Organic Waste Recycling

Land treatment of wastewater 419

Nitrogen removal is also related to infiltration rate and an application rate of
15 cm/day is recommended as a maximum where 80% N removal is needed
with secondary effluent (Figure 8.8). When primary effluent is used, the
maximum application rate is recommended not to exceed 20 cm/day.

Phosphorus removal

There is no crop uptake in RI systems and the nature of the soils and high
hydraulic rates used require greater travel distances in the soil for effective P
removal.
An equation to predict phosphorus removal at RI sites has been developed
from data collected at number of operating systems (Reed and Crites 1984).

Px = Po .exp(-kt) (8.6)

Where:

Px = total P concentration at a distance x along the percolate flow path, mg/L

P 0 = total P concentration in the applied wastewater, mg/L

k = instantaneous rate constant, 0.002/hr at neutral pH

t = detention time = x.ș/I, hr

x = distance along the flow path, in

ș = volumetric water content, usually 0.4

I = infiltration rate, in/hr

8.5.2 OF process

Based on a pilot study conducted by Smith and Schroeder (1985), it was found
that the removal of organic material from primary effluent is accomplished in
two stages as a function of slope length (z) and application rate (q).
In the first stage a rapid removal of organic matter takes place within the first
6 to 10 meters of slope length and in the second stage a slower rate of organic
removal takes place in the remainder of the slope.
From a practical design and operation standpoint only the second stage would
be the basis for design and operation.
The probable reason for the observation of the two distinct rates of organic
removal over the length of the overland flow slope is that settleable matter is
removed readily by sedimentation in the first few meters, leaving colloidal and
soluble organic material to be removed at a slower rate by adsorption and,
ultimately, bacterial assimilation over the remaining length of the slope.
The mathematical model was developed from the studies made on lands with
down-slope grade of 2% and cross-slope grade of 0.2% with a mixture of