Dear Sir. Concerning your issue about the calculation of organic loading rate for the vertical flow constructed wetland. The effect of different operational conditions on wetland performance was evaluated on the basis of percent removal, mass removal rate, areal removal rate constant and volumetric removal rate constant.

The percent removal (removal efficiency) was calculated as follows:

Removal efficiency(%)=(Cin-Cout)/Cin×100

where Cin and Cout = inflow and outflow concentrations, respectively (mg L−1).

The mass removal rate (r, in g m−2d−1) was calculated as follows:

r = q(Cin-Cout)

r = mass removal rate (g m−2 d−1).

q = hydraulic loading rate (m d−1).

Removal rate constants: A first-order degradation approach has been used to predict the removal performance of COD, BOD, TSS, N and P in the constructed wetlands. The rate constants for this model can be defined on either an areal (kA) or a volumetric (kV) basis.

The areal removal rate constant (kA) was calculated using the equation proposed by [25]:

ln(Cout/Cin)=-kA/q

where

q = hydraulic loading rate (m day−1) = Q/A,

Q = flow rate through the wetland (m3 d−1),

A = area of the wetland (m2), and

kA = areal removal rate constant (m d−1).

The volumetric removal rate constant (kV) was calculated using the equation :

ln(Cout/Cin)=-kvt

where

kv = volumetric removal rate constant (d−1),

t = hydraulic retention time in the wetland (d) = VƐ/Q,

V = volume of the wetland (m3), and

Ɛ = wetland porosity.

I think the following below links may help you in your analysis:

https://www.sciencedirect.com/science/article/pii/S2090123215001265

https://pdfs.semanticscholar.org/5bc4/1006ca87ccc30d105c55fd15bd8a85c3d832.pdf

Thanks

https://www.sciencedirect.com/search?qs=Organic%20loading%20rate%20for%20vertical%20flow%20constructed&show=25&sortBy=relevance

Agree with Isam Eldin Hussein Elgailani.