You know Christoph...
as we all deal in all this sewage-calculations very often with rough assumptions, rules of thumb and made personal experiences, I by my self try to keep very much on the conservative side of sewage-line. Having many unsure numbers involved I go even for up to 30% additional security, offering a possible customer a more robust/flexible system, rather to win by all means e.g. a tender. This extra %'s I mention transparent to the possible costumer.

If more numbers are trustful & O&M is in good hands a 30% extra security is not necessary. This is just my personal opinion/taste not a recommendation to you and others.

Thanks again
Detlef

Hi Detlef,
To stick with your example:

600 l /d with 600 mg/L COD -60% by improved pre-tretment = 240 mg/l = 144 g/d COD Organic loading per day 144 g/d : 60 gCOD/(m²•d) = 2,4 m² -> 250 mm/d!!

110 gCOD/pe,d 200 l/pe,d = 550 mg COD/l normal value

With rain influence still this goes down.
Therefore: Yes the hydraulic loading is a relevant limit for many cases here in Brazil for me.

Don´t forget the oxygen balance – it is very relevant especially as well for the hydraulic load.

Obs: When I come to 3,2 I use 3,2 and not 4, the difference is 25%!

All the best
Christoph

Thanks Christoph,

Now I understand, the limiting factor will never a possible 40% higher hydraulic loading. It is always driven by organic loading 60-70 gCOD/(m²•d) and filterable solids loading 5 g/(m²•d). Therefore, the CW-area will be by all means always large enough for the hydraulic limit 200 mm/d.

I had to write it down to understand better :

warm: (200 mm/d)
600 l /d with 800 mg/L COD -30% by septic tank = 560 mg/l = 336 g/d COD Organic loading per day 336 g/d : 60 gCOD/(m²•d) = 5,6 m² --> 6 m² = 100 mm/d

600 l /d with 800 mg/L COD -60% by improved pre-tretment = 320 mg/l = 192 g/d COD Organic loading per day 192 g/d : 60 gCOD/(m²•d) = 3,2 m² --> 4 m² = 150 mm/d

cold: (100 mm/d)
600 l /d with 800 mg/L COD -30% by septic tank = 560 mg/l = 336 g/d COD Organic loading per day 336 g/d : 20 gCOD/(m²•d) = 16,8 m² ---> 17 m² = 36 mm/d

600 l /d with 800 mg/L COD -50% by improved pre-tretment = 50 mg/l = 240 g/d COD Organic loading per day 240 g/d : 20 gCOD/(m²•d) = 12 m² = 50 mm/d

Maybe only a CW-area sizing for grey water with low organic loadings will go to the limit of 200 mm/d hydraulic area loading.

All the Best
Detlef

Hi Detlef,
baffeld reactors are more common in warm climates. Please have a look at section 5.2, there the limits to be respected are explained.
Including references.

In case of any further doubt please feel free to ask.

Christoph

Dear Günter and dear Christoph,

May I have your valuable advices/suggestions on that?

See: 2011, Technology review of constructed wetlands Subsurface flow constructed wetlands for greywater and domestic wastewater treatment by GIZ
page 21:

…
Baffled tanks have a higher removal ratio for organic matter (40-60% BOD removal) compared to septic tanks (typically 30%) due to the more efficient clarification and sludge retention in the baffled tank.

Also, a constructed wetland treating wastewater which has been pre-treated in a baffled tank needs only about 60% of the area of a constructed wetland which is treating wastewater after pre-treatment in a septic tank. Therefore, the higher costs of pre-treatment for the baffled tank are partially or fully offset by the lower cost for the constructed wetland.
…

In this review is not mentioned the maybe possible problematic of a hydraulic overloading (Maybe “killing” the Biofilm by too much water?) of the then up to approx 40% “smaller” CW-“filter”-area under tropical or cold conditions. I regret, not mentioned any made studies on this important suggestion („unabhängige und bestätigte Untersuchungen“ fehlen m.e.) by the authors.

What are your experiences on possible hydraulic overloading by sizing CW’s up to 40% smaller, regardless in cold or warm climates, if higher removal ratio for organic matter due to the more efficient clarification and sludge retention by any improved pre-treatment devices?

Thanks in advance.
Detlef