Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

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  • sararamos
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Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Dear SuSanA,

I am researching to find a methodology to determine the best combination between centralised and decentralised systems in a rural area of ca. 500 km2 in India that comprises more than 150 small villages. The questions in these cases are: how many WWTP should be installed? How the villages should be grouped (or clustered) to connect to one WWTP? Would it be better that each of the villages has its WWTP?

Nowadays is known that the extremes of fully centralised or fully decentralised are not always the best solution, but a flexible combination according to local conditions. 

Therefore, I would appreciate having your opinion on which factors should be essential or important to define the clusters or groups to be connected to a common WWTP. Until now, I have considered the following, but I am pretty sure more factors need to be considered. 

- Location in the same river basin, which favours the transport of water by gravity, reducing pumping costs
- Similar characterisation of the wastewater produced
- Proximity between villages
- Cost differences between centralised and decentralised systems

Thanks a lot for your contributions. 

Sara Ramos
Master Student
Technical University Dresden
Sara Ramos
Master students Hydro Science and Engineering - Technical University of Dresden
Research Assistant Helmholtz-Centre for Environmental Research – UFZ, Germany
Co-Founder SWIM - Solutions for Water Integrity and Management
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  • goeco
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Hi Sara,
in my mind the most important factor is the reuse of the wastewater. That is, centralised plants tend to discharge to water bodies, which is the design "behaviour" that must change and therefore be influenced by the scale of the plant. Land disposal should be a prerequisite, with design at the optimum scale to make that happen and to retain the nutrients for the application e.g. crops, forestry depending on treatment level and land use.
cheers
Dean
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Thank you Dean for your answer. 

Yes definitely the use of the treated water is key for selecting the best option, even more in the study area where I am doing the analysis as it is a dry area with not many water courses. And, you make a good point about retention of nutrients through irrigation . This might give benefits to the population and make more feasible the installation of treatment plants with acceptance from the community. The challenge is to guarantee an adequate treatment of the wastewater. 

Regards, 

Sara
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  • Heiner
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Hi Sara,

in addition to Dean's words: beside the retention and reuse of water look always for the amount of nitrogen which is left after treatment. Nitrogen is the most important nutrient for good yields and especially in semi arid or arid climates very often in shortage. This is because legumes suffer from water shortages and can't produce nitrogen then.
So whenever someone declares his wwtp as the best and most of nitrogen is lost.....guide him to the door ;-)

Heiner
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  • goeco
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Yes indeed, Heiner's point relates again to systems set up to discharge to water bodies, these remove as much nitrogen as possible because it becomes a pollutant in water (causing algal blooms). For crop irrigation re-use the nitrogen should be retained. Anaerobic systems such as dewats remove much of the nitrogen whereas aerobic systems such as vermifiltration do not remove the nitrogen and are better suited to crop irrigation.
Another important consideration is cost-efficiency and how this relates to scale (for a given level of treatment). Some treatment methods remain cost efficient at small scale (e.g. vermifiltration), which is important because a higher level of decentralisation offers greater opportunities for small communities to use their own resources (nutrients, water).
Level of treatment will depend on the application. Irrigating green leaf crops to be consumed raw is the most difficult, whereas tree crops and forestry is easy. Modular treatment systems offer versatility whereby each module in series provides additional treatment, so can be tailored to the crop being grown.
cheers
Dean
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

I think this is the most important change of thinking required from our sanitation engineers, consultants, universities:
don't treat the waste water to the extend removing as much nutrients so that the effluent can be returned to a river ..
BUT
treat the waste water such that it maintains or even increases its nutrient content so that it can be used as fertilizer in irrigation water...
PLEASE NOTE
that we humans are the only creatures who put their excreta into surface waters (via a WWTP) while all other land-living animals put their excreta back on the soil where they support growing the food which the animals consume...
ONLY
we humans mine phosphates to grow our food and contaminate rivers, lakes and oceans with our excreta.... thus, spoiling our environment on both ends.....

Since Sara's project are villages in rural, dry India, her project should look at putting the nutrients of human excreta back on the fields which will be a triple win-win solution: dealing with the excreta in the right way can help increasing the yields on the field and will reduce the contamination of surface waters.... thus, the question Sara has to answer is not only how to group/connect the villages, but more importantly 'what treatment technology to use'....

ciao Hajo
We can't solve problems by using the same kind of thinking we used when we created them.
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Dear Dean, thank you for your answer. In fact, some publications have showed that economies of scale apply for wastewater treatment plants. The higher the population served, the lower the cost investments per person. However you mentioned that it does not always works like this? Did I understand correctly? It would be useful for my research to further elaborate this topic. In case you have in mind references on this respect I would appreciate if you share them ;) 

Regards, 

Sara
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Dear Hajo, 

The points you make are very valid, thank you! The issue of choosing the best wastewater treatement system (centralised, decentralised, clustered) is complex and involves many variables. In my opinion we are still lacking a holistic approach that includes costs analysis considering economies of scale in wastewater treatment plants, but also the appropiate technologies and thus use of the treated wastewater as you mentioned in your post. 

For the costs related analysis, I consider the lenght of pipelines, the amount of wastewater that needs to be transported and the pumping requirements as the main parameters that determine the costs of the system and then can be a decisive factor to chose how to group villages. And, additionally of course, the required technology to treat the water according to the potential uses in different regions and contexts. 

Regards, 

Sara 
Sara Ramos
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Dear Sara,

it seems from your response that you are looking in your research more at the transport system (centralized, decentralized, condominium, ..) than at the treatment system. I (and Dean) want to draw your attention also to the selection of treatment: do you want to apply state-of-the-art WWTP, where the treatment tries to reduce nutrients as much as possible so that prescribed parameters (COD, BOD) are achieved which then legally permit the release of the effluent into surface waters? I underline some words because from my experience, WWTP in developing countries rarely achieve what they are supposed to (for a number of technical, financial, organizational reasons).

