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

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.

Reliability analysis as a support tool for projects focusing on non-potable uses of treated wastewater in steel industries

“…The large difference between the means obtained in the treatment (μx = 0.37 MPN 100 mL -1 ) and the design mean (mx = 1320 MPN 100 mL -1 ) for thermotolerant coliforms indicates that the WWTP can meet more restrictive standards. This parameter had the highest CV (which is due to the microbiological indicator count varying according to many orders of magnitude) and the lowest COR value, as observed elsewhere (Alves et al, 2021).…”

“…The high CV for metals is justified by the low concentration means of the components, smaller than the standard deviation. Such behavior does not reveal a bad treatment performance, just unstable operational levels, as previously observed (Alves et al, 2021). Since, for the same reliability level, higher CVs indicate lower COR, WWTPs in this situation will need to operate with lower design concentrations (Oliveira and Von Sperling, 2008).…”

“…In the reliability study, a lognormal distribution was assumed for all analyzed parameters. This distribution is considered the most adequate for most of the wastewater constituents (Alderson et al, 2015;Alves et al, 2021;Oliveira and Von Sperling, 2008).…”

“…Silveira (2011) analyzed the performance, efficiency, and reliability of treatment plants located in Fortaleza, Ceará, Brazil, to accomplish the discharge limits on surface water. In a similar objective, Alves et al (2021) evaluated the reliability of stabilization ponds of 10 full-scale systems operating in Northeastern Brazil, finding that the 95% reliability scenario is not very feasible for these systems. The reliability concept can be used for different purposes.…”

Performance evaluation of wastewater treatment plants under the sewage variations imposed by COVID-19 spread prevention actions

“…Hotels, restaurants, and educational centers were some activities with the highest water reduction rate during COVID-19 pandemic [4]. WWTPs, respectively, it can be claimed that WWTPs were not be affected considerably during pandemic period [25,26]. It is good news for the users of the productions of WWTPs.…”