Organic Waste Recycling

Aquatic weeds and their utilization 345

In general, chemical adsorption and precipitation are more significant
mechanisms of removal (Whigham et al. 1980) because:

• the potential rates of phosphorus removal by these mechanisms are
  greater than that achievable by plant uptake, and


• harvesting and disposal of plants is not a necessary part of removing
  phosphorus by chemical means.

However, the removal of phosphorus by chemical reactions cannot be
predicted accurately because of the many competing and interacting reactions
occurring simultaneously in the water column, detrital layer and sediments. The
major factors determining how much and at what rate phosphorus will be stored
in an aquatic system as a result of chemical reactions are pH, redox potential,
and the concentrations of iron, aluminium, calcium, and clay minerals. For
example, at pH greater than 8, metal phosphates precipitate from solution.
While, at pH less than 6 and redox potential greater than 200 mv, phosphate
adsorbs strongly to ferric oxyhydroxides and similar compounds. Many other
phosphate retaining reactions occur to a greater or lesser extent depending on
the aquatic environment. General conclusions as to what aquatic environmental
conditions will maximize phosphorus removal have not been reached, and
phosphorus removals from various aquatic treatment research projects and
systems have not been consistent (Cornwell et al. 1977; Dinges 1979; Reddy
and Sutton 1984; Wolverton and McDonald 1976a).
Ultimate disposal of phosphorus from aquatic system is by:

• harvesting the plants;


• dredging the sediments, and


• desorbing/ re-solubilizing phosphorus stored in the sediments and
  releasing it to the receiving water when it will have the least
  environmental impact.

Selection of the method will depend on design considerations such as the
discharge permit, environmental factors, and the rate of sediment build-up as
compared to the rate of phosphorus accumulation.

Heavy metals removal

Heavy metals are removed from wastewater during aquatic treatment by:

• Plant uptakes;


• Precipitation as oxides, hydroxides, carbonates, phosphates, and
  sulfides, and


• Ion exchange with and adsorption to sedimented clay and organic
  compounds.