Environmental Engineering Reference
In-Depth Information
greatest with the mixture. The greatest removal of P by plants growing on
full-strength wastewater was by the cattail.
Mean total K removed by the cattail ranged from a low of 10.4 g m
-2
on 30 Sept.
2002 for the wastewater treatment to 213.5 g m
-2
for the inorganic treatment on 3
June 2002 (Table 9.4). There were no statistically significant differences between
treatments for the cattail cuttings on specific dates except for the cutting of 20
Aug. 2001, when the plants grown on the inorganic treatment had significantly less
removal of K than the other two treatments, and the cutting of 15 Oct. 2001, when
the inorganic treatment had significantly less K removal than the plants grown on the
mixture. Mean total removal of K per cutting by the rush was much lower than that
removed by the cattail and maidencane. Mean total K removal by the maidencane
was generally greatest for the mixture. The total removal of K by the cattail was
significantly greater than that removed by the rush and the maidencane except for
the mixture treatment of the maidencane. Overall, the cattail removed the greatest
K from full-strength wastewater.
Mass balance calculations were made of the total percent nutrient removal by
the floating vegetated mats (Table 9.5). For these calculations, it was assumed
that the root zone depth was 15 cm for both the rush and maidencane, and that
it was 30 cm for the cattail. It was also assumed that the total available nutri-
ents to the plants was the sum of the nutrients contained in the 26 applications
of the solutions for the total wastewater volume corresponding to the thickness
of this root zone. In reality, this is not true, since nutrients in solution below
the assumed root zone thickness could freely mix with the solution in the root
zone. Hence, where the mass balance calculations (Table 9.5) show percent-
ages greater than 100, natural mixing of the solutions within the tanks clearly
provided nutrients in excess of those calculated using our root zone thickness
assumptions.
Mass balance calculations for N removal ranged from a low of 4% for the rush
grown on wastewater to a high of 157% for the maidencane grown on the mixture.
For both the cattail and rush, the percent N removal decreased in the order inorganic
> mixture > wastewater. For P, the percent removal ranged from 3% for the rush
grown on wastewater to 130% for the maidencane grown on the mixture. For K,
mass balance calculations showed a different pattern than that observed for N and
P. Both the cattail and maidencane removed as much or more K than that contained
in the root zone thickness.
Overall, the mass balance calculations showed that we were meeting the nutrient
needs for the cattail and maidencane with the inorganic treatment, had an excess
of N and P with the full-strength wastewater, and that with greater growth on
1
/
2
-
strength wastewater, the maidencane utilized N in excess of that from the assumed
root zone thickness. For both the cattail and rush, more K was removed than was
available from the assumed root zone thickness. Clearly, a wastewater lagoon with
continuous inputs of fresh animal waste will in most cases provide nutrient amounts
such that the plants on the floating mats are in a luxury uptake situation, and
removal of nutrients from the lagoon will be a function of biomass produced and
the maximum plant nutrient uptake levels.
