Stabilisation pond (SP) and high-rate algal pond (HRAP) wastewater treatment systems are based on the same processes, but differ in their capacity to remove ammonia. This study analyzes the processes concerning ammonia elimination.

The different factors studied are: (i) light irradiance and temperature, (ii) phytoplankton uptake, and (iii) chemical characteristics affecting ammonia concentration. The variables relevant to these processes have been used in path analysis studies.

Different variables describing the studied processes have been used to quantify the different phenomena. A conceptual model has been developed, and applied to each pond. Path analysis technique has been applied. It is a causal modelling technique. The evaluation of causal models involves theoretical, methodological and statistical analysis. Theoretical considerations have been defined in the list of the processes studied. Methodological considerations involve the variable measurements on a defined time-scale. The statistical aspect consists of the evaluation of structural equation models.

The results of these studies show that the principal phenomena affecting ammonia in SP systems are temperature, pH, and total nitrogen input. In HRAP systems ammonia variability is directly linked to pH, temperature, nitrite and total nitrogen input, and indirectly linked to chlorophyll a concentration and pH.

The percentage of observed variability given by the models (R2 =coefficient of determination) ranges between 46% and 60.8%.

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