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.
Post-treatment of anaerobic reactor effluent with maturation ponds is a good option for small to medium-sized communities in tropical climates. The treatment line investigated, operating in Brazil, with an equivalent capacity to treat domestic sewage from 250 inhabitants, comprised a upflow anaerobic sludge blanket reactor followed by two shallow maturation ponds (unbaffled and baffled) and a granular rock filter (decreasing grain size) in series, requiring an area of only 1.5 m inhabitant. With an overall hydraulic retention time of only 6.7 days, the performance was excellent for a natural treatment system. Based on over two years of continuous monitoring, median removal efficiencies were: biochemical oxygen demand = 93%, chemical oxygen demand = 79%, total suspended solids = 87%, ammonia = 43% and Escherichia coli = 6.1 log units. The final effluent complied with European discharge standards and WHO guidelines for some forms of irrigation, and appeared to be a suitable alternative for treating domestic sewage for small communities in warm areas, especially in developing countries.
The results of tests with a sodium chloride tracer in a shallow maturation pond in Brazil are presented. The pond – a full-scale effluent treatment unit for around 250 p.e – was 0.80 m deep with a length/width ratio of 4.3:1, and had accumulated sludge for 12 years to about 40% of the total volume. The pond is subject to daily cycles of thermal stratification/destratification, as shown by vertical profiles of temperature, dissolved oxygen, pH and redox potential, and by Computational Fluid Dynamics simulations. Four tracer tests were undertaken. There was a tendency to complete mixing with high dispersion, and daily vertical cycles were evident in all tracer response curves, a distinctive feature of these studies. However, the reliability of the hydraulic retention time calculation, and the short-circuit and mixing indicators, is doubtful, primarily because of the liquor's high density. The study's results enabled practical consideration of the use of saline tracers in pond systems. Sodium chloride acquisition and semi-continuous sensor measurements of effluent concentrations (electrical conductivity) are both easy, but the large amount of salt required (around 300 kg for a 125 m3 pond) and solution density are limiting factors, and can yield unrepresentative results, making this tracer potentially impractical, especially for larger ponds. However, the diurnal cycles of effluent tracer concentration were important in this study and assisted in verification of hydraulic behavior in the liquid column, associated with the stratification and mixing patterns.
The study covers different investigations related to the upgrading of a post-treatment system for the effluent from a upflow anaerobic sludge blanket reactor. The original post-treatment scheme comprised three ponds in series and a small coarse rock filter inserted in the last pond. Upgrading involved reducing the pond depths, applying baffles in the second pond and converting all of the third pond into a rock filter (three decreasing grain sizes). The system was conceived for 250 population equivalents, occupied an area of only 1.5 m2/inhabitant and aimed at very good removals of all major wastewater constituents. Overall final effluent concentrations for Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Total Suspended Solids (TSS) were very good, and lower than those from the previous treatment line. Hydraulic flow patterns in both ponds showed daily thermal stratification and destratification periods. The conversion of the third pond into a rock filter increased the removal capacity of TSS (algae), but could potentially result in clogging, a phenomenon that was evaluated by a geophysical method called Georadar. The influence of accumulated sludge on treatment performance was tested before starting the operation of the upgraded system by operating the two ponds in parallel (one with sludge accumulated after 11 years of operation and the other without sludge). In this test, ammonia removal was virtually the same in both ponds.
This study aimed at evaluating the occurrence of stratification/equalization cycles in two full-scale shallow maturation ponds in Brazil, with different operational configurations and different periods of the year, through monitoring of temperature and other constituents in the vertical profile of the ponds. The study comprised two operational phases: one phase in which both ponds had no baffles and operated in parallel (one pond had sludge accumulated on the bottom while the other did not); and another phase in which the ponds operated in series, the first pond without baffles and with accumulated sludge on the bottom while the second pond had two longitudinal baffles, a shallower depth and no accumulated sludge. Overall, there were systematic daily periodic events of thermal stratification followed by temperature equalization and vertical mixing in both ponds and operating phases. Vertical temperature gradients were predominantly in the 0–7 °C m−1 range. Statistical tests showed a significant positive correlation between the thermal gradient in the pond and air temperature, but not between the thermal gradient and wind speed. Data suggested that ponds remained 56% of the time under thermal stratification and 44% in vertical mixing. The data also highlighted the importance of sludge in the thermal balance of ponds.
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