TM 5-814-3/AFM 88-11, Volume III
(9) Recarbonation. This subject is discussed in detail in the EPA Process Design Manual for
15-8. Land application Systems.
a. Background. The use of land and biomass growth upon and within the soil has a long and interesting
history. This history, as well as a much more detailed treatment of land application of wastewaters is covered
in EPA 625/1-81-013. It should be noted that this manual is intended to be supplemented by the U.S. EPA
manual for detailed design criteria. This has been done because of the broad site-specific design conditions
that exist for land application systems.
b. Health hazards and regulatory limitations. Because land treatment of wastewater entails a higher
risk than other treatment processes of introducing pathogenic micro-organisms and toxic chemicals into
groundwater and surface water, land treatment system design must carefully consider all possible means to
prevent water supply contamination. Additionally, state and local health regulations often dictate land
treatment process design criteria. Therefore, these regulations must be consulted early in the design phase
and frequently throughout construction and operation to ensure consistent compliance.
c. Treatment capabilities and objectives. Land treatment of domestic wastewater which has undergone
secondary treatment and sludges from wastewater treatment plants may involve one of the following modes
(Land treatment of wastewater after primary treatment is acceptable for isolated locations with restricted
access when limited to crops which are not for direct human consumption.):
-- Slow rate filtration;
-- Rapid infiltration;
-- Overland flow;
-- Use of wetlands; and
-- Subsurface incorporation.
Two methods of land treatment apply to sludges:
-- Composting and land spreading; and
-- Subsurface incorporation.
d. Slow rate processes. Slow rate processes essentially mean irrigation of crops, grassland or forest land
based upon the demand of the vegetation. Typical application methods involve pipeline to row crops, surface
distribution along furrows and ridges on the contour, sprinkler irrigation, or drip irrigation. Sprinklers and
drip irrigation require that wastewater is quite free of solid suspended matter In arid to semi-arid areas,
utilization of such wastewaters-even if only to recreational areas on a military compound-should be seriously
considered both for conservation and to improve local aesthetics.
e. Rapid infiltration. Rapid infiltration, often called "infiltration percolation," involves almost complete
saturation of the soil column and potentially also the rock beneath. A thick, sandy regolith with low water
table is required. Fresh water wells, well points and springs must be sufficiently far away as to not receive
contamination. Often the object is to renovate water and to recapture the effluent again with special wells
or underdrains for re-use in cooling or irrigation. Rapid infiltration may often be used to prevent the intrusion
of saline water on an atoll or sandy coastal plain site. Although vegetation utilization is not planned for rapid
infiltration systems, studies have shown that use of deep rooted plants, an active root and humus mat, and
tolerant vegetation will much improve the quality of the recovered water. Vegetation must be carefully
selected and, of course, some water will be lost to evapotranspiration and to production of biomass but the
"living filter" will produce excellent quality water beneath the surface. (See D'Itri et al., 1982.)
f. Overland flow. This process involves a surface phenomena and depends strongly on vegetation and
the myriad organisms in the humus layer of a sloped field. Wastewater is applied over the upper reaches of
sloped terraces carefully constructed to match the contour of the land. Runoff after surface flow is collected
in ditches. Application may be from linear pipeline sprayers, plastic trickle irrigators, or using rotating
sprinklers. Overland flow could be used in forested land or to produce forage. Like the bio-filter concept,
such systems not only remove suspended solids, kill pathogens and lower biochemical oxygen demand, but
dramatically lower levels of nitrogen and phosphorus. (See D'Itri et al., 1984.)