What is the best decentralized treatment solution for a city which is already built without a centralized sewage system?

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Most of the cities in Afghanistan are built without a proper sewage system, for example Kabul is a metropolitan city with only 3% of the houses connected to sewage and treatment plant.

97% of the city does not have sewage systems and treatment plant for wastewater and they only collect the toilet wastewater: in septic tanks (18%), pit latrine/cesspit (65%), and dry toilets (14%).

All other domestic wastewater flows to the open channels which are connected to the river, therefore in dry season the river becomes a sewage channel. Most of the time people illegally drop the sludge from their cesspits, septic tanks and dry toilets to the river and landfill. 

What is the best decentralized household wastewater treatment solution?
Are there any case studies of cities which have successfully implemented such a solution? All experiences are much appreciated. 

 

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29 Answers

  2. The Water Network research team has consolidated an answer to the above question. 
    Further input from members is welcome and appreciated.  
     

    Both centralized and decentralized sewage systems have their advantages and disadvantages. Deciding which one to use depends on various factors.
     

    Decentralized system can be:

    • Economical – it reduces operation and maintenance costs
    • Green and sustainable – using the resources for agriculture and energy, preserving space
    • Environmental – reduced pollutants and nutrients, mitigated contamination and health risks
       

    Decentralized sewage is a good solution for rural areas in which the housing is periodically placed or for smart alternative communities.
    Ultimately, decentralized wastewater system can be applied in communities of any size and demographic. However, it must be designed properly, maintained and operated in order to provide best results.
     

    Site-specific conditions must be evaluated before deciding on the appropriate type of treatment system.

    Some of the parameters that need to be considered before deciding on the best system are the existing infrastructure, climate, water supply, position and depth of groundwater.

    It is best to dissect the city into smaller, local zones and analyze the topography and the availability and amount of water supply.

     

                * Since we have insufficient knowledge on the details of these parameters The Water Network team has decided to focus on a couple of the successfully implemented decentralized wastewater treatment solutions.

     

    It is possible to make a completely efficient decentralized system in which

    • blackwater (urine and faecal matter) is used as a fertilizer or for energy production
    • greywater (water from kitchen sinks and showers) is treated should be treated in low maintenance systems.
       

    Greywater doesn’t contain as many nutrients as blackwater. However, it may contain over 50% of organic matter and high levels of bacteria, even viruses.

    Greywater should not, under any circumstances be discharged into rivers without treatment!


     

       -    One of the possible solutions is to make a model like or similar to Klosterenga in Oslo, Norway .

    At Klosterenga greywater is treated in a nature-based treatment system.

    klost.png
    Diagram 1: Klosterenga
    Diagram source: umb.no/statisk/ecosan/publications/Klosterenga.pdf

    The system was built in 2000 and successfully treats water from around 100 people. The area requirement is 1 m2/person.

    The system consists of

    • a septic tank
    • vertical biofilter (single pass)
    • horizontal subsurface flow constructed wetland (porous media filter).

    Treated greywater from this system can be used for irrigation or groundwater recharge or discharged to rivers and streams.

    The system doesn’t require a lot of space and produces high-quality effluent.  

     

    septic tank.png
    Diagram 2: Septic Tank
    Diagram source: sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatments/s

    The septic tank is the most used small-scale decentralized unit for grey and blackwater treatment.

    Septic tanks are used for wastewater with a high content of settleable solids. Heavy particles accumulate at the bottom and oil and grease float on the top, while the liquid floats through.
    The settled solids are anaerobically degraded (but periodical removal is necessary since the accumulation is faster than degradation).

    If the removed sludge is dried or composted, it can be used as a fertilizer.

     

    Biofilter captures and biologically degrades pollutants by a bioreactor which contains a living material.

    wetl.jpg
    Diagram 3: Horizontal Subsurface Wetland Constructed Flow
    Diagram source: sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatments/h

    Horizontal Subsurface Wetland Constructed Flow are relatively cheap, easy to maintain and require no electricity or chemicals.

    They are used as a secondary or tertiary wastewater treatment.

    The filter media removes the remaining solids. Most organics are degraded by anaerobic bacteria but wetland vegetation a small amount of oxygen comes from the roots therefore enabling aerobic bacteria to degrade the organics. Plant roots play an important role in maintaining the permeability of the filter.  

