Environmental Engineering Reference
In-Depth Information
WSPS consists of series of artificial ponds, which are subject to a continuous flow and
are arranged in such a way that successive ponds receive their flow from the previous
one. The degree of treatment achieved is a function of the number and types of the ponds
in the system and their configuration. Thus the performance of each type of pond would
influence the performance of the subsequent one and the output of the system as a whole.
The most commonly applied types of ponds are the anaerobic (AP), facultative and
maturation ponds (M). Facultative ponds could be classified as primary (PF), which
receive directly screened sewage and secondary (SF), which receive treated effluent in an
AP or PF. The basic principles of the functioning of these three types of ponds have been
described in Chapter 2. In general, the main reduction of pollutants occurs in the
facultative stage. The maturation ponds have the primary objective of pathogen removal,
with some additional polishing functions with respect to nutrients and organic matter
removal. Some more contemporary modifications of the classic types of ponds are the
high rate algal pond (HRAP) and the macrophytes pond.
The HRAP is a modification of the facultative ponds, where conditions for accelerated
algae growth have been created in order to intensify the organic matter assimilation and
removal. These ponds are designed as very shallow (up to 0.5 m depth) long channels to
allow for light penetration. Furthermore, conditions for regular mixing have been
provided to prevent sludge layer formation. HRAP ponds could produce a high volume of
algae mass, which could be used as a protein source. It should be noted, however, that
these treatment units require a constant removal of the excess algae growth from the
suspension. The new alga mass generated should be removed regularly from the system
with the effluent. If not removed from the pond, the excess algae mass would settle and
decay in the pond, thus contributing an additional pollution load and jeopardizing the
treatment process. In general, HRAP have been developed as experimental units with the
primary objective of protein generation. However, the thickening and dewatering of the
algae mass would be a complex and expensive process, which would influence the
economic effectiveness of such a system. For this reason, HRAP have not found wide
application internationally.
Macrophyte ponds could be considered as an alternative of facultative ponds, which
have the primary objective of polishing the effluents by removing algae from the pond
effluent and nutrients reduction. Macrophytes, also known as water hyacinths, are aquatic
plants, which grow on the pond surface, forming a dense mat and preventing sunlight
from penetrating the water. Algae are also removed by biological means. A micro-
invertebrate, known also as
Daphnia
or water flea, breeds on macrophytes, and consume
algae in large quantity through grazing, thus helping in the algae removal process. It
should be emphasized that macrophyte ponds do not provide for pathogen removal. The
water hyacinth reproduces rapidly in tropical conditions and could double its mass in 6
days. Thus, such types of treatment units require a frequent harvesting of the biomass
generated in order to achieve their treatment effect, which in most cases should be done
mechanically or by hand. The harvested material could be used as a protein source to
complement animal feeding or as a substrate for anaerobic digestion and biogas
production. However, such applications have not been widely applied in practice. One of
the main disadvantages of macrophyte ponds is the fact that they provide a breeding
environment not only to the water flea but to other insects as well, including mosquitoes.