We want you to look also for alternative ways of treatment, like vermitechnology. This technology will have some advantages: it is less sophisticated, cheaper to build and maintain, using natural processes. Being an aerobic process it does not produce green house gases! And it avoids the risk of contaminating surface waters but produces effluent which must be used as irrigation water and solids which are used as soil conditioner, thus not waste but products (--> closing the loop, Ecosan).

And - although I am not a friend of patenting processes of nature: the use of soil and worms in waste water treatment is quite advanced in India. If you search the following link, you will find Indian firms with information how to plan vermitechnology waste water treatment for your project:   https://innoqua-project.eu/conference-documents/   Dean may forgive me, but if we want to promote vermitechnology at larger scale, we may have to 'support' firms which make money with it. Money rules the world...  or not?

good luck, Sara, with your project,
ciao
Hajo
We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
Any intelligent fool can make things bigger and more complex... It takes a touch of a genius - and a lot of courage to move in the opposite direction.
E.F. Schumacher
Everything should be made as simple as possible, but not simpler. :-)
Albert Einstein
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  • Decentral
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Dear Sara,

Sorry to enter this discussion late, but the topic you are dealing with is very interesting and challenging. Your points are very well formulated and you are on the right path. I would like to add some additional considerations, which could help your system analysis.
  1. It would be recommendable to consider "the system" and its boundaries - e.g. how many villages, population to be served, extrapolation for future growth, topography, and very importantly, the existing water supply system with future developments. If these villages have a borehole for a water supply source, it means, that you would not need a sewer system, but on-site sanitation options only should be considered (this is also a form of a decentralized system). These include pit latrines, dry toilets, and the discussed above treatment units.
  2. If there is an existing water supply system, or it is planned in the future, then your approach should include the grouping of the villages, or "the levels of decentralization". As you correctly mention, it is very dependent on the distance between the sources, topography and pumping costs. So for this scenario, you could identify at least three possible solutions.
  3. Water reuse is very important, as already mentioned, so the choice of suitable treatment technologies depends very much on the type of reuse, usually for irrigation. The Problem with this is, what to do with the treated water when it is not used for irrigation because it will not meet the requirements for discharge into a water body. An interesting practice from Zimbabwe is to irrigate crops + pastures, where the pastures irrigated area would vary, depending on the water availability. But this of course requires large areas and pumping costs (depending on topography).
  4. Having identified the main scenarios, a simplified cost assessment could be done, and based on the cost estimates and other technical, economic, and social considerations, you could choose the best option. Also, as an alternative, it is possible to develop a simple optimization model, with several variables, which would allow you to play with different technological solutions, especially as related to the treatment system.
For more information about decentralized systems, you can visit: sites.google.com/site/decentralizedwastewatersystems/

Best regards,
Roumiana Hranova
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  • sararamos
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Dear Roumiana, 

Thanks a lot for sharing your thoughs. It can be seen that there are several variables to consider, and the bigger the area of analysis the more complex it becomes. 

I have a doubt about what you mentioned. Why it would not be required to have a sewer system if there is not water supply? is it because the amount of water is way lesser if it is obtained from boreholes than from a water supply connection?. In fact, many small villages in India still lack water supply systems. Nevertheless, for the sake of finding a good approach to determine a cost-effective level of centralisation, I am assuming the water supply will provided. 

Best regards, 

Sara
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Re: Between the extremes of centralised and decentralised systems. What should be the most important criteria for grouping cities and connecting them to a common wastewater treatment plant?

Hello Sara,

I understand you are doing a theoretical research how 150 villages over 500 km2 in rural dry India can be provided best with sewer systems and (de-)centralized WWTP. Make sure that you write in bold letters in your report that the proposal should only be considered AFTER piped water supplies with house connections to 80% of the households have been installed. Otherwise some donor may mistake your research and finance sewers before water supply.

Doesn't happen? It did, in a city in southern Africa where a sewer system was installed in a low-income area and stakeholders only noted after, that a basic water supply with water kiosks is not sufficient to sustain a sewer system. Yes, and to answer your question: where majority of people are provided with water through point sources like water kiosks and boreholes, they will not and cannot carry enough water to their homes to flush their WC properly.

Other example: after unification of Germany in some East German towns the sewer system were rehabilitated and extended anticipating a population increase...  which did not happen. And now the utilities are faced with blocked sewers due to insufficient flush water and must regularly flush the pipes using fresh water from fire fighting hydrants (ask your utility in Dresden!).

Other example: I supervised construction of a sewer system (the authorities objected to on-site-sanitation) in a German holiday area and had to provide an automatic time-controlled water flush system which had to ensure that the sewers would not block regularly. To have sufficient water in the sewer is also the reason for the 'compulsory connection to sewer services' (Anschlusszwang) in Germany!

And because I have my doubts that 80% of households in those 150 villages in rural, dry India will have piped water at their houses very soon, please make sure that your research is recognized as theoretical sewer project and only be implemented after water supply construction.

YES, sewer systems require piped water AND house connections for 80% of served households. Sewer systems do not function with water supply from boreholes and water kiosks. Nobody wants to carry water over long distance only to flush the toilet.

ciao
Hajo
We can't solve problems by using the same kind of thinking we used when we created them.
Albert Einstein
Any intelligent fool can make things bigger and more complex... It takes a touch of a genius - and a lot of courage to move in the opposite direction.
E.F. Schumacher
Everything should be made as simple as possible, but not simpler. :-)
Albert Einstein
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