Land treatment of wastewater 407concentration in the percolating water below allowable limits, generally less than
10 mg/L nitrate nitrogen or 45 mg/L nitrate to prevent methamoglobenemia, a
disease caused by drinking water that contains too high nitrate concentrations.
The factors that favor denitrification in the soil are: high organic matter, fine
textured soil, frequent wetting or high groundwater table, neutral to slightly
alkaline pH, vegetative cover, and warm temperature.
Denitrification losses of nitrogen can range up to 50% depending upon how
the land disposal site is managed. A conservative estimate would be to assume
denitrification and volatilization losses to be 20 to 30% of the applied nitrogen.
Consideration of plant uptake and these losses as the nitrogen control
mechanisms will reduce the risk of excessive nitrogen in the percolate
(Broadbent et al. 1977) as shown in example 8.2.
Pre-application approaches related to biological mechanisms could include
nitrogen removal prior to land application where the nitrogen application rate is
the limiting factor and the required land area is excessive. Such removal could
include nitrification followed by denitrification in preliminary wastewater
storage ponds.
8.4 System design and operation
Three methods can be used to determine wastewater application rates in land
treatment systems: field measurements; comparison with application rates from
similar existing projects; and combination of the above two methods together
with previous experience and judgement. In view of the present state of the art
in land treatment technology, the third method is recommended, so as to
maximise the land treatment efficiency and minimise operation and maintenance
costs.
In SR or irrigation systems, application rates are determined by using the
water balance, the nitrogen balance, and measured or estimated percolation
rates. In rapid infiltration systems, field measurements are necessary, coupled
with a detailed knowledge of the subsurface hydrology and comparisons with
existing systems. In overland flow systems, application rates depend mostly on
required wastewater treatment and are currently determined by comparison with
existing treatment systems or by calculation from kinetic equations.