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Maturation (polishing) ponds operating after UASB (upflow anaerobic sludge blanket) reactors are considered a highly suitable system for treating domestic sewage for small to medium communities in warm-climate developing countries. Important attributes are operational simplicity, good organic matter and nitrogen removal, excellent pathogen removal and lower land requirements compared to traditional pond systems. However, most papers in the literature focus on single studies investigating one particular aspect of this system. The objective of the current paper is to present a broad evaluation of one particular system (UASB reactor, three shallow ponds and a coarse rock filter in series) in Brazil treating sewage from 250 inhabitants over a period of 10 years. The study covers in an integrated manner the removal and behaviour of several variables, such as organic matter (BOD and COD), solids, nitrogen fractions, phosphorus, coliforms, pathogenic bacteria, helminth eggs, sulphides, surfactants and organic micropollutants (diethylphthalate, bis(2ethylhexyl)phthalate and bisphenol A). Even with short hydraulic retention times in each pond (2 to 6 days), the results endorse the good capacity for organic matter and ammonia removal and excellent removal of coliforms, helminth eggs and sulphides. Removal of phosphorus, surfactants and selected micropollutants was limited.

Polishing ponds are natural systems used for the post-treatment of upflow anaerobic sludge blanket (UASB) effluents. They are designed as maturation ponds and their main goal is the removal of pathogens and nitrogen and an additional removal of residual organic matter from the UASB reactor. This study aimed to evaluate organic matter and suspended solids removal as well as sludge accumulation in two shallow polishing ponds in series treating sanitary effluent from a UASB reactor with a population equivalent of 200 inhabitants in Brazil, operating since 2002. For this evaluation, long-term monitoring of biochemical oxygen demand and total suspended solids and bathymetric surveys have been undertaken. The ponds showed an irregular distribution of total solids mass in the sludge layer of the two ponds, with mean accumulation values of 0.020 m3 person−1 year−1 and 0.004 m3 person−1 year−1 in Ponds 1 and 2, leading to around 40% and 8% of the liquid volume occupied by the sediments after 11 years of operation. The first pond showed better efficiency in relation to organic matter removal, although its contribution was limited, due to algal growth. No simple input–output mass balance of solids can be applied to the ponds due to algal growth in the liquid phase and sludge digestion in the sludge.

Stabilization ponds are a highly appropriate system for treating sewage in small to medium size communities. However, sludge accumulation at the pond bottom occurs with the passage of time, reducing the net pond volume, which, in principle, could affect its performance. The objective of this paper is to compare the behaviour of two equal ponds in parallel treating the same flow of municipal wastewater from an upflow anaerobic sludge blanket reactor in Brazil. Each pond treated a population equivalent of around 125 inhabitants. One pond had approximately 40% of its net volume occupied by sludge after 11 years of operation, while the other pond had previously undergone complete desludging. The study covers the removal of biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), nitrogen fractions and coliforms. Owing to the presence of a sludge layer, the theoretical hydraulic retention time (HRT) was lower in the pond without sludge. For BOD, COD, SS and Escherichia coli there were no significant differences (Wilcoxon matched-pairs test) between both ponds. The pond without sludge had significantly better removal efficiencies in terms of total Kjeldahl nitrogen and ammonia-N. The sludge layer probably allowed the occurrence of removal mechanisms that compensated for the reduction caused in the HRT.