     

     

       -    Another example of a decentralized greywater treatment system is implemented in Xi'an, China .

    It uses a dual-pipe system for separate collection of grey and blackwater.

    The system collects water at a flow rate of 100 m3/day from 6 buildings.

    Blackwater is treated in a septic tank and greywater is treated by a hybrid physicochemical process using a fluidized pellet bed separator as the first stage and by ozone enhanced flotation as the second stage.

    All treated water is used for artificial pond replenishment, gardening, road-washing and other non-potable purposes. 


     

    Documents on Decentralized water systems on TWN: 

  3. Very interesting with everyones expereinced and I totally see great value of information..the contributing factor is the cost and water...added problem will be transportation......I would also like to introduce mobile sludge removal system that can treat grey water and dry solids onsite or some paricular spot...This is very affective.....what it does mobile can move around the city and pump out these septic and treated vacuum dewatering device...80/20 DS clean cake for transport and ultra water for reuse......it can be a WWTP unit or a busness venture..

  4. Stick to the basics, in line with the boundary conditions

    Dear Qasem,

    Back in 2007-08 I had an opportunity to spend couple of months in Kabul Municipality and study the challenges of sanitation. I must say the boundary conditions for a conventional sanitary engineering solution are rather challenging - low water supply and coverage, low to very low level of affordability (extreme poverty), low availability of energy, low skill base for operation and maintenance, severe winters, etc. Therefore network solutions coupled with decentralised treatment, however promising they may sound, will be hard to sustain. Further, given the severe cold winters, I am sure you will not be tempted by the promise of 'biogas' (at the other end of the rainbow!), because the bacteria will not sing the song unless you pamper them with a blanket (insulation) and hot water !!

    However, what is very favourable under the given circumstances is the fact that the local population is very much accustomed to dry on-site sanitation (pit latrines have been in vogue for long time).  Secondly, there is very high demand of stabilised sludge among the farmers within and outside the cities for a wide range of horticulture and agriculture crops ( I know the grapes and pomegranates of Kabul are amzing and so are the large roses, among others...). Thirdly, there is a thriving market for the sludge and an active micro-entrepreneur community of service providers. Therefore my suggestion is that a more basic approach is adopted rather than attempting to emulate advanced tech solutions which could be perfect in other geo-climatic and socio-economic settings. It will be far more sustainable if the local agencies embrace and perfect the (prevailing) non-network on-site sanitation paradigm. It will be ideal to include urine diverting systems - look at the local market for fertiliser and the opportunity to create several micro-entrepreneurs out of the local population which otherwise does not have very many sources of livelihood. 

    You would need to perfect the design of the dry toilets with urine diversion and storage features; you will need to evolve a robust faecal sludge collection and motorised transport system (currently done manually and on mules/donkeys) as well as urine collection and transport system; and you will need to set up simple septage treatment facilities (primarily stabilisation ponds and drying beds - you have good Sun during summmers; occasional dosing of lime)  on the outskirts of the cities. If urine volumes and cost of logistics permit, you could consider setting up struvite production unit(s) in decentralised format. Struvite gives long life to urine derived nutrients and makes it lot more economical to transport to farms. 

    Hope this helps. Best regards. 

    Asit Nema

    Director, Foundation for Greentech Environmental Systems

  5. Aerobic On-Site Treatment

    I would recommend starting with the limited infrastructure you have. Rather than building a large sewage treatment plant, treat the sewage at the source. Convert the existing septic tanks to aerobic treatment. Route the grey water into the septic tanks. If desired, the effluent can be polished with ozone and discharged sub surface or pumped by small forced main to detention pond.

    The equipment is relatively inexpensive and the process is robust. http://www.aero-stream.com/

    This solution will quickly remedy 18% of the issues promptly. The latrines can be connected to septic tanks providing an 80% solution. Over time, new septic tanks can be installed in small clusters to treat the wastewater from multiple homes.

    Keep the solution simple and it will be successful.

  6. decentralised solution.

