Organic Waste Recycling

Land treatment of wastewater 421

Then,

BOD 5 model: (C - 5)/Co = A.exp (-k.z/qn) (8.11)

TOC model : (C - 7)/Co = A'.exp (-k’.z/qn) (8.12)

The values of coefficients k and n, obtained for q values ranging from 0.1 -
0.37 m^3 / (hr-m) are summarized in Table 8.9.
A family of curves that describe BOD 5 and TOC removal as functions of
slope length (up to 64 m) and application rates (0.1, 0.16, 0.25, and 0.37 m^3 /(hr-
m)) for raw wastewaters and primary effluent are shown in Figures 8.9 – 8.12
(application periods were up to 16 hr/day). Either application rate or slope
length may be used as the independent design variable. The variation of A and
A' with q was found to be inconsistent. Therefore, for design purposes, it would
be satisfactory to use directly the family of curves given in Figures 8.9 to 8.12.
When designing a system, the required or desired level of treatment is
known. Thus a value for (C - c)/Co, according to Equation 8.8 or Equation 8.9
can be established. If slope length is used as the independent variable then a
design value for slope length, z, is selected. The required value of application
rate, q, is then determined by entering the appropriate curve with the known
values of z and (C - c)/Co. Alternatively, if application rate q is the independent
design variable, then a value for q is selected and the required value of slope
length, z is taken from the curves. The independent design parameter used
depends on the individual case.

Table 8.9 Summary of coefficients for BOD 5 and TOC removal models (Smith and
Schroeder 1985)

Coefficient value

Applied wastewater k N

Primary effluent

BOD 5

TOC

Screened raw wastewater

BOD 5

TOC

0.043

0.038

0.030

0.032

0.136

0.170

0.402
0.350
From Smith and Schroeder (1985); reproduced by the permission of the Water Pollution
Control Federation, U.S.A.

A later study by Witherow and Bledsoe (1986) confirmed the removal of
BOD 5 , TOC, total suspended solids (TSS) and NH 4 -N to follow the models
given in Equation 8.11 and 8.12. By setting n at 0.5 and using field data from a
4-ha OF site in Oklahoma, U.S.A., the intercept constant A and rate constant k
of equation 8.13 for the four pollutants are given in Table 8.10.