356 Organic waste recycling: technology and management
Organic loading rate
Organic loading rate, expressed as kg / (m^2 -day), is the mass of applied organic
material per unit surface area of the system per unit time. It is a function of flow
rate and concentration of organic matter. Theoretically, organic loading rates are
dictated by a balance between the applied carbon and available oxygen based on
the oxygen/carbon stoichiometry of bacterial conversion. In practice, organic
loading rates, based on experience, are dependent on effective distribution of
wastewater to the system. To avoid odour problems due to uneven organic
loading, the wastewater should be distributed evenly over the entire pond
system.
A relationship between average BOD 5 loading and removal rates for aquatic
systems obtained from 24 different studies is shown in Figure 7.15. These data
suggest that aquatic systems using either emergent or floating plants could be
designed at BOD 5 loading rates up to approximately 110 kg / (ha-day) with 80%
or more of the applied BOD 5 being removed. It is apparent that a low effluent
BOD 5 concentration from aquatic systems can be achieved with a reduced
BOD 5 loading rate. BOD 5 removal rates naturally decrease during the winter
period if the influent wastewater has a low temperature (Figure 7.11).
Nitrogen loading rate
Nitrogen loading rate, expressed as kg / (m^2 -day), is defined as the mass of
applied nitrogen to the system per unit surface area per unit time. If plants are
being harvested on a regular basis, the nitrogen requirement for new plant
reproduction can be matched with the system nitrogen loading rate. Early in the
development of aquatic systems, nitrogen removal by plant uptake and
subsequent harvesting was suggested as the principal removal mechanism. More
recently the potential for nitrogen removal through nitrification/denitrification
has been identified. If nitrification is the major step for total nitrogen removal,
then nitrogen loading rate is not a valid design parameter. System environmental
conditions favoring nitrification are different from those that would enhance
denitrification. As a result, a loading rate based on a lumping of the various
forms of influent nitrogen into a single unit is not fundamentally sound.
From their experimental study Weber and Tchobanoglous (1985) found
ammonium nitrogen removal rate in an aquatic (water hyacinth) system to be a
function of the hydraulic application rate and reactor length. The ammonium
nitrogen conversion rate was independent of the ammonium loading rate; and
hydraulic retention time was not recommended as a design parameter. As shown
in Figure 7.16, ammonium removal rates were observed to be inversely
proportional to the hydraulic application rates, while Figure 7.17 shows the