Aquatic weeds and their utilization 343Solids removal
Aquatic systems have long hydraulic residence times, generally several days.
Consequently, virtually all settleable and floatable solids of wastewater origins
are removed.
Non-settling/colloidal solids are removed, at least partially, by a number of
mechanisms. Colloidal solids tend to be the foci for bacterial growth. Such
growth during the residence time of the water in the aquatic system will result in
the settling of some solids and the microbial decay of others. Colloidal solids
will also be removed as a result of collisions (inertial and brownian) with an
adsorption to other solids such as plants, pond bottom and suspended solids.
Ultimate removal of suspended solids will be by bacterial metabolism:
anaerobic decay of settled solids and aerobic decay of floating solids entangled
in the surfaces of vegetation. The annual build-up of stable residues from these
decay processes at the bottom of aquatic systems will be quite less, so that
frequent dredging of the pond bottom will not be necessary.
Solids in aquatic system effluents will be decayed or detrital to aquatic plant
matter and bacterial flocs. Effluent SS concentrations are a function of water
velocity and turbulence in the aquatic system, the type(s) of plant being grown,
and the time of the year. An important function of the plants in aquatic systems
is attentuation of sufficient light to prevent algae growth. The plants also help
reduce the effects of wind (e.g., water turbulence) on aquatic systems. Effluent
SS concentrations are normally less than 20 mg/L, typically, and often less than
10 mg/L, particularly during summer and fall.
Nitrogen removal
Nitrogen is removed from wastewater by a number of mechanisms:
- Uptake by plants and subsequent harvesting of them.
- Volatilization of ammonia.
- Bacterial nitrification/denitrification.
Of these, bacterial nitrification/denitrification has the greatest nitrogen
removal potential. To maintain significant populations of the slowly
reproducing nitrifying bacteria in an aquatic system, the aquatic plants provide
structure to which these organisms can attach. For bacterial nitrification to
occur, the DO concentration of the wastewater must be above 0.6-1.0 mg/L
(Metcalf and Eddy Inc. 2003). Thus, the depth of zone below the water surface
in which nitrification will occur is a function of BOD loading rate and oxygen
flux into the aquatic environment. In estimating the depth of this zone for the
purpose of providing sufficient support structure for the design nitrification rate,