Reed bed for highly variable output

Hello Feidhlim!
What kind of feautures in your view constructed werland (reed bed) should have in order to cope with high viarabilities of inflow  volume? I am talking about for instance the case of small hotel.  
Thank you,
Bogdan

Hi Bogdan,

Variability occurs for different instances, including for example seasonal businesses like you mentioned, or for variable rainfall inputs from roads, yards or full stream or river catchments etc. Each will need to be designed differently.

For seasonal variability in visitor numbers, your design should cater for the largest population size. Not necessarily for any single day (which may be a single spike in a year of otherwise low average numbers), but at least averaged out over a few days to a week (a longer average is ok if you have a larger wetland). There are different types of wetland design, most commonly: vertical flow gravel reed beds, horizontal flow gravel reed beds and soil based constructed wetlands. The vertical flow systems are not so good for dealing with seasonal flows because they run the risk of drying out when visitor numbers are low. (An exception this may be in Ireland and other places where the winters are good and wet to make up for the lower visitor numbers). For HF reed beds and constructed wetlands, the use of a plastic or heavy clay subsoil liner and outlet flow control weir (usually a pipe on an elblow, set at the right height) will ensure that when the population size is low, the water depth remains constant - it's just that the outlet flow volume will stop or slow down during the quiet season.

I know you're not asking this one, but I'll throw it in to answer the topic title rather than your question: For stormwater fed systems the designs are a little different. Here you'll need to have a restricted outlet to allow flood water to accumulate in the wetland and then drain down slowly afterwards. This means that the soil based wetlands are much more suitable than the gravel reed bed options. By virtue of their design, constructed wetlands survive occasional flooding better than gravel reed beds survive having plants left high and dry for much of the year, if that makes sense to you.

Scroll down through this article http://www.engineersjournal.ie/2017/11/07/reed-beds-natural-solution-irelands-waste-treatment-problems/   to get a quick look at the basic differences between the layout of the different treatment wetland types.

Feel free to shout out some of your visitor numbers, location etc. and I'll have a look at the sizing needed.

Hello Feidhlim!

Thank you very much for the nice explanation!  

The problem with this specific hotel I am talking about is that there are prolonged periods of the no vistitors at all (uo to 2 weeks). Basically it works with the organized groups of diffrent  size by prearrangment and this doesn't depend much on the time of the year. The hotel itself has maximum capacity of 24 people  but usually the groups are smaller.  The flow is mixed  grey and black .  Toilets are non urine separating .  Kitchen sink has a grease 40 l  trap. The stormwater   from the roof i is harvested.  Sometimes events are held in the hotel  (parties and such) without people  staying overnight  and taking showers but heavily using the toilet especially for urination (up to 50 people during event)
Climate data is given in the jpg  chart attached.

There is existing two chamber septic tank with total volume of 8 cubic meters.

I was thinking about the Horizontal Flow RB since it is simpler, there is no much head for gravity dosing and also getting the right substrate for VFRB might be problematic  while gravel of diffrent size (from 5 to 30 mm )is available. An option with artficially aerated RB was also considered.  
Generally the idea is to built a waste water treatment systemcompetitive and alternative to  activated sludge based SBR packaged plant which are on the market and usually used for decentralized sanitation.

Best wishes,
Bogdan

Hi Bogdan,

Let's leave out the occasional 50 people using the toilet, and focus on the hotel capacity of 24 people. Can I also include 6 staff to bring the population size up to 30 people?

Given the variable nature of the inputs, and the seasonal temperature variations, I'd propose a marsh-type, soil based constructed wetland system rather than a horizontal flow gravel reed bed system. Mind you, your rainfall seems sufficiently high to keep a HF reed bed hydrated, so perhaps that would be fine too.  

Here in Ireland the EPA Code of Practice specifies 20m2/person for constructed wetlands, or 5m2/person for HF reed beds. This size gives the equivalent of secondary treatment (same as a standard mechanical/electrical aerated system). For tertiary treatment add 10m2 of wetland or 1m2 of reed bed. Now, having said that I'd be much happier to build a larger reed bed than our codes propose. Thus I recommend 10m2/person for secondary treatment (at gravel surface, reducing towards the base, given the 45' bank slope and system depth of 6-700mm); and 5m2/person for tertiary treatment. This is still half the size of the constructed wetland option.

What is your final destination for the treated effluent? If it can go to a percolation area then secondary treatment should be sufficient in most cases. If you are discharging to a watercourse you may need tertiary treatment and may also need a soil polishing filter (like a raised percolation area to filter bacteria out prior to discharge).

A gap of no people for 2 weeks will be fine given that rainfall will keep the system topped up in the interval, and when the toilets are flushed for the next event, the wetland will simply discharge the treated effluent from the 2 weeks previously. Best case scenario really.

Where are you based?

Regards

Féidhlim

Hello Feidhlim!


Thank you very much!

We are in western Ukraine, Carpathian mountains.

The stuff doesn't live in the hotel.  There are only 2 people coming to cook and clean the place.  


The effluent will go to the open ditch and then the meadow.    
There is no space  on property to have a soil based CW with the size you mentioned (20 m2/person).   I think maximum what we can have is 70 m2 in total.  

What do you think about the option  preatremnt of efflluent prior to CW with compact recirculating media filter to get the eflluent cleaner, more aerobic and thus reduce the size of HFCW
needed for final discharge?

Best,

Bogdan

Hi Bogdan,

If you have 24 people using the hotel, then in order to fit within 70m2 I have the following suggestions.

Media filter unit for secondary treatment (as per your suggestion) followed by a pump-fed vertical flow reed bed (24m2) or gravity-fed horizontal flow gravel reed bed system (c.40m2). As you say, the soil based constructed wetland option will be too large (based on Irish sizing requirements at any rate).

Note that in Ireland and Britain, certain minimum separation distances apply; for example our EPA requires 10m between a septic tank, percolation or wetland and any dwelling; 4m to a public road; 3m to a site boundary; 15-60m+ to a well; etc. These guidelines may significantly reduce your 70m2 area.

If you are sending the final effluent to a ditch and then a meadow, will it be able to percolate into the ground, or is the soil very heavy clay?

Hello Feidhlim!

Thank you for the good advice!  I like the idea of combining compact media filter and  Horizontal CW.   It might work out n our case with the space figures you mentioned.  

The meadow usually absorbs the water quite well unless gets really saturated and/or frozen in winer.  However actually many households send their basically untreated waste water to this meadow through this or other ditch (standart practise in rural Ukraine).   One of our goals is to change this situation through own example.  But solutions should be affordable both in terms of cost and space.  Probably the best strategy actually would be to turn this meadow into  a communal CW that treats wasre water from the whole neighbourhood.  But we are too far from this now.
Best,
Bogdan

Very welcome Bogdan. Is there any potential to convert the meadow to a willow plantation to soak up liquid. Depending on the layout of the meadow you may be able to simply plant it and let the trees grow; or you may need to dig a series of trenches to get an even flow of liquid across the meadow surface; or you may wish to cover these trenches with trench infiltrator units (1' piping, cut in half as 2 gutters and placed face down on the trench base and then covered in soil). This keeps people away from the effluent, which is safer vis a vis contamination. It makes harvesting the willows safer too, since otherwise the risk of contamination from the cut firewood can be very high unless you leave the willows out in the open for a year or two to kill off bacteria that way.