Search

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.