Comparative performance of macrophyte and algal-based waste water continuous flow treatment systems

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lines using duckweed (Spirodella), water lettuce (Pistia stratiotes) and algae (natural colonization) was set up to determine the environmental conditions, faecal coilform profiles and general performance. A flow rate of 0.01m3/day was maintained in all treatment systems. Each treatment line consisted of four ponds in series with 7 days retention in each pond and was fed by diluted sewage (1part sewage to 2parts tap water). Parameters measured included temperature, pH, DO, TDS, turbidity, BOO, COD, nitrate, nitrite, ammonia, phosphate, faecal coliforms, mosquito larvae, total suspended solids, specific gravity, settleable solids and sludge accumulations. The studies were conducted one month after algae colonization, and lasted from February to August, 2000. After six months of operation, the containers were emptied and the sludge accumulation measured. Faecal coliforms, pH, DO and temperature profiles were determined in the sediments (0. 63m), suspensions (0. 35m) and surfaces (0. 0lm) of each pond. BOD removal was highest in the duckweed treatment line, followed by water lettuce and algae at 93%, 70% and 24% respectively; COD removals were 65 and 59% respectively for duckweed and water lettuce while the loves rather increased in the algae ponds. Nitrate nitrogen removals were 36, 71 and 72% respectively for algae, water lettuce and duckweed. Nitrite levels increased in all the treatment lines. Total phosphorus removals were 49, 9.6 and 32% respectively for algae, duckweed and water lettuce lines. Ammonia removals wore 92, 89 and 87% respectively in the algae, water lettuce and duckweed lines. TDS was highest in the duckweed ponds and least in the Pistia ponds. Removals of TDS were 9, 15, and 70% for algae, duckweed and Pistia respectively in the effluent discharges. Total suspended solids, turbidity, specific gravity, settleable solids and sludge accumulations were higher in the algal ponds than in the macrophytes. All the parameters decreased with increasing retention period in all the systems. Alkaline conditions and high dissolved oxygen characterised algae ponds, while acidic and neutral conditions were associated with water lettuce and duckweed ponds respectively. Dissolved oxygen levels were characteristically lower in the macrophyte ponds than in the algae ponds. Prevailing temperatures were similar in all the treatment ponds, though slightly higher values were observed in the algae ponds. Sedimentation accounted for about 99% feacal coilform removal in the algae and water lettuce ponds. Log removals of 5, 3 and 2 were observed in algae, duckweed and water lettuce ponds in the final effluents respectively. Average mosquito larvae in all four ponds of each treatment line were 19/100ml for water lettuce, 6/100ml for algae and 0.15/100mls in duckweed ponds. Low turbidity and low sludge accumulation were associated with the macrophyte ponds.
A thesis submitted to the Department of Biological Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of the Degree of Master of Science in Environmental Science, 2000