EFFECT OF ALGAL DENSITY ON NUTRIENT REMOVAL FROM PRIMARY SETTLED WASTE-WATER

Laboratory scale batch experiments were employed to examine the effect of initial inoculum sizes of algae on their activities and reduction of nutrients form the primary settled municipal sewage. the microalgae, Chlorella vulgaris, with four initial inoculum sizes, namely 'superconcentrated' (1 x 10(7) cells ml(-1)) 'concentrated' (5 x 10(6) cells ml(-1)), 'medium' (1 x 10(6) cells ml(-1)) and 'low' (5 x 10(5) cells ml(-1)), were applied to primarily treated sewage. the cell number and chlorophyll content of all algal cultures increased gradually in the first week and the rates of increase were similar in all cultures. During the experimental period, even in the 'superconcentrated' cultures, the self-shading problem was not found probably due to good mixing. At the end of the 10 days study, over 90% HN4+-N and 80% PO43+-P were removed from wastewater in all cultures except the 'low' one. the residual concentrations of NH4+-N, PO43--P, TKN and total P in wastewater were negatively correlated with the cell numbers and chlorophyll content of the cultures. These results indicated that the efficiency of reducing wastewater-borne nutrients by an algal system was directly related to the physiological activity and growth of the Chlorella cells which in turn were affected by the initial inoculum size. The super-concentrated culture with an initial algal density of 1 x 10(7) cells ml(-1) seemed to be more beneficial as this treatment achieved satisfactory nutrient removal within 7 days instead of 10. the sharp initial reduction of COD (>50%) and total organic-N (>60%) was not related to the algal number or chlorophyll content, the performance in the control (without algal cells) was similar to that of the algal cultures. This suggests that the removal of COD and TON was mainly due to the metabolism of the indigenous bacteria. Under the open system, the interaction between algal and bacterial cells was significant which could enhance the simultaneous removal of N, P and organic matter from primary settled sewage.