basis of expected performance.

If one considers the evolution of wastewater treatment, one would see that the pioneering performance objective was to remove solids and organic matter. 

Then, translated into todays's terminology, primary treatment is the type of treatment that removes SS and COD without the aid of biological processes.

Conversely, secondary treatment is the type of treatment that removes SS and COD  using biological/biochemical processes.  The focal point in this case is COD and/or organic carbon

All additional treatment steps designed to remove other pollutants, should be lumped into "tertiary treatment", i.e. biological N & P removal is a tertiary treatment scheme  as the main focus is N or P and not COD, although it still relies on biological processes.

Dear Derin: First of all, thanks very much for your answer and for sharing your large expertise. I agree with you, with respect to your conventional delimitation. But if you do not see a clear question, how do you interpret different delimitations or zones of overlap found in technical literature?

As mere example, in Metcalf and Eddy, Wastewater Engineering Treatment, Disposal, Reuse, Third Edition McGraw-Hill, 1995, on page 126 is said that:

“Historically, the term “preliminary” and/or “primary” referred to physical unit operations; “secondary” referred to chemical and biological unit processes; and “advanced” or “tertiary” referred to combination of all three.”

And on page 128:

 “Disinfection is included frequently in the definition of conventional secondary treatment.”

(…)

“In many cases, the nutrient removal processes are coupled with secondary treatment”

(…)

“Advanced wastewater treatment is a term that has many definitions. In the context of this book, advanced wastewater treatment is defined as the level of treatment required beyond conventional secondary treatment to remove constituents of concern, including nutrients, toxic compounds, and increased amounts of organic material and suspended solids.”

“The "old school" of wastewater treatment, still embraced by most government regulators and many academics, considers water to be a vehicle for the routine transfer of waste from on place to another. It also considers the accompanying organic material to be of little or no value. The "new school", on the other hand, sees water as a dwindling, precious resource that should not be polluted with waste; organic materials are seen as resources that should be constructively recycled. My research for this chapter included reviewing hundreds of research papers on alternative wastewater systems. I was amazed at the incredible amount of time and money that has gone into studying how to clean the water we have polluted with human excrement. In all of the research papers, without exception, the idea that we should simply stop defecating in water was never suggested.”

― Joseph Jenkins, The Humanure Handbook: A Guide to Composting Human Manure

http://www.goodreads.com/quotes/232929-the-old-school-of-wastewater-treatment-still-embraced-by-most

Thanks very much for keeping the discussion alive, Dear Subhash. Metcalf and Eddy is a book of reference in Wastewater Engineering and Wastewater Treatment. Demonstration by absurd is a challenging procedure that I always liked enthusiastically, but that needs normally to be didactically explained. I believe that you can assist me in this matter. What did you intend with the quote you cited? Are you against the current water supply systems serving the populations and the subsequent utilities in the edifications and in their homes? I do not think so. Are you proclaiming by absurd the return to medieval age or to the age of caves?

Dear Antonio, please summarize the responses from the most rational attitude to the primary and secondary wastewater treatment. Thanks in advance.

Dear Vladimir: You are correct. Typically, the delimitation between Primary Treatment and Secondary Treatment is very clear (and can be found everywhere). Primary Treatment (after the Preliminary Treatment that removes normally the larger materials transported in the wastewaters by screening and may sometimes remove also grit (sands) and also less frequently oils, grease and fats) is basically the physical removing of separable (or settleable) solids (fluctuating and weighty materials). Total Suspended Solids (TSS) are typically reduced in an order between 40% and 60% and Biochemical Oxygen Demand (BOD) is removed (indirectly) in about 30% (roughly). Interesting definitions of primary, secondary, and tertiary treatment are given in the links below:

https://en.wikipedia.org/wiki/Secondary_treatment

http://water.worldbank.org/shw-resource-guide/infrastructure/menu-technical-options/wastewater-treatment

Dear Antonio Thanks for the detailed and clear information. I realized that the cost of primary, secondary and tertiary wastewater treatment may well pay off. Especially in countries where there is a catastrophic problem with drinking water. Thank you for your enlightenment.

Dear Vladimir: Another important aspect that you raised up sagaciously with your answer. Reuse of treated wastewater is an environmentally sustainable rational approach, given that the municipal wastewater needs to be treated before the restitution to nature, in order to control the pollution in the receiving bodies (according to the water quality patterns established in the current legislations of the countries). However the costs of treatment may be relatively high, particularly when tertiary treatment or advanced treatments are required for satisfying high quality exigencies as those needed for drinking water, and risk factors of possible punctual operation deficiencies need to be seriously considered, given that, in the case of potable water, we are speaking about public health. Specifically for the origin of drinking water for the water supply systems to the populations in coastal regions with scarce fresh water resources, desalination of sea water or of brackish water is still currently the more used origin, the most accepted and economically, in normal conditions, (in terms absolute) more advantageous and reliable.

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

Tertiary treatment is used at municipal wastewater treatment plants when receiving water conditions or other uses require higher quality effluent than that produced by secondary wastewater treatment. Disinfection for control of pathogenic microorganisms and viruses is the most common type of tertiary treatment.

Again plagiarism by Chuck A Arize!!! He is champion of plagiarism!

Copy/paste is bad doing without mentioning the source!

Source:

https://www3.epa.gov/npdes/pubs/mstr-ch3.pdf

In municipal wastewater treatment, there is a clear delineation between secondary and tertiary treatment processes. Secondary treatment in wastewater treatment plants typically involves the use of biological processes to further purify the wastewater after primary treatment, which removes solids from the wastewater. Biological processes in secondary treatment, such as activated sludge or trickling filter systems, help to break down organic matter and remove a significant portion of pollutants.

On the other hand, tertiary treatment refers to an additional purification step beyond secondary treatment, aimed at achieving higher levels of removal of specific contaminants or improving the overall quality of the treated wastewater. Tertiary treatment processes can include filtration, disinfection, nutrient removal, and advanced oxidation processes. These processes are employed to further enhance the quality of the effluent, making it suitable for reuse, discharge into sensitive environments, or meeting stringent regulatory standards.

The clear delimitation between secondary and tertiary treatment lies in the specific processes employed and the objectives of each treatment stage. Secondary treatment primarily focuses on the biological degradation of organic matter and the reduction of pollutants, while tertiary treatment involves additional steps to achieve specific water quality goals.

Fine resource: The four stages of wastewater treatment plants

https://www.idrica.com/blog/stages-of-wastewater-treatment-plants/