Performance evaluation and coefficients of reliability for waste stabilization ponds in northeast Brazil

Matheus Sales Alves,Fernando José Araújo Da Silva,Erlon Lopes Pereira,André Luís Calado Araújo

doi:10.4136/ambi-agua.2571

Matheus Sales Alves, Fernando José Araújo Da Silva + Show 2 more

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https://doi.org/10.4136/ambi-agua.2571

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Abstract

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.

Highlights

Benedetti et al (2010) defined the reliability of a wastewater treatment plant (WWTP) as the probability of the effluent concentrations to comply with specified discharge standards or treatment objectives during a given period of time
Niku et al (1979) based themselves on the lognormal distribution in order to develop the coefficient of reliability (COR), an index that relates the mean concentration of a parameter to the quality standards to be complied with a required reliability level
This study evaluated the performance and the coefficients of reliability (COR) for full scale Waste Stabilization Ponds (WSP) systems operating in Northeast Brazil

Summary

Introduction

Benedetti et al (2010) defined the reliability of a wastewater treatment plant (WWTP) as the probability of the effluent concentrations to comply with specified discharge standards or treatment objectives during a given period of time. The probability (p) of the standard or a target being exceeded in the effluent depends on the distribution function describing the quality parameters concentrations In this sense, the right-skewed probabilistic distributions are common when the analyzed mean data are low, present large variance, and cannot be negative (Limpert et al, 2001), such as WWTP effluents. The COR was developed based on TSS and BOD data from activated sludge systems, wastewater treatment processes with high degree of mechanization and operational flexibility This method is recommended for use by the USEPA (Niku and Schroeder, 1981) and recognized in the technical literature (Tchobanoglous et al, 2003; Oliveira and Von Sperling, 2008; OwusuAnsah et al, 2015; Andraka, 2019)

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