Effects of power level, organic loading and temperature on the performance of facultative aerated lagoons

Abstract

Facultative aerated lagoons are aerated lagoons operated at low power levels and are wastewater stabilization ponds with artificial aeration. These lagoons are used more commonly than aerobic aerated lagoons because a satisfactory effluent can be produced with a lower power input. The power level applied to facultative aerated lagoons is sufficient only to satisfy the oxygen demand but not adequate to keep all the solids in suspension and settled solids will decompose aerobically and anaerobically. In the study reported, laboratory-model facultative aerated lagoons of 81 litres volume, aerated with diffused air, were used to study the performance of such lagoons in the treatment of municipal wastewater. Different combinations of four power levels, 0.25,0.5,1 and 2.0 W/m3, three different organic loadings, 20,33 and 62 g BOD5/m3d. , and two temperature levels, 20°C and 30°C, were applied in twenty four experimental runs. Influent and effluent were sampled on a regular basis and their characteristics were determined. The effluent from the model facultative aerated lagoons was always of reasonable quality, with respect to BOD5, COD and suspended solids. This was achieved with no provision for effluent settling or additional treatment. Removals of 91 percent BOD5 and 67 percent COD could be achieved for unfiltered samples. Effluent BOD5 of 13 mg/l in the filtered samples and 31 mg/l in the unfiltered samples was attainable in these lagoons. Effluent suspended solids levels as low as 41 mg/i were also obtained. Thus facultative aerated lagoons will provide both biological and physical treatment operations in a single earthen tank. Because suspended solids in the effluent from facultative aerated lagoons are low, no sludge disposal or processing is needed on a continuous basis. Other performance criteria; nitrogen, phosphorus, iii chlorophyll "a", Escherichia. coli and faecal streptococci, are reported on in the thesis. It was observed that the level of power introduced into the facultative aerated lagoon had positive and significant effects on some performance parameters, including BOD5 and COD filtered removal rate coefficients, removal efficiencies for BOD5 and COD (except for COD removal in facultative aerated lagoons operated at high temperature, 30°C) and effluent suspended solids and negative and significant effects for others, such as suspended solids removal efficiency. The effect of power was insignificant for other parameters, especially BOD5 and COD unfiltered removal rate coefficients. The effect of power level on filtered removal efficiencies was higher than on unfiltered ones. BOD5 and COD removal efficiencies were negatively affected by organic loading (or positively by retention time) and the effect was found to be significant. The organic loading effect was significant and positive on filtered BOD5 and both filtered and unfiltered COD removal rate coefficients whereas it was negative on unfiltered BOD5 removal rate coefficients. Temperature had significant and positive effects on some parameters, including removal rate coefficients both filtered BOD5 and COD as well as unfiltered BOD5 and BOD5 and COD removal efficiencies, and insignificant effects on others, such as unfiltered COD removal rate coefficient. The effect of temperature on the removal rate coefficients, except the unfiltered COD removal rate coefficient, was higher at higher organic loadings (shorter retention times) whereas its effect on BOD5 and COD removals efficiencies was higher at lower power levels. The temperature correction coefficient for BOD5 at low power levels was higher than at higher levels of power. iv The effect of power level on the temperature correction coefficient was significant whereas the organic loading (or retention time) effect was insignificant. Relationships between the individual operating parameters and performance parameters are presented in the form of empirical equations and the combined effects of these operating parameters and performance parameters were also modelled. High organic loading (short retention time) in facultative aerated lagoons operated at low power levels proved to remove more organic material per day per unit of power introduced into the lagoon. Therefore, the optimum conditions of organic loading and power level at which a single facultative aerated lagoon used as a sole treatment process for treating settled sewage should be operated are 0.25 W/m3 for power level and 62 g BOD5 /m3. d for organic loading (3 to 4 days retentiom time). The effect of mixing, represented by the parameters in a simulation model, on performance was also modelled in the form of empirical equations. Nitrogen, phosphorus, Escherichia coli and faecal streptococci removals were considerable. Variations of their removal performance as well as algal concentration variations with the operating parameters were also studied and discussed. A mathematical equation was developed for the determination of the mean solids retention time (SRT) in facultative aerated lagoons. The relationship between SRT and power level and organic loading was determined and represented by an empirical equation. A power level of around 0.5 W/m3 was the threshold for settleable solids suspension below which no significant decrease in effluent or mixed liquor suspended solids concentration would take place.