3. Results and discussion
3.1. Entire treatment system efficiency
During the studied period, approximately 1140 m3 of swine
waste was treated. The entire treatment system showed a significant
efficiency of nutrient reduction (greater than 99% for TKN
and TP) and DO increase (Tables 2 and 3). Most likely, this is because
of the long HRT (more than 200 days), the high concentration
of raw influent and the use of different treatment stages (with aerobic
and anaerobic conditions) (Bortone, 2009). In addition, the pH
values remained nearly neutral, suffering a mild acidification along
the system stages (from 7.52 to 6.68). This pH range is expected for
swine wastes; however, duckweed ponds usually have low pH levels
compared to maturation ponds because of the algae growth
inhibition (Skillicorn et al., 1993; Costa et al., 2009). Phosphorus
retention in anaerobic units occurs mainly because of the settling
of inorganic phosphate compounds under anaerobic conditions.
Additionally, a high concentration of iron ions that are present in
this region could contribute to the formation of these compounds.
However, the greatest nutrient removal efficiency was observed in
the duckweed ponds, which is discussed in the following sections.
A wide variation in the raw manure composition was found
throughout the studied period. This range is primarily caused by
the management and hog production cycle, such as the number
and age of animals, diet composition and quantity of water used,
but this was expected. This variation in raw waste composition
can be seen in Table 3, as the high standard deviation from the
median. Nevertheless, high treatment efficiency was observed
through the stages of the system.