Ammonia removal was monitored in a waste stabilisation pond complex comprising ponds of different geometries and depths under two different operational regimes. It was found that a high degree of ammonia removal commenced in the secondary maturation ponds, with the highest removals occurring in the shallowest ponds as a consequence of improved aerobic conditions. The tertiary maturation ponds produced effluents with mean ammonia concentrations of < 5 mg N/l, the maximum permitted recommended by Brazilian environmental legislation for the discharge of effluents of wastewater treatment plants into surface waters. Ammonia removal in the secondary facultative and maturation ponds could be modelled using equations based on the volatilization mechanism proposed by Middlebrooks et al. (1982).
-This paper investigated the influence of organic loading on BOD and COD removal in primary facultative ponds. The study was based on six full-scale pond plants in which average removals of unfiltered biochemical oxygen demand (BOD) and chemical oxygen demand (COD) were 72 and 50%, respectively. For filtered samples, the removals were 89 and 83%, respectively. First-order removal rates assuming ideal hydraulic patterns (completely mixed and plug-flow) decreased with increments in the mean hydraulic retention time (HRT). Reduction in organic loading also caused a decrease in removal rates. The results emphasized that HRT and surface organic loading are more reliable to estimate first-order removal rates than traditional Arrhenius-style equations. Thus, HRT and surface organic loading can be used to compute more realistic first-order removal rates and surface removal rates. An alternative design procedure based on HRT and surface organic loading was proposed and demonstrated.
This paper assesses the reliability of Waste Stabilization Ponds (WSP) and proposes an alternative approach to WSP design based on the calculation of coefficient of reliability (COR) from an acceptable measure of violation of discharge standards. For that, data were collected from 10 full-scale systems operating in Northeast Brazil. All systems receive predominantly domestic effluent and are composed of one facultative pond and two serial maturation ponds. Different levels of restriction for effluent discharge were considered regarding the parameters: BOD, COD, total suspended solids, ammonia and thermotolerant coliforms. The Log-normal Probability Distribution Function (PDF) was able to represent the behavior of the concentration data in the effluent and, therefore, allowed the COR calculation. The COR was obtained from the coefficient of variation (CV) of the concentrations and the standardized normal variable associated with a 95% probability of non-exceedance. The observed dispersion of the results proved to be detrimental to the adoption of a single COR value for the evaluated parameters. In addition, the comparison between observed and design/operational concentration for optimal performance showed that the 95% reliability scenario represents a less achievable target for WSP systems.
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