El Halouani, H., Picoti, B., Casellas, C., Pena, G. and
J. Bontoux (1993). Removal of nitrogen and phosphorus by high rate algal
pond system. Rev. Sci. Eau, 6 (1) : 47-61. [article in
Original title : Elimination de l'azote et du phosphore
dans un lagunage à haut rendement.
High raie algal ponds (HRAP) for wastewater treatment have
been the locus of a lot of attention since their creation by Professor W.J.
OSWALD in the early 1960' s. These aquatic ecosystems are hypereutrophic
because of massive nutrient introduction with the wastewater influent. This
pond is very different from oxidation ponds because al short residence times
(2 to 12 days), shallow depths (0.30 to 0.60 m) and constant mechanical mixing
which improves algal growth.
This study has been carried out in a small town in the department of Herault
(France). The objective is to establish the efficiencies and mechanisms for
nitrogen and phosphorus remval from secondary domestic wastewater by treatment
by HRAP. A high rate algal pond of 48 m2 surface area and 35 cm
depth, constantly mixed with paddle wheels was studied (fig. 1). Residence
limes changed from 2 te 12 days with solar radiations and influent loading.
The samples were collected
once a week from the in- and outflow at 3 p.m. - Dissolved and particulate
COD, varions nitrogen and phosphorus compounds, suspended solide, chlorophyll-a,
pH, dissolved oxygen, temperature and irradiation were determined.
The results of principal component analysis show a negative correlation between
(1) the climatic and photosynthetic parameters and (2) inorganic nitrogen and
phosphorus compounds, as well as the influence of organic loading on the HRAP
The chronological clustering analysis was performed on the data
taking into account the discontinuities of effluent autrient contents (fig.
4). During the first year, from February to September 1988, good climatic
conditions were shown to favour photosynthetic activity and consequently
a good ammonia
and orthophosphate removal. Front September 1988 to February 1989, the effluent
was characterized by high ammonia and orthophosphate levels because of unfavourable
climatic conditions and low photosynthetic activity. During the second year,
the objective was to improve removal efficiencies ; the residence time was
changed with solar radiations and influent loading. The results of the chronological
clustering analysis of ammonia and orthophosphate levels showed only one
sequence (February 1989-January1990) because the seasonal variations of nutrient
removal were attenuated. The optimal residence time was found to be 4 days
in summer and 12 days in winter.
The evolution of dissolved chemical oxygen demand is independent of seasonal
conditions but decreases with the maturity of the system (< 50 mg . l-1).
The mass balance of nitrogen and phosphorus compounds has been established
(fig. 5). In high rate algal pond influent, the main nitrogen form
is ammonia. In the effluent the main nitrogen form is suspended organic
oxidized forms (NO2- et NO3) present very low levels
; nitrification is a minor process
in the system. Ammonia is reduced by both assimilation and stripping ; suspended
organic nitrogen in the effluent is an indicator of assimilation by algal
biomass and loss of nitrogen is an indicator of ammonia stripping. With
regard to directive
91/271 EEC concerning domestic wastewater treatment, partial algal separation
is necessary in order to achieve 15 mg • l-1 total nitrogen, which
is the limit for zones sensitive to eutrophication.
The residual phosphate levels are controlled by pH, which is responsible
for instantaneous calcium phosphate precipitation. As the dissolved calcium
are high in the calcareous region near the site (Ca = 125 mg • l-1),
the pH of the water will be the main contraint on phosphate elimination in
the HRAP. Theoretical calculations indicate that the precipitate is probably
an amorphous tricalcium phosphate in pseudoequilibrium (fig.6).
Phosphate concentrations are much higher titan the values expected for calcium
(MOUTIN et al., 1992). The limit of 2 mg • l-1 for phosphores can only
be attained when pH reaches or exceeds 9.
Nitrogen and phosphorus removal is correlated mainly with phytoplanktonic
activity, which controls biological nutrients assimilation and pH levels.
High pH values
are responsable for ammonia stripping and calcium phosphate precipitation.
High rate algal pond, wastewater treatment nitrogen,
El Fialouani, H., Département Sciences de l'environnement
et Santé Publique, URA CNRS 1355, Faculté de pharmacie, Avenue
Charles Flahault, 34060 Montpellier cedex