o_19f3m1n7k170b14e9u0oqipgu4a.pdf

are called rapid infiltration basins. Basin infiltration
rates may sometimes be enhanced or maintained
by creation of ridges within the basin
(Peyton, 2002). The advantage of ridges within
the basin is that materials that cause basin clogging
accumulate in the bottom of the ridges while
the remainder of the ridge maintains high infiltration
rates.
Rapid infiltration basins require permeable soil
for high hydraulic loading rates, yet the soil must
be fine enough to provide sufficient soil surfaces
for biochemical and microbiological reactions,
which provide additional treatment to the reclaimed
water. Some of the best soils are in the
sandy loam, loamy sand, and fine sand range.
When the reclaimed water is applied to the
spreading basin, the water percolates through
the unsaturated zone to the saturated zone of
the groundwater table. The hydraulic loading
rate is preliminarily estimated by soil studies,
but final evaluation is completed through operating
in situ test pits or ponds. Hydraulic loading
rates for rapid infiltration basins vary from
65 to 500 feet per year (20 to 150 meters per
year), but are usually less than 300 feet per year
(90 meters per year) (Bouwer, 1988).
Though management techniques are site-specific
and vary accordingly, some common principles
are practiced in most infiltration basins. A
wetting and drying cycle with periodic cleaning
of the bottom is used to prevent clogging. Drying
cycles allow for desiccation of clogging layers
and re-aeration of the soil. This practice helps
to maintain high infiltration rates, and microbial
populations to consume organic matter, and
helps reduce levels of microbiological constituents.
Re-aeration of the soil also promotes nitrification,
which is a prerequisite for nitrogen removal
by denitrification. Periodic maintenance
by cleaning of the bottom may be done by deep
ripping of the soils or by scraping the top layer of
soil. Deep ripping sometimes causes fines to
migrate to deeper levels where a deep clogging
layer may develop. The Orange County Water
District (California) has developed a device to
continuously remove clogging materials during
a flooding cycle.
Spreading grounds can be managed to avoid nuisance
conditions such as algae growth and insect
breeding in the percolation ponds. Generally,
a number of basins are rotated through filling,
draining, and drying cycles. Cycle length is
dependent on both soil conditions and the distance
to the groundwater table. This is determined
through field-testing on a case-by-case basis.
Algae can clog the bottom of basins and reduce
infiltration rates. Algae further aggravate soil clogging
by removing carbon dioxide, which raises
the pH, causing precipitation of calcium carbonate.
Reducing the detention time of the reclaimed
water within the basins minimizes algal growth,
particularly during summer periods where solar
intensity and temperature increase algal growth
rates. The levels of nutrients necessary to stimulate
algal growth are too low for practical consideration
of nutrient removal as a method to control
algae. Also, scarifying, rototilling, or discing
the soil following the drying cycle can help alleviate
clogging potential, although scraping or “shaving”
the bottom to remove the clogging layer is
more effective than discing it. Removing the hard
precipitant using an underwater machine has also
been accomplished (Mills, 2002).
2.5.1.2 Soil-Aquifer Treatment Systems
Soil-Aquifer Treatment (SAT) systems usually are designed
and operated such that all of the infiltrated water
is recovered via wells, drains, or seepage into surface
water. Typical SAT recharge and recovery systems are
shown in Figure 2-7. SAT systems with infiltration basins
require unconfined aquifers, vadose zones free of
restricting layers, and soils that are coarse enough to
allow high infiltration rates, but fine enough to provide
adequate filtration. Sandy loams and loamy or fine sands
are the preferred surface soils in SAT systems. Recent
work on SAT removal of dissolved organic carbon (DOC),
trace organics, and organic halides has shown positive
results (Fox et al., 2001; Drewes et al., 2001). The majority
of trace organic compounds are removed by biodegradation
and organic chlorine and organic bromine are
removed to ambient levels. Short-term DOC removal is
enhanced by maintaining aerobic conditions in the unsaturated
zone (Fox, 2002).
In the U.S., municipal wastewater usually receives conventional
primary and secondary treatment prior to SAT.
However, since SAT systems are capable of removing
more BOD than is in secondary effluent, efficient secondary
treatment may not be necessary in cases where
the wastewater is subjected to SAT and subsequently
reused for nonpotable purposes. Higher organic content
may enhance nitrogen removal by denitrification in
the SAT system and may enhance removal of synthetic
organic compounds by stimulating greater microbiological
activity in the soil. However low hydraulic loading
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