Organic Waste Recycling

Land treatment of wastewater 401

process, large suspended solids are removed in the surface soil and smaller
particulates including bacteria are removed in the upper few centimeters of most
soils, except the very coarse soils.
Constituents in the applied wastewater can be diluted by rain and, for cold
climates, snowmelt. Chemical and biological transformations and removals in
the soil also can reduce concentrations of specific constituents. Where
evaporation losses are high, as in arid climates, increases in the concentration of
conservative constituents such as salts can occur.
Excessive suspended solids can clog the soil pores as well as the wastewater
distribution systems. Clogging of the soil will reduce the soil infiltration rate.
Natural decomposition of the organic solids during non-application of
wastewater or "resting" periods will allow the infiltration rate to recover.
The design hydraulic application rate for a land treatment system should be
less than the infiltration rate of the soil. At this application rate, soil clogging
due to suspended solids will not be a significant problem. Thus preapplication
methods for suspended solids removal should be limited to methods such as
screening or primary sedimentation that will avoid clogging of the irrigation
distribution equipment or avoid excessive wear of pumps and piping.

Figure 8.6 Waste management relationships that occur with land treatment (from Loehr
and Overcash 1985; reproduced by the permission of the American Society of Civil
Engineers)

Nutrient removal due to

vegetation

Biological treatment in

upper soil layers

Physical and chemical

treatment throughout the

soil

Percolate contains very small

concentrations of BOD or COD

Nitrogen losses due to

volatilization and

denitrification are

possible

Aeration and aerobic

conditions due to

proper non-application

(resting) period

• Hydraulic retention time = long


• Solids retention time = infinite


• No sludge requiring further treatment and disposal