Tertiary Physico-chemical Treatment of Secondary Effluent from the Chania Municipal Wastewater Treatment Plant

The present study investigated tertiary physico-chemical treatment of the secondary effluent
from the Chania municipal Wastewater Treatment Plant (WTP). Laboratory experiments were
carried out with the aim of studying coagulation efficiency regarding reduction of turbidity,
soluble COD and phosphorus both in a conventional Coagulation-Settling treatment scheme,
as well as by means of Contact Filtration. The results showed that high doses of coagulants
(0,5 mmol Me+3 l-1 or higher) are required to achieve significant removals of turbidity after
settling. At these high doses, soluble COD can be removed by about 50%, while soluble
Phosphorus by 80-95%. Ferric Chloride demonstrated slightly better removal ability as
compared to Alum. The Chania WTP effluent was also treated by Contact Filtration, using a
very low dose of coagulants, 0,1 mmol Me+3 l-1. Turbidity was removed by around 50%, while
at this low coagulant dose removals of COD and Phosphorus were insignificant. Filtration was
effective in the first 35cm of the filter bed. No significant differences were observed between
the coagulants Alum and FeCl3 in the elimination of turbidity. Nevertheless, with the use of
Alum a smaller filter headloss was observed, during the first two hours of continuous filtration,
in comparison with the use of FeCl3 (nearly double). No difference was observed between the
headloss developed at a filter depth of 5cm as compared to that developed at a depth of
70cm. This indicates that the headloss increase was due to the accumulation of suspended
and colloidal solids within the first layers of the sand filter.