    Fully scalable, portable and efficient. http://soneerawater.com 

  7. Dear Qasem, I think that the drying beds...are a great solution for Areas like Kabul..then you can use the sludge as a fertilizer...........and then you can use the proper technology for the treatment..such as oxidation ditches..or activated sludge.....Don't be stick to a specific technology..you can add or remove stages of treatment according to your case...but try to unify the teatment technology all over the country

  8. Decentralized Sewage Treatment Plant

    Hi,
    DEWATS is one of the best decentralized sewage treatment. Very low maintenance and can work for any required size and easy to construct. One can generate Bio gas also from the same. Recycled water can be used for flushing, gardening or farming. There is no mechanized part in system unless one required for feeding sludge or pumping the recycle water. We have worked on up to 1 MLD in size.

  9. To answer the final question of the presentation There is no situtation any country any city whose sanitation wastewater soity performance. The purification performance of public sanitation is almost zero, near zero cost for proébitif The question that we are entitled to ask: Why? All siemplement because the principle of collective and individual sanitation and elsewhere is to transform feces into mud. More than 40 million tons of human waste are transformed into mud annually. The Biological Remediation ELIMINATES him fully feces without sludge production. The elimination process produces a liquid that can fertilize all agricultural land in abundance.

  10. French English L'Assainissement Biologique correspond entièrement à la problématique du sujet. Son premier objectif est de supprimer tout système d'assainissement collectif avec une station d'épuration. Trop couteux et totalement inefficace. Ce document démontre que l'urbanisme comme de partout dans le monde entier est totalement incapable d'assurer l'assainissement urbaine. Les constructions s'implantent avec un assainissement ou pas d'assainissement. De par ce fait , l'administration doit prendre à sa charge cette partie importante de la salubrité urbaine. C'est l'administration qui implante systèmatiquement un système d'assainissement pour toute nouvelle construction. Les utilisateurs remboursent mensuellement le montant de cette implantation sous la forme d'une taxe d'assainissement jsuqu'à concurence du complet remboursement de la somme investie. Pour les habitations déjà en place Quand le terrain privé permet une implantation individuelle, l'état l'installe et se fait rembourser Quand le terrain privé est trop faible, l'état installe un assainissement semi collectif Une partie du système est privé sur le terrain, l'autre partie est sur le terrain de la communauté. English The Biological Remediation fully corresponds to the problem of the subject. Its primary objective is to remove any sewerage system with a treatment plant. Too expensive and totally ineffective. This paper demonstrates that urban planning as from all over the world is totally incapable of providing urban sanitation. The buildings are implanted with a sanitation or no sanitation. Because of this, the administration must take charge of this important part of urban sanitation. It is the administration that implements prompted systematically a sanitation system for new construction. Users monthly repay the amount of the settlement in the form of a sanitation tax jsuqu'à concurence full refund of the amount invested. For homes already in place When private property allows individual locations, the state installs and is reimbursed When private property is too low, the state sets up a semi sewerage Part of the system is private on the ground, the other part is in the field of community.

  11. Dear Qasem, I am so glad you asked that question. Like many of the people have already mentioned, decentralized sewage treatment is very common in Europe and has worked well. We have been doing this in Dubai as well as in Karachi for the past several years using very good SBR type Sewage Treatment Plants, and have had excellent results. I will be happy to share our experiences with you. I do go to Kabul from time to time, so please let me know if you're interested and we can meet when I am there next. Please write to me directly at; jawad@claritywater.ae. I look forward to hearing from you. Regards, Jawad.

  12. Decentralized Sewage Collection and Treatment

    Hi Qasem;

    As part 2 to my answer earlier; I would suggest that you first treat Kabul in the same manner we look at managing water networks; - ie: first break it up into smaller and easier to manage Sewage zones based on local features such as topography and quantum of water supply.

    This will allow you to figure out what parts can be decentralized and what cannot not. The idea of many small mini-STP's does not work as we still have to transport the solid component of the sewage to them which is difficult when water supply is low and variable as I previously mentioned. The best solution I have encountered in 15 years off working is the one I currently represent as it digests the organic load and traps the inorganic in multiple biodigester tanks. As a result you have smaller quantities of inorganics distributed in each tank instead of large amounts accumalating at a single location and choking the network. Eliminating the requirement of transporting solids means that the network can have shallower slopes and will be easier to install. This also eliminates the need for manholes. the piping can now be HDPE or even PVC-O whose flexibility allows you to navigate obstructions and corners in Asia's organically grown cities...

