Aerobes and Effluents 125Table 6.3 Horticultural waste process liquor analysis before and after 85-day aeration
treatment and the associated percentage reductions achieved
Determinant Baseline Post-treatment % reduction
pH 5.8 8.8 –
Conductivity @20◦C 6 950 6 320 9.1
BOD total+ATU 15 800 198 98.7
COD 27 200 1 990 92.7
Solids particulate 105◦C 6 200 28 99.5
Total dissolved solids 13700 293 97.9
Ammoniacal nitrogen 515 316 38.6
Total oxidised nitrogen 1.7 0.3 82.4
Kjeldahl nitrogen 926 435 53.0
Nitrite 0.79 0.04 94.9
Nitrate 0.9 0.3 66.7
Sulphate 194 63.4 67.3
All in mg/l except pH (in pH units) and conductivity (in mS/cm). Results courtesy of Rob Heap,
unpublished project report.
The UFB system is the most expensive, both to install in the first place and
subsequently to run, as it requires comparatively high maintenance and needs
a filtered air supply to avoid air-borne particulates blocking the narrow diffuser
pores. Illustrative UFB aeration results, based on operational data, obtained from
the amelioration of post-anaerobic digestion liquor from a horticultural waste
processing plant, are shown in Table 6.3.
Though the comparatively simple approaches which produce large to medium
sized bubbles are the least efficient, they are commonly encountered in use since
they offer a relatively inexpensive solution.
Mechanical aeration systems
In this method, a partly submerged mechanically driven paddle mounted on floats
or attached to a gantry vigorously agitates the liquid, drawing air in from the
surface and the effluent is aerated as the bubbles swirl in the vortex created.
Other variants on this theme are brush aerators, which are commonly used to
provide both aeration and mixing in the sewage industry and submerged turbine
spargers, which introduce air beneath an impeller, which again mixes as it aer-
ates. This latter approach, shown in Figure 6.2, can be considered as a hybrid
between mechanical and diffused systems and though, obviously, represents a
higher capital cost, it provides great operational efficiency. A major factor in this
is that the impeller establishes internal currents within the tank. As a result the
bubbles injected at the bottom, instead of travelling straight up, follow a typi-
cally spiral path, which increases their mean transit time through the body of the
liquid and hence, since their residence period is lengthened, the overall efficacy
of oxygen diffusion increases.