Variations in BOD, algal biomass and organic matter biodegradation constants in a wind-mixed tropical facultative waste stabilization pond.

This study considered the impact of wind mixing on the efficiency of BOD removal and the first order biodegradation constant for organic matter in a primary facultative pond. Wind speeds of 1-4 m/s blowing from the effluent end of the pond towards the influent created surface-water flows of up to 0.94 m/s as determined by orange and coconut drogues moving in the opposite direction to the bulk hydraulic flow of 0.217 m/s. This was sufficient to cause mixing of the water column resulting in loss of stratification in terms of chlorophyll a, temperature and dissolved oxygen. BOD and chlorophyll a concentrations were spatially and temporally homogeneous throughout this large pond. BOD removal efficiency was only 50.30% as opposed to a projected value of 79% despite an acceptable surface organic loading of 350 kgBOD5/ha/d and an actual k value for BOD removal using influent sewage samples of 0.29 d-1 close to the projected value of 0.30 d-1. It would seem that wind mixing reduced pond efficiency by destroying stratification and thus reducing the microbial activity necessary to consume organic material. Mixing also increased the mean chlorophyll a concentration compared to stratified facultative ponds receiving similar loads and non-motile algae dominated the water column.

Determination of the sedimentation constants for total suspended solids and the algal component in a full-scale primary facultative pond operating at high wind velocities under tropical conditions.

This study evaluated the amount, distribution and sedimentation constant of solids in a full-scale primary facultative pond operating mostly under high wind conditions and the contribution made by the algal biomass. Solids deposition rates were measured using sedimentation traps placed in the inlet and outlet zones of the pond. Most sludge accumulation occurred, not surprisingly, in the inlet zone A1 with a sludge volume of 9072.m3 accumulating over an operating time of approximately 3 years. However, sludge deposition within this zone was uneven and affected by wind action. Mean proportionality constant (K) values for solids sedimentation were 3.02 and 5.70 for depths of 50 cm and 100 cm respectively for A1. In contrast in zone A3, (the outlet zone), reduced K values of 1.38 and 3.22 were obtained for depths of 50 cm and 100 cm respectively. The algal sedimentation constant varied from 0.8 d(-1) in zone A1 to 0.02 d(-1) in A3. These data suggest that in this large facultative pond the wind, blowing predominantly from the direction of the outlets towards the pond inlets, had a greater influence on solids deposition than the bulk hydraulic flow and also kept the pond completely mixed for most of the time.