    Hope this helps, I think it is the only solution that at one go eliminates the problems you mentioned as both the greywater and sewage are dealt with. best of luck and please feel free to reach out to me via email at sdeshpande@clearford.com for any further help...

     

  14. Looking at the Sewage Treatment Problem Forces You Into Limited Outcomes

    Before I start one thing I should comment on right away is the concept of using sewage for food production. Though I agree 100% in taking organic waste and using it for food production - sewage is something completely different and filled with many problems if you do not use a system that sophisticated enough to kill pathogens and deal with such toxins as oil, chemo therapy, detergents, and the millions of chemicals humans dispense. 

    Cities such as Kabul have far more complex issues that should also be looked at as well versus using systems that will eventually collapse or pollute ground water which may already be thought of as a lost cause due to the current level of pollution.

    Corruption can also be a major factor that is blocking solutions that would be far superior for the community and of course greed is always an issue. 

    If one was able to look at the situation openly it might be possible to come up with radical solutions that in the end are far more practical. For example a proper sewage system is possible if all infrastructure is dealt with at the same time - this can be done by leveling one part of the city at a time when it become impossible to deal with the sins of the past. There is an opportunity to deal with sewage, clean water, renewable energy, and disease control using technology that is extremely beneficial to the community as a whole versus a few companies. Even industry should be looked at for ways to optimize water usage which as one writer commented on impacts how sewage is handled.

    Good luck with your project.

    . 

  15. Decentralized Sewerage Collection and Treatment

    Unfortunately, while seeming straightforward, the issue of putting sewage collection and treatment solution into villages, towns and smaller cities is difficult for two reasons:

    1. Organic unplanned urban growth
    2. Unreliable, Low or Variable water supply often below hydraulic cleansing velocity

    Conventional sewer networks are designed to be built in areas where there are neat straight roads and cannot therefore be easily placed in areas with short or curving roads. In addition; the low and often seasonally variable water supply means that conventional sewers cannot effectively transport sewage without the solid component falling out of suspension, choking the pipe network and increasing maintenance costs. This problem can often only be rectified by increasing the slope of the pipes which then results in pipes having greater depths and deeper manholes which increase the risks and costs associated with maintenance as additional water is not available. Combined this with the additional risk to lives caused by houses collapsing as deep trenches are dug in narrow lanes and one begins to understand why open drains, septic tanks and soak pits are employed to convey wastewater away.

    Full disclosure: I run the India operations of Clearford Water Systems Inc., (www.clearford.com) a Canadian company specializing in just such solutions and the below is our solution.

    The Clearford One system removes sewage solids at source in ClearDigest™ Smart Digesters™ before releasing liquid effluent into the SBS® ClearConvey™, a network of smaller diameter pipes that carries liquid sewage to an optimized Waste Water Treatment Plant (WWTP) facility for final processing.  The reduced contaminants present in the liquid effluent allows for more efficient recovery of reusable water using the ClearRecover™ WWTP package.  Recovered water can be safely returned to the environment or redistributed for non-potable uses such as toilet water, fire protection, irrigation, and groundwater recharge. 

    ClearDigest™ Smart Digester™ tanks provide decentralised pre-treatment of the organic component of wastewater through anaerobic digestion, while retaining inert solids in the tank.  These Smart Digesters™ greatly reduce the solids and organic loading in the liquid effluent while continuously decomposing sewage solids.  Clearford patented technologies enhance the natural processes in the tank such that they are pumped out only after decades of normal use.  Peak flow is also reduced, resulting in a consistent trickle of wastewater to the SBS® ClearConvey™ collection network. This process does not use cultured bacteria, chemicals or electricity to operate.

    ClearConvey™ Small Bore Sewers® (SBS®) use modern HDPE piping technologies to achieve a watertight network of small diameter pipes for the collection and conveyance of the liquid effluent by gravity.  The flexibility of small diameter pipes allows for adaptable installations in shallow trenches with less material requirements.  Additionally, the seamless fusion of pipe sections eliminates infiltration to the system and stops leakage of sewage to the environment.  Thus, ClearConvey™ overcomes many of the constraints associated with conventional sewer systems. The network also eliminates the need for manholes which are the source of the majority of inert solids found at WWTP bar screens from entering pipe networks.

    At the end of the system treatment requirements are significantly reduced compared to conventional systems, as solids and organic matter are reduced by a typical 65-75% in terms of BOD, COD and TSS loads in the liquid effluent reaching the WWTP facility, and peak flows are reduced to a factor of 2x.  This allows for the water to be treated and recycled at significantly lower O&M costs.

  16. We suggest modular decentralized sewage treatment plants ranging from capacities of 5 kl to 100 kl batch processed and return treated waste water for non potable uses like irrigation etc which run on need based requirements....soak pit / septic tank being the lift station..... Madan Iyengar www.ozosciences.com

  17. What is the best decentralized treatment solution for a city which is already built without a centralized sewage system

    Dear Questioner: Please see these two papers which also have many references on the same subject.

    (1) Wastewater Management Strategy: centralized v. decentralized technologies for small communities 

    (2) Decentralized Wastewater Treatment Systems    Kind regards, Prof. Dr. Hilmi S. Salem  

  18. It is an opportunity for a better system

    Cities without a centralised sewage system have the opportunity to use a more ecologically appropriate system where the nutrients present in sewage are used to produce food. Localised septic tanks can be used in conjunction with a system for pumping the effluent to a place where further biological treatment like the German constructed wetland technology can be used. Ideally, the waste water should be used for growing vegetables locally. 

  19. Same situation here in Sudan, in my country the govenment have not any willness to do it so because they realized it will need a lot of money and there is no immediate benefits of it so they just ignored although the coming generations will suffer due to complexity of problem taking into accout the increase of population.

  21. handling sewage water without central system

    The best and only solution may be 

    Trucking the seweage water to treatment plant out of city,

    They are doing it at Dubai

     

    Rgds

    M Jahangir

    Pakistan

  23. Guidelines and material for strategic planning

    Dear Qasem,
    the best decentralized system for a city like Kabul depends a lot on local conditions, as :
    -climate
    -local culture and what is socially accepted
    -already existing infrastructure
    -the costs local households can pay
    -local political and communal organisation
    -etc.
    As local conditions are really important, there is not an unique answer.

    To start, I would like to recommend you some material done by EAWAG in Switzerland which can help you to design a full system :
    CLUES : Community-Led Urban Environmental Sanitation Planning
    http://www.sandec.ch/forschung/sandec/gruppen/SESP/projects_sesp/clues/i...
    This guidelines are mostly about planning and implementing a sanitation system.

    Compendium : http://www.eawag.ch/forschung/sandec/publikationen/compendium_e/index_EN
    Mostly about more technical aspects

    There also has been a open online course on this. Videos can be found here :
    https://www.youtube.com/channel/UCgHWg270mPystIe5rVFOvCA

    Finally, I can also recommend you the SUSANA Forum where you can find a lot of people having worked in this kind of projects.

    I hope this information will help you.

    Best regards,
    Patrick

  26. Check out the Integrated Rural Sanitation System (SISAR) in the Brazilian state of Ceará and the ‘condominial sanitation’ system in the city of Recife. You might also want to contact the EU-funded FP7 project DESAFIO which is looking into low tech sanitation solutions and community-led sanitation too.

  27. Consider the installation of a low pressure sewage system. these system utilize a small pump chamber with a high pressure pump. A common force main, generally less than 3" in size is installed in the ground below frost depth (42" below grade commonly). Individual lines from the septic tank to the common force main is 1.25" or 1.5". the pumps are grinder pumps so the raw sewage is converted to a slurry and pumped to a treatment plant. these systems can be used in flat or hilly terrain. Please contact me if you have any additional questions about these types of systems.

  28. We need to know the total volume of sewage generated? Also why is there consideration for a decentralized system? An answer can be given after studying the topography, land availability and presumably availability of energy. If I presume right Afghanistan is a low per capita energy producer and consumer. If that is the case none of the activated sludge systems will be successful. There will be a need for low energy need technologies to deal with the Wastewater. MAST Technology would be a good way to go about it. If you wish to get more information about MAST please send me an email at advenvtech@gmail.com.