Removal of Nutrients in Constructed Wetlands

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Dear wise friends, we are planning to construct a wetland as a tertiary treatment after an activated sludge treatment unit (0.8 Ml/day domestic sewage). 

The activated sludge is not designed to remove phosphates, but reasonable denitrification will occur (in an orbal raceway configuration with aeration and anoxic conditions occuring before re-aeration).  We are planning to use a broken shale gravel bed which according to the literature will absorb excess phosphates, and planting vetiver grass into the bed (horizontal flow). We realise the shale may become saturated after a time, and no longer act as a sink for phosphates. 

I would appreciate other's experience and comments.

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10 Answers

  1. Dear Ian,

    Biological P-removal can also be improved in AS plants, by improving sedimentation in the clarifier, and avoiding microbial cell autolysis which releases cellular-P in water.

    Ask me if this is of any interest concerning your project.

    François

  2. Hi,

    There are two main ways you can go about P-removal for a more long term solution (chemically or biologically). As you have a problem with ferric storage (chemical), maybe you should consider biological p-removal instead using an EBPR system and PAO's. 

    If your focus is ammonia removal then aeration is sufficient.

    Seeing as you have denitrification occurring means you have anoxic zones and are also removing nitrates.

    For an enhanced biological phosphate removal process, you will require an anerobic zone as well.

  4. for phosphorus, we have a high surface area iron media, which traps phosphorus.  the neat thing is you can recover the phosphorus.  very green

  5. Wow, thank you all for your responses so far - that is very helpful.  To respond to some of the issues and questions: the water supply is sourced from a dolomitic spring (karst) in South Africa so is quite a hard water.  There is provision made for dosing ferric chloride, however the local authority has a very low revenue base and a poor reputation on O&M, so we are not confident that they will keep stock of the ferric, and secondly mechanical breakdowns on the works may take some months to repair.  The effluent receiving stream is fed by the same dolomitic spring and is pristine.  The purpose of the wetland is then  to reduce P and remaining N, but also as a buffer when there are failures on the activated sludge works. 

    We have considered floating wetlands as they have been recently piloted in another municipality.  Will definitely re-visit this option.  We also have waste products generated by mining operations (e.g. ferric sludge from acid mine water treatment) which we could consider. I will also follow up on the other leads.  Many thanks

    2 Comments

  6. Dear Ian,

    I agree with Alexandros, the fisrt question is: what is the tertiary CW designed for?

    If it is for P removal, a P precipitation step added to the activated sludge willprobably be the most efficient/reliable, and cheapest. To my point of view, P adsorption/precipitation on specific substrates, either natural or engineered, should be used mainly on smaller units, where activated sludge are not advisable.

    If total N removal is the goal, there again, a proper management of aeration cycle in the AS will do the job.

    To my point of view, the best use of CW's as a tertiary treatment after an AS is to securize the outlet in terms of suspended solids, by entrapping any sludge loss. A properly designed vertical flow CW, with relatively fine filtration sand, would be perfect for that purpose.

    Best regards,

    François

  7. Dear Ian,

    I would recommend the material of Metamatria PO4-sponge with high capacity and possible to install in nets and can be regenerated if loaded.

    secondly I would recommend the floating wetlands from BGS  from Germany https://www.bestmann-green-systems.de/produktgruppen/schwimmmatten-und-begruenungssysteme-repofloat-repotex.html

    you can use just only the floating system or with plants. Fare as i know patented and the cheapest solution on the market.

    I also recommend a design with lines to use the floating wetlands as a guiding for the flow and so that all the biofilm on the rises is always used and you have no dead zones. 40 % of the surface of the pond is minimum recommended for the effect you want to have.

    In addition I recommmed to use the OLOID for a movement and flow with the lowest energy demand.

    if you need help with the design please let me know.

    All the best

    Eric Schieblich

    eric.schieblich@inversions-technik.ch

  9. First,  what is your main goal for the tertiary wetland, to only remove P from the secondary effluent or also to have some denitrification? If P is the case, I am not sure a horizontal flow wetland (suppose subsurface) can indeed be an option. If P is the target pollutant, it will be cheaper however to simply add eg ferric chloride in a small tank to precipitate P. The use of shale gravel in the wetland will indeed remove some P, but after a short period of time the material will get saturated and the P removal will start again deteriorating.

    What I usually do to improve P in my constructed wetland designs (if I want to avoid chemicals) is (a) install a simple gravity filter with a special media (specific to P removal, eg steel slag, bauxite etc) to polish the effluent, so that I can have easy access and easily replace the material when it gets saturated. However, although effective, this is not a very cheap option and involves some additional operation tasks, (b) I go for a combination of subsurface flow + surface flow wetland (which is cheaper to build) and exploit biological processes (eg plant uptake) and filtration/precipitation, so that I can also improve the quality in general (eg nitrate removal). Of course, the level of inflow P and the desired P effluent limit you want to achieve play a crucial role in the selection of the most appropriate solution, as well as the available carbon source, should denitrification also be desired.

    1 Comment

    1. Vertiver is mentioned in one answer above, they must be an indigenous species that will survive the environmental and agronomical conditions of the location of the wetland, look at alternating beds, with other species with an affinity for metabolizing P in their roots.

      1 Comment reply

      1. I suppose vetiver is native in that area that's why they are using it. But again, I dont expect any significant effect on P removal in the long run. If you really want to see an effect, then longer retention times are needed, which is not cost-effective.

  10. This is not a simple answer , it depends on the quality of the water and the country.  Our expert  is Dr Dinesh Kumar  (IIT Bombay)  Rebound Enviro Tech Pvt Ltd  see www.rebound.co.in.  He is a world class  expert in Wetland constrcutions . tel +919930105108 - but very busy