Page 68 - March 2020 - WT Site

which also supply oxygen to the biological suspension . Natural Biological Treatment Systems
Aeration devices commonly used include submerged diffusers Natural low-rate biological treatment systems are available for the
that release compressed air and mechanical surface aerators that treatment of organic wastewaters such as municipal sewage and
introduce air by agitating the liquid surface. Hydraulic retention tend to be lower in cost and less sophisticated in operation and
time in the aeration tanks usually ranges from 3 to 8 hours but maintenance. Although such processes tend to be land intensive by
can be higher with high BOD5 wastewaters. Following the comparison with the conventional high-rate biological processes
aeration step, the microorganisms are separated from the liquid already described, they are often more effective in removing
by sedimentation and the clarified liquid is secondary effluent. A pathogens and do so reliably and continuously if properly designed
portion of the biological sludge is recycled to the aeration basin and not overloaded. Among the natural biological treatment
to maintain a high mixed-liquor suspended solids (MLSS) level. systems available, stabilization ponds and land treatment have
The remainder is removed from the process and sent to sludge been used widely around the world and a considerable record
processing to maintain a relatively constant concentration of of experience and design practice has been documented. The
microorganisms in the system. nutrient film technique is a fairly recent development of the
hydroponic plant growth system with application in the treatment
ii. Trickling Filters and use of wastewater.

A trickling filter or biofilter consists of a basin or tower filled with Wastewater Stabilization Ponds:
support media such as stones, plastic shapes, or wooden slats.
Wastewater is applied intermittently, or sometimes continuously, Facultative Ponds
over the media. Microorganisms become attached to the media
and form a biological layer or fixed film. Organic matter in The effluent from anaerobic ponds will require some form of
the wastewater diffuses into the film, where it is metabolized. aerobic treatment before discharge or use and facultative ponds
Oxygen is normally supplied to the film by the natural flow will often be more appropriate than conventional forms of
of air either up or down through the media, depending on the secondary biological treatment for application in developing
relative temperatures of the wastewater and ambient air. Forced countries.
air can also be supplied by blowers but this is rarely necessary.
The thickness of the biofilm increases as new organisms grow. Primary facultative ponds will be designed for the treatment
Periodically, portions of the film ‘slough off the media. of weaker wastes and in sensitive locations where anaerobic
pond odours would be unacceptable. Solids in the influent to a
The sloughed material is separated from the liquid in a secondary facultative pond and excess biomass produced in the pond will
clarifier and discharged to sludge processing. iii. Rotating Biological settle out forming a sludge layer at the bottom. The benthic
Contactors: They are fixed-film reactors similar to biofilters in layer will be anaerobic and, as a result of anaerobic breakdown
that organisms are attached to support media. In the case of the of organics, will release soluble organic products to the water
RBC, the support media are slowly rotating discs that are partially column above.
submerged in flowing wastewater in the reactor. Oxygen is Floating Aquatic Macrophyte Systems
supplied to the attached biofilm from the air when the film is out
of the water and from the liquid when submerged, since oxygen Floating macrophyte species, with their large root systems, are
is transferred to the wastewater by surface turbulence created by very efficient at nutrient stripping. Although several genera have
the discs’ rotation. been used in pilot schemes, including Salvinia, Spirodella, Lemna
and Eichornia has been studied in much greater detail. In tropical
Tertiary And/Or Advanced Treatment regions, water hyacinth doubles in mass about every 6 days and
a macrophyte pond can produce more than 250 kg/ha d (dry
Tertiary and/or advanced wastewater treatment is employed weight). Nitrogen and phosphorus reductions up to 80% and
when specific wastewater constituents which cannot be removed 50% have been achieved.
by secondary treatment must be removed.

70 Water Today - The Magazine March 2020