Organic Waste Recycling

418 Organic waste reuse and recycling: technology and management

8.5 LAND TREATMENT- DESIGN EQUATIONS

Mathematical models for the three land treatment processes have not been
extensively developed. This is due partly to the complex nature of land
treatment involving the physical, chemical and biological reactions. Efficiency
of a land treatment system also depends on climate and soil conditions at the
specific site. SR process is normally designed based on practical experiences.
Some empirical and rational models for the RI and OF processes have been
developed.

8.5.1 RI process

Treatment performance of RI systems with respect to nitrogen and phosphorus
removal is given below.

Nitrogen removal

Application rates up to 30 cm/day with 20 mg/L of ammonia will result in a
nitrified effluent. As wastewater temperatures drop, the rate of nitrification will
also decrease. For nitrification, the loading cycle should consist of short
application periods (1 day or so) and relatively long drying periods (5 to 10
days) to allow for aeration and the occurrence of aerobic conditions in the soil
favourable for the growth of nitrifying bacteria.
Nitrogen removal by denitrification requires both adequate organic carbon
and adequate detention time. Nitrogen removal can be expressed by the
following equation (Reed and Crites 1984).

TOC - 5

Nt= (8.5)

2

Where:

Nt = total nitrogen removal, mg/L

TOC = total organic carbon in the applied wastewater, mg/L

The 5 mg/L of residual TOC is typical for municipal wastewater passage
through about 5 ft (1.5 m) of soil. The coefficient 2 in the denominator is based
on experimental data where 2 g wastewater carbon was required to denitrify 1 g
of wastewater nitrogen.