Organic Waste Recycling

Land treatment of wastewater 389

Methods of irrigation

Spray, ridge and furrow, and flood are the wastewater irrigation methods
commonly adopted in this process (Figure 8.2). Spray irrigation may be
accomplished by using a variety of systems from portable to solid-set sprinklers
(Figure 8.2c). Ridge and furrow irrigation consists of applying water by gravity
flow into furrows. The relatively flat land is groomed into alternating ridges and
furrows with crops grown on the ridges (Figure 8.2b). Flood irrigation is
accomplished by inundation of land with water. The type of irrigation system to
be used depends upon the soil drainability, the crop, the topography, process
objective and the economics. Table 8.5 gives the methods of surface irrigation
(flood and ridge and furrow) and their conditions of use applicable for
wastewater and other irrigation water.

Table 8.4 Comparison of site characteristics for land treatment processes (U.S. EPA
1981)

Slow rate Rapid infiltration Overland flow

Grade

Soil

permeability

Depth to

groundwater

Climatic

restrictions

Less than 20 % on

cultivated land;

less than 40 % on

non-cultivated land

Moderately slow to

moderately rapid

(clay loam to sand

loam), > 0.15 cm/h

0.6 – 1 m

(minimum)b

Storage often

needed for cold

weather and during

heavy precipitation

Not critical; excessive

grades require much

earthwork

Rapids (sands, sandy

loams), >5.0 cm/h

1 m during flood

cycle;b 1.5 – 3 m

during drying cycle

None (possibly modify

operation in cold

weather)

Finish slopes 2- 8 %

Slow ( clays, silts and

soil with impermeable

barriers), < 0.5 cm/h

Not critical

Storage usually

needed for cold

weather

a Steeper grades might be feasible at reduced hydraulic loadings.
b Under drains can be used to maintain this level at sites with high ground water table.
c Impact on ground water should be considered for more permeable